In teleosts, estradiol-17β (E2) is an important hormone responsible for oocyte development. To elucidate the molecular mechanisms underlying E2 biosynthesis, we characterized the structure of red seabream (Pagrus major) cytochrome P450 aromatase (P450arom) that is directly involved in E2 biosynthesis and found changes in mRNA levels of P450arom during oocyte development induced by implantation of gonadotropin-releasing hormone analogue. A cDNA clone encoding P450arom is 1779 base pairs in length and encodes a protein of 519 amino acids in length, with a calculated molecular weight of 58.9 kDa. Northern blot analysis showed that P450arom mRNA levels increased gradually from Day 8, when oocytes reached the secondary yolk globule stage, and were maintained at high levels at the day of spawning (Day 15). The P450arom mRNA levels increased in association with an increase of the gonadosomatic index (gonad weight/body weight × 100%), serum E2, and P450arom enzyme activity (in vitro conversion of testosterone to E2 in the ovarian fragments). Furthermore, an increase in mRNA levels of the LHβ, but not FSHβ, correlated with increased P450arom mRNA levels during the course of ovarian development. In addition, the levels of P450arom mRNA increased in isolated ovarian follicles during the course of vitellogenic oocyte growth and became undetectable in follicles at the migratory nucleus and the mature stages. These findings, together with those of the previous studies, suggest that LH, not FSH, may regulate E2 biosynthesis via increased levels of P450arom mRNA during oocyte development of red seabream.
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