Lixin Mi

Isothiocyanates are versatile cancer-preventive compounds. Evidence from animal studies indicates that the anticarcinogenic activities of ITCs involve all the major stages of tumor growth: initiation, promotion and progression. Epidemiological studies have also shown that dietary intake of ITCs is associated with reduced risk of certain human cancers. A number of mechanisms have been proposed for the chemopreventive activities of ITCs. To identify the molecular targets of ITCs is a first step to understand the molecular mechanisms of ITCs. Studies in recent years have shown that the covalent binding to certain protein targets by ITCs seems to play an important role in ITC-induced apoptosis and cell growth inhibition and other cellular effects. The knowledge gained from these studies may be used to guide future design and screen of better and more efficacious compounds. In this review, we intend to cover all potential protein targets of ITCs so far studied and summarize what are known about their binding sites and the potential biological consequences. In the end, we also offer discussions to shed light onto the relationship between protein binding and reactive oxygen species generation by ITCs.

Isothiocyanates (ITCs), including benzyl isothiocyanate (BITC), phenethyl isothiocyanate (PEITC) and sulforaphane, compounds found in cruciferous vegetable, are highly effective in inducing cell cycle arrest and apoptosis in a variety of cancer cells and animal models. Although some studies indicate that ITC-induced reactive oxygen species (ROS) generation may underlie apoptosis induction, our recent studies show that covalent binding to target proteins may be an important event triggering apoptosis. In this study, we report that BITC and PEITC significantly inhibit proteasome activity in a variety of cell types. Further studies show that ITCs inhibit both the 26S and 20S proteasomes, presumably through direct binding, and that this inhibition is unrelated to either ROS generation or ITC-induced protein aggregation. The potency of ITC-induced proteasome inhibition correlates with the rapid accumulation of p53 (tumor suppressor) and IκB nuclear factor-kappaB (nuclear factor-kappaB inhibitor). Finally, our results demonstrate that BITC and PEITC, the two strongest proteasome inhibitors, significantly suppress growth of multiple myeloma (MM) cells through induction of cell cycle arrest at G ₂ /M phase and apoptosis. This study suggests that proteasome, like tubulin, is a potential molecular target of ITCs, thus providing a novel mechanism by which ITCs strongly inhibit growth of MM cells and new leads in identifying compounds with therapeutic and preventative efficacies for MM. It also supports the future studies of ITCs as therapeutic and preventive agents for MM.