Regulation of the epithelial Na ⁺ channel by the mTORC2/SGK1 pathway

Abstract

The epithelial Na ⁺ channel (ENaC) is decisive for sodium reabsorption by the aldosterone-sensitive distal nephron (ASDN) of the kidney. ENaC is regulated by the serum- and glucocorticoid-inducible kinase 1 (SGK1), a kinase genomically upregulated by several hormones including glucocorticoids and mineralocorticoids. SGK1 is activated by the serine/threonine kinase mammalian target of rapamycin (mTOR) isoform mTORC2. SGK1 knockout ( sgk1 ^−/− mice) impairs renal Na ⁺ retention during salt depletion. The mTOR catalytic site inhibitor, PP242, but not mTORC1 inhibitor rapamycin, inhibits ENaC, decreases Na ⁺ flux in isolated perfused tubules and induces natriuresis in wild-type mice. PP242 does not lead to further impairment of Na ⁺ reabsorption in sgk1 ^−/− mice. The mTORC2/SGK1 sensitive renal Na ⁺ retention leads to extracellular volume expansion with increase of blood pressure. A SGK1 gene variant (prevalence ∼3–5% in Caucasians, ∼10% in Africans) predisposes to hypertension, stroke, obesity and type 2 diabetes. Future studies will be required to define the role of mTORC2 in the regulation of further SGK1 sensitive transport proteins, such as further ion channels, carriers and the Na ⁺/K ⁺-ATPase. Moreover, studies are required disclosing the impact of mTORC2 on SGK1 sensitive disorders, such as hypertension, obesity, diabetes, thrombosis, stroke, inflammation, autoimmune disease, fibrosis and tumour growth.