Dexamethasone's prosurvival benefits in podocytes require extracellular signal-regulated kinase phosphorylation

Background: The reduction in podocyte number is a critical determinant in the development of glomerular diseases. Our recent study demonstrated that glucocorticoids, which are widely used for the treatment of various forms of glomerular injury characterized by proteinuria, protect podocytes from undergoing apoptosis induced by puromycin aminonucleoside (PA). However, the precise mechanisms underlying the beneficial effects of glucocorticoids on podocytes remain to be fully elucidated. Methods: To clarify the role of p53 in apoptosis-inducing factor (AIF) translocation associated with podocyte apoptosis, we performed immunostaining for AIF on cultured mouse podocytes in the presence of the p53 inhibitor pifithrin-alpha. Extracellular signal-regulated kinase (ERK) phosphorylation in podocytes was measured by Western blot analysis. The role of ERK phosphorylation in podocyte apoptosis was also investigated utilizing MEK1/2 inhibitor U0126. Results: AIF translocation to nuclei was p53 dependent. Furthermore, phosphorylated ERK was reduced in podocytes exposed to PA, and this was prevented by dexamethasone (DEX). Inhibition of ERK phosphorylation by U0126 enhanced podocyte apoptosis induced by PA. Interestingly, when ERK phosphorylation was inhibited, DEX exerted a proapoptotic effect on podocytes, and this effect was also associated with AIF translocation. Our results showed that DEX did not prevent caspase-3-dependent podocyte apoptosis induced by transforming growth factor-beta 1 (TGF-beta 1) or UV-C. Conclusion: These results suggest that ERK phosphorylation and the subcellular localization of AIF are important determinants in the protective effect of DEX in podocytes. Copyright c 2008 S. Karger AG, Basel.