Expression and role of serum and glucocorticoid-regulated kinase 2 in the regulation of Na⁺/H⁺exchanger 3 in the mammalian kidney

Abstract

Serum and glucocorticoid-regulated kinase 2 (sgk2) is 80% identical to the kinase domain of sgk1, an important mediator of mineralocorticoid-regulated sodium (Na(+)) transport in the distal nephron of the kidney. The expression pattern and role in renal function of sgk2 are virtually uncharacterized. In situ hybridization and immunohistochemistry of rodent kidney coupled with real-time RT-PCR of microdissected rat kidney tubules showed robust sgk2 expression in the proximal straight tubule and thick ascending limb of the loop of Henle. Sgk2 expression was minimal in distal tubule cells with aquaporin-2 immunostaining but significant in proximal tubule cells with Na(+)/H(+) exchanger 3 (NHE3) immunostaining. To ascertain whether mineralocorticoids regulate expression of sgk2 in a manner similar to sgk1, we examined sgk2 mRNA expression in the kidneys of adrenalectomized rats treated with physiological doses of aldosterone together with the glucocorticoid receptor antagonist RU486. Northern blot analysis and in situ hybridization showed that, unlike sgk1, sgk2 expression in the kidney was not altered by aldosterone treatment. Based on the observation that sgk2 is expressed in proximal tubule cells that also express NHE3, we asked whether sgk2 regulates NHE3 activity. We heterologously expressed sgk2 in opossum kidney (OKP) cells and measured Na(+)/H(+) exchange activity by Na(+)-dependent cell pH recovery. Constitutively active sgk2, but not sgk1, stimulated Na(+)/H(+) exchange activity by >30%. Moreover, the sgk2-mediated increase in Na(+)/H(+) exchange activity correlated with an increase in cell surface expression of NHE3. Together, these results suggest that the pattern of expression, regulation, and role of sgk2 within the mammalian kidney are distinct from sgk1 and that sgk2 may play a previously unrecognized role in the control of transtubular Na(+) transport through NHE3 in the proximal tubule.

mTORC2 regulates renal tubule sodium uptake by promoting ENaC activity

“…Thus, whether ENaC is selectively regulated by SGK1, or whether other signaling molecules can also regulate ENaC in a physiologically relevant manner, becomes a question of considerable importance. There have been conflicting reports based on cell culture and oocyte studies regarding the roles of several SGK1 relatives (31,32,(44)(45)(46). Furthermore, there have been conflicting reports regarding the effects of SGK1 on transporters other than ENaC.…”

Sgk-1 is a Positive Regulator of Constitutive Albumin Uptake in Renal Proximal Tubule Cells

“…Unlike Sgk-2, which is not reported to interact with NHERF2 effective regulation of NHE3 by Sgk-1 requires NHERF2 [20]. The OKP cells used in the Sgk-2 study are not reported to express adequate levels of endogenous NHERF2 [43] hence the lack of effect of Sgk-1 that these investigators observed may be simply due to the absence of NHERF2.…”

“…Conversely, the absence of Sgk-1 can be protective as shown in studies using DOCA/high salt treated mice where glomerulosclerosis and tubulointerstitial fibrosis was blunted in Sgk-1 -/-mice compared to wild type controls [51]. Sgk-2 is also expressed in the proximal tubule and a recent study showed that overexpression of constitutively active Sgk-2 in OKP cells increased NHE3 activity but were unable to demonstrate any effect with Sgk-1 [43]. Unlike Sgk-2, which is not reported to interact with NHERF2 effective regulation of NHE3 by Sgk-1 requires NHERF2 [20].…”

“…The dynamic nature and regulated nature of the interactions between NHERF2 and NHE3 is highlighted in recent studies using methods such as fluorescence resonance energy transfer and photo-bleaching recovery [20,42]. Sgk-1 is widely expressed in the kidney and although it is predominantly expressed in the distal nephron there is evidence to support its expression and presence in the proximal tubule, albeit at lower levels [43]. In addition, cultured proximal tubule cells (both primary and clonal) express Sgk-1 [44,45].…”

SGK1 regulation by miR-466g in cortical collecting duct cells

“…Reactions were amplified using the Applied Biosystems Prism 7900HT Fast Real-Time PCR system. Results were expressed as 2 Ϫ⌬⌬Ct values, with ⌬Ct ϭ CtmiR Ϫ Ctsno202 (29). Expression of miR-466g was normalized to expression of the control miRNA sno-202.…”