Previous attempts to delineate the consequences of Galpha (q) activation in cardiomyocytes relied largely on molecular strategies in cultures or transgenic mice. Modest levels of wild-type Galpha(q) overexpression induce stable
cardiac hypertrophy, whereas intense Galpha(q) stimulation induces cardiomyocyte apoptosis. The precise mechanism(s) whereby traditional targets of Galpha (q) subunits that induce
hypertrophy also trigger cardiomyocyte apoptosis is not obvious and is explored with recombinant
Pasteurella multocida toxin (rPMT, a Galpha(q) agonist). Cells cultured with rPMT display cardiomyocyte enlargement, sarcomeric organization, and increased
atrial natriuretic factor expression in association with activation of
phospholipase C, novel
protein kinase C (PKC)
isoforms, extracellular signal-regulated
protein kinase (ERK), and (to a lesser extent) JNK/
p38-MAPK. rPMT stimulates the ERK cascade via
epidermal growth factor (
EGF) receptor transactivation in cardiac fibroblasts, but
EGF receptor transactivation plays no role in ERK activation in cardiomyocytes. Surprisingly, rPMT (or novel PKC
isoform activation by PMA) decreases basal Akt phosphorylation; rPMT prevents Akt phosphorylation by
EGF or
IGF-1 and functionally augments cardiomyocyte apoptosis in response to H2O2. These results identify a Galpha(q)-PKC pathway that represses basal Akt phosphorylation and impairs Akt stimulation by survival factors. Because inhibition of Akt enhances cardiomyocyte susceptibility to apoptosis, this pathway is predicted to contribute to the transition from
hypertrophy to cardiac decompensation and could be targeted for
therapy in
heart failure.