Hexabromocyclododecane (HBCD) and polybrominated diphenyl ethers (PBDEs) are additive flame retardants used in a wide range of consumer products. Both compounds have been detected in free-living avian species, but toxicological and molecular end points of exposure are limited. An in vitro approach was used to compare concentration-dependent effects of HBCD and the commercial penta-brominated diphenyl ether mixture DE-71 on cytotoxicity and mRNA expression in cultured hepatocytes derived from embryonic chickens. Neither HBCD-alpha, HBCD-technical mixture (TM), nor DE-71 effected hepatocyte viability at the highest concentrations assessed (30-100 microM). Real-time RT-PCR assays were developed to quantify changes in mRNA abundance of genes associated with chicken xenobiotic-sensing orphan nuclear receptor activation, the thyroid hormone (TH) pathway, and lipid regulation. Exposure to >or= 1 microM HBCD-alpha and HBCD-TM resulted in significant upregulation of cytochrome P450 (CYP) 2H1 (fourfold to sevenfold) and CYP3A37 (5- to 30-fold) at 24 and 36 h. In contrast, 30 microM DE-71 caused a twofold increase of CYP2H1 only. UGT1A9 expression was only upregulated by HBCD-alpha to a maximum of fourfold at >or= 1 microM. Transthyretin, thyroid hormone-responsive spot 14-alpha, and liver fatty acid-binding protein were all significantly downregulated (up to sevenfold) for cells exposed to >or= 1 microM HBCD-alpha and HBCD-TM. DE-71 also downregulated these three target genes twofold to fivefold at concentrations >or= 3 microM. Taken together, our results indicate that xenobiotic-metabolizing enzymes and genes associated with the TH pathway and lipid regulation are vulnerable to HBCD and DE-71 administration in cultured avian hepatocytes and might be useful molecular markers of exposure.