Hypotonic induction of SGK1 and Na⁺ transport in A6 cells

Serum and glucocorticoid-regulated kinase-1 (SGK1) is a serine-threonine kinase that is regulated at the transcriptional level by numerous regulatory inputs, including mineralocorticoids, glucocorticoids, follicle-stimulating hormone, and osmotic stress. In the distal nephron, SGK1 is induced by aldosterone and regulates epithelial Na⁺ channel-mediated transepithelial Na⁺ transport. In other tissues, including liver and shark rectal gland, SGK1 is regulated by hypertonic stress and is thought to modulate epithelial Na⁺ channel- and Na⁺-K⁺-2Cl^- cotransporter-mediated Na⁺ transport. In this report, we examined the regulation of SGK1 mRNA and protein expression and Na⁺ currents in response to osmotic stress in A6 cells, a cultured cell line derived from Xenopus laevis distal nephron. We found that in contrast to hepatocytes and rectal gland cells, hypotonic conditions stimulated SGK1 expression and Na⁺ transport in A6 cells. Moreover, a correlation was found between SGK1 induction and the later phase of activation of Na⁺ transport in response to hypotonic treatment. When A6 cells were pretreated with an inhibitor of phosphatidylinositol 3-kinase (PI3K), Na⁺ transport was blunted and only inactive forms of SGK1 were expressed. Surprisingly, these results demonstrate that both hypertonic and hypotonic stimuli can induce SGK1 gene expression in a cell type-dependent fashion. Moreover, these data lend support to the view that SGK1 contributes to the defense of extracellular fluid volume and tonicity in amphibia by mediating a component of the hypotonic induction of distal nephron Na⁺ transport.