Toxicity of brominated flame retardants with focus on retinoid system disturbances

Abstract

Background: Brominated flame retardants (BFR) are detected in the environment and biota all over the world. They contribute to the human body burden of industrial chemicals and exposure is mainly via food. Indoor dust contributes substantially in some exposure situations, which involve small children. Regulatory restrictions and bans have been introduced as some BFRs can impact proper development, potentially via the endocrine system.

Objectives: The study aim was to clarify the role of retinoid system disturbances in BFR toxicity. Specific aims were to identify retinoid forms, which are sensitive to BFR exposure, and to identify genes, which can explain observed changes in specific retinoid forms. An additional aim was to evaluate the relevance of using retinoid system endpoints for risk characterization of human BFR exposure.

Methods: Technical decabromodiphenyl ether (decaBDE), hexabromocyclododecane (HBCD), tetrabromobisphenol-A (TBBPA), or pentabromodiphenyl ether (pentaBDE) mixtures, were given orally to male and female Wistar rats at 3-7 different dose-levels. The OECD 28-day repeat dose toxicity study (TG407) and/or the 1-generation toxicity study (TG415) protocols, enhanced to detect endocrine effects were used. Retinoid concentrations and gene expression profiles were determined by HPLC and RT-PCR, respectively. Benchmark doses were established for all endpoints except gene expressions of the decaBDE study. Mode of action and margin of exposure evaluations were performed for decaBDE and HBCD and partly for pentaBDE.

Results and discussion: Reduction of hepatic all-trans RA levels was the most sensitive physiological endpoint after decaBDE exposure, and was connected to increased expressions of Cyp1b1, Cyp26a1, Crabp1 and CYP2B. HBCD exposure resulted in reductions of hepatic 9-cis-4-oxo-13,14-dihydro-retinoic acid, retinyl palmitate, and all-trans RA levels, which were connected to increased expressions of Adh1, Aox1, Aldh1, Cyp26a1, Lrat, Ugt1a1, Ugt1a6, and Ugt1a9. The retinoid system characterization of pentaBDE and TBBPA was explorative, and the obtained results confirm that a full characterization is needed; in particular so for pentaBDE. Several lines of evidence, including gender and compound differences in response, as well as previously published data, provide support for key roles of CYP2B and CYP3A in the observed modulations of the retinoid system suggesting that CAR and/or PXR mediated mechanisms are involved. MOE calculations suggest that the observed retinoid system changes are of relevance to human exposure scenarios.

Conclusion: Retinoid forms analyzed in this thesis were sensitive to BFR-exposure and showed gender and compound specificities in response patterns. Most of the 25 selected target genes were needed to explain the observed changes of the analyzed retinoid forms. Additional gene data from the literature was required to provide a good fit for the mechanistic interpretation. Further research is needed to clarify the exact role of CAR, PXR, AhR and/or AhRR, in BFR-induced modulation of the retinoid system.