A novel targeting modality for renal cell carcinoma: Human osteocalcin promoter-mediated gene therapy synergistically induced by vitamin C and vitamin D₃

Background: Advanced renal cell carcinoma (RCC) frequently develops skeletal metastasis and is highly resistant to conventional therapies. We hypothesized that the osteocalcin (OC) promoter may be a promising gene delivery system for RCC targeted gene therapy because osteotropic tumors gain osteomimetic properties and thrive in the new environment by exhibiting a bone-like gene expression profile. Human OC (hOC) expression is highly regulated by vitamins and hormone. In the present study, we tested the feasibility of vitamin-regulatable hOC promoter for RCC-specific transcriptional targeting, and examined the anti-tumor effect of vitamins C and D₃ with hOC-based adenoviral vectors towards RCC. Methods: Real-time reverse transcriptase-polymerase chain reaction measured OC expression induced by vitamins C and D₃, either alone or in combination, in RCC and normal human renal epithelial cells (HRE). The RCC-cytotoxic effects of concomitant vitamins and hOC promoter-based adenoviral vectors, Ad-hOC-TK and Ad-hOC-E1, were evaluated in both cell culture and a xenograft murine model. Results: We found that high doses of vitamin C induced H₂O₂-dependent apoptosis in RCC but not HRE. Treatment of RCC cells with combined vitamins C and D₃ treatment significantly increased OC promoter activity compared to single reagent treatment. Combined vitamin therapy reduced tumor size (85%) and complete tumor regression occurred in 38% of mice co-administrated Ad-hOC-E1. Conclusions: The results obtained in the present study demonstrate that vitamins C and D₃ synergized with the anti-tumor effects of therapeutic genes driven by hOC promoter through direct cytotoxicity as well as transcriptional targeting. This combined gene therapy provides a promising modality for advanced RCC targeted therapy.