Cellular gp96 upregulates AFP expression by blockade of NR5A2 SUMOylation and ubiquitination in HCC.

AFP is the most widely used biomarker for the diagnosis of hepatocellular carcinoma. However, a substantial proportion of HCC patients have either normal or marginally increased AFP levels in serum, and the underlying mechanisms are not fully understood. In the present study, we provided in vitro as well as in vivo evidence that heat shock protein gp96 promoted AFP expression at the transcriptional level in HCC. NR5A2 was identified as a key transcription factor regulated by AFP and its stability was enhanced by gp96. A further mechanistic study by CO-IP, GST-pull down and molecular docking showed the competitive binding of gp96 and SUMO E3 ligase RanBP2 to NR5A2 at the sites spanning from aa 507 to 539. The binding of gp96 inhibited SUMOylating, ubiquitination, and subsequent degradation of NR5A2. In addition, clinical analysis of HCC patients indicated that gp96 expression was positively correlated to serum AFP levels in tumors. Therefore, our study uncovered the novel regulatory mechanism of gp96 on the stability of its client proteins by directly affecting their SUMOylation and ubiquitination. These findings will help in designing more accurate AFP-based HCC diagnosis and progression monitoring approaches.