Aldosterone regulation of intestinal Na absorption involves SGK-mediated changes in NHE3 and Na+ pump activity

Aldosterone-induced intestinal Na+ absorption is mediated by increased activities of apical membrane Na+/H+ exchange (aNHE3) and basolateral membrane Na+-K+-ATPase (BLM-Na+-K+-ATPase) activities. Because the processes coordinating these events were not well understood, we investigated human intestinal Caco-2BBE cells where aldosterone increases within 2-4 h of aNHE3 and alpha-subunit of BLM-Na+-K+-ATPase, but not total abundance of these proteins. Although aldosterone activated Akt2 and serum glucorticoid kinase-1 (SGK-1), the latter through stimulation of phosphatidylinositol 3-kinase (PI3K), only the SGK-1 pathway mediated its effects on Na+-K+-ATPase. Ouabain inhibition of the early increase in aldosterone-induced Na+-K+-ATPase activation blocked most of the apical NHE3 insertion, possibly by inhibiting Na+-K+-ATPase-induced changes in intracellular sodium concentration ([Na](i)). Over the next 6-48 h, further increases in aNHE3 and BLM-Na+-K+-ATPase activity and total protein expression were observed to be largely mediated by aldosterone-activated SGK-1 pathway. Aldosterone-induced increases in NHE3 mRNA, for instance, could be inhibited by RNA silencing of SGK-1, but not Akt2. Additionally, aldosterone-induced increases in NHE3 promoter activity were blocked by silencing SGK-1 as well as pharmacological inhibition of PI3K. In conclusion, aldosterone-stimulated intestinal Na+ absorption involves two phases. The first phase involves stimulation of PI3K, which increases SGK-dependent insertion and function of BLM-Na+-K+-ATPase and subsequent increased membrane insertion of aNHE3. The latter may be caused by Na+-K+-ATPase-induced changes in [Na] or transcellular Na flux. The second phase involves SGK-dependent increases in total NHE3 and Na+-K+-ATPase protein expression and activities. The coordination of apical and BLM transporters after aldosterone stimulation is therefore a complex process that requires multiple time-and interdependent cellular processes.