Expression of 3beta-hydroxysteroid dehydrogenase, cytochrome p450 17alpha-hydroxylase/17,20-lyase and cytochrome p450 aromatase enzymes in corpora lutea of diestrous and early pregnant mares.

This research paper explores the mechanism of estrogen production in pregnant mares, particularly examining the role of the equine corpus luteum (CL) and its steroidogenic enzymes. Results reveal that equine chorionic gonadotropin (eCG) causes differential expression of these enzymes which in turn leads to an increased estrogen production, not due to increased aromatase expression but due to increased availability of aromatizable androgen.

Understanding the Hormones and Enzymes

• The focus of the study is to better understand the regulation of steroidogenesis in the equine corpus luteum (CL) during pregnancy. Steroidogenesis refers to the biological process of producing steroid hormones from cholesterol in the body.
• The CL is a temporary endocrine structure involved in ovulation and early pregnancy, secreting progesterone to maintain pregnancy in mammals.
• The hormones and enzymes examined in this research include 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20 lyase (P450(17alpha)), and cytochrome P450 aromatase (P450(arom)) – all essential in the process of steroidogenesis.
• Particularly, the P450(17alpha) enzyme is responsible for converting progesterone into androstenedione, a precursor of estrogen, while P450(arom) catalyses the conversion of androstenedione into estrone, a form of estrogen. 3beta-HSD helps convert pregnenolone to progesterone, and androstenediol to testosterone, contributing to the pool of aromatizable androgens.

Experimental Procedures and Observations

• The team collected luteal tissue samples during different phases – diestrus and early stages of pregnancy. Diestrus refers to the phase in the estrous/menstrual cycle when the corpus luteum is functional.
• They analyzed protein levels corresponding to each enzyme using immunoblotting. A steady-state level refers to the equilibrium state where the concentration of the enzyme/protein remains constant due to a balance between its synthesis and degradation.
• Observations showed that 3beta-HSD were lower in diestrus than pregnancy but did not change during pregnancy. But, P450(17alpha) expression significantly increased following the onset of eCG secretion. However, P450(arom) decreased after the eCG onset.

Key Findings and Implications

• The findings suggest that eCG influences the function of the CL, especially the expression of steroidogenic enzymes. One key outcome is that eCG is associated with increased luteal estrogen production.
• This increased estrogen production was found not to be a result of an increase in aromatase expression, which was originally believed to be the case. Instead, it is due to an increase in availailable aromatizable androgen that results from increased P450(17alpha) expression and activity.
• The understanding of these mechanisms, and the roles of different enzymes and hormones, can greatly contribute to reproductive biology and may have practical applications in equine breeding and pregnancy management.