Regulation of expression of the genes encoding steroidogenic enzymes

In recent years it has become apparent that tropic hormones involved in steroidogenesis act to regualte the expression of the enzymes involved in the various steroidogenic pathways. This is particularly evident in the ovary where the episodic secretion of steroids throughout the ovarian cycle is regulated largely by changes in the levels of the particular enzymes involved in each step of the steroid biosynthetic pathways. Recently, the genes for the various cytochrome P450 species involved in ovarian steroidogenesis, namely cholesterol side-chain cleavage P450 (P450_SCC), 17α-hydroxylase P450 (P450_17α), and aromatase cytochrome P450 (P450_AROM) have been isolated and characterized, making it possible to study the regulation of expression at the molecular level. To this end, a series of chimeric constructs have been prepared in which fragments of the 5′-untranslated region of bovine P450_17α and P450_SCC have been inserted upstream of the chloramphenicol acetyl transferase (CAT) and β-globin reporter genes. These constructs have been used to transfect primary cultures of bovine luteal and thecal cells. The results indicate that cAMP responsiveness lies within defined regions of genes which do not contain a classical CRE, similar to previous results utilizing adrenal cells in culture. Furthermore, although constructs containing both the P450_17α and P450_SCC 5′-upstream regions are expressed in both luteal and thecal cell cultures, only those containing the P450_SCC sequences are expressed in luteal cells. Studies on the expression of P450_AROM indicate that the promoter which is responsible for its expression in human placenta is not operative in the corpus luteum. Thus estrogen biosynthesis may be regulated by the differential use of tissue specific promoters, thus accounting for the complexity and multifactorial nature of the expression of this activity.