Measurement of parathyroid hormone-related protein in extracts of fetal parathyroid glands and placental membranes

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

VITAMIN D AND HUMAN PREGNANCY

At the end of 2007, Time magazine listed the “benefits of vitamin D” as one of its top 10 medical breakthroughs for that year. Since then there has been a remarkable upsurge of interest in vitamin D, with new research advances seemingly published on a weekly basis. In particular, there has been increasing awareness of the variability of vitamin D status in populations across the globe and, significantly, a growing debate about the need for revised parameters for vitamin D supplementation. Although sub-optimal vitamin D is likely to be a widespread problem for 21stcentury societies, it is also clear that some groups are at much greater risk of low vitamin D status. Prominent amongst these are pregnant women and the aim of the following review article will be to discuss this problem in further detail with specific emphasis on its potential physiological and clinical impact.

Calcium metabolism in pregnancy and lactation

Homeostatic adaptation to maternal calcium metabolism is a prerequisite for optimal delivery of sufficient calcium to the fetus and neonate during pregnancy and lactation, respectively. This article outlines the major adaptations known to occur and the physiological regulators likely to be principally involved. Importantly, different adaptive responses are used in pregnancy and lactation. The rarity of calcium disorders in pregnancy underscores the successful implementation of these adaptations in most women. For those few women with either pre-existing or pregnancy-acquired disorders of calcium metabolism, a knowledge of normal physiology is essential to understand the implications for managing these disorders in pregnant women.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Messenger RNA stability of parathyroid hormone-related protein regulated by transforming growth factor-beta1

Humoral hypercalcemia of malignancy (HHM), a paraneoplastic syndrome associated with epithelial cancers, including squamous cell carcinoma (SCC), is due to expression and secretion of parathyroid hormone-related protein (PTHrP). Transforming growth factor-beta1 (TGFbeta1), expressed by many tumors, has been demonstrated in vitro to increase the half-life of PTHrP mRNA. In this study, oral squamous carcinoma cells (SCC2/88) had a two-fold increase in PTHrP mRNA stability (from 45 to 90 min) in response to treatment with TGFbeta1. In order to examine the mechanism of TGFbeta1-mediated PTHrP mRNA stability, a cell-free assay of mRNA degradation was utilized in which the degradation of in vitro-transcribed mRNA incubated with cytoplasmic protein extracts from SCC2/88 treated with vehicle or TGFbeta1 was measured. In this assay, full-length PTHrP mRNA was not significantly stabilized in TGFbeta1-treated samples when compared to vehicle treated samples. However, there was a striking (>5-fold) increase in PTHrP mRNA half-life in TGFbeta1-treated samples when PTHrP mRNA lacked the 3'-untranslated region (3'-UTR). In contrast, the degradation of 3'-UTR-truncated PTHrP mRNA using the cell-free assay was not altered in vehicle-treated samples. UV cross-linking of PTHrP mRNA and cytoplasmic proteins from cells treated with either vehicle or TGFbeta1 revealed numerous mRNA-binding proteins. TGFbeta1 treatment resulting in decreased binding of 33, 31, 27, 20 and 18 kDa binding proteins to the terminal coding region. These studies revealed that TGFbeta1-induced PTHrP mRNA stability might be, in part, the result of cis-acting sequences within the coding region of the PTHrP mRNA.

Regulation of secretion of PTHrP by Ca(2+)-sensing receptor in human astrocytes, astrocytomas, and meningiomas

Parathyroid hormone-related protein (PTHrP) is the major mediator of the humoral hypercalcemia of malignancy and of malignant osteolysis associated with skeletal metastases of common epithelial cancers. PTHrP secretion is regulated by the extracellular calcium concentration ([Ca(2+)](o)) in several types of normal and malignant cells. Because the [Ca(2+)](o)-sensing receptor (CaR) is a key mediator of [Ca(2+)](o)-regulated hormone secretion [e.g., of parathyroid hormone (PTH) by parathyroid chief cells], we investigated the expression of the CaR and PTHrP in normal and neoplastic glial cells and studied the effects of [Ca(2+)](o) on PTHrP secretion. Our results show that primary embryonic human astrocytes (HPA) express CaR mRNA and protein as detected by RT-PCR and Western analysis, respectively. Furthermore, astrocytomas and meningiomas also express the CaR at similar levels as assessed by RT-PCR and Northern and Western blot analyses. HPA and astrocytomas express transcripts encoding all three known isoforms of PTHrP [PTHrP(139), PTHrP(141), and PTHrP(173), comprising 139, 141, and 173 predicted amino acid residues, respectively] as assessed by RT-PCR, whereas meningiomas express only the first two of these. Finally, elevated levels of [Ca(2+)](o) and other polycationic CaR agonists dose dependently stimulate PTHrP secretion from HPA, astrocytomas, and meningiomas, although both basal and high [Ca(2+)](o)-stimulated rates of PTHrP secretion are approximately 2. 5-fold higher in HPA than in the glial tumors studied here. Therefore, our results show that HPA, astrocytomas, and meningiomas express both the CaR and PTHrP and that CaR agonists stimulate PTHrP secretion.

Immunoreactive parathyroid hormone-related protein: its association with preterm labor

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a 141 amino acid protein which contains a 1-36 N-terminal domain resembling parathyroid hormone which has smooth muscle relaxant activity and a mid (67-86) domain which reportedly alters placental calcium transport. Using specific antibodies to these regions of PTHrP, the objective of this study was to determine changes in the levels and localization of the peptides in placenta and membranes that might be indicative of their biological activity and role during term and preterm labor.

STUDY DESIGN

Placenta and fetal membranes were collected from patients with preterm delivery (PTL) (n = 16), term cesarean section in the absence of labor (n = 10) and term vaginal delivery (n = 5). Immunohistochemistry was performed with specific antisera visualized by the avidin-biotin peroxidase method and the staining intensity was quantified with an image analysis system MCID.

RESULTS

Immunoreactive (ir)-PTHrP(1-34) and ir-PTHrP(67-86) were localized to the amnionic epithelium chorionic trophoblasts, decidual cells and placental syncytiotrophoblast. Intense immunostaining was observed for ir-PTHrP(67-86) but not for ir-PTHrP(1-34) in the endothelial lining of the villous capillaries. Ir-PTHrP(1-34) staining was lower in placenta and fetal membranes of PTL patients compared with term cesarean section in the absence of labor (P < 0.05 Mann-Whitney test). In contrast, there was no difference in ir-PTHrP [67-86] staining intensity between delivery categories.

CONCLUSION

These results showing differential localization of PTHrP(1-34) and PTHrP(67-86) suggest cell specific processing of PTHrP precursor in the human placenta. Moreover, the changes in ir-PTHrP(1-34) but not ir-PTHr(67-86) with labor are indicative of a particular role for this peptide in the delivery process.

The expression of the gene coding for parathyroid hormone-related protein (PTHrP) during tooth development in the rat

By means of in situ hybridisation studies, it is shown that parathyroid hormone-related protein (PTHrP) mRNA is strongly expressed in the developing enamel organs of rat teeth. In particular, the cervical loop hybridises strongly with the PTHrP probe and expression is maintained at this site throughout life in the permanently erupting incisor teeth. In mature molar teeth, expression is downregulated to low levels and confined to the epithelial cell rests of Malassez and/or cementoblasts which may derive from these. The gene is also expressed at low levels in the tissue overlying the erupting molars and, thereafter, in the junctional epithelia and connective tissue cells of the epithelial attachment on all tooth surfaces. The premise that PTHrP may undergo post-translational processing and that the resultant products could act in different ways raises the possibility of its exerting multiple paracrine actions during tooth development. These could include the control of cell division and local vascular dilation during development.

[Parathyroid hormone related peptide (PTHrP) and bone metabolism]

The discovery of PTHrP was the result of research on the mechanisms by which some cancers cause hypercalcemia (humoral hypercalcemia of malignancy) without necessarily metastasizing to bone. PTHrP is also present in various normal adult and fetal tissues. Its concentration is normally very low (picomolar) in blood, but it is more abundant in milk (nanomolar concentration). PTHrP seems able to exert autocrine/paracrine as well as endocrine effects on bone metabolism. A major role for PTHrP in regulation of fetal bone metabolism has been demonstrated in mice. Homologous recombination has been used in these rodents to remove the major coding exon from one copy of the mouse PTHrP gene in embryonic stem cells. Subsequently generated chimeric mice transmit the mutant PTHrP allele through the germline. Homozygous mutants died immediately after birth and had a multitude of skeletal abnormalities. So PTHrP seems necessary to embryonic development of the skeleton. PTHrP (1-34), like PTH (1-34) fragments, might be responsible for both bone resorption and formation. Although the effects of the carboxyl-terminal fragments are still controversial, PTHrP (107-111) fragment seems able to inhibit osteoclast activity. PTHrP (1-34), whose 8 of the first 13 amino-acids are identical with those in PTH (1-34), acts through the same receptor as PTH on osteoblasts and renal cells membrane. The PTHrP/PTH receptor sequence is now well established. PTHrP-receptor coupling is mediated by cyclic AMP and/or inositols-phosphate. The consequent activation of protein kinase A and intracellular calcium or protein kinase C, respectively, locally induces growth factors or cytokines secretion, responsible for the observed effects. The role of PTHrP appears important during pregnancy and lactation, when it stimulates fetal bone growth by increasing calcium transport from the dam to its fetus and maternal bone resorption allowing calcium supply for milk production, respectively. Such a role would be particularly important in domestic ruminants, which are often simultaneously pregnant and lactating. The role of PTHrP during aging (especially in post-menopausal women in which bone loss may induce osteoporosis) remains unknown and might be of peculiar interest since PTHrP (1-34) and (107-111) are able to restore bone loss induced by ovariectomy in rats.

Parathyroid hormone-related protein

We review the current state of knowledge of the molecular properties and actions of parathyroid hormone-related protein (PTHrP) both in cancer patients and in normal physiology. PTHrP is a common product of squamous cancers and is the major mediator of the syndrome of humoral hypercalcemia of malignancy (HHM) by its actions through parathyroid hormone receptors in bone and kidney. Recently developed radioimmunoassays and tissue localization techniques indicate that PTHrP is produced by many more cancers than was originally indicated by clinical studies and that it contributes significantly to malignancy-related hypercalcemia associated with other etiologies, for example, cancers metastatic to bone and hematological malignancies. The gene encoding PTHrP is complex, with multiple exons coding for up to 12 alternate transcripts and three different length proteins, potentially in a tissue-specific manner, by the use of three promoters. Its expression is regulated by hormones and growth factors, and the untranslated exons display features in common with many cytokine genes. Although potential endocrine actions of PTHrP are evident in fetal development, further evidence suggesting that the normal physiological role of PTHrP is predominantly as a locally produced regulator/cytokine comes from localization studies and investigations of its actions in a variety of tissues. Such studies indicate that in addition to its parathyroid hormone-like actions, PTHrP has multiple activities, including those in fetal development, placental calcium transfer, lactation, smooth muscle relaxation, and on epithelial cell growth. Although PTHrP was discovered because of its production by cancers, evidence for its actions as a local regulator highlights the importance of understanding its roles not only in the etiology of HHM in cancer patients but also in normal tissues.

Effect of parathyroid hormone-related peptide supplementation of soy protein formulas in the neonatal pig model

PTH-related peptide (PTHrP) is found in all milks, including human and pig. To define a role for PTHrP in milk, 2-day-old piglets were randomized to receive soy formula devoid of PTHrP or supplemented with 1 nM synthetic PTHrP(1-86) (n = 8 per group). The number of serum samples with detectable PTHrP by immunoassay (Incstar) and radiometric assay (Nichols) was 9 of 33 and 3 of 13 in PTHrP- and 8 of 27 and 3 of 15 in PTHrP+ formula-fed piglets and 8 of 14 and 7 of 12 in naturally suckling piglets, respectively. Serum and urine concentrations of calcium and magnesium and total and bone alkaline phosphatase were similar in both groups at 3, 6, 10, and 17 days of age. No differences were seen in bone mineral content of the tibia measured by single-photon absorptiometry (BMC 0.22 +/- 0.06 and 0.22 +/- 0.10) or dual x-ray absorption (BMC 1.43 +/- 0.36 and 1.31 +/- 0.78) either in vivo or on excised bone or by measurement of Ca, Mg, or P content or total bone ash (1.26 +/- 0.26 and 1.38 +/- 0.28 mg). Intestinal histology, serum intestinal alkaline phosphatase, and net absorption and retention of Ca, Mg, and P in balances from age 11-17 days were all similar. As in humans, however, a developmental pattern was seen for phosphorus regulation in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-related protein gene expression in invasive cervical tumors

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is the main tumor-derived factor responsible for the hypercalcemia of malignancy.

METHODS

Using a polyclonal serum to the 37-67 region of PTHrP, and 35S-labeled riboprobes, the authors investigated the cellular expression of PTHrP mRNA and peptide in formalin fixed, paraffin embedded sections from 16 invasive cervical tumors. In addition, the relationship among the histologic cell type, degree of differentiation, pattern of invasion, and tumor expression of PTHrP were examined.

RESULTS

PTHrP mRNA and peptide were identified in 10 of 10 and 16 of 16 tumors examined, respectively. Overall strong mRNA expression with moderate to intense intracellular staining for peptide was associated, with adenosquamous carcinoma displaying a spray pattern of invasion.

CONCLUSION

PTHrP mRNA and peptide were observed in all cervical tumors studied. Despite their high frequency of expression of PTHrP, cervical tumors seldom give rise to humoral hypercalcemia of malignancy, but the autocrine/paracrine effects of PTHrP may be important in the growth and dedifferentiation of the malignant cell population.

Short communication: parathyroid hormone-related peptide does not stimulate phosphate placental transport

It has been demonstrated that parathyroid hormone-related peptide (PTHrP) stimulates calcium (Ca) placental transport. Ca and inorganic phosphorous (P) metabolisms are intimately linked. Thus we have studied the influence of PTHrP on P placental transport in ewes. In our experimental paradigm, synthetic human PTHrP (1-86) fragment intravenously injected in six single chronically catheterized fetuses (3 x 1 micrograms/fetus/day, from day 129 to day 140 of gestation) had no significant effect upon P placental transport which was 131 +/- 6 and 129 +/- 9 mg/kg fetal weight/day in treated and control fetuses, respectively.

Idiopathic osteoporosis during pregnancy

Among the idiopathic forms of osteoporosis, the one developing during pregnancy is the least common and scarcely studied. Poorly understood, it seems to stem from transient failure of calcitropic hormones and decreased osteoblast activity. A 25-year-old patient presented with coxofemoral pain during the last three months of pregnancy, followed by multiple vertebral compression fractures at postpartum. Laboratory, radiological, densitometric and histological examinations led to a diagnosis of idiopathic osteoporosis in pregnancy, once other causes of osteopenia had been ruled out. Bone densitometries performed 12 and 24 months later showed an increase in mineral density, thus demonstrating the self-limited nature of this entity.

Effects of epidermal growth factor on parathyroid hormone-related protein production by mammary epithelial cells

Parathyroid hormone-related protein (PTHrP) plays a prominent role in the pathogenesis of humoral hypercalcemia of malignancy. However, it is also expressed in various nonmalignant tissues, particularly during fetal organogenesis and tissue differentiation. Thus, PTHrP is synthesized in skin, placenta, and mammary gland during lactation. Little is known, however, about the regulation of PTHrP synthesis and release in nontumoral cells. We investigated the regulation of PTHrP production by epidermal growth factor (EGF), a factor of major importance in the development of lactating breast, in primary cultures of rat mammary epithelial cells. EGF stimulated the production of immunoreactive and bioactive PTHrP in a time- and concentration-dependent manner. A 12 h incubation with 10 ng/ml of EGF increased PTHrP production by 36.0 +/- 7.1% (n = 7 experiments, p < 0.01). This was accompanied by an increase in PTHrP mRNA steady-state levels. The production of PTHrP was stimulated by the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA) by 82.9 +/- 9.7% (n = 4 experiments, p < 0.01). The effects of PMA and EGF were additive. The EGF-induced stimulation appeared to be independent of extracellular calcium concentration, prostaglandin, or cAMP synthesis, but may have involved tyrosine kinase-mediated mechanisms. These results indicate that EGF was capable of increasing the production of PTHrP by cultured mammary epithelial cells. They also suggest that factors activating the PKC pathway are involved in the upregulation of PTHrP expression in mammary epithelial cells.

Parathyroid hormone-related protein in healthy pregnant women

The object of this study was to determine whether increased circulating levels of parathyroid hormone-related protein (PTH-rp) may explain the increased parathyroid hormone (PTH) bioactivity in pregnancy. In 41 healthy pregnant women (age 19-41 years), PTH-rp and corrected calcium levels were measured and compared with those of nonpregnant control women (n = 18, age 20-39 years). PTH-rp and corrected calcium levels were significantly higher in pregnant women (PTH-rp 21.9 +/- 7.9 pg/ml, P < 0.001; corrected calcium 2.38 +/- 0.07 mmol/liter, P = 0.001) than in nonpregnant women (PTH-rp 10.3 +/- 7.8 pg/ml; corrected calcium 2.30 +/- 0.10 mmol/liter). Our data indicate that circulating PTH-rp levels may significantly increase in pregnancy, suggesting a possible role of this peptide in the modification of calcium homeostasis in pregnant women.

Parathyroid hormone-related protein and calcium phosphate metabolism

There is marked homology between the parathyroid hormone (PTH) and PTH-related protein (PTHrP) molecules at the amino terminal but the rest of the molecules are quite different, providing immunologically distinct peptides. However, they interact with the same receptor. Thus, PTHrP mediates biological actions reminiscent of PTH. PTHrP gene is a single copy gene, producing one to three mRNA transcripts through alternative splicing of the carboxy terminal, encoding peptides of 139, 141 or 173 amino acids. Having been recently isolated from malignant tumours, PTHrP is now considered to be the major mediator of humoral hypercalcaemia of malignancy (HHM). The PTH-like effects of PTHrP on the kidney and bone have been well characterized. The increase in renal tubular calcium reabsorption and the reduction in tubular phosphate reabsorption with a concomitant rise in nephrogenous cyclic AMP constitute the pathophysiological changes in the renal handling of calcium and phosphate in HHM. The osteotropic contribution to the malignant hypercalcaemia has been validated by enhanced osteoclastic bone resorption--an indirect effect of the amino terminal portion of the PTHrP molecule on osteoblasts. However, PTHrP has also been detected in a large number of normal adult tissues/organs as well as in human and animal fetuses. Fetal plasma calcium is higher than maternal and this is achieved by active transport of calcium across the placenta. Using ovine placental perfusion models, PTHrP, which is believed to originate from fetal parathyroid glands and the placenta itself, has been demonstrated to sustain this calcium gradient. Active placental transport of magnesium, but not phosphate, was also shown to be enhanced by PTHrP.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct evidence of a parathyroid related protein gradient between the mother and the newborn in humans

Umbilical cord plasma has increased parathormone (PTH)-like bioactivity in comparison with that in maternal plasma, but suppressed PTH levels. Previous attempts to detect elevated levels of PTHrP(1-34) in the umbilical cord were unsuccessful, whereas PTHrP was detected by immunohistochemistry in both fetal parathyroid glands and placental membranes. At the time of delivery, plasma samples were drawn simultaneously from 47 normal mothers (mean age, 26 years) and from the umbilical cord of 25 female and 22 male newborns and assessed for calcium adjusted for albumin, magnesium, creatinine, intact PTH and PTHrP. PTHrP was measured using a new 2-site immunoradiometric assay recognizing separately 1-40 and 60-72 residues with a sensitivity of 0.3 pmol/l. Mean (+/- S.D.) plasma calcium values were 2.35 +/- 0.10 in the mothers versus 2.63 +/- 0.12 mmol/l in the newborns (P < 0.001). PTH values were significantly higher in the mothers, as expected (22.3 +/- 14.8 vs. 6.2 +/- 0.9 pg/ml). In contrast, PTHrP values were significantly higher in the newborns (1.50 +/- 0.39 versus 0.84 +/- 0.28 pmol/l, P < 0.001). Only 1/47 mother had a PTHrP level higher than her child (0.9 vs. 0.8 pmol/l). Only 10/47 mothers had PTHrP higher than 1 pmol/l whereas only 1/47 newborn had a value below 1 pmol/l. There was no correlation between Ca and PTHrP in either group. Serum creatinine values were all within the normal range. Serum magnesium levels were not different between the mothers and the children.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of (1-34) parathyroid hormone-related peptide on the composition and turnover of phospholipids in syncytiotrophoblast brush border and basal plasma membranes of human placenta

The effect of parathyroid hormone-related peptide on the lipid composition and the turnover of phosphoinositides was studied in brush border and basal plasma membranes of human placenta syncytiotrophoblasts. Lipid composition of the two polar membranes differed markedly with respect to the cholesterol/phospholipid ratio (0.57 +/- 0.04 and 0.91 +/- 0.05 in basal plasma membranes and brush border membranes, respectively). Sphingomyelin was the major phospholipid in both membranes. Except for the phosphoinositide-phosphatidylserine complex which was higher in basal plasma membranes, the phospholipid composition was comparable in the brush border membrane and basal plasma membranes. Incubation of the tissue with 10(-8) M parathyroid hormone-related peptide (1-34) resulted in a significant increase in the phosphatidylinositol phosphate content of the two membranes and in the phosphatidylinositol biphosphate concentration in the basal plasma membranes. Finally, when the tissue was preincubated with [3H]myo-inositol in the presence of 10(-8) M parathyroid hormone-related peptide (1-34), the hormone significantly stimulated the inositol phosphate release by the two membranes. These results demonstrate that: (1) in the placental syncytiotrophoblast, as found in other transport epithelia, the lipid composition of the polar membranes is different; (2) parathyroid hormone-related peptide stimulates the phosphoinositide turnover in both membranes.

Effects of thyroparathyroidectomy, parathyroid hormone, and PTHrP on kidneys of ovine fetuses

The fetal parathyroid glands and parathyroid hormone-related protein (PTHrP) have been shown to be important regulators of fetal calcium metabolism through their actions on the placenta and bone. This study examined the effects of fetal thyroparathyroidectomy (with thyroxine replacement) and exogenous infusion of human parathyroid hormone [PTH-(1-34)], PTHrP-(1-34), and PTHrP-(1-141) on the urinary excretion of calcium in chronically cannulated ovine fetuses during the last one-fifth of gestation. Fetal plasma total and ionized calcium concentrations were significantly lower in thyroparathyroidectomized (TxPTx) fetuses when compared with intact fetuses, but there were no significant differences in urinary excretion rates of total calcium. However, TxPTx produced a significant increase in the fractional excretion rate of total calcium and a significant decrease in the excretion of adenosine 3',5'-cyclic monophosphate (cAMP) compared with intact fetuses. Infusions of PTH-(1-34), PTHrP-(1-34), and PTHrP-(1-141) into the jugular vein of TxPTx fetuses (n = 5) at the rate of 1 nmol/h for 2 h, after a 1-nmol loading dose, significantly decreased the excretion rate of total calcium and increased the excretion rate of cAMP in fetal urine. Infusions of all three peptides resulted in significant increases in the concentration of total calcium in fetal plasma but had no effect on the plasma concentrations or urinary excretion rates of phosphate. Infusion of either PTH-(1-34), PTHrP-(1-34), or PTHrP-(1-141) also resulted in an increase in fetal urine osmolality and pH and a decrease in free water clearance in TxPTx fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-related protein: a possible endocrine function in lactation

OBJECTIVE

Parathyroid hormone-related protein (PTHrP), initially discovered as the factor responsible for the syndrome of humoral hypercalcaemia of malignancy, has also been found to be expressed in placenta, in pregnant uterus, in the fetus at many locations, and in the lactating mammary gland. This study sought to establish whether PTHrP reaches the maternal circulation when it is expressed in mammary tissue during lactation or in the maternal reproductive tract during gestation.

DESIGN

Blood samples were collected from 53 subjects: 18 pregnant women in all stages of gestation, 19 lactating mothers and 16 non-lactating post-partum controls.

MEASUREMENTS

PTHrP was measured using a specific and validated radioimmunoassay. Parathyroid hormone was measured by two-site immunoradiometric assay. Total calcium was measured by atomic absorption spectrophotometry.

RESULTS

Circulating levels of PTHrP were readily detectable in 12 of 19 nursing mothers (range 2.7-7.8 pmol/l) but in none of the mothers who were bottle feeding. PTHrP was also detected in one of 18 pregnant subjects. Parathyroid hormone concentrations were lower in lactating mothers (2.3 +/- 1.0 pmol/l), than in non-lactating mothers (3.5 +/- 1.2 pmol/l) (P < 0.01).

CONCLUSION

PTHrP reaches the maternal circulation during lactation in amounts which could produce a systemic effect.

Abundant expression of parathyroid hormone-related protein in human amnion and its association with labor

In animal models, parathyroid hormone-related protein (PTHrP) increases placental calcium transport and inhibits contraction of uterine smooth muscle. The present studies were undertaken to characterize the expression of PTHrP in human uteroplacental tissues. PTHrP mRNA was identified by Northern analysis as a single species (approximately 1.8 kilobases) in human amnion, chorion, placenta, decidua, and myometrium. The most abundant signal was seen in amnion, where it was 10-400 times that in the other uteroplacental tissues. PTHrP mRNA abundance was decreased in amnion (but not in the other tissues) following the onset of labor (P less than 0.001). PTHrP mRNA in amnion appeared to be translated to a bioactive peptide, as PTHrP bioactivity and immunoreactive PTHrP in amnion correlated closely with PTHrP mRNA content (r = 0.86 and 0.95, respectively; P less than 0.05 and P less than 0.01). Amniotic fluid contained PTHrP, 21 +/- 6 pmol/liter (n = 10) at 16 weeks and 41 +/- 9 pmol/liter (n = 7) at 38 weeks (P = 0.05). These concentrations equaled or exceeded those found in plasma of patients with hypercalcemia secondary to PTHrP. After rupture of the fetal membranes, PTHrP mRNA in amnion was decreased by 78% (P less than 0.0001). This decrease appeared to be specific for PTHrP mRNA, as glyceraldehyde-3-phosphate dehydrogenase mRNA was unchanged following rupture of membranes. Like PTHrP mRNA, PTHrP bioactivity and immunoreactive PTHrP in amnion decreased significantly following rupture of membranes (P less than 0.03 and P less than 0.01, respectively). Since PTHrP is a potent antagonist of uterine muscle contraction, the decrease of PTHrP following rupture of the fetal membranes may play a key role in the onset of labor.

Parathyroid-like regulation of parathyroid-hormone-related protein release and cytoplasmic calcium in cytotrophoblast cells of human placenta

Immunohistochemical staining of human placenta revealed intense reactivity for amino terminal and midregional parathyroid-hormone-related protein (PTHrp) in the cytotrophoblast cells and weaker staining in the syncytiotrophoblasts. The cytotrophoblasts also displayed conspicuous surface staining with the monoclonal antibodies E11 and G11, which recognize a Ca2+ receptor mechanism regulating hormone release of parathyroid cells. Cytotrophoblasts enriched on Percoll gradients or by linking surface-bound E11 to magnetic beads revealed biphasic elevation of cytoplasmic Ca2+ ([Ca2+]i) upon a stepwise rise of external Ca2+ from 0.5 to 3.0 mM, with a half-maximal effect at 1.75 mM. Individual cytotrophoblasts identified by their E11 reactivity disclosed a temporary increase of [Ca2+]i upon elevation of external Mg2+, while Mn2+ triggered both a [Ca2+]i transient and an influx of itself. These effects were efficiently blocked by the G11 antibody. Depolarization with K+ or addition of the voltage-dependent Ca2+ channel blocker verapamil had only marginal effects on [Ca2+]i. Raised extracellular calcium inhibited release of PTHrp from the cells, and this inhibition was blocked by the G11 antibody. The virtually parathyroid-identical Ca2+ regulation of [Ca2+]i may mediate feedback control of PTHrp release from the cytotrophoblasts and thereby participate in the regulation of placental Ca2+ transport.

Regulation of parathyroid hormonelike protein production in cultured normal and malignant keratinocytes

We have recently demonstrated that parathyroid hormone-like protein (PLP) production by cultured human squamous carcinoma cells (SCC) can be modulated by co-culture with fibroblasts. The interaction of SCC with fibroblasts, possibly occurring during the invasive phase of SCC, may be the stimulus for enhanced PLP production, thus contributing to the genesis of humoral hypercalcemia of malignancy in this type of cancer (Cancer Res 50:3589-3594, 1990). In the present study we show that the fibroblast-induced increase in PLP level in the medium of SCC-4 cells is paralleled by an increase in PLP messenger ribonucleic acid (mRNA) expression in these cells. We also found that the inhibition of secretion of PLP by monensin for 2 h resulted in a marked increase in immunodetectable PLP intracellularly, suggesting that secretion of PLP was a fast process. The modulation of the production of PLP by calcium and hydrocortisone was further examined in SCC-4 cells and was compared to that in normal keratinocytes and in SCC-9 cells. PLP levels in conditioned media were highest in poorly differentiating SCC-4 cells, intermediate in moderately differentiating SCC-9 cells, and lowest in normal keratinocytes showing high differentiating capacity. Furthermore, in each of the cell types used, PLP production was highest in cultures grown under low calcium conditions; at both calcium concentrations used, the presence of hydrocortisone reduced the PLP release into the medium. This reduction was probably due to a direct effect of hydrocortisone on PLP synthesis because the expression of PLP mRNA was also reduced in the presence of hydrocortisone when tested in SCC-4 cells. In conclusion, our findings indicate that the induction of differentiation in both normal and malignant keratinocytes is associated with the inhibition of PLP production.

Parathyroid hormone-related protein production by primary cultures of mammary epithelial cells

Parathyroid hormone-related protein (PTHrP) plays a major role in the pathogenesis of malignant hypercalcemia, but has also been found in fetal and adult non-neoplastic tissues. Among them, lactating mammary gland was shown to produce PTHrP, and high levels of PTHrP were measured in milk. However, the regulation of PTHrP production by breast cells is still unknown. Primary cultures of mammary cells isolated from rat lactating glands were grown on collagen gels in an insulin/epidermal growth factor (EGF)-supplemented medium. Under these conditions, mammary cells displayed an epithelial phenotype and their number increased more than twofold after 1 week in culture. At that time, the cells were capable of producing immunoreactive PTHrP (range: 25 to 150 pg/10(5) cells x 24 h) and PTH-like bioactivity, as indicated by a 60% increase in cyclic adenosine monophosphate (cAMP) production induced by mammary epithelial cell conditioned medium in the PTH-responsive osteoblast-like UMR-106 cell line. When cell proliferation was hindered by lowering plating density, by removing medium supplements, or by adding transforming growth factor (TGF)-beta, a well-known autocrine inhibitor of mammary epithelial cell growth. PTHrP production was increased. In contrast, the omission of EGF or addition of specified anti-EGF antibodies decreased PTHrP production. In conclusion, primary cultures of mammary epithelial cells isolated from lactating rat were shown for the first time to produce PTHrP in vitro. This production was higher in the presence of EGF and could be modulated by cell growth rate.

Parathyroid hormone-related protein(50-69) and response to pamidronate therapy for tumour-induced hypercalcaemia

A region-specific radioimmunoassay has been employed to measure levels of immunoreactive parathyroid hormone-related protein(50-69) (iPTHrP(50-69)) in patients with tumour-induced hypercalcaemia (TIH). This assay is based on an antiserum raised against synthetic human PTHrP(50-69). The assay showed no cross-reactivity with human or bovine parathyroid hormone(1-84). The effect of a single dose (60 mg) of pamidronate was studied in 25 consecutive patients with TIH. All were rehydrated prior to treatment. All but 2 patients (8%) became normocalcaemic after treatment; both of these had very high levels of iPTHrP(50-69). Time to achieve normocalcaemia, as an index of relative resistance to pamidronate, correlated positively with pretreatment level of iPTHrP(50-69). Absence of radiological evidence of bone metastases also predicted relative resistance to pamidronate. In this study, iPTHrP(50-69)-induced osteoclastic bone resorption was a more important mechanism in the causation of TIH than PTHrP-induced renal reabsorption of calcium as assessed by the renal thresholds for calcium and phosphate.

Adenylate cyclase stimulating activity immunologically similar to parathyroid hormone-related peptide can be extracted from fetal rat long bones

We observed that culture medium conditioned with fetal rat long bones contained peptides immunologically related to the parathyroid hormone-related peptide of malignancy (PTHrP) and stimulated cyclic AMP production in canine renal cortical membranes. Because the adenylate cyclase stimulating activity (CSA) of the medium increased when bone resorption was stimulated, it was suspected that these peptides were stored in the matrix and released during the resorption process. In this work, we extracted the noncollagenous proteins of fetal rat long bones and found that the extract contained significant amounts of CSA. The biologic activity of the extract was abolished after trypsin digestion and eluted at 24 and 37 kD on filtration HPLC. The CSA of bone extract and of both HPLC peaks could be inhibited by 3-34 and 7-34 parathyroid hormone analogs. It was not blocked by an antiserum directed against the N-terminal region of parathyroid hormone, but it was significantly inhibited after an overnight preincubation with an antiserum directed against the 1-11 fragment of PTHrP. One band migrating at 18 kD could be visualized after SDS-PAGE electrophoresis of bone extract and immunoblotting with the anti-PTHrP antiserum. We conclude that an adenylate cyclase stimulator immunologically similar to PTHrP is present in the matrix of fetal rat long bones. Adenylate cyclase stimulating peptides of lower molecular weight found in bone-conditioned medium could be active fragments formed by proteolysis during the resorption process.

Production of calcitonin gene related peptide, calcitonin and PTH-related protein by a prostatic adenocarcinoma

PTH and calcitonin are the two major hormones controlling calcium metabolism. Recently two new substances related to these hormones have been isolated: calcitonin gene related peptide (CGRP) and PTH-related protein (PTHrP). CGRP is a potent vasodilator and stimulant of intestinal secretion while PTHrP is probably the agent responsible for humoral hypercalcaemia of malignancy. We report here a patient with a prostatic tumour presenting with vasodilation, diarrhoea and hypercalcaemia. Our investigations revealed that the primary prostatic and liver secondary tumour contained CGRP, calcitonin and PTHrP. Most of the immunoreactive CGRP in the tumour and plasma co-eluted with the biologically active form of CGRP. The circulating levels of CGRP correlated with the presence of the diarrhoea. PTHrP concentration in the tumours was one of the highest reported for any tumour although previous studies may have utilized less than optimal extraction procedures. The somatostatin analogue, octreotide (SMS 201-995), did not reduce the plasma CGRP or the diarrhoea, a finding similar to that seen in patients with medullary thyroid carcinoma and high plasma CGRP. The hypercalcaemia was also unaffected by octreotide administration. This is the first report of a prostatic tumour associated with over-production of calcitonin, PTHrP and CGRP. The major life-threatening effects of this unusual case of prostatic carcinoma were diarrhoea and hypercalcaemia. Both these effects could be tentatively ascribed to newly discovered substances, CGRP and PTHrP. With the greater availability of assays to measure CGRP and PTHrP in plasma, a detailed examination of the incidence of over-production of these substances in various cancers will be possible.

Parathyroid hormone-related protein: biochemistry and molecular biology

This article critically reviews the current state of knowledge regarding the recently identified and cloned novel hormone parathyroid hormone-related protein (PTHrP). PTHrP is produced by tumors associated with the syndrome of humoral hypercalcemia of malignancy giving rise to the parathyroid hormone (PTH)-like symptoms characteristic of the syndrome. Areas that will be reviewed include identification, purification and cloning, localization, actions, and significance of PTHrP in cancers and normal physiology. The structure and regulation of the PTHrP gene that may be ancestrally related to the PTH gene will also be discussed. Studies in vivo and in vitro with synthetic and recombinant PTHrP sequences and antibodies developed against them have established that the PTH-like actions of PTHrP are mediated via the N-terminal sequences, which show some limited sequence homology with PTH. Evidence for PTH and non-PTH-like actions of PTHrP in normal physiology, which implicate a role for PTHrP in fetal and neonatal development, is also presented.

Development of endocrine pathways in the regulation of calcium homeostasis

The mammalian fetus is maintained hypercalcaemic relative to its mother by the action of a calcium pump believed to be located at the basement membranes of the epithelial cells of the fetal chorion. It has recently been demonstrated that the activity of this putative pump is stimulated by a new fetal hormone, parathyroid hormone-related protein, described originally as the product of a human BEN cell line which was derived from a lung tumour associated with hypercalcaemia of malignancy. Whereas the circulating level of immunoreactive parathyroid hormone in the fetus is very low, in keeping with the hypercalcaemia, the plasma concentrations of bioactive parathyroid hormone and parathyroid hormone-related protein can be measured using a sensitive cytochemical bioassay and the separate concentrations assessed by pre-incubation with appropriate antisera. The total plasma concentration of both hormones is inversely related to the prevailing calcium ion concentration but the set point of parathyroid hormone-related protein is probably higher than that for parathyroid hormone. Probably as a result of the hypercalcaemia, the circulating concentration of calcitonin is also higher than in maternal plasma and may serve to limit bone resorption to favour net bone accretion as part of the overall growth of the fetus. Vitamin D and its most active metabolite, 1,25(OH)2D, can pass across the placenta in either direction, in contrast to most peptide hormones. In addition to the supply of some 1,25(OH)2D by the mother to her fetus, the fetal placenta and fetal kidneys can all synthesize 1,25(OH)2D. The relative concentrations circulating in maternal and fetal plasma pools vary with the species, presumably as a result of differing importance of the three sources of supply to the fetus and the relative concentrations of vitamin D-binding protein circulating in mother and fetus. The importance of parathyroid hormone-related protein derived from fetal parathyroid glands has been clearly demonstrated in the fetal sheep. Such animals develop rickets following the removal of their parathyroid glands, despite the demonstration of this substance in fetal placental membranes. However, the relative importance of the parathyroid glands versus the placenta and its membranes as the principal source of parathyroid hormone-related protein remains to be elucidated and may vary with species.