Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Transforming growth factor beta (TGF-beta) has been shown to participate in the pathophysiology of diabetic complications. As shown most recently, TGF -beta stimulates the expression of a distinct serine/threonine kinase (hSGK ) which had previously been cloned as an early gene transcriptionally regul ated by cell volume alterations. The present study was performed to elucida te transcription and function of hSGK in diabetic nephropathy. As shown by Northern blotting, an increase of extracellular glucose concentration incre ased hSGK mRNA levels in cultured cells, an effect qualitatively mimicked b y osmotic cell shrinkage or treatment with TGF-beta (2 mu g/liter), phorbol 12,13-didecanoate (1 mu M), or the Ca2+ ionophore ionomycin (1 CIM) and bl unted by high concentrations of nifedipine (10 and 100 mu M). In situ hybri dization revealed that hSGK transcription was markedly enhanced in diabetic nephropathy, with particularly high expression in mesangial cells, interst itial cells, and cells in thick ascending limbs of Henle's loop and distal tubules. According to voltage clamp and tracer flux studies in Xenopus oocy tes expressing the renal epithelial Na+ channel ENaC or the mouse thick asc ending limb Na+,K+,2Cl(-) cotransporter BSC-1, coexpression with hSGK stimu lated ENaC and BSC-1 11-fold and 6-fold, respectively, effects reversed by kinase inhibitors staurosporine (1 mu M) and chelerythrine (1 mu M) and not elicited by inactive hSGK. In conclusion, excessive extracellular glucose concentrations enhance hSGK transcription, which in turn stimulates renal t ubular Na+ transport. These observations disclose an additional element in the pathophysiology of diabetic nephropathy.