Oxidative stress-induced inhibition of adrenal steroidogenesis requires participation of p38 mitogen-activated protein kinase signaling pathway

in Journal of Endocrinology

Previous studies from this laboratory identified excessive oxidative stress as an important mediator of age-related decline in steroid hormone production. Here, we investigated whether oxidative stress exerts its antisteroidogenic action through modulation of oxidant-sensitive mitogen-activated protein kinase (MAPK) signaling pathways. To accomplish these studies, we employed a highly responsive mouse adrenocortical cell line, Y1-BS1 cells that secrete large quantities of steroids when stimulated with lipoprotein plus hormone. Treatment of these cells with superoxide, H2O2 or 4-hydroxy-2-nonenal (HNE) significantly inhibited steroid production and increased phosphorylation and activation of p38 MAPK. None of the treatments altered the phosphorylation of either extracellular signal-regulated kinases or c-Jun N-terminal kinases (JNKs). Pretreatment of Y1-BS1 cells with MnTMPyP, a cell-permeable superoxide-dismutase/catalase mimetic reactive oxygen species (ROS scavenger), completely prevented the superoxide- and H2O2-mediated inhibition of steroid production. Likewise, antioxidant N-acetylcysteine completely blocked the HNE-induced loss of steroidogenic response. Incubation of Y1-BS1 cells with either MnTMPyP or NAC also upregulated Bt2cAMP and Bt2cAMP+hHDL3-stimulated steroid synthesis, indicating that endogenously produced ROS can inhibit steroidogenesis. Inhibition of p38 MAPK with SB203580 or SB202190 upregulated the basal steroid production and also prevented the oxidant-mediated inhibition of steroid production. mRNA measurements by qPCR indicated that Y1-BS1 adrenal cells predominantly express p38 MAPKα isoform, along with relatively low-level expression of p38 MAPKγ. By contrast, little or no expression was detected for p38 MAPKβ and p38 MAPKδ isoforms in these cells. Transfection of Y1-BS1 cells with either caMKK3 or caMMK6 construct, the upstream p38 MAPK activators, decreased steroidogenesis, whereas transfection with dnMKK3 or dnMKK6 plasmid DNA increased steroidogenesis. Similarly, transfection of cells with a dnp38 MAPKα or dnp38 MAPKβ construct also increased steroid hormone production; however, the effect was less pronounced after expression of either dnp38 MAPKγ or dnp38 MAPKδ construct. These results indicate that activated p38 MAPK mediates oxidant (excessive oxidative stress)-induced inhibition of adrenal steroidogenesis.

Abstract

Previous studies from this laboratory identified excessive oxidative stress as an important mediator of age-related decline in steroid hormone production. Here, we investigated whether oxidative stress exerts its antisteroidogenic action through modulation of oxidant-sensitive mitogen-activated protein kinase (MAPK) signaling pathways. To accomplish these studies, we employed a highly responsive mouse adrenocortical cell line, Y1-BS1 cells that secrete large quantities of steroids when stimulated with lipoprotein plus hormone. Treatment of these cells with superoxide, H2O2 or 4-hydroxy-2-nonenal (HNE) significantly inhibited steroid production and increased phosphorylation and activation of p38 MAPK. None of the treatments altered the phosphorylation of either extracellular signal-regulated kinases or c-Jun N-terminal kinases (JNKs). Pretreatment of Y1-BS1 cells with MnTMPyP, a cell-permeable superoxide-dismutase/catalase mimetic reactive oxygen species (ROS scavenger), completely prevented the superoxide- and H2O2-mediated inhibition of steroid production. Likewise, antioxidant N-acetylcysteine completely blocked the HNE-induced loss of steroidogenic response. Incubation of Y1-BS1 cells with either MnTMPyP or NAC also upregulated Bt2cAMP and Bt2cAMP+hHDL3-stimulated steroid synthesis, indicating that endogenously produced ROS can inhibit steroidogenesis. Inhibition of p38 MAPK with SB203580 or SB202190 upregulated the basal steroid production and also prevented the oxidant-mediated inhibition of steroid production. mRNA measurements by qPCR indicated that Y1-BS1 adrenal cells predominantly express p38 MAPKα isoform, along with relatively low-level expression of p38 MAPKγ. By contrast, little or no expression was detected for p38 MAPKβ and p38 MAPKδ isoforms in these cells. Transfection of Y1-BS1 cells with either caMKK3 or caMMK6 construct, the upstream p38 MAPK activators, decreased steroidogenesis, whereas transfection with dnMKK3 or dnMKK6 plasmid DNA increased steroidogenesis. Similarly, transfection of cells with a dnp38 MAPKα or dnp38 MAPKβ construct also increased steroid hormone production; however, the effect was less pronounced after expression of either dnp38 MAPKγ or dnp38 MAPKδ construct. These results indicate that activated p38 MAPK mediates oxidant (excessive oxidative stress)-induced inhibition of adrenal steroidogenesis.

Introduction

Advancing age in humans (Shifren & Schiff 2000, Burger et al. 2002, Arlt 2004, Dharia & Parker 2004, Harman 2005, Kaufman & Vermeulen 2005) and experimental animals (Zirkin & Chen 2000, Medicherla et al. 2002, Wang & Stocco 2005) is associated with profound changes in the synthesis and secretion of steroid hormones. In rats, this steroidogenic decline is found in adrenocortical (Malamed & Carsia 1983, Popplewell et al. 1986, Cheng et al. 1990) and testicular Leydig cells (Bethea & Walker 1979, Tsitouras et al. 1979, Liao et al. 1993, Chen et al. 1994) of several strains and is a progressive defect increasing in impact as the rat ages from 5 months to 12, 18, and 24 months (Malamed & Carsia 1983, Popplewell et al. 1986, Liao et al. 1993, Medicherla et al. 2002). The major functional alteration occurs distal to sites of tropic hormone interaction and cAMP generation (Tsitouras et al. 1979, Popplewell et al. 1986, Liao et al. 1993) and is primarily related to a defect in the transport of free cholesterol to CYP11A1 (P450scc) sites within the mitochondria where the conversion of cholesterol to pregnenolone (parent steroid) begins with side-chain cleavage (Popplewell & Azhar 1987, Liao et al. 1993).

Although the cellular and molecular mechanisms controlling this aging defect (i.e. impaired cholesterol transport to mitochondria) have not been completely defined, considerable evidence from this laboratory points to excessive free radical formation and oxidative damage (especially from lifelong continued processing of cholesterol for steroid production (Azhar et al. 1995, Hanukoglu 2006)) to the cell machinery regulating the functional expression of key proteins involved in mitochondrial cholesterol transport (Azhar et al. 1995, Leers-Sucheta et al. 1999, Abidi et al. 2004, Cao et al. 2004). For example, previous studies from this laboratory have shown that age-induced loss of steroidogenic function is accompanied by a significant alteration in oxidative status of rat adrenal and testicular tissues including enhanced oxidative stress, loss of enzymatic and non-enzymatic antioxidants and increased membrane lipid peroxidation (Azhar et al. 1995, Cao et al. 2004). In addition, there is evidence that aging leads to alterations in the expression of two oxidant-sensitive transcription factors, AP-1 and NF-κB (Medicherla et al. 2001, 2002). Additional studies further demonstrated that expression of two cellular cholesterol transport/delivery proteins, StAR (Stocco & Clark 1996) and peripheral-type benzodiazepine receptor (PBR) (now renamed translocator protein (18 kDa) (TSPO) (Papadopoulos et al. 2006)), that assist in the intracellular delivery of cholesterol to mitochondrial CYP11A1 are also reduced during aging (Leers-Sucheta et al. 1999, Luo et al. 2001, Culty et al. 2002). However, the underlying mechanism(s) by which excessive oxidative stress leads to loss of steroid production during aging remain unclear. Oxidative stress has been implicated in the development of various pathophysiological changes that occur during aging. One important consequence of oxidative stress is the altered phosphorylation and activation of MAP kinases (Lewis et al. 1998, Martindale & Holbrook 2002, Matsuzawa & Ichijo 2005, McCubrey et al. 2006).

Mitogen-activated protein kinase (MAPK) signal transduction pathways are well-characterized signaling networks that regulate a variety of cellular processes including gene transcription, protein synthesis, cell cycle, apoptosis, cell differentiation, inflammation, and cytoskeletal rearrangements (Lewis et al. 1998, Widmann et al. 1999, Kyriakis & Avruch 2001, Edmunds & Mahadevan 2004, Raman et al. 2007, Whitmarsh 2007). MAPK signaling pathways can be triggered by a variety of stimuli including growth factors, cytokines, oxidative stress, environmental stress, and toxic chemical insults (Lewis et al. 1998, Martindale & Holbrook 2002, Matsuzawa & Ichijo 2005, McCubrey et al. 2006). There are three major groups of MAPKs: extracellular signal-regulated kinase (ERK)1/2, p38 (α/β/γ/δ), and c-Jun N-terminal kinase (JNK1/2/3) (Widmann et al. 1999, Kyriakis & Avruch 2001, Raman et al. 2007). These serine/threonine kinases are phosphorylated/activated by distinct upstream dual specificity MAP kinases (MKKs or MEKs), which selectively target ERK (MKK1/2), JNK (MKK4/7), and p38 (MKK3/6) and phosphorylate both threonine and tyrosine in a regulatory T-X-Y motif present in all MAPKs (Widmann et al. 1999, Kyriakis & Avruch 2001, Raman et al. 2007). Once activated, MAPKs can be translocated to the nucleus and regulate gene expression at transcriptional, translational, and posttranslational levels. Presently it is believed that growth and differentiation factors preferentially activate ERK signaling pathway, whereas the JNK and p38 pathways are more responsive to environmental and physiochemical stresses.

It is well documented that StAR protein (presumably in cooperation with PBRT/TSPO) plays a major role in steroid hormone synthesis by enhancing the translocation of cholesterol substrate from the outer to inner mitochondrial membranes, where the cholesterol side-chain cleavage enzyme, P450scc (CYP11A1), resides (Stocco & Clark 1996, Stocco 2000). The regulation of StAR protein is executed primarily at the transcription level (Manna et al. 2003, 2004, Hiroi et al. 2004, Clem & Clark 2006, Manna & Stocco 2007), but posttranscriptional (phosphorylation) modification of StAR also plays a significant role in its functional expression (Stocco & Clark 1996). Transcriptional regulations of StAR have been extensively studied, and a cluster of transcription factors is involved in the regulatory process (Manna et al. 2003, 2004, Hiroi et al. 2004, Clem & Clark 2006, Manna & Stocco 2007). One of these transcription factors is AP-1, a dimeric complex composed primarily of the Fos and Jun family of proteins (Chinenov & Kerppola 2001, Hess et al. 2004), which play a critical role in the regulation of StAR gene transcription (Manna et al. 2004). Since CRE2 site overlaps with AP-1 site in the proximal StAR promoter region, its activity is also subject to regulation via the crosstalk between CREB and Fos/Jun proteins (Manna et al. 2004, Manna & Stocco 2007). Interestingly, studies on the transcriptional regulation of another cholesterol transport protein, PBR/TSPO (which is believed to work in concert with StAR (Miller 2007, Papadopoulos et al. 2007)), have also identified an AP-1 binding site in its proximal promoter region (Giatzakis et al. 2007). Given the oxidant-sensitive nature of AP-1 (Dalton et al. 1999, Klaunig & Kamendulis 2004), involvement of stress-activated kinases, the MAPKs, in the regulation of AP-1 (Whitmarsh & Davis 1996, Pramanik et al. 2003, Tanos et al. 2005, Humar et al. 2007, Whitmarsh 2007) and the fact that functional expression of AP-1 is substantially reduced (Medicherla et al. 2001), while p38 MAPK is specifically activated in intact rat adrenal glands (Abidi et al. 2008) and Leydig cells (unpublished observations) during aging, it seems likely that oxidant stressors acting on p38 MAPK and AP-1 contribute to the age-related decline in StAR (and PBR/TSPO) expression and subsequently loss of steroidogenic function.

The present studies were initiated to further expand on the observations that p38 MAPK is specifically activated in intact rat adrenal glands (Abidi et al. 2008) and testicular Leydig cells (unpublished observations) during aging and to critically evaluate whether excessive oxidative stress-induced inhibition of steroid synthesis is dependent upon activation of p38 MAPK. These studies were conducted using mouse hormone- and lipoprotein-sensitive adrenocortical cell line Y1-BS1 as an in vitro adrenal model system. Our data indicate that exposure of cells to oxidants leads to a significant reduction in hormone-stimulated steroid secretion, providing a direct demonstration that oxidative stress negatively impacts steroidogenesis. We also present data showing that both pharmacological and molecular inhibitors of p38 MAPK attenuate oxidant-dependent inhibition of steroid secretion, demonstrating that oxidative stress-mediated suppression of steroidogenesis is mediated through activation of p38 MAPK. These results define a novel mechanism for oxidative stress-mediated inhibition of steroid synthesis and identify p38 MAPK activation as a signaling pathway mediating oxidative stress-induced inhibition of steroidogenesis.

Materials and Methods

Reagents and antibodies

The following chemicals were purchased from Calbiochem (La Jolla, CA, USA): MEK/ERK inhibitors PD098059 and U0126, the p38 MAP kinase inhibitors SB203580, SB202190, and SB202474 (inactive analog), the JNK inhibitor SP600125, MnTMPyP (Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin pentachloride), and MnTBAP (Mn(III)tetrakis(4-benzoic acid)porphyrin). Bt2cAMP, fatty acid poor BSA, hydrogen peroxide (H2O2), N-acetylcysteine (NAC), xanthine and xanthine oxidase were supplied by Sigma–Aldrich. 4-Hydroxy-2-nonenal (HNE) was purchased from Cayman Chemical Co. (Ann Arbor, MI, USA), and 6-carboxy-2′7′-dichlorodihydrofluorescein diacetate (DCFDA) was obtained from Molecular Probes/Invitrogen Corporation. All other reagents used were of analytical grade. Polyclonal antibodies against total ERK1/ERK2, JNK1/JNK2, and p38 MAPKα were purchased from Cell Signaling Technology (Beverly, MA, USA). Phospho-specific antibodies that recognize ERKs phosphorylated at Thr202 and Tyr204, and p38 MAPK phosphorylated at Thr180 and Tyr182 were also supplied by Cell Signaling Technology. Phospho-JNKs (Thr183 and Tyr185) antibody was obtained from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA, USA). Anti-myc and anti-FLAG M2 monoclonal antibodies were purchased from Sigma–Aldrich.

Plasmids

Expression vectors of wild-type human FLAG-MKK3, constitutively active human FLAG-MKK3(Glu) (Ser189→Glu/Thr193→Glu), dominant-negative human FLAG MKK3(Ala) (Ser189→Ala/Thr193→Ala), wild-type human FLAG-MKK6, constitutively active human FLAG MKK6(Glu) (Ser207→Glu/Thr221→Glu), and dominant-negative human FLAG MKK6(K82A) (Lys82→Ala), were generously provided by Dr Roger Davis, University of Massachusetts Medical School, Worcester, MA, USA (Raingeaud et al. 1996). Wild-type myc-tagged human MKK7, constitutively active myc-tagged human MKK7(MKK73E) (Ser271→Glu/Thr275→Glu/Ser277→Glu), dominant-negative mutant form of MKK7(MKK73A) (Ser271→Ala/Thr275→Ala/Ser277→Ala), and dominant-negative (kinase-dead) mutant form of MKK7(MKK7K149M) (Lys149→Met) were kindly provided by Dr Michael Kracht, Medical School, Hannover, Germany (Holtmann et al. 1999). The cloning of wild-type FLAG-tagged p38α, p38β, p38γ, and p38δ MAPK constructs have been described previously (Pramanik et al. 2003). Dominant-negative mutant forms of FLAG-tagged human p38α (AF) (Thr180→Ala/Tyr182→Phe), p38β (AF) (Thr188→Ala/Tyr190→Phe), p38γ (AF) (Thr183→Ala/Tyr185→Phe), and p38δ (AF) (Thr180→Ala/Tyr182→Phe) were created by substituting threonine and tyrosine with alanine and phenylalanine respectively in TGY motif (Pramanik et al. 2003).

Culture and treatment of Y1-BS1 mouse adrenocortical cells with oxidants, and/or antioxidants

The Y1-BS1 mouse adrenocortical cell line was initially obtained from Dr David William's laboratory (State University at Stony Brook, Stony Brook, NY, USA) in 1998. The cells were grown in F-10 medium supplemented with 12.5% horse serum, 2.5% fetal bovine serum, 2 mM glutamine, 100 units/ml penicillin, and 100 μg/ml streptomycin at 37 °C under 5% CO2.

To induce oxidative stress, cells cultured in 60 mm dishes were treated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate a flux of superoxide anion, , and H2O2), H2O2 (0.05, 0.1, 0.25, 0.5, or 1.0 mM) or a lipid peroxidation product, HNE (10, 20, 50, or 100 μM) for an appropriate time as specified under each figure and table. In other experiments, cells were co-incubated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml)±the superoxide-dismutase (SOD)/catalase mimetic MnTMPyP (10 μM), H2O2 (0.1 mM)±MnTMPyP (10 μM), or HNE (50 μM)±antioxidant NAC (20 mM) for 1 h. Following treatment, cell samples were washed and were either used immediately for the measurement of steroid hormone secretion or stored frozen until analyzed for total and phosphorylated forms of ERK1/ERK2, p38 MAPK, and JNK1/JNK2. Cell viability was assessed by following the conversion of MTT (3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide) to formazan crystals (Mosmann 1983), a reaction dependent upon mitochondrial respiratory chain activity that reflects mitochondrial redox state. After treatment with oxidants and washing, an MTT solution was added to the dishes at a final concentration of 500 μg/ml and the induction continued at 37 °C for 3 h. Following incubation, the medium was aspirated and accumulated formazan product was solubilized with 1:1 solution of dimethyl sulfoxide/absolute ethanol. Cell viability was determined by the differences in absorbance at wavelength 570 nm minus 690 nm using a microplate reader.

Measurement of ROS

Oxidant-induced intracellular reactive oxygen species (ROS) generation and oxidative stress was monitored by measuring changes in fluorescence resulting from intracellular oxidation of 2′7′-dichlorofluorescein (DCFH) (Keller et al. 2004). The probe DCFDA enters the cell, is hydrolyzed by cellular esterases, where upon nonfluorescent DCFH is trapped inside the cell. Subsequent oxidation by ROS yields DCF. Y1-BS1 cells grown in 35 mm dishes were pre-loaded with DCFDA (10 μM) for 60 min, washed and further incubated with buffer alone (control), xanthine (X, 0.25 mM)+xanthine oxidase (XO, 20 mU/ml), H2O2 (100 μM), or HNE (50 μM) in the presence of Bt2cAMP (2.5 mM) for 1 h. At the end of incubation the media were removed and cells were immediately lysed and centrifuged. DCF fluorescence of the cell lysates was measured using a microplate reader with an excitation wavelength of 488 nm and emission wavelength of 515 nm. Results are expressed as percent change from medium-treated controls.

Transfection of Y1-BS1 adrenocortical cells

For transient transfection experiments, Y1-BS1 cells were sub-cultured at a density of 1.5×105 cells/dish in 60 mm tissue culture dishes the day prior to transfection. Each plate was transfected with 5 μg vector alone or various MAPK constructs as noted under specific tables and figures using a FuGENE transfection reagent according to the manufacturer's instructions (Roche Molecular Diagnostics). After 48 h of transfection, cells were utilized for various measurements.

Measurement of steroid secretion by Y1-BS1 mouse adrenocortical cells

To examine hormone-stimulated and lipoprotein-supported steroid production, native Y1-BS1 cells, oxidant/antioxidant-treated Y1-BS1 cells, or Y1-BS1 cells transiently transfected with various MAPK constructs were incubated for 5 h with Bt2cAMP (2.5 mM)±hHDL3 (500 μg/ml), and following incubation the media were collected and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard.

To evaluate the ability of MAPKs to modulate hormone-stimulated and lipoprotein-supported steroidogenesis, we utilized the specific MEK/ERK inhibitors PD098059 and U0126, the p38 MAP kinase inhibitors SB203580, SB202190, and SB202474 (inactive analog), and the JNK inhibitor SP600125. Triplicate dishes of Y1-BS1 cells were pretreated with vehicle alone (control), PD098059 (30 μM), U0126 (10 μM), SB203580 (10 μM), SB202190 (10 μM), SB202474 (10 μM), or SP600125 (10 μM) for 1 h, and incubations continued for an additional 5 h following the addition of Bt2cAMP±hHDL3 (500 μg protein/ml). At the end of incubation, the media were collected, frozen, and stored frozen until analyzed for 20α-dihydroprogesterone levels as described above. Cell viability after treatment was monitored using the MTT assay as described above.

Preparation of lipoproteins

Human low-density lipoprotein (hLDL) and human apoE-free high-density lipoproteins3 (hHDL3) were isolated and characterized as previously described (Reaven et al. 1990).

Western blot analysis of total and phosphorylated forms of ERKs, p38 MAPK, and JNKs

The Y1-BS1 adrenal cell samples were solubilized with lysis buffer (20 mM HEPES (pH 7.4), 1% Triton X-100 (vol/vol), 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 20 mM NaF, 20 mM β-glycerophosphate, 10 mM sodium pyrophosphate, 1 mM sodium vanadate, 10 nM okadaic acid, 1 mM dithiothreitol, 10 μg/ml leupeptin, 10 μg/ml pepstatin, 10 μg/ml aprotinin, 0.5 mM 4-(2-aminoethyl)benzylsulfonyl fluoride, 10 μM E-64, 50 μM bestatin, 10 μg/ml tosyl-l-phenylalanine chloromethyl ketone and 10 mg/ml Nα-tosyl-l-lysine chloromethyl ketone) and incubated for 15 min at 4 °C. Subsequently, each cell lysate was sonicated briefly to disrupt chromatin (DNA), centrifuged at 15 000 g for 10 min at 4 °C and protein concentration of each clarified lysate was determined. All cell lysate samples were stored frozen until analyzed by western blotting (Kelley et al. 2004).

Samples containing an equal amount of protein (30–40 μg) were fractionated by SDS-PAGE (10% polyacrylamide gel with 4% stacking gel) and transferred to Immobilon polyvinylidene difluoride membrane (Millipore Corp., Bedford, MA, USA). After transfer, the membrane was washed in TBS containing 0.1% Tween 20 (TBS) and incubated in blocking buffer (TTBS containing 5% nonfat dry milk) for 90 min at room temperature, followed by overnight incubation at 4 °C with the primary antibody in blocking buffer (against total or phosphorylated forms of ERK1/ERK2, p38 MAPK, or JNK1/JNK2, or anti-FLAG monoclonal antibody). Subsequently, the membrane was washed in TTBS and incubated for 2 h with horseradish peroxidase-conjugated anti-rabbit or anti-mouse IgG in blocking buffer. The immunoreactive bands were then visualized using a LumiGLO Chemiluminescent Detection System (Kirkegaard and Perry Laboratories Inc., Gaithersburg, MD, USA) followed by exposure to X-ray film (10–30 min) and quantified by Fluor-S-MultiImager scanning densitometry system (Bio-Rad).

Determination of p38 MAPK activity by western blotting

The activity of p38 MAPK was determined using a kit supplied by Cell Signaling Technology (catalog #9820). Briefly, cell lysates were incubated cold overnight with an immobilized primary phospho-p38 MAPK (Thr180/Tyr182) monoclonal primary antibody to selectively immunoprecipitate active p38 MAPK. Subsequently, immobilized immunoprecipitates were incubated in a buffer supplemented with ATP (200 μM) and ATF-2 fusion protein. The reactions were terminated by the addition of a 3× SDS sample buffer and subjected to SDS-PAGE followed by western blotting. The blots were developed with a polyclonal phospho-ATF-2 (Thr71) antibody and HRP-linked anti-rabbit IgG and if necessary, signals were quantified by the Fluor-S-MultiImager scanning densitometry system (Bio-Rad).

RNA isolation

Total RNA was isolated from young and old Leydig cells (pre-incubated with or without 10 ng/ml hCG) using TRIzol (Invitrogen Life Technologies) according to the manufacturer's protocol. The purity and concentration of RNA samples were determined by following absorbance (A260/A280) ratios. The integrity of the purified RNA samples was confirmed by 1.2% formaldehyde-agarose gel electrophoresis.

Measurement of mRNA levels by quantitative PCR (qPCR)

The specific primer sets used to detect the mRNA expression of p38 MAPKα, p38 MAPKβ, p38 MAPKγ and p38 MAPKδ were developed using Primer Express software (Applied Biosystems, Foster City, CA, USA) according to the recommended guidelines based on the sequences accessed through GenBank. Table 1 shows the primer sequences for qPCR assay used. First-strand cDNA was synthesized from 2 μg total RNA by incubating for 1 h at 50 °C with SuperScript II reverse transcriptase (Invitrogen Life Technologies) and random primers. Amplification of cDNAs was performed with an ABI Prism 7900 system according to the manufacturer's instructions. Each sample consisted of 1 μl cDNA, 4 mM MgCl2, 0.2 mM deoxynucleoside triphosphate, 500 nM of each sense and antisense primer, 2 μl 10× PCR buffer, TaqMan polymerase, and SYBR Green master mix in a final volume of 20 μl. As an internal quantitative control for gene expression, ribosomal protein 36B4 gene expression was also determined. Real-time PCR was performed with an ABI Prism. The relative mass of p38 MAPKα, p38 MAPKβ, p38 MAPKγ, p38 MAPKδ, or 36B4 mRNA was calculated by the comparative cycle of threshold detection method according to the manufacturer's instruction. Three independent sets of Taqman real-time PCR were performed using different RNA preparations; each run of Taqman real-time PCR was conducted in triplicate. The final data were normalized to 36B4 and the ratios of p38 MAPKα, p38 MAPKβ, p38 MAPKγ and p38 MAPKδ to 36B4 represented the normalized relative levels of each mRNA.

Table 1

Mouse-specific primer pairs used for quantitative PCR (qPCR)

Forward primerReverse primer
Gene
p38α5′-CCCCAGAGATCATGCTGAAT-3′5′-ACAACGTTCTTCCGGTCAAC-3′
p38β5′-ATGTAGCGGTGAACGAGGAC-3′5′-CCACATATCCGGTCATCTCC-3′
p38γ5′-GAGACTTGAAGCCTGGCAAC-3′5′-CCGGGTTACCACATATCCTG-3′
p38δ5′-CGCCACACAGACACTGAGAT-3′5′-ATGATGCAACCAACAGACCA-3′
36B45′-CACTGGTCTAGGACCCGAGAAG-3′5′-GGTGCCTCTGGAGATTTTCG-3′

Analytical procedures

Protein was measured with a bicinchoninic acid protein assay kit (Pierce Chemical Co., Rockford, IL, USA). The procedure of Markwell et al. (1978) was used to quantify the protein content of hHDL3 and hLDL. Cholesterol content of hHDL3 and hLDL was determined according to the procedure of Tercyak (1991).

Statistical analysis

All results are expressed as mean±s.e.m. Data were analyzed using a nonpaired two-tailed Student's t-test using GraphPad Prism version 3.00 software for Windows (GraphPad Software, San Diego, CA, USA). Differences between groups was considered significant if P<0.05.

Results

To investigate the molecular basis of the potential interaction between oxidative stress, p38 MAPK activation, and loss of steroidogenesis, we employed Y1-BS1 cells, a stable subclone of the Y1 mouse adrenocortical tumor cell line (Schimmer 1979). The Y1-BS1 cell line provides a useful in vitro model system for studying hormone-stimulated and lipoprotein-supported steroidogenesis (Faust et al. 1977, Rae et al. 1979, Temel et al. 1997). Unlike normal rodent adrenal cells that secrete corticosterone, Y1-BS1 cells chiefly secrete 20α-dihydroprogesterone and 11β-hydroxy-20α-dihydroprogesterone.

Effect of various additives on steroid production by Y1-BS1 cells

To examine the effects of hormones and exogenous cholesterol on steroid production, Y1-BS1 cells were incubated without (basal) or with adrenocorticotrophin (ACTH) or its second messenger analog, Bt2cAMP±LDL or HDL for 5 h before harvesting aliquots of medium for quantification of the 20α-dihydroprogesterone content. As expected, both ACTH and Bt2cAMP treatment increased steroid production by ∼10- to 11-fold (Fig. 1). Incubation of Y1-BS1 cells with either hLDL or hHDL also significantly increased the production of 20α-dihydroprogesterone. Steroid production further enhanced after treatment with ACTH or Bt2cAMP+lipoprotein (hLDL or hHDL) (Fig. 1).

Figure 1
Figure 1

ACTH- or Bt2cAMP-stimulated and lipoprotein-supported steroid production by the Y1-BS1 mouse adrenocortical tumor cells. Results are mean±s.e.m. of three independent experiments. Y1-BS1 mouse adrenocortical tumor cells were incubated in a serum-free medium in the absence or presence of ACTH (10 ng/ml), Bt2cAMP (2.5 mM), hHDL3 (500 μg protein/ml), hLDL (100 μg protein/ml), or both ACTH (10 ng/ml) or Bt2cAMP (2.5 mM), and hHDL3 (500 μg protein/ml) or hLDL (100 μg protein/ml) at 37 °C for 5 h. Following incubation, a suitable aliquot of the medium from each sample was collected and steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as standard.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

Oxidant-mediated inhibition of steroidogenesis

Next, we examined the inhibitory actions of enhanced oxidative stress on steroidogenesis. In the first set of experiments, we studied the effects of three oxidants, superoxide anion (Englert & Shacter 2002), H2O2 (Nair et al. 2004), and HNE (Uchida et al. 1999) on steroid production in response to Bt2cAMP or Bt2cAMP+hHDL3. HNE, an end product of the lipid peroxidation of membrane-associated polyunsaturated fatty acids, exhibits high reactivity towards various macromolecules (Uchida 2000, Esterbauer et al. 1991). The results in Fig. 2 show that exposure of Y1-BS1 cells to generated from xanthine oxidase (XO) and xanthine (X) significantly inhibited both Bt2cAMP and Bt2cAMP+hHDL3-stimulated steroid synthesis. Likewise, treatment of cells with either H2O2 or HNE, an end product of lipid peroxidation (Uchida et al. 1999) also caused a reduction in 20α-dihydroprogesterone production in response to both Bt2cAMP alone and Bt2cAMP+hHDL3 (Fig. 2). To ensure that the oxidants used to induce oxidative stress were indeed causing such an effect, simultaneous measurements of ROS production using DCFDA were performed during oxidant-induced oxidative stress. As shown in Fig. 3, the extent of DCF oxidation was significantly increased in cells treated with H2O2 or HNE. Relatively, very little fluorescence was noted in cells exposed to X/XO (superoxide anion, ); this is most likely due to the fact that reacts very poorly with the dye and the observed fluorescence represents only a fraction of the that reacted with NO to form peroxynitrite (ONOO−) or have been dismutated to H2O2 by cellular superoxide dismutases (SODs) (Keller et al. 2002). To rule out the possibility that oxidant-induced inhibition of steroidogenesis was due to increased cell death, cell viability was measured under identical experimental conditions using a sensitive MTT assay (Mosmann 1983). None of these three oxidants, however, had any significant effect on the cell viability, which remained in the range of 85–90% (data not shown).

Figure 2
Figure 2

Effect of the ROS scavengers MnTMPyP and NAC on oxidant-mediated inhibition of adrenocortical cell steroidogenesis. Results are mean±s.e.m. of four independent experiments. (A) Y1-BS1 cells cultured in 60 mm dishes were treated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate a flux of superoxide anion, , and H2O2)± the superoxide-dismutase (SOD)/catalase mimetic MnTMPyP (10 μM), H2O2 (0.1 mM)±MnTMPyP (10 μM), or a lipid peroxidation product, HNE (50 μM)±antioxidant NAC (20 mM) for 1 h. Following treatment, dishes were rapidly washed and incubated with Bt2cAMP (2.5 mM)±hHDL3 (500 μg/ml) for 5 h. (B) Following incubation, media was collected and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The basal amount of 20α-dihydroprogesterone produced was 32.5±4.6 ng/mg cell protein/5 h.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

Figure 3
Figure 3

Oxidant-induced reactive oxygen species (ROS) production and oxidative stress in Y1-BS1 cells. Triplicate dishes of cultured Y1-BS1 adrenal cells were pre-loaded with 10 mM 6-carboxy-2′7′-dichlorodihydrofluorescein diacetate (DCFDA) for 60 min. Subsequently, dishes were washed and incubated with in situ generated superoxide from xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (in situ generated superoxide), H2O2 (100 μM) or HNE (50 μM) for 1 h. At the end of incubation, the media were removed and cells were immediately lysed and centrifuged. DCF fluorescence of the cell lysates was measured using a microplate reader with excitation and emission wavelengths of 488 and 515 nm respectively.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

To examine whether oxidants directly mediate inhibition of steroidogenesis, we measured 20α-dihydroprogesterone production in the presence of ROS scavengers, manganese (III) tetrakis(1-methyl-4-pyridyl) porphyrin (MnTMPyP), and N-acetyl cysteine (NAC). MnTMPyP functions as a cell-permeable SOD/catalase mimetic (Pimental et al. 2001), while antioxidant NAC is the precursor of cellular glutathione (Jonassen et al. 1999) and is known to interfere with HNE action (Uchida et al. 2004). As shown in Fig. 2, pretreatment of Y1-BS1 cells with MnTMPyP attenuated the X/XO- or H2O2-induced inhibition of steroid production. To address the possibility that MnTMPyP prevented the inhibition of steroidogenesis via a nonspecific effect, control experiments were conducted with light-inactivated, Mn(III)tetrakis (4-benzoic acid) porphyrin (MnTBAP). As expected, X/XO- or H2O2-induced inhibition in 20α-dihydroprogesterone production was unaffected in cells pretreated with light-inactivated MnTBAP (10 μM; data not shown). The results of these experiments suggest that blockade of X/XO- or H2O2-induced inhibition of steroidogenesis is not due to a nonspecific effect of MnTMPyP, but occurs in response to the scavenging of superoxide and H2O2 and suggest that these oxidants directly impair the steroidogenic function. In a similar manner, to investigate whether NAC could prevent an HNE-induced reduction in steroid formation, cells were co-incubated with NAC±HNE. Addition of NAC to the incubation medium also effectively blocked the HNE-induced suppression of steroidogenesis, further confirming a direct inhibitory action of HNE (Fig. 2). Since steroidogenic tissues are constantly exposed to ROS as byproducts of both oxidative phosphorylation and steroid biosynthetic pathways, we also incubated normal Y1-BS1 cells with these two antioxidants to determine whether such manipulations would also impact steroidogenesis in a positive manner. Upon incubation of Y1-BS1 cells with MnTMPyP or NAC, steroid synthesis in response to either Bt2cAMP alone or in the presence of HDL3 was significantly upregulated indicating inhibition of steroidogenesis by the endogenously produced ROS (Table 2). Overall, these results suggest that steroidogenic pathway is highly sensitive to and negatively impacted by ROS possibly through enhanced lipid peroxidation and oxidative damage to membranes crucial for cholesterol to mitochondria or other events connected with the steroidogenesis.

Table 2

Effect of reactive oxygen species (ROS) scavengers on steroid hormone synthesis and secretion by the mouse Y1-BS1 mouse adrenocortical cells. Results are mean±s.e.m. of four independent experiments. Y1-BS1 mouse adrenocortical tumor cells were incubated in a serum-free medium±hHDL3 (500 μg protein/ml), ±Bt2cAMP (2.5 mM), ±MnTMPyP (10 μM), or ±NAC (20 mM) at 37 °C for 5 h. Following incubation, medium was collected separately from each dish and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The cell lysates were analyzed for protein content

20α-dihydroprogesterone (ng/mg cell protein/5 h±s.e.m.)
ControlMnTMPyP (10 mM)NAC (20 mM)
Additions
None27.5±4.826.0±6.723.3±4.3
hHDL3 (500 mg/ml)56.3±5.866.8±9.468.3±9.3
Bt2cAMP (2.5 mM)694±511023±105*1094±121
Bt2cAMP+hHDL31673±1542663±3102463±276§

*P=0.0306; P=0.0229; P=0.0416; §P=0.0467.

Effects of superoxide- and HNE-induced oxidative stress on the activation state of MAPKs

Enhanced oxidative stress is known to activate multiple MAPK signaling cascades in many cellular systems (Finkel & Holbrook 2000, Martindale & Holbrook 2002, Matsuzawa & Ichijo 2005, McCubrey et al. 2006). To identify the MAPK signaling pathways activated in Y1-BS1 cells in response to superoxide, H2O2, and HNE, we assessed the activation of ERK1/2, JNK1/2, and p38 MAPK using western blot analysis with antibodies that recognize either the phosphorylated or total content of each of the three MAPKs. Figure 4 shows total and phosphorylated forms of MAPKs assayed in cell extracts from Y1-BS1 cells pretreated with or without oxidants (X/XO or HNE). In control (vehicle-treated) cells, significant expression of the phosphorylated form of ERK1/2 and p38 MAPK was detected but very little of JNK1 (JNK-46) and JNK2 (JNL-54). The XO/X-generated superoxide treatment led to a marked increase in the amount of the phosphorylated form of p38 MAPK, but not of phosphorylated ERK1/2 and JNK1/2. Superoxide and HNE also increased the levels of phospho-p38 MAPK. Although, HNE treatment somewhat enhanced the phosphorylation of ERK1/2 and JNK1 (JNK-46) and JNK2 (JNK-54), the degree of their phosphorylation was several fold lower than that seen with p38 MAPK. Interestingly, the total amount (protein level) of p38 MAPK remained constant following treatment of cells with superoxide, or HNE for up to 1 h (Fig. 4), suggesting that oxidant effects on p38 MAPK are due to changes in the phosphorylation (activation) status of this kinase.

Figure 4
Figure 4

Phosphorylation of ERK1/2, JNK1/2 and p38 MAPK by oxidants in cultured Y1-BS1 cells. Triplicate dishes of cultured Y1-BS1 adrenal cells were incubated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate superoxide in situ), or HNE (50 μM) for 1 h. Equal aliquots of solubilized samples (30–40 μg protein) were subjected to SDS-PAGE and immunoblotted with antibodies against ERK1/2 phosphorylated at Thr202/Tyr204 (P-ERK1/2), against p38 MAPK phosphorylated at Thr180/Tyr182 (P-p38 MAPK), or JNK1/2 phosphorylated at Thr183/Tyr185 (P-JNK1/2). The blots were stripped and re-probed using the antibodies that recognize total (phospho- plus non-phospho forms) ERK1/2, p38α MAPK, and JNK1/2 proteins. The blots shown represent three independent experiments. Numerical results were obtained by densitometric scanning of the individual blots.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

Kinetic dose–response studies were carried out to further establish the stimulatory actions of these three oxidants on p38 MAPK phosphorylation and the results are presented in Fig. 5A–C. The activation of p38 MAPK by XO/X generated superoxide was rapid (10 min) and sustained (Fig. 5A). Dose–response studies demonstrated that H2O2 significantly increased the phosphorylation of p38 MAPK (approximately fivefold) at 0.1 mM reaching a maximum around 0.25 mM (Fig. 5C). Likewise, the stimulatory action of HNE also showed concentration dependence with a maximal effect observed around 50 μM (Fig. 5B).

Figure 5
Figure 5

Kinetics and dose–response effects of superoxide, H2O2, and HNE on p38 MAPK activation. Y1-BS1 adrenal cells were treated with (A) xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) for varying incubation time (10, 20, 30 or 60 min), (B) 0, 10, 20, 50, or 100 μM HNE for 1 h and (C) 0, 0.1, 0.25, 0.5, or 1 mM H2O2 for 1 h. The cell lysates were analyzed by immunoblotting with antibodies against phosphorylated or total p38 MAPK (Fig. 4). Data shown are representative of two independent experiments.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

Involvement of p38 MAPK during oxidant-mediated inhibition of steroidogenesis

Data shown in Figs 4 and 5 suggest possible involvement of the p38 MAPK signaling cascade in oxidant-induced loss of adrenal steroid hormone production. We next investigated the potential relationship between oxidative stress-induced activation of p38 MAPK and the ability of excessive oxidative stress to inhibit hormone-stimulated and lipoprotein-supported steroid synthesis. One way to determine the involvement of p38 MAPK in mediating the inhibitory actions of oxidative stress on adrenal steroidogenesis is to examine the effects of two specific and cell-permeable chemical inhibitors of p38 MAPK, SB203580, and SB202190 (Lee et al. 1994, Cueda et al. 1995). Treatment of Y1-BS1 adrenal cells with a maximal effective dose of SB203580 (10 μM) potentiated the stimulatory action effects of Bt2cAMP and Bt2cAMP + hHDL3 (Fig. 6). The effect of SB202190 (10 μM), a more potent inhibitor of p38 MAPK, was also examined in the face of Bt2cAMP or Bt2cAMP + hHDL3-stimulated steroid production. In agreement with the effect of SB203580, SB202190 also increased the steroid production over that observed with Bt2cAMP alone or Bt2cAMP + hHDL3 (Fig. 6). By contrast, SB202474, the biologically inactive inhibitor of p38 MAPK had no effect on steroidogenesis. To determine the potential involvement of ERK1/2 and JNK1/2 in the regulation of adrenal steroidogenesis, two MEK 1/2 inhibitors PD98059 (Alessi et al. 1995) and U0126 (Favata et al. 1998), and SP600125 a JNK1/2 inhibitor (Bennett et al. 2001) were utilized. (MEK1/2 are upstream kinases that activate ERKs). Treatments with either PD98059 (30 μM), U0126 (10 μM), inactive analog U0124 (10 μM), or SP600125 were ineffective in upregulating Bt2cAMP or Bt2cAMP plus lipoprotein-stimulated steroid production (Fig. 6). None of the various inhibitors had any significant effect on the overall viability of the treated cells.

Figure 6
Figure 6

Effect of MAPK signaling pathway inhibitors on (A) Bt2cAMP and (B) Bt2cAMP+hHDL3-stimulated 20α-dihydroprogesterone production. Y1-BS1 cells were incubated for 5 h in culture medium supplemented with Bt2cAMP (2.5 mM) or Bt2cAMP + hHDL3 (500 μg/ml)±SB203580 (10 μM), SB202190 (10 μM), SB202474 (10 μM), PD98059 (30 μM), U0126 (10 μM) or SP600125 (10 μM). At the end of incubation, medium samples were quantified for the production of 20α-dihydroprogesterone. In some cases, cell viability was determined using a sensitive MTT assay as described under the experimental section. The results are mean±s.e.m. of four independent experiments. The amount of 20α-dihydroprogesterone produced under basal condition was 26.5±3.7 ng/mg cell protein/5 h. P<0.005, Bt2cAMP vs Bt2cAMP+SB203580; P<0.02, Bt2cAMP+hHDL3 versus Bt2cAMP+hHDL3+SB203580; P<0.02, Bt2cAMP versus Bt2cAMP+SB202190; P<0.01, Bt2cAMP+hHDL3 versus Bt2cAMP+hHDL3+SB202190.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

As shown above, treatment of Y1-BS1 cells with oxidants leads to increased stimulation of p38 MAPK activity, but at the same time inhibits hormone- and/or lipoprotein-supported steroid production (Figs 4 and 5). To investigate whether inhibition of the p38 MAPK cascade contributed to the protective effect conferred by ROS scavengers and p38 MAPK inhibitors in response to oxidant treatment, the levels of phospho-p38 MAPK were examined by western blot analysis. As described above, the level of phospho-p38 MAPK was enhanced in cell extracts prepared from superoxide (X/XO)- or HNE-treated Y1-BS1 cells. However, pretreatment with MnTMPyP and NAC completely suppressed the superoxide- and HNE-induced increase in the levels of phosphorylation of p38 MAPK respectively (data not shown). Likewise, use of p38 MAPK inhibitors completely prevented the superoxide- or HNE-induced enhanced phosphorylation of p38 MAPK. We, therefore, investigated the effect of p38 MAPK inhibition on superoxide- and HNE-inhibited steroidogenesis. In agreement with previous results, treatment with X/XO or HNE alone caused a significant reduction in Bt2cAMP+hHDL3-stimulated 20α-dihydroprogesterone production (Fig. 7). Treatment of cells with either X/XO or HNE in the presence of SB203580 or SB202190 not only attenuated but restored steroidogenic function to almost control level (Fig. 7). Under identical experimental conditions, both compounds also prevented oxidant-stimulated increases in p38 MAPK as indicated by inhibition of p38 MAPK-catalyzed threonine phosphorylation of ATF2 (data not shown).

Figure 7
Figure 7

Effect of p38 MAPK inhibitors on superoxide or HNE inhibited steroid production. Results are mean±s.e.m. of three independent experiments. Y1-BS1 cells cultured in 60 mm dishes were treated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate a flux of superoxide anion, ) or HNE (50 μM)±SB203580 or SB202190 (10 μM) for 1 h. Following treatment, dishes were rapidly washed and incubated with Bt2cAMP (2.5 mM)±hHDL3 (500 μg/ml)±respective SB203580 or SB202190 (10 μM) for 5 h. Following incubation, the incubation media was collected and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The amount of 20α-dihydroprogesterone produced under basal condition was 22.9±4.0 ng/mg cell protein/5 h. (A) P<0.002, Control versus X/XO; P<0.00, control versus HNE; P<0.007, control vs SB203580; P<0.01, control versus X/XO+SB203580; P<0.01, control vs HNE+SB203580. (B) P<0.0005, Control versus X/XO; P<0.0008, control vs HNE; P<0.003, control vs SB202190; P<0.01, control versus X/XO+SB202190; P<0.03, control versus HNE+SB202190.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

Since SB203580 and SB202190 inhibit only p38 MAPKα/β isoforms, it raised the possibility that antisteroidogenic actions of excessive oxidative stress is primarily mediated by either p38 MAPKα, p38 MAPKβ or both. To address this issue, qPCR was used to assess endogenous mRNA levels of p38 MAPKα, p38 MAPKβ, p38 MAPKγ and p38 MAPKδ isoforms. The results presented in Fig. 8 indicate that Y1-BS1 cells predominantly express p38 MAPKα isoform along with only low levels of p38 MAPKγ. By contrast, little or no expression of p38 MAPKβ and p38 MAPKδ isoforms was noted. These results, thus, strongly support the possibility that p38 MAPKα is the predominant, if not the sole mediator of oxidative stress inhibition of steroid synthesis.

Figure 8
Figure 8

p38 MAPKα, p38 MAPKβ, p38MAPKγ and p38 MAPKδ mRNA levels in cultured Y1-BS1 cells. Total cellular RNA was extracted, cDNA synthesized, and aliquots of cDNA were used as template reactions containing either primers for p38 MAPKα, p38 MAPKβ, p38MAPKγ, p38 MAPKδ or 36B4. Real-time PCR was performed with an ABI Prism 7900 System using SYBR Green master mix reagent and specific primers as listed in Table 1. Data are presented as ratio of p38 MAPK genes normalized to 36B4. Results are mean±s.e.m. of three independent experiments performed in triplicates.

Citation: Journal of Endocrinology 198, 1; 10.1677/JOE-07-0570

To further substantiate the above findings, we also employed various dominant-negative mutant forms of p38 MAPKs and consecutively active and dominant-negative MKK3/MKK6 constructs to genetically alter the function of p38 MAPK. MKK3 and MKK6 are upstream kinases that phosphorylate and activate p38 MAPK isoforms (Jiang et al. 1997, Enslen et al. 1998). Initially, Y1-BS1 cells were transiently transfected with FLAG- or c-myc-tagged dominant-negative(dn) dnMKK3, dnMKK6, dnMKK7, and dnMKK7, constitutively active (ca) caMKK3, caMKK6, or caMKK7 constructs and were subsequently evaluated for their steroidogenic potential. As a negative control, cells were transfected with an empty vector (pcDNA3.1). The immunoblot analysis of cell lysates prepared from transfected cells confirmed a robust expression of the respective MKK proteins (data not shown). Moreover, transfection of Y1-BS1 cells with a dn-MKK3 (Ala) plasmid construct increased steroid production by ∼50% (Table 3). Expression of dnMKK6(K82A) also enhanced steroid synthesis in the range of 50–60%. Conversely, introduction of either caMKK3 (Glu) or caMkk6(Glu) constructs reduced the 20α-dihydroprogesterone production to approximately half as compared with vector control (Table 3). By contrast, expression of either a constitutively active (caMKK7(73E)) or a dominant-negative form of MKK7 (dnMKK7(73A) or dnMKK7(7K149M)), an upstream activator of JNKs (Tournier et al. 1999), had no effect on the levels of steroid produced (Table 3). These results clearly establish that p38 MAPK acts as a negative modulator of steroidogenesis.

Table 3

Overexpression of dominant-negative MKK3 and MKK6 upregulate while constitutively active MKK3 and MKK6 suppress steroidogenic response in Y1-BS1 mouse adrenocortical tumor cells. Results are expressed as mean±s.e.m. of four independent experiments. Y1-BS1 cells were transiently transfected with an empty vector (control) or expression vectors for dnMKK3(Ala), caMKK3(Glu), dnMKK6(K82A), caMKK6(Glu), dnMKK7(73A), or caMKK7(73E) for 48 h. Bt2cAMP (2.5 mM) + hHDL3 (500 μg protein/ml) were then added, and the incubation continued for another 5 h. At the end of incubation, the medium samples were collected and stored frozen until analyzed for steroid content. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The cells were lysed and cell lysates were used for protein determination and immunoblotting to detect the expression of myc- or FLAG-tagged proteins

20α-dihydroprogesterone (ng/mg cell protein/5 h±s.e.m.)P
Expression constructs
Control (vector DNA alone)8327±1167
dnMKK3(Ala)12 210±6570.0275
caMKK3(Glu)4889±7230.0463
dnMKK6(K82A)13 760±11670.0148
caMKK6(Glu)4095±5020.0158
dnMKK7(73A)9112±1138NS
dnMKK7(7K149M)8891±1037NS
caMKK7(73E)8022±1285NS

ca, catalytically active; dn, dominant negative.

We next sought to define the contribution of individual p38 MAPK isoforms that may mediate the inhibition of steroidogenesis. Presently, there are four known isoforms of p38 MAPK identified as p38 MAPKα, p38 MAPKβ, p38 MAPKγ and p38 MAPKδ (Jiang et al. 1997, Enslen et al. 1998). To test their relative actions on steroidogenesis, we transfected Y1-BS1 cells with the dominant-negative (AF) constructs of each of these isoforms and subsequently measured steroid production. The dnp38 MAPKα (AF) and dnp38 MAPKβ (AF) significantly increased steroidogenic function compared to vector transfected cells (Table 4). The other two dominant-negative mutants, dnp38 MAPKγ (AF) and dnp38 MAPKδ (AF) also slightly, but reproducibly, increased steroid production, although the data did not achieve statistical significance. From these detailed studies, we conclude that excessive oxidative stress-induced inhibition of steroidogenesis requires the activation of and is mediated by p38 MAPK.

Table 4

Overexpression of dominant-negative p38 mitogen-activated protein kinase (MAPK)α and p38 MAPKβ upregulate steroidogenic response in Y1-BS1 mouse adrenocortical tumor cells. Results are expressed as mean±s.e.m. of four independent experiments. Y1-BS1 cells were transiently transfected with an empty vector (control) or expression vectors for dnp38 MAPKα, dnp38 MAPKβ, dnp38 MAPKγ, or dnp38 MAPKδ for 48 h. Bt2cAMP (2.5 mM) + hHDL3 (500 μg protein/ml) were then added and the incubation continued for another 5 h. At the end of incubation, the medium samples were collected and stored frozen until analyzed for steroid content. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The cells were lysed and cell lysates used for protein determination and immunoblotting of FLAG-tagged p38 MAPK proteins

20α-dihydroprogesterone (ng/mg cell protein/5 h±s.e.m.)P
Expression constructs
Control (vector DNA alone)9775±883
dnp38 MAPKα (AF)14 580±12760.0212
dnp38 MAPKβ (AF)15 380±17090.0268
dnp38 MAPKγ (AF)11 810±812NS
dnp38 MAPKδ (AF)11 470±1598NS

dn, dominant negative.

Discussion

Recently we have shown that p38 MAPK is specifically activated in intact rat adrenals during aging and that this activation is linked to age-induced excessive oxidative stress and subsequent loss of steroidogenic response (Abidi et al. 2008). This study provides further evidence that oxidant-induced inhibition of adrenal steroidogenesis requires participation of p38 MAPK and that pharmacological and genetic inhibition of p38 MAPK activity prevents the oxidant-induced loss of steroid hormone production. This is the first demonstration of involvement of p38 MAPK in the negative regulation of steroidogenesis.

Initially our efforts were directed at determining the functional consequences of a potential interaction between oxidative stress and p38 MAPK using oxidant-treated mouse adrenocortical Y1-BS1 cells as an in vitro cell model system. Three different oxidants, namely superoxide, H2O2, and HNE were selected on the basis that superoxide anion and H2O2 are the main oxidants generated as a byproduct of the oxidative phosphorylation and other cellular oxidative pathways (Azhar et al. 1995, Finkel & Holbrook 2000), while aldehydic product HNE is produced during lipid peroxidation of membrane polyunsaturated fatty acids (Esterbauer et al. 1991). Our results indicate that treatment of cells with any of the three types of oxidants caused a similar but significant reduction in steroidogenesis. Moreover, inhibitory effects of both superoxide anion and H2O2 were completely attenuated when cell incubations were conducted in the presence of MnTMPyP, a cell-permeable SOD/catalase mimetic (Pimental et al. 2001). Likewise, blockade of the HNE effect was observed when the cells were simultaneously exposed to the antioxidant NAC. These results suggest that superoxide anion, H2O2, and HNE specifically inhibit steroid hormone production and that such oxidants are likely to be involved in the molecular mechanisms underlying the age-related decline in adrenal steroidogenesis. Furthermore, the fact that all three oxidants were equally effective in inhibiting steroid synthesis implies that they either act in concert or in a sequential manner to interfere with the steroidogenic process. While relatively less reactive, and H2O2 can individually cause oxidative damage under certain conditions. However, they elicit more damaging effects when present together or in combination with other reactive species (Girotti 1998). The most likely possibility is the iron-mediated reduction of H2O2 by by a Haber–Weiss reaction (a); (b); (c)], leading to formation of hydroxyl radical (OH), a highly reactive and strong oxidant (Kehrer 2000, Welch et al. 2002). Indeed, OH is the well-accepted initiator of membrane lipid peroxidation and is responsible for the generation of lipid oxidation products including HNE. In this scenario, we believe that the combined actions of H2O2 and may facilitate the enhanced production of OH, thereby potentially increasing the extent of peroxidative damage of membrane lipids leading to the increased formation of HNE and consequently, inhibition of steroidogenesis.

Treatment of mouse adrenal Y1-BS1 cells with either , H2O2, or HNE also activated p38 MAPK in a time- and concentration-dependent manner. These results are in agreement with recent data from our laboratory showing increased stimulation of adrenal p38 MAPK in response to excessive oxidative stress that occurs during aging (Abidi et al. 2008). To elucidate the role of the p38 MAPK signaling cascade in oxidant-induced inhibition of steroid synthesis, we made use of both cell-permeable chemical inhibitors of p38 MAPK as well as genetic approaches involving either mutant cDNA constructs of p38 MAPKs or their upstream activators MKK3 and MKK6. Treatment of Y1-BS1 cells with specific inhibitors of p38 MAPK, SB203580, and SB202190 completely prevented oxidant-dependent inhibition of steroid production. Likewise, both inhibitors completely prevented oxidant-mediated increases in p38 MAPK activity as indicated by inhibition of ATF2 phosphorylation (a p38 MAPK substrate). By contrast, SB202474, a control compound that does not inhibit p38 MAPK activity had no effect on oxidant-mediated inhibition of steroidogenic response (Lee et al. 1994). Interestingly, the ability of two p38 MAPK inhibitors to enhance steroid production in untreated control cells further suggests that p38 MAPK functions as a negative modulator of adrenal steroidogenesis even under normal physiological conditions. To establish the specificity of p38 MAPK inhibitors, we also investigated the effects of PD98059 and U0126, structurally unrelated, specific inhibitors of the ERK activators, MEK1 and MEK2. Treatment of cells with either of the two inhibitors did not prevent oxidant-induced loss of steroidogenic function. Likewise, a specific JNK inhibitor, SP600125, had no significant effect on steroid synthesis following treatment of cells with either or HNE.

The specificity of SB203580 and SB202190 noted above indicates that prevention of the oxidant-mediated inhibition of steroidogenesis resulted from the blockade of p38 MAPK activity. To further elucidate whether the p38 MAPK signaling cascades functions as a negative regulator of and directly interferes with adrenal steroidogenic function, we directly examined the effects of various dominant-negative (dn) and constitutively active (ca) constructs of MKK3, MKK6, and p38 MAPK isoforms; MKK3 and MKK6 kinases are specific upstream activators of p38 MAPKs (Jiang et al. 1997, Enslen et al. 1998). Transient expression of a dominant-negative form of either MKK3(Ala) or MKK6(K82A) significantly increased the steroidogenic response, whereas expression of constitutively active forms of MKK3(Glu) and MKK6(Glu) reduced the steroidogenic response by ∼50%. A similar inhibitory effect was observed with the use of dominant-negative mutants of p38 MAPK isoforms (referred to here as dnp38 MAPKα (AF), dnp38 MAPKβ (AF), dnp38 MAPKγ (AF), and dnp38 MAPKδ (AF)), although we noticed subtle differences in the relative contributions of the four isoforms. For instance, treatment with dnp38 MAPKα (AF) and dnp38 MAPKβ (AF) increased stimulation of steroid production. On the other hand, although the use of dnp38 MAPKγ (AF) and dnp38 MAPKδ (AF) also enhanced steroidogenesis, the results were not statistically significant. The qPCR measurements further indicated that Y1-BS1 adrenal cells express extremely high levels of p38 MAPKα mRNA; approximately eightfold higher than second most expressed p38 MAPKγ mRNA. For p38 mAPKβ and p38 MAPKδ, there was little or no mRNA expression. Taken together, these various observations suggest that activation of p38 MAPK by oxidative stress is a necessary event in the oxidant-mediated inhibition of steroid production. This is the first report implicating p38 MAPK signaling cascade as a negative modulator of adrenal steroidogenesis.

The exact role played by p38 MAPK in oxidative stress-dependent inhibition of steroidogenesis remains unclear at the present time. However, because multiple kinases and transcription factors have been identified as p38 MAPK substrates, this suggests that any of these processes could potentially contribute to the negative actions of p38 MAPK on steroidogenesis (Lewis et al. 1998, Widmann et al. 1999, Kyriakis & Avruch 2001, Raman et al. 2007, Whitmarsh 2007). One line of investigation supports a role for p38 MAPK in positively modulating cyclooxygenase-2 (COX-2), which is known to negatively impact steroid hormone synthesis (Nakamura & Sakamoto 2001, Wang et al. 2003, Frungieri et al. 2006). Several studies have demonstrated that expression of COX-2 is positively regulated by oxidants such as superoxide anion, H2O2, and HNE (Nakamura & Sakamoto 2001, Kiritoshi et al. 2003, Kumgai et al. 2004, Yang et al. 2005, 2006). Furthermore, a number of studies have shown that engagement of p38 MAPK is necessary for oxidant-dependent induction of COX-2 (Guan et al. 1998, Lasa et al. 2000, Hendrickx et al. 2003, Zarrouki et al. 2007). When considering the possibility that COX-2 is a well-established negative modulator of steroid biosynthesis (Nakamura & Sakamoto 2001, Wang et al. 2003, Frungieri et al. 2006), there is a likely possibility that COX-2 may be involved in the p38 MAPK-mediated downregulation of steroidogenesis. Indeed, it has been recently demonstrated that COX-2 downregulates the gene expression of StAR (Wang et al. 2003) (the StAR protein, in concert with several other proteins, especially TSPO facilitate the rate-limiting transfer of cholesterol to the mitochondrial inner membrane where the substrate cholesterol is converted to steroid precursor, pregnenolone by CYP11A1 (P450scc; Liu et al. 2003, Manna & Stocco 2005)). Furthermore, our own unpublished preliminary data suggest that p38 MAPK functions as a suppressor of StAR promoter activity in adrenal and Leydig cell lines (S M Zaidi, unpublished observations). Recently, S M Zhao et al. (2006) have provided evidence for stabilization of 3.5 kb StAR mRNA transcript by arsenite/anisomycin- (non-specific stimulators of p38 MAPK) sensitive putative protein kinase(s) in an SB203580-independent manner. Use of p38 MAPK inhibitor, SB203580 also stabilized the 3.5 kb StAR mRNA transcript, which was completely blocked by an inhibitor of ERK pathway (MEK innibitor PD98059). Interestingly, arsenite/anisomycin-mediated stabilization of 3.5 kb StAR mRNA transcript occurred in the absence of any detectable increases in StAR phosphorylation. Moreover, incubation of adrenal or Leydig cells with higher arsenite concentrations or longer incubation times resulted in inhibition of StAR expression. Thus, although physiological significance of these disjointed observations and their relevance to StAR function is not apparent, they are in line with our findings that p38 MAPK negatively impacts steroidogenesis.

In conclusion, our data provide direct evidence that activation of the p38 MAPK signaling pathway is functionally linked to the oxidative stress response and mediates its inhibitory effect on adrenal steroid production. This represents a novel cellular mechanism that allows negative modulation of steroidogenesis in response to excessive oxidative insult. Additional studies are underway in this laboratory to further delineate the mechanism by which oxidative stress-mediated activation of p38 MAPK signaling pathway negatively impacts steroidogenesis, particularly impact of oxidants/oxidative stress on the expression of other StARD proteins and potential interactions beween StAR and PBR.

Declaration of Interest

The authors declare that there is no conflict of interest that would prejudice the impartiality of this scientific work.

Funding

This work was supported by the Office of Research and Development, Medical Service, Department of Veterans Affairs, and a grant (DK56339) from the National Institutes of Health.

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(S Leers-Sucheta is now at Pfizer Animal Health, Veterinary Medicine R & D, Pfizer Inc., 7000 Portage Road, Kalamazoo, Michigan 49001, USA)

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    ACTH- or Bt2cAMP-stimulated and lipoprotein-supported steroid production by the Y1-BS1 mouse adrenocortical tumor cells. Results are mean±s.e.m. of three independent experiments. Y1-BS1 mouse adrenocortical tumor cells were incubated in a serum-free medium in the absence or presence of ACTH (10 ng/ml), Bt2cAMP (2.5 mM), hHDL3 (500 μg protein/ml), hLDL (100 μg protein/ml), or both ACTH (10 ng/ml) or Bt2cAMP (2.5 mM), and hHDL3 (500 μg protein/ml) or hLDL (100 μg protein/ml) at 37 °C for 5 h. Following incubation, a suitable aliquot of the medium from each sample was collected and steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as standard.

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    Effect of the ROS scavengers MnTMPyP and NAC on oxidant-mediated inhibition of adrenocortical cell steroidogenesis. Results are mean±s.e.m. of four independent experiments. (A) Y1-BS1 cells cultured in 60 mm dishes were treated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate a flux of superoxide anion, , and H2O2)± the superoxide-dismutase (SOD)/catalase mimetic MnTMPyP (10 μM), H2O2 (0.1 mM)±MnTMPyP (10 μM), or a lipid peroxidation product, HNE (50 μM)±antioxidant NAC (20 mM) for 1 h. Following treatment, dishes were rapidly washed and incubated with Bt2cAMP (2.5 mM)±hHDL3 (500 μg/ml) for 5 h. (B) Following incubation, media was collected and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The basal amount of 20α-dihydroprogesterone produced was 32.5±4.6 ng/mg cell protein/5 h.

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    Oxidant-induced reactive oxygen species (ROS) production and oxidative stress in Y1-BS1 cells. Triplicate dishes of cultured Y1-BS1 adrenal cells were pre-loaded with 10 mM 6-carboxy-2′7′-dichlorodihydrofluorescein diacetate (DCFDA) for 60 min. Subsequently, dishes were washed and incubated with in situ generated superoxide from xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (in situ generated superoxide), H2O2 (100 μM) or HNE (50 μM) for 1 h. At the end of incubation, the media were removed and cells were immediately lysed and centrifuged. DCF fluorescence of the cell lysates was measured using a microplate reader with excitation and emission wavelengths of 488 and 515 nm respectively.

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    Phosphorylation of ERK1/2, JNK1/2 and p38 MAPK by oxidants in cultured Y1-BS1 cells. Triplicate dishes of cultured Y1-BS1 adrenal cells were incubated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate superoxide in situ), or HNE (50 μM) for 1 h. Equal aliquots of solubilized samples (30–40 μg protein) were subjected to SDS-PAGE and immunoblotted with antibodies against ERK1/2 phosphorylated at Thr202/Tyr204 (P-ERK1/2), against p38 MAPK phosphorylated at Thr180/Tyr182 (P-p38 MAPK), or JNK1/2 phosphorylated at Thr183/Tyr185 (P-JNK1/2). The blots were stripped and re-probed using the antibodies that recognize total (phospho- plus non-phospho forms) ERK1/2, p38α MAPK, and JNK1/2 proteins. The blots shown represent three independent experiments. Numerical results were obtained by densitometric scanning of the individual blots.

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    Kinetics and dose–response effects of superoxide, H2O2, and HNE on p38 MAPK activation. Y1-BS1 adrenal cells were treated with (A) xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) for varying incubation time (10, 20, 30 or 60 min), (B) 0, 10, 20, 50, or 100 μM HNE for 1 h and (C) 0, 0.1, 0.25, 0.5, or 1 mM H2O2 for 1 h. The cell lysates were analyzed by immunoblotting with antibodies against phosphorylated or total p38 MAPK (Fig. 4). Data shown are representative of two independent experiments.

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    Effect of MAPK signaling pathway inhibitors on (A) Bt2cAMP and (B) Bt2cAMP+hHDL3-stimulated 20α-dihydroprogesterone production. Y1-BS1 cells were incubated for 5 h in culture medium supplemented with Bt2cAMP (2.5 mM) or Bt2cAMP + hHDL3 (500 μg/ml)±SB203580 (10 μM), SB202190 (10 μM), SB202474 (10 μM), PD98059 (30 μM), U0126 (10 μM) or SP600125 (10 μM). At the end of incubation, medium samples were quantified for the production of 20α-dihydroprogesterone. In some cases, cell viability was determined using a sensitive MTT assay as described under the experimental section. The results are mean±s.e.m. of four independent experiments. The amount of 20α-dihydroprogesterone produced under basal condition was 26.5±3.7 ng/mg cell protein/5 h. P<0.005, Bt2cAMP vs Bt2cAMP+SB203580; P<0.02, Bt2cAMP+hHDL3 versus Bt2cAMP+hHDL3+SB203580; P<0.02, Bt2cAMP versus Bt2cAMP+SB202190; P<0.01, Bt2cAMP+hHDL3 versus Bt2cAMP+hHDL3+SB202190.

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    Effect of p38 MAPK inhibitors on superoxide or HNE inhibited steroid production. Results are mean±s.e.m. of three independent experiments. Y1-BS1 cells cultured in 60 mm dishes were treated with xanthine (0.25 mM)+xanthine oxidase (20 mU/ml) (to generate a flux of superoxide anion, ) or HNE (50 μM)±SB203580 or SB202190 (10 μM) for 1 h. Following treatment, dishes were rapidly washed and incubated with Bt2cAMP (2.5 mM)±hHDL3 (500 μg/ml)±respective SB203580 or SB202190 (10 μM) for 5 h. Following incubation, the incubation media was collected and stored frozen until analyzed. Steroids were extracted from the medium using methylene chloride and quantified by fluorescence in 65% sulfuric acid–35% ethanol using 20α-dihydroprogesterone as a standard. The amount of 20α-dihydroprogesterone produced under basal condition was 22.9±4.0 ng/mg cell protein/5 h. (A) P<0.002, Control versus X/XO; P<0.00, control versus HNE; P<0.007, control vs SB203580; P<0.01, control versus X/XO+SB203580; P<0.01, control vs HNE+SB203580. (B) P<0.0005, Control versus X/XO; P<0.0008, control vs HNE; P<0.003, control vs SB202190; P<0.01, control versus X/XO+SB202190; P<0.03, control versus HNE+SB202190.

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    p38 MAPKα, p38 MAPKβ, p38MAPKγ and p38 MAPKδ mRNA levels in cultured Y1-BS1 cells. Total cellular RNA was extracted, cDNA synthesized, and aliquots of cDNA were used as template reactions containing either primers for p38 MAPKα, p38 MAPKβ, p38MAPKγ, p38 MAPKδ or 36B4. Real-time PCR was performed with an ABI Prism 7900 System using SYBR Green master mix reagent and specific primers as listed in Table 1. Data are presented as ratio of p38 MAPK genes normalized to 36B4. Results are mean±s.e.m. of three independent experiments performed in triplicates.

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