Involvement of protein kinase N and GLUT1 in the regulation of cardiomyocyte hypertrophy

The heart responds to chronic increases in cardiovascular demand by undergoing hypertrophic rather than hyperplastic cell growth. Enlargement of the terminally differentiated cardiac myocytes results in increased heart size. While initially beneficial, this growth response can progress to a decompensated state of heart failure if the chronic stress is not removed. Many of the signaling pathways responsible for establishing compensated hypertrophy also appear to be involved in the progression of heart failure. A well-established in vitro model of cardiac hypertrophy has been successfully used to delineate hypertrophic signaling pathways. Neonatal rat ventricular (NRVMs) stimulated with agonists that activate Gq-coupled receptors recapitulate many aspects of in vivo hypertrophy such as ANF gene expression and myofibrillar organization. Previous work from our lab has shown a role for the low molecular weight GTPase RhoA in hypertrophy. To extend this work my first objective was to determine the effector and cis-promoter element through which RhoA increases ANF expression. Our findings using transient transfections of NRVMs indicated that RhoA and its effector, Protein kinase N, mediate ANF expression through a common serum response element. RhoA signaling pathways have also been implicated in the acute regulation of glucose uptake via GLUT4. In cardiomyocytes RhoA-dependent effects on GLUT4 were not detectable. However GLUT1 transcription was increased in association with cardiomyocyte hypertrophy and by PKN, and was attenuated by inhibition of Rho signaling with C3 toxin. GLUT1 protein expression was increased by pressure overload, Galphaq, and hypertrophic agonists (PE, PGF2alpha, LIF). PI3-Kinase and ERK1/2 were necessary for PE-induced increases in GLUT1 protein expression. Overexpression of GLUT1 was not sufficient to cause cardiomyocyte hypertrophy but antisense GLUT1 prevented that induced by agonist. Addtionally increased GLUT1 expression prevented myocyte apoptosis, and was necessary for maintained survival of hypertrophic myocytes. Surprisingly these effects were independent of glucose, however GLUT1 expression increased phosphorylation of GSK3beta, ERK, and Akt, suggesting activation of a signaling cascade. The studies presented here contribute mechanistic insight into the role of Rho in hypertrophic gene expression and demonstrate the functional impact of transcriptional activation and increased expression of GLUT1 in cardiomyocyte hypertrophy and survival.