A role for cytochrome P450 17alpha-hydroxylase/c17-20 lyase during shift in steroidogenesis occurring in ovarian follicles prior to oocyte maturation.

Cytochrome P450 17alpha-hydroxylase/c17-20 lyase (P450c17) is regarded as one of the key enzymes involved in the steroidogenic shift that occurs prior to oocyte maturation in teleosts. Role of P450c17 in the shift in steroidogenesis during oocyte maturation is a contentious issue even after identification of a novel type of P450c17 that lacks lyase activity. To understand the role of P450c17 in steroidogenic shift explicitly, a full length cDNA encoding p450c17 from ovary of air-breathing catfish, Clarias gariepinus was cloned. p450c17 transiently expressed in COS-7 cells converted progesterone to androstenedione through 17alpha-hydroxyprogesterone and catfish p450c17 was found to be expressed ubiquitously with relatively higher levels in gonads, brain, kidney and gills. Immunocytochemical analysis revealed the presence of P450C17 in follicular layer of ovarian follicle, interstitial cells and spermatocytes of testis. p450c17 expression and ratio of lyase to hydroxylase was high in preparatory and pre-spawning phases of ovary and low in spawning phase. Expression of p450c17 correlated well with testicular recrudescence with maximum expression in preparatory and spawning phases. Neither protein expression nor lyase/hydroxylase activity changed significantly during hCG-induced oocyte maturation, in vitro and in vivo though mRNA levels increased. These results tend to suggest that the ovarian follicles attains capacity to produce maximum precursor steroid levels before spawning that might contribute to the shift in steroidogenesis.