Sgk1-dependent stimulation of cardiac Na+/H+ exchanger Nhe1 by dexamethasone

The serum- and glucocorticoid-inducible kinase Sgk1 contributes to cardiac remodeling and development of heart failure, which is paralelled by Sgk1-dependent stimulation of the cardiac Na(+)/H(+) exchanger Nhe1. Glucocorticoids are powerful stimulators of Sgk1 expression and influence cardiac remodeling. The present study thus explored whether the glucocorticoid receptor agonist dexamethasone influenced cardiac Sgk1 expression, as well as activity, expression and phosphorylation at Ser(703) of the cardiac Na(+)/H(+) exchanger Nhe1.

Experiments were performed in HL-1 cardiomyocytes and gene targeted mice lacking functional Sgk1 (sgk1(-/-)) and respective wild type mice (sgk1(+/+)). Gene expression was determined by quantitative RT-PCR and Nhe1 phosphorylation was determined utilizing a specific antibody against a 14-3-3 binding motif at P-Ser(703), which represents a putative phosphorylation site recognition motif for Sgk1 and is involved in Nhe1 activation. Cytosolic pH (pHi) was determined utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence and Nhe activity by the Na(+)-dependent realkalinization after an ammonium pulse.

Treatment of HL-1 cardiomyocytes with dexamethasone was followed by a significant increase in Sgk1 mRNA expression, parallelled by increased Na(+)/H(+) exchanger activity. Furthermore, dexamethasone significantly increased Nhe1 and Spp1 mRNA expression. The effects of dexamethasone were blunted by cotreatment of HL-1 cardiomyocytes with the Sgk1 inhibitor EMD638683. Cotreatment with Nhe1 inhibitor cariporide similarly prevented dexamethasone-stimulated Spp1 mRNA expression. In sgk1(+/+) mice, dexamethasone significantly increased cardiac Sgk1 mRNA levels. In sgk1(+/+) mice, but not in sgk1(-/-) mice, dexamethasone significantly increased cardiac Nhe1 mRNA expression and Nhe1 phosphorylation at Ser(703). Furthermore, cardiac Spp1, Ctgf, Nppa and Nppb mRNA levels were significantly increased in dexamethasone treated sgk1(+/+) mice, effects significantly blunted in sgk1(-/-) mice.

Sgk1 is critically involved in the phosphorylation and activation of the cardiac Na(+)/H(+) exchanger Nhe1.