Department of Molecular Signaling, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Chuo, Yamanashi, Japan.
Am J Physiol Renal Physiol. 2010 Jun;298(6):F1436-44. doi: 10.1152/ajprenal.00512.2009. Epub 2010 Mar 17.
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 D(3), 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-alpha. In the present investigation, we identified that TNF-alpha selectively represses activity of RAR but not VDR. To elucidate mechanisms underlying this observation, we tested involvement of downstream targets for TNF-alpha: nuclear factor-kappaB (NF-kappaB), mitogen-activated protein (MAP) kinases, phosphatidylinositol 3-kinase (PI3K)-Akt, and cAMP-protein kinase A (PKA). TNF-alpha caused activation of NF-kappaB, MAP kinases, and PI3K-Akt in podocytes, whereas blockade of these molecules did not affect inhibition of RAR by TNF-alpha. In contrast, TNF-alpha 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-alpha 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-alpha is caused, at least in part, through selective inhibition of this pathway.
足细胞裂孔隔膜的重要组成部分 Nephrin 在包括肾小球肾炎在内的蛋白尿性肾小球疾病中表达下调。我们之前报道过：1）视黄酸（RA）和 1,25-二羟维生素 D（3）可增强培养的足细胞中 Nephrin 的表达；2）这些作用是由视黄酸受体（RAR）和维生素 D 受体（VDR）介导的；3）TNF-α 下调 Nephrin 的基础和诱导表达。在本研究中，我们发现 TNF-α 选择性抑制 RAR 的活性而不影响 VDR。为了阐明这一观察结果的机制，我们检测了 TNF-α的下游靶点：核因子-κB（NF-κB）、丝裂原激活蛋白（MAP）激酶、磷脂酰肌醇 3-激酶（PI3K）-Akt 和环磷酸腺苷-蛋白激酶 A（PKA）的参与情况。TNF-α可引起足细胞中 NF-κB、MAP 激酶和 PI3K-Akt 的激活，而阻断这些分子并不影响 TNF-α对 RAR 的抑制作用。相反，TNF-α 抑制 cAMP-PKA 的活性，而 PKA 的阻断则抑制 RAR 的基础和 RA 诱导的激活。此外，cAMP 可显著上调 RAR 的活性，而 TNF-α 对 RAR 的抑制作用可被 cAMP 升高剂逆转。这些结果表明：1）足细胞中 Nephrin 的表达受 cAMP-RAR 通路的调节；2）TNF-α 对 Nephrin 的抑制作用至少部分是通过选择性抑制该通路引起的。
