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

Abstract

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-␣. In the present investigation, we identified that TNF-␣ selectively represses activity of RAR but not VDR. To elucidate mechanisms underlying this observation, we tested involvement of downstream targets for TNF-␣: nuclear factor-B (NF-B), mitogen-activated protein (MAP) kinases, phosphatidylinositol 3-kinase (PI3K)-Akt, and cAMP-protein kinase A (PKA). TNF-␣ caused activation of NF-B, MAP kinases, and PI3K-Akt in podocytes, whereas blockade of these molecules did not affect inhibition of RAR by TNF-␣. In contrast, TNF-␣ 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-␣ 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-␣ is caused, at least in part, through selective inhibition of this pathway.tumor necrosis factor-␣; protein kinase A NEPHRIN, a podocyte-specific protein, is a key regulator that maintains normal structure and function of the slit diaphragm in the glomerulus (26). Loss of nephrin fails to form functional complexes in the slit membrane, resulting in dysfunction of the filtration barrier. In Finnish-type congenital nephrotic syndrome, massive proteinuria is caused by mutation of the nephrin gene (14). Similarly, disruption of the nephrin gene in mice results in abnormal foot process formation in podocytes and consequent nephrosis (21,22). Previous reports showed that the level of nephrin decreases in several proteinuric glomerular diseases including glomerulonephritis (9, 10, 15). However, information is limited regarding mechanisms underlying the downregulation of nephrin under pathological conditions.Cultured podocytes easily lose expression of nephrin in vitro. We previously reported (31, 38) that physiological ligands of nuclear receptors including all-trans-retinoic acid (ATRA) and 1,25-dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ] allow podocytes to maintain expression of nephrin in culture at both mRNA and protein levels. We also identified recently (19) that the nephrin gene promoter is activated by ATRA and 1,25(OH) 2 D 3 , which is mediated by the retinoic acid (RA) receptor (RAR) and the vitamin D receptor (VDR) but not the retinoid X receptor (RXR). On the other hand, activity of the nephrin gene promoter is downregulated by bystander macrophages as well as macrophage-derived cytokines including tumor necrosis factor-␣ (TNF-␣) and IL-1␤. This repression is reversible and obs...

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Transcription of nephrin-Neph3 gene pair is synergistically activated by WT1 and NF- B and silenced by DNA methylation

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5′-AMP-Activated Protein Kinase Attenuates Adriamycin-Induced Oxidative Podocyte Injury through Thioredoxin-Mediated Suppression of the Apoptosis Signal-Regulating Kinase 1–P38 Signaling Pathway

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