Hypotonic induction of SGK1 and Na⁺transport in A6 cells

Abstract

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.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

“…Nuclear TonEBP is increased throughout the medulla in dehydrated versus water-loaded animals (26). It is noteworthy that Rozansky et al showed that sgk1, in addition to being stimulated by increases in extracellular tonicity, is also stimulated by decreases in extracellular tonicity (47). The latter has been demonstrated using hypo-osmotic induction of A6 cells and confirmed here using rat IMCD cells.…”

Multiple Translational Isoforms Give Functional Specificity to Serum- and Glucocorticoid-induced Kinase 1

“…The different molecularweight bands of Sgk1 have been attributed exclusively to aberrant migration, owing to phosphorylation of residues T253 and S422 upon activation of Sgk1 by insulin or other agonists (Park et al, 1999;Rozansky et al, 2002). We demonstrate that additional bands of lower molecular weight than Sgk1 correspond to different translational isoforms.…”

Aldosterone acts via an ATP autocrine/paracrine system: The Edelman ATP hypothesis revisited

“…This regulation also involves serum-and glucocorticoid-kinase (SGK1) that prevents ENaC degradation and mitogen-activated protein kinase (MAPK) that in contrast stimulates channel degradation (31,32). In A6 cells, hypotonicity, as well as aldosterone, induces SGK1 expression (7). However, studies in the SGK1 knockout mouse demonstrated that renal salt handling is normal when the animals are on a standard salt intake but impaired with dietary salt restriction, suggesting that SGK1 participates in, but does not fully account for, mineralocorticoid regulation of sodium reabsorption (33).…”

“…In addition to hormonal stimulation (aldosterone, vasopressin, insulin), other factors are known to increase epithelial sodium transport, including hypotonic and mechanical stimulation (7). In recent work, Jans et al (8) have shown that hypotonic treatment of renal epithelia induces a transient increase in intracellular Ca 2ϩ through a mechanism that is sensitive to the purinergic P2 receptor antagonist suramin applied to the basolateral border only.…”