Efficacy of HCC treatment with Proteasome Inhibitor Bortezomib is increased by Oncolytic Virotherapy mediated downregulation of Mcl-1 and Bip

Virotherapy using conditionally replicating adenoviral vectors provides a challenging tool for the tumor treatment. The proteasome inhibitor Bortezomib has been established as anti-cancer agent but potential interference with virotherapy if applied in combination is unkown. Therefore we investigated cytolytic efficacy of either single or combined treaments of hTERT-Ad virotherapy (MOI 25) and/or Bortezomib treatment (10 nM) in Huh-7 HCC cells. Bortezomib killed cells by ER-stress induced apoptosis as shown by Caspase-3 activation and increase of UPR-markers such as CHOP and BIP. Apoptosis was confirmed by downregulation of the antiapoptotic XIAP. After combined treatment adenoviral infection significantly increased Bortezomib-induced caspase-3 activity. Caspase-3 was activated in a synergistic manner as adenovirus alone did not result in caspase-3 activation. We could show that virus-induced proteasomal degradation of the antiapoptotic factor Mcl-1 could not be prevented by Bortezomib and synergistic caspase-activation could be rescued in transgenic Huh-7 cells expressing a degradation-resistant mutant of Mcl-1. Additionally, we could observe a virus-induced elimination of the protective chaperone Bip that could not be inhibited by Bortezomib. We found that Bortezomib neither influenced viral protein expression nor viral DNA replication, and only slightly reduced viral particle numbers. Increased treatment efficacy was confirmed by oncolysis assays in vitro and in vivo in s.c. Huh7-xenografts on nude mice. In summary, we could show that low dose Bortezomib does not significantly interfere with adenoviral replication but acts synergistically in inducing apoptosis in HCC virotherapy. Coapplication of proteasome inhibition by Bortezomib together with virotherapy could thus provide a promising approach HCC treatment.

Bortezomib Virotherapy Mcl-1