Regulation of the tumor suppressor PML influences HCC development and growth
Aims: Hepatocellular carcinoma (HCC) is characterised by a very poor prognosis and is associated with high mortality. This accounts for the molecular mechanisms of development of this tumor, which are herterogenous and only incompletely understood. We could previously show that the promyelocytic leukaemia (PML) protein, a nuclear phosphoprotein which has been shown to inhibit cell growth and transformation of tumor cells, plays a role in the regulation of apoptotic factors in hepatocellular carcinoma (HCC). To clarify the clinical implication of PML in HCC, the regulation and expression of PML was analysed in a large series of human HCCs.
Methods: Resected liver tissue of 90 patients undergoing partial liver resection or LTx because of HCC was analysed. Expression of PML was analysed by immunohistochemistry, qrt-PCR and western blot. Markers of proliferation and apoptosis were determined. The same was done in several hepatoma cell lines. In addition, hepatoma cell lines were investigated for PML expression before and after treatment with proteasome inhibitors.
Results: Expression of the PML protein was reduced or abolished in liver tumors compared to normal liver tissue. Level of down regulation was depending on the growth and correlated inversely with tumor grade. Both mRNA and DNA was present in HCC and hepatoma cells as well as in normal liver tissue, suggesting that the decreased protein expression was due to either a defect in translation or protein instability, rather than the consequence of decreased transcription or gene deletion. Double staining showed that PML expression was inversely correlated with the proliferation marker Ki67 and is positively correlated with levels of apoptotic cells in these tumors. Upon treatment of hepatoma cells with proteasome inhibitors MG132 and bortezomib, we receive a considerable increase of PML protein expression up to levels in normal liver tissue.
Conclusions: PML protein loss is an essential feature of liver carcinogenesis and HCC progression, what is rather due to a proteasome-dependent pathway than to transcriptional processes. PML protein expression is lost in advanced HCC which is due to an increased degradation of the protein.