Activation of the intracellular renin-angiotensin system in cardiac fibroblasts by high glucose: role in extracellular matrix production

Abstract

The occurrence of a functional intracellular renin-angiotensin system (RAS) has emerged as a new paradigm. Recently, we and others demonstrated intracellular synthesis of ANG II in cardiac myocytes and vascular smooth muscle cells that was dramatically stimulated in high glucose conditions. Cardiac fibroblasts significantly contribute to diabetes-induced diastolic dysfunction. The objective of the present study was to determine the existence of the intracellular RAS in cardiac fibroblasts and its role in extracellular matrix deposition. Neonatal rat ventricular fibroblasts were serum starved and exposed to isoproterenol or high glucose in the absence or presence of candesartan, which was used to prevent receptor-mediated uptake of ANG II. Under these conditions, an increase in ANG II levels in the cell lysate represented intracellular synthesis. Both isoproterenol and high glucose significantly increased intracellular ANG II levels. Confocal microscopy revealed perinuclear and nuclear distribution of intracellular ANG II. Consistent with intracellular synthesis, Western analysis showed increased intracellular levels of renin following stimulation with isoproterenol and high glucose. ANG II synthesis was catalyzed by renin and angiotensin-converting enzyme (ACE), but not chymase, as determined using specific inhibitors. High glucose resulted in increased transforming growth factor-beta and collagen-1 synthesis by cardiac fibroblasts that was partially inhibited by candesartan but completely prevented by renin and ACE inhibitors. In conclusion, cardiac fibroblasts contain a functional intracellular RAS that participates in extracellular matrix formation in high glucose conditions, an observation that may be helpful in developing an appropriate therapeutic strategy in diabetic conditions.

Intracellular Angiotensin II Production in Diabetic Rats Is Correlated With Cardiomyocyte Apoptosis, Oxidative Stress, and Cardiac Fibrosis

“…6, activation of the RAS appears to be a major event in hyperglycemia as a result of increased oxidative stress, increased protein kinase C (PKC) levels, and/or increased activity of the hexosamine biosynthesis pathway. iAng II could directly produce oxidative stress and cellular apoptosis through unidentified mechanisms and/or could enhance expression of RAS components through a positive feedback mechanism (52), resulting in enhanced extracellular Ang II levels as well, particularly via cardiac fibroblasts (13). Extracellular Ang II in turn causes oxidative stress, cardiac myocyte apoptosis, and cardiac fibrosis through the AT 1 receptor.…”

“…In cardiac myocytes and fibroblasts, we demonstrated the presence of RAS components and synthesis of Ang II intracellularly (13,14). Hyperglycemia selectively upregulates the intracellular system in cardiac myocytes, vascular smooth muscle cells (VSMCs), and renal mesangial cells, where Ang II synthesis is largely catalyzed by chymase, not ACE (14 -18).…”

New insights into the renoprotective actions of the renin inhibitor aliskiren in experimental renal disease

“…However, some of the label seen in glomeruli may have reflected aliskiren bound to renin that possibly was present in the mesangial matrix. In addition, as aliskiren can penetrate cultured cardiomyocytes, 98 the drug may have entered mesangial cells and podocytes, both of which express renin 20,[24][25][26] to which aliskiren could bind. Furthermore, the unequivocal evidence for labeled aliskiren in the walls of renal vessels prompts the proposal that this DRI can enter JG cells, and possibly incorporate into forming or formed renin granules.…”

Caged ligands to study the role of intracellular GPCRs

“…Ang-II immunoreactivity is increased 3-fold in cardiomyocytes and cardiac endothelial cells from patients with type 2 diabetes and a further 2-fold increase is detected in diabetic patients who are also hypertensive [37]. Elevated glucose increases iAng II in rat neonatal and adult cardiomyocytes [38,39], fibroblasts [40], vascular smooth muscle cells (VSMC) [41], and mesangial cells [42]. Furthermore, cardiomyocytetargeted overexpression of Ang-II or angiotensinogen in mice increases iAng-II and induces hypertrophy [43,44]: this elevated iAng-II is not associated with increases in blood pressure or plasma Ang-II concentrations and the resulting hypertrophic effect is no prevented by the AT 1 antagonist losartan [43].…”