SGK1 increases Na,K-ATP cell-surface expression and function in Xenopus laevis oocytes

Regulation of the excitatory amino acid transporter EAAT5 by the serum and glucocorticoid dependent kinases SGK1 and SGK3

In the mammalian retina, glutamate re-uptake is mediated by the sodium dependent cotransport systems EAAT1-5 thus terminating neuronal excitation and preventing neuroexcitotoxicity. In retinal amacrine and ganglion cells, EAAT5 is colocalized with the serum and glucocorticoid inducible kinase SGK1, a serine/threonine kinase known to regulate transport. The study explored the possible regulation of EAAT5 by SGK1, its isoform SGK3, and the closely related protein kinase B. EAAT5 was coexpressed in Xenopus laevis oocytes with or without the respective kinases. Transport activity was quantified by electrophysiology and cell surface expression was determined by chemiluminescence. Both EAAT5 mediated currents and EAAT5 protein abundance at the cell surface were increased by a factor of 1.5-2 upon coexpression of SGK1 or SGK3 but not following coexpression of PKB. In conclusion, the kinases SGK1 and SGK3 increase EAAT5 activity by increasing cell surface abundance of the carrier.

Role of Sgk1 in salt and potassium homeostasis

Aldosterone plays a pivotal role in NaCl and K(+) homeostasis by stimulation of Na(+) reabsorption and K(+) secretion in the aldosterone-sensitive distal nephron (ASDN). Recent studies demonstrated that the serum- and glucocorticoid-regulated kinase 1 (Sgk1) is induced by aldosterone in the ASDN and that polymorphisms of the kinase associate with arterial blood pressure in normotensive subjects. This review discusses the role of Sgk1 in NaCl and K(+) homeostasis as evidenced by in vivo studies, including those in Sgk1-deficient mice. The studies indicate that Sgk1 is not absolutely required for Na(+) reabsorption and K(+) secretion in the ASDN. On a standard NaCl and K(+) diet, modestly enhanced plasma aldosterone concentrations appear sufficient to establish a compensated phenotype in the absence of Sgk1. The kinase is necessary, however, for upregulation of transcellular Na(+) reabsorption in the ASDN. This may involve Sgk1-mediated stimulation of basolateral Na(+)-K(+)-ATPase as well as retention of epithelial Na(+) channel, ENaC, in the apical membrane. Such an upregulation is a prerequisite for adequate adaptation of 1) renal NaCl reabsorption during restricted dietary NaCl intake, as well as 2) K(+) secretion in response to enhanced K(+) intake. Thus gain-of-function mutations of Sgk1 are expected to result in renal NaCl retention and enhanced K(+) secretion. Further studies are required to elucidate renal and nonrenal aldosterone-induced effects of Sgk1, the role of other Sgk1 activators, as well as the link of Sgk1 polymorphisms to arterial hypertension in humans.

Mechanisms of mineralocorticoid action: determinants of receptor specificity and actions of regulated gene products

The mineralocorticoid receptor (MR) and its close cousin, the glucocorticoid receptor (GR), share considerable structural and functional similarity, including indistinguishable DNA binding properties, yet they mediate distinct physiological responses in some tissues. Specificity is determined by their distinct interactions with other protein factors and modification by peptides, including the small ubiquitin modifier SUMO1. Serum and glucocorticoid-regulated kinase 1 (sgk1) is one key target gene of both MR and GR, and encodes a serine-threonine kinase that stimulates the apical membrane localization of the epithelial sodium channel ENaC. Sgk1 exerts its effects, at least in part, by inhibiting an isoform of the ENaC inhibitory ubiquitin ligase Nedd4-2. This review briefly summarizes two areas of mineralocorticoid research: molecular determinants of MR specificity, and the role of Sgk1 in mediating the effects of aldosterone on epithelial Na(+) transport.