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

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.

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Role of MAPK pathways in light chain-induced cytokine production in human proximal tubule cells

AJP Renal Physiology ◽

10.1152/ajprenal.00350.2002 ◽

2003 ◽

Vol 284 (6) ◽

pp. F1245-F1254 ◽

Cited By ~ 55

Author(s):

Sule Sengul ◽

Craig Zwizinski ◽

Vecihi Batuman

Keyword(s):

Proximal Tubule ◽

Light Chain ◽

Cytokine Production ◽

Tubulointerstitial Injury ◽

Proximal Tubule Cells ◽

Mapk Pathways ◽

Cytokine Responses ◽

Tubule Cells ◽

Human Proximal Tubule

We previously demonstrated that light chain (LC) endocytosis by human proximal tubule cells (PTCs) leads to production of cytokines through activation of NF-κB. Here, we examined the role of MAPK pathways in these responses using four species of myeloma LCs (κ1, κ2, κ3, and λ1) previously shown to induce cytokine production by PTCs. Among these, κ1-LC, which yielded the strongest cytokine responses, was selected for detailed studies. Activation of MAPKs was probed by Western blot analysis for the active kinases, ERK 1/2, JNK 1/2, and p38 in κ1-LC-exposed human PTCs. To evaluate the functional role of MAPKs in LC-induced cytokine responses, we tested the effects of U-0126, an ERK inhibitor; SP-600125, an inhibitor of JNK; SB-203580, a p38 inhibitor; and curcumin, a JNK-AP-1 inhibitor, all added to media before 4-h exposure to 1.5 mg/ml κ1-LC. IL-6 and monocyte chemotactic protein-1 (MCP-1) were determined by ELISA. Both LC and human serum albumin (HSA) activated ERK, although the HSA effect was weaker. κ1-LC stimulated all three MAPKs, although phosphorylation of ERK was more pronounced and sustained than others. Inhibitors of ERK, JNK, and p38 reduced LC-induced IL-6 and MCP-1 production. These findings suggest that activation of MAPKs plays a role in LC-induced cytokine responses in PTCs. Activation of MAPKs may be involved in cytokine responses induced by other proteins as well as LCs and may be pivotal in the pathophysiology of tubulointerstitial injury in proteinuric diseases.

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