NUMB facilitates autophagy initiation through targeting SCFβ-TrCP2 complex.

Given the critical role of SCF E3 ligases in autophagy by modulating the protein stability of various autophagic components, the activity of SCF should be tightly controlled to maintain the autophagic flux. We here showed that Numb, a multifunctional adaptor protein, increased the protein abundance of DEPTOR, which is an inhibitor of mTORC1, leading to increased autophagy flux. In vitro ubiquitination assay demonstrated that Numb inhibited SCFβ-TrCP2 mediated ubiquitination of DEPTOR. Mechanistically, Numb interrupted the interaction between β-TrCP2 and SKP1 by directly binding with SKP1. In the presence of wild type β-TrCP2, Numb overexpression inhibited DEPTOR degradation. Whereas, in the presence of the mutant β-TrCP2 which lacks the F-box domain, Numb overexpression did not affect the protein abundance of DEPTOR. In mouse model of renal fibrosis induced by unilateral ureteral obstruction, the expression of Numb was significantly increased. Consistently, the upregulation of Numb was observed in renal fibrotic lesions of chronic kidney disease patients. Specifically depleting Numb in proximal renal tubules decreased the protein abundance of DEPTOR, attenuated autophagy in fibrotic lesions and protected the kidney from development of renal fibrosis in vivo. Taken together, both in vitro and in vivo data indicated that Numb functions as a novel regulator to fine tuning the activity of SCFβ-TrCP2 in modulating autophagy.