Identification of a major prolactin-regulated protein as 20 alpha-hydroxysteroid dehydrogenase: coordinate regulation of its activity, protein content, and messenger ribonucleic acid expression.

We have previously reported that an abundant 37,000 mol wt protein with a pI of 6.15 (37K) is expressed specifically in the corpus luteum and is markedly inhibited by PRL. To identify the 37K, amino acid sequence analysis of the protein was performed. The 37K protein showed sequence similarity with rabbit 20 alpha-hydroxysteroid dehydrogenase (20 alpha HSD), chlordecone reductase, prostaglandin synthase, and 3 alpha-hydroxysteroid dehydrogenase, which are members of the aldo-keto reductase group of enzymes that catalyze the NADPH-dependent reduction of carbonyl compounds. Comparison of 20 alpha HSD activity with the level of 37K in the corpus luteum throughout pregnancy demonstrated a close correlation between enzyme activity and luteal levels of the protein. Both protein and enzyme activity were low early in pregnancy, reached a nadir between days 5-19, and reappeared abruptly between days 19-21 of pregnancy. To establish that the enzyme activity is intrinsic to the 37K, the protein was purified from sodium dodecyl sulfate-polyacrylamide electrophoresis gels (SDS-PAGE), renatured, and assayed for 20 alpha HSD activity. The renatured protein exhibited substantial 20 alpha HSD activity. As 20 alpha HSD is known to play a major role in the termination of pregnancy in the rat, it was of interest to examine whether the rapid appearance of the 37 K protein at the end of pregnancy is accompanied by the induction of 20 alpha HSD gene expression. Northern blot analysis using a rabbit cDNA for 20 alpha HSD indicated that the pattern of 20 alpha HSD mRNA expression in the corpus luteum closely paralleled the ontogeny of 20 alpha HSD enzyme activity as well as 37K protein levels. Our studies demonstrated that 20 alpha HSD protein and mRNA levels are coordinately regulated, and that the profound inhibitory effect of PRL on 20 alpha HSD activity is apparently due to inhibition of 20 alpha HSD gene expression, leading to the disappearance of the protein from the corpus luteum.