Calcitonin inhibits prolactin gene transcription in rat pituitary cells

A synthetic peptide derived from mouse pituitary calcitonin cDNA sequence exhibits potent inhibition of prolactin secretion and prolactin mRNA abundance in primary mouse pituitary cells

We have shown that gonadotrophs synthesize and secrete immunoreactive calcitonin (CT)-like peptide, and CT is a potent inhibitor of prolactin (PRL) secretion and gene transcription. CT cDNA cloned from LssT2 cells (pit-CT cDNA) exhibits 99% homology with mouse CT cDNA sequence, but exhibits four mismatches in the coding region of CT peptide (347-485 bp) with consequent changes in the amino acids at positions 5 and 17 of mouse CT. We have synthesized a putative 23 amino acid pit-CT peptide based on pit-CT cDNA sequence, and tested its effect on PRL secretion and mRNA abundance in primary mouse pituitary cells. The results suggest that synthetic pit-CT attenuates PRL mRNA abundance and inhibits PRL release from mouse anterior pituitary cells. Moreover, pit-CT is remarkably more potent than salmon (S)CT in attenuating PRL mRNA abundance. These results raise a possibility that this endogenous pituitary peptide may potentially serve as a therapeutic molecule for the treatment of prolactinomas.

Antiproliferative action of calcitonin on lactotrophs of the rat anterior pituitary gland: evidence for the involvement of transforming growth factor beta 1 in calcitonin action

Calcitonin-like pituitary peptide, which is synthesized and secreted by gonadotrophs of the rat anterior pituitary (AP) gland, is a potent inhibitor of prolactin biosynthesis and lactotroph cell proliferation. Because TGF-beta 1 is an autocrine inhibitor of lactotroph cell proliferation, we investigated a possibility that calcitonin (CT) interacts with TGF-beta 1 to inhibit lactotroph cell proliferation. The actions of CT on GGH3 cell proliferation were examined in the absence or presence of anti-TGF-beta 1 serum. Subsequent experiments tested the effects of CT on TGF-beta 1 mRNA abundance as well as TGF-beta 1 synthesis. The studies also tested whether the stimulatory action of CT on TGF-beta 1 mRNA expression involves stabilization of TGF-beta 1 mRNA. Finally, the experiments investigated in vivo actions of CT on TGF-beta 1 synthesis in the AP gland. This was accomplished by studying the changes induced by i.v. administered CT in TGF-beta 1-immunopositive cell populations of adult female rat AP glands. The results have shown that the inhibitory action of CT on proliferation of GGH3 cells was attenuated by rabbit anti-TGF-beta 1 serum. Moreover, CT stimulated TGF-beta 1 mRNA expression, as well as TGF-beta 1 synthesis, in a dose-dependent fashion. Stimulatory action of CT on TGF-beta 1 expression may be posttranscriptional, because it significantly increased TGF-beta 1 mRNA stability. When administered in vivo, CT significantly increased TGF-beta 1-immunopositive cell populations of adult female rat AP gland. Colocalization studies for prolactin and TGF-beta 1 suggest that CT increased TGF-beta 1 synthesis in lactotrophs, and possibly in nonlactotroph cell populations. These results suggest that antiproliferative action of CT on lactotrophs may, at least in part, be mediated by CT-induced TGF-beta 1 expression.

Calcitonin expression in rat anterior pituitary gland is regulated by ovarian steroid hormones

Gonadotroph-derived calcitonin-like peptide (pit-CT) is a potent inhibitor of lactotroph function. We investigated the effect of ovarian hormones on pit-CT mRNA expression in the anterior pituitary (AP) gland of cycling female rats. Levels of mRNAs for pit-CT, CT receptor, prolactin (PRL), and beta-LH during 4-d estrous cycle were determined. In a second study, the effects of estrogens and progesterone on pit-CT and PRL mRNA levels were investigated. In a third group, the effect of estrogen or progesterone depletion on pit-CT mRNA expression was studied. In a fourth group, the effect of passive pit-CT immunization on PRL and LH mRNA expression was examined. Pit-CT mRNA levels varied during estrous cycle. They were highest in diestrus, but lowest in the evening of proestrus. CT-receptor mRNA levels displayed smaller fluctuations. Estrogen repletion caused a decline in pit-CT mRNA expression in ovariectomized rats, but progesterone produced a marked increase. ICI 182,780 prevented the decline of pit-CT mRNA levels during late proestrus-estrus, but RU 486 attenuated pit-CT mRNA levels. Passive CT immunization in diestrus altered PRL and LH mRNA expression, and advanced the estrus cycle. These results suggest that pit-CT mRNA expression is regulated by ovarian hormones, and depletion of pit-CT advances their estrous cycle.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Calcitonin inhibits anterior pituitary cell proliferation in the adult female rats

Previous studies have shown that CT-like immunoreactive peptide(s) (pit-CT) is synthesized by the anterior pituitary (AP) gland, and exogenously added salmon(s) CT inhibits PRL release and PRL gene transcription in cultured AP cells. Anti-sCT serum, which immunoreacts with pit-CT, stimulates PRL secretion, suggesting pit-CT is a physiologically relevant PRL-inhibiting hormone. Using proliferating cell nuclear antigen (PCNA) staining and 5-bromo-2'deoxyuridine (BrdU) incorporation into newly replicated DNA, the effect of calcitonin (CT) on cellular proliferation in the rat anterior pituitary gland (AP) was examined. CT significantly attenuated PCNA-immunopositive as well as BrdU-positive AP cell populations in dispersed rat AP cells. A second series of experiments tested the effects of CT on AP cell proliferation in vivo. OVX + E2 rats were injected with 200 microg CT (iv), the rats killed at various time points, and the APs were processed for BrdU staining. CT inhibited BrdU incorporation at all time points up to 15 h after the injection, and this inhibitory effect was reversed at later time points. The effect of CT was concentration dependent, and a maximal inhibition was observed 10 h after the CT injection. Subsequent experiments identified CT-responsive AP cell populations using double immunofluorescence for BrdU and either PRL or FSH. The number of BrdU-labeled lactotropes in the AP gland declined by 74% in the CT-treated rats. Neutralization of endogenous pit-CT by passive immunization with anti-sCT serum caused a 2-fold increase in AP cell proliferation. These results suggest an important role for the endogenous pit-CT in regulation of lactotrope population of the AP gland.

Calcitonin-like peptide containing gonadotrophs are juxtaposed to cup-shaped lactotrophs

Calcitonin (CT) is known to inhibit basal and TRH-stimulated prolactin release in cultured anterior pituitary cells in vitro and pituitary CT-like peptide (pit-CT) is synthesized and released by isolated anterior pituitary cells. However, the specific cell type containing pit-CT has not been identified. To determine this, double label immunohistochemistry was performed on pituitary sections from male rats using antisera for specific marker peptides of gonadotrophs, thyrotrophs, lactotrophs, somatotrophs, corticotrophs, and folliculo-stellate cells. CT was only colocalized with gonadotroph-specific markers and the distribution of pit-CT immunoreactive (IR) cells followed the patterns of gonadotroph distribution in male and female rats. Double and triple label immunohistochemistry using antiserum for CT, FSH, and PRL showed an apposition of calcitonin-like peptide containing gonadotrophs to cup-shaped lactotrophs. To examine whether pit-CT IR was altered, similarly to gonadotrophs, with known changes in PRL serum levels, studies were extended to ovariectomized, pregnant, and lactating rats. The area covered by pit-CT immunoreactivity and the tissue content of pit-CT significantly differed between physiological states and the pit-CT level was inversely related to the known PRL status. Pit-CT containing gonadotrophs were in all cases apposed to cup-shaped lactotrophs. These results provide histological support for previous studies proposing that pit-CT serves as a paracrine inhibitor of PRL release.