Identification of a cDNA encoding a parathyroid hormone-like peptide from a human tumor associated with humoral hypercalcemia of malignancy

Re-Evaluating the Role of PTHrP in Breast Cancer

Parathyroid-hormone-related protein (PTHrP) is a protein with a long history of association with bone metastatic cancers. The paracrine signaling of PTHrP through the parathyroid hormone receptor (PTHR1) facilitates tumor-induced bone destruction, and PTHrP is known as the primary driver of humoral hypercalcemia of malignancy. In addition to paracrine signaling, PTHrP is capable of intracrine signaling independent of PTHR1 binding, which is essential for cytokine-like functions in normal physiological conditions in a variety of tissue types. Pre-clinical and clinical studies evaluating the role of PTHrP in breast cancer have yielded contradictory conclusions, in some cases indicating the protein is tumor suppressive, and in other studies, pro-growth. This review discusses the possible molecular basis for the disharmonious prognostic indications of these studies and highlights the implications of the paracrine, intracrine, and nuclear functions of the protein. This review also examines the current understanding of the functional domains of PTHrP and re-evaluates their role in the unique context of the breast cancer environment. This review will expand on the current understanding of PTHrP by attempting to reconcile the functional domains of the protein with its intracrine signaling in cancer.

Hypercalcemia of Malignancy

Hypercalcemia of malignancy (HCM) is considered an oncologic emergency associated with significant symptom burden and increased comorbid conditions and mortality. Underlying pathologic processes most often stimulate osteoclast-mediated bone resorption. Although long-term control of HCM depends on effective management of the underlying cancer, temporizing management strategies for acute and/or symptomatic HCM include hydration and antiresorptive bone-modifying agents. Although most patients respond well to the antiresorptive therapies available, further investigation into other agents for those who are refractory to both bisphosphonates and denosumab is needed.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

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.

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.

Deletion of the nuclear localization sequences and C-terminus of PTHrP impairs embryonic mammary development but also inhibits PTHrP production

Parathyroid hormone-related protein (PTHrP) can be secreted from cells and interact with its receptor, the Type 1 PTH/PTHrP Receptor (PTHR1) in an autocrine, paracrine or endocrine fashion. PTHrP can also remain inside cells and be transported into the nucleus, where its functions are unclear, although recent experiments suggest that it may broadly regulate cell survival and senescence. Disruption of either the PTHrP or PTHR1 gene results in many abnormalities including a failure of embryonic mammary gland development in mice and in humans. In order to examine the potential functions of nuclear PTHrP in the breast, we examined mammary gland development in PTHrP (1-84) knock-in mice, which express a mutant form of PTHrP that lacks the C-terminus and nuclear localization signals and which can be secreted but cannot enter the nucleus. Interestingly, we found that PTHrP (1-84) knock-in mice had defects in mammary mesenchyme differentiation and mammary duct outgrowth that were nearly identical to those previously described in PTHrP-/- and PTHR1-/- mice. However, the mammary buds in PTHrP (1-84) knock-in mice had severe reductions in mutant PTHrP mRNA levels, suggesting that the developmental defects were due to insufficient production of PTHrP by mammary epithelial cells and not loss of PTHrP nuclear function. Examination of the effects of nuclear PTHrP in the mammary gland in vivo will require the development of alternative animal models.

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.

Small cell ovarian carcinoma: genomic stability and responsiveness to therapeutics

Background

The biology of small cell ovarian carcinoma of the hypercalcemic type (SCCOHT), which is a rare and aggressive form of ovarian cancer, is poorly understood. Tumourigenicity, in vitro growth characteristics, genetic and genomic anomalies, and sensitivity to standard and novel chemotherapeutic treatments were investigated in the unique SCCOHT cell line, BIN-67, to provide further insight in the biology of this rare type of ovarian cancer.

Method

The tumourigenic potential of BIN-67 cells was determined and the tumours formed in a xenograft model was compared to human SCCOHT. DNA sequencing, spectral karyotyping and high density SNP array analysis was performed. The sensitivity of the BIN-67 cells to standard chemotherapeutic agents and to vesicular stomatitis virus (VSV) and the JX-594 vaccinia virus was tested.

Results

BIN-67 cells were capable of forming spheroids in hanging drop cultures. When xenografted into immunodeficient mice, BIN-67 cells developed into tumours that reflected the hypercalcemia and histology of human SCCOHT, notably intense expression of WT-1 and vimentin, and lack of expression of inhibin. Somatic mutations in TP53 and the most common activating mutations in KRAS and BRAF were not found in BIN-67 cells by DNA sequencing. Spectral karyotyping revealed a largely normal diploid karyotype (in greater than 95% of cells) with a visibly shorter chromosome 20 contig. High density SNP array analysis also revealed few genomic anomalies in BIN-67 cells, which included loss of heterozygosity of an estimated 16.7 Mb interval on chromosome 20. SNP array analyses of four SCCOHT samples also indicated a low frequency of genomic anomalies in the majority of cases. Although resistant to platinum chemotherapeutic drugs, BIN-67 cell viability in vitro was reduced by >75% after infection with oncolytic viruses.

Conclusions

These results show that SCCOHT differs from high-grade serous carcinomas by exhibiting few chromosomal anomalies and lacking TP53 mutations. Although BIN-67 cells are resistant to standard chemotherapeutic agents, their sensitivity to oncolytic viruses suggests that their therapeutic use in SCCOHT should be considered.

Parathyroid hormone-related protein: an update

PTHrP was identified as a cause of hypercalcemia in cancer patients 25 yr ago. In the intervening years, we have learned that PTHrP and PTH are encoded by related genes that are part of a larger "PTH gene family." This evolutionary relationship permits them to bind to the same type 1 PTH/PTHrP receptor, which explains why humoral hypercalcemia of malignancy resembles hyperparathyroidism. This review will outline basic facts about PTHrP biology and its normal physiological functions, with an emphasis on new findings of the past 5-10 yr. The medical and research communities first became aware of PTHrP because of its involvement in a common paraneoplastic syndrome. Now, research into the basic biology of PTHrP has suggested previously unrecognized connections to a variety of disease states such as osteoporosis, osteoarthritis, and breast cancer and has highlighted how PTHrP itself might be used in therapy for osteoporosis and diabetes. Therefore, the story of this remarkable protein is a paradigm for translational research, having gone from bedside to bench and now back to bedside.

Twenty-five years of PTHrP progress: from cancer hormone to multifunctional cytokine

Twenty-five years ago a "new" protein was identified from cancers that caused hypercalcemia. It was credited for its ability to mimic parathyroid hormone (PTH), and hence was termed parathyroid hormone-related protein (PTHrP). Today it is recognized for its widespread distribution, its endocrine, paracrine, and intracrine modes of action driving numerous physiologic and pathologic conditions, and its central role in organogenesis. The multiple biological activities within a complex molecule with paracrine modulation of adjacent target cells present boundless possibilities. The protein structure of PTHrP has been traced, dissected, and deleted comprehensively and conditionally, yet numerous questions lurk in its past that will carry into the future. Issues of the variable segments of the protein, including the enigmatic nuclear localization sequence, are only recently being clarified. Aspects of PTHrP production and action in the menacing condition of cancer are emerging as dichotomies that may represent intended temporal actions of PTHrP. Relative to PTH, the hormone regulating calcium homeostasis, PTHrP "controls the show" locally at the PTH/PTHrP receptor throughout the body. Great strides have been made in our understanding of PTHrP actions, yet years of exciting investigation and discovery are imminent. © 2012 American Society for Bone and Mineral Research.

Evolution of the vertebrate pth2 (tip39) gene family and the regulation of PTH type 2 receptor (pth2r) and its endogenous ligand pth2 by hedgehog signaling in zebrafish development

In mammals, parathyroid hormone (PTH), secreted by parathyroid glands, increases calcium levels in the blood from reservoirs in bone. While mammals have two PTH receptor genes, PTH1R and PTH2R, zebrafish has three receptors, pth1r, pth2r, and pth3r. PTH can activate all three zebrafish Pthrs while PTH2 (alias tuberoinfundibular peptide 39, TIP39) preferentially activates zebrafish and mammalian PTH2Rs. We know little about the roles of the PTH2/PTH2R system in the development of any animal. To determine the roles of PTH2 and PTH2R during vertebrate development, we evaluated their expression patterns in developing zebrafish, observed their phylogenetic and conserved synteny relationships with humans, and described the genomic organization of pth2, pth2r, and pth2r splice variants. Expression studies showed that pth2 is expressed in cells adjacent to the ventral part of the posterior tuberculum in the diencephalon, whereas pth2r is robustly expressed throughout the central nervous system. Otic vesicles express both pth2 and pth2r, but heart expresses only pth2. Analysis of mutants showed that hedgehog (Hh) signaling regulates the expression of pth2 transcripts more than that of nearby gnrh2-expressing cells. Genomic analysis showed that a lizard, chicken, and zebra finch lack a PTH2 gene, which is associated with an inversion breakpoint. Likewise, chickens lack PTH2R, while humans lack PTH3R, a case of reciprocally missing ohnologs (paralogs derived from a genome duplication). The considerable evolutionary conservation in genomic structure, synteny relationships, and expression of zebrafish pth2 and pth2r provides a foundation for exploring the endocrine roles of this system in developing vertebrate embryos.

Role of RANKL-RANK/osteoprotegerin molecular complex in bone remodeling and its immunopathologic implications

Bone remodeling is a cyclic and continuous physiological process, which ensures the conservation and renewal of the bone matrix. Osteosynthesis of the bone matrix is achieved by osteoblasts and coordinated within this complex machinery of bone remodeling with resorption of extracellular bone matrix performed by osteoclasts. The mismatch between the activities of osteoblasts and osteoclasts has immunopathologic implications associated with either a decrease or increase of bone mass mineral density. The balance of the trimolecular control factor complex composed of osteoprotegerin (OPG), RANKL (osteoprotegerin ligand) and RANK maintains physiologic bone remodeling. This trimolecular complex functions as receptors and ligands and belongs to the superfamily of tumor necrosis factor (TNF). This mini review highlights the complex interplay of the RANKL-RANK/OPG axis and their immunopathologic implications in clinical medicine.

Severe hypercalcemia associated with hepatocellular carcinoma secreting intact parathyroid hormone: a case report

Here we describe a 73-year-old woman with hypercalcemia caused by a hepatocellular carcinoma (HCC) secreting intact parathyroid hormone (iPTH). Serum tumor markers and dynamic CT findings indicated a diagnosis of HCC. The source of the elevated serum iPTH was not obvious. Transarterial chemoembolization (TACE) was effective against the HCC, and the serum iPTH level fell to within the normal range, suggesting a correlation between the carcinoma and the iPTH. About 2 months later, the tumor had grown and the serum calcium level increased leading to physical deterioration and death. This clinical course suggested that HCC can ectopically secrete iPTH.

Parathyroid hormone related protein (PTHrP) in tumor progression

Parathyroid hormone-related protein (PTHrP) is widely expressed in fetal and adult tissues and is a key regulator for cellular calcium transport and smooth muscle cell contractility, as well as a crucial control factor in cell proliferation, development and differentiation. PTHrP stimulates or inhibits apoptosis in an autocrine/paracrine and intracrine fashion, and is particularly important for hair follicle and bone development, mammary epithelial development and tooth eruption. PTHrP's dysregulated expression has traditionally been associated with oncogenic pathologies as the major causative agent of malignancy-associated hypercalcemia, but recent evidence revealed a driving role in skeletal metastasis progression. Here, we demonstrate that PTHrP is also closely involved in breast cancer initiation, growth and metastasis through mechanisms separate from its bone turnover action, and we suggest that PTHrP as a facilitator of oncogenes would be a novel target for therapeutic purposes.

PTHrP P3 promoter activity in breast cancer cell lines: role of Ets1 and CBP (CREB binding protein)

Parathyroid hormone-related protein (PTHrP) is produced by many tumors including breast cancer. We have reported that Ets1 factor activates P3 PTHrP promoter in our model of tumorigenic breast cancer cell and not in pre- or non-tumorigenic cell lines, thus contributing to an increased PTHrP production. In this study, gel retardation assays revealed that Etsl and its promoter binding site (EBS) specifically formed complexes whose abundance correlates with Ets1 levels in the three cell lines. Coexpression of Etsl and CBP induced a synergistic activation of the P3 promoter only in the tumorigenic cell line. This synergism required the integrity of the EBS and was abrogated by E1A. All breast cancer cell lines showed high basal concentrations of phosphorylated CREB. Moreover a CRE-like sequence was also required for Ets1/CBP synergy and, finally, CREB expression was found to enhance the PTHrP P3 promoter activity. Thus a multipartite complex of transcription factors and coactivators seems to regulate PTHrP transcription and contribute to the alterations that promote tumorigenic behavior in breast epithelial cells.

Hypercalcaemia in relapsed medulloblastoma 8 years post-diagnosis; evidence to support PTHrP production by medulloblastoma cells

BACKGROUND

A 19-year-old male presented with symptomatic hypercalcaemia as the first manifestation of relapsed metastatic medulloblastoma. Management at the time of the initial presentation 8 years earlier was with surgical excision and craniospinal radiotherapy. His biochemistry at the time of relapse and studies of medulloblastoma cell lines provide an insight into the pathogenesis of his hypercalcaemia.

METHODS

Parathyroid hormone-related protein (PTHrP) was measured by immunoradiometric assay in blood, and in conditioned and control media from three medulloblastoma cell lines following 72 h growth.

RESULTS

The histology at initial presentation (11 years of age) and at the time of relapse (with bone marrow infiltration and widespread bony metastases) demonstrated medulloblastoma. Ionised calcium concentrations at relapse were 2.89 mmol/l and serum PTHrP levels were increased at the same time (2.7 pmol/l; normal range: 0.7-1.8 pmol/l). There was evidence of PTHrP production by one cell line (MHH-MED-8A) while results for both other lines tested were below the limit of detection.

CONCLUSIONS

Relapse 8 years after diagnosis is unusual in medulloblastoma and for this relapse to manifest as hypercalcaemia is also very uncommon. Our investigations suggest that the clinical picture was a reflection of PTHrP production by medulloblastoma cells.

Small cell carcinoma of the ovary with hypercalcemia and ectopic parathyroid hormone production

Small cell carcinoma of the ovary is a rare malignant tumor of the ovary. It is the most common undifferentiated ovarian carcinoma in young women. Approximately two thirds of patients with ovarian small cell carcinoma have hypercalcemia. The mechanism of development of hypercalcemia is unclear, although parathyroid hormone-related protein has been found in some of the cases. Parathormone expression in tumor cells, rarely reported, was seen in this case, suggesting that ectopic parathyroid hormone production by the tumor cells may be the cause of hypercalcemia.

Parathyroid hormone expression in a patient with metastatic nasopharyngeal rhabdomyosarcoma and hypercalcemia

Ectopic PTH secretion by tumor cells has been described as the cause of hypercalcemia associated with malignancy in the absence of osteolytic bone lesions. Although there have been case reports of elevated PTH and hypercalcemia in patients with rhabdomyosarcoma, to date ectopic PTH secretion by malignant cells has not been definitively shown. The possibility of PTH production by pleural-based metastatic nasopharyngeal rhabdomyosarcoma cells in a 62-yr-old Japanese male with hypercalcemia was investigated. The patient's serum PTH level was found to be elevated at 62.22 pmol/L, and pleural fluid PTH level was 47.28 pmol/L and PTHrP level was 3.7 pmol/L. RT-PCR of mRNA extracted from rhabdomyosarcoma cells in the pleural fluid was performed with the addition of PTH and PTHrP exonic primer sets yielded only a cDNA fragment of approx 150 bp consistent with the expected PTH fragment. Sequence analysis of a nested primer PCR fragment confirmed PTH mRNA sequence. We believe this patient to have had hypercalcemia secondary to ectopic PTH secretion, as we have identified the presence of PTH mRNA in tumor cells. We speculate that the overexpression of PTH in rhabdomyosarcoma cells results from molecular rearrangement of the PTH gene. The finding of a normal PTH DNA sequence of the PCR fragment suggests the likelihood of alterations in regulatory sequences.

Altered renal hemodynamics in mice overexpressing the parathyroid hormone (PTH)/PTH-related peptide type 1 receptor in smooth muscle

PTH-related protein (PTHrP) is an autocrine/paracrine peptide expressed in renal tubules and vasculature and may play an important role in regulating overall renal function. To evaluate the potential role of endogenous PTHrP in the control of renal hemodynamics, we performed clearance measurements in transgenic (TG) mice in which the SMP8 alpha-actin promoter was used to drive overexpression of the PTH/PTHrP type 1 receptor in smooth muscle. In protocol I, responses to acute saline volume expansion (SVE, 0.75 microl/min.g body weight) were measured in TG and nontransgenic (NTG) mice. Mean arterial pressure was significantly lower in TG mice throughout the experiment, and it decreased comparably in both groups in response to SVE. SVE significantly increased effective renal plasma flow in both groups of mice, but the increase was greater in TG than in NTG. Glomerular filtration rate decreased in response to SVE in NTG but did not change in TG animals. In protocol II, renal responses to angiotensin II (ANG II) infusion were determined (0.5 ng/min.g body weight). Baseline arterial pressure was again significantly lower in TG, compared with NTG mice, and TG mice had a blunted pressor response to ANG II. Also, ANG II decreased effective renal plasma flow and glomerular filtration rate in both groups of animals, but the reductions were less in TG than in NTG mice. Our findings indicate that smooth-muscle-specific overexpression of the PTH/PTHrP type 1 receptor resulted in augmentation of the vasodilatory response to SVE and attenuation of the vasoconstrictor response to ANG II. We conclude that endogenous PTHrP can act as an endogenous vasorelaxant factor to modulate renal responses to vasoactive stimuli.

Autoregulation in the parathyroid glands by PTH/PTHrP receptor ligands in normal and uremic rats

BACKGROUND

The secretion of parathyroid hormone (PTH) from the parathyroid glands might be regulated by autocrine/paracrine factors. We have previously shown that N-terminal parathyroid hormone-related protein (PTHrP) enhanced the secretory PTH response to low calcium in vivo and in vitro in rat parathyroid glands. N-terminal PTHrP fragments are equipotent to N-terminal PTH as ligands for the PTH/PTHrP receptor that is demonstrated in parathyroid tissue. This supports the possibility that the parathyroid cells respond to PTH/PTHrP receptor ligands and as such are target for an autoregulatory action of PTH and PTHrP. Our aim was to search for the PTH/PTHrP receptor in rat parathyroid glands and to examine the effects of PTHrP 1-40 on PTH secretion in in vivo models of secondary hyperparathyroidism (HPT) in uremic rats.

METHODS

PTH secretion was examined during ethyleneglycol tetraacetic acid (EGTA)-induced hypocalcemia both with and without PTHrP. Five groups, each of six normal rats, received a bolus of increasing doses of 0.1, 1.0, 10, and 100 microg of PTHrP 1-40, or vehicle only. Chronic renal failure (CRF) was induced by 5/6 nephrectomy. One group of 12 CRF rats received a standard diet, while another CRF group of 18 rats received a high phosphorus diet to induce more severe HPT. After 8 weeks of uremia, the rats were infused with EGTA and PTHrP 1-40 or vehicle. The presence of the PTH/PTHrP receptor in the rat parathyroid glands was examined by reverse transcription-polymerase chain reaction (RT-PCR) technique. PTH was measured by a rat PTH assay that does not cross-react with PTHrP.

RESULTS

In a dose-related manner, PTHrP enhanced the PTH response to hypocalcemia in normal rats. A similar rate of decrease of plasma Ca++ was induced by EGTA in all experimental groups. In CRF rats, plasma creatinine (0.99 +/- 0.10 mmol/L vs. 0.33 +/- 0.01 mmol/L, P < 0.001) and plasma PTH (226 +/- 32 pg/mL vs. 69 +/- 16 pg/mL, P < 0.001) levels were significantly increased. The CRF rats on high phosphorus diet had significant hypocalcemia (Ca++, 1.04 +/- 0.02 mmol/L vs. 1.28 +/- 0.03 mmol/L, P < 0.001), hyperphosphatemia (3.48 +/- 0.3 mmol/L vs. 2.25 +/- 0.1 mmol/L, P < 0.001) and severe secondary HPT, PTH (984 +/- 52 pg/mL vs. 226 +/- 32 pg/mL, P < 0.001) compared to CRF rats on a standard phosphorus diet. The maximal PTH response to hypocalcemia was enhanced in CRF rats (maximum PTH 382 +/- 58 pg/mL vs. 196 +/- 29 pg/mL in normal rats, P < 0.01) and further enhanced by PTHrP 1-40 to 826 +/- 184 pg/mL (P < 0.01). The secretory capacity of the parathyroid glands in response to low Ca++ was severely diminished in uremia. In CRF rats given a high phosphorus diet, the basal PTH levels were at the upper part of the calcium/PTH curve, and the induction of more marked hypocalcemia did not stimulate PTH secretion further (maximum PTH 1475 +/- 208 pg/mL vs. basal 1097 +/- 69 pg/mL, NS). PTHrP, however, further enhanced the maximal PTH levels significantly (maximum PTH 3142 +/- 296 pg/mL, P < 0.01). The presence of the PTH/PTHrP receptor in the rat parathyroid glands was confirmed by RT-PCR technique.

CONCLUSION

PTHrP enhanced significantly, in a dose-related manner, the low Ca++-stimulated PTH secretion in normal rats. The PTH/PTHrP receptor is present in rat parathyroid glands. The impaired secretory capacity of the parathyroid glands in uremic rats is significantly enhanced by PTHrP. An autocrine/paracrine role in the parathyroid glands of the PTH/PTHrP receptor targeting peptides, PTHrP and PTH, is suggested. Thus, it is hypothesized that PTH during hypocalcemia might have a positive auto-feedback regulatory role on its own secretion.

Ets-1 activates parathyroid hormone-related protein gene expression in tumorigenic breast epithelial cells

Parathyroid hormone-related protein (PTHrP) is produced by many tumors not associated with humoral hypercalcemia, including breast cancers. In this study, we used three human immortalized mammary epithelial cell lines that differ in tumorigenicity and PTHrP expression. Using RT-PCR we investigated 5' and 3' alternative splicing of PTHrP transcripts and promoter usage in the lines. Increased levels of P3-derived transcripts and the 1-139 mRNA isoform were observed in the most tumorigenic cell line. Transient transfection experiments identified elements close to P3 promoter that appeared to account for a portion of differential PTHrP expression among the three cell lines. Using site-directed mutagenesis, a previously described Ets-1/Sp1 binding site upstream of P3 was determined to be crucial for full activity of this promoter. RT-PCR and western blot evaluation of Ets family member expression found that Ese-1 was present in all three lines, but that appreciable levels of Ets-1 protein were present exclusively in the most tumorigenic line. Cotransfection of Ets-1 expression vectors activated PTHrP reporter constructs in the most tumorigenic line but not in the other cell lines. These findings suggest a potential mechanism by which PTHrP transcription may be regulated as a consequence of events that promote tumorigenic behavior in breast epithelial cells.

Hypercalcemic complication in patients with oral squamous cell carcinoma

Hypercalcemia is one of the metabolic complications associated with cancer. To assess the frequency of hypercalcemia in patients with squamous cell carcinoma (SCC), 242 patients who were evaluated as having SCC in the oral cavity between July 1995 and June 2001 were investigated. All patients were periodically monitored for their serum level of calcium (Ca). Hypercalcemia was defined as a serum Ca concentration higher than 11 mg/dl. By this definition, hypercalcemia was detected in 12 of the 242 patients (5.0%). All 12 patients were at an advanced stage of oral SCC. In these 12 patients, the serum level of parathyroid hormone-related protein (PTH-rP) was also significantly elevated. Therefore, we diagnosed these diseases as humoral hypercalcemia of malignancy (HHM). Moreover, we studied the efficacy of anti-hypercalcemic therapy on the quality of life (QOL). The European Organization for Research and Treatment of Cancer (EORTC) QLQ-C30 was used for estimation of QOL. The patients with HHM who were administrated drugs such as bisphosphonate and calcitonin showed a reduction in their Ca and PTH-rP levels, and the six of ten EORTC QLQ-C30 subscales (emotional functioning, cognitive functioning, fatigue, dyspnoea, nausea/vomiting and appetite loss) were also improved after the anti-hypercalcemic therapy. However, these suppressive effects were temporary. The median survival time after the diagnosis of HHM was only 54.9+/-18.3 days (range 27-86 days). Therefore, HHM in SCC appears to be an ominous prognostic sign. Although anti-hypercalcemic therapy has a palliative role, the patients may be in less discomfort during the terminal stage of their illness.

Expression of parathyroid hormone (PTH)/PTH-related peptide receptor messenger ribonucleic acid in mice hair cycle

There is increasing evidence that parathyroid hormone (PTH) and PTH-related peptide (PTHrP) are involved in normal skin cell growth, influence the proliferation and differentiation of the epidermis and hair follicle. PTHrP and PTH/PTHrP receptor show prominent cutaneous expression, may exert important paracrine and/or autocrine functions. The expression of PTH/PTHrP receptor in different stages of hair cycle is unknown. Therefore, we examined the amount of PTH/PTHrP mRNA in C57BL/6 mice skin at different stages of hair cycle by relatively quantitative reverse transcription-polymerase chain reaction (RT-PCR), and investigated the localization of this receptor in mice skin by in situ hybridization. The expression of PTH/PTHrP receptor mRNA were higher in anagen, but significantly lower in catagen and telogen. Then, the PTH/PTHrP receptor mRNA was located in the inner root sheath (IRS) in anagen and catagen, but was not detected in telogen hair follicles, although it was expressed weakly in dermis. The variety of the PTH/PTHrP receptor mRNA expression during hair cycling suggest that PTH, PTHrP and their receptors might participate in the regulation of hair cycle in mice skin.

Molecular cloning and functional characterization of the canine parathyroid hormone/parathyroid hormone related peptide receptor (PTH1)

Parathyroid hormone (PTH) is a major mediator of calcium and phosphate metabolism through its interactions with receptors in kidney and bone. PTH binds with high affinity to PTH1 and PTH2, members of the superfamily of G protein-coupled receptors. In order to clone the canine PTH1 receptor, a canine kidney cDNA library was screened using the human PTH1 receptor cDNA and two clones were further characterized. The longest clone was 2177 bp and contained a single open reading frame of 1785 bp, potentially encoding a protein of 595 amino acids with a predicted molecular weight of 66.4 kD. This open reading frame exhibits >91% identity to the human PTH1 receptor cDNA and >95% identity when the putative canine and human protein sequences are compared. Competition binding following transfection of the canine PTH1 receptor into CHO cells demonstrated specific displacement of 125I-human PTH1-34 by canine PTH1-34, human PTH1-34, and canine/human parathyroid hormone related peptide (PTHrP) 1-34. Treatment of canine PTH1 receptor transfected cells, but not mock transfected cells, with these ligands also resulted in increased levels of intracellular cAMP. In contrast, the non-related aldosterone secretion inhibiting factor 1-35 neither bound nor activated the canine PTH1 receptor. Northern blot analysis revealed high levels of PTH1 receptor mRNA in the kidney, with much lower, but detectable, levels in aorta. heart, lung, prostate, testis, and skeletal muscle. Together, these data indicate that we have cloned the canine PTH1 receptor and that it is very similar, both in sequence and in functional characteristics, to the other known PTH1 receptors.

Clues from hypercalcaemia

British Journal of Cancer (2002) 86, 1021–1022. DOI: 10.1038/sj/bjc/6600220www.bjcancer.com

© 2002 Cancer Research UK

Paraneoplastic syndromes in urologic malignancy: the many faces of renal cell carcinoma

Renal cell carcinoma is unique among the genitourinary malignancies in that close to one third of affected patients show signs and symptoms of a paraneoplastic syndrome. The paraneoplastic syndromes associated with renal cell carcinoma range from those manifesting in constitutional symptoms (ie, fever, cachexia, and weight loss) to those that result in specific metabolic and biochemical abnormalities (ie, hypercalcemia, nonmetastatic hepatic dysfunction, amyloidosis, etc). The presence of a paraneoplastic syndrome in a patient with renal cell carcinoma is neither a marker of metastatic disease nor necessarily indicative of a poor prognosis. The importance of understanding the pathophysiology and biology behind the many paraneoplastic syndromes associated with renal cell carcinoma lies in the fact that the presence of these protean symptoms may be the initial presentation of either primary or recurrent disease. In this review, we will describe the proposed mechanisms of action of the many paraneoplastic syndromes associated with renal cell carcinoma as well as outline the clinical evaluation and treatment options currently available for these noteworthy disorders.

Hypercalcemia associated with parathyroid hormone-related protein at the terminal stage of uncomplicated squamous cell carcinoma in the head and neck region

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is mainly responsible for hypercalcemia in squamous cell carcinomas (SCCs).

METHODS

We retrospectively checked the appearance of hypercalcemia among 33 patients who died with head and neck SCC. Serum concentrations of C-terminal region of PTHrP (C-PTHrP) were measured in 15 of them. The intracellular PTHrP expression was immunohistochemically stained in 42 SCC sections obtained from the 33 before the appearance of hypercalcemia.

RESULTS

Hypercalcemia appeared in 24 of the 33, and increased serum C-PTHrP levels were confirmed in 11 of 12 hypercalcemic patients. PTHrP was identified in all SCC sections, and a stronger intensity than in normal squamous epithelia was observed in 50% of those obtained within 1 year before the onset of hypercalcemia.

CONCLUSION

A high incidence of PTHrP-induced hypercalcemia was shown among patients dying with head and neck SCCs. The intracellular increase in PTHrP might be observed preceding hypercalcemia.

Higher expression of K-ras is associated with parathyroid hormone-related protein-induced hypercalcaemia in renal cell carcinoma

OBJECTIVES

To determine whether the K-ras oncogene is associated with parathyroid hormone-related protein (PTHrP) production in renal cell carcinoma (RCC) and whether the serum value of PTHrP is related to the patients' survival.

PATIENTS AND METHODS

The serum levels of PTHrP and corrected serum calcium levels were analysed in 51 consecutive patients (29 men and 22 women, mean age 63.7 years, range 33-82) with newly diagnosed RCC. Matched pairs were analysed of the mRNA levels of K-ras and PTHrP in tumour and in corresponding non-tumour tissue originating from the same patient, using the polymerase chain reaction after reverse transcription.

RESULTS

Seven patients had elevated serum PTHrP values at the diagnosis of RCC. The mRNA expression of K-ras and PTHrP were detected in both tumour and non-tumour tissues, with K-ras mRNA levels being higher in the former than the latter (P < 0.05), and correlated with tumour stage (P < 0.05). There were no differences in PTHrP mRNA levels between the tissues. Furthermore, the mRNA levels of K-ras and PTHrP in seven tumours from patients with high serum values of PTHrP were higher than in tumours from those with normal values (both P < 0.01). The expression of mRNAs of K-ras and PTHrP was positively correlated (r = 0.771, P < 0.001). In seven patients with high serum PTHrP values the mRNA levels of PTHrP correlated with serum values of PTHrP and calcium (r = 0.875, P < 0.01 and r = 0.762, P < 0.05, respectively). Kaplan-Meier plots of survival rate in patients with elevated or normal serum PTHrP showed that high serum PTHrP was associated with a shorter overall survival (P < 0.05). The Cox proportional hazards model showed that serum PTHrP was an independent predictor of overall survival (P < 0.05).

CONCLUSIONS

These findings suggest that K-ras may be associated with PTHrP-induced hypercalcaemia and that PTHrP levels may reflect the aggressiveness of tumour cells through the K-ras oncogene in RCC.

Production of parathyroid hormone-related protein in two new cell lines of renal cell carcinoma

BACKGROUND

Hypercalcemia is the most common of all paraneoplastic syndromes and has been reported to appear in up to 20% of patients with renal cell carcinoma (RCC). Humoral hypercalcemia of malignancy is believed to be induced when parathyroid hormone-related protein (PTHrP) is excessively produced in cancer cells and impairs the homeostasis of serum calcium concentrations.

METHODS

Cancer cells were isolated from a surgical specimen and successfully cultured in a monolayer. The present study describes the establishment and characterization of new cell lines of RCC.

RESULTS

Two different cell lines, designated SMRC-1 and SMRC-3, were established from human RCC, each of which had been continuously secreting PTHrP in vitro. The patient from whom the SMRC-3 cells were obtained was shown to have elevated levels of PTHrP and resultant hypercalcemia. Cultured SMRC-1 was spindle-shaped in morphology. SMRC-3 had pleomorphic polygonal shapes and formed typical epithelial monolayers. Both cell types secreted intact, C-terminal PTHrP and interleukin-6 in the culture medium. Cellular messenger RNA of PTHrP was analyzed by reverse transcriptase-polymerase chain reaction. The SMRC-1 cells showed chromosome numbers ranging from 42 to 47 with consistent structural abnormalities of add(4)(q23~25) and add(6)(q13). The chromosomal analysis of SMRC-3 revealed a modal number of 95 with consistent structural abnormalities of add(1)(p36) and der(1;3)(q10;p10).

CONCLUSIONS

These cell lines could be good models for investigating the mechanism of PTHrP production and the relationship between this hormone and hypercalcemia.

Involvement of parathyroid hormone-related protein in experimental cachexia induced by a human lung cancer-derived cell line established from a bone metastasis specimen

Cachexia often causes deterioration in the quality of life in cancer patients; however, its mechanism remains poorly understood. Cachexia has often been observed in experimental animals with bone metastases, and parathyroid hormone-related protein (PTHrP) plays an important role in the formation of such metastases. We therefore investigated the possible involvement of PTHrP in an experimental cachexia model using human lung-cancer cells (HARA-B). HARA-B cells produce a high amount of PTHrP but no TNF-alpha, IL-6 or leukemia inhibitory factor. The s.c. inoculation of HARA-B cells into nude mice caused reductions in body weight, adipose tissue weight, muscle weight and serum glucose levels. Serum levels of calcium and PTHrP increased. Neutralization of PTHrP with antibody caused rapid weight gain along with a rapid decrease in serum calcium levels. Our findings suggest that PTHrP plays an important role in the development of cancer cachexia. PTHrP therefore is a possible target molecule for the treatment of cancer cachexia.

Hormonal regulation and implication of cell signaling in calcium transfer by placenta

During pregnancy, the human placenta transfers about 30 g of calcium (Ca2+) from the mother to the fetus. This transfer is mainly done during the third trimester, at a rate of 140 mg/(kg x d). It allows adequate growth and development of the fetus, since Ca2+ is vital for the mineralization of the fetus's skeleton and many cellular functions. Because Ca2+ flows through the placenta against an electrochemical gradient, calcemic hormones could also be involved to overcome this gradient. Hormones such as calcitonin, parathyroid hormone (PTH), and PTH-related peptide (PTHrP) have been found in maternal and fetal circulation, and they originate from both parties, as well as from the placenta in the case of PTHrP. As the placenta possesses most of the G-protein-coupled receptors to bind these hormones, it is likely that they play an important role in maternal-fetal Ca2+ homeostasis. More studies are needed to assess the importance of these hormones in the regulation of Ca2+ management during pregnancy, and to understand better the cell-signaling pathways involved. This article addresses the current knowledge in this field to guide future investigations on the roles, functions, and localizations of the components involved during Ca2+ transfer by syncytiotrophoblasts.

Parathyroid hormone-related peptide stimulates proliferation of highly tumorigenic human SV40-immortalized breast epithelial cells

Parathyroid hormone-related protein (PTHrP) is the main mediator of humoral hypercalcemia of malignancy (HHM) and it is produced by many tumors, including breast cancers. Breast epithelial cells as well as breast cancer tumors and cell lines have been reported as expressing PTHrP and the PTH/PTHrP receptor, suggesting that PTHrP may act as an autocrine factor influencing proliferation or differentiation of these cell types. We investigated PTHrP gene expression, PTH/PTHrP receptor signaling, and PTHrP-induced mitogenesis in three immortalized human mammary epithelial cell lines that exhibit differential tumorigenicity. The most tumorigenic cells expressed the highest levels of PTHrP messenger RNA (mRNA) and protein. We used reverse-transcription polymerase chain reaction (RT-PCR) and immunoblotting to detect the PTH/PTHrP receptor transcripts and proteins in all of the three cell lines. Treatment with human PTHrP(1-34) [hPTHrP(1-34)] and hPTH(1-34) increased intracellular cyclic adenosine monophosphate (cAMP) but not free Ca2+ in the nontumorigenic line. These agonists increased both cAMP and free Ca2+ levels in the moderately tumorigenic line, but only increased free Ca2+ in the highly tumorigenic line. Application of the PTH/PTHrP receptor antagonist [Asn10,Leu11,D Trp12]PTHrP(7-34) or PTHrP antibodies reduced [3H]thymidine incorporation in a dose-dependent fashion in the highly tumorigenic cell line but did not affect the other lines. Thus, treatment with a PTH/PTHrP receptor antagonist reduced cell proliferation, suggesting that PTHrP signaling mediated by the phospholipase C (PLC) pathway stimulates proliferation of a highly tumorigenic immortalized breast epithelial cell line.

Increased upstream methylation has no influence on the overexpression of the parathyroid hormone-related protein gene in squamous cell carcinoma of the lung

Humoral hypercalcaemia of malignancy (HHM) commonly results from the excessive production of a parathyroid hormone-related protein (PTHrP) by tumours. We have previously shown malignancy is associated with increased DNA methylation in the 5' region of the PTHrP gene. In a series of patients with lung carcinoma and relatively high serum calcium levels, 3 patients showed substantially increased PTHrP gene methylation while 5 patients showed no change in methylation status in this region. Patients showed marked tumour-specific expression of PTHrP through the P1 and P3 promoters with more general tumour and non-tumour expression through the P2 promoter. The lack of potential key regulatory CpG sites in the P1 promoter and the complete demethylation in the P2 and P3 promoters suggests methylation does not influence tumour-specific expression of PTHrP. Although demethylation may be a prerequisite for P2 and P3 expression, the overexpression of the PTHrP gene in cancer cells must be mediated through mechanisms other than DNA methylation.

Regulation of granulocyte colony-stimulating factor and parathyroid hormone-related protein production in lung carcinoma cell line OKa-C-1

Previously we have established a clonal squamous cell carcinoma cell line OKa-C-1 derived from lung cancer of a patient with marked leukocytosis and hypercalcemia. OKa-C-1 cells simultaneously produce granulocyte colony-stimulating factor (G-CSF) and parathyroid hormone-related protein (PTHrP) at the single cell level and cause paraneoplastic syndromes in nude mice bearing the tumor. It is known that the production of G-CSF and PTHrP is individually regulated by inflammatory cytokines in various malignant cells. To investigate the common factors in the regulation of G-CSF and PTHrP production in OKa-C-1 cells, we examined the effects of some inflammatory agents [lipopolysaccharide (LPS), phorbol-12-myristate-13-acetate (PMA), tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1) beta and IL-6] on G-CSF and PTHrP production, by means of enzyme-linked immunosorbent assay (ELISA), immunoradiometric assay (IRMA) and quantitative reverse transcription-polymerase chain reaction (RT-PCR). TNF-alpha or IL-1beta induced both G-CSF and PTHrP production in the conditioned medium. TNF-alpha synergized with IL-1beta to significantly increase G-CSF production. In addition, TNF-alpha and IL-1beta strongly induced G-CSF mRNA with peaks at 2 and 6 h respectively. Although PTHrP production was also strongly induced by TNF-a PTHrP mRNA expression was more strongly induced by PMA than by TNF-alpha. Thus, TNF-alpha and IL-1beta could be common factors that individually and synergistically regulate G-CSF and PTHrP production in OKa-C-1 cells. Moreover, G-CSF and PTHrP production could be not only transcriptionally, but also posttranscriptionally regulated by other factors.

Expression of parathyroid hormone-related protein in human and experimental atherosclerotic lesions: functional role in arterial intimal thickening

We investigated the expression of parathyroid hormone-related protein (PTHrP) in atherosclerotic lesions and the role of PTHrP in the development of arterial neointima formation. Immunohistochemical staining of PTHrP in the neointima of rat aorta produced by balloon injury and of rat femoral artery produced by non-obstructive polyethylene cuff placement, and in the atherosclerotic lesion of human coronary artery was performed using anti-human PTHrP-(1-34) antibody. Anti-muscle actin antibody, HHF-35, and anti-macrophage antibody, HAM-56, were used to identify smooth muscle cells and macrophages, respectively. Immunoreactivity of PTHrP was detected in the thickened intima of rat and human lesions where the predominant cell types were smooth muscle cells or macrophages dependently on the lesion type. In the next series of experiments, we examined the effect of PTHrP on the development of cuff-induced intimal thickening of rat femoral artery. Either PTHrP-(1-34) or PTHrP-(7-34), a PTH/PTHrP receptor antagonist, suspended in pluronic F-127 gel was locally applied around the rat femoral artery. Intimal thickening induced by cuff placement was evaluated 2 weeks later. PTHrP-(1-34) dose-dependently inhibited intimal thickening determined as intima/media ratio and % stenosis whereas PTHrP-(7-34) dose-dependently enhanced that. These results suggest that PTHrP, which is expressed in atherosclerotic lesions, inhibits the development of neointimal formation.

Inefficient function of the signal sequence of PTHrP for targeting into the secretory pathway

Parathyroid hormone-related peptide (PTHrP) is not only secreted out of cells, but also targeted to the nucleoli due to a nucleolar targeting signal (NTS). We assessed the molecular mechanism underlying the dual targeting of PTHrP by constructing a series of truncated forms of rat PTHrP cDNA and expressing them in CHO cells. Immunostaining was observed in both the Golgi apparatus and nucleoli in the same cell expressing PTHrP with the N-terminal full-length signal sequence. When PTHrP molecules were translated from CUGs downstream of the AUG-initiator codon in the signal sequences, potential alternative initiators of the translation, they were exclusively localized in the nucleoli. In contrast, when a construct containing only the ATG-initiator codon was expressed, PTHrP was found to localize in both the nucleolus and the Golgi apparatus. No nucleolar staining of PTHrP was observed in the CHO cells transfected with PTH/PTHrP receptors even after incubating with a conditioned medium containing PTHrP, ruling out a possibility that PTHrP is, once secreted, internalized via receptor-mediated endocytosis and subsequently conveyed to nucleoli. Compatible with these morphological observations, a preproform of PTHrP was found in the cells expressing PTHrP in addition to proPTHrP, indicative of molecules along the secretory pathway. These results strongly indicate that the signal sequence of PTHrP is not sufficient to direct all the newly synthesized molecules across the endoplasmic reticulum, resulting in part of it being delivered to the nucleoli due to the NTS.

Parathyroid hormone-related peptide and Indian hedgehog

Normal endochondral bone development requires temporal and spatial coordination of various cell types. Parathyroid hormone-related peptide and Indian hedgehog interact with each other and form a feedback loop that plays a major role in this coordination. Defects in the signalling of either of the two molecules cause severe bone malformations.

PTHrP enhances the secretory response of PTH to a hypocalcemic stimulus in rat parathyroid glands

BACKGROUND

The secretion of parathyroid hormone (PTH) from the parathyroid glands might be regulated by autocrine/paracrine factors, and a feedback regulatory mechanism of PTH on the secretion of PTH has been suggested. Because of the existence of a common receptor between PTH and PTH-related peptide (PTHrP), the aim of the present study was to examine the possible effects of PTHrP 1-40 and 1-86 on PTH secretion in rats.

METHODS

In vivo, the effect of PTHrP on Ca++-regulated PTH secretion was examined by the induction of hypocalcemia and hypercalcemia by an infusion of EGTA and Ca++, with and without PTHrP. The eventual effects of PTHrP on the peripheral metabolism of PTH were examined by infusion of human PTH (hPTH) with and without PTHrP. hPTH was measured by an intact hPTH assay not cross reacting with rat PTH or PTHrP. To examine whether near physiological levels of circulating PTH have an autoregulatory effect in vivo on PTH secretion from the parathyroid gland, an acute reduction of the circulating PTH was induced by an acute unilateral parathyroidectomy (UPTX). PTH secretion from the remaining parathyroid gland was followed in response to EGTA-induced hypocalcemia. In vitro investigations on the effect of PTHrP 1-40 on PTH secretion from whole rat parathyroid glands incubated in media containing a calcium concentration of 0.6 or 1.35 mmol/L were performed to confirm whether the effect of PTHrP was directly on the gland. The rat PTH assay was examined for cross reaction with PTHrP.

RESULTS

In vivo, the same rate of decrease of plasma Ca++ was induced in the experimental groups. The maximal response of PTH to hypocalcemia (218 +/- 16 pg/mL, N = 6) was significantly enhanced by PTHrP 1-40 (525 +/- 79 pg/mL, N = 6) and by PTHrP 1-86 (465 +/- 29 pg/mL, N = 6, P < 0.001). No effect of PTHrP on PTH secretion was found during normocalcemia or hypercalcemia. UPTX resulted in a 50% reduction of PTH secretion, and no compensatory increase of PTH was observed. PTHrP had no effect on the metabolism of PTH. In vitro, low-Ca++-induced PTH secretion was significantly augmented by 300% (P < 0.01) when the medium contained PTHrP 1-40. PTHrP did not cross react with the PTH assay.

CONCLUSIONS

PTHrP significantly enhanced the low-Ca++-stimulated PTH secretion in vivo and in vitro. An autocrine/paracrine role of PTHrP in the parathyroid glands is suggested. An autoregulatory effect of circulating PTH on the PTH secretion from parathyroid glands seems unlikely. The "maximal secretory capacity" of the parathyroid glands induced by hypocalcemia in vivo and in vitro is not the maximum, as PTH secretion can be increased even further, by several-fold.