Nuclear localization of BRCA1-associated protein 1 is important in suppressing hepatocellular carcinoma metastasis via CTCF and NRF1/OGT axis

Germline mutations of the deubiquitinase BRCA1-associated protein 1 (BAP1) lead to the "BAP1 cancer syndrome" characterized by development of cancers. However, the role of BAP1 in hepatocellular carcinoma (HCC) is unclear. We found that BAP1 was upregulated at mRNA level in human HCCs and significantly correlated with a more aggressive tumour behaviour. Intriguingly, we observed cytoplasmic but no or minimal nuclear BAP1 in human HCC samples by immunohistochemistry. We observed that, while BAP1 protein was found mainly in the cytoplasm and less in the nuclei of HCC cell lines, BAP1 expression was predominantly nuclear in HepG2 cells, by cell fractionation and immunofluorescence analyses. Functionally, in the orthotopic liver injection mouse model, silencing the BAP1 predominant nuclear expression of HepG2 cells promoted intrahepatic tumor metastasis, with more frequent tumor microsatellite formation and venous invasion. With transcriptomic profiling, we identified RHOJ amongst the downregulated targets in HepG2 cells upon BAP1 knockdown. Subsequent overexpression of RHOJ suppressed cell migration in HCC cells, suggesting that BAP1 might upregulate RHOJ resulting in reduced cell migratory ability of HCC cells. Furthermore, we identified two transcription factors, CTCF and NRF1, which activated BAP1 transcription by binding to BAP1 promoter region. On the other hand, we uncovered that O-linked N-acetylglucosamine (GlcNAc) transferase (OGT) physically bound to BAP1 in the nucleus, resulting in diminished stability of the nuclear BAP1. Intriguingly, OGT transcription was upregulated and was also under the control of CTCF and NRF1 in human HCC, acting as a negative regulator of BAP1. To summarize, this study uncovered the underlying mechanisms of the regulation of BAP1 and that loss of the nuclear localization of BAP1 protein contributed to enhanced cell migration in vitro and more aggressive tumor behavior in human HCCs.