Decidual-type prolactin expression by the human myometrium

The human myometrium, in addition to the decidualized endometrium of the late luteal phase and of pregnancy, has been proposed as a second source of uterine PRL, since immunoreactive PRL was found in supernatants from myometrial explant cultures. We demonstrate here that: 1) the human (h) PRL gene is expressed in the myometrium in vivo; 2) myometrial PRL is identical to pituitary hPRL; 3) the encoding transcript differs from pituitary hPRL messenger (m) RNA but is homologous to decidual and IM-9-P3 lymphoid hPRL mRNA; and 4) the expression of myometrial hPRL mRNA is inhibited by progestin. hPRL mRNA was detected in freshly isolated myometrium by Northern blot hybridization and was larger than the pituitary message. Sequence and primer extension analyses revealed that the transcript is identical to pituitary hPRL mRNA downstream of the pituitary cap site and carries an extension of the 5'-untranslated region homologous to that of decidual/IM-9-P3 lymphoid hPRL mRNA. This mRNA species results from alternative transcription initiation and comprises exon 1a of the hPRL gene, which is not transcribed in the pituitary. hPRL mRNA steady state levels and hPRL secretion increased dramatically when myometrial explants were maintained in long term culture. In addition to the 23,000 mol wt form, myometrial explants synthesized a glycosylated hPRL variant (G-hPRL) which was approximately 500 Daltons larger than pituitary G-hPRL but of similar size as lymphoid G-hPRL (26,500). Lactogenic activity of myometrial conditioned medium paralleled that of pituitary hPRL in the Nb2 lymphoma bioassay and was neutralized by the addition of monoclonal antibody to hPRL. hPRL secretion and hPRL mRNA abundance were not affected by estrogen but were markedly reduced by medroxy-progesterone acetate, which was maximally effective at a dose as low as 10(-10) M.