Sulforaphane, a naturally occurring isothiocyanate, induces cell cycle arrest and apoptosis in HT29 human colon cancer cells

Sulforaphane is an isothiocyanate that is present naturally in widely consu med vegetables and has a particularly high concentration in broccoli. This compound has been shown to block the formation of tumors initiated by chemi cals in the rat. Although sulforaphane has been proposed to modulate the me tabolism of carcinogens, its mechanism of action remains poorly understood. We have previously demonstrated that sulforaphane inhibits the reinitiatio n of growth and decreases the cellular viability of quiescent human colon c arcinoma cells (HT29). Moreover, the weak effect observed on differentiated CaCo2 cells suggests a specific anticancer activity for this compound. Here we investigated the effect of sulforaphane on the growth and viability of HT29 cells during their exponentially growing phase. We observed that s ulforaphane induced a cell cycle arrest in a dose-dependent manner, followe d by cell death. This sulforaphane-induced cell cycle arrest was correlated with an increased expression of cyclins A and B1. Moreover, we clearly dem onstrated that sulforaphane induced cell death via an apoptotic process. In deed, a targe proportion of treated cells display the following: (a) transl ocation of phosphatidylserine from the inner layer to the outer layer of th e plasma membrane; (b) typical chromatin condensation; and (c) ultrastructu ral modifications related to apoptotic cell death. We also showed that the expression of p53 was not changed in sulforaphane-treated cells. In contras t, whereas bcl-2 was not detected, we observed increased expression of the proapoptotic protein bar, the release of cytochrome c from the mitochondria to the cytosol, and the proteolytic cleavage of poly(ADP-ribose) polymeras e. In conclusion, our results strongly suggest that in addition to the acti vation of detoxifying enzymes, induction of apoptosis is also involved in t he sulforaphane-associated chemoprevention of cancer.