Thyroid disrupting effects and associated mechanisms of TCPP in GH3 cell line and zebrafish (Danio rerio) larva and adult

Abstract

Since polybrominated diphenyl ethers (PBDEs) have been phased out due to their persistence and potential adverse effect, uses of tris(1-chloro-2-propyl) phosphate (TCPP) have been increased as an alternative flame retardant. TCPP, one of the most frequently detected organophosphate flame retardants (OPFRs) in various environments and biota, has been regarded as an important contaminant because of its high potency of human exposure. However, information on toxicological effects of TCPP is limited, so far. This study was conducted to investigate the adverse effects of TCPP on the thyroid endocrine system in zebrafish larvae and adult, and the underlying mechanisms related hypothalamic-pituitary-thyroid (HPT) axis were determined using a rat pituitary (GH3) cell line. In the GH3 cell, down-regulated expression of Type 2 deiodinase (dio2) following TCPP exposure was verified, which was not the same manner of T3. In zebrafish larvae at 120 hours post-fertilization (hpf), significant increased T4 concentration was observed following 3.16 or 10 mg/L TCPP exposure. Significantly decreased T3/T4 ratio was also observed. TCPP exposure significantly up-regulated the gene expressions involved in thyroid hormone synthesis (tshβ, tshr, trα and trβ) and metabolism/elimination (dio1 and ugt1ab). Increasing whole-body thyroid level following TCPP exposure can be at least part of theses altered gene expressions. In addition, thyroid hormone metabolism led to be up-regulated as a response to maintain their hormone homeostasis. In adult, however, the thyroid hormone levels were not altered following TCPP exposure for 14 days. This can be interpreted that TCPP exposure is more sensitive to disrupt thyroid hormone regulation in zebrafish at the early developmental stage than in adult. These results showed that TCPP exposure could disrupt the thyroid hormone level in zebrafish at early developmental stage through the HPT axis. Potential adverse effects on thyroid hormone disruption of TCPP should be considered for better understanding of alternative flame retardants. This study will be important for determining which flame retardants are safer alternatives for PBDEs.