Many test-tube and animal studies have found sulforaphane to be particularly helpful for suppressing cancer development by inhibiting enzymes that are involved in cancer and tumor growth (27, 28, 29).
Sulforaphane appears to be most protective against colon and prostate cancer but has also been studied for its effects on many other cancers, such as breast, leukemia, pancreatic and melanoma (29).
Finally, animal studies suggest that sulforaphane may also play a role in diabetes prevention and reducing the risk of diabetes-induced complications, such as kidney disease (30).
It is common that incorporation of cruciferous vegetables including broccoli, cabbage, cauliflower, and kale into the diet is associated with reduced incidence of some types of cancers (162). Sulforaphane (SFN; Fig. 3C) is an organosulfur compound found at high concentrations in these vegetables (163).
SFN has extremely high bioavailability in vitro and in vivo (153). Human and rat perfusion experiments showed that a large fraction of broccoli extract can be absorbed in the small intestine, and SFN can be distributed in the body (164). Animal studies showed that the maximum urinary elimination ofSFNinratsiswithin24h,andinthenext24h,a minor fraction of SFN is detectable (165).
The mechanisms of SFN effects on cancer cells have been well studied. It suppresses the proliferation of cancer cells via diverse mechanisms including cell-cycle arrest, apoptosis induction, ROS production, and manipulation of some signaling pathways (166). SFN inhibits proliferation of PC-3 cells in culture in concentrationand time-dependent manner. Singh et al. (167) showed that oral administration of SFN led to >50% reduction in PC-3 xenograft tumor volume in SFN-treated mice in 10 days and more than 70% reduction in 20 days after starting treatment with no effect on body weight.
They also reported that SFN changes the Bax: Bcl-2 ratio, activates caspases 3, 8, and 9, and cleaves and inactivates PARP protein. The authors proposed that SFN induces apoptosis in PC-3 xenograft tumors in a p53-independent manner through cytoplasmic and mitochondrial pathways. Liquid chromatography–mass spectrometry (LC-MS) analyses performed by Rose et al. (17) showed the presence of 7-methylsulphinylheptyl isothiocyanates in watercress (Rorippa nasturtium aquaticum) extract and 4-methylsulfinylbutyl nitrile and 4-methylsulfinylheptyl isothiocyanates in the broccoli extract. Their investigations showed that these compounds contribute to the inhibitory effects of broccoli and watercress extracts on the invasion of MDA-MB-231 cancer cells through suppression of MMP-9 activity.
Treatment of HEK293 cells with different concentrations of SFN with and without TSA, as a HDAC1 inhibitor, leads to the increase in TOPflash reporter activity without affecting b-catenin protein levels. Further studies showed that this increase is due to the decrease in HDAC activity and consequently the increase in histone acetylation following SFN treatment (168).
It has been demonstrated that mamosphere formation in breast cancer cells is dependent on E-cadherin expression (168). It is showed that SFN could target breast cancer stem cells. The mammosphere formation test on two cancer cell lines, MCF7 and SUM195, indicated that SFN could reduce the proportion of cell with stem cell properties, and this was further supported by ALDEFLUOR assay. In vivo examination results of SFN effects on xenograft SUM159 cells in NOD/SCID mice were consistent with the in vitro results. More importantly, cells derived from SFN-treated primary tumors could not produce secondary tumors, while cells derived from the nontreated primary tumors rapidly produced the secondary tumors in the contralateral mammary fat pad of the same mice (168).
Aldehyde dehydrogenase activity is a stem cell marker for enriching tumorigenic stem/progenitor cells (169,170). Five mmol/L of SFN led to >80% reduction of ALDH-positive SUM159 cells in vitro, and daily treatment of xenograft of SUM159 tumors with 50 mg/kg of SFN for 2 weeks led to 50% reduction in tumor size through the reduction in ALDH-positive SUM159 cells by 50%, with no effect on body weight (171). ApcMin/C mice consumed SFN in their diet have fewer tumors with lower sizes in comparison with a control group, albeit, immunohistochemical (IHC) staining revealed that the b-catenin expression was not affected by SFN consumption (172).
Furthermore, the effect of SFN treatment on selfrenewal contributing to signaling pathway, Wnt pathway, was examined by analysis of b-catenin and some other downstream genes at mRNA and/or protein levels (171).
Treatment of T24 bladder cancer cells with SFN results in induction of miR-200c expression (173).
Previous studies demonstrated that miR-200c targets the E-cadherin repressors ZEB1 and ZEB2. Ectopic expression of miR-200c resulted in upregulation of E-cadherin in cancer cells (174). Therefore, treatment of T24 cells with SNF led to E-cadherin induction and EMT suppression (173). However, it seems that these results depend upon cell type and treatment conditions. Although clinical trials seem necessary, there is a large body of investigations about anticancer effects of SFN, and the explicit point is that SFN inclusion into the diet promises a safe and confident strategy.
Another active ingredient of broccoli and other cruciferous vegetables is Indole-3-carbinol (I3C) that has anticancer effects too. Meng et al. (175,176) reported despite a somehow prohibiting effect of I3C on cell attachment in vitro, and I3C could also suppress the invasion and motility of cells. The effect of I3C on cellular metastasis was also evaluated by injecting treated cells into the tail vein of mice and tracing surface metastasis in the lung of the sacrificed animal. Their results indicated that I3C treatment reduced the metastatic capability of the cells.