Subcellular Localization Of Serum And Glucocorticoid Inducible Protein Kinase(SGK) In Renal Cells And Relationship Between SGK And Aldose Reductase

Serum and glucocorticoid inducible protein kinase, SGK is a novel member of the serine/threonine protein kinase gene family. Unlike the vast majority of protein kinases, which are predominantly regulated at the enzymatic level by posttranslational phosphorylation and dephosphorylation, the transcript expression, activity and subcellular localization of SGK1 can be acutely controlled by a diverse set of stimuli. SGK is an important focal point of intracellular cross-talk to control many cellular processes, including cell proliferation, osmoregulation, ion channel regulation and cell survival and/or apoptotic responses. SGK1 is also found to contributes to the development of many disease such as diabetic nephropathy and hypertension.However, the precise mechanism by which SGK1 plays important roles in those signal transduction pathways has not been established. The substrates of SGK1's physiological effects also remain to be identified. The subcellular localization of SGK1 has not yet been conclusively established and the studies of subcellular localization of transiently expressed SGK1 using expression construct of SGK1 and endogenous SGK1 have provided controversial results. In this study, Immunocytochemistry and western blotting were performed to detect the expression and subcellular localization of SGK1 in mIMCD3 cells. We discovered that SGK1 was predominantly localized to the nucleus that can be greatly up-regulated by hypertonicity. This result was confirmed by SGK1 siRNA which reduced the expression of SGK1. Then the expression construct of SGK1 with Flag tag at the C-terminus was used to determined the subcellular localization of transiently expressed SGK1. We found that transiently expressed SGK1 can be detected in both nucleus and cytoplasm, although the positive signals were stronger in nucleus than that in cytoplasm.Aldose reductase, AR as rate-limiting enzyme in the polyol pathway plays a important role in diabetic nephropathy. Recently, some experimental studies have suggest that SGK1 also contributes to the development of diabetic nephropathy. In this study, we provided the first evidence that the expression of SGK1 can be significantly down-regulated when cells were treated with AR inhibitor. The conclusion was also confirmed by AR siRNA which reduced the expression of AR. In summary the expression of SGK1 can be significantly regulated by AR in cultured mIMCD3 cells. These results have important implications for SGK's roles in renal physiology and pathophysiology.