PKB/SGK-resistant GSK-3 signaling following unilateral ureteral obstruction

Background/Aims: Renal tissue fibrosis contributes to the development of end-stage renal disease. Causes for renal tissue fibrosis include obstructive nephropathy. The development of renal fibrosis following unilateral ureteral obstruction (UUO) is blunted in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1. Similar to Akt isoforms, SGK1 phosphorylates and thus inactivates glycogen synthase kinase GSK-3. The present study explored whether PKB/SGK-dependent phoshorylation of GSK-3/β impacts on pro-fibrotic signaling following UUO.

Methods: UUO was induced in mice carrying a PKB/SGK-resistant GSK-3/β (gsk-3^KI) and corresponding wild-type mice (gsk-3^WT). Three days after the obstructive injury, expression of fibrosis markers in kidney tissues was analyzed by quantitative RT-PCR and western blotting.

Results: GSK-3 and GSK-3β phosphorylation was absent in both, the non-obstructed and the obstructed kidney tissues from gsk-3^KI mice but was increased by UUO in kidney tissues from gsk-3^WT mice. Expression of -smooth muscle actin, type I collagen and type III collagen in the non-obstructed kidney tissues was not significantly different between gsk-3^KI mice and gsk-3^WT mice but was significantly less increased in the obstructed kidney tissues from gsk-3 ^KI mice than from gsk-3^WT mice. After UUO treatment, renal β-catenin protein abundance and renal expression of the β-catenin sensitive genes: c-Myc, Dkk1, Twist and Lef1 were again significantly less increased in kidney tissues from gsk-3^KI mice than from gsk-3 ^WT mice.

Conclusions: PKB/SGK-dependent phosphorylation of glycogen synthase kinase GSK-3 contributes to the pro-fibrotic signaling leading to renal tissue fibrosis in obstructive nephropathy.