Human response to dioxin: aryl hydrocarbon receptor (AhR) molecular structure, function, and dose-response data for enzyme induction indicate an impaired human AhR.

The aryl hydrocarbon receptor (AhR) mediates nearly all studied adverse effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and many related compounds. Binding of TCDD or related ligands to AhR is the key initiating event in downstream biochemical responses. The binding affinity of AhR for TCDD is specific to species and strain, and studies of human AhR demonstrate binding affinities approximately an order of magnitude or more lower than those observed in the most sensitive laboratory strains and species. Molecular genetic studies confirmed that human AhR shares key mutations with the DBA mouse strain that result in an "impaired" AhR (with respect to TCDD binding and responsiveness). Despite a number of polymorphisms in human AhR, the key "DBA-type" mutations appear to be a constant feature of the human AhR, and no polymorphisms have been identified that compensate for the impaired binding function conferred by these mutations. Consistent with the impaired binding status of the human AhR, human cells have consistently required approximately 10-fold higher concentrations of TCDD in vitro than rodent cells to respond with enzyme induction. Recent studies of in vivo enzyme induction-related endpoints in human populations with moderately and highly increased TCDD body burdens detected no relationship between these endpoints and TCDD body burdens at body-burden levels up to 250 ng TEQ/kg body weight, or approximately 25 times above the upper range of current general population background body burdens, while marked elevations in enzyme activity were observed in persons with body burdens above 750 ng TEQ/kg. In contrast, the more sensitive laboratory rodent strains and species exposed to TCDD exhibit significant enzyme induction at body burdens below 50 ng/kg. These interspecies data on the most sensitive and best understood response to binding of TCDD and related compounds to the AhR are consistent with the binding affinity and molecular structure data and support the hypothesis that the human AhR is less functional than the AhR of the more sensitive laboratory animals at a molecular level. Quantitative risk assessments involving interspecies extrapolation from sensitive laboratory species and strains should take these fundamental differences into account when margins of exposure and safety factors are considered.

Quantitation of polychlorinated biphenyls in fish for human cancer risk assessment: a comparative case study.

A range of analytical options is available for the quantification of environmental polychlorinated biphenyls (PCBs). The PCBs can be quantified as Aroclor mixtures, as individual PCB congeners, or as PCB homologues. The methodological choice is driven by many considerations, including cost, but it is important to note that the risk assessment methods recommended by the U.S. Environmental Protection Agency (U.S. EPA) have specific and varying requirements for these analytical endpoints. The toxic equivalency approach is recommended for settings where the original Aroclor mixtures have been heavily degraded or weathered. Because this method only addresses the dioxin-like fraction of the PCB mass, the remaining PCB congeners need to be quantified and assessed separately, in a manner that is consistent with current U.S. EPA guidance. In this present analysis, we examined various methods for estimating a total nondioxin-like PCB mass in fish tissue samples from a New Jersey waterway using congener and homologue data: The National Oceanic and Atmospheric Administration estimate of the PCB total based on 18 congeners sigma dioxin-like congeners, an estimate of the PCB total from 38 analyzed congeners--sigma dioxin-like congeners, and the total of nine homologue groups--sigma dioxin-like congeners. These three approaches yielded similar estimates of the total nondioxin-like PCBs for a variety of fish species, whereas the totals quantified as Aroclor 1248, 1254, or 1260 were typically more than fivefold lower. Based on these results, the selection of the PCB analytical endpoints obtained from field investigations of fish and shellfish should be guided by the appropriate risk assessment methodology and by the analytical limitations.

The use of toxic equivalency factor distributions in probabilistic risk assessments for dioxins, furans, and PCBs.

Toxic equivalency factors (TEFs) for 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDFs) and coplanar polychlorinated biphenyl (PCB) congeners have been developed by the World Health Organization (WHO). Each TEF was derived from a range of relative potency (REP) estimates obtained from in vivo and in vitro studies wherein the potency of the congener was evaluated relative to 2,3,7,8-TCDD (or some other appropriate benchmark). For most congeners, the range of REP values spans several orders of magnitude, and the degree of conservatism varies widely among the congeners. Although some TEFs are greater than the maximum REP value, others are less than the minimum. This suggests that the point estimate TEFs introduce a significant amount of variability and uncertainty into the PCB and PCDD/F risk assessment process. The use of REP data distributions, rather than point estimate TEFs, would permit a more informed evaluation of the variability and uncertainty in the attendant risk estimates. Further, a standardized method of choosing a TEF from an REP distribution would ensure a uniform degree of conservatism in the TEF values. In this analysis, distributions of REP values were derived for the coplanar PCBs and 2,3,7,8-substituted PCDD/Fs. There are 936 REP values in the WHO database; the number of values per congener ranges from 1 (1,2,3,7,8,9-HxCDF) to 117 (PCB126). Twenty REP values qualified by WHO as "<" were replaced with one-half the stated value; 65 values qualified as ">" were not used. Fit tests indicate that most distributions are lognormal. Mean, standard deviation, and 50th and 95th percentile values for each REP distribution are presented. In general, the WHO TEFs for the PCDD/Fs are at the upper bound (75th percentile or greater) of the underlying REP distributions, while the PCB TEFs tend to be more representative of the central tendency of the underlying REP distribution. A simplistic weighting scheme that emphasizes long-term in vivo studies suggests that the REP distributions may not be overly sensitive to weighting techniques--that is, the statistical descriptors of the weighted distributions were similar to those of the unweighted distributions. A case study using fish tissue PCB and PCDD/F data suggests that in some settings the use of WHO TEFs may understate upper bound PCB risks relative to PCDD/F risks. A preliminary sensitivity analysis suggests that measurement endpoint, tissue-type and species (or strain) may be significant contributors to the variability and heterogeneity in the underlying REP data for some congeners. Although there are fundamental shortcomings inherent in any TEF scheme, evidence suggests that some form of REP distributions should be used in lieu of or in addition to point estimate TEFs in settings where PCBs and PCDD/Fs are commingled.

AH receptor agonist activity in human blood measured with a cell-based bioassay: evidence for naturally occurring AH receptor ligands in vivo.

In the present study, an aryl hydrocarbon receptor (AHR)-driven reporter gene bioassay was used to measure the activity, measured as an induction equivalent (IEQ) as compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), or IEQ concentration in human blood samples from 10 volunteers under different dietary regimens. Blood concentrations of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) and polychlorinated biphenyls (PCBs), as determined by analytical chemistry (HR-GC/MS), and expressed as toxic equivalents (TEQs) with the use of TCDD equivalency factors (TEFs), were within a range that has been reported in the general US population, ranging from 0.022 to 0.119 ppt (whole blood basis). However, the human blood IEQ measured directly via bioassay ranged from 13.4 to 218 ppt (whole blood basis). These order of magnitude greater IEQs compared to the TEQs for dioxins, furans, and certain PCBs suggests that human blood contains a relatively high level of AHR agonists able to activate the CYP1A1 dioxin response element (DRE)-linked reporter gene bioassay and that this AHR activity is not accounted for by PCDDs/Fs and dioxin-like PCBs based on standard HR-GC/MS and TEF analysis. When study participants switched from a "baseline" to a high-vegetable diet, increases in bioassay IEQ were observed that were statistically significant (P<0.05). In addition, IEQ activity was elevated above levels observed following dietary intervention in two subjects given indole-3-carbinol (I3C) supplements. We conclude that a substantial portion of the IEQ activity occurred as a result of the increased intake of natural AHR agonists (NAHRAs) present in many fruits, vegetables. and herbs. Our findings also suggest that dietary NAHRAs constitute a substantial daily dietary intake of AHR-active compounds, and these NAHRAs could influence AHR status in humans and play a role in a basal level of AHR activation.