Abstract SY07-04: Diet and xenobiotic-metabolizing enzymes: Learning from controlled feeding studies in humans
Authors: Lampe Johanna W. 
Published: May 2019

Nutrients, other dietary constituents, foods, and food patterns all have been shown to be associated with cancer risk in epidemiologic studies and in animal models of carcinogenesis. Modulation of xenobiotic-metabolizing (i.e., biotransformation) enzymes is one mechanism by which diet is hypothesized to influence risk of cancer. Phase I and phase II enzymes in the gut, liver, and other tissues are involved in the metabolism of exogenous compounds, including drugs, carcinogens, and dietary constituents, and endogenous compounds, such as steroid hormones. Phase I metabolic activation of carcinogens is typically catalyzed by the cytochrome P450 (CYP) family of oxidation enzymes and other classes of enzymes, such as peroxidases (e.g., the cyclooxygenases), and transferases, such as N-acetyltransferase. Phase II enzyme-catalyzed conjugation reactions produce more water-soluble products that can be excreted in urine or bile. Phase II enzymes include the glutathione S-transferases (GSTs), UDP-glucuronosyltransferases (UGTs), sulfotransferases, and quinone reductase.

The human diet is a complex mixture of carcinogens, mutagens, and protective agents, many of which are metabolized by xenobiotic-metabolizing enzymes, and which also can influence the expression and activities of these enzymes. Modulation of these enzymes in a fashion associated with more favorable handling of carcinogens may, at the same time, be associated with less favorable handling of chemopreventive agents. Many of the genes that encode for these enzymes are highly polymorphic and SNPs in both phase I and phase II enzymes have been shown to modify cancer risk, particularly in individuals exposed to the particular substrates for enzymes. Thus, given the complexity of the human diet, as well as the genetic variation that can influence enzyme activities, teasing out the positive and negative contributions of foods, dietary patterns, and specific dietary constituents in relation to this cancer-related mechanism is particularly challenging.

Certain botanical families of plant foods are often unique, rich sources of specific classes of phytochemicals. For example the allium or onion family contains high amounts of allyl sulfides; apiaceous or carrot-family vegetables are sources of furanocoumarins; certain legumes are major sources of isoflavones; and cruciferous or broccoli-family vegetables are the primary dietary sources of precursors of isothiocyanates and indoles. Recently, we conducted a series of studies to examine the effect of various botanical groups of plant foods on phase I and phase II enzyme activities. We used randomized crossover controlled feeding studies in healthy volunteers, recruited based on specific genotypes, to compare metabolizing enzyme response to supplemented cruciferous vegetables (at two doses) and mixtures of cruciferous and apiaceous vegetables as part of a plant-food free, basal diet.

CYP1A2, a phase I enzyme, mediates the activation of several procarcinogens, including heterocyclic amines. In vitro, individually, the isothiocyanates and indoles in cruciferous vegetables have varying capacities to induce CYP1A2 expression, with the isothiocyanate sulforaphane not having any effect (1). However, in vivo, cruciferous vegetable consumption increases CYP1A2 activity in humans. Our previous studies have also shown that intake of apiaceous vegetables decreases activity of this enzyme (2), due to the inhibitory effects of the furanocoumarins. When fed at two doses, cruciferous vegetables increased CYP1A2 activity in a dose-responsive manner and when cruciferous vegetables were fed concurrently with apiaceous vegetables there was no change in CYP1A2 compared with the basal diet (3).

GSTs detoxify polycyclic aromatic hydrocarbons present in grilled meats. GSTs also conjugate isothiocyanates, the chemopreventive compounds found in cruciferous vegetables. Polymorphisms in the GSTM1 and GSTT1 genes result in complete lack of GSTM1-1 and GSTT1-1 proteins, respectively. A growing number of studies show effect modification of GSTM1 and GSTT1 genotypes on the association between cruciferous vegetable intake (or isothiocyanate exposure) and the risk of several cancers, including lung, prostate, bladder, and colon. Based on these epidemiologic data, investigators have hypothesized that GSTM1-null or GSTT1-null individuals have reduced metabolism of isothiocyanates and therefore greater and longer exposure to these compounds. Interestingly, studies of sulforaphane pharmacokinetics show that, individuals who lack the GSTM1 enzyme (i.e., are GSTM1-null) have slightly higher areas under the curve for plasma sulforaphane metabolite concentrations, but also more rapid and higher percentages of sulforaphane excretion than do individuals who have at least one intact allele (i.e., GSTM1-positive) (4). These results support that the GSTM1-positive individuals may actually retain a greater amount of ingested sulforaphane. However, response of intermediate biomarkers to short-term interventions with cruciferous vegetables suggest that GSTM1-null individuals tend to have a greater response to isothiocyanate exposure. Supplementation with cruciferous vegetables at two doses significantly increased serum GST-alpha, primarily in GSTM1-null/GSTT1-null men (5). Serum GST-alpha concentrations have been shown to be associated inversely with prostate cancer risk (6).

UGTs catalyze the addition of UDP-glucuronic acid to xenobiotics and a variety of endogenous compounds, enhancing their water solubility and elimination in urine and bile. We showed that, in a controlled feeding study, cruciferous vegetable supplementation also increases UGT1A1 activity, as measured by serum bilirubin concentrations, in a dose-dependent manner, and to a greater extent in individuals with the UGT1A1*28/*28 (low-UGT1A1 expression genotype) compared with those with one or more *1 allele (7). Further, combined feeding of the lower dose of cruciferous vegetables and added apiaceous vegetables resulted in an increase in UGT activity similar to that with the high-dose crucifer feeding; this demonstrated that constituents of apiaceous vegetables also have the capacity to increase UGT1A1 activity. Nonetheless, concurrent feeding of more complex mixtures of plant foods that are rich sources of UGT-inducers does not necessarily further enhance UGT expression. When we supplemented the basal diet with a combined mixture of citrus fruits, soy foods and cruciferous vegetables, UGT1A1 activity increased only moderately, but still significantly, after a week of feeding (8). Other compounds in the same foods that increase UGT activity also have been shown to inhibit UGTs in vitro, which may explain in part the modest effects in this study.

In conclusion, the results of these controlled feeding studies point to the complex interactions among types of foods consumed, genetic variation, and modulation of xenobiotic metabolism that may not be apparent in observational studies. Feeding studies allow for testing effects of diet on biologic pathways and elucidation of mechanisms in humans. While findings of observational studies can be followed up using animal models, species differences in metabolism, disposition, and action of bioactive food constituents requires direct verification that similar modes of action exist in humans too. This type of upfront evaluation of intermediate endpoints in humans may help inform the study designs and doses and types of intervention for future larger intervention studies with cancer endpoints.

References

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Citation Format: Johanna W. Lampe. Diet and xenobiotic-metabolizing enzymes: Learning from controlled feeding studies in humans [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY07-04