Effects of 17β-estradiol on levels and distribution of metallothionein and zinc in squirrelfish

Females of the squirrelfish family (Holocentridae) accumulate higher levels of zinc in the liver than any other known animal. This zinc accumulation is made possible by high expression of the zinc-binding protein, metallothionein (MT). In the present study, the squirrelfish (Holocentrus ascensionis) MT cDNA was cloned and sequenced. The deduced amino acid sequence was very similar to other teleost MT. The role of estrogens on zinc metabolism was investigated by injecting male and immature female squirrelfish with 17β-estradiol (E₂). E₂ treatment triggered transient increases in plasma zinc and vitellogenin (VTG) levels, and both of these variables showed very similar time courses. These results suggest that E₂ is responsible for the large hepatoovarian translocation of zinc observed in female squirrelfish and that VTG might be a vehicle for zinc. E₂ did not directly alter the levels of zinc or MT mRNA in the liver. However, the hepatic MT protein concentration increased differentially in the nuclear fraction. Thus E₂ is probably responsible for the association of MT with the nuclear fraction previously observed in untreated mature female squirrelfish.