Suppression of nephrin expression by TNF-α via interfering with the cAMP-retinoic acid receptor pathway

Nephrin, a crucial component of the slit diaphragm, is downregulated in proteinuric glomerular diseases including glomerulonephritis. We previously reported that 1) expression of nephrin in cultured podocytes is reinforced by retinoic acid (RA) and 1,25-dihydroxyvitamin D3, 2) these effects are mediated by retinoic acid receptor (RAR) and vitamin D receptor (VDR), and 3) basal and inducible expression of nephrin is downregulated by TNF-α. In the present investigation, we identified that TNF-α selectively represses activity of RAR but not VDR. To elucidate mechanisms underlying this observation, we tested involvement of downstream targets for TNF-α: nuclear factor-κB (NF-κB), mitogen-activated protein (MAP) kinases, phosphatidylinositol 3-kinase (PI3K)-Akt, and cAMP-protein kinase A (PKA). TNF-α caused activation of NF-κB, MAP kinases, and PI3K-Akt in podocytes, whereas blockade of these molecules did not affect inhibition of RAR by TNF-α. In contrast, TNF-α depressed activity of cAMP-PKA, and blockade of PKA inhibited basal and RA-induced activation of RAR. Furthermore, activity of RAR was significantly upregulated by cAMP, and the suppressive effect of TNF-α on RAR was reversed by cAMP-elevating agents. These results suggest that 1) expression of nephrin in podocytes is regulated by the cAMP-RAR pathway and 2) suppression of nephrin by TNF-α is caused, at least in part, through selective inhibition of this pathway.