Hypertension and myocardial infarction are associated with the onset of hypertrophy. Hypertrophy is a compensatory response mechanism to increases in mechanical load due to pressure or volume overload. It is characterized by extracellular matrix remodeling and hypertrophic growth of adult cardiomyocytes. Production of Vascular Endothelial Growth Factor (VEGF), which acts as an angiogenic factor and a modulator of cardiomyocyte function, is regulated by mechanical stretch. Mechanical stretch promotes VEGF secretion in neonatal cardiomyocytes. Whether this effect is retained in adult cells and the molecular mechanism mediating stretch-induced VEGF secretion has not been elucidated. Our objective was to investigate whether cyclic mechanical stretch induces VEGF secretion in adult cardiomyocytes and to identify the molecular mechanism mediating VEGF secretion in these cells. Isolated primary adult rat cardiomyocytes (ARCMs) were subjected to cyclic mechanical stretch at an extension level of 10% at 30 cycles/min that induces hypertrophic responses. Cyclic mechanical stretch induced a 3-fold increase in VEGF secretion in ARCMs compared to non-stretch controls. This increase in stretch-induced VEGF secretion correlated with NFkB activation. Cyclic mechanical stretch-mediated VEGF secretion was blocked by an NFkB peptide inhibitor and expression of a dominant negative mutant IkBα, but not by inhibitors of the MAPK/ERK1/2 or PI3K pathways. Chromatin immunoprecipitation assays demonstrated an interaction of NFkB with the VEGF promoter in stretched primary cardiomyocytes. Moreover, VEGF secretion is increased in the stretched myocardium during pressure overload-induced hypertrophy. These findings are the first to demonstrate that NFkB activation plays a role in mediating VEGF secretion upon cyclic mechanical stretch in adult cardiomyocytes. Signaling by NFkB initiated in response to cyclic mechanical stretch may therefore coordinate the hypertrophic response in adult cardiomyocytes. Elucidation of this novel mechanism may provide a target for developing future pharmacotherapy to treat hypertension and heart disease.

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Cardiac pressure and volume overload is a potent stimulus for cardiac hypertrophy. The adaptive value of hypertrophic remodeling is largely determined by the ability of the heart to develop new vasculature, to accommodate the growth of myocardial tissue. This study focused on the production of proangiogenic vascular endothelial growth factor (VEGF) by cardiomyocytes. We cultured primary adult rat cardiomyocytes (ARCM) under hypoxic conditions, and collected culture media for VEGF measurements. Hypoxia caused a 3‐fold increase in VEGF release from ARCM. Primary adult mouse cardiomyocytes (AMCM) responded to hypoxia in a similar way. Hypoxia upregulated the expression of genes in AMCM that are controlled by hypoxia‐response elements. Expression of vegfa increased 5 to 8‐fold in hypoxic AMCM. Cobalt chloride, an agent that induces hypoxia‐like response, increased both VEGF production and vegfa expression in these cells. In separate experiments, mechanical stretch of ARCM caused a 3‐fold increase in VEGF release, as compared to non‐stretched cells. Thus, we conclude that cultured primary cardiomyocytes respond to hypoxia and mechanical stretch, factors that are present in overloaded hearts, with a significantly upregulated release of VEGF. The study suggests that the model may be valuable in designing agents that regulate the production of angiogenic factors by cardiomyocytes in the heart.