Steroidogenic acute regulatory protein in bovine corpora lutea

Abstract

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 (P450(scc)) 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. P450(scc) 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). P450(scc) mRNA abundance did not vary through the luteal phase except for its disappearance in regressed CL. Corpora lutea from intact animals treated with prostaglandin F(2α) 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 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.