Isothiocyanates (ITCs) found in cruciferous vegetables, including benzyl-ITC (BITC), phenethyl-ITC (PEITC), and sulforaphane (SFN), inhibit carcinogenesis in animal models and induce apoptosis and cell cycle arrest in various cell types. The biochemical mechanisms of cell growth inhibition by ITCs are not fully understood. Our recent study showed that ITC binding to intracellular proteins may be an important initiating event for the induction of apoptosis. However, the specific protein target(s) and molecular mechanisms were not identified. In this study, two-dimensional gel electrophoresis of human lung cancer A549 cells treated with radiolabeled PEITC and SFN revealed that tubulin may be a major in vivo binding target for ITC. We examined whether binding to tubulin by ITCs could lead to cell growth arrest. The proliferation of A549 cells was significantly reduced by ITCs, with relative activities of BITC > PEITC > SFN. All three ITCs also induced mitotic arrest and apoptosis with the same order of activity. We found that ITCs disrupted microtubule polymerization in vitro and in vivo with the same order of potency. Mass spectrometry demonstrated that cysteines in tubulin were covalently modified by ITCs. Ellman assay results indicated that the modification levels follow the same order, BITC > PEITC > SFN. Together, these results support the notion that tubulin is a target of ITCs and that ITC-tubulin interaction can lead to downstream growth inhibition. This is the first study directly linking tubulin-ITC adduct formation to cell growth inhibition.

Although it is conceivable that cancer preventive isothiocyanates (ITCs), a family of compounds in cruciferous vegetables, induce cell cycle arrest and apoptosis through a mechanism involving oxidative stress, our study shows that binding to cellular proteins correlates with their potencies of apoptosis induction. More recently, we showed that ITCs bind selectively to tubulins. The differential binding affinities toward tubulin among benzyl isothiocyanate, phenethyl isothiocyanate, and sulforaphane correlate well with their potencies of inducing tubulin conformation changes, microtubule depolymerization, and eventual cell cycle arrest and apoptosis in human lung cancer A549 cells. These results support that tubulin binding by ITCs is an early event for cell growth inhibition. Here we demonstrate that ITCs can selectively induce degradation of both alpha- and beta-tubulins in a variety of human cancer cell lines in a dose- and time-dependent manner. The onset of degradation, a rapid and irreversible process, is initiated by tubulin aggregation, and the degradation is proteasome-dependent. Results indicate that the degradation is triggered by ITC binding to tubulin and is irrelevant to oxidative stress. This is the first report that tubulin, a stable and abundant cytoskeleton protein required for cell cycle progression, can be selectively degraded by a small molecule.

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.

Mutations in the p53 tumor-suppressor gene are prevalent in human cancers. The majority of p53 mutations are missense, which can be classified into contact mutations (that directly disrupts the DNA-binding activity of p53) and structural mutations (that disrupts the conformation of p53). Both of the mutations can disable the normal wild-type (WT) p53 activities. Nevertheless, it has been amply documented that small molecules can rescue activity from mutant p53 by restoring WT tumor-suppressive functions. These compounds hold promise for cancer therapy and have now entered clinical trials. In this study, we show that cruciferous-vegetable-derived phenethyl isothiocyanate (PEITC) can reactivate p53 mutant under in vitro and in vivo conditions, revealing a new mechanism of action for a dietary-related compound. PEITC exhibits growth-inhibitory activity in cells expressing p53 mutants with preferential activity toward p53^R175, one of the most frequent ‘hotspot’ mutations within the p53 sequence. Mechanistic studies revealed that PEITC induces apoptosis in a p53^R175 mutant-dependent manner by restoring p53 WT conformation and transactivation functions. Accordingly, in PEITC-treated cells the reactivated p53^R175 mutant induces apoptosis by activating canonical WT p53 targets, inducing a delay in S and G2/M phase, and by phosphorylating ATM/CHK2. Interestingly, the growth-inhibitory effects of PEITC depend on the redox state of the cell. Further, PEITC treatments render the p53^R175 mutant sensitive to degradation by the proteasome and autophagy in a concentration-dependent manner. PEITC-induced reactivation of p53^R175 and its subsequent sensitivity to the degradation pathways likely contribute to its anticancer activities. We further show that dietary supplementation of PEITC is able to reactivate WT activity in vivo as well, inhibiting tumor growth in xenograft mouse model. These findings provide the first example of mutant p53 reactivation by a dietary compound and have important implications for cancer prevention and therapy.

Hepatoma Research is an open access journal and focuses on all topics related to hepatoma. The following articles are especially welcome: pathogenesis, clinical examination and early diagnosis of hepatoma, complications of hepatoma, and their preventions and treatments, etc.