Secretion of Specific Nonphosphorylated and Phosphorylated Rat Prolactin Isoforms at Different Stages of the Estrous Cycle

Abstract

The biological activity and immunoreactivity of serum prolactin (PRL) has been shown to fluctuate throughout the estrous cycle of the rat. Since the 24-kDa nonphosphorylated and phosphorylated isoforms from several species have also been shown to differ in their biological and immunoreactivities, we have investigated the possibility that the 24-kDa monomer isoform profile varied throughout the estrous cycle of the rat. The PRL isoform profile was assessed in homogenates of pituitaries and in short-term incubation media. Comparisons between homogenates, which always contained isoforms 1, 2, 3, and 3’ (numbered according to increasing acidity), and media showed nonproportional release of the isoforms at all stages. Of great interest were the release of isoform 1 (a nonphosphorylated form) only at estrus and the lack of release of isoform 3’ (a phosphorylated form) only in the afternoon of proestrus. This lack of release of 3’ was accompanied by a marked increase in the release of isoform 2 (the unmodified polypeptide). These results suggest a unique function for isoform 1 during estrus and a role for increased isoform 2 and absent isoform 3’ during the proestrus surge of PRL. Moreover, they suggest that fluctuations in the biological activity and immunoreactivity of serum PRL during the estrous cycle could be due, at least in part, to fluctuations in the isoform profile.

Autocrine and Paracrine Regulation of Prolactin Secretion by Prolactin Variants and by Hypothalamic Hormones

“…Therefore, the present study shows that the lactating rat pituitary produces PRL variants that are absent or deficient in the male pituitary gland and in the PRL Standard, even though the AP of male and of other types of rats, do respond to stimulatory factors released from the anterior pituitary of lactating rats (Mena et al, 2010). The results presented here, together with those in our previous study, also indicate that several PRL variants are produced and released by the lactating rat pituitary (Denef, 1988;Sinha 1992;Asawaroengchai et al, 1978); this hormonal heterogeneity may be physiologically very relevant in the context of autoregulatory mechanisms determining the wide range of PRL effects under different physiological conditions (Schwartz & Cherny, 1992;Schwartz, 2000;Sinha, 1992) and upon different structures (Ben-Jonathan et al, 2001;Lorenson & Walker, 2001;Ho et al, 1993;Celotti et al, 1997).…”

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

“…Ho et al [16] showed that four isoforms (isoforms 1, 2, 3, and 3'; 3' has the most acidity) exist in the rat pituitary. Isoform 1 and 2 are non-phosphorylated PRLs and 3 and 3' are phosphorylated PRL.…”

“…Of all the stages of the estrous cycle, isoform 3 was only absent from mice during proestrus, and the ratios of isoforms 2 and 3 to isoform 1 did not differ during any other stage of estrous. Ho et al [16] previously reported that the rat at proestrus is deficient in one PPRL isoform. Thus, the differences in the ratios of PPRL to nonphosphorylated PRL between male and female mice have become interesting subjects of study.…”

S179D Prolactin Increases Vitamin D Receptor and p21 through Up-regulation of Short 1b Prolactin Receptor in Human Prostate Cancer Cells

“…Short PRLR Up-Regulates VDR and p21/waf1 www.aacrjournals.org serum. The proportion of unmodified to phosphorylated prolactin has been shown to be physiologically regulated in rodent systems where this can be analyzed (51,52). Using human prostate cancer cell lines, we previously showed that the predominant form of prolactin produced by these cell lines is unmodified prolactin because it is able to stimulate Nb2 cell proliferation (17), whereas phosphorylated prolactin is not (50).…”