A previous investigation has demonstrated that plasma 5′-AMP (pAMP) exacerbates and causes hyperglycemia in diabetic mice. However, the crosstalk between pAMP and insulin signaling to regulate glucose homeostasis has not been investigated in depth. In this study, we showed that the blood glucose level was more dependent on the ratio of insulin to pAMP than on the absolute level of these two factors. Administration of 5′-AMP significantly attenuated the insulin-stimulated insulin receptor (IR) autophosphorylation in the liver and muscle tissues, resulting in the inhibition of downstream AKT phosphorylation. A docking analysis indicated that adenosine was a potential inhibitor of IR tyrosine kinase. Moreover, the 5′-AMP treatment elevated the ATP level in the pancreas and in the isolated islets, stimulating insulin secretion and increasing the plasma level of insulin. The insulin administration decreased the 5′-AMP-induced hyper-adenosine level by the up-regulation of adenosine kinase activities. Our results indicate that blood glucose homeostasis is reciprocally regulated by pAMP and insulin.
Lin Xia, Zhongqiu Wang, Ying Zhang, Xiao Yang, Yibei Zhan, Rui Cheng, Shiming Wang, and Jianfa Zhang
Crystal Ying Wang, Yajun Wang, Amy Ho Yan Kwok, and Frederick C Leung
In this study, two novel GHRHR receptor splice variants, named chicken GHRHR-v1 (cGHRHR-v1) and cGHRHR-v2 respectively, were identified from chicken pituitary using RT-PCR assay. cGHRHR-v1 is characterized by an N-terminal deletion of 36 amino acid residues, including an aspartate at position 56 (Asp56) conserved in G protein-coupled receptor B-I subfamily. cGHRHR-v2 is a carboxyl-terminal truncated receptor variant with four putative transmembrane domains, which arose from alternative use of a splice acceptor site on intron 8. Using the pGL3-CRE-luciferase reporter system, the functionality of the two variants was examined in Chinese hamster ovary cells. cGHRHR-v1 was shown to be capable of transmitting signal upon agonist stimulation, but cGHRHR-v2 could not. Both GHRH and pituitary adenylate cyclase-activating peptide (PACAP) could activate cGHRHR-v1 at high dosages (GHRH ≥10−8 M; PACAP ≥10−6 M) and GHRH was much more potent than PACAP, suggesting that cGHRHR-v1 is a functional membrane-spanning receptor with an impairment in high-affinity ligand binding, rather than in receptor activation and ligand-binding specificity. This finding also points out the possibility that Asp56 is not a critical determinant for receptor activation and direct ligand–receptor interaction. To substantiate this hypothesis, using site-directed mutagenesis, two receptor mutants with replacement of Asp56 by Ala or Gly were generated. Expectedly, chicken or human GHRH could still activate both receptor mutants with reduced potencies (about 2- to 14-fold less potent). Taken together, our findings not only suggest that cGHRHR variants may play a role in controlling normal pituitary functions, but also support that Asp56 is nonessential for receptor activation and direct ligand–receptor interaction.
Ying Wang, Xiao-Hui Wang, Deng-Xuan Fan, Yuan Zhang, Ming-Qing Li, Hai-Xia Wu, and Li-Ping Jin
Mammalian proprotein convertases (PCs) play an important role in folliculogenesis, as they proteolytically activate a variety of substrates such as the transforming growth factor beta (TGFβ) superfamily. PC subtilism/kexin 6 (PCSK6) is a member of the PC family and is ubiquitously expressed and implicated in many physiological and pathological processes. However, in human granulosa cells, the expression of the PC family members, their hormonal regulation, and the function of PCs are not clear. In this study, we found that PCSK6 is the most highly expressed PC family member in granulosa cells. LH increased PCSK6 mRNA level and PCSK6 played an anti-apoptosis function in KGN cells. Knockdown of PCSK6 not only increased the secretion of activin A and TGFβ2 but also decreased the secretion of follistatin, estrogen, and the mRNA levels of FSH receptor (FSHR) and P450AROM (CYP19A1). We also found that, in the KGN human granulosa cell line, TGFβ2 and activin A could promote the apoptosis of KGN cells and LH could regulate the follistatin level. These data indicate that PCSK6, which is regulated by LH, is highly expressed in human primary granulosa cells of pre-ovulatory follicles and plays important roles in regulating a series of downstream molecules and apoptosis of KGN cells.
Qidi Wang, Ying Cai, Mark Van de Casteele, Daniel Pipeleers, and Zhidong Ling
Sulfonylurea and metformin are used in the treatment of diabetes. Their chronic effects on β cells are not well known. We have shown that sustained exposure of rat β cells to glibenclamide increased their protein synthesis activity, while metformin caused an inhibition. The effect of glibenclamide was attributed to an activation of translation factors. This study examines whether both drugs interact at the level of protein translation in β cells. Purified rat β cells were cultured with and without glibenclamide and metformin before measurement of protein and insulin synthesis, abundance of (phosphorylated) translation factors, and cell viability. A 24 h exposure to metformin stimulated AMP-activated protein kinase (AMPK), suppressed activation of translation factors- both the mammalian target of rapamycin (mTOR; also known as mechanistic target of rapamycin, MTOR)-dependent ones (eukaryotic initiation factor 4E-binding protein 1 and ribosomal protein S6) and the mTOR-independent eukaryotic elongation factor 2-, and inhibited protein synthesis; a 72 h exposure resulted in 50% dead cells. These effects were counteracted by addition of glibenclamide, the action of which was blocked by the mTOR inhibitor rapamycin and the protein kinase A (PKA) inhibitor Rp-8-Br-cAMPs. In conclusion, metformin activates AMPK in β cells leading to suppression of protein translation through mTOR-dependent and -independent signaling. Glibenclamide antagonizes these metformin effects through activation of mTOR- and PKA-dependent signaling pathways.
Hao Wu, Junduo Wu, Shengzhu Zhou, Wenlin Huang, Ying Li, Huan Zhang, Junnan Wang, and Ye Jia
Endothelial dysfunction contributes to diabetic macrovascular complications. Sirtuin 1 (SIRT1) protects against diabetic vasculopathy. SRT2104 is a novel SIRT1 activator and was not previously studied for its effects on diabetes-induced aortic endothelial dysfunction. Additionally, whether or to what extent deacetylation of P53, a substrate of SIRT1, is required for the effects of SIRT1 activation was unclear, given the fact that SIRT1 has multiple targets. Moreover, little was known about the pathogenic role of P53 in diabetes-induced aortic injury. To these ends, diabetes was induced by streptozotocin in C57BL/6 mice. The diabetic mice developed enhanced aortic contractility, oxidative stress, inflammation, P53 hyperacetylation and a remarkable decrease in SIRT1 protein, the effects of which were rescued by SRT2104. In HG-treated endothelial cells (ECs), P53 siRNA and SRT2104 produced similar effects on the induction of SIRT1 and the inhibition of P53 acetylation, oxidative stress and inflammation. Interestingly, SRT2104 failed to further enhance these effects in the presence of P53 siRNA. Moreover, P53 activation by nutlin3a completely abolished SRT2104’s protection against HG-induced oxidative stress and inflammation. Further, forced activation of P53 by nutlin3a increased aortic contractility in the healthy mice and generated endothelial oxidative stress and inflammation in both the normal glucose-cultured ECs and the aortas of the healthy mice. Collectively, the present study demonstrates that P53 deacetylation predominantly mediates SRT2104’s protection against diabetes-induced aortic endothelial dysfunction and highlights the pathogenic role of P53 in aortic endothelial dysfunction.
Xiang Zhou, Ying Wang, Luisina Ongaro, Ulrich Boehm, Vesa Kaartinen, Yuji Mishina, and Daniel J Bernard
Pituitary follicle-stimulating hormone (FSH) synthesis is regulated by transforming growth factorβsuperfamily ligands, most notably the activins and inhibins. Bone morphogenetic proteins (BMPs) also regulate FSHβ subunit (Fshb) expression in immortalized murine gonadotrope-like LβT2 cells and in primary murine or ovine primary pituitary cultures. BMP2 signals preferentially via the BMP type I receptor, BMPR1A, to stimulate murine Fshb transcription in vitro. Here, we used a Cre–lox approach to assess BMPR1A’s role in FSH synthesis in mice in vivo. Gonadotrope-specific Bmpr1a knockout animals developed normally and had reproductive organ weights comparable with those of controls. Knockouts were fertile, with normal serum gonadotropins and pituitary gonadotropin subunit mRNA expression. Cre-mediated recombination of the floxed Bmpr1a allele was efficient and specific, as indicated by PCR analysis of diverse tissues and isolated gonadotrope cells. Furthermore, BMP2 stimulation of inhibitor of DNA binding 3 expression was impaired in gonadotropes isolated from Bmpr1a knockout mice, confirming the loss of functional receptor protein in these cells. Treatment of purified gonadotropes with small-molecule inhibitors of BMPR1A (and the related receptors BMPR1B and ACVR1) suppressed Fshb mRNA expression, suggesting that an autocrine BMP-like molecule might regulate FSH synthesis. However, deletion of Bmpr1a and Acvr1 in cultured pituitary cells did not alter Fshb expression, indicating that the inhibitors had off-target effects. In sum, BMPs or related ligands acting via BMPR1A or ACVR1 are unlikely to play direct physiological roles in FSH synthesis by murine gonadotrope cells.
Qinglei Yin, Liyun Shen, Yicheng Qi, Dalong Song, Lei Ye, Ying Peng, Yanqiu Wang, Zhou Jin, Guang Ning, Weiqing Wang, Dongping Lin, and Shu Wang
SIRT1, a class III histone/protein deacetylase (HDAC), has been associated with autoimmune diseases. There is a paucity of data about the role of SIRT1 in Graves’ disease. The aim of this study was to investigate the role of SIRT1 in the pathogenesis of GD. Here, we showed that SIRT1 expression and activity were significantly decreased in GD patients compared with healthy controls. The NF-κB pathway was activated in the peripheral blood of GD patients. The reduced SIRT1 levels correlated strongly with clinical parameters. In euthyroid patients, SIRT1 expression was markedly upregulated and NF-κB downstream target gene expression was significantly reduced. SIRT1 inhibited the NF-κB pathway activity by deacetylating P65. These results demonstrate that reduced SIRT1 expression and activity contribute to the activation of the NF-κB pathway and may be involved in the pathogenesis of GD.
Beata Bak, Laura Carpio, Jinjing L Kipp, Pankaj Lamba, Ying Wang, Ren-Shan Ge, Matthew P Hardy, Kelly E Mayo, and Daniel J Bernard
Activins are pleiotropic members of the TGFβ superfamily and were initially characterized based on their abilities to stimulate FSH synthesis and secretion by gonadotrope cells of the anterior pituitary gland. Here, we identified the gene encoding the steroidogenic enzyme, 17β-hydroxysteroid dehydrogenase type I (17β-HSD1; Hsd17b1), as an activin-responsive gene in immortalized gonadotrope cells, LβT2. 17β-HSD1 catalyzes the conversion of estrone to the more active 17β-estradiol, and activin A stimulated an increase in this enzymatic activity in these cells. We demonstrated that activins signaled via the type I receptor, activin receptor-like kinase (ALK4), and the intracellular signaling protein, SMAD2, to regulate Hsd17b1 transcription in immediate-early fashion. Critical cis-elements, including a minimal SMAD-binding element, were mapped to within 100 bp of the start of transcription. Activin/ALK4 signaling also regulated Hsd17b1 transcription in both immortalized and primary cultured murine granulosa cells. The promoter regions mediating basal and activin/ALK4-regulated promoter activity were generally conserved across the different cell types. The data show that activin A rapidly regulates Hsd17b1 transcription in gonadotrope and granulosa cells and may thereby regulate local 17β-estradiol synthesis.
Xiaoyi Ma, Fei Gao, Qi Chen, Xiuping Xuan, Ying Wang, Hongjun Deng, Fengying Yang, and Li Yuan
The angiotensin-converting enzyme 2 (ACE2)/angiotensin 1–7 (A1–7)/MAS axis and glutamate decarboxylase 67 (GAD67)/gamma-aminobutyric acid (GABA) signal both exist in the islet and play important roles in regulating blood glucose metabolism. It has been reported that the activation of ACE2 in the brain increases GABA expression to improve biological effects; however, it is unclear whether there is functional correlation between the ACE2/A1–7/MAS axis and GAD67/GABA signal in the islet. In this study, we showed that the ACE2/A1–7/MAS and GABA signaling systems decreased in the islet of different metabolic stress models. In ACE2-knockout mice, we found that GAD67 and GABA expression decreased significantly, which was reversed by exogenous administration of A1–7. Furthermore, A1–7 mediated PDX1 and AKT activation was inhibited by allylglycine (a specific GAD67 inhibitor) in MIN6 cells. Moreover, giving A1–7 and GABA could significantly reduce beta-cell dedifferentiation and improved glucose metabolism during metabolic stress in vivo and in vitro. In conclusion, our study reveals that the ACE2/A1–7/MAS axis improves beta-cell function through regulating GAD67/GABA signal in beta cells and that up-regulating the ACE2/A1–7/MAS axis and GABA signals delays the development of obesity-induced diabetes.
Harn-Shen Chen, Jia Jia, Hou-Fen Su, Hong-Da Lin, Jaw-Wen Chen, Shing-Jong Lin, Jia-Ying Yang, Hui-Chin Lai, Ruben Mestril, and Ping H Wang
The 70 kDa heat shock protein family plays important cardiac protective roles against myocardial injuries. Reduced myocardial protection is a common feature of diabetic myocardium. This study was carried out to define the changes in the 70 kDa heat shock protein family in the myocardium in the of streptozotocin-diabetes rats, and to explore the mechanisms through which diabetes alters the abundance of Hsp70/Hsc70 in cardiac muscle. In the diabetic myocardium, the abundance of Hsc70 was significantly reduced. The abundance of Hsp70 was low in cardiac muscle and was not induced in the diabetic myocardium. Unlike Hsp60, Hsp70 and Hsc70 did not augment insulin-like growth factor-I receptor signaling in cardiac muscle cells. In cultured cardiomyocytes, insulin directly increased the abundance of Hsc70, whereas insulin could not modulate Hsp70. Treating diabetic rats with insulin restored myocardial Hsc70 level, but phlorizin treatment failed to restore myocardial Hsc70. These in vivo and in vitro studies showed that downregulation of Hsc70 in diabetic myocardium was secondary to insulin deficiency. Thus, insulin played a major role in maintaining adequate expression of Hsc70 in cardiac muscle.