Abstract 94: Nitric Oxide--Regulated RGS4 Loss of Function Coordinates Vascular Growth with Cardiomyocyte Growth in the Absence of External Hypertrophic Stimuli

Abstract

The mechanisms that coordinate an increase in vasculature with cardiomyocyte growth in adult heart are not known. We report that induction of angiogenesis is sufficient to promote cardiomyocyte growth and myocardial hypertrophy through a novel nitric oxide (NO)-dependent mechanism. Our hypothesis is that an increase in endothelial-derived NO release promotes the proteosomal degradation of negative regulator of G protein signaling RGS4 in cardiomyocytes (CM) thus relieving repression of hypertrophic stimulation through G proteins. Methods include cell culture systems and transgenic mouse models. We generated three conditional cardiac inducible mouse models: (1) to stimulate angiogenesis by transgenic expression of placental growth factor (PlGF); (2) to stimulate angiogenesis by PlGF and compensate for RGS4 degradation by transgenic expression of RGS4 in CM; and (3) to stimulate angiogenesis by PlGF in eNOS -/- mice. Results. In cultured rat neonatal CM, the treatment with a slow release NO donor resulted in increased degradation of RGS4 through the N-end rule pathway of protein degradation. RGS4 loss of function led to increase in cell size through Gbetagamma/PI3Kgamma-dependent Akt/mTORc1 activation. In mice, PlGF expression induced an increase in capillary/myocyte ratio and arteriolar branches (determined by microCT angiograms). Myocardial hypertrophy was induced subsequent to angiogenic stimulation and concurrent with increased in NO production. Consistent with in vitro data hypertrophic stimulation was associated with reduced RGS4 protein levels and robust activation of Akt/mTORc1 signaling. This hypertrophy was prevented by concomitant transgenic expression of RGS4 in CM, despite the persistence of angiogenesis, arteriogenesis, and increased NO release. Furthermore, the NOS inhibitor L-NAME or mTORc1 inhibitor rapamycin significantly attenuated RGS4 degradation, Akt/mTORc1 signaling, and heart size in PlGF-induced mice. A similar effect was observed upon conditional heart-specific PlGF expression in eNOS -/- mice. In conclusion, these findings support a novel NO/RGS4/G protein/Akt mechanism that couples vessel growth with myocyte growth and heart size in the absence of traditional hypertrophic stimuli.