Rapid in Vitro Metabolism of the Flame Retardant Triphenyl Phosphate and Effects on Cytotoxicity and mRNA Expression in Chicken Embryonic Hepatocytes

The organophosphate flame retardant, triphenyl phosphate (TPHP), has been detected with increasing frequency in environmental samples and its primary metabolite is considered to be diphenyl phosphate (DPHP). Information on the adverse effects of these compounds in avian species is limited. Here, we investigate the effects of TPHP and DPHP on cytotoxicity and mRNA expression, as well as in vitro metabolism of TPHP, by use of a chicken embryonic hepatocyte (CEH) screening assay. After 36 h of exposure, CEH cytotoxicity was observed following exposure to >10 μM TPHP (LC50 = 47 ± 8 μM), whereas no significant cytotoxic effects were observed for DPHP concentrations up to 1000 μM. Using a custom chicken ToxChip polymerase chain reaction (PCR) array, the number of genes altered by 10 μM DPHP (9 out of 27) was greater than that by 10 μM TPHP (4 out of 27). Importantly, 4 of 6 genes associated with lipid/cholesterol metabolism were significantly dysregulated by DPHP, suggesting a potential pathway of importance for DPHP toxicity. Rapid degradation of TPHP was observed in CEH exposed to 10 μM, but the resulting concentration of DPHP accounted for only 17% of the initial TPHP dosing concentration. Monohydroxylated-TPHP (OH-TPHP) and two (OH)2-TPHP isomers were identified in TPHP-exposed CEH, and concentrations of these metabolites increased over 0 to 36 h. Overall, this is the first reported evidence that across 27 toxicologically relevant genes, DPHP altered more transcripts than its precursor, and that TPHP is also metabolized via a hydroxylation pathway in CEH.