Anticarcinogenesis by dietary phytochemicals: cytoprotection by Nrf2 in normal cells and cytotoxicity by modulation of transcription factors NF-kappa B and AP-1 in abnormal cancer cells.

Cancer statistics from the American Cancer Society and other sources are a stark reminder of our failure to combat this deadly disease. Chemoprevention entails the use of specific naturally occurring dietary or synthetic agents to thwart cancer development and progression. Some of these agents are believed to do so by protecting the cells or tissues from the malicious attack of exogenous carcinogens and/or endogenous reactive oxygen/nitrogen species (RONS) by inducing several detoxifying/antioxidant enzymes that appear to form stable conjugates such as glutathione, glucuronides or sulfates thus rendering the carcinogenic species harmless. This process of inducing the cellular defense enzymes is believed to be mediated by the antioxidant response elements (ARE) within the promoter regions of these genes. Nrf2, a redox sensitive transcription factor has been documented to play a central role in ARE-driven gene expression. Nrf2, under normal unstimulated conditions, remains sequestered in the cytosol by Keap1. The putative chemopreventive agents disrupt the Nrf2-Keap1 association, thereby releasing Nrf2 which then translocates to the nucleus and drives the gene expression of detoxifying enzymes. The role of other transcription factors such as NF-kappaB and AP-1 in carcinogenesis is well established. By modulating the activity of these transcription factors and their upstream signaling molecules, naturally occurring dietary phytochemicals appear to cause apoptosis in abnormal cells that over-express these factors, thereby inhibiting the promotion and progression. This review discusses the most current and up to date understanding of the possible signaling mechanisms by which these naturally dietary phytochemicals can differentially modulate signal transduction cascades such that they can bring about apoptosis/cell death in abnormal cancer cells but at the same time induce defensive enzymes to protect against carcinogenesis in normal cells.

Modulation of activator protein-1 (AP-1) and MAPK pathway by flavonoids in human prostate cancer PC3 cells.

In last couple of decades the use of natural compounds like flavonoids as chemopreventive agents has gained much attention. Our current study focuses on identifying chemopreventive flavonoids and their mechanism of action on human prostate cancer cells. Human prostate cancer cells (PC3), stably transfected with activator protein 1 (AP-1) luciferase reporter gene were treated with four main classes of flavonoids namely flavonols, flavones, flavonones, and isoflavones. The maximum AP-1 luciferase induction of about 3 fold over control was observed with 20 microM concentrations of quercetin, chrysin and genistein and 50 microM concentration of kaempferol. At higher concentrations, most of the flavonoids demonstrated inhibition of AP-1 activity. The MTS assay for cell viability at 24 h showed that even at a very high concentration (500 microM), cell death was minimal for most of the flavonoids. To determine the role of MAPK pathway in the induction of AP-1 by flavonoids, Western blot of phospho MAPK proteins was performed. Four out of the eight flavonoids namely kaempferol, apigenin, genistein and naringenin were used for the Western Blot analysis. Induction of phospho-JNK and phospho-ERK activity was observed after two hour incubation of PC3-AP1 cells with flavonoids. However no induction of phospho-p38 activity was observed. Furthermore, pretreating the cells with specific inhibitors of JNK reduced the AP-1 luciferase activity that was induced by genistein while pretreatment with MEK inhibitor reduced the AP-1 luciferase activity induced by kaempferol. The pharmacological inhibitors did not affect the AP-1 luciferase activity induced by apigenin and naringenin. These results suggest the possible involvement of JNK pathway in genistein induced AP-1 activity while the ERK pathway seems to play an important role in kaempferol induced AP-1 activity.

Combined inhibitory effects of curcumin and phenethyl isothiocyanate on the growth of human PC-3 prostate xenografts in immunodeficient mice.

Earlier studies using prostate cancer cells in culture showed that phenethyl isothiocyanate (PEITC) and curcumin have significant chemopreventive and possibly chemotherapeutic effects. However, their in vivo effects are still lacking. Hence, this study was undertaken to determine the possible in vivo efficacy of prostate cancer-prevention as well as cancer-therapeutic treatment by PEITC and curcumin alone or in combination. We evaluated the effects on tumor growth in vivo, using NCr immunodeficient (nu/nu) mice bearing s.c. xenografts of PC-3 human prostate cancer cells. Molecular biomarkers representing proliferation and apoptosis were determined. Continued i.p. injection of curcumin or PEITC (6 and 5 mumol; thrice a week for 28 days), beginning a day before tumor implantation significantly retarded the growth of PC-3 xenografts. Combination of i.p. administration of PEITC (2.5 mumol) and curcumin (3 mumol) showed stronger growth-inhibitory effects. Next, we evaluated the cancer-therapeutic potential of curcumin and PEITC in mice with well-established tumors, and the results showed that PEITC or curcumin alone had little effect, whereas combination of curcumin and PEITC significantly reduced the growth of PC-3 xenografts. Immunohistochemistry staining and Western blot analysis revealed that the inhibition of Akt and nuclear factor-kappaB signaling pathways could contribute to the inhibition of cell proliferation and induction of apoptosis. Taken together, our results show that PEITC and curcumin alone or in combination possess significant cancer-preventive activities in the PC-3 prostate tumor xenografts. Furthermore, we found that combination of PEITC and curcumin could be effective in the cancer-therapeutic treatment of prostate cancers.