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Involvement of the renin–angiotensin system (RAS) in human pathophysiology has expanded to include several diseases beyond a traditional role in salt and water homeostasis. In diabetes, there is significant overactivity of the RAS, which is inhibited by treatment with RAS blockers, thus decreasing diabetic complications. Activation of the RAS in diabetes includes several unique aspects, such as elevation of circulating prorenin levels and angiotensin (Ang) II-independent effects, mediated through interaction of pro(renin), with the pro(renin) receptor. Ang II-independent RAS actions suggest that efficacy of angiotensin receptor blockers (ARBs) and ACE inhibitors would have limitations in the treatment of diabetic patients. Recent meta-analyses of clinical trials have suggested that currently used RAS blockers may not provide additional benefits in diabetics compared to non-diabetics. We recently reported another novel aspect of the RAS, the intracellular system, which is dramatically activated in hyperglycemic conditions. In cardiac myocytes and fibroblasts, we demonstrated the presence of RAS components and synthesis of Ang II intracellularly. Hyperglycemia selectively upregulated the intracellular system in cardiac myocytes, vascular smooth muscle cells (VSMC), and renal mesangial cells where Ang II synthesis was largely catalyzed by chymase, not ACE. We also demonstrated elevation of intracellular Ang II (iAng II) levels in diabetic rat hearts, which resulted in increased cardiac myocyte apoptosis, oxidative stress, and cardiac fibrosis, suggesting a significant role of iAng II in diabetic cardiomyopathy. Others and we have previously reported that iAng II elicits multiple biological effects, some of which are not blocked by ARBs. Using Chinese hamster ovary (CHO) cells that do not express AT1 receptor, we confirmed that the latter are not required for intracellular actions of Ang II. The AT1-independent effects of iAng II are likely mediated by novel interactions between Ang II and intracellular proteins. The mechanism of RAS activation and intracellular accumulation of components by cardiac myocytes in high glucose (HG) conditions is not known. There is a possibility that an increased influx of glucose into the hexosamine biosynthesis pathway (HBP) and resultant O-glycosylation of proteins/transcription factors is responsible for the activation of the RAS and intracellular synthesis of Ang II.

Identification and elucidation of the roles of humoral factors involved in cardiogenesis and other aspects of cardiac growth will be important for the overall understanding of how the heart develops and functions. Angiotensin (Ang) II has been demonstrated to have important in vivo and in vitro growth-related effects in the neonate and adult myocardium. However, very little is known regarding the regulation of the cardiac renin-angiotensin system (RAS) and the role of Ang II receptors at any developmental stage. Enhanced expression of RAS during fetal and neonatal development suggests that locally produced Ang II exerts autocrine and paracrine influences on cardiogenesis and cardiac growth in these stages of development. Utilization of transgenic models and specific nonpeptide Ang II receptor antagonists will be useful in determining the roles of AT1 and AT2 receptors, respectively, in the regulation of the cardiac RAS, myocardial growth, and apoptosis in the developing rat heart. These studies may provide a better understanding of the effects of Ang II receptors on cellular and molecular aspects of cardiac development and thus lead to improved therapeutic interventions for cardiovascular disease.

The Signal Transducers and Activators of Transcription (STAT) proteins are a novel class of transcription factors activated by cytokines (Interleukin-6, IL-6) and growth factors (platelet-derived growth factor, PDGF). In cultured neonatal rat cardiac fibroblasts and in CHO-K1 cells expressing angiotensin (Ang) II type 1 receptors (AT1A), called T3CHO/AT1A calls, Ang II stimulated predominandy the tyrosine phosphorylation of STAT3. In these cells, Ang II also tyrosine-phosphorylated STAT1, but tyrosine phosphorylation of STAT1 was significantly lower compared with that of STAT3. Angiotensin II-mediated activation of STAT3 compared with the rapid activation by the cytokine IL-6 was delayed (maximal 2h). However, like cytokines, Ang II rapidly induced serine phosphorylation of STAT3. Using T3CHO/AT1A cells, we examined the potential reasons for the delayed tyrosine phosphorylation of STAT3 by Ang II. We tested the possibility that the delayed tyrosine phosphorylation of STAT3 was due to the induction of an inhibitory pathway, prior to stimulation. A short pretreatment of cells with Ang II transiently inhibited the rapid STAT3 tyrosine phosphorylation by IL-6, and this inhibition could be blocked by pre-exposing the cells with the AT1 antagonist EXP3174. PD98059, a specific inhibitor of MAPK kinase 1, attenuated the inhibitory effects of Ang II. These results suggest that the inhibition by Ang II is a MAPK kinase 1-dependent process. The ability of Ang II to cross-talk with IL-6 signaling suggests a modulatory role for Ang II in cytokine-induced cellular responses.

Angiotensin II receptors present in cardiomyocytes, nonmyocytes (predominantly fibroblasts), nerve terminals, and the heart vasculature mediate the multiple actions of angiotensin II (AII) in the heart, including modulation of normal and patho-physiological cardiac growth. Although the cellular processes that couple AII receptors (principally the AT1 subtype) to effector responses are not completely understood, recent studies have identified an array of signal transduction pathways activated by AII in cardiac cells. These include: the stimulation of phospholipase C which results in the activation of protein kinase C and the release of calcium from intracellular stores; an enhancement of phosphaditic acid formation; the coupling to soluble tyrosine kinase phosphorylation events; the initiation of the mitogen activated protein kinase (MAPK) cascade; and the induction of the STAT (Signal Transducers and Activators of Transcription) signaling pathway. It is tempting to speculate that these latter responses, which have been previously associated with growth factor signaling pathways, are involved in AII-induced cardiac growth. Interestingly, some of these novel pathways are apparently not under the same strict control imposed upon the more classical signaling pathways. Thus, while AII-induced calcium transients are rapidly (within minutes) desensitized following exposure to AII, the MAP kinase pathway is not, and activation of the STAT pathway requires hours of agonist exposure for maximal induction. These observations support an emerging picture in which the downstream signal transduction pathways of AII receptors are initiated and terminated with a distinct temporal arrangement. This organization allows appropriate rapid responses (e.g. vascular contraction) to transient AII exposure, some of which are rapidly terminated, perhaps for protective reasons, and others not. In contrast, additional responses (e.g. growth) probably require prolonged exposure to agonist.