Expression of the growth hormone-releasing hormone gene and its peptide product in the rat ovary.

Abstract

GH-releasing hormone (GHRH) is a potent cAMP-mediated agonist in the rat ovary, where it binds to a common vasoactive intestinal peptide/GHRH receptor and enhances the actions of FSH on granulosa cell maturation. A GHRH-like peptide has been detected by immunocytochemistry in the human ovary and by RIA in follicular fluid, suggesting local synthesis of the peptide. In rat ovarian poly(A)+ RNA, Northern blot hybridization analysis with a 32P-labeled 48-nucleotide (nt) rat GHRH oligonucleotide probe revealed the presence of one major and two minor mRNA species. The major ovarian GHRH mRNA (1750 nt) was much larger than that present in hypothalamus and placenta (750 nt), but was similar to that observed in the rat testis. Two well defined higher mol wt forms of 3.2 and 3.6 kilobases were also present and probably represent unprocessed precursors of the 1750-nt mRNA. Further evidence of GHRH gene expression in the ovary and testis was provided by reverse transcription polymerase chain reaction of ovarian mRNA and restriction enzyme analysis of the amplified product. In addition, immunoreactive (ir) GHRH was detected in ovarian extracts and in the incubation medium of cultured rat granulosa cells. Ovaries from PMSG-treated female rats, aged 22-27 days, contained 400 +/- 25 pg/g ir-GHRH. The GHRH content of the hypothalamus of the same animals was 2.9 +/- 0.1 ng/g. Cultured rat granulosa cells released 20 +/- 0.1 pg ir-GHRH/4 x 10(5) cells.3 h into the incubation medium. The GHRH immunoreactivity detected in ovarian extracts coeluted on gel filtration chromatography with authentic rGHRH (5.2 kilodaltons). A larger form of ir-GHRH (approximately 16.5 kilodaltons) was also present. These data demonstrate that the rat ovary contains a 1750-nt transcript that could arise from the GHRH gene by tissue-specific initiation, alternative splicing, or transcript termination. The translation product of this mRNA is the same similar size as the rat hypothalamic neuropeptide and may promote follicular maturation by autocrine or paracrine modulation of the stimulatory action of FSH on granulosa cell function.