Effects of chronic exposure to an environmentally relevant mixture of brominated flame retardants on the reproductive and thyroid system in adult male rats

Abstract

Brominated flame retardants (BFRs) are incorporated into a wide variety of consumer products, are readily released into home and work environments, and are present in house dust. Studies using animal models have revealed that exposure to polybrominated diphenyl ethers (PBDEs) may impair adult male reproductive function and thyroid hormone physiology. Such studies have generally characterized the outcome of acute or chronic exposure to a single BFR technical mixture or congener but not the impact of environmentally relevant BFR mixtures. We tested whether exposure to the BFRs found in house dust would have an adverse impact on the adult male rat reproductive system and thyroid function. Adult male Sprague Dawley rats were exposed to a complex BFR mixture composed of three commercial brominated diphenyl ethers (52.1% DE-71, 0.4% DE-79, and 44.2% decaBDE-209) and hexabromocyclododecane (3.3%), formulated to mimic the relative congener levels in house dust. BFRs were delivered in the diet at target doses of 0, 0.02, 0.2, 2, or 20 mg/kg/day for 70 days. Compared with controls, males exposed to the highest dose of BFRs displayed a significant increase in the weights of the kidneys and liver, which was accompanied by induction of CYP1A and CYP2B P450 hepatic drug-metabolizing enzymes. BFR exposure did not affect reproductive organ weights, serum testosterone levels, testicular function, or sperm DNA integrity. The highest dose caused thyroid toxicity as indicated by decreased serum thyroxine (T4) and hypertrophy of the thyroid gland epithelium. At lower doses, the thickness of the thyroid gland epithelium was reduced, but no changes in hormone levels (T4 and thyroid-stimulating hormone) were observed. Thus, exposure to BFRs affected liver and thyroid physiology but not male reproductive parameters.

FIG. 1.

Effects of 70-day BFR exposure on body weights and liver and kidney weights in male rats. (A) Change in body weight during the BFR treatment. Male rats were randomly divided into five treatment groups: 0 (control), 0.02, 0.2, 2, and 20 mg/kg/day. Animals were weighed weekly for 70–72 days and the overall change in body weight during this period is shown. Bars represent the means ± SEM (n = 15 per group). At the end of BFR treatment, (B) liver and (C) kidneys were weighed and normalized to body weight (BW, g/kg BW). Values are expressed as mean ± SEM (n = 15 per group). *p < 0.05 compared with control.

FIG. 2.

Effects of BFR exposure on liver Cyp1a1 mRNA levels. Relative levels of hepatic Cyp1a1 mRNA were determined by qRT-PCR and normalized to both Actb and Hprt, then compared with control levels. Values are expressed as mean ± SEM (n = 9–10 per group). *p < 0.01 compared with control.

FIG. 3.

Effects of BFR exposure on the hepatic activities of drug-metabolizing enzymes. The activities of ethoxy-(EROD) (A), methoxy-(MROD) (B), propyloxy-(PROD) (C), and benzyloxy-(BROD) (D)-resorufin-O-deethylation were measured in hepatic S9 fractions prepared from frozen liver tissues, as described in the text. Activities were corrected for S9 protein levels and are expressed as mean ± SEM (n = 9–10 per group). *p < 0.05 compared with control.

FIG. 4.

Reproductive organ weights and serum testosterone levels in control and BFR-treated males. At the end of treatment, (A) epididymis, (B) testes, (C) empty seminal vesicles, (D) prostate, and (E) vas deferens were weighed. Blood was collected and (F) serum testosterone concentrations were measured by ELISA. Values are expressed as mean ± SEM (n = 8–10 per group).

FIG. 5.

Effects of BFR exposure on circulating (A) thyroxine (T4) and (B) TSH. Serum collected at termination was assayed for both hormones, as described in the text. Values are expressed as mean ± SEM (n = 9–10 per group). *p < 0.05 compared with control.

FIG. 6.

Light micrographs (×200, periodic acid Schiff staining) of the inner follicles of thyroid glands of control (A) and BFR-treated males (B) 0.02, (C) 0.2, (D) 2.0, or (E) 20 mg/kg/day. “c” indicates follicular colloid in the acellular lumen, whereas the arrow indicates the single layer of cuboidal epithelium that surrounds and encloses the follicle. Bar = 50 μm.

FIG. 7.

Effects of BFR exposure on height of the (A) inner and (B) outer thyroid follicular epithelium. Values are expressed as mean ± SEM (n = 9–10 per group). Bars with the same letter superscript are not significantly different.