025. Prolactin signaling through the short form of its cognate receptor causes severe ovarian defect

Abstract

Extensive investigations from our laboratory have clarified the action and interaction of estradiol (E) and prolactin (PRL) on corpus luteum (CL) function. Our research has led us to discover and isolate a CL specific gene that encodes a protein we named PRAP, that associates with the intracellular domain of the short form (PRLRS) but not the long form (PRLRL) and whose expression is tightly regulated by E. Our laboratory and others have established that this protein, expressed in CL of every species investigated, is a novel 17 beta hydroxysteroid dehydrogenase (17bHSD-7) whose function is to catalyze the transformation of estrone to E. Our results with cells expressing only PRLRS revealed that PRL acting through PRLRS leads to phosphorylation of PRAP/17bHSD-7 (PRAP/17b) by JAK2 establishing for the first time that a steroidogenic enzyme can be phosphorylated through its association with a membrane bound protein. The association of PRAP/17b with the PRLRS and its phosphorylation leads to its stabilization. To further investigate the role of PRL signaling through PRLRS, we used PRLR(–/–) mice expressing the PRLRS as a transgene. The results obtained were totally unexpected and of great interest. The follicles of the ovaries, expressing PRLRS only, underwent premature development followed by severe granulosa and oocyte death leaving holes surrounding collapsed zona pellucida and premature ovarian failure. The observations that: (1) the expression of PRLRS in the ovaries of PRL null mice leads to inhibition in Foxo3a and of GALT, two proteins whose deletion/mutation causes similar premature ovarian failure; and (2) that GALT promoter activity is stimulated by Foxo3a transcription factor led us to hypothesize that PRL acting through PRLRS prevents the expression of Foxo3a, which normally stimulates GALT transcriptional activity. Absence of Foxo3a then leads to inhibition of GALT and increases in galactose and its metabolites, causing galactose toxicity and granulosa as well as oocyte cell death.