IGF-I is synthesized in the human and rat anterior pituitary glands. The present study was designed to clarify the growth-promoting action of IGF-I on mouse pituitary cells in a primary serum-free culture system. Proliferation of pituitary cells was detected by monitoring the cellular uptake of bromodeoxyuridine (BrdU). BrdU labelling in the nucleus was found in all types of secretory cells: corticotrophs, thyrotrophs, gonadotrophs (LH cells and FSH cells), somatotrophs and mammotrophs. IGF-I (75 ng/ml) stimulated the proliferation of corticotrophs and mammotrophs among the pituitary secretory cells. IGF-I receptor mRNA was detected in the cultured pituitary cells using reverse transcription (RT)-PCR, indicating that mouse pituitary cells expressed IGF-I receptors. Insulin (100 ng/ml) or IGF-I (7.5 ng/ml) failed to increase the percentage of BrdU-labelled cells. However, treatment with insulin (100 ng/ml) plus IGF-I (7.5 ng/ml) increased the percentage of BrdU-labelled cells in a synergistic-like manner. Genistein, a tyrosine kinase specific inhibitor, decreased the IGF-I-induced cell proliferation, indicating that IGF-I acts through IGF-I receptors. IGF-I mRNA was also detected in the cultured pituitary cells by RT-PCR, and its peptides were immunocytochemically detected. The present results demonstrate that all types of pituitary secretory cells have the ability to proliferate in our serum-free culture system. IGF-I synthesized in the pituitary gland may stimulate the growth of pituitary cells, in particular corticotrophs and mammotrophs, by an autocrine or paracrine mechanism.

ABSTRACT

Carbonic anhydrase (CA) and Mg²⁺-dependent ATPase and Mg²⁺-dependent, HCO₃ ⁻-dependent ATPase (Mg²⁺-HCO₃ ⁻-ATPase) activities in rat duodenal mucosa and kidney cortex were examined with respect to thyroidal status. Administration of 50 and 150 μg thyroxine (T₄)/kg per day s.c. for 7 days decreased duodenal cytosol CA activity to 66% of control with the former and 43% with the latter dose, while Mg²⁺-HCO₃ ⁻-ATPase activity in brush borders of duodenal mucosa was increased to 116% of control by 150 μg T₄/kg. CA and Mg²⁺-HCO₃ ⁻-ATPase activities in the cytosol and brush border of kidney cortex did not change after administration of T₄. Hypothyroidism induced by thyroidectomy for 2 and 4 weeks or administration of methimazole (2·5–20 mg/kg per day s.c. or peroral) for 2, 3 and 4 weeks all increased duodenal cytosol CA activity, to about 140% at 2 weeks and 153% at 4 weeks after thyroidectomy, and to about 136% after the oral administration of 10 mg methimazole/kg per day for 4 weeks, while brush border Mg²⁺-HCO₃ ⁻-ATPase activity was decreased to 56% of control 4 weeks after thyroidectomy and to 74% after the s.c. administration of 20 mg methimazole/kg per day for 3 weeks. The increase in CA activity and the decrease in ATPase activity after thyroidectomy were restored to normal levels by replacement with T₄. Neither enzyme activity in the kidney changed in hypothyroidism. Serum concentrations of T₄ and cortisol-like material increased after administration of T₄, and serum concentrations of T₄, aldosterone and cortisol-like material all decreased in hypothyroidism. Correlations were observed between duodenal CA and Mg²⁺-HCO₃ ⁻-ATPase activities and serum concentrations of T₄ (P < 0·01). These results reveal that the decrease in CA activity and the increase in Mg²⁺-HCO₃ ⁻-ATPase activity of duodenal mucosa in hyperthyroidism are reversed in hypothyroidism, while both enzyme activities in the kidney are unrelated to thyroidal status.

Journal of Endocrinology (1990) 126, 119–129

IGF-I is expressed in somatotrophs, and IGF-I receptors are expressed in most somatotrophs and some corticotrophs in the mouse pituitary gland. Our recent study demonstrated that IGF-I stimulates the proliferation of corticotrophs in the mouse pituitary. These results suggested that somatotrophs regulate corticotrophic functions as well as somatotrophic functions by the mediation of IGF-I molecules. The present study aimed to clarify factors regulating pituitary IGF-I expression and also the roles exerted by IGF-I within the mouse anterior pituitary gland. Mouse anterior pituitary cells were isolated and cultured under serum-free conditions. GH (0.5 or 1 microg/ml), ACTH (10(-8) or 10(-7) M), GH-releasing hormone (GHRH; 10(-8) or 10(-7) M), dexamethasone (DEX; 10(-8) or 10(-7) M) and estradiol-17beta (e2; 10(-11) or 10(-9) M) were given for 24 h. IGF-I mRNA levels were measured using competitive RT-PCR, and GH and pro-opiomelanocortin (POMC) mRNA levels were measured using Northern blotting analysis. GH treatment significantly increased IGF-I mRNA levels (1.5- or 2.1-fold). ACTH treatment did not alter GH and IGF-I mRNA levels. IGF-I treatment decreased GH mRNA levels (0.7- or 0.5-fold), but increased POMC mRNA levels (1.8-fold). GH treatment (4 or 8 microg/ml) for 4 days increased POMC mRNA levels. GHRH treatment increased GH mRNA levels (1.3-fold), but not IGF-I mRNA levels. DEX treatment significantly decreased IGF-I mRNA levels (0.8-fold). e2 treatment did not affect IGF-I mRNA levels. GH receptor mRNA, probably with GH-binding protein mRNA, was detected in somatotrophs, and some mammotrophs and gonadotrophs by in situ hybridization using GH receptor cDNA as a probe. These results suggested that IGF-I expression in somatotrophs is regulated by pituitary GH, and that IGF-I suppresses GH expression and stimulates POMC expression at the transcription level. Pituitary IGF-I produced in somatotrophs is probably involved in the regulation of somatotroph and corticotroph functions.

Oestrogen stimulates the proliferation of pituitary cells. The present study was designed to clarify the involvement of transforming growth factor-alpha (TGF-alpha) in the oestrogen-induced growth of mouse pituitary cells in vitro. Anterior pituitary cells obtained from ICR male mice were cultured in a primary serum-free culture system. Proliferation of pituitary cells was detected by monitoring the cellular uptake of bromodeoxyuridine. Secretory cell types were immunocytochemically determined. Treatment with TGF-alpha (0.1 and 1 ng/ml) for 5 days stimulated cell proliferation. Since TGF-alpha binds to the epidermal growth factor (EGF) receptor, this action may be exerted through the EGF receptor. Oestradiol-17beta (OE(2), 10(-)(9) M) stimulated mammotrophic and corticotrophic cell proliferation. RG-13022, an EGF receptor inhibitor, inhibited the cell proliferation induced by EGF or OE(2), showing that the EGF receptor was involved in the growth response in mammotrophs and corticotrophs. Treatment with antisense TGF-alpha oligodeoxynucleotide (ODN) inhibited the cell proliferation induced by OE(2), but treatment with antisense EGF ODN did not. RT-PCR analysis revealed that OE(2) stimulated TGF-alpha mRNA and EGF receptor mRNA expression. These results indicate that TGF-alpha mediates the stimulatory effect of oestrogen on the pituitary cell proliferation in a paracrine or autocrine manner, and that EGF receptor expression is stimulated by oestrogen.

The presence and possible physiological roles of alpha-melanocyte-stimulating hormone (alpha-MSH) in the peripheral tissues of birds have not been established. By a combination of RT-PCR, immunocytochemistry and in situ hybridization, we have examined alpha-MSH expression in the eye of the chicken during development. In the 1-day-old chick, alpha-MSH was expressed in the retinal pigment epithelial (RPE) cells, and also at a lower level in the cone cells. The melanocortin receptor subtypes, CMC1, CMC4 and CMC5, were expressed in the layers of the choroid and the neural retina, but not in the RPE cells. It is probable that the RPE cells secrete alpha-MSH to exert paracrine effects on the choroid and neural retina. During embryonic development, alpha-MSH immunoreactivity in the RPE cells was initially detected at embryonic day 10, and increased in intensity as development proceeded. No cone cells were stained with anti-alpha-MSH antiserum in any of the embryonic stages tested. The immunoreactivities for two prohormone convertases, PC1 and PC2, were co-localized to the RPE cells with a pattern of staining similar to that of alpha-MSH. Despite containing alpha-MSH immunoreactivity, the RPE cells in 1-day-old chicks expressed no immunoreactivity for the endoproteases. Furthermore, in a 3-day-old chick, pro-opiomelanocortin mRNA was detectable by in situ hybridization only in the photoreceptor layer and not in the RPE cells. These results suggest that the RPE cells and the cone cells are intraocular sources of alpha-MSH in the embryonic and postnatal life of the chicken respectively. Embryonic expression of alpha-MSH in the RPE cells implies a possible role for the peptide in ocular development.

Abstract

The plasma concentration and liver mRNA content of IGF-I are regulated by the quantity and quality of dietary proteins. To determine whether the synthesis of IGF-binding proteins (BPs) is also affected by protein nutrition, we assessed plasma concentration, tissue mRNA content and liver transcription rate of each BP after rats were fed either a 12% casein or a protein-free diet for 1 week. Protein deprivation reduced the plasma concentration of IGFBP-3 and IGFBP-4 and increased that of IGFBP-1 and IGFBP-2. The mRNA content in tissues and liver transcription rates of IGFBP-3 and IGFBP-4 did not change in response to protein deprivation although their plasma concentrations decreased. The increased plasma IGFBP-1 and IGFBP-2 concentrations were explained by the increased mRNA content and transcription rate of their genes in the liver. Although IGFBP-1 mRNA was increased by protein deprivation not only in liver but also in kidney, IGFBP-2 mRNA was increased only in liver and did not increase in any other tissue examined. In addition, the liver mRNA content of the acid-labile subunit, which can form a ternary complex with IGFs and IGFBP-3, was not affected by protein deprivation. These results show that tissue-specific synthesis of each BP is regulated in a distinct way in response to protein deprivation.

Journal of Endocrinology (1996) 150, 33–41

Abstract

Large changes in the responsiveness of target organs to oxytocin are thought to originate from alteration of the number of oxytocin receptors (OTR). To elucidate the molecular mechanisms regulating the synthesis of the OTR, we developed a competitive reverse transcription-PCR protocol to measure OTR mRNA. We synthesized cRNA comprising a small stuffer introduced into the target mRNA. Using this cRNA as an internal standard, we made a quantitative estimation of OTR mRNA. Application of this method to the rat uterus revealed that the mean levels of OTR mRNA remained unchanged until 1030–1100 h on day 21 of pregnancy, increased significantly after 2200–2230 h on the same day and declined rapidly after parturition. A similar rapid increase in uterine OTR mRNA content was observed in rats given prostaglandin on day 18, inducing premature delivery on day 19 of pregnancy. All parturient rats had higher OTR mRNA levels regardless of whether parturition was spontaneous or prostaglandin induced. However, in a few rats, OTR mRNA remained as low as that observed during mid pregnancy even on day 22 of gestation, the expected day of parturition in about 70% of the rats in our colony. A similar increase in uterine OTR mRNA content to that observed at parturition was induced by oestrogen treatment for 3 days in ovariectomized virgin rats, but concomitant injection of progesterone did not influence the effect of oestrogen. The present results revealed that the large increase of uterine OTR at the peripartum period is accompanied by an increase in OTR mRNA content that may be brought about, at least in part, by increased oestrogen secretion following luteolysis.

Journal of Endocrinology (1996) 150, 479–486

Proliferation of vascular smooth muscle cells (VSMC) plays a major role as an initiating event of atherosclerosis. Although estrogen directly inhibits the proliferation of VSMC, the mechanism has not been firmly established. In addition, the effect of raloxifene on VSMC remains unknown. 17Beta-estradiol (E(2)) and raloxifene significantly inhibited the growth of VSMC under growth-stimulated conditions. Since mitogen-activated protein (MAP) kinases have been implicated in VSMC proliferation, the role of MAP kinases in both the E(2)- and raloxifene-induced growth inhibition of VSMC was studied. Both E(2) and raloxifene caused rapid, transient phosphorylation and activation of p38 that was not affected by actinomycin D and was blocked by ICI 182,780. In contrast with p38 phosphorylation, extracellular signal-regulated protein kinase (ERK) phosphorylation was significantly inhibited and c-Jun N-terminal kinase (JNK) phosphorylation was not changed by E(2). Because VSMC expressed both estrogen receptor (ER) alpha and ERbeta, it is not known which of them mediates the E(2)-induced phosphorylation of p38. Although E(2) did not affect the p38 phosphorylation in A10 smooth muscle cells, which express ERbeta but not ERalpha, transfection of ERalpha expression vector into A10 cells rendered them susceptible to induction of p38 phosphorylation by E(2). We then examined whether E(2) and raloxifene induce apoptosis through a p38 cascade. Both E(2) and raloxifene induced apoptosis under growth-stimulated conditions. The p38 inhibitor SB 203580 completely blocked the E(2)-induced apoptosis. Our findings suggest that both E(2)- and raloxifene-induced inhibition of VSMC growth is due to induction of apoptosis through a p38 cascade whose activation is mediated by ERalpha via a nongenomic mechanism.

Abstract

The binding of insulin to its receptor rapidly induces intrinsic insulin receptor tyrosine kinase activity, resulting in tyrosine phosphorylation of various cytosolic substrates, such as insulin receptor substrate-1 (IRS-1) which, in turn, associates with a p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) followed by activation of this enzyme.

In the present study, we have examined these early steps of insulin signalling in rat liver in vivo after food ingestion. After fasting for 22 h, a 12% casein diet was available ad libitum throughout the 8-h experimental period. Plasma insulin concentrations increased within 45 min after feeding, reached a maximum at 1·5 h and gradually decreased until 8 h. Autophosphorylation of the insulin receptor β-subunit in liver was detected even during fasting and increased about 1·5-fold at 1·5 h after feeding. Basal tyrosine phosphorylation of IRS-1 was detectable during starvation, increased about twofold at 3 h after feeding and levels were maintained until 8 h. The content of the p85 subunit of PI 3-kinase associated with IRS-1 also increased after feeding in parallel with the changes in tyrosine phosphorylation of IRS-1.

Because tyrosine phosphorylation of the insulin receptor β-subunit and IRS-1 and the association of the p85 subunit of PI 3-kinase with IRS-1 in liver were closely correlated with the changes in the plasma concentration of insulin, we concluded that endogenous insulin secreted in response to eating caused these insulin-dependent intracellular changes in the liver.

Journal of Endocrinology (1997) 154, 267–273

ABSTRACT

The factors associated with high concentrations of circulating plasma immunoreactive endothelin in patients with diabetes mellitus are unknown. Plasma and tissue (lung and kidney) immunoreactive endothelin levels were therefore measured by radioimmunoassay in three animal models of diabetes mellitus: dexamethasone-treated rats (2 mg/kg per day for 12 days), streptozotocin-treated rats (100 mg/kg, 4 days before being killed) and rats treated with both dexamethasone and streptozotocin. Plasma concentrations of immunoreactive endothelin in the dexamethasone-treated rats (3·13±0·28 pmol/l, mean ± s.e.m., n = 15) were significantly (P < 0·005) higher than those in controls (1·33±0·18 pmol/l, n = 15), while plasma concentrations of immunoreactive endothelin in streptozotocin-treated rats (n = 8) and rats treated with both dexamethasone and streptozotocin (n= 14) were undetectable (< 0·5 pmol/l). Fast protein liquid chromatographic analysis of the plasma immunoreactive endothelin of dexamethasone-treated rats showed four peaks: one in the void volume, one eluting before endothelin-3, one eluting after endothelin-3 and before endothelin-1 and one eluting in a position identical with that of endothelin-1. Pulmonary concentrations of immunoreactive endothelin in the three groups of rats with diabetes mellitus were lower (P < 0·005) but no significant change was found in renal immunoreactive endothelin. These findings indicate that short-term dexamethasone treatment increases plasma levels of immunoreactive endothelin while streptozotocin treatment decreases them. Thus, multiple factors may influence plasma concentrations of immunoreactive endothelin in diabetes mellitus.

Journal of Endocrinology (1991) 130, 123–127