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 a and beta
isoforms of GSK3. Western blotting using an antibody specific for the PKB/SGK1 consensus phosphorylation site in GSK3a/beta (
serine 21 and 9 respectively) revealed an increase in GSK3a/beta 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
GSK3beta phosphorylation and, to a much lesser extent, GSK3a phosphorylation. However, under this treatment
GSK3beta phosphorylation was apparent even in mice lacking functional SGK1, indicating that the phosphorylation of
GSK3beta was not exclusively mediated by this
kinase. Despite similar cardiac
GSK3beta 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
GSK3beta, 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.