Calcineurin-mediated hypertrophy protects cardiomyocytes from apoptosis invitro and in vivo - An apoptosis-independent model of dilated heart failure

We have previously shown that the calcium-calmodulin-regulated phosphatase calcineurin (PP2B) is sufficient to induce cardiac hypertrophy that transit ions to heart failure in transgenic mice. Given the rapid onset of heart fa ilure in these mice, we hypothesized that calcineurin signaling would stimu late myocardial cell apoptosis. However, utilizing multiple approaches, we determined that calcineurin-mediated hypertrophy protected cardiac myocytes from apoptosis, suggesting a model of heart failure that is independent of apoptosis. Adenovirally mediated gene transfer of a constitutively active calcineurin cDNA (AdCnA) was performed in cultured neonatal rat cardiomyocy tes to elucidate the mechanism whereby calcineurin affected myocardial cell viability. AdCnA infection, which induced myocyte hypertrophy and atrial n atriuretic factor expression, protected against apoptosis induced by 2-deox yglucose or staurosporine, as assessed by terminal deoxynucleotidyltransfer ase;mediated dUTP nick end labeling (TUNEL) labeling, caspase-3 activation, DNA laddering, and cellular morphology. The level of protection conferred by AdCnA was similar to that of adenoviral Bcl-x(L) gene transfer or hypert rophy induced by phenylephrine. In vivo, failing hearts from calcineurin-tr ansgenic mice did not demonstrate increased TUNEL labeling and, in fact, de monstrated a resistance to ischemia/reperfusion-induced apoptosis, We deter mined that the mechanism whereby calcineurin afforded protection from apopt osis was partially mediated by nuclear factor of activated T cells (NFAT3) signaling and partially by Akt/protein kinase B (PKB) signaling. Although c alcineurin activation protected myocytes from apoptosis, inhibition of calc ineurin with cyclosporine was not sufficient to induce TUNEL labeling in Gq alpha-transgenic mice or in cultured cardiomyocytes. Collectively, these d ata identify a calcineurin-dependent mouse model of dilated heart failure t hat is independent of apoptosis.