Telomerase-Dependent Virotherapy Overcomes Resistance of Hepatocellular Carcinomas against Chemotherapy and Tumor Necrosis Factor–Related Apoptosis-Inducing Ligand by Elimination of Mcl-1

Abstract

Hepatocellular carcinomas (HCC) are drug-resistant tumors that frequently possess high telomerase activity. It was therefore the aim of our study to investigate the potential of telomerase-dependent virotherapy in multimodal treatment of HCC. In contrast to normal liver, HCC xenografts showed high telomerase activity, resulting in tumor-restricted expression of E1A by a telomerase-dependent replicating adenovirus (hTERT-Ad). Neither tumor necrosis factor-related apoptosisinducing ligand (TRAIL) or chemotherapy alone nor the combined treatment with both agents resulted in significant destruction of HCC cells. Application of hTERT-Ad at low titers was also not capable to destroy HCC cells, but telomerase-dependent virotherapy overcame the resistance of HCC against TRAIL and chemotherapy. The synergistic effects are explained by a strong down-regulation of Mcl-1 expression through hTERT-Ad that sensitizes HCC for TRAILand chemotherapy-mediated apoptosis. To investigate whether down-regulation of Mcl-1 alone is sufficient to explain synergistic effects observed with virotherapy, Mcl-1 expression was inhibited by RNA interference. Treatment with Mcl-1-siRNA significantly enhanced caspase-3 activity after chemotherapy and TRAIL application, confirming that elimination of Mcl-1 is responsible for the drug sensitization by hTERT-Ad. Consistent with these results, heterologous overexpression of Mcl-1 significantly reduced the sensitization of hTERT-Ad transduced cells against apoptosisinducing agents. Chemotherapy did not interfere with quantitative hTERT-Ad production in HCC cells. Whereas hTERT-Ad virotherapy alone was only capable to inhibit the growth of Hep3B xenografts, virochemotherapy resulted in vast destruction of the drug-resistant HCC. In conclusion our data indicate that telomerase-dependent virotherapy is an attractive strategy to overcome the natural resistance of HCC against anticancer drugs by elimination of Mcl-1. (Cancer Res 2005; 65(16): 7393-402)

Endogenous Bak inhibitors Mcl-1 and Bcl-xL: differential impact on TRAIL resistance in Bax-deficient carcinoma

“…Evidence for a differential role of Bak versus Bax in cell death regulation also comes from analyses on BH3-only proteins or bacterial toxins (Cartron et al, 2003b;Gillissen et al, 2003Gillissen et al, , 2007Lindenboim et al, 2005;Pardo et al, 2006). Interestingly, recent studies have shown involvement of Mcl-1, an endogenous Bak inhibitor, in the regulation of TRAIL-induced apoptosis (Henson et al, 2003;Taniai et al, 2004;Wirth et al, 2005;Han et al, 2006;Meng et al, 2007). Similar data are available for Bcl-x L , the second major endogenous inhibitor of Bak .…”

Functional characterization of the Bcl-2 gene family in the zebrafish

“…In zebrafish embryos, knockdown of zMcl1a and zMcl-1b substantially increased apoptosis triggered by ectopic expression of the Apo2L/TRAIL homolog zDL1b. Previous work suggests that in human hepatocellular carcinoma-derived cells, 32 human cholangiocarcinoma cells, 31 and NIH3T3 mouse fibroblasts, 30 Mcl-1 is a critical modulator of sensitivity to apoptosis signaling by Apo2L/TRAIL. Hence, endogenous Mcl-1 may be particularly important among prosurvival Bcl-2 family members in controlling apoptosis induction by stimuli that engage both the extrinsic and intrinsic apoptosis pathways.…”

“…[30][31][32] Thus, Mcl-1 may be a key modulator of DLinduced apoptosis in cells that require engagement of both the extrinsic and intrinsic pathways for commitment to apoptosis. Furthermore, these results indicate that endogenous zMcl-1a and zMcl-1b play an important role in curtailing apoptosis induction through the cell-intrinsic pathway in early zebrafish embryos.…”

VSV virotherapy improves chemotherapy by triggering apoptosis due to proteasomal degradation of Mcl-1

“…19,20 Recently, it has been reported that the E1A protein of human Adenovirus type 5 is capable of downregulating protein levels of Mcl-1 21 followed by induction of apoptosis. Additionally, we showed that this decrease can be therapeutically exploited by combined virochemotherapy, 22 indicating that virus-induced Mcl-1 degradation is a promising tool to support conventional chemotherapy.…”