THE DIFFERENT FORMS OF THE PROLACTIN RECEPTOR IN THE RAT CORPUS-LUTEUM - DEVELOPMENTAL EXPRESSION AND HORMONAL-REGULATION IN PREGNANCY

The corpora lutea of pregnancy in the rat are highly dependent on the action of PRL and PRL-like hormones to hypertrophy and to produce prog esterone needed for the maintenance of gestation. Two forms of the PRL receptor (PRL-R),designated as long (PRL-R-L) and short (PRL-R-S), ha ve been described in rat tissues. To determine whether both forms are present in the corpus luteum during pregnancy and to examine the devel opmental and hormonal regulation of their expression, total RNA isolat ed fi om corpora lutea at different stages of pregnancy and from highl y luteinized granulosa cells subjected to different hormonal treatment s were analyzed by semiquantitative RT-PCR. Immunoblotting of luteal p roteins from early and late pregnancy was also performed to determine if the pattern of PRL-R proteins follows that of PRL-R messenger RNA ( mRNA) expression. In addition, the correlation between the well charac terized PRL-regulated gene, 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD), and PRL-R gene expression was investigated during the time of luteolysis. Both PBL-R-L and PRL-R-S mRNA and protein were express ed in corpora lutea of pregnancy, with the long form being the most do minant at all stages. Whereas no changes in mRNA level of either PRL-R -L or PRL-R-S were found until day 20 of gestation, a profound decline in PRL-R mRNA and protein for both receptor types occurred at the end of pregnancy. This drop in PRL-R expression was accompanied by a shar p and abrupt expression of 20 alpha-HSD mRNA. Studies per formed in vi vo and in luteinized cells in culture indicate that PRL can up-regulat e the expression of the PRL-R-L mRNA, an effect prevented by the tyros ine kinase inhibitor, genistein. PRL-R-L mRNA was also selectively inc reased by cAMP. In summary, the results of this investigation have est ablished that: 1) the corpus luteum of pregnancy expresses both the sh ort and long forms of the PRL-R with the long form being more abundant ; 2) the mRNA for both forms of the PRL-R remains at constant levels t hroughout pregnancy but drops before parturition; 3) the decline in PR L-R mRNA at the end of pregnancy is accompanied by a dramatic rise in 20 alpha-HSD; 4) PRL is able to increase the expression of PRL-R mRNA; and that 5) both A kinase and tyrosine kinase mediated pathways appea r to participate in the up-regulatory mechanism involved in PRL-R mRNA expression.