Natural dietary anti-cancer chemopreventive compounds: redox-mediated differential signaling mechanisms in cytoprotection of normal cells versus cytotoxicity in tumor cells

Nrf2-mediated transcriptional induction of antioxidant response in mouse embryos exposed to ethanol in vivo: implications for the prevention of fetal alcohol spectrum disorders

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that is important in protection against oxidative stress. This study was designed to determine the role of Nrf2 signaling in transcriptional activation of detoxifying and antioxidant genes in an in vivo mouse fetal alcohol syndrome model. Maternal ethanol treatment was found to increase both Nrf2 protein levels and Nrf2-ARE binding in mouse embryos. It also resulted in a moderate increase in the mRNA expression of Nrf2 downstream target detoxifying and antioxidant genes as well as an increase in the expression of antioxidant proteins. Pretreatment with the Nrf2 inducer, 3H-1,2 dithiole-3-thione (D3T), significantly increased Nrf2 protein levels and Nrf2-ARE binding, and strongly induced the mRNA expression of Nrf2 downstream target genes. It also increased the expression of antioxidant proteins and the activities of the antioxidant enzymes. Additionally, D3T pretreatment resulted in a significant decrease in ethanol-induced reactive oxygen species generation and apoptosis in mouse embryos. These results demonstrate that Nrf2 signaling is involved in the induction of antioxidant response in ethanol-exposed embryos. In addition, the potency of D3T in inducing antioxidants as well as in diminishing ethanol-induced apoptosis suggests that further exploration of the antiteratogenic effect of this compound will be fruitful.

Cancer chemoprevention: a radical perspective

Cancer chemopreventive agents block the transformation of normal cells and/or suppress the promotion of premalignant cells to malignant cells. Certain agents may achieve these objectives by modulating xenobiotic biotransformation, protecting cellular elements from oxidative damage, or promoting a more differentiated phenotype in target cells. Conversely, various cancer chemopreventive agents can encourage apoptosis in premalignant and malignant cells in vivo and/or in vitro, which is conceivably another anticancer mechanism. Furthermore, it is evident that many of these apoptogenic agents function as prooxidants in vitro. The constitutive intracellular redox environment dictates a cell's response to an agent that alters this environment. Thus, it is highly probable that normal cells, through adaption, could acquire resistance to transformation via exposure to a chemopreventive agent that promotes oxidative stress or disrupts the normal redox tone of these cells. In contrast, transformed cells, which typically endure an oxidizing intracellular environment, would ultimately succumb to apoptosis due to an uncontrollable production of reactive oxygen species caused by the same agent. Here, we provide evidence to support the hypothesis that reactive oxygen species and cellular redox tone are exploitable targets in cancer chemoprevention via the stimulation of cytoprotection in normal cells and/or the induction of apoptosis in transformed cells.

Initial results in the development of a reporter cell line for toxicology studies at gene expression level: activation of the electrophile-responsive element by copper and methyl parathion

Induction of many genes encoding detoxifying enzymes and antioxidant proteins is mediated through a common mechanism, which is controlled by electrophile-responsive elements (EpRE) within the regulatory region of those genes. Copper and methyl parathion are environmental pollutants known to induce the expression of EpRE-mediated genes. In order to evaluate the molecular response triggered by these pollutants, a stable cell line was produced, which carries a transgene comprised of the green fluorescent protein (GFP) reporter gene under transcriptional control of the mouse glutathione-S-transferase (gst1) electrophile-responsive element fused to the mouse metallothionein (mt1) minimal promoter. The rat HTC hepatoma cells were transfected with the EpREmt-GFP construct and successfully selected with G418 antibiotic. EpREmt-GFP HTC cells were treated with 0.002 mg L(-1), 0.02 mg L(-1), 0.2 mg L(-1) and 2 mg L(-1) copper sulfate and 0.001 mg L(-1), 0.01 mg L(-1), 0.1 mg L(-1) and 1 mg L(-1) methyl parathion for 48 h. GFP expression was directly quantified in living cells using a microplate fluorimeter. GFP expression was significantly increased in higher concentrations of both pollutants, showing a 1.80- and 2.58-fold induction of GFP at 2mg copper L(-1) and 1mg methyl parathion L(-1), respectively (p<0.01). The results obtained in the present study demonstrate that the EpREmt-GFP HTC cell line can be an interesting model for further development for the study of the cellular response to aquatic pollutants as well as a new tool for environmental monitoring at the molecular level.

Hormetic dietary phytochemicals

Compelling evidence from epidemiological studies suggests beneficial roles of dietary phytochemicals in protecting against chronic disorders such as cancer, and inflammatory and cardiovascular diseases. Emerging findings suggest that several dietary phytochemicals also benefit the nervous system and, when consumed regularly, may reduce the risk of disorders such as Alzheimer's and Parkinson's diseases. The evidence supporting health benefits of vegetables and fruits provide a rationale for identification of the specific phytochemicals responsible, and for investigation of their molecular and cellular mechanisms of action. One general mechanism of action of phytochemicals that is emerging from recent studies is that they activate adaptive cellular stress response pathways. From an evolutionary perspective, the noxious properties of such phytochemicals play an important role in dissuading insects and other pests from eating the plants. However at the subtoxic doses ingested by humans that consume the plants, the phytochemicals induce mild cellular stress responses. This phenomenon has been widely observed in biology and medicine, and has been described as 'preconditioning' or 'hormesis.' Hormetic pathways activated by phytochemicals may involve kinases and transcription factors that induce the expression of genes that encode antioxidant enzymes, protein chaperones, phase-2 enzymes, neurotrophic factors, and other cytoprotective proteins. Specific examples of such pathways include the sirtuin-FOXO pathway, the NF-kappaB pathway, and the Nrf-2/ARE pathway. In this article, we describe the hormesis hypothesis of phytochemical actions with a focus on the Nrf2/ARE signaling pathway as a prototypical example of a neuroprotective mechanism of action of specific dietary phytochemicals.

Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health

Plant natural products derived from phenylalanine and the phenylpropanoid pathway are impressive in their chemical diversity and are the result of plant evolution, which has selected for the acquisition of large repertoires of pigments, structural and defensive compounds, all derived from a phenylpropanoid backbone via the plant-specific phenylpropanoid pathway. These compounds are important in plant growth, development and responses to environmental stresses and thus can have large impacts on agricultural productivity. While plant-based medicines containing phenylpropanoid-derived active components have long been used by humans, the benefits of specific flavonoids and other phenylpropanoid-derived compounds to human health and their potential for long-term health benefits have been only recognized more recently. In this part of the review, we discuss the diversity and biosynthetic origins of phenylpropanoids and particularly of the flavonoid and stilbenoid natural products. We then review data pertaining to the modes of action and biological properties of these compounds, referring on their effects on human health and physiology and their roles as plant defense and antimicrobial compounds. This review continues in Part II discussing the use of biotechnological tools targeting the rational reconstruction of multienzyme pathways in order to modify the production of such compounds in plants and model microbial systems for the benefit of agriculture and forestry.

Cancer chemoprevention by phytochemicals: potential molecular targets, biomarkers and animal models

Recent studies have strongly indicated that certain daily-consumed dietary phytochemicals could have cancer protective effects against transgenic mice cancer models and cancers mediated by carcinogens, irradiations and carcinogenic metabolites derived from exogenous or endogenous sources. The cancer-protective effects elicited by these dietary compounds are believed to be due at least in part to the induction of cellular defense systems including the detoxifying and antioxidant enzymes system, as well as the inhibition of anti-inflammatory and anti-cell growth signaling pathways culminating in cell cycle arrest and/or celldeath. In this review, we summarize the potential mechanisms including the modulation of nuclear factor kappaB (NF-kappaB), cyclooxygenases-2 (COX-2), activator protein-1 (AP-1), mitogen-activated protein kinases (MAPKs) and the induction of phase II cellular detoxifying and antioxidant enzymes mediated mainly by the antioxidant response elements (ARE) within the promoter regions of these genes through nuclear factor-erythroid 2-related factor 2 (Nrf2), a member of the Cap "n" collar (CNC) family of the basic region-leucine zipper transcription factor. In addition, we also review several animal models of carcinogenesis and cancer chemopreventive efficacy studies of these animal models using dietary chemopreventive compounds. Finally, we discuss the cellular signaling cascades mediated by Nrf2, NF-kappaB, AP-1, MAPKs and COX-2, which have been considered to play pivotal roles in tumor initiation, promotion and progression processes, and could be promising molecular targets for the design of drugs targeting cancer prevention and therapy.

Synergistic effects of a combination of dietary factors sulforaphane and (-) epigallocatechin-3-gallate in HT-29 AP-1 human colon carcinoma cells

PURPOSE

The objective of this study was to investigate combinations of two chemopreventive dietary factors: EGCG 20 microM (or 100 microM) and SFN (25 microM) in HT-29 AP-1 human colon carcinoma cells.

METHODS

After exposure of HT-29 AP-1 cells to SFN and EGCG, individually or in combination, we performed AP-1 luciferase reporter assays, cell viability assays, isobologram analyses, senescence staining, quantitative real-time PCR (qRT-PCR) assays, Western blotting, and assays for HDAC activity and hydrogen peroxide. In some experiments, we exposed cells to superoxide dismutase (SOD) or Trichostatin A (TSA) in addition to the treatment with dietary factors.

RESULTS

The combinations of SFN and EGCG dramatically enhanced transcriptional activation of AP-1 reporter in HT-29 cells (46-fold with 25 microM SFN and 20 microM EGCG; and 175-fold with 25 microM SFN and 100 microM EGCG). Isobologram analysis showed synergistic activation for the combinations with combination index, CI < 1. Interestingly, co-treatment with 20units/ml of SOD, a free radical scavenger, attenuated the synergism elicited by the combinations (2-fold with 25 muM SFN and 20 muM EGCG; and 15-fold with 25 microM SFN and 100 microM EGCG). Cell viability assays showed that the low-dose combination decreased cell viability to 70% whereas the high-dose combination decreased cell viability to 40% at 48 h, with no significant change in cell viability at 24 h as compared to control cells. In addition, 20 microM and 100 microM EGCG, but not 25 microM SFN, showed induction of senescence in the HT-29 AP-1 cells subjected to senescence staining. However, both low- and high-dose combinations of SFN and EGCG attenuated the cellular senescence induced by EGCG alone. There was no significant change in the protein levels of phosphorylated forms of ERK, JNK, p38, and Akt-Ser473 or Akt-Thr308. Besides, qRT-PCR assays corroborated the induction of the luciferase gene seen with the combinations in the reporter assay. Relative expression levels of transcripts of many other genes known to be either under the control of the AP-1 promoter or involved in cell cycle regulation or cellular influx-efflux such as cyclin D1, cMyc, ATF-2, Elk-1, SRF, CREB5, SLCO1B3, MRP1, MRP2 and MRP3 were also quantified by qRT-PCR in the presence and absence of SOD at both 6 and 10 h. In addition, pre-treatment with 100 ng/ml TSA, a potent HDAC inhibitor, potentiated (88-fold) the synergism seen with the low-dose combination on the AP-1 reporter transcriptional activation. Cytoplasmic and nuclear fractions of treated cells were tested for HDAC activity at 2 and 12 h both in the presence and absence of TSA, however, there was no significant change in their HDAC activity. In addition, the H2O2 produced in the cell system was about 2 microM for the low-dose combination which was scavenged to about 1 microM in the presence of SOD.

CONCLUSION

Taken together, the synergistic activation of AP-1 by the combination of SFN and EGCG that was potentiated by HDAC inhibitor TSA and attenuated by free radical scavenger SOD point to a possible multifactorial control of colon carcinoma that may involve a role for HDACs, inhibition of cellular senescence, and SOD signaling.