Glycogen synthase kinase-3 (GSK-3) is a master regulator of growth and death in cardiac myocytes.
GSK-3 is inactivated by hypertrophic stimuli through phosphorylation-dependent and -independent mechanisms. Inactivation of
GSK-3 removes the negative constraint of
GSK-3 on
hypertrophy, thereby stimulating
cardiac hypertrophy. N-terminal phosphorylation of the
GSK-3 isoforms GSK-3alpha and
GSK-3beta by upstream
kinases (e.g., Akt) is a major mechanism of
GSK-3 inhibition. Nonetheless, its role in mediating
cardiac hypertrophy and failure remains to be established. Here we evaluated the role of
Serine(S)21 and S9 phosphorylation of
GSK-3alpha and
GSK-3beta in the regulation of
cardiac hypertrophy and function during pressure overload (PO), using
GSK-3alpha S21A knock-in (alphaKI) and
GSK-3beta S9A knock-in (betaKI) mice. Although inhibition of S9 phosphorylation during PO in the betaKI mice attenuated
hypertrophy and
heart failure (HF), inhibition of S21 phosphorylation in the alphaKI mice unexpectedly promoted
hypertrophy and HF. Inhibition of S21 phosphorylation in
GSK-3alpha, but not of S9 phosphorylation in
GSK-3beta, caused phosphorylation and down-regulation of G1-cyclins, due to preferential localization of
GSK-3alpha in the nucleus, and suppressed E2F and markers of cell proliferation, including phosphorylated
histone H3, under PO, thereby contributing to decreases in the total number of myocytes in the heart. Restoration of the E2F activity by injection of adenovirus harboring
cyclin D1 with a
nuclear localization signal attenuated HF under PO in the alphaKI mice. Collectively, our results reveal that whereas S9 phosphorylation of
GSK-3beta mediates pathological
hypertrophy, S21 phosphorylation of
GSK-3alpha plays a compensatory role during PO, in part by alleviating the negative constraint on the cell cycle machinery in cardiac myocytes.