PROLACTIN-MEDIATED INHIBITION OF 20-ALPHA-HYDROXYSTEROID DEHYDROGENASE GENE-EXPRESSION AND THE TYROSINE KINASE SYSTEM

The rat luteal 2O alpha-hydroxysteroid dehydrogenase plays a key role at catabolizing progesterone and at decreasing the level of this stero id secreted by the ovaries. Throughout pregnancy and before parturitio n neither the mRNA nor the protein for this enzyme could be detected. In this investigation we set to examine whether PRL and PRL-like hormo ne from placental origin silence the expression of this gene and wheth er PRL action involves tyrosine kinase activity and/or de novo protein synthesis. The results revealed that PRL and PRL-like hormone from ra t placental origin (rPL-1 and rPL-2), but not rat growth hormone, caus ed a rapid and profound inhibition of 2O alpha-HSD mRNA expression in highly luteinized granulosa cells. Immunoprecipition and western blot analysis indicate that PRL-R associates with JAK2 and Stat5, and this association is increased whithin 30 seconds with PRL treatment. Althro ugh both JAK2 and Stat5 were phosphorylated on tyrosine upon PRL treat ment, the PRL mediated inhibition of 2O alpha-HSD was not reversed by either tyrosine kinase inhibitors, AG18 and genistein, but was largely reversed by the protein synthesis inhibitor cycloheximide, In summary , results of this investigation indicate that although PRL can activat e the JAK2/Stat5 system in the corpus luteum, the down regulation of 2 O alpha-HSD mRNA by PRL does not appear to involve tyrosine kinase act ivity but depends on de novo synthesis of protein(s). (C) 1997 Academi c Press.