Regulation of autophagy by Ubiquitin Specific Peptidase 1 (USP1)

Abstract

Ubiquitination is a post-translation modification of cellular proteins that occurs through a coordinated enzymatic cascade ending in the attachment of ubiquitin’s C-terminal glycine (Gly76) to an acceptor lysine residue, via an isopeptide bond. In opposition to ubiquitination lies deubiquitination, the hydrolysis of the isopeptide bond and subsequent release of ubiquitin from its substrate. This is achieved by a ~100-membered group of enzymes termed deubiquitinases, or DUBs. They are further divided into five families and they contribute to regulate levels, activity and localization of many cellular proteins and maintain the pool of free ubiquitin. The ubiquitin specific proteases (USP) family is the largest family of DUBs in humans and Ubiquitin specific peptidase1 (USP1) is one of the best-characterized. USP1 is the main player in the cellular response to DNA damage and it is deregulated in several types of human cancer, suggesting that this enzyme could represent a favourable therapeutic target for cancer treatment. Autophagy is a cellular mechanism for the degradation of cytoplasmic material, damaged organelles and protein aggregates in the lysosomes. Autophagy has a complex role in tumorigenesis and cancer treatment, since it has a role in both tumor prevention and in tumor survival and in resistance to treatment. The serine/threonine kinase ULK1, is one of the most upstream autophagy-related factors. It forms a complex with ATG13, FIP200 and ATG101, commonly considered as an initiator of the autophagic cascade. However, how the deubiquitinases may be involved in the regulation of autophagy is still unclear. In the present work, we show that ULK1, critical for autophagy initiation, is a target of USP1. To understand what is the biological effect exerted by USP1 upon ULK1, we depleted USP1 in several cell lines and followed the fate of endogenous ULK1. We transfected USP1 specific siRNA, or treated cells with pimozide, a chemical inhibitor of USP1, and we observed a sharp reduction of ULK1 in Triton X-100 soluble cellular lysate and its redist ribution in a 5M urea soluble fraction. Additionally, in USP1-depleted cells this fraction shows an enrichment of p62/SQSTM1 protein and HDAC6, an aggresome marker. Furthermore, by both immunofluorescence and co-immunoprecipitation assays, we confirmed that in USP1 depleted cells, ULK1 aggresomes contain p62/SQSTM1. Moreover, we observed that ULK1 redistribution did not affect ULK1 kinase activity. Notably, depletion of USP1 impairs the autophagic flux. In breast tumors, it is still a matter of debate whether autophagy suppresses or promotes tumor progression. In this study, we describe the existence of a collection of breast tumors co-expressing USP1 and LC3 proteins, suggesting that USP1-autophagy axis represents a promising tool for the treatment of tumors relying on this axis for survival