VSV virotherapy of tumors enhances chemotherapeutic efficacy by proteasomal degradation of MCL-1

Over-expression of myeloid cell leukemia 1 protein (Mcl-1), an anti-apoptotic Bcl-2 family member, contributes to chemotherapy resistance of tumors. The short half-life of Mcl-1 makes it an interesting target for therapeutic agents that negatively interfere with cellular protein biosynthesis such as oncolytic viruses (Wirth et al. 2005). Vesicular stomatitis virus (VSV) has been established as oncolytic virus that efficiently disrupts de novo protein biosynthesis of infected cells on different levels (RNA-polymerases I-III, nuclear mRNA export, CAP-dependent translation). Here, we demonstrate that following VSV infection, Mcl-1 protein levels rapidly declined in the hepatoma cell line Huh-7 and HCC cell line HepG2 whereas expression of other members of the Bcl-2 family remained unchanged. Mcl-1 elimination was a consequence of inhibited protein biosynthesis and proteasomal degradation since over-expression of a degradation-resistant Mcl-1 mutant restored Mcl-1 levels. Supporting the assumption that rapid Mcl-1 degradation might be based on the short half-life and the suppression of de novo synthesis of Mcl-1 protein, a VSV mutant (VSV-AV1) attenuated in blocking host protein biosynthesis was incapable to decrease protein levels of Mcl-1 in vitro (Tumilasci et al. 2008). Regarding cell death, Mcl-1 rescue inhibited apoptosis and thereby confirmed that Mcl-1 down-regulation contributes to VSV-induced apoptosis. In vitro, VSV virotherapy in combination with chemotherapy revealed an enhanced therapeutic effect compared to the single treatments which could be reverted by Mcl-1 rescue or RNAi-mediated knockdown of pro-apoptotic Bax and Bak proteins. Finally, in a tumor mouse model combinations of Doxorubicin and VSV showed a superior therapeutic efficacy compared to VSV or Doxorubicin alone. In summary, our data indicate that VSV virotherapy is an attractive strategy to overcome tumor resistance against conventional chemotherapy by elimination of Mcl-1.

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Mcl-1 - VSV - combination therapy - virochemotherapy