Tubular Numb Promotes Renal Interstitial Fibrosis Via Modulating Hif-1 Alpha Protein Stability

Tubulointerstitial fibrosis is the ultimate common pathway of all manners of chronic kidney disease. We previously demonstrated that specific deletion of Numb in proximal tubular cells (PTCs) prevented G2/M arrest and attenuated renal fibrosis. However, how Numb modulates cell cycle arrest remains unclear. Here, we showed that Numb overexpression significantly increased the protein level of hypoxia-inducible factor-1 alpha (HIF-1 alpha). Numb overexpression-induced G2/M arrest was blocked by silencing endogenous HIF-1 alpha, subsequently downregulated the expression of cyclin G1 which is an atypical cyclin to promote G2/M arrest of PTCs. Further analysis revealed that Numb-augmented HIF-1 alpha protein was blocked by simultaneously overexpressing MDM2. Moreover, silencing Numb decreased TGF-beta 1-induceded HIF-1 alpha protein expression. While endogenous MDM2 was knocked down this reduction was reversed, indicating that Numb stabilized HIF-1 alpha protein via interfering MDM2mediated HIF-1 alpha protein degradation. Interestingly, HIF-1 alpha overexpression significantly upregulated the expression of Numb and silencing endogenous HIF-1 alpha blocked CoCl2 or TGF-beta 1-induced Numb expression. Chromatin immunoprecipitation (ChIP) assays demonstrated that HIF-1 alpha binded to the promoter region of Numb. This binding was significantly increased by TGF-beta 1. Collectively, these data indicate that Numb and HIF-1 alpha cooperates to promote G2/M arrest of PTCs, and thus aggravates tubulointerstitial fibrosis. Numb might be a potential target for the therapy of tubulointerstitial fibrosis.