Abstract 17136: A Novel RhoA-Dependent Calcineurin/NFAT Activation Pathway Mediates Leptin-Induced Cardiomyocyte Hypertrophy
Introduction: Leptin, a product of the obesity gene, has been shown to produce cardiac hypertrophy. Although leptin's mechanism of action is poorly understood activation of the RhoA/ROCK pathway has been proposed as a contributing mechanism. Activation of the Ca2+-dependent calcineurin-NFAT pathway is a well established process underlying the hypertrophic program although it is not known whether leptin can activate this signaling pathway or whether there is a relationship between RhoA activation and calcineurin. Accordingly, we determined the effect of leptin on calcineurin activation in cultured cardiomyocytes and assessed the possible role of RhoA.
Methods and Results: Experiments were performed using cultured neonatal rat ventricular myocytes exposed to 50 ng/ml leptin for 24 h. This treatment increased cell surface area by 28% (P<0.05), rate of protein synthesis by 25% (P<0.05) and expression of the α-skeletal actin gene by 6 fold (P<0.05). Moreover, leptin increased intracellular Ca2+ concentrations by 32% (P< 0.05). The hypertrophic responses were completely abrogated by C3 exoenzyme (C3; 30 ng / ml), a RhoA inhibitor, whereas C3 had no effect on leptin-induced elevations in intracellular Ca2+ concentrations. Leptin also activated the calcineurin-NFAT pathway as determined by a three- fold elevation in expression levels of modulatory calcineurin-interacting protein 1 (MCIP1), an indicator of calcineurin activation (P<0.05) and markedly enhanced NFAT3 nuclear translocation as measured by fluorescence microscopy. RhoA inhibition with C3 blocked leptin-induced calcineurin activation and NFAT3 translocation. We next assessed the importance of calcineurin activation/NFAT translocation to the hypertrophic effect of leptin by determining the effect of calcineurin inhibition with FK506 (2 nM) and NFAT3 translocation with NFAT activation inhibitor III (20 nM). Both agents completely prevented leptin induced cardiomyocyte hypertrophy.
Conclusions: Our study demonstrates a critical role for the calcineurin pathway in mediating leptin-induced hypertrophy. Moreover, we report a novel RhoA-dependent leptin-induced calcineurin activation which acts independently of changes in intracellular Ca2+ concentrations.
- © 2010 by American Heart Association, Inc.