The need for cellular, biochemical, and mechanistic studies

The results of diverse in vitro neurotoxicity studies demonstrate that there are variations in cellular responsiveness between different types of neural cells. In contrast to experimental systems that have reported cellular responses to relatively high concentrations of various PCB congeners, our studies with rat hippocampal neural cells indicate that neurons and astroglia are responsive to relatively low levels of TCDD. However, these responses are probably not mediated through the classical Ah receptor pathway, which involves nuclear Ah receptor-mediated modulation of gene expression. It has recently been reported that TCDD-induced phosphorylation and other responses can be observed in some cell lines within minutes after treatment (20), and that cell membrane or cytosolic receptors may also play a role in mediating these effects. Future studies are required to determine both Ah receptor-dependent and -independent pathways associated with the neurotoxicity of PCBs, TCDD, and related compounds. The report that low-level dietary or background exposure to HAHs (32) results in neurobehavioral deficits is still a perplexing problem also requiring additional research and consideration of other dietary factors that may contribute to these effects. For example, we have recently been comparing the toxic effects and relative potencies of TCDD (exodioxins) and other "natural occurring" compounds (endodioxins) such as indole-3-carbinol (vegetables) and polynuclear aromatic hydrocarbons (PAHs, cooked foods), which also bind to the Ah receptor. Dr. Clynn Wilker has shown that in utero exposure of rats to indole-3-carbinol and chrysene (a PAH) cause demasculinization of the adult offspring as previously reported for TCDD (19). Thus, the neurotoxicity of low-level dietary exposure to HAHs should at least consider other possible confounding factors, including dietary endodioxins.

Effects of Aroclor 1254-induced rat liver S-9 fraction on benzo[a]pyrene-mediated DNA damage and benzo[a]pyrene metabolism in cultured human skin fibroblasts

Incubation of cultured normal human skin fibroblasts with benzo[a]pyrene (B[a]P) resulted in covalent binding of B[a]P to protein, RNA and DNA. B[a]P adduct formation was increased when the cultured cells were also treated with the 9,000 g hepatic microsomal supernatant (S-9) fraction from rats pretreated with Aroclor 1254, a commercial preparation of polychlorinated biphenyls (PCBs). Comparison with control experiments which included the incubation with corn oil-induced S-9 fraction indicated that the enhanced B[a]P-adduct formation in the host cells was related to the induction of hepatic NADPH-dependent microsomal monooxygenase activity by Aroclor 1254 in the S-9 fraction. Parallel experiments showed that this Aroclor-induced enzymic activity was responsible for B[a]P-mediated DNA damage as measured by unscheduled DNA synthesis (UDS) in the cells. Similarly, administration of rat hepatic S-9 induced by 3-methylcholanthrene (MC) and phenobarbitone (PB) caused B[a]P-mediated DNA damage in cultured human skin fibroblasts.