Abstract 143: Caveolae Regulate PI3K/Akt/RhoA-Mediated p38 MAPK Nuclear Translocation in Leptin-Induced Cardiomyocyte Hypertrophy
Background: Obesity is associated with increased leptin production which may contribute to cardiac hypertrophy. However, the mechanism of leptin-induced cardiac hypertrophy remains incompletely understood. Previous studies have shown that the RhoA/ROCK/cofilin pathway and p38 MAPK but not ERK1/2 activation are major contributors to leptin-induced cardiac hypertrophy. In this study we explored the roles of caveolae and the PI3K/Akt pathway in regulating RhoA and p38 MAPK activation during leptin-induced cardiomyocyte hypertrophy.
Methods and Results: Neonatal rat ventricular myocytes were cultured with 3.1 nmol/L leptin for 24 hours. Caveolae number and expression of caveolin-3 were significantly increased after leptin treatment (2 and 3 fold, respectively; p<0.01). These effects were associated with a 29% (p<0.05) increase in cell surface area and a 40% (p<0.05) increase in leucine incorporation, indicating cardiomyocyte hypertrophy. Disruption of cardiomyocyte caveolae with 5 mM methyl-beta-cyclodextrin (MβCD) significantly inhibited leptin-induced hypertrophy. RhoA was detected in caveolae fractions of a sucrose gradient after cardiomyocytes were treated with leptin for 5 min, demonstrating subcellular translocation of RhoA. Treatment with MβCD, 50 ng/ml C3 exoenzyme (a RhoA inhibitor) or 50 nM latrunculin B (actin filaments depolymerization agent) significantly attenuated leptin-induced RhoA translocation into caveolae fractions. Moreover, Western blot analysis showed that leptin-dependent activation of PI3K (116%; p<0.05), Akt (115%; p<0.05) and RhoA (330%; p<0.05) were significantly inhibited by 50 μM LY294002, a specific PI3K inhibitor. In addition, LY294002 significantly attenuated leptin-induced increases in cell surface area and in leucine incorporation. Furthermore, we found that leptin-induced activation of p38 (189%; p<0.05) and ERK1/2 (220%; p<0.05) was associated with p38 MAPK but not ERK1/2 nuclear translocation. MβCD, C3 exoenzyme and LY294002 potently attenuated leptin-induced p38 MAPK nuclear translocation.
Conclusions: Our results demonstrate that caveolae, the PI3K/Akt/RhoA pathway and p38 MAPK nuclear translocation play a pivotal role in leptin-induced cardiomyocyte hypertrophy.