The Serum-‐ and Glucocorticoid-‐Inducible Kinase SGK1 and SGK3 Regulate hERG Channel Expression via Ubiquitin Ligase Nedd4-‐2 and GTPase Rab11
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The rapidly activating delayed rectifier potassium channel (IKr) encoded by the human ether-a-go-go related gene (hERG) is a crucial component of cardiac repolarization of the action potential. The gating kinetics of the channel and the number of channels on the membrane determine the degree of current flowing through hERG channels. Membrane channel density is directly related to a balance between protein trafficking and degradation. While hERG channel trafficking has been studied in the past decade, recently our lab and others have made new discoveries regarding hERG degradation. These studies show that Neural Down Regulated Gene 4 subtype 2 (Nedd4-‐2) is the ligase that covalently attaches ubiquitin to the hERG channel and facilitates degradation (Albesa et al., 2011;Guo et al., 2012). Given these findings, my goal/objective was to study the molecular mechanisms that control the destructive effects of Nedd4-‐2 on the hERG channel. In the present study, I demonstrated that overexpression of the stress-‐responsive serum-‐ and glucocorticoid-‐inducible kinase (SGK) isoforms SGK1 and SGK3 increase the current and expression level of the membrane-‐ localized mature proteins of hERG channels stably expressed in HEK 293 (hERG-‐HEK) cells. I have found that the overexpression of SGK1 and SGK3 increased Nedd4-‐2 phosphorylation, which is known to inhibit Nedd4-‐2 activity. Furthermore, the synthetic glucocorticoid, dexamethasone, increased the current and abundance of mature ERG proteins in both hERG-‐HEK cells and neonatal rat cardiac myocytes through the enhancement of SGK1 but not SGK3 expression. Additionally, disruption of Rab11 proteins led to a complete elimination of SGK-‐mediated increase in hERG expression. These results indicate that SGK enhances the expression level of mature hERG channels by inhibiting Nedd4-‐2 as well as by promoting Rab11-‐mediated hERG recycling.