Antioxidative and antigenotoxic properties of vegetables and dietary phytochemicals: the value of genomics biomarkers in molecular epidemiology.

There is considerable evidence that consumption of fruits and vegetables may contribute to the prevention of cancer. It is however remarkable that evidence for such a preventive action arising from mechanistic studies is becoming stronger, whereas results of some recent prospective studies are less convincing. This apparent discrepancy may be overcome, or at least understood, by introducing molecular markers in future epidemiological studies, taking modulation of molecular processes as well as genetic variability in human populations into account. Both human and animal studies demonstrated that vegetable intake modulates gene expression in the gastrointestinal tract of many genes involved in biological pathways in favor of cancer risk prevention. Gene sets identified in this type of studies can be further evaluated, linked to the biological effects of phytochemicals and developed into biomarkers for larger human studies. Human dietary intervention studies have demonstrated that, apart from target tissues, also peripheral lymphocytes can be used for biomonitoring of chemopreventive effects. Transcriptomic responses and metabolite profiling may link phenotypic markers of preventive effects to specific molecular processes. The use of genomics techniques appears to be a promising approach to establish mechanistic pathways involved in chemoprevention by phytochemicals, particularly when genetic variability is taken into account.

A human intervention study with foods containing natural Ah-receptor agonists does not significantly show AhR-mediated effects as measured in blood cells and urine.

Binding and activation of the aryl hydrocarbon receptor (AhR) is thought to be an essential step in the toxicity of the environmental pollutants dioxins and dioxin-like PCBs. However, also a number of natural compounds, referred to as NAhRAs (natural Ah-receptor agonists), which are present in, for example, fruits and vegetables, can bind and activate this receptor. To study their potential effects in humans, we first investigated the effect of the prototypical AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on gene expression in ex vivo exposed freshly isolated human lymphocytes, and compared the resulting gene expression profile with those caused by the well-known NAhRA indolo[3,2-b]carbazole (ICZ), originating from cruciferous vegetables, and by a hexane extract of NAhRA-containing grapefruit juice (GJE). Only ICZ induced a gene expression profile similar to TCDD in the lymphocytes, and both significantly up-regulated CYP1B1 and TIPARP (TCDD-inducible poly (ADP-ribose) polymerase) mRNA. Next, we performed a human intervention study with NAhRA-containing cruciferous vegetables and grapefruit juice. The expression of the prototypical AhR-responsive genes CYP1A1, CYP1B1 and NQO1 in whole blood cells and in freshly isolated lymphocytes was not significantly affected. Also enzyme activities of CYP1A2, CYP2A6, N-acetyltransferase 2 (NAT2) and xanthine oxidase (XO), as judged by caffeine metabolites in urine, were unaffected, except for a small down-regulation of NAT2 activity by grapefruit juice. Examination of blood plasma with DR CALUX showed a 12% increased AhR agonist activity 3 and 24 h after consumption of cruciferous vegetables, but did not show a significant effect of grapefruit juice consumption. We conclude that intake of NAhRAs from food may result in minor AhR-related effects measurable in human blood and urine.

Influence of TCDD and natural Ah receptor agonists on benzo[a]pyrene-DNA adduct formation in the Caco-2 human colon cell line.

Several compounds originating from cruciferous vegetables and citrus fruits bind to and activate the aryl hydrocarbon receptor (AhR). This receptor plays an important role in the toxicity of the known tumour promoter and potent AhR-agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, vegetables and fruits are generally considered as healthy. Therefore, besides the AhR activation, the natural AhR agonists (NAhRAs) are assumed to show other health-concerning effects. AhR activation induces several cytochrome P450 phase I enzymes involved, e.g. in the bioactivation of carcinogenic polycyclic aromatic hydrocarbons, like benzo[a]pyrene (BaP), and may as such stimulate DNA adduct formation of those compounds. Therefore, the influence of TCDD, indolo[3,2-b]carbazole (ICZ, an NAhRA originating from cruciferous vegetables) and an NAhRA-containing extract of grapefruit juice (GJE) on BaP-DNA adduct formation in the human Caco-2 cell line was studied. Also, we investigated if different effects of TCDD, ICZ and GJE on adduct formation could be related to the modulation of transcription of biotransformation- and DNA-repair enzymes. Co-exposure to high AhR-activating concentrations of both TCDD and ICZ significantly reduced the amount of BaP-DNA adducts at 0.1 microM BaP, while at higher concentrations of BaP no influence was observed. In contrast, exposure to 0.1 microM BaP combined with GJE showed a significant increase in BaP-DNA adducts, and a significant decrease at 0.3 and 1 microM BaP. These differences could not be related to transcription of the phase I and II enzymes CYP1A1, CYP1B1, NQO1, GSTP1 and UGT1A6 or to transcription of the nucleotide excision repair enzymes ERCC1, XPA, XPC, XPF and XPG. We conclude that ICZ showed a similar effect on BaP-DNA adduct formation than TCDD, while GJE influenced the adduct formation in a different way. The difference in the influence on adduct formation may be due to effects at the level of enzyme activity, rather than gene expression.