Targeting Cyclooxygenase 2 and Nuclear Factor Erythoid 2-Related Factor 2 with Sulforaphane for Bladder Cancer Chemoprevention
Sulforaphane (SF) is a well-known cancer-preventive phytochemical. SF-rich broccoli sprout extracts can inhibit bladder cancer development in a rat model. It may be particularly useful in bladder cancer chemoprevention because oral administration of SF can achieve extremely high bioavailability and is selectively delivered through urinary excretion to the bladder epithelium, the principal site of bladder cancer development. My thesis research is focused on determining the ability of SF to target both nuclear factor erythoid 2-related factor 2 (Nrf2) and cyclooxygenase-2 (COX-2) for bladder cancer prevention and to elucidate the molecular mechanisms by which SF targets COX-2. COX-2 is an oncogene and over-expression of COX-2 is an important step in bladder tumorigenesis. On the other hand, Nrf2 is a transcriptional factor and Nrf2 activation can stimulate many cytoprotective Phase 2 enzymes which catalyze detoxification reactions of carcinogens and oxidants. Both COX-2 and Nrf2 play important roles in the bladder cancer development. 4-aminobiphenol (4-ABP) is a major human bladder carcinogen and cigarette smoke is a major source of human exposure. To assess the effect of SF on ABP, human bladder cancer RT-4 cells and mice without Nrf2 along with matched wide-type mice were treated with SF followed by ABP treatment. DNA damage was measured by using N -(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-4-ABP) as a biomarker, which is the predominant ABP-DNA adduct formed in human bladder cells and tissues. We have shown that SF significantly inhibited 4-ABP-induced DNA damage through activating Nrf2 signaling pathway in human bladder cells in vitro and in mouse bladder tissue in vivo. Moreover, Nrf2 activation by SF specifically occurs in the bladder epithelium. Surprisingly, in the absence of SF, Nrf2 faces a serious dilemma in defense against 4-ABP. Nrf2 protects the liver against 4-ABP-induced DNA damage by stimulating UDP-glucuronosyltransferase (UGT), promoting UGT-catalyzed glucuronidation of 4-ABP, and increasing urinary excretion of such metabolites. Thus, Nrf2 renders the bladder more susceptible to 4-ABP, as the metabolites dissociate in acidic urine, thereby delivering more carcinogenic parent compounds to the bladder tissues. These findings have a significant implication for cancer chemoprevention, as potential bladder toxicity may occur when Nrf2-activating chemopreventive agents are used in humans exposed to ABP, especially in smokers. SF not only targets Nrf2, but also targets COX-2. When three human bladder cancer cells were treated with SF for 24 hours, SF significantly down-regulated COX-2 mRNA and protein expression as well as PGE2 levels, which is the key downstream product of COX-2. Further, we found that SF targeted COX-2 mRNA stability via its 3'-untranslated region by decreasing COX-2 RNA binding protein CUG triplet repeat RNA-binding protein 2 (CUGBP2). Moreover, by using MTT assay and a subcutaneous xenograft model, we showed that SF inhibited bladder cancer cell proliferation and the growth of human bladder cancer xenografts in nude mice. Thus, our studies indicate that SF is a highly promising agent for bladder cancer prevention and/or treatment.