In vivo modulation of nonylphenol-induced zonagenesis and vitellogenesis

Zonagenesis and vitellogenesis (eggshell zona radiata protein (Zrp) and vitellogenin (Vtg) production, respectively), are two estrogen-regulated processes in oviparous vertebrates that are crucial for oocyte maturation. Treatment of juvenile Atlantic salmon (Salmo salar) with nonylphenol (NP; 25 mg kg(-1)) alone or in combination with 3,3',4,4'-tetrachlorobiphenyl (TCB; 0.1 mg kg(-1)) resulted in pronounced elevations of plasma eggshell Zrp and Vtg and their respective liver mRNA levels in two separate experiments. TCB treatment alone caused the elevation of CYP1A mRNA, protein and enzyme levels (7-ethoxyresorufin O-deethylase (EROD)). In experiment 3, which also included the time factor, exposure of juvenile salmon to 10 and 25 mg NP per kg in combination with TCB generally resulted in reduced plasma Zrp and Vtg levels, compared with NP treatments alone. In a fourth experiment, juvenile salmon were exposed to different doses of TCB either 2 days before or 2 days after a single dose (25 mg kg(-1)) of NP. Samples were always collected 5 days after the NP exposure and analyzed for mRNA and protein levels. Generally, TCB doses given 2 days after NP exposure resulted in the elevation of Vtg and Zrp protein and mRNA levels. Vtg and Zrp mRNA levels were also elevated in the groups treated with 0.1 mg TCB 2 days before NP exposure. In all experiments, TCB injection resulted in the induction of liver CYP1A mRNA, CYP1A protein and EROD activity, but no Zrp or Vtg protein/mRNA inductions were observed when given alone. The present study documents for the first time the apparent stimulation of xenoestrogen-induced responses by an antiestrogenic CYP1A-inducer, in fish or any other lower vertebrate. However, the stimulatory or inhibitory effect of TCB on NP-induced responses appear to be dependent on the ratio of NP and TCB doses, and temporal sequence of exposure. Fish hepatic zonagenesis and vitellogenesis continue to provide interesting models for further studies on the mechanisms and possible interactions between endocrine disruptors and CYP1A-inducers, their antiestrogenic and/or estrogen potentiating effects.