Effects of polychlorinated biphenyls on metamorphosis of a marine fish Japanese flounder (Paralichthys olivaceus) in relation to thyroid disruption

Highlights

• Aroclor 1254 retarded metamorphic processes of Paralichthys olivaceus.

• Aroclor 1254 delayed the increase in THs at metamorphic climax of P. olivaceus.

• Inhibition of Aroclor 1254 on metamorphosis was related to lower T₄ and T₃ levels.

• P. olivaceus provides a good model for studying interference of thyroid function.

Abstract

This study examined the influence of environmental concentrations of Aroclor 1254 (10, 100, and 1000 ng/L) on metamorphosis of Paralichthys olivaceus, and analyzed the mechanisms in relation to thyroid disruption. Results showed that 100 and 1000 ng/L Aroclor 1254 delayed metamorphosis and that 1000 ng/L Aroclor 1254 caused abnormal morphology. Thyroxine and triiodothyronine levels in the control group were significantly elevated at metamorphic climax, but treatment with 100 and 1000 ng/L delayed the increase in thyroid hormones (THs) and retarded metamorphic processes. In larvae exposed to 1000 ng/L Aroclor 1254, TH levels at metamorphic climax were significantly lower than those of the control group at the same metamorphic stage. We suggest that the effects of Aroclor 1254 on larval metamorphosis can be explained by disruption of thyroid homeostasis. These findings provide a new perspective and biological model for thyroid-disrupting chemicals (TDCs) screening and investigating interference of thyroid function by TDCs.

Introduction

Thyroid-disrupting chemicals (TDCs) such as planar halogenated aromatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), heavy metals, and steroids have the potential to affect thyroid status (Brown et al., 2004, Rolland, 2000). Administration of ammonium perchlorate, sodium arsenate, potassium-perchlorate and polychlorinated biphenyls (PCBs) to fish influences the morphology of the thyroid gland (Crane et al., 2005, Liu et al., 2008, Schmidt et al., 2012) and can have positive or negative effects on levels of circulating thyroid hormones (THs) (Coimbra et al., 2005, LeRoy et al., 2006, Schnitzler et al., 2011). PCBs are frequently detected in aquatic environments. For example, the PCB content of surface water in the Minjiang River Estuary and Bohai Bay, China, average 985 and 210 ng/L, respectively (Wang et al., 2007, Zhang et al., 2003). The total concentration of PCBs ranged from 2.33 to 44 μg/kg in marine sediments in Barcelona, Spain (Castells et al., 2008), and from 10 to 899 μg/kg in surface sediments of Naples Harbor, Italy (Sprovieri et al., 2007). It has been reported that PCBs can alter thyroid histopathology, combine transthyretin (TTR), deiodinase activity, and metabolism of THs, thereby interfering with TH levels in fish (Adams et al., 2000, Brown et al., 2004, Coimbra et al., 2005, Ishihara et al., 2003, Klaassen and Hood, 2001, Schnitzler et al., 2011).

Several studies in fish have highlighted the important role of TH during flatfish metamorphosis (Inui and Miwa, 1985, Miwa and Inui, 1987, Power et al., 2001, Shao et al., 2017). Exogenous THs, administered by simple immersion protocols to gravid females, larvae, or pre-metamorphic juveniles, can enhance maturation of oocytes, improve larval survival, synchronize metamorphosis, and produce uniform cohorts in flatfish (Gavlik et al., 2002, Schreiber and Specker, 1998, Solbakken et al., 1999). Japanese flounder (Paralichthys olivaceus) were treated with exogenous thiourea (TU), an inhibitor of thyroxine (T₄) synthesis, significantly reduced their T₄ levels and retarded the metamorphosis (Okada et al., 2005). Thus, TDCs that alter larval TH levels may affect larval metamorphosis. It has been confirmed that TDCs can retard metamorphosis and increase rates of offspring deformity in amphibians. Croteau et al. (2009) found that the estrogenic chemical 4-tert-octylphenol caused a significant delay in development starting from Gosner stage 29, by observing that fewer Rana pipiens tadpoles developed past this stage when they had lowered levels of deiodinase type 2 or increased mRNA levels of deiodinase type 3. Balch et al. (2006) found significant inhibition of tail resorption, delayed metamorphosis, and effects on skin pigmentation in Xenopus laevis exposed to polybrominated diphenyl ethers. Exposure of eggs or tadpoles of Bufo boreas, R. pipiens, and Rana clamitans to 73 ng/g 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused minor morphological abnormalities (Jung and Walker, 1997). In contrast, no changes in T₄ levels or obvious inhibition of metamorphosis was observed in X. laevis and Rana temporaria exposed to 200 mg/kg Clophen A50 (Gutleb et al., 2000). Accordingly, the amphibian metamorphosis assay has been adopted for testing of TDCs in Tier 1 testing of the endocrine screening program developed by the United States Environmental Protection Agency (USEPA) (USEPA, 2009); however, it appeared that exposure to chemicals at doses greater than the actual environmental contamination can affect amphibians metamorphosis (Miyata and Ose, 2012). Indeed, early life stage exposure to 3, 3′, 4, 4′, 5-pentachlorobiphenyl (PCB 126) at 1, 3, and 10 ng/L was reported to cause delayed development in common sole (Solea solea) (Foekema et al., 2008). Soffientino et al. (2010) also found 15 ng/g of PCB 126 delayed metamorphic progress and resulted in abnormal gastric gland morphology in larval summer flounder (Paralichthys dentatus). Regarding the definitive evidence that TH plays a major role in initiating metamorphosis in flounder larvae (Inui and Miwa, 1985, Inui et al., 1994), we hypothesized that the thyroid-mediated metamorphosis in this flatfish model would be more suitable for detecting potential TDCs in the aquatic environment.

Metamorphosis is a crucial developmental phase in flatfish species and the transformation from symmetric pelagic larva to asymmetric benthic juveniles most conspicuously involves eye migration and craniofacial remodeling (Klaren et al., 2008). Defects in metamorphosis of flatfish, such as migration of the wrong eye, arrest of metamorphosis, and various pigmentation flaws could result in maldevelopment of juveniles and adults (Bisbal and Bengston, 1993, Ellis et al., 1997, Pittman et al., 1998). Japanese flounder (P. olivaceus) inhabits coastal waters of Asia, has significant economic potential, and is cultured at a commercial scale. Furthermore, P. olivaceus is the first teleost species in which a role for TH in metamorphosis has been demonstrated (Inui and Miwa, 1985), and a recent study also demonstrated the molecular basis of metamorphosis in this flatfish (Shao et al., 2017). Here, we used premetamorphic P. olivaceus larvae in an experimental examination of the influence of environmental concentrations of Aroclor 1254 on metamorphosis (Castells et al., 2008, Wang et al., 2007, Zhang et al., 2003). Since PCBs exposure has been shown to exert thyroid-disrupting properties in teleosts by influencing TH levels (Coimbra et al., 2005, LeRoy et al., 2006, Schnitzler et al., 2011) and cause developmental delay in larval flatfish (Foekema et al., 2008, Soffientino et al., 2010), changes in whole-body T₄ and triiodothyronine (T₃) levels during metamorphosis were determined in P. olivaceus larvae exposed to Aroclor 1254 to analyze the mechanisms behind abnormal metamorphosis. This study provided a new understanding for study of thyroid interference by TDCs in fish and suggested a potential teleost model for testing of TDCs in aquatic environments.