SYNTHESIS AND MESSENGER-RIBONUCLEIC-ACID EXPRESSION OF APOLIPOPROTEIN-E AND A-I BY THE BOVINE CORPUS-LUTEUM DURING THE ESTROUS-CYCLE AND PREGNANCY

In an attempt to characterize proteins secreted by the corpus luteum, explant cultures of luteal slices from cows taken on Days 3, 7, 11, 14 , 17, and 19 of the estrous cycle, and Days 17, 88, 180, and > 240 of pregnancy were incubated with H-3-leucine for 24 h. Proteins in luteal -conditioned medium were separated by two-dimensional PAGE, transferre d to polyvinylidene fluoride membrane, and subjected to N-terminal ami no acid microsequencing. Microsequence analysis revealed that the bovi ne corpus luteum synthesized and released de novo synthesized apolipop roteins (Apo) E and A-I in culture during the estrous cycle and pregna ncy. Release of Apo E was observed only on Day 3 of the estrous cycle. Release of Apo A-I was observed on Days 11, 14, 17, and 19 of the est rous cycle, and on all days of pregnancy examined. To demonstrate the presence of the appropriate mRNA and characterize the temporal relatio nship for these identified proteins, total RNA was isolated from corpo ra lutea on Days 2, 3, 7, 16, 17, and 20 of the estrous cycle, and on Days 17, 90, 170, 180, and 272 of pregnancy, and submitted to Northern and dot blot analysis. Apo E mRNA was expressed only on Days 2-3 of t he estrous cycle and was not expressed on the other days of the cycle or during pregnancy. A single Apo E mRNA transcript about 1.0 kilobase (kb) in size was observed. Expression of Apo A-I mRNA was detected on all days of the estrous cycle and pregnancy examined. Apo A-I cDNA hy bridized with a single mRNA transcript about 1.0 kb in size. Apo A-I m RNA levels did not differ among days of the estrous cycle, although hi gher levels of Apo A-I mRNA were observed during later stages of pregn ancy. Serum concentrations of Apo A-I and progesterone were correlated across the estrous cycle but not during the prepartum period or after parturition. This study demonstrates for the first time that the corp us luteum synthesizes Apo E and Apo A-I and expresses their respective mRNAs. The pattern of expression of Apo E and Apo A-I mRNAs parallele d that of de novo synthesis of their respective proteins after incubat ion of luteal tissue with [H-3]leucine. The role of luteal apolipoprot eins may involve an autocrine/paracrine function influencing luteal de velopment, tissue remodeling, and steroidogenesis.