Deubiquitinating enzyme USP11 promotes renal tubular cell senescence and fibrosis via inhibiting the ubiquitin degradation of TGF-β receptor II

Transforming growth factor-β1 (TGF-β1) is regarded as a key factor in promoting renal fibrosis during chronic kidney disease (CKD). Signaling transduction of TGF-β1 starts with binding to TGF-β type II receptor (Tgfbr2), a constitutively activated kinase that phosphorylates TGF-β type I receptor (Tgfbr1), and then activates downstream Smad2/3 or noncanonical pathways. Previous studies show that cellular senescence is associated with the progression of CKD, and accelerated tubular cell senescence is implicated in promoting renal fibrosis. In the present study we investigated the renal parenchymal cell senescence in fibrosis from the sight of posttranslational regulation and focused on Tgfbr2, the important gatekeeper for TGF-β1 downstream signaling. In mice with unilateral ureteral obstruction (UUO) and folic acid (FA)-induced fibrotic kidneys, we found that Tgfbr2 was markedly elevated without obvious change in its mRNA levels. As an important member of deubiquitinating enzymes, ubiquitin-specific protease 11 (Usp11) was also significantly increased in fibrotic kidneys, and co-distributed with Tgfbr2 in tubular epithelial cells. Pretreatment with Usp11 inhibitor mitoxantrone (MTX, 30 mg · kg⁻¹ · d⁻¹, i.p.) twice a week, for 2 weeks significantly attenuated the elevation of Tgfbr2, activation in downstream senescence-related signaling pathway, as well as renal senescence and fibrosis. In cultured mouse tubular epithelial cells (MTECs), treatment with angiotensin II (Ang-II, 10⁻⁷, 10⁻⁶ M) dose-dependently elevated both Tgfbr2 and Usp11 levels. Inhibition or knockdown on Usp11 attenuated Ang-II-induced elevation in Tgfbr2 level, and attenuated the activation of downstream senescent-related signaling pathway and as well as cell senescence. We conducted Co-IP experiments, which revealed that Usp11 was able to interact with Tgfbr2, and inhibition of Usp11 increased the ubiquitination of Tgfbr2. Taken together, these results demonstrate that the elevation of Usp11 under pathological condition is implicated in promoting renal fibrosis. Usp11 promotes the development of renal fibrosis by deubiquitinating Tgfbr2, reducing Tgfbr2 ubiquitination degradation, and then facilitating the activation of downstream senescent signaling pathway.