Transcriptional regulation of ubiquitin specific peptidase 2b by farnesoid X receptor and its implications in the pathogenesis of hepatocellular carcinoma.

Farnesoid X receptor (FXR) signaling plays an important role in liver regeneration and carcinogenesis. Dysregulation of FXR signaling is linked to the pathogenesis of hepatocellular carcinoma (HCC) while FXR knockout mice spontaneously develop HCC as they age. Ubiquitin specific peptidase 2b (USP2b) is a deubiquitinating enzyme regulating protein stability and other activities. In our recent study, we found that USP2b was significantly downregulated in subjects with HCC and exhibited both tumor-promoting and tumor suppressive activity in a context-dependent manner. However, the mechanistic link between FXR signaling and USP2b and its implications in the pathogenesis of HCC remain to be determined. In this study, we revealed that USP2b was transcriptionally regulated by FXR. Activation of FXR significantly increased while antagonizing FXR reduced USP2b mRNA and protein expression in vitro and in vivo. Consistently, USP2b expression was significantly reduced in FXR knockout (FXR-KO) mice. Further investigation showed that USP2b was transcriptionally regulated by FXR in an isoform-specific manner, predominantly by FXRα2 but not FXRα1. The transcription starting sites (TSS) of USP2 isoforms were determined by 5'RACE, which leads to the identification of the USP2b-specific promoter. An FXR response element (FXRE) was identified in the USP2b promoter and functionally characterized. FXR directly bound to the FXRE in vitro and recruited to the USP2b promoter in intact cells and in vivo. Further study revealed that upon FXR activation, co-activator-associated arginine methyltransferase 1 (CARM-1) was significantly recruited to the human USP2b promoter while coactivators glutamate receptor-interacting protein 1 (GRIP-1) and peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) were significantly recruited to the mouse USP2b promoter. Over-expression of USP2b in FXR-KO mice significantly slowed down tumor growth with reduced tumor size while maintained comparable tumor numbers consistent with its tumor suppressive activity. In summary, this study demonstrated that USP2b was transcriptionally regulated by FXR in an isoform-dependent manner and USP2b played a tumor-suppressive role in the pathogenesis of FXR deficiency-induced HCC.