The Effect of Estrogen on Phosphorylation of Prolactin in the Mouse Pituitary Gland

Abstract

Abstract. Several studies have indicated that prolactin (PRL) assumes oligomeric, proteolytically cleaved, phosphorylated and glycosylated forms. Phosphorylated PRL (PPRL) is considered to be the most important posttranslationally modified form in the rat. In the present study, we examined whether or not PRL is present in the mouse pituitary gland in the phosphorylated form. Mouse pituitary PRL was digested with acid phosphatase, resolved by two-dimensional gel electrophoresis, stained with Coomassie brilliant blue, and then immunoblotted against the anti-PRL, antiphosphoserine and anti-phosphothreonine antibodies. We also examined whether PRL is phosphorylated by protein kinases and semi-quantified the ratios of PPRL to PRL in the pituitary gland. The results indicated that three types of PRL are present in the pituitary glands of both male and female mice. One was non-phosphorylated (isoform 1), and the other two were immunoreactive to anti-phosphoserine (isoform 2) and/or anti-phosphothreonine (isoform 3) antibodies. The ratio between isoforms 2 and 1 of the 30-day-old female mice was higher than that of the 20-day-old female mice. However, the ratios among the three isoforms in the male pituitary glands did not differ with age. The ratio of PPRL to isoform 1 was obviously reduced after ovariectomy (OVX), and it recovered with estrogen replacement. These results suggest that estrogen influences PRL phosphorylation in female mice.

Estrogen Regulates the Serum Level of Phosphorylated Prolactin in Mice

“…However, no one has been able to show the function of serine/ threonine PPRL. We previously identified that serine/threonine PPRL is relatively abundant in the pituitary gland of the male mouse [7]; however, it was not present in male serum in the present study. Thus, serine/threonine PPRL might have a role in storage in secretory granules or an affect on neighboring PRL cells through an autocrine and/or paracrine pathway.…”

“…Sex differences have been observed in regard to the ratio of PPRL to PRL in the mouse pituitary gland [7]. However, the ratio of PPRL in serum has not been determined.…”

“…The ratio between serine PPRL and non-phosphorylated PRL in adult female mice is higher than that of adult male mice; however, the ratio between serine/ threonine PPRL and non-phosphorylated PRL is higher in adult male mice than in adult female mice. Furthermore, we suggested that estrogen might influence serine phosphorylation of PRL [7]. However, it is unclear whether PPRLs were secreted into the blood circulation and which factors, besides estrogen, promote phosphorylation of PRL.…”

“…This phosphorylated PRL (PPRL) antagonizes the growth effect of unmodified PRL in the Nb2 T lymphoma assay [4,6]. In a recent study, we identified three different charge types of PRL in mice [7]. Isoform 1 is non-phosphorylated PRL, while isoforms 2 and 3 are phosphorylated at serine or serine and threonine, respectively.…”

Prolactin Expression in the Cochlea of Aged BALB/c Mice Is Gender Biased and Correlates to Loss of Bone Mineral Density and Hearing Loss

“…In addition, loss of ERß has also been linked to a hearing loss pathology [33]. The relationship between estrogen as an effector of PRL expression has been well documented [34]–[36]. This provides a strong indication that estrogen may play a role in the expression of cochlea PRL, and provides a clue as to why there is a gender bias in its expression.…”