[Show abstract][Hide abstract] ABSTRACT: 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-3α and GSK-3β 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-3α and GSK-3β in the regulation of cardiac hypertrophy and function during pressure overload (PO), using GSK-3α S21A
knock-in (αKI) and GSK-3β S9A knock-in (βKI) mice. Although inhibition of S9 phosphorylation during PO in the βKI mice attenuated
hypertrophy and heart failure (HF), inhibition of S21 phosphorylation in the αKI mice unexpectedly promoted hypertrophy and
HF. Inhibition of S21 phosphorylation in GSK-3α, but not of S9 phosphorylation in GSK-3β, caused phosphorylation and down-regulation
of G1-cyclins, due to preferential localization of GSK-3α 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 αKI mice. Collectively, our results reveal that whereas S9 phosphorylation of GSK-3β mediates pathological
hypertrophy, S21 phosphorylation of GSK-3α plays a compensatory role during PO, in part by alleviating the negative constraint
on the cell cycle machinery in cardiac myocytes.
Full-text · Article · Dec 2008 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Cyclins and other cell-cycle regulators have been used in several studies to regenerate cardiomyocytes in ischaemic heart failure. However, proliferation of cardiomyocytes induced by nuclear-targeted cyclin D1 (D1NLS) stops after one or two rounds of cell cycles due in part to accumulation of p27Kip1, an inhibitor of cyclin-dependent kinase (CDK). Thus, expression of S-phase kinase-associated protein 2 (Skp2), a negative regulator of p27Kip1, significantly enhances the effect of D1NLS and CDK4 on cardiomyocyte proliferation in vitro. Here, we examined whether Skp2 can also improve cardiomyocyte regeneration and post-ischaemic cardiac performance in vivo.
Wistar rats underwent ischaemia/reperfusion injury by ligation of the coronary artery followed by injection of adenovirus vectors for D1NLS and CDK4 with or without Skp2. Enhanced proliferation of cardiomyocytes in the presence of Skp2 was demonstrated by increased expression of Ki67, a marker of proliferating cells (1.95% vs. 4.00%), and mitotic phosphorylated histone H3 (0.24% vs. 0.58%). Compared with rats that received only D1NLS and CDK4, expression of Skp2 improved left ventricular function as measured by the maximum and minimum rates of change in left ventricular pressure, the left ventricle end-diastolic pressure, left ventricle end-diastolic volume index, and the lung/body weight ratio.
Expression of Skp2 enhanced the effect of D1NLS and CDK4 on the proliferation of cardiomyocytes and further contributed to improved post-ischaemic cardiac function. Skp2 might be a versatile tool to improve the effect of cyclins on post-ischaemic regeneration of cardiomyocytes in vivo.
Full-text · Article · Aug 2008 · Cardiovascular Research