STEROIDOGENIC ACUTE REGULATORY PROTEIN IN BOVINE CORPORA-LUTEA

Steroidogenic acute regulatory protein (StAR) transfers cholesterol fr om the outer to the inner mitochondrial membrane to initiate steroidog enesis. Our purpose was to determine the tissue distribution of StAR m RNA and the occurrence of StAR gene products in the bovine corpus lute um (CL). Tissues were taken from the slaughterhouse or by ovariectomy of cattle at specific times after estrus or ovulation. StAR mRNA was i dentified by Northern analysis employing a 1.6-kb cDNA mouse StAR prob e, and polyclonal antiserum against mouse StAR was used in Western ana lysis of StAR protein in bovine luteal tissue. The mRNA for cytochrome P450 side-chain cleavage enzyme (P450(scc)) was also evaluated by mea ns of a homologous cDNA probe. Two isoforms of StAR mRNA, approximatel y 2.9 and 1.8 kb, were present in bovine CL, adrenal, theca, and granu losa cells and caruncles and cotyledons. One, or sometimes two, protei n bands were recognized by the mouse StAR antiserum. P450(scc) mRNA co localized 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 th e mid- to late-luteal phase, and disappeared in CL that had regressed. StAR protein concentrations were highly correlated with StAR mRNA thr oughout the estrous cycle (r = 0.93, p < 0.05). P450(scc) mRNA abundan ce did not vary through the luteal phase except for its disappearance in regressed CL. Corpora lutea from intact animals treated with prosta glandin F-2 alpha displayed a 50% decline in StAR mRNA over 12 h while P450(scc) mRNA remained unchanged. At 24 h StAR mRNA was undetectable , while P450(scc) 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 regress ed 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.