HCC development is supported by proteasomal degradation of the tumor suppressor PML

Abstract

Background: Hepatocellular carcinoma (HCC), which accounts for 80%-90% of primary liver tumors, is characterised by a very poor prognosis and is associated with high mortality. The molecular mechanisms responsible for development of this tumor are 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 expression of PML was analysed in a large series of human HCCs. Methods: 90 patients undergoing partial liver resection or LTx because of HCC were included. Liver tissue was macrodissected and analysed for expression of PML (immunohistochemistry, qrt-PCR, western blot), proliferation- and apoptosismarkers. 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 all tumors compared to normal liver tissue 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: Our results suggest that PML protein loss occurs in HCCs during carcinogenesis and progression, what is due to a proteasome-dependent pathway. PML protein expression is lost in advanced HCC which is due to an increased degradation of the protein. Thus, a decrease in PML expression may play an important role in HCC development.