Chemoprevention by isothiocyanates: molecular basis of apoptosis induction

An important and promising group of compounds that have a cancer-chemopreventive property are organosulfur compounds, such as isothiocyanates (ITCs). Various ITCs are effective chemoprotective agents against chemical carcinogenesis in experimental animals. Several epidemiological studies also indicated that the dietary consumption of ITCs or ITC-containing foods inversely correlates with the risk of developing lung, breast, and colon cancers, providing evidence that they have a potential to prevent cancer in humans. Mechanistically, ITCs are capable of inhibiting both the formation and development of a cancer cell through multiple pathways; i.e. the inhibition of carcinogen-activating cytochrome P450 mono-oxygenases, induction of carcinogen-detoxifying phase 2 enzymes, induction of apoptosis, and inhibition of cell cycle progression. We have clarified the molecular mechanisms underlying the relationship between cell cycle regulation and apoptosis induced by benzyl ITC (BITC), a major ITC compound isolated from papaya (Carica papaya) fruit. We identified phosphorylated Bcl-2 as a key molecule linking p38 MAPK-dependent cell cycle regulation with the c-Jun N-terminal kinase activation by BITC. We also established that BITC exerts the cytotoxic effect more preferentially in the proliferating cells than in the quiescent cells. Furthermore, p53 was found to be a potential negative regulator of apoptosis induction by BITC in normal epithelial cells through inhibition of cell cycle progression at the G(0)/G(1) phase. In contrast, treatment with an excessive concentration of BITC resulted in necrotic cell death in an ATP-dependent manner. This review addresses the biological impact of cell death induction by BITC as well as other ITCs and the involved molecules regulating signal pathways.