Mixtures of four organochlorines enhance human breast cancer cell proliferation

In view of the large differences between the concentrations of estrogenic c hemicals needed to elicit effects in in vitro assays and their levels in hu man tissues, it is hard to explain possible health risks in terms of exposu re to individual compounds. Human populations, however, are exposed to mixt ures of estrogenic and estrogen-like agents and it is necessary to consider the impact of combined effects. We assessed the combined effects of 1-(o-c hlorophenyl)-1-(p-chlorophenyl)-2,2,2-trichloroethane (o,p'-DDT), 2,2-bis(p -chlorophenyl)-1,1-dichloroethylene (p,p'-DDE), beta -hexachlorocyclohexane (beta -HCH), and 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (p,p'-DDT) on the induction of cell proliferation in MCF-7 cells. All four compounds a re persistent organochlorines that carl be found in human tissues. We perfo rmed extensive concentration-response analyses with the single agents to pr edict the effects of two mixtures of all four compounds with different mixt ure ratios. We calculated the predictions by using the pharmacologically we ll-founded models of concentration addition and independent action and then tested them experimentally. o,p'-DDT, p,p'-DDE, beta -HCH, and p,p'-DDT ac ted together to produce proliferative effects in MCF-7 cells, The combined effect of the four agents could be predicted on the basis of data about sin gle agent concentration-response relationships. Regression analysis demonst rated that there were combination effects even when each mixture component was present at levels at or below its individual no-observed-effect-concent ration. We assessed combination effects in two ways: First, evaluations in relation to the proliferative responses induced by single mixture component s revealed that the combination effects were stronger than the effects of t he most potent constituent. Thus, according to this method of evaluation, t he combined effects may be termed synergistic. Second, comparisons with the expected effects, as predicted by concentration addition and independent a ction, showed excellent agreement between prediction and observation. With this approach, the combined effect of all four compounds can be termed addi tive.