Prostate cancer targeting using replication-selective adenoviruses in combination with phytochemicals

Abstract

Oncolytic adenoviral mutants have demonstrated good safety profiles but, despite some encouraging clinical results, efficacy as a single agent was limited. Combinations with conventional chemo- or radiotherapy significantly enhanced the anti-tumour effect. We investigated the possibility of enhancing prostate cancer (PCa) cell killing using adenovirus type 5 (Ad5) with phytochemicals. Phytochemicals are chemopreventive and can modulate intracellular signalling pathways, including the mitogen-activated protein kinase (MAPK) pathway which regulates the expression of the coxsackie-adenovirus receptor (CAR). Equol and resveratrol synergistically enhanced cell death in both androgen receptor (AR)- positive and AR-negative PCa cell lines. On the other hand, curcumin, epigallocatechin-gallate (EGCG) and genistein had either synergistic and antagonistic responses with Ad5, depending on the dose and timing of addition. We therefore decided not to pursue the use of these compounds. Although we found that equol and resveratrol increased adenoviral receptor expression and viral uptake, this was not paralleled by enhanced viral replication. Treatment of DU145 and PC-3 cells with equol or resveratrol decreased viral titres, but did not block cell cycle progression. E1A expression in these cells was lower at 18h post-infection, but levels were normal by 72h. The exact mechanism behind the repression of adenovirus replication remains unclear. Equol and resveratrol induced moderate apoptotic responses. Mitochondrial membrane depolarization and caspase-3 activation were further increased by the addition of Ad5 in DU145 and PC-3 cells, but was reduced in 22Rv-1 cells. Caspase inhibition could not prevent sensitisation of 22Rv-1 cells to combination-induced cell death. The involvement of additional cell death mechanisms was therefore considered. Equol and resveratrol triggered autophagy while Ad5 acted as an autophagy repressor. Modulation of autophagy using pharmacological inducers and repressors indicates that autophagy may play a protective role in AR-negative cell lines.