Xenopus laevis Ovarian CYP17 Is a Highly Potent Enzyme Expressed Exclusively in Oocytes

Abstract

Progesterone has long been considered the primary mediator of Xenopus oocyte maturation. We have recently shown, however, that androgens, which are equal or more potent promoters of maturation and are present at higher levels in ovulating frogs, may also be playing an important physiologic role in mediating maturation. Here, we examined the role of CYP17, a key enzyme mediating sex steroid synthesis, in Xenopus ovarian androgen production. We found that the 17,20-lyase activities of Xenopus CYP17 exceeded the 17␣-hydroxylase activities in both the ⌬4 and ⌬5 pathways; thus, Xenopus CYP17 rapidly converted pregnenolone and progesterone to dehydroepiandrosterone (DHEA) and androstenedione, respectively. This remarkably robust activity exceeds that of CYP17 from most higher vertebrates, and likely explains why virtually no progesterone is detected in ovulating frogs. Additionally, ovarian CYP17 activity was present exclusively in oocytes, although all other enzymes involved in sex steroid production were expressed almost entirely in surrounding follicular cells. This compartmentalization suggests a "two-cell" model whereby Xenopus ovarian androgen production requires both follicular cells and oocytes themselves. The requirement of oocytes for ovarian androgen production further introduces the unusual paradigm whereby germ cells may be responsible for producing important steroids used to mediate their own maturation.The phenomenon of steroid-induced maturation of Xenopus oocytes has served as a model for studying cell cycle and nongenomic steroid signaling for several decades (1-5). During this time, progesterone has been considered the primary physiologic mediator of oocyte maturation, perhaps through interactions with classical progesterone receptors expressed in the oocyte (6, 7). We have recently shown, however, that testosterone, rather than progesterone, may be the primary physiologic mediator of Xenopus oocyte maturation (8). This dominant role of testosterone is evidenced by the observation that testosterone is a substantially more potent activator of oocyte maturation than progesterone, and that testosterone levels are significantly higher than progesterone (which is nearly undetectable) in the serum and ovaries of ␤-HCG 1 -stimulated frogs. In addition, we have shown that, in vitro, isolated oocytes rapidly convert exogenously added progesterone to the androgen androstenedione (AD), which is an equally potent promoter of maturation when compared with progesterone. These data suggest that both AD and progesterone are likely inducing maturation under in vitro conditions typical for "progesteronemediated" maturation.The high levels of ovarian testosterone in the absence of detectable progesterone imply that Xenopus ovaries are extremely efficient at metabolizing progesterone. Additionally, the ability of isolated oocytes to rapidly convert progesterone to androgens suggests that the oocytes themselves may be important contributors to progesterone metabolism in the ovary. To explain the lack of in vivo progeste...

Aromatase expression in Xenopus oocytes: a three cell-type model for the ovarian estradiol synthesis

“…In order to further investigate whether oocyte contribution to E 2 synthesis is specific to a few vitellogenic species or, in contrast, a mechanism shared by evolutionary distant vertebrates, it would be interesting to perform such studies in other non-mammalian species. The expression in the Xenopus oocyte of both Cyp17 (Yang et al 2003) -the enzyme able to metabolize progestins into androgens -and p450-arom -the enzyme able to aromatize androgens -is consistent with an autonomous production of E 2 by the oocyte. It is also very likely that steroid exchanges exist between oocyte and surrounding somatic follicular compartments, leading to difficulties to access the specific role of E 2 synthesized by one compartment in comparison to the other.…”

“…The regulation of aromatase expression and activity in the germinal compartment of the ovarian follicle during final oocyte maturation would be in favor of an active participation of E 2 from oocyte origin in the intra-follicular dialog. Therefore, our findings and a previous report (Yang et al 2003) clearly indicate that a third participant -the oocyte -has to be added to the two cell-type model of E 2 synthesis in Xenopus. Nevertheless, determining the physiological role of E 2 synthesized by the oocyte in comparison to the role of E 2 of somatic origin is difficult to perform for technical reasons as local inhibition of oocyte aromatase within each compartment of the whole follicle cannot be performed by microinjection or use of aromatase inhibitors.…”

“…Star is involved in cholesterol shuttling to the inner mitochondrial membrane and is thus necessary for androgen and estrogen synthesis. As Cyp17 (Yang et al 2003) and p450-arom are expressed by the oocyte, it suggests that a constant interaction exists between the somatic and the germinal follicular compartments for ovarian steroidogenesis to occur. Therefore, the contribution of the oocyte compartment to the overall follicular aromatase activity appears more important than initially believed.…”

“…Nevertheless, in Xenopus laevis, some steroidogenic activity had been previously reported in oocytes (Yang et al 2003). Besides oocyte ability to metabolize exogenous steroid precursors (Reynhout & Smith 1973, Sanyal et al 1973, Fouchet et al 1975, Thibier-Fouchet et al 1976, 17-hydroxylase/ 17,20-lyase (Cyp17) appeared to be expressed only by the oocyte in Xenopus ovary (Yang et al 2003). Cyp17 is a key enzyme mediating androgen production, suggesting that Xenopus oocytes participate in synthesis of estrogen precursors.…”

Xenopus laevis CYP17 Regulates Androgen Biosynthesis Independent of the Cofactor Cytochrome b5

“…Interestingly, Xenopus ovarian androgen production is dependent upon both ovarian germ and somatic cells, as CYP17 activity is found almost exclusively in oocytes, whereas all other steroidogenic enzymes are expressed in the surrounding follicle cells. Initial characterization of CYP17 activity in frog oocytes revealed remarkably high 17,20-lyase activity in both the ⌬4 and the ⌬5 pathways, with lyase rates at least equaling those of the 17␣-hydroxylase reactions (14). Thus, Xenopus ovaries produced virtually no detectable progesterone, 17-hydroxyprogesterone, or 17-hydroxypregnenolone upon stimulation with gonadotropins, whereas ovarian androgens such as androstenedione and testosterone levels were very high.…”