- Prostate cancer is the second leading cause of cancer related death in American men. Epidemiologic studies suggest that cruciferous vegetable intake may lower the risk for many cancers, including prostate and colon. Isothiocyanates (ITC) are phytochemicals derived from cruciferous vegetables such as broccoli, Brussels sprouts, cauliflower, and cabbage that may have health promoting properties. Broccoli and broccoli sprouts are a good source of sulforaphane (SFN), a well studied chemopreventive ITC. SFN is known to inhibit histone deacetylase (HDAC) activity and alter epigenetic endpoints. A key factor in understanding the efficacy of SFN as a chemoprevention agent is to determine the metabolism, distribution and bioavailability of SFN, and the factors that alter these parameters. The present study was undertaken to provide further evidence that SFN can alter HDAC activity, alter prostate cancer cell proliferation, both in vitro and in vivo and expands our understanding of SFN metabolism and tissue distribution.
We characterized the effects of SFN in normal (PrEC), benign hyperplasia (BPH1) and cancerous (LnCap and PC3) prostate epithelial cells. We observed that 15 µM SFN differentially induced cell cycle arrest and apoptosis in BPH1, LnCap and PC3 cells but not PrEC cells. SFN treatment also differentially decreased HDAC activity, and Class I and II HDAC proteins, increased acetylated histone H3 at the promoter for P21, induced p21 expression and increased tubulin acetylation in prostate cancer cells. In PrEC cells, SFN caused only a transient reduction in HDAC activity with no change in any other endpoints tested. Therefore, normal prostate cells were refractory to the cytotoxic and epigenetic effects of SFN.
In order for SFN to be an effective chemopreventive agent it must be metabolized and reach target tissues. Nrf2 wild-type and Nrf2 knockout (Nrf2-/-) mice were treated with 5 or 20 µmoles of SFN, and SFN metabolites were detected in all tissues tested at 2 and 6 h in a dose dependent manner. Genotype only had marginal effects at 5 µmoles, whereas, at 20 µmoles the female Nrf2-/- mice had dramatically higher levels. The relative abundance of each metabolite was not strikingly different between genders and genotypes, although different ratios between tissues were observed. In the transgenic adenoma of the mouse prostate model dietary SFN, fed as freeze-dried broccoli sprouts, increased SFN content in the prostate and decreased the severity of prostate cancer at 12 and 28 weeks of age.
In humans, the differences in metabolism of isothiocyanates between whole food and broccoli supplements have yet to be determined. Two separate human trials were conducted; the first was a randomized 7 day feeding study where subjects consumed either broccoli sprouts or a broccoli supplement, and the second study was a randomized single dose cross-over study with broccoli sprouts, followed by a washout period, then broccoli supplement. In plasma and urine, the total amounts of SFN and erucin (ERN) metabolites were greater and the peak concentration occurred sooner in subjects who consumed broccoli sprouts. Glutathione-S-transferase pi-1 polymorphisms did not affect ITC metabolism. Interconversion of SFN to ERN was observed. Histone deacetylase activity in peripheral blood mononuclear cells was inhibited only in subjects who consumed sprouts.
In conclusion, these data provide evidence that SFN alters HDAC activity and protein acetylation in cancerous prostate cells but not normal prostate cells. For the first time we show that SFN is bioavailable to many tissues types, including the prostate and are largely found as SFN metabolites not the parent SFN compound. We also show that the bioavailability of ITCs is markedly lower in human subjects who consume a broccoli supplement. Decreased HDAC activity in the peripheral blood of subjects who consumed sprouts indicates that higher ITC plasma concentrations can alter HDAC activity in vivo. Taken together, these data show that SFN is an effective prostate cancer chemopreventive agent that can easily be utilized in the diet from whole food.