Blockade of an angiotensin type I receptor enhances effects of radiation on tumor growth and tumor-associated angiogenesis by reducing vascular endothelial growth factor expression

Objective: Angiogenesis, the formation of new capillary blood vessels, is essential for tumor progression. We had reported that Type 1 angiotensin receptor (AT1-R) antagonist reduced tumor-associated angiogenesis. Since antiangiogenic agents were reported to enhance efficacy of radiation therapy, we tested here whether or not AT1-R blockade facilitates the effects of radiation. Methods: 1 × 10⁶ LLC cells were injected into the subcutaneous tissue of male C57BL/6 mice, and when the average tumor volume reached around 0.1 cm³, radiation doses (3, 5, 10, and 15 Gy) were given on day 1. Results: The mean tumor volumes at day 22 were 6.39 (3 Gy), 6.15 (5 Gy), 5.15 (10 Gy), and 3.07 (15 Gy) cm³, respectively. Combination of 10 Gy radiation with AT1R antagonist TCV-116 (30 mg/kg) significantly inhibited tumor growth by 83% (1.47 ± 0.11 cm³, P < 0.01) in comparison with its inhibition of control tumors (8.81 ± 0.45 cm³). The same was true for mean vessel density, and the combination therapy markedly reduced tumor-associated angiogenesis. This was confirmed by the reduced expression of CD31. LLC tumor growth was blocked by neutralizing antibody against vascular endothelial growth factor (VEGF). Real-time PCR analysis of VEGF disclosed a marked reduction in the mice under combination therapy, compared with control mice. Conclusions: These results suggest that combination of radiation with AT1-R blockade markedly reduced the LLC growth rate, and that this was due to reduction of neovascularization by reducing VEGF levels. Combination therapy consisting of radiation and AT1R blockade may become an effective novel strategy for cancer treatment.