Eradication of breast cancer xenografts by hyperthermic suicide gene therapy under the control of the heat shock protein promoter

To investigate the usefulness of heat shock protein (HSP) promoter for brea st cancer gene therapy, hyperthermia and HSV thymidine kinase (tk) suicide gene combination therapy was examined with mouse mammary cancer cell line F M3A. HSP promoter activity was markedly increased after heat shock (41-45 d egreesC), with maximum activation (about 400-fold) at 3 hr. An in vitro cyt otoxic assay showed that HSP-tk-transduced FM3A cells became more sensitive (more than 50,000 times) to ganciclovir (GCV) with heat shock, but untreat ed cells showed no increased cytotoxic sensitivity to GCV compared with con trol FM3A cells. In addition to promoter-oriented selective cell killing, a "chemosensitization effect" as a bystander effect was demonstrated by hype rthermia and suicide gene combination therapy, using a non-heat-inducible p romoter. Immunohistochemical analysis revealed that this synergistic killin g effect was dependent on apoptotic cell death with upregulation of both Fa s and FasL (Fas ligand) expression. We also examined the efficacy of HSP-tk gene therapy in vivo by implanting breast cancer in subcutaneous and intra peritoneal models of BALB/c nude mice targeted by the HVJ-anionic liposome method. Significant tumor regression was observed in HSP-tk-transduced tumo rs followed by hyperthermia therapy, but no such inhibition was noted in ei ther the mock vector transfection or hyperthermia group compared with contr ol tumor-bearing mice. Our results demonstrate that this combination system is synergistically effective in mediating Fas-dependent apoptosis for a sp ecific gene therapy targeting HSP-expressing mammary carcinomas, even in ad vanced and heat-resistant breast cancer.