Glycogen synthase kinase-3 (GSK-3) is a
serine/threonine kinase having multiple functions and consisting of two
isoforms,
GSK-3alpha and
GSK-3beta. Pressure overload increases expression of
GSK-3alpha but not
GSK-3beta. Despite our wealth of knowledge about
GSK-3beta, the function of
GSK-3alpha in the heart is not well understood. To address this issue, we made cardiac-specific
GSK-3alpha transgenic mice (Tg). Left ventricular weight and cardiac myocyte size were significantly smaller in Tg than in non-Tg (NTg) mice, indicating that
GSK-3alpha inhibits cardiac growth. After 4 weeks of aortic banding (transverse aortic constriction (TAC)), increases in left ventricular weight and myocyte size were significantly smaller in Tg than in NTg, indicating that
GSK-3alpha inhibits
cardiac hypertrophy. More severe cardiac dysfunction developed in Tg after TAC. Increases in
fibrosis and apoptosis were greater in Tg than in NTg after TAC. Among signaling molecules screened, ERK phosphorylation was decreased in Tg. Adenovirus-mediated overexpression of
GSK-3alpha, but not
GSK-3beta, inhibited ERK in cultured cardiac myocytes. Knockdown of
GSK-3alpha increased ERK phosphorylation, an effect that was inhibited by
PD98059,
rottlerin, and
protein kinase Cepsilon (PKCepsilon) inhibitor
peptide, suggesting that
GSK-3alpha inhibits ERK through PKC-
MEK-dependent mechanisms. Knockdown of
GSK-3alpha increased
protein content and reduced apoptosis, effects that were abolished by
PD98059, indicating that inhibition of ERK plays a major role in the modulation of cardiac growth and apoptosis by
GSK-3alpha. In conclusion, up-regulation of
GSK-3alpha inhibits cardiac growth and pressure overload-induced
cardiac hypertrophy but increases
fibrosis and apoptosis in the heart. The anti-hypertrophic and pro-apoptotic effect of
GSK-3alpha is mediated through inhibition of ERK.