Prolactin control of receptor for estradiol in corpora lutea of pregnant rats

PRL-induced ERalpha gene expression is mediated by Janus kinase 2 (Jak2) while signal transducer and activator of transcription 5b (Stat5b) phosphorylation involves Jak2 and a second tyrosine kinase

In the rat corpus luteum of pregnancy, PRL stimulation of ER expression is a prerequisite for E2 to have any luteotropic effect. Previous work from our laboratory has established that PRL stimulates ERalpha expression at the level of transcription and that the transcription factor Stat5 (signal transducer and activator of transcription 5) mediates this stimulation. Since it is well established that PRL activates Stat5 through the tyrosine kinase, Janus kinase 2 (Jak2), the role of Jak2 in PRL regulation of ERalpha expression was investigated. In primary luteinized granulosa cells, the general tyrosine kinase inhibitors, genistein and AG18, and the Jak2 inhibitor, AG490, prevented PRL stimulation of ERalpha mRNA levels, suggesting that PRL signaling to the ERalpha gene requires Jak2 activity. However, using an antibody that recognizes the tyrosine-phosphorylated forms of both Stat5a and Stat5b (Y694/Y699), it was found that AG490 could inhibit PRL-induced Stat5a phosphorylation only and had little or no effect on Stat5b phosphorylation. These effects of AG490 were confirmed in COS cells overexpressing Stat5b. Also in COS cells, a kinase-negative Jak2 prevented PRL stimulation of ERalpha promoter activity and Stat5b phosphorylation while a constitutively active Jak2 could stimulate both in the absence of PRL. Furthermore, kinase-negative-Jak2, but not AG490, could inhibit Stat5b nuclear translocation and DNA binding. Therefore, it seems that in the presence of AG490, Stat5b remains phosphorylated, is located in the nucleus and capable of binding DNA, but is apparently transcriptionally inactive. These findings suggest that PRL may activate a second tyrosine kinase, other than Jak2, that is capable of phosphorylating Stat5b without inducing transcriptional activity. To investigate whether another signaling pathway is involved, the src kinase inhibitor PP2 and the phosphoinositol-3 kinase inhibitor (PI3K), LY294002, were used. Neither inhibitor alone had any major effect on PRL regulation of ERalpha promoter activity or on PRL-induced Stat5b phosphorylation. However, the combination of AG490 and LY294002 largely prevented PRL-induced Stat5b phosphorylation. These findings indicate that PRL stimulation of ERalpha expression requires Jak2 and also that PRL can induce Stat5b phosphorylation through two tyrosine kinases, Jak2 and one downstream of PI3K. Furthermore, these results suggest that the role of Jak2 in activating Stat5b may be through a mechanism other than simply inducing Stat5b phosphorylation.

The luteotrophic complex in hysterectomized and pregnant goats

The pituitary hormones responsible for the maintenance of progesterone secretion by the corpus luteum have been determined by replacement therapy in hysterectomized-hypophysectomized goats and in goats hypophysectomized on day 40 of pregnancy. Progesterone concentrations were 7.2 +/- 0.81 ng/ml plasma (mean +/- S.E. of mean) in intact goats during the luteal phase of the cycle and were 5.9 +/- 0.56 ng/ml plasma after hysterectomy. After hypophysectomy of hysterectomized goats, treatment with 25 or 150 i.u. prolactin/day failed to maintain the corpora lutea, and progesterone concentrations fell to below 1.0 ng/ml plasma. Treatment with 1.0 mg luteinizing hormone (LH)/day after hypophysectomy maintained progesterone concentrations at 2.5 +/- 0.48 ng/ml and treatment with 1.0 mg LH and 150 i.u. prolactin/day resulted in increased peripheral concentrations of 12.9 +/- 1.66 ng progesterone/ml plasma. In goats hypophysectomized on day 40 of pregnancy treatment with 1.0 mg LH/day or 1.0 mg LH and 150 i.u. prolactin/day resulted in maintenance of the pregnancy and maintenance of high peripheral concentrations of 15.0 +/- 1.77 ng progesterone/ml plasma, with no difference between the two treatments. The results show that LH is the pituitary hormone primarily responsible for the maintenance of progesterone secretion by the corpora lutea in the goat and that prolactin is synergistic with LH in stimulating this function. The difference in the pituitary luteotrophic complex between non-pregnant and pregnant animals may be explained if placental lactogen can substitute for prolactin in the luteotrophic complex of pregnant goats.

Oestrogens regulate divergent effects of prolactin in the ovary

Prolactin exerts well established stimulatory effects both in vitro and in vivo in the rat ovary. However, in the ovary of higher mammals, prolactin action is not well characterised, and may differ from that observed in rodents. For example, in human clinical conditions of physiological or pathological prolactin excess, ovarian function is depressed, and in isolated, human granulosa cells in vitro, prolactin seems to inhibit progesterone production. To examine direct prolactin effects in the higher mammalian ovary, we have used an in vitro porcine granulosa-cell model. In this system, prolactin action is bipotential, depending critically on the degree of granulosa-cell differentiation attained in vivo. Prolactin suppresses steroid production by cultured granulosa cells isolated from small (1-2 mm), immature follicles, but stimulates progesterone secretion by granulosa cells collected from large (greater than 6 mm) mature follicles. The present studies show that oestrogens may play an important part in regulating these divergent actions of prolactin in the ovary.