Calcitonin is expressed in gonadotropes of the anterior pituitary gland: its possible role in paracrine regulation of lactotrope function

Crosstalk within a brain-breast-bone axis regulates mineral and skeletal metabolism during lactation

To support the increased calcium demands for milk production during lactation, a dramatic and reversible physiological response occurs to alter bone and mineral metabolism. This coordinated process involves a brain-breast-bone axis that integrates hormonal signals that allow for adequate calcium delivery to milk yet also protects the maternal skeletal from excessive bone loss or decreases in bone quality or function. Here, we review the current knowledge on the crosstalk between the hypothalamus, mammary gland, and skeleton during lactation. We discuss the rare entity of pregnancy and lactation associated osteoporosis and consider how the physiology of bone turnover in lactation may impact the pathophysiology of postmenopausal osteoporosis. Further understanding of the regulators of bone loss during lactation, particularly in humans, may provide insights into new therapies for osteoporosis and other diseases of excess bone loss.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

Presentation and management of osteoporosis presenting in association with pregnancy or lactation

In this review, we summarize our current understanding of the pathophysiology of fragility fractures that occur for the first time during pregnancy and lactation, and provide guidance on appropriate investigations and treatment strategies. Most affected women will have had no prior bone density reading, and so the extent of bone loss that may have occurred during pregnancy or lactation is uncertain. During pregnancy, intestinal calcium absorption doubles in order to meet the fetal demand for calcium, but if maternal intake of calcium is insufficient to meet the combined needs of the mother and baby, the maternal skeleton will undergo resorption during the third trimester. During lactation, several hormonal changes, independent of maternal calcium intake, program a 5-10 % loss of trabecular mineral content in order to provide calcium to milk. After weaning the baby, the maternal skeleton is normally restored to its prior mineral content and strength. This physiological bone resorption during reproduction does not normally cause fractures; instead, women who do fracture are more likely to have additional secondary causes of bone loss and fragility. Transient osteoporosis of the hip may affect one or both femoral heads during pregnancy but it involves localized edema and not skeletal resorption. Case reports have described the use of calcitonin, bisphosphonates, strontium ranelate, teriparatide, vertebroplasty, and kyphoplasty to treat post-partum vertebral fractures. However, the need for such treatments is uncertain given that a progressive increase in bone mass subsequently occurs in most women who present with a fracture during pregnancy or lactation.

Calcium and bone metabolism disorders during pregnancy and lactation

Pregnancy and lactation cause a substantial increase in demand for calcium that is met by different maternal adaptations within each period. Intestinal calcium absorption more than doubles during pregnancy, whereas the maternal skeleton resorbs to provide most of the calcium content of breast milk during lactation. These maternal adaptations also affect the presentation, diagnosis, and management of disorders of calcium and bone metabolism. Although some women may experience fragility fractures as a consequence of pregnancy or lactation, for most women, parity and lactation do not affect the long-term risks of low bone density, osteoporosis, or fracture.

Identification of Calcitonin Expression in the Chicken Ovary: Influence of Follicular Maturation and Ovarian Steroids1

Calcitonin (CALCA), a hormone primarily known for its role in calcium homeostasis, has recently been linked to reproduction, specifically as a marker for embryo implantation in the uterus. Although CALCA expression has been documented in several tissues, there has been no report of production of CALCA in the ovary of any vertebrate species. We hypothesized that the Calca gene is expressed in the chicken ovary, and its expression will be altered by follicular maturation or gonadal steroid administration. Using RT-PCR, we detected Calca mRNA and the calcitonin receptor (Calcr) mRNA in the granulosa and theca layers of preovulatory and prehierarchial follicles. Both CALCA and Calca mRNA were localized in granulosa and thecal cells by confocal microscopy. Using quantitative PCR analysis, F1 follicle granulosa layer was found to contain significantly greater Calca mRNA and Calcr mRNA levels compared with those of any other preovulatory or prehierarchial follicle. The granulosa layer contained relatively greater Calca and Calcr mRNA levels compared with the thecal layer in both prehierarchial and preovulatory follicles. Progesterone (P(4)) treatment of sexually immature chickens resulted in a significantly greater abundance of ovarian Calca mRNA, whereas estradiol (E(2)) or P(4) + E(2) treatment significantly reduced ovarian Calca mRNA quantity. Treatment of prehierarchial follicular granulosa cells in vitro with CALCA significantly decreased FSH-stimulated cellular viability. Collectively, our results indicate that follicular maturation and gonadal steroids influence Calca and Calcr gene expression in the chicken ovary. We conclude that ovarian CALCA is possibly involved in regulating follicular maturation in the chicken ovary.

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.

Calcitonin is expressed in the chicken pituitary gland: influence of gonadal steroids and sexual maturation

Calcitonin (CT) is primarily produced by the thyroid C cells in mammals or by the ultimobranchial gland in chickens. CT is also expressed by the pituitary gland in rats in which it functions as a paracrine factor causing decreased lactotroph proliferation and prolactin (PRL) secretion. Gonadal steroids influence CT expression in the rat pituitary gland. However, the expression of the CT gene in the pituitary gland of chickens or of any other avian species has not previously been reported. We have tested the hypotheses that CT is expressed in the chicken pituitary gland, and that its expression is influenced by sexual maturation or in response to ovarian steroid administration. We have detected robust expression of CT cDNA in the chicken pituitary gland by reverse transcription/polymerase chain reaction (PCR). The sequence of the pituitary-derived CT cDNA is identical to that of the ultimobranchial gland. CT-immunoreactive (ir) cells have been observed throughout the anterior pituitary gland by confocal microscopy. Many of the PRL-ir cells show co-localization with CT-ir cells. Quantitative real-time PCR analysis has revealed an inverse relationship between the quantities of PRL mRNA and CT mRNA in the pituitary gland: sexually mature hens contain lower amounts of CT mRNA but larger quantities of PRL mRNA compared with sexually immature chickens. Estradiol and/or progesterone treatment of sexually immature chickens leads to a significant decrease in the quantity of pituitary CT mRNA relative to that in the vehicle-treated chickens. We conclude that pituitary CT plays an important paracrine/autocrine role in the control of lactotroph function and PRL secretion in the chicken.

Calcitonin stimulates multiple stages of angiogenesis by directly acting on endothelial cells

Although a strong correlation between neuroendocrine differentiation and angiogenesis of prostate cancer has been reported, no mechanistic link between the two events has been established. Because neuropeptide calcitonin is secreted by prostate tumors and endothelial cells are known to express calcitonin receptor-like receptor, we examined the potential action of calcitonin on endothelial cells. The presence of calcitonin receptor, calcitonin receptor-like receptor, and receptor activity-modifying proteins in human microvessel endothelial-1 cells was tested by reverse transcriptase-PCR (RT-PCR). The proangiogenic action of calcitonin was examined in several in vitro models of angiogenesis using HMEC-1 cells and also in vivo using dorsal skinfold assays. Calcitonin expression of PC-3M cells was modulated, and its effect on angiogenesis was examined in in vitro as well as in vivo models. The results of RT-PCR and radioligand receptor assays showed the presence of functional calcitonin receptor in HMEC-1 cells. Calcitonin stimulated all phases of angiogenesis through the calcitonin receptor, but its effect on tube morphogenesis by endothelial cells occurred at the concentration of the Kd of calcitonin receptor. Silencing of calcitonin receptor expression in HMEC-1 cells abolished calcitonin-induced tube formation. Vascular endothelial growth factor antibodies attenuated but did not abolish calcitonin-induced tube morphogenesis. PC-3M prostate cancer cells induced angiogenesis in in vivo and in vitro models. Overexpression of calcitonin in PC-3M cells increased their angiogenic activity, whereas the silencing of calcitonin expression abolished it. These results show that prostate tumor-derived calcitonin may play an important role in prostate tumor growth by regulating intratumoral vascularization.

Targeted overexpression of calcitonin in gonadotrophs of transgenic mice leads to chronic hypoprolactinemia

It was previously shown that calcitonin-like pituitary peptide (pit-CT) is synthesized and secreted by gonadotrophs, and pit-CT inhibits PRL gene transcription and lactotroph cell proliferation. Present studies examined long-term consequences of pit-CT overexpression on the functioning of mouse anterior pituitary (AP) gland. Targeted overexpression of pit-CT in gonadotrophs of mouse pituitaries was achieved by generating mice overexpressing bovine luteinizing hormone (LH)-alpha subunit promoter-pit-CT cDNA transgene. Transgenic (pit-CT+) mice displayed chronic but selective overexpression of pit-CT in gonadotrophs. The mice also displayed a dramatic decline in PRL gene expression as assessed by PRL mRNA abundance, PRL immunohistochemistry (IHC) and serum PRL levels. LH secretion in pit-CT+ mice was also reduced, without any change in FSH secretion. Reproductive abnormalities such as prolonged estrous cycles, reduced pregnancy rate, delivery of smaller litters, increased neonatal mortality and deficient lactation were also observed. Administration of PRL during early pregnancy significantly increased the pregnancy rate and neonatal survival of newborns. These results demonstrate that overexpression of pit-CT leads to chronic hypoprolactinemia and reproductive dysfunction in female mice, and reinforces the possibility that gonadotroph-derived pit-CT is an important paracrine regulator of lactotroph function.

Calcitonin increases invasiveness of prostate cancer cells: Role for cyclic AMP-dependent protein kinase A in calcitonin action

Calcitonin (CT) is synthesized and secreted in prostate epithelium, and its secretion from malignant prostates is several-fold higher than from benign prostates. CT receptor (CTR) is expressed in malignant prostate epithelium, and its activation stimulates growth of prostate cancer (PC) cells via activation of adenylyl cyclase and calcium/phospholipid pathways. To identify the role of "CT System" in prostate cancer, we tested the expression of CT and CTR mRNAs in invading tumor cells of prostate cancer specimens. The effect of CT on in vitro invasion of PC cell lines and on activation of gelatinases was also examined. The cells of primary tumors and those invading stroma co-expressed CT/CTR mRNAs. Exogenously added CT increased in vitro invasion of PC cell lines and caused a rapid, several-fold but transient increase in protein kinase A activity. In contrast, anti-CT serum caused a dose-dependent inhibition of in vitro invasion of PC-3M cells. CT also increased the concentration and activities of MMP-2 and MMP-9. Rp.cAMP, a competitive inhibitor of cAMP-dependent protein kinase A, myristoylated protein kinase A inhibitory peptide (PKI) as well as the expression of dominant negative form of PKA all attenuated basal in vitro invasion of PC-3M cells, and CT could not increase in vitro invasiveness in their presence. These results suggest that overexpression of "CT System" in invasive PC tumors significantly contributes to increased invasiveness of prostate cancer cells. The action of CT may be mediated by protein kinase A signaling, which subsequently leads to increased cell invasion and secretion of gelatinases.

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.

Molecular cloning and expression of equine calcitonin, calcitonin gene-related peptide-I, and calcitonin gene-related peptide-II

In this study, we describe the cloning and tissue expression of equine calcitonin (CT), calcitonin-gene related peptide (CGRP)-I, and CGRP-II cDNA. We also describe a novel divergent form of CGRP (CGRP-I). Equine CT has greatest homology (>85%) to human, rat and mouse subgroups of calcitonins. Equine CGRP-I has low homology (<59%) to CGRPs of other species. The signal and N-terminal peptides for equine CT and CGRP-I were identical, indicating that these peptides are encoded by a gene equivalent to the human CALC-I gene. Equine CGRP-II has >80% homology to chicken, human, rat, ovine, swine, and bovine CGRPs. The homology between equine CGRP-I and CGRP-II is low (56%). The high homology of equine CGRP-II and the low homology of equine CGRP-I to CGRP in other species were unexpected findings. Northern blot analysis revealed that CT mRNA expression was restricted to the thyroid gland; however, RT-PCR revealed that CT mRNA expression was also present in the pituitary gland and in the liver. CGRP-I and CGRP-II mRNA expression was present in several regions of the nervous system and other tissues of neuroectodermal origin. An unexpected finding was CGRP-I expression in the kidney by both Northern analysis and by RT-PCR. Based on these results, CT gene expression in the horse was not restricted to the thyroid gland, and CT may be important in regulating pituitary cell function. CGRPs are widely expressed in tissues of the central and peripheral nervous system. Information from this study will be valuable to study the role of CT, CGRP-I, and CGRP-II in equine health and disease.

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.

Calcitonin gene expression induced by lipopolysaccharide in the rat pituitary

Procalcitonin (PCT), the precursor protein of calcitonin (CT), has been considered recently as a significant indicator of bacterial infection and sepsis. However, the major source of PCT in sepsis remains unclear. The hypothalamic-pituitary-adrenal axis is activated during sepsis. Moreover, immunoreactive CT (iCT) can be detected in the pituitary. Therefore, we examined the effects of lipopolysaccharide (LPS) administration on CT mRNA expression in the pituitary. After administration of LPS, CT mRNA expression in the pituitary was increased significantly. The increase of CT mRNA was associated with significant elevations of the iCT levels in the serum. These results imply that the pituitary is one of the sources of the serum PCT during sepsis.