A diet of an isothiocyanate metabolite from cruciferous vegetables inhibits growth of human prostate cancer cell xenografts by apoptosis and cell cycle arrest

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Epidemiological surveys indicate that intake of diets rich in cruciferous vegetables such as broccoli, cabbage, and watercress, etc. is inversely related to prostate cancer incidence, although the responsible dietary factors have not been clearly identified. When the vegetables are cut or masticated, the enzyme myrosinase is released to hydrolyze glucosinolates to several compounds including isothiocyanates. We have described that exposure of human prostate cancer cells in culture to the N-acetylcysteine (NAC) conjugate of phenethyl isothiocyanate (PEITC-NAC), the major metabolite of PEITC that is rich in watercress, inhibited proliferation and tumorigenesis. The PEITC-NAC is known to mediate cytoprotection at initiation of carcinogenesis. To further evaluate the efficacy of PEITC-NAC to regulate the growth of tumor cells, the effects of PEITC-NAC in the diet have been investigated in immunodeficient mice with xenografted tumors of human prostate cancer PC-3 cells. The daily PEITC-NAC (8 μmol/g) supplemented diet group showed a significant reduction in tumor size in 100% of the mice during nine-week treatment period. Tumor weight at autopsy was reduced by 50% compared with mice on diet without PEITC-NAC (P=0.05). Mitosis and in vivo BrdU labeled proliferating cells were reduced in these tumors. The PEITC-NAC diet up-regulated the inhibitors of cyclin-dependent kinases p21^WAF-1/Cip-1 and p27^Kip1, and reduced the expression of cyclins D and E, indicating they were potential molecular targets. As a result, phosphorylated Rb was significantly decreased and the G₁- to S-phase transition retarded. The treated tumors also showed a significant increase in apoptosis as determined by in situ end-labeling, and by PARP cleavage. Inhibition of the xenografts from rapidly growing human prostate cancer cells by PEITC-NAC diet is significant, especially since it is the first time such effects are reported in vivo using a conjugate of an isothiocyanate. The results strongly suggest that the chemopreventive effects of isothiocyanates could occur at two levels. One is cytoprotection at the level of initiation of carcinogenesis, and the other is growth suppression at postinitiation of carcinogenesis. The effects of growth suppression could block the clonogenesis and the growth of emerging prostate cancer cells, thus delaying the onset of clinically significant prostate tumors. The hypothesis for the postinitiation effects is corroborated by the recent epidemiological studies. They concluded that the intake of cruciferous vegetables was inversely related to especially those advanced cases of prostate cancer, indicating that the development of prostate cancer beyoind initiation could be affected by the dietary factors from the vegetables.