Transplantation of fetal cardiomyocytes has been utilized in studies on myocardial repair in the damaged hearts of rodents and other species. Angiotensin II (Ang II) is a potent mediator in the cardiovascular system, acting via its type I (AT1) and type II (AT2) receptors. Studies in rat fetal cardiomyocytes revealed that activation of AT1 and AT2 receptors positively regulate Ang II-induced ERK1/2 activation via Gq. Also, that Ang II-induced ERK1/2 activation is mediated by tyrosine kinases such as Src and the epidermal growth factor receptor (EGFR), but not by intracellular calcium. Interestingly, reduction of phospholipase C (PLC) and protein kinase Cα(PKCα) markedly increased ERK1/2 activation through c-Raf, and AT1 receptor-mediated protein kinase A (PKA) activation negatively regulated ERK1/2 activation by other signaling pathways in fetal cardiomyocytes stimulated by Ang II. Observations that PLC, PKCαand PKA mediate the feedback inhibition of ERK1/2 activation provided insights into the complex interactions between MAPK activation and other pathways in Ang II receptor signaling. These findings revealed that the mechanism of Ang II-induced ERK1/2 activation has distinct features in fetal cardiomyocytes, in which activation of the AT2 receptor could contribute to such physiological functions as differentiation, proliferation and survival. The observed mechanism of ERK1/2 activation via the synergy of both AT1 and AT2 receptors could also be relevant to clinical approaches using cell-based therapy for diseases such as heart failure and cardiac hypertrophy. Insulin resistance is an important factor in the development of Type 2 diabetes mellitus (DM2). At the molecular level, insulin resistance is linked to reduced tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1). The mechanisms involved in this process have not been completely identified, but the role of serine/threonine phosphorylation of IRS-1 in desensitization of insulin action is well established. Studies with Dr. Alberto Olivares-Reyes revealed that blockade of the renin-angiotensin system by inhibiting angiotensin-converting enzyme or the angiotensin II (Ang II) AT1 receptor reduces insulin resistance and may substantially decrease the risk for DM2. The potential interactions between the Ang II and insulin signaling systems in hepatocytes, and the regulation of IRS-1 phosphorylation and insulin-Akt activation by Ang II were examined in clone 9 (C9) hepatocytes. In these cells, Ang II treatment specifically desensitized insulin-induced Akt Thr308 activation in a time-dependent manner. This inhibition was associated with increased IRS-1 phosphorylation on Ser636/Ser639 that was prevented by inhibition of EGFR tyrosine kinase activity by AG1478. Insulin-induced phosphorylation of IRS-1 on Ser636/639 is mediated mainly by PI3K/mTOR/S6K-1. Inhibition of PI3K (wortmannin) and mTor (rapamycin) revealed that Ang II stimulates IRS-1 phosphorylation on Ser636/639 via the PI3K/mTOR/S6K-1 pathway. Both agents also blocked the inhibitory effect of Ang II on insulin-induced activation of Akt. Also, preliminary studies suggested that MAPK/S6K-1 is a secondary pathway involved in the Ang II-induced inactivation of insulin signaling. In summary, Ang II inhibits insulin signaling by increasing ser636/ser639 IRS phosphorylation via a mechanism dependent on EGFR trans-activation that leads to MAPK/S6K1 and PI3-K/Akt/mTor/S6K-1 activation. These findings indicate that Ang II has a significant role in the development of insulin resistance via the EGFR. Ang II promotes cell growth and proliferation, and has been implicated in several forms of tumorigenesis. The role of Ang II in prostate cancer was investigated in collaborative studies performed in the laboratory of Dr. Simon Louis in Melbourne, Australia. Ang II is present in the basal cell layer of the normal prostate gland and in benignprostatic hyperplasia (BPH), and stimulates prostate cell growth via the AT1R. Furthermore, AT1R blockers have been shown to reduce prostate-specific antigen and to inhibit prostate cancer cell growth. An analysis of Ang II expression in BPH and prostate cancer, including high grade prostatic intraepithelial neoplasia (HGPIN), showed its presence in basal epithelial cells in BPH and also in proliferating malignant cells in prostate cancer (Gleason grades 2-5), and in the cytoplasm of LNCaP, DU145, and PC3 prostate cancer cell lines. These data demonstrated Ang II staining in malignant cells in all grades of prostate cancer, and indicate that Ang II expression in non-basal epithelial cells is an early index of pre-malignant and malignant changes. In view of its mitogenic activity, it is probable that Ang II contributes to the growth and infiltration of malignant epithelial cells in the prostate. Furthermore, based on the observation by Baker et al. (2006) that elevated levels of cytoplasmic Ang II can increase cell proliferation via a non-AT1R mechanism, it is possible that angiotensin converting enzyme (ACE) inhibitors would also be of value in the treatment of prostate cancer by reducing intracellular Ang II formation. Kisspeptins, the endogenous ligands of G protein-coupled receptor 54 (GPR54), are structurally related peptides encoded by the KiSS-1 gene, and were originally identified by their ability to inhibit tumour metastasis. There is abundant evidence that the KiSS-1/GPR54 system has a major role in regulation of the gonadotropic axis by stimulating GnRH secretion at the hypothalamic level and consequently activating gonadotropin release. Recently, the KiSS-1/GPR54 system has been found to be expressed in both human and mouse pancreas, indicating that the neuroendocrine actions of KiSS-1 and GPR54 are not restricted to the hypothalamic/pituitary system, and might participate in the regulation of pancreatic islet function. Studies with Dr. Antonio Martinez-Fuentes confirmed the expression of KiSS-1 and GPR54 transcripts in the rat pancreas and Ins-1 beta cells. Analysis of the direct effects of kisspeptin-10 on calcium kinetics in single cultured beta cells in a microfluorimetric system revealed that about 70% of insulin-producing cells responded with a prominent calcium increase. Given the known coupling between calcium increase and exocytosis, these data suggested that kisspeptin directly induces insulin release in pancreatic beta cells. Furthermore, kisspeptin-10 administration caused a significant increase in FM5-95 incorporation in single beta cells, similar to that elicited by glucose administration and thus supporting a role for kisspeptins in insulin secretion. Analysis of KiSS-1 and GPR54 expression under high glucose culture conditions revealed a significant reduction in the number of KiSS-1 transcripts, while those for GPR54 remained unaltered. In summary, the presence of KiSS-1 and GPR54 transcripts suggest that endogenous kisspeptins may regulate insulin production through an autocrine mechanism in pancreatic beta cells. Furthermore, regulation of their expression by one of the main stimuli of insulin secretion supports the participation of the KiSS-1/GPR54 system in the pathophysiology of the endocrine pancreas. Relatively little is known about the protein-protein interactions that regulate the trafficking of the AT1R through the biosynthetic pathway. The membrane-proximal region of the cytoplasmic tail of the AT1R has been defined by site-directed mutagenesis studies as a site required for normal AT1R folding and surface expression. Based on yeast two-hybrid screening of a human embryonic kidney cDNA library with the AT1R carboxyl-terminal tail as bait, the Invariant chain (Ii) was identified as a novel receptor-interacting protein.Ang II promotes cell growth and proliferation, and has been implicated in several forms of tumorigenesis.
| Louis, Simon N S; Chow, Laurie; Rezmann, Linda et al. (2010) Expression and function of ATIP/MTUS1 in human prostate cancer cell lines. Prostate 70:1563-74 |
| Arellano-Plancarte, Araceli; Hernandez-Aranda, Judith; Catt, Kevin J et al. (2010) Angiotensin-induced EGF receptor transactivation inhibits insulin signaling in C9 hepatic cells. Biochem Pharmacol 79:733-45 |
