DOCA-induced phosphorylation of glycogen synthase kinase 3β

Abstract

Mineralocorticoid excess leads to cardiac fibrosis, a leading cause of morbidity and mortality. Cardiac hypertrophy and fibrosis are inhibited by the glycogen synthase kinase GSK3 which itself is a target of protein kinase B (PKB) and the serum and glucocorticoid inducible kinase SGK1. Phosphorylation of GSK3 by PKB or SGK1 inhibits GSK3 activity and should thus favour the development of cardiac hypertrophy and fibrosis. As SGK1 is transcriptionally upregulated by mineralocorticoids and has been recently shown to play an important role in the pathogenesis of mineralocorticoid-induced cardiac fibrosis, the present study explored whether mineralocorticoid excess had any effect on the phosphorylation status of the α and β isoforms of GSK3. Western blotting using an antibody specific for the PKB/SGK1 consensus phosphorylation site in GSK3α/β (serine 21 and 9 respectively) revealed an increase in GSK3α/β phosphorylation in human embryonic kidney 293 (HEK293) cells overexpressing wild type SGK1, constitutively active SGK1, but not catalytically inactive SGK1. The effect of SGK1 was mimicked by PKB and SGK3. Furthermore, DOCA/high salt treatment of wild type mice induced a robust increase in cardiac GSK3β phosphorylation and, to a much lesser extent, GSK3α phosphorylation. However, under this treatment GSK3β phosphorylation was apparent even in mice lacking functional SGK1, indicating that the phosphorylation of GSK3β was not exclusively mediated by this kinase. Despite similar cardiac GSK3β phosphorylation cardiac fibrosis following DOCA/high salt treatment was significantly blunted in SGK1 knockout mice. In conclusion, mineralocorticoid excess leads to phosphorylation and thus inactivation of GSK3β, an effect not only due to upregulation of SGK1 but as well due to activation of additional kinases. The inactivation of GSK3 may play a permissive role in the stimulation of cardiac fibrosis but may by itself not be sufficient to trigger cardiac fibrosis. Copyright © 2006 S. Karger AG.