Functional role of CBX2 in the pathogenesis of hepatocellular carcinoma

Abstract

Chromobox 2 (CBX2) is an essential component of the canonical Polycomb Repressive Complex 1 (cPRC1), which plays a role in epigenetic regulation and gene silencing. Earlier findings suggested that aberrant expression of CBX2 is associated with poor prognosis in hepatocellular carcinoma (HCC). However, the underlying mechanisms remain elusive. We confirmed that CBX2 is overexpressed in a subpopulation of HCC patients. While CBX2 expression is low in non-tumorous liver cell lines, it is overexpressed to varying levels in most HCC cell lines examined. CBX2-overexpressing and -knockout cell models were generated to determine the functions of CBX2 in HCC pathogenesis. We found that growth and migratory ability of HCC cells are directly correlated with CBX2 expression levels. Gene knockout of CBX2 also profoundly reduced HCC tumor growth in vivo. Transcriptomic and bioinformatics analyses further suggested that genes of the metabolic pathways are enriched in the CBX2-knockout cells. Concordantly, cell growth inhibition of CBX2 knockout cells were associated with reduction in rate of glycolysis and oxidative phosphorylation, with a concomitant reduction in ATP and NAD+ level. Bioinformatics analysis and functional screen led to the identification of phosphoenolpyruvate carboxykinase 2 (PCK2), a mitochondrial enzyme that catalyzes the conversion of oxaloacetate to phosphoenolpyruvate in the gluconeogenic pathway, was induced in CBX2 knockdown cells and responsible for the growth-inhibition found in these cells. Importantly, knockdown of PCK2 in CBX2-knockout cells restored the rate of glycolysis and NAD+ level, and completely rescued the cell growth defects due to CBX2 deficiency. Interestingly, overexpression of PCK1, the cytosolic from of PCK, did not have any effect on CBX2-knockout cells. Together these data suggested that PCK2 potentially regulates cell growth through yet-unknown activity. Our findings uncovered a novel functional role of CBX2-PCK2 regulatory axis in HCC growth, revealed a novel target for the development of novel HCC therapeutics.