The ubiquitin-specific protease 8 (USP8) is a prototypic multidomain deubiquitinating enzyme with pleiotropic functions. We investigated the role of USP8 in hepatocellular carcinoma (HCC) by analyzing expression patterns of USP8 in HCC patients, and evaluating its functions and underlying signaling. Among 20 HCC patients investigated, we found that USP8 protein upregulation was a common phenomenon (17 out of 20) in HCC compared to normal liver tissue. Furthermore, the upregulation of USP8 was not associated with any clinicopathology. USP8 inhibition via genetic and pharmacological approaches resulted in growth inhibition and apoptosis induction in both sensitive and doxorubicin-resistant HCC cells. Of note, USP8 inhibition significantly enhanced doxorubicin or sorafenib's efficacy in HCC cells and mouse models. We further found that USP8 inhibition decreased levels of multiple receptor tyrosine kinases (RTKs) by ~90%, such as epidermal growth factor receptor (EGFR) and c-Met. Consistently, the downstream signaling regulated by RTKs was disrupted in HCC cells after USP8 inhibition, as shown by the decreased p-Akt, p-STAT3 and p-Raf. Our findings demonstrate that USP8 is a novel therapeutic target in HCC. Inhibiting USP8 has potential to overcome current drug resistance, particularly on HCC patients with high USP8 expression.
Keywords: HCC; USP8; chemoresistance; receptor tyrosine kinase.