Pesticides and breast cancer: fact or fad?

Breast cancer incidence has risen steadily over most of the last 50 years. Multiple explanations have been invoked for this rise and include increased mammographic screening, an aging population, and changes in the prevalence of accepted risk factors associated with prolonged exposure to unopposed estrogen (e.g., delayed childbearing and earlier menarche). However, no explanation is more contentious than the putative role of weakly estrogenic environmental contaminants such as the pesticide DDT and the industrial products polychlorinated biphenyls (PCBs). Although widespread use of both types of compounds effectively ceased in the United States during the 1970s, their ability to accumulate and to persist in adipose tissues has raised fears about their contribution to breast cancer development in recent years. This has not been an easy area to study. One need only recall two conflicting reports (1,2) on the association between breast cancer risk and blood levels of organochlorines published in this Journal. Wolff et al. (1) initially reported higher levels of the DDT metabolite DDE but not of PCBs in serum specimens taken from women around the time of breast cancer diagnosis than in serum specimens taken from matched, unaffected control subjects, whereas Krieger et al. (2) were unable to document any relationship between DDE or PCB levels in serum specimens obtained some years before breast cancer diagnosis and subsequent breast cancer development. A 1994 meta-analysis of available data (3) appears to confirm the lack of an association. However, the small number of women (fewer than 750) in the aggregated studies, the uncertain value of serum organochlorine levels as a surrogate marker for tissue levels, and the unanswered question of the appropriate interval between serum or tissue sampling and breast cancer development leave this an open area of research (3). Similar controversies abound in the laboratory. A report (4) that combinations of environmental chemicals, including PCBs, can synergistically activate the human estrogen receptor in a yeast reporter system could not be confirmed (5) and was recently withdrawn (6). The relevance of some findings in rodent systems to humans was challenged when species differences were documented in the ability of certain DDT isomers and metabolites to bind competitively to the estrogen receptor (7). In this issue of the Journal, Shekhar et al. (8) enter the fray with an analysis of the ability of members of the DDT and PCB families, alone and in combination, to affect estrogen receptormediated events in preneoplastic MCF10AT and malignant MCF-7 human mammary epithelial cells. They suggest that both o,p8-DDT and p,p8-DDT can stimulate cell growth and can increase expression of a reporter construct containing an estrogen response element as well as of the endogenous estrogenresponsive pS2 gene. Shekhar et al. found that p,p8-DDT was more effective than o,p8-DDT in eliciting these responses, even though o,p8-DDT was more active in competitive binding assays for the human estrogen receptor. Most importantly, the potential for interaction between different organochlorines was suggested by their studies showing that equimolar mixtures of the two DDT isomers were more effective than either isomer used alone at concentrations some 10to 100-fold higher. Blocking studies with the antiestrogen ICI 182,780 as well as negative results using similar reporter assays in estrogen receptor-negative MCF10A cells (the latter data not shown) were said to imply that interaction between organochlorines and the estrogen receptor is a critical event (8). The strength of the analysis by Shekhar et al. (8) lies in its use of a relevant model system—both premalignant and malignant mammary cells of human origin. However, their conclusions about organochlorine regulation of estrogen receptor-mediated cellular responses must be tempered somewhat in the face of the data as presented. First, it is surprising that p,p8-DDT, the more effective DDT isomer in the functional assays, had limited ability to compete with tritiated 17b-estradiol for estrogen receptor binding. It is noteworthy that a separate study by Chen et al. (7) documented that p,p8-DDT and o,p8-DDT bound equally well to recombinant human estrogen receptor and activated to a similar extent a yeast cell human estrogen receptor-reporter system. However, in MCF-7 cells, o,p8-DDT was a more effective inducer than p,p8-DDT of another endogenous estrogenresponsive gene, the progesterone receptor gene. Taken together, these findings raise the possibility that the effects of organochlorines are mediated, at least in part, through pathways other than the estrogen receptor pathway. Second, the strategy of using the pure antiestrogen ICI 182,780 at 100-fold molar excess could be flawed; it seems unlikely that this drug at concentrations of 100 mM to 1 mM will function solely as an estrogen receptor antagonist. Rather, nonspecific and unexpected effects