Parathyroid hormone-related protein: a regulated calcium-mobilizing product of the mammary gland

TRIENNIAL LACTATION SYMPOSIUM/BOLFA: Serotonin and the regulation of calcium transport in dairy cows

The mammary gland regulates maternal metabolism during lactation. Numerous factors within the tissue send signals to shift nutrients to the mammary gland for milk synthesis. Serotonin is a monoamine that has been well documented to regulate several aspects of lactation among species. Maintenance of maternal calcium homeostasis during lactation is a highly evolved process that is elegantly regulated by the interaction of the mammary gland with the bone, gut, and kidney tissues. It is well documented that dietary calcium is insufficient to maintain maternal calcium concentrations during lactation, and mammals must rely on bone resorption to maintain normocalcemia. Our recent work focused on the ability of the mammary gland to function as an accessory parathyroid gland during lactation. It was demonstrated that serotonin acts to stimulate parathyroid hormone-related protein (PTHrP) in the mammary gland during lactation. The main role of mammary-derived PTHrP during mammalian lactation is to stimulate bone resorption to maintain maternal calcium homeostasis during lactation. In addition to regulating PTHrP, it was shown that serotonin appears to directly affect calcium transporters and pumps in the mammary gland. Our current working hypothesis regarding the control of calcium during lactation is as follows: serotonin directly stimulates PTHrP production in the mammary gland through interaction with the sonic hedgehog signaling pathway. Simultaneously, serotonin directly increases calcium movement into the mammary gland and, subsequently, milk. These 2 direct actions of serotonin combine to induce a transient maternal hypocalcemia required to further stimulate PTHrP production and calcium mobilization from bone. Through these 2 routes, serotonin is able to improve maternal calcium concentrations. Furthermore, we have shown that Holstein and Jersey cows appear to regulate calcium in different manners and also respond differently to serotonergic stimulation of the calcium pathway. Our data in rodents and cows indicate that serotonin and calcium are working through a unique feedback loop with PTHrP during lactation to regulate milk calcium and maternal calcium homeostasis.

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.

Renal mechanisms of calcium homeostasis in sheep and goats

In small ruminants, the renal excretion of calcium (Ca) and phosphate (Pi) is not modulated in response to dietary Ca restriction. Although this lack of adaptation was observed in both sheep and goats, differences in renal function between these species cannot be excluded. Recent studies demonstrated that compared with sheep, goats have a greater ability to compensate for challenges to Ca homeostasis, probably due to a more pronounced increase in calcitriol production. Therefore, the aim of the present study was to examine the effect of 1) dietary Ca restriction, 2) administration of calcitriol, and 3) lactation on Ca and Pi transport mechanisms and receptors as well as enzymes involved in vitamin D metabolism in renal tissues of sheep and goats. Whereas RNA expression of renal transient receptor potential vanilloid channel type 5 was unaffected by changes in dietary Ca content, a significant stimulation was observed with administration of calcitriol in both sheep (P < 0.001) and goats (P < 0.01). Calbindin-D28K was downregulated during dietary Ca restriction in goats (P < 0.05). Expression of the sodium/Ca exchanger type 1 was decreased by low Ca intake in sheep (P < 0.05) and upregulated by calcitriol treatment in goats (P < 0.05). A significant reduction in RNA expression of the cytosolic and the basolateral Ca transporting proteins was also demonstrated for lactating goats in comparison to dried-off animals. Species differences were found for vitamin D receptor expression, which was stimulated by calcitriol treatment in sheep (P < 0.01) but not in goats. As expected, expression of 1α-hydroxylase was upregulated by dietary Ca restriction (P < 0.001; P < 0.05) and inhibited by exogenous calcitriol (P < 001; P < 0.05) in both sheep and goats. However, whereas 24-hydroxylase expression was stimulated to the same extent by calcitriol treatment in sheep, irrespective of the diet (P < 0.001), a modulatory effect of dietary Ca supply on 24-hydroxylase induction was observed in goats (P < 0.05). Taken together, our results confirm observations that modulation of renal Ca excretion does not contribute to maintenance of Ca homeostasis in these ruminants during restricted dietary supply, unlike responses in monogastric animals. The interesting species differences related to vitamin D metabolism might explain the greater capacity of goats to compensate for challenges of Ca homeostasis and should be further investigated.

Expression of PTHrP and PTH/PTHrP receptor 1 in the superior cervical ganglia of rats

PTHrP and its receptor PTHR1 are found in the CNS and peripheral nervous system. The presence of PTHrP mRNA has been detected in the superior cervical ganglion (SCG), but there are no data on the cellular distribution of PTHrP and PTHR1 in the SCG. Although it is known that ovarian activity and reproductive status influence sympathetic activity, and the PTHrP/PTHR1 system is influenced by estrogens in different tissues, it is not known whether these factors have a similar effect on expression of PTHrP and PTHR1 in the nervous system. Hence, we investigated the presence and distribution of PTHrP and PTHR1 in neurons and glia of the SCG of rats, as well as the influence of ovariectomy on their expression, by using immunohistochemistry. PTHrP and PTHR1 immunoreactivity was observed in cytoplasm as well as in nuclei of almost all neurons in the SCG. In male rats, intensity of PTHrP fluorescence was significantly higher in cytoplasm of NPY-, in comparison to NPY+ neurons (p < 0.05). In female rats, 2 months post-ovariectomy, significantly lower intensity of PTHrP fluorescence in cytoplasm of the SCG neurons was observed in comparison to sham operated animals (p < 0.05). In addition to neurons, PTHrP and PTHR1 immunoreactivity was observed in most of the glia and was not influenced by ovariectomy. Results show the expression of PTHrP and its receptor, PTHR1, in the majority of neurons and glial cells in the SCG of rats. Expression of PTHrP, but not PTHR1 in the cytoplasm of SCG neurons is influenced by ovarian activity.

New concepts of breast cell communication to bone

Lactation is the most extreme case of normal physiological bone loss during a lifetime, and breast cancers have a strong tendency to metastasize to bone. In both the physiological and pathological circumstances, parathyroid hormone-related peptide (PTHrP) plays a central role. Until recently there were no regulatory mechanisms to explain the induction of endocrine PTHrP secretion from breast cells during lactation. The mammary epithelium possesses a local serotonin signaling system which drives PTHrP expression during lactation and in breast cancer cells. The mammary gland serotonin system is highly induced in response to alveolar dilation due to milk secretion. Discovery of serotonergic control of PTHrP suggests that it may be possible to manipulate the breast-to-bone axis by targeting serotonin signaling.

Mammary gland serotonin regulates parathyroid hormone-related protein and other bone-related signals

Breast cells drive bone demineralization during lactation and metastatic cancers. A shared mechanism among these physiological and pathological states is endocrine secretion of parathyroid hormone-related protein (PTHrP), which acts through osteoblasts to stimulate osteoclastic bone demineralization. The regulation of PTHrP has not been accounted for fully by any conventional mammotropic stimuli or tumor growth factors. Serotonin (5-HT) synthesis within breast epithelial cells is induced during lactation and in advancing breast cancer. Here we report that serotonin deficiency (knockout of tryptophan hydroxylase-1) results in a reduction of mammary PTHrP expression during lactation, which is rescued by restoring 5-HT synthesis. 5-HT induced PTHrP expression in lactogen-primed mammary epithelial cells from either mouse or cow. In human breast cancer cells 5-HT induced both PTHrP and the metastasis-associated transcription factor Runx2/Cbfa1. Based on receptor expression and pharmacological evidence, the 5-HT2 receptor type was implicated as being critical for induction of PTHrP and Runx2. These results connect 5-HT synthesis to the induction of bone-regulating factors in the normal mammary gland and in breast cancer cells.

Evolution of the parathyroid hormone family and skeletal formation pathways

Bone is considered to be a feature of higher vertebrates and one of the features that was required for the movement from water onto land. But there are a number of evolutionarily important species that have cartilaginous skeletons, including sharks. Both bony and cartilaginous fish are believed to have a common ancestor who had a bony skeleton. A number of factors and pathways have been shown to be involved in the development and maintenance of bony skeleton including the Wnt pathway and the parathyroid hormone gene family. The study of these pathways and factors in cartilaginous animals may shed light on the evolution of the vertebrate skeleton.

Expression of parathyroid hormone-related protein (PTHrP) mRNA in mammary gland of periparturient cows

The expression of parathyroid hormone-related protein (PTHrP) mRNA was examined in mammary gland with or without lactation, and during periparturient period in a Holstein cow and a Jersey cow. In the lactating mammary gland, PTHrP was detected in alveolar epithelial cells and the lumen by immunohistochemical analysis. The relative expression levels of PTHrP mRNA in mammary gland from lactating cows were significantly higher than those from non-lactating cows (P<0.05). During periparturient period, relative PTHrP mRNA level was remarkably low before the parturition in a Jersey and a Holstein cow, however, both levels were gradually increased and reached a peak level at 5-6 weeks after the parturition. In addition, the peak level in a Jersey cow was approximately 3-fold higher than that in a Holstein cow. From these results, PTHrP was synthesized and secreted in alveolar epithelial cells in mammary gland and increased subsequently with the lactation, suggesting a possible mechanism for the regulation of local calcium homeostasis.

Expression of bovine parathyroid hormone/parathyroid hormone- related protein (PTH/PTHrP) receptor mRNA in the mammary gland of cows

To examine the PTH/PTHrP receptor in the mammary gland, molecular cloning of bovine PTH/PTHrP receptor and measurement of its mRNA expression were carried out in cows during the periparturient period. The PTH/PTHrP receptor gene was partially cloned, and expression of bovine PTH/PTHrP receptor mRNA was detected in various tissues of the cow. In the mammary gland, PTH/PTHrP receptor mRNA expression was constantly low during the periparturient period, whereas PTHrP mRNA expression dramatically increased after parturition. This suggested that expression of PTH/PTHrP receptor mRNA in the mammary gland is not affected by lactation in cows.

Parathyroid Hormone-Related Protein (PTHrP) and Ca Levels in the Milk of Lactating Cows

To investigate the roles of mammary PTHrP in calcium uptake and/or release in the mammary gland of cows, plasma PTHrP and Ca levels, and their arterial-venous differences were examined in a Jersey cow during the periparturient period. Levels of Ca in both abdominal aorta and abdominal subcutaneous vein blood slightly decreased around the parturition and at 24 days after the parturition, however, no remarkable arterial-venous differences were observed. Plasma PTHrP levels in both arterial and venous samples were below the detection limit (0.57 pmol/l) during the experimental period. Milk PTHrP and Ca levels were measured in 9 Holstein dairy cows. Although plasma PTHrP levels in all arterial and venous samples were also below the detection limit, milk PTHrP and Ca levels were remarkably high, ranging from 14,900 pmol/l to 41,200 pmol/l and from 772 mg/l to 1,200 mg/l, respectively. In addition, a significant positive correlation (P<0.01) was observed between milk PTHrP and Ca levels. These results suggested that mammary PTHrP is closely related to Ca concentration in the milk.

Calcium transport and signaling in the mammary gland: targets for breast cancer

The mammary gland is subjected to extensive calcium loads during lactation to support the requirements of milk calcium enrichment. Despite the indispensable nature of calcium homeostasis and signaling in regulating numerous biological functions, the mechanisms by which systemic calcium is transported into milk by the mammary gland are far from completely understood. Furthermore, the implications of calcium signaling in terms of regulating proliferation, differentiation and apoptosis in the breast are currently uncertain. Deregulation of calcium homeostasis and signaling is associated with mammary gland pathophysiology and as such, calcium transporters, channels and binding proteins represent potential drug targets for the treatment of breast cancer.

Secretion of estradiol-17β by porcine mammary gland of ovariectomized steroid-treated sows

Although the mammary gland of many species secretes estradiol (E(2)), nothing is known of E(2) secretion in the porcine gland. The present study was designed to investigate whether porcine mammary gland was a source of E(2), and to test the influence of individual and combined effects of exogenous progesterone and estradiol benzoate (EB) on the secretion of E(2). Immature crossbred gilts were ovariectomized at 7 months of age followed by 4 weeks later by steroid hormone replacement therapy to produce estradiol and progesterone (P(4)) blood concentrations similar to those observed during a normal estrous cycle. Arterial and venous blood plasma (from carotid artery and anterior mammary vein, respectively) were sampled for 2h at 10 min intervals. Plasma concentrations of progesterone, androstenedione (A(4)), testosterone (T), estrone (E(1)) and estradiol were determined by RIA. In all gilts treated with progesterone alone or in combination with EB, concentrations of P(4), A(4) and E(1) in blood collected from venous outflow were lower compared to concentrations in arterial blood, whereas concentrations of E(2) were higher in blood plasma from the anterior mammary vein compared to plasma from the carotid artery. The results indicated that the porcine mammary gland secreted E(2). Increased concentrations of plasma E(2) collected only from P(4)-treated animals suggested that progesterone activated enzymes involved in steroidogenesis in porcine mammary gland, or those utilized in its metabolism.

Expression of PTHrP and the PTH/PTHrP receptor in purified alveolar epithelial cells, myoepithelial cells, and stromal fibroblasts derived from the lactating rat mammary gland

Parathyroid hormone-related protein (PTHrP) is produced by the lactating mammary gland and secreted into the milk; however, the function of PTHrP during lactation is unknown. Since messenger RNA for both PTHrP and the PTH/PTHrP receptor have been demonstrated within mammary tissue, a paracrine or autocrine function for PTHrP has been proposed. To investigate this hypothesis in lactating tissue, the expression of PTHrP and the PTH/PTHrP receptor was examined in purified subpopulations of cells derived from lactating rat mammary glands. Subpopulations of stromal, myoepithelial, and alveolar epithelial cells were isolated from mammary tissue using enzymatic digestion and immunomagnetic purification. Isolated cells were phenotypically characterized by immunohistochemistry and ultrastructural morphology. The purity of the separated alveolar and myoepithelial cells was assessed ultrastructurally and ranged from 91 to 96%. Messenger RNA and protein expression of PTHrP and the PTH/PTHrP receptor were examined using reverse transcription polymerase chain reaction, immunohistochemistry, and Western blot analysis, respectively. PTHrP mRNA and protein were expressed in alveolar epithelial cells and stromal fibroblasts, whereas PTH/PTHrP receptor mRNA and protein were expressed in all three cell types. The expression patterns for PTHrP and the PTH/PTHrP receptor support an autocrine or paracrine function for PTHrP in alveolar epithelial cells and stromal fibroblasts and a paracrine function for PTHrP in myoepithelial cells in the rat mammary gland during lactation.

Targeted overexpression of parathyroid hormone-related protein (PTHrP) to vascular smooth muscle in transgenic mice lowers blood pressure and alters vascular contractility

PTH-related protein (PTHrP) and its receptor are expressed in vascular smooth muscle cells and are believed to participate in the local regulation of vascular tone. To explore the function of locally produced PTHrP in vascular smooth muscle in vivo, we developed transgenic mice that overexpress PTHrP in smooth muscle using a smooth muscle alpha-actin promoter to direct expression of the transgene. In the PTHrP-overexpressing mice, messenger RNA expression was mainly restricted to smooth muscle-containing tissues. Several founders also expressed the transgene in bone and heart and exhibited striking abnormalities in the development of these tissues. In PTHrP-overexpressing mice, blood pressure was significantly lower than that in wild-type controls (121 +/- 3 vs. 135 +/- 2 mm Hg; P < 0.01). Moreover, the magnitude of the vasorelaxant response to iv infusions of PTHrP-(1-34)NH2 was significantly attenuated in the transgenic animals. A similar desensitization to PTHrP was observed in aortic ring and portal vein preparations. Surprisingly, PTHrP-overexpressing mice were also significantly less responsive to the hypotensive action of infused acetylcholine in vivo and to the relaxant actions of acetylcholine on aortic vessel preparations in vitro. In summary, we have successfully targeted overexpression of PTHrP to the smooth muscle of transgenic mice. When expressed in its normal autocrine/paracrine setting, PTHrP lowers systemic blood pressure and decreases vascular responsiveness to further relaxation by PTHrP and other endothelium-dependent vasorelaxants such as acetylcholine. We postulate that the heterologous desensitization to acetylcholine-induced relaxation in PTHrP-overexpressing blood vessels involves desensitization of second messenger/effector signaling pathways common to PTHrP and acetylcholine.

Calcium and vitamin D metabolism during lactation

Calcium transfer to the fetus in late pregnancy and the subsequent transfer of calcium to milk represent the greatest challenges to calcium homeostasis in adult animals. The adaptation of the maternal calcium homeostatic mechanisms is the result of a complex interplay between calciotropic hormones and the tissues, intestine, bone, and kidney, responsible for providing the large amounts of calcium needed to support fetal skeletal growth and lactation. In this review, we will discuss general calcium homeostasis followed by a review of the specific adaptations required by the human, rat, and cow to meet fetal and lactational demands for calcium. Finally, we will review what is known about the regulation of calcium transfer from the plasma to the milk.

In vitro model of parathyroid hormone-related protein secretion from mammary cells isolated from lactating cows

Parathyroid hormone-related protein (PTHrP) is produced by the lactating mammary gland and is present in milk in a biologically active form. The goal of this investigation was to determine if cells cultured from the lactating mammary glands of cows would secrete PTHrP in vitro. Mammary acini were isolated from lactating cows at 1-6 wk after calving, and fresh or cryopreserved mammary acini were cultured for 14 d on Type I collagen. Cultures on thick layers of collagen (2.5 mm) were detached and allowed to contract on Day 6. PTHrP production was measured by N-terminal radioimmunoassay and bioassay (increased cAMP levels in ROS 17/2.8 osteoblast-like cells). The mammary cells reached confluence at Day 6. PTHrP production was low at Day 2 (< 0.5 ng/ml) but increased to peak production (2-4 ng/ml) at approximately Day 6 and remained constant until Day 14. Immunoreactive and bioactive PTHrP levels in the culture medium correlated well. The cultures produced lactoferrin (2,000-2,300 ng/ml and alpha s1-casein (14-19 ng/ml). Prolactin stimulated PTHrP production approximately 50% on Days 6-14. PTHrP production was increased approximately 100% by treatment with epidermal growth factor (10 ng/ml) for 2 d. Morphologic evaluation of cultures on thick, contracted collagen at Day 14 revealed an inner layer of mammary epithelial cells overlying myoepithelial cells and an outer layer of collagen containing stromal cells. Immunohistochemistry demonstrated positive staining for PTHrP and cytokeratin in both mammary epithelial and myoepithelial cells and alpha-smooth muscle actin in myoepithelial cells. These data demonstrated that cryopreserved mammary tissue from lactating cows could be cultured in vitro and secreted PTHrP in a regulated manner. This in vitro model will be useful to investigate the function and regulation of PTHrP in the lactating mammary gland.

Regulation of blood flow in the mammary microvasculature

Although milk yield of cows and goats is known to be closely related to the total flow of blood through the udder, a number of studies suggest that milk yield can vary independently. No studies have attempted to measure the proportion of total flow that is nutritive. Within the mammary gland, capillary networks form a basket-like architecture surrounding each alveolus. Notably, flow in individual capillaries is not constant and varies among capillaries. Capillary flow (measured by intravital microscopy) was decreased by oxytocin, which generally increased total flow in the mammary artery, suggesting that the proportion of total flow that is nutritive can vary. In addition to classic metabolic regulators (e.g., carbon dioxide and oxygen) of tissue blood flow, the mammary gland produces a number of vasodilatory compounds, including parathyroid hormone-related protein, insulin-like growth factor-I, prostacyclin, nitric oxide, and endothelin. All of these compounds have been shown to alter mammary blood flow. Mammary tissue also contains kallikrein and angiotensin-converting enzyme, which convert circulating kinins and angiotensin, respectively, into potent vasoactive compounds. A number of these compounds are produced by epithelial cells themselves, providing a mechanism for the functioning epithelium to control its own blood supply and, hence, nutrient flow for milk synthesis. In this review, we examine the nature of the mammary microcirculation, its behavior under different conditions, and some of the regulatory features of the mammary microvasculature.

Distribution and functions of parathyroid hormone-related protein in vertebrate cells

Parathyroid hormone-related protein (PTHrP) was isolated from tumors and identified as the agent of humoral hypercalcemia of malignancy (HHM) in 1987. Since then its gene structure in several mammalian and an avian species has been analyzed and its gene expression demonstrated in many adult and embryonic tissues derived from all three germ layers. The composition and structure of PTHrP peptide depends on both differential gene splicing and posttranslational processing, which result in a range of peptides of potentially diverse functions. This chapter describes the distribution of PTHrP in both normal and neoplastic adult and embryonic tissues. PTHrP is of fundamental importance to cell survival because the absence of the gene is fatal; this aspect of PTHrP function in cell physiology becomes overwhelmingly important in neoplasia. Intracrine or paracrine actions for PTHrP seem to be most likely in mammalian and avian physiology, but in fishes high circulating levels suggest classic endocrine functions as well. Much remains to be learned of the biology of this fascinating protein.