Deubiquitinating enzyme USP44 suppresses hepatocellular carcinoma progression through inhibiting Hedgehog signaling and PDL1 expression

Abstract Hepatocellular carcinoma (HCC) is one of the deadliest malignancies in the world. Research into the key genes that maintain the malignant behavior of cancer cells is crucial for the treatment of HCC. Here, we identify ubiquitin‐specific peptidase 44 (USP44), a member of deubiquitinase family, as a novel regulator of HCC progression. The tumor suppressive function of USP44 was evaluated in a series of in vitro and in vivo experiments including using USP44 flox/flox mice and orthotopic xenograft models.Through Quantitative Proteomics examination, we demonstrated that USP44 inhibits HCC PDL1 expression through downregulating Hedgehog (Hh) signaling pathway. Mechanistically, we found USP44 directly interacts with Itch, an E3 ligase involved in Hh signaling, and promotes the deubiquitination and stabilization of Itch.These events result in the proteasomal degradationof Gli1 and subsequent inactivation of Hh signaling, which ultimately suppresses the PDL1 expression and progression of HCC. Furthermore, HCC tissue microarray was analyzed by immunohistochemistry to evaluate the pathological relevance of the USP44/Itch/Gli1/PDL1 axis. At last, Gli1 inhibitor GANT61 was found to act in synergy with anti-PDL1 therapy. Overall, USP44 can act as a suppressive gene in HCC by modulating Hh signaling and co-inhibition of Gli1 and PDL1 might be an effective novel combination strategy for treating HCC patients.