Prostate cancer is highly prevalent in Western society, and its early stages can be controlled by androgen ablation therapy. However, the cancer eventually regresses to an androgen-independent state for which there is no effective treatment. The renin-angiotensin system (RAS), in particular the octapeptide angiotensin II, is now recognized to have important effects on growth factor signaling and cell growth in addition to its well-known actions on blood pressure, fluid homeostasis and electrolyte balance. All components of the RAS have been recently identified in the prostate, consistent with the expression of a local RAS system in this tissue. The presence of the AT1-R in vascular smooth muscle cells suggests that Ang II could contribute to prostatic blood flow. Activation of AT1-Rs in LNCaP and DU145 and PrSC cells resulted in increased mitogen-activated protein kinase (MAPK) activation, Janus tyrosine kinase-signal transducers, activators of transcription (STAT), and cell proliferation. The AT1-R blocker, telmisartan, which is structurally similar to the peroxisome proliferation activated receptor (PPAR ) ligand, inhibited AT1-R expression and prostatic cancer cell proliferation through PPYR signalling activation. Moreover, Ang II, acting via AT1-R, caused a dose-dependent increase in the growth of both hPCPs and PrSC cells. Taken together, these results suggest that Ang II acting via the AT1-R could have a role in the development and progression of prostate cancer. Although Ang II does not directly stimulate PC3 cell growth, it has been observed that MAPK activation was induced in PC3 using PrSC cell conditioned medium. This supports the concept that prostatic stromal cells are involved in the promotion of growth factor activity, and may contribute to the initiation and development of prostate cancer. Of interest was the finding that when both functional AT1- and AT2-Rs were present (LNCaP cells), the dominant effect of Ang II was proliferative, whereas in the presence of non-functional AT1-Rs (PC3 cells) the action of Ang II on cell growth became inhibitory. This indicates that the autocrine effect of Ang II on cell growth can vary and is dependent on the functional states of both AT1- and AT2-Rs. Although AT1- and AT2-stimulation usually results in opposing effects on cell growth, there are also reports that AT1- and AT2-Rs act synergistically to activate the fibroblast growth factor-2 (FGF-2) gene through a unique Ang II-responsive promoter element in bovine adrenal medullary cells. This takes on special importance in prostate cancer, as recent reports suggest that deregulated FGF signaling in mouse models of prostate cancer leads to cancer progression and epithelialmesenchymal transition, which is believed to have a central role in tumour progression. Activation of Akt by insulin is a multistep process that for full activation requires hierarchical phosphorylation of two residues, the Thr308 in the activation loop within the kinase domain, and the Ser473 in the C-terminal of the kinase domain. The phosphorylation of both sites is mediated by the actions of PDK1 and PDK2 (mTORC2), respectively. Interestingly, several reports have suggested that each phosphorylation site performs different tasks in insulin action. Mutation of Thr308 to Ala blocks its activation, indicating that phosphorylation of this residue is required for Akt activation. On the other hand, mutation of Ser473 into Ala only partially inhibits Akt activation, suggesting that Ser473 phosphorylation is required for maximal kinase activation. In this context, Akt phosphorylation at Thr308 has been implicated in the metabolic actions of insulin. Knock-in mice expressing a mutant of PDK1 incapable of binding phosphoinositides were unable to activate Akt. Inhibition of Akt action resulted from reduced phosphorylation of Thr308, without affecting Ser473 phosphorylation. Interestingly, the knock-in mice are significantly smaller, insulin resistant, and hyperinsulinemic, correlating these abnormalities with defective PDK1 and the impairment of Akt phosphorylation at Thr308. One of the major targets of activated Akt is GSK-3, which has an important role in the regulation of glycogen synthesis via inhibitory phosphorylation of glycogen synthase (GS). Upon insulin-mediated phosphorylation on Ser21/9 of the two isoforms of GSK-3, GSK-3 and GSK-3, respectively, GSK-3 is inactivated. This inactivation, in parallel with protein phosphatase-1 activation, relieves the inhibitory phosphorylation of GS, which becomes activated and promotes glycogen synthesis. In summary, our findings indicate that Ang II impairs insulin-induced Akt Thr308 phosphorylation by increasing IRS-1 Ser636/Ser639 phosphorylation and IRS-1 protein degradation, by a mechanism dependent on EGF transactivation that leads to PI3K/ERK1/2/mTOR/S6K-1 activation. These findings provide evidence that defines a role of Ang II in the development of insulin resistance. Growth factors such as Ang II are known to stimulate cAMP-dependent products of PKA via Gs-adenylate cyclase interactions. PKA might regulate ERK1/2 through Raf-dependent or -independent signaling pathways, according to the cell type. Our confocal microscopy studies revealed that inhibition of PLC and PKC significantly increase the perinuclear accumulation of Ang II-induced ERK1/2 phosphorylation, suggesting that they might mediate the feedback inhibition of the ERK1/2 activation in Ang II receptor signaling. In most cell types, including cardiomyocytes, PLC has been reported to mediate the production of inositol trisphosphate (IP3) and diacylglycerol, which results in PKC activation during Ang II receptor signaling. In particular, PLC1 as one of PLC1-4 appears to be mainly expressed in the heart, and to have unspecified roles in cellular proliferation and differentiation. We therefore further characterized the roles of PLC and PKC in mediating Ang II-induced ERK1/2 activation by immunoblot studies. This demonstrated that both enzymes have negative regulatory roles in the signaling pathways of Ang II-induced ERK1/2 activation in fetal cardiomyocytes, in contrast to previous reports in newborn cardiomyocytes. On this basis, our studies included identification of the PLC and PKC isoforms as specific mediators of the Ang II-induced ERK1/2 signaling cascade. These findings demonstrated that PLC1/PKC is at least one of the PLC/PKC isoforms that mediates Ang II-induced ERK1/2 activation via the feedback inhibition of c-Raf activation. Recently, others have reported that PKA and PKC mediate ERK1/2 activation via the feedback inhibition of agonist-induced c-Raf and EGFR activations in other cell types. In addition to ERK1/2 activation via the cAMP/PKA and PLC/PKC/c-Raf signaling pathways in fetal cardiomyocytes, these findings have also indicated that ERK1/2 activation derived from Ang II receptor signaling is regulated through c-Raf-dependent signaling pathways. In AT2 receptor signaling, ERK1/2 is phosphorylated through c-Raf, which is positively activated by unknown intermediate signaling molecules, but not via EGFR transactivation. In AT1 receptor signaling, EGFR transactivation is required for ERK1/2 phosphorylation through c-Raf. This is consistent with our previous report that defined the roles of EGFR in Ang II-induced ERK1/2 activation in other cell types. The present studies reveal that multiple and parallel signaling pathways are involved in the mechanism of signaling pathways of Ang II-induced ERK1/2 activation in fetal cardiomyocytes. These results are likely to be accounted for balancing the contribution of multiple signaling pathways to Ang II-induced ERK1/2 activation in fetal cardiomyocytes.
| 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 |
