Search

In vivo and in vitro luteinization were investigated in the porcine ovary, with emphasis on expression of steroidogenic acute regulatory protein (StAR). StAR mRNA and protein as well as cytochrome P450 side-chain cleavage mRNA (P450scc) increased during the luteal phase in the corpus luteum (CL) and were absent in regressed CL. Cytochrome P450 aromatase mRNA (P450arom) was not detectable at any time in CL. In vitro luteinization of granulosa cells occurred over 96 h in culture, during which P450arom mRNA was present at 1 h after cell isolation but not detectable at 6 h; and P450scc and StAR mRNAs were first detectable at 6 h and 48 h, respectively. Incubation of cultures with insulin-like growth factor I (IGF-I, 10 ng/ml), dibutyryl cAMP (cAMP, 300 microM), or their combination, induced measurable StAR mRNA at 24 h (p < 0.05), increased progesterone accumulation at 48 h, and elevated both StAR and P450scc expression through 96 h. Incubation of luteinized granulosa cells with epidermal growth factor (EGF, 10 nM) changed their phenotype from epithelioid to fibroblastic, eliminated steady-state StAR expression, and interfered with cAMP induction of StAR mRNA and progesterone accumulation. EGF had little apparent effect on P450scc mRNA abundance. It is concluded that StAR expression characterizes luteinization, and early luteinization is induced by cAMP and IGF-I in vitro. Further, EGF induces a morphological and functional phenotype that appears similar to an earlier stage of granulosa cell function.

Steroidogenic acute regulatory protein (StAR) transfers cholesterol from the outer to the inner mitochondrial membrane to initiate steroidogenesis. Our purpose was to determine the tissue distribution of StAR mRNA and the occurrence of StAR gene products in the bovine corpus luteum (CL). Tissues were taken from the slaughterhouse or by ovariectomy of cattle at specific times after estrus or ovulation. StAR mRNA was identified by Northern analysis employing a 1.6-kb cDNA mouse StAR probe, and polyclonal antiserum against mouse StAR was used in Western analysis of StAR protein in bovine luteal tissue. The mRNA for cytochrome P450 side-chain cleavage enzyme (P450scc) was also evaluated by means of a homologous cDNA probe. Two isoforms of StAR mRNA, approximately 2.9 and 1.8 kb, were present in bovine CL, adrenal, theca, and granulosa cells and caruncles and cotyledons. One, or sometimes two, protein bands were recognized by the mouse StAR antiserum. P450scc mRNA colocalized in all sites where StAR mRNA was found, and in bovine liver. StAR mRNA was low in developing CL, increased 9- to 15-fold during the mid- to late-luteal phase, and disappeared in CL that had regressed. StAR protein concentrations were highly correlated with StAR mRNA throughout the estrous cycle (r = 0.93, p < 0.05). P450scc mRNA abundance did not vary through the luteal phase except for its disappearance in regressed CL. Corpora lutea from intact animals treated with prostaglandin F2 alpha displayed a 50% decline in StAR mRNA over 12 h while P450scc mRNA remained unchanged. At 24 h StAr mRNA was undetectable, while P450scc mRNA had declined to 50% of pretreatment values. We conclude that StAR mRNA and protein are tightly coupled in the bovine CL, being present at low levels during CL development and in elevated concentrations during the midluteal phase, and disappearing in regressed CL within 24 h of prostaglandin-induced luteolysis. We have further shown, for the first time, that StAR mRNA is present in the mammalian placenta.