Munc18-1-interacting protein 3 mitigates renal fibrosis through protection of tubular epithelial cells from apoptosis

Background: Tubulointerstitial fibrosis is a hallmark of chronic kidney disease (CKD), and is initiated by tubular epithelial cell (TEC) injury. Hypoxia promotes tubular cell death, fibrosis and CKD progression. Munc18-1-interacting protein 3 (Mint3) is a molecule that activates hypoxia-inducible factors (HIFs) by binding and suppressing factor inhibiting HIF-1 (FIH). However, the role of Mint3 in tubulointerstitial fibrosis remains unknown. Methods: We induced fibrosis of the kidney after unilateral ischemia-reperfusion injury (uIRI) in Mint3-knockout and littermate wild-type mice. The duration of ischemia was 23 min and the kidneys were harvested at 24 h and 7 days after ischemia-reperfusion. The function of Mint3 was further investigated by using mouse cortical tubular (MCT) cells, which were treated with Mint3 and/or FIH small interfering RNA and exposed to normoxia or hypoxia. Results: Knockout of Mint3 did not affect the acute injury induced by uIRI, but exacerbated the tubulointerstitial fibrosis, accompanied by an increase in TEC apoptosis. Consistently, hypoxia-induced apoptosis of MCT cells was aggravated by Mint3 knockdown. Unexpectedly, the additional knockdown of FIH did not suppress the increase in apoptosis by Mint3 knockdown, demonstrating the irrelevance of the FIH/HIF pathway. Therefore, we next focused on nuclear factor (NF)-kappa B, which has an anti-apoptotic role. Indeed, not only the expression of the inhibitory NF-kappa B p50 but also the DNA-binding activity of p50/p50 homodimer was increased by knockdown of Mint3 in the TECs, along with the decreased expressions of the NF-kappa B-targeted anti-apoptotic genes. An increase in NF-kappa B p50 was also confirmed in Mint3-knockout kidneys. Conclusions: Mint3 in epithelial cells protects the cells from apoptosis by up-regulating anti-apoptotic effects of NF-kappa B, leading to fibrosis suppression. This new pathophysiology of tubulointerstitial fibrosis could be a target of future therapy for CKD.