Targeting Mcl-1 and Bak As A Therapeutic Tool to Selectively Induce Apoptosis in Hepaptocellualr Carcinoma

In this study we seek to identify novel drug targets to induce apoptosis in hepatocellular carcinoma (HCC) cells thus providing opportunities to develop novel treatments to improve the prognosis of liver cancer patients. Several apoptotic pathways are mediated through cleavage of Bid (a BH3 domain-only, pro-apoptotic protein) to produce truncated Bid (tBid). tBid induces apoptosis through induction of outer mitochondrial membrane (OMM) permeabilization by activation of pro-apoptotic Bak that resides in the OMM or cytoplasmic Bax. Due to its localization, Bak can mediate the early phase of the response to tBid. We have recently demonstrated that OMM targeting of Bak and the sensitivity to tBid-induced OMM permeabilization is dependent on the expression of voltage-dependent anion channel isoform 2 (VDAC2). Here, we show that HCC induced by carcinogenic compounds, aflatoxin or DEN is highly sensitized to tBid-induced OMM permeabilization and cyto c release when compared to normal liver. Expression levels of VDAC2 and Bak were also higher in HCC than in control liver. The role of the VDAC2-Bak pathway in the differential tBid sensitivity was validated by overexpression of VDAC2 in primary hepatocytes. In HCC cells, elevation of Bak was also associated with an increased level of Mcl-1, an inhibitor of Bak. Furthermore, both constitutive and drug-induced genetic deletion of Mcl-1 in mouse embryonic fibroblasts caused sensitization to tBid-induced OMM permeabilization in the absence of a change in Bcl-xL or Bax. Based on these results, we are evaluating the possibility of selective killing of HCC cells by the combination of a Bak activator and an Mcl-1 inhibitor.