Background, aim, and scope
2,2-bis(chlorophenyl)-1,1,1-trichloroethane (DDT) metabolites, other than those routinely measured [i.e., 2,2-bis(chlorophenyl)-1,1-dichloroethylene (DDE) and 2,2-bis(chlorophenyl)-1,1-dichloroethane (DDD)], have recently been detected in elevated concentrations not only in the surface water of Teltow Canal, Berlin, but also in sediment samples from Elbe tributaries (e.g., Mulde and Havel/Spree). This was paralleled by recent reports that multiple other metabolites could emerge from the degradation of parent DDT by naturally occurring organisms or by interaction with some heavy metals. Nevertheless, only very few data on the biological activities of these metabolites are available to date. The objective of this communication is to evaluate, for the first time, the cytotoxicity, dioxin-like activity, and estrogenicity of the least-studied DDT metabolites.
Four DDT metabolites, p,p′-2,2-bis(chlorophenyl)-1-chloroethylene (DDMU), p,p′-2,2-bis(chlorophenyl)-1-chloroethane (DDMS), p,p′-2,2-bis(4-ch1oropheny1)acetonitrile (DDCN), and p,p′-2,2-bis(chlorophenyl)acetic acid (DDA), were selected based on their presence in environmental samples in Germany such as in sediments from the Mulde River and Teltow Canal. O,p′-DDT was used as reference in all assays. Cytotoxicity was measured by neutral red retention with the permanent cell line RTG-2 of rainbow trout (Oncorhynchus mykiss). Dioxin-like activity was determined using the 7-ethoxyresorufin-O-deetylase assay. The estrogenic potential was tested in a dot blot/RNAse protection-assay with primary hepatocytes from male rainbow trout (O. mykiss) and in a yeast estrogen screen (YES) assay.
All DDT metabolites tested revealed a clear dose–response relationship for cytotoxicity in RTG-2 cells, but no dioxin-like activities with RTL-W1 cells. The dot blot/RNAse protection-assay demonstrated that the highest non-toxic concentrations of these DDT metabolites (50 μM) had vitellogenin-induction potentials comparable to the positive control (1 nM 17β-estradiol). The estrogenic activities could be ranked as o,p′-DDT > p,p′-DDMS > p,p′-DDMU > p,p′-DDCN. In contrast, p,p′-DDA showed a moderate anti-estrogenic effect. In the YES assay, besides the reference o,p′-DDT, p,p′-DDMS and p,p′-DDMU displayed dose-dependent estrogenic potentials, whereas p,p′-DDCN and p,p′-DDA did not show any estrogenic potential.
The reference toxicant o,p′-DDT displayed a similar spectrum of estrogenic activities similar to 17β-estradiol, however, with a lower potency. Both p,p′-DDMS and p,p′-DDMU were also shown to have dose-dependent estrogenic potentials, which were much lower than the reference o,p′-DDT, in both the vitellogenin and YES bioassays. Interestingly, p,p′-DDA did not show estrogenic activity but rather displayed a tendency towards anti-estrogenic activity by inhibiting the estrogenic effect of 17β-estradiol. The results also showed that the p,p′-metabolites DDMU, DDMS, DDCN, and DDA do not show any dioxin-like activities in RTL-W1 cells, thus resembling the major DDT metabolites DDD and DDE.
All the DDT metabolites tested did not exhibit dioxin-like activities in RTL-W1 cells, but show cytotoxic and estrogenic activities. Based on the results of the in vitro assays used in our study and on the reported concentrations of DDT metabolites in contaminated sediments, such substances could, in the future, pose interference with the normal reproductive and endocrine functions in various organisms exposed to these chemicals. Consequently, there is an urgent need to examine more comprehensively the risk of environmental concentrations of the investigated DDT metabolites using in vivo studies. However, this should be paralleled also by periodic evaluation and monitoring of the current levels of the DDT metabolites in environmental matrices.
Recommendations and perspectives
Our results clearly point out the need to integrate the potential ecotoxicological risks associated with the “neglected” p,p′-DDT metabolites. For instance, these DDT metabolites should be integrated into sediment risk assessment initiatives in contaminated areas. One major challenge would be the identification of baseline data for such risk assessment. Further studies are also warranted to determine possible additive, synergistic, or antagonistic effects that may interfere with the fundamental cytotoxicity and endocrine activities of these metabolites. For a more conclusive assessment of the spectrum of DDT metabolites, additional bioassays are needed to identify potential anti-estrogenic, androgenic, and/or anti-androgenic effects.