Parathyroid Hormone-related Peptide Secretion Research Articles

Hormonal regulation of mammary gland development and lactation.

Lactation is critical to infant short-term and long-term health and protects mothers from breast cancer, ovarian cancer and type 2 diabetes mellitus. The mammary gland is a dynamic organ, regulated by the coordinated actions of reproductive and metabolic hormones. These hormones promote gland development from puberty onwards and induce the formation of a branched, epithelial, milk-secreting organ by the end of pregnancy. Progesterone withdrawal following placental delivery initiates lactation, which is maintained by increased pituitary secretion of prolactin and oxytocin, and stimulated by infant suckling. After weaning, local cytokine production and decreased prolactin secretion trigger large-scale mammary cell loss, leading to gland involution. Here, we review advances in the molecular endocrinology of mammary gland development and milk synthesis. We discuss the hormonal functions of the mammary gland, including parathyroid hormone-related peptide secretion that stimulates maternal calcium mobilization for milk synthesis. We also consider the hormonal composition of human milk and its associated effects on infant health and development. Finally, we highlight endocrine and metabolic diseases that cause lactation insufficiency, for example, monogenic disorders of prolactin and prolactin receptor mutations, maternal obesity and diabetes mellitus, interventions during labour and delivery, and exposure to endocrine-disrupting chemicals such as polyfluoroalkyl substances in consumer products and other oestrogenic compounds.

Mammary homeostasis in the mother-offspring dyad

Homeostasis during lactation is a special case in which the unit for regulation is a dyad comprising the mother and her currently nursing offspring (the mother–offspring dyad). This arrangement is not a trivial. A litter of mice can have a mass greater than the mother and nutrient demands that far exceed her. Homeostasis for milk secretion, appetite, and calcium metabolism must come under integrated control, responding seamlessly to the needs of the mother and the offspring.Serotonin (5-HT) is a primary local regulator of mammary homeostasis. 5-HT synthesis in the mammary epithelium is high during lactation and increases during milk stasis. Two important functions are attributed to the 5-HT system. Firstly, when alveolar spaces are filled with milk 5-HT inhibits milk secretion and opens tight junctions. This feedback induces early phases of involution. Secondly, 5-HT induces synthesis and secretion of parathyroid hormone-related peptide (PTHrP).

Abstract #1181995: A rare case of humoral hypercalcemia of malignancy caused by a Gallbladder Adenocarcinoma

Humoral hypercalcemia of malignancy (HHM) is a common paraneoplastic syndrome occurring secondary to oversecretion of parathyroid hormone-related peptide (PTHrP). It is usually seen in solid tumors particularly squamous cell carcinoma of the head, neck, and lungs. Gallbladder adenocarcinoma is an unusual cause of HHM. We present a rare case of hypercalcemia secondary to elevated PTHrP secretion in a patient with gallbladder adenocarcinoma.

Fetomaternal Acid-Base Balance and Electrolytes during Pregnancy.

A pregnant mother undergoes significant changes in acid-base status as well as sodium and calcium metabolism to combat her physiological needs of pregnancy. Pregnant patients experience mild respiratory alkalosis due to the stimulation of the respiratory center by progesterone. This is associated with a corresponding increase in bicarbonate excretion by kidneys; as a result, the pH remains slightly high (7.40–7.45) but within the normal range. Pregnant women are predisposed to starvation ketosis as compared to nonpregnant states due to relative insulin resistance and increased production of the counter-regulatory hormone. Physiological mild hyponatremia occurs during pregnancy due to increased AVP secretion caused by resetting of osmoreceptors in the hypothalamus at a lower osmolality, but values below 130 mEq/L require a diagnostic workup and intervention. Gestational diabetes insipidus can occur due to increased production or decreased destruction of enzyme vasopressinase. Secretion of parathyroid hormone-related peptide by the placenta and breasts and two- to three-fold increased calcium and phosphate absorption in the maternal gut are the key changes in calcium metabolism during pregnancy. Though rare, both hypo- and hypercalcemia in pregnancy are associated with significant maternofetal morbidity and mortality.How to cite this articleAhmed A. Fetomaternal Acid–Base Balance and Electrolytes during Pregnancy. Indian J Crit Care Med 2021; 25(Suppl 3):S193–S199.

Severe hypercalcemia caused by parathyroid hormone in a rectal cancer metastasis: a case report

BackgroundHypercalcemia of malignancy is relatively common in several cancers. However, in colorectal cancer, paraneoplastic phenomena that cause hypercalcemia is uncommon. In the few cases that are reported, secretion of parathyroid hormone-related peptide mediates the effect. We describe the first case of severe hypercalcemia mediated by intact parathyroid hormone secretion from a bone metastasis of colorectal origin. This was a diagnostic and therapeutic challenge.Case presentationA 68-year-old male treated for rectal adenocarcinoma 10 years earlier developed a bone metastasis. After initial treatment of the metastasis with surgery and irradiation, he developed a relapse with severe hypercalcemia and corresponding elevated parathyroid hormone levels. The workup showed no signs of parathyroid adenomas, but the metastasis produced intact parathyroid hormone. The hypercalcemia was successfully treated by irradiation and osteoclast inhibitor, and the patient received chemotherapy. Survival was 24 months from the onset of hypercalcemia.ConclusionsProper diagnosis of the uncommon endocrine disturbance allowed targeted therapy and avoidance of neck exploration for wrongly suspecting primary hyperparathyroidism. Intact parathyroid hormone should be measured in cases of malignant hypercalcemia.

Overexpression of CENPF correlates with poor prognosis and tumor bone metastasis in breast cancer

BackgroundCentromere Protein F (CENPF) associates with the centromere–kinetochore complex and influences cell proliferation and metastasis in several cancers. The role of CENPF in breast cancer (BC) bone metastasis remains unclear.MethodsUsing the ONCOMINE database, we compared the expression of CENPF in breast cancer and normal tissues. Findings were confirmed in 60 BC patients through immunohistochemical (IHC) staining. Microarray data from GEO and Kaplan–Meier plots were used analyze the overall survival (OS) and relapse free survival (RFS). Using the GEO databases, we compared the expression of CENPF in primary lesions, lung metastasis lesions and bone metastasis lesions, and validated our findings in BALB/C mouse 4T1 BC models. Based on gene set enrichment analysis (GSEA) and western blot, we predicted the mechanisms by which CENPF regulates BC bone metastasis.ResultsThe ONCOMINE database and immunohistochemical (IHC) showed higher CENPF expression in BC tissue compared to normal tissue. Kaplan–Meier plots also revealed that high CENPF mRNA expression correlated to poor survival and shorter progression-free survival (RFS). From BALB/C mice 4T1 BC models and the GEO database, CENPF was overexpressed in primary lesions, other target organs, and in bone metastasis. Based on gene set enrichment analysis (GSEA) and western blot, we predicted that CENPF regulates the secretion of parathyroid hormone-related peptide (PTHrP) through its ability to activate PI3K–AKT–mTORC1.ConclusionCENPF promotes BC bone metastasis by activating PI3K–AKT–mTORC1 signaling and represents a novel therapeutic target for BC treatment.

Trends in mortality due to hypercalcemia in breast cancer.

e18252 Background: Hypercalcemia (HC) in metastatic breast cancers results from various mechanisms including increased bone turnover and secretion of Parathyroid hormone-related peptide. The presence of HC has traditionally been associated with poor prognosis but recent advances in the treatment of both HC with bisphosphonates and RANK-L inhibitors, and metastatic breast cancers with chemotherapy and immunotherapy, have improved symptoms and progression free survival. It has yet to be determined whether HC remains a poor prognostic indicator. Methods: Adult admissions ( > 18 years) between 1999 and 2014 with a primary diagnosis of breast cancer (BC) were extracted from the Nationwide Inpatient Sample using ICD-9 code 174.9. They were stratified for the presence of HC (ICD-9 code: 275.42), and further according to the year of admission. Chi-Square test and cox proportional hazard regression were used to determine in-hospital mortality and hazard of death based on the presence of HC. Results: A total of 484,859 (N = 98,631, weighted N = 484,859) admissions with a primary diagnosis of BC were obtained. The prevalence of HC increased from 0.52% in 1999 to 1.32% in 2014, yet inpatient mortality and adjusted Hazard Ratios (HR) of mortality declined over time as shown in Table. Despite this, patients with HC continued to have a significant increase in hospital mortality and HR (p < .001). Conclusions: Due to advances in medical therapy, in hospital mortality for patients with metastatic breast cancer has been declining over the last 15 years, however, the presence of HC continues to carry an independent risk for mortality. More research is needed to elucidate why HC continues to confer worse prognosis despite new calcium-lowering treatment modalities.[Table: see text]

SAT-494 A Very Rare Case: Hypercalcemia Caused by Ectopic PTH Secretion in Endometrial Cancer

Up to 40% of cancer patients develop hypercalcemia due to malignancy. Parathyroid hormone related peptide (PTHrP) secretion accounts for >80% of the cases. Here, we describe an unusual case of hypercalcemia due to PTH secretion by endometrial cancer. A 46-year old female presented with nausea, vomiting and right lower quadrant pain. She was noted to be tachycardic, with a pulse of 110 bpm; physical exam was significant only for mild abdominal distention. Laboratory analysis showed a serum calcium of 12.8 mg/dl (8.5-10.5), albumin of 3.8 g/dl (3.5-5), mild transaminitis and normal renal function. A contrasted CT of the abdomen and pelvis revealed a 26 cm right ovarian mass, hypodense lesions throughout the liver, moderate ascites and omental fat nodularity worrisome for ovarian malignancy and peritoneal carcinomatosis. Total abdominal hysterectomy with bilateral salpingo-oophorectomy with biopsy of small bowel mesentery and liver were performed. Pathology showed dedifferentiated endometroid adenocarcinoma with metastasis to the ovaries, liver and small bowel mesentery. Post-operatively, the calcium normalized but then gradually climbed to 12.6 mg/dl despite adequate intravenous hydration. Workup showed a PTH of 135 pg/ml (10-65), PTHrP was undetectable, 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D were normal. Neck ultrasound was negative for an obvious parathyroid adenoma. However, immunohistochemical staining of the surgical pathology was strongly positive for PTH. Pamidronate 30 mg IV was administered. Serum calcium normalized , and the patient was discharged for outpatient neo-adjuvant chemotherapy. Unfortunately, she passed away from septic shock and multi-organ failure a few weeks after diagnosis. Hypercalcemia in malignancy may arise via three distinct mechanisms: (1) humoral hypercalcemia arising from tumor secretion of PTHrP or PTH; (2) osteolytic hypercalcemia from direct skeletal involvement, and (3) over secretion of 1,2 dihydroxyvitamin D. Very rarely, ectopic PTH production has been described in the literature as the cause of humoral hypercalcemia. By 2018, there were only 26 cases in the reported literature. Three were from gynecological cancer, with two related to ovarian cancer. Only one reported case was related to endometroid adenocarcinoma. To our knowledge, this is the second reported case. The presence of hypercalcemia in malignancy has been associated with increased mortality; the survival was two years in patients with mild hypercalcemia versus 1-2 months with more severe hypercalcemia. Though unusual, this case highlights the importance of considering ectopic PTH secretion rather than the default assumption of primary hyperparathyroidism in cancer patients with high calcium and PTH levels.

Hypercalcemia in metastatic GIST caused by systemic elevated calcitriol: a case report and review of the literature

BackgroundHypercalcemia is the most common oncologic metabolic emergency but very rarely observed in patients with gastrointestinal stromal tumour, which is a rare mesenchymal malignancy of the gastrointestinal tract. We describe a case of hypercalcemia caused by elevated levels of activated vitamin D in a patient with gastrointestinal tumour. Prior to this case report, only one paper has reported an association between hypercalcemia, gastrointestinal stromal tumours and elevated levels of vitamin D.Case presentationAn otherwise healthy 70-year-old Caucasian woman, previously treated for duodenal gastrointestinal stromal tumour, was diagnosed with liver metastasis, and relapse of gastrointestinal stromal tumour was confirmed by biopsy. At presentation, the patient suffered from severe symptoms of hypercalcemia. The most common causes of hypercalcemia, hyperparathyrodism, parathyroid hormone-related peptide secretion from tumour cells, and metastatic bone disease, were all dismissed as the etiology. Analysis of vitamin D subtypes revealed normal levels of both 25-OH Vitamin D2 and 25-OH Vitamin D3, whereas the level of activated vitamin D, 1,25 OH Vitamin D3, also referred to as calcitriol, was elevated.ConclusionThe fact that plasma calcitriol decreased after initiation of oncological treatment and the finding that hypercalcemia did not recur during treatment support the conclusion that elevated calcitriol was a consequence of the gastrointestinal stromal tumour. We suggest that gastrointestinal stromal tumours should be added to the list of causes of humoral hypercalcemia in malignancy, and propose that gastrointestinal stromal tumour tissue may have high activity of the specific enzyme 1α-hydroxylase, which can lead to increased levels of calcitriol and secondarily hypercalcemia.

Squamous Cell Carcinoma (Marjolin's Ulcer) Arising in a Sacral Decubitus Ulcer Resulting in Humoral Hypercalcemia of Malignancy.

Long-standing burns, fissures, and ulcers that undergo malignant transformation into a variety of malignancies, including squamous cell carcinoma, is commonly referred to as a Marjolin's ulcer. It is well recognized that squamous cell carcinomas of the lung and esophagus can cause humoral hypercalcemia of malignancy secondary to paraneoplastic secretion of parathyroid hormone-related peptide. However, it is extremely rare for a squamous cell carcinoma developing in a sacral decubitus ulcer to cause humoral hypercalcemia of malignancy. We describe the first case of a patient found to have elevated serum levels of parathyroid hormone related peptide related to his Marjolin's ulcer. A 45-year-old African American man with T6 paraplegia and a sacral decubitus ulcer present for 20 years was admitted for hypercalcemia of unclear etiology. He was subsequently found to have elevated parathyroid hormone related peptide and an excisional biopsy from the ulcer showed invasive squamous cell carcinoma suggestive of humoral hypercalcemia of malignancy. The patient ultimately succumbed to sepsis while receiving chemotherapy for his metastatic squamous cell carcinoma. Humoral hypercalcemia of malignancy is a rare and likely underrecognized complication that can occur in a Marjolin's ulcer.

A Case of a Giant Glucagonoma with Parathyroid Hormone-related Peptide Secretion Showing an Inconsistent Postsurgical Endocrine Status

A 53-year-old woman was admitted because of a giant pancreatic tumor. Hypercalcemia and a high serum parathyroid hormone-related peptide (PTHrP) level were observed. A hypoglycemic attack occurred during pancreatectomy, and the surgical specimen revealed a PTHrP-secreting glucagonoma. Liver metastases developed 1 and 5.5 years later, and bone metastases appeared 6 years after surgery. Her serum PTHrP concentrations remained normal after surgery, despite re-elevation of the serum glucagon concentration after recurrence. The clinical course of this case illustrates the process of development of neuroendocrine tumors secreting two or more hormones.

Effects of calcium-sensing receptor on the secretion of parathyroid hormone-related peptide and its impact on humoral hypercalcemia of malignancy

The extracellular calcium-sensing receptor (CaR) plays a key role in the defense against hypercalcemia by "sensing" extracellular calcium (Ca2+(o)) levels in the parathyroid and kidney, the key organs maintaining systemic calcium homeostasis. However, CaR function can be aberrant in certain pathophysiological states, e.g., in some types of cancers known to produce humoral hypercalcemia of malignancy (HHM) in humans and animal models in which high Ca2+(o), via the CaR, produces a homeostatically inappropriate stimulation of parathyroid hormone-related peptide (PTHrP) secretion from these tumors. Increased levels of PTHrP set a cycle in motion whereby elevated systemic levels of Ca2+(o) resulting from its increased bone-resorptive and positive renal calcium-reabsorbing effects give rise to hypercalcemia, which in turn begets worsening hypercalcemia by stimulating further release of PTHrP by the cancer cells. I review the relationship between CaR activation and PTHrP release in normal and tumor cells giving rise to HHM and/or malignant osteolysis and the actions of the receptor on key cellular events such as proliferation, angiogenesis, and apoptosis of cancer cells that will favor tumor growth and osseous metastasis. I also illustrate diverse signaling mechanisms underlying CaR-stimulated PTHrP secretion and other cellular events in tumor cells. Finally, I raise several necessary questions to demonstrate the roles of the receptor in promoting tumors and metastases that will enable consideration of the CaR as a potential antagonizing/neutralizing target for the treatment of HHM.

Subchondral bone osteoblasts induce phenotypic changes in human osteoarthritic chondrocytes

To determine the influence of osteoarthritic (OA) phenotype of subchondral osteoblasts on the phenotype of human chondrocytes. Human chondrocytes were isolated from OA cartilage and cultured in alginate beads for 4 or 10 days in the absence or in the presence of osteoblasts in monolayer. The osteoblasts were either isolated from non-sclerotic (N) or sclerotic (SC) zones of human subchondral bone. Before co-culture, osteoblasts were incubated for 72 h with or without 1.7 ng/ml interleukin (IL)-1beta, 100 ng/ml IL-6 with its soluble receptor (50 ng/ml) or 10 ng/ml oncostatin M. SOX9, type I, II and X collagen (COL1, COL2, COL10), osteoblasts-stimulating factor (OSF)-1, bone alkaline phosphatase (ALP), parathyroid hormone related peptide (PTHrP) and its receptor (PTH-R) messenger RNA (mRNA) levels in chondrocytes were quantified by real-time polymerase chain reaction. In comparison with chondrocytes cultured alone in alginate beads, chondrocytes after 4 days in co-culture with N or SC osteoblasts expressed significantly less SOX9 and COL2 mRNA. The decrease of SOX9 and COL2 gene expression was significantly more pronounced in the presence of SC than in the presence of N osteoblasts (P<0.001). OSF-1 mRNA level in chondrocyte was increased by both N and SC osteoblasts, but to a larger extent by SC osteoblasts (P<0.001). PTHrP expression in chondrocytes was 21-fold increased by N osteoblasts but four-fold inhibited by SC osteoblasts. PTHrP secretion was also increased by N but reduced by SC osteoblasts. SC, but not N osteoblasts, induced a significant decrease of PTH-R gene expression in chondrocyte. In our experimental conditions, chondrocytes did not express COL1, COL10 or ALP, even after 10 days of co-culture with osteoblasts. In co-culture, SC subchondral osteoblasts decrease SOX9, COL2, PTHrP and PTH-R gene expression by chondrocytes but increase that of OSF-1. These findings suggest that SC osteoblasts could initiate chondrocyte phenotype shift towards hypertrophic differentiation and subsequently further matrix mineralization.

The Hedgehog-inducible ubiquitin ligase subunit WSB-1 modulates thyroid hormone activation and PTHrP secretion in the developing growth plate.

WSB-1 is a SOCS-box-containing WD-40 protein of unknown function that is induced by Hedgehog signalling in embryonic structures during chicken development. Here we show that WSB-1 is part of an E3 ubiquitin ligase for the thyroid-hormone-activating type 2 iodothyronine deiodinase (D2). The WD-40 propeller of WSB-1 recognizes an 18-amino-acid loop in D2 that confers metabolic instability, whereas the SOCS-box domain mediates its interaction with a ubiquitinating catalytic core complex, modelled as Elongin BC-Cul5-Rbx1 (ECS(WSB-1)). In the developing tibial growth plate, Hedgehog-stimulated D2 ubiquitination via ECS(WSB-1) induces parathyroid hormone-related peptide (PTHrP), thereby regulating chondrocyte differentiation. Thus, ECS(WSB-1) mediates a mechanism by which 'systemic' thyroid hormone can effect local control of the Hedgehog-PTHrP negative feedback loop and thus skeletogenesis.

Severe hypercalcaemia from secretion of parathyroid hormone-related peptide

A 47-year-old woman was referred to our institution with severe hypercalcaemia. 5 years before referral she was diagnosed with liver metastases, and a biopsy sample showed a neuroendocrine tumour. Initial assessment did not include immunohistochemical analysis for production of parathyroid hormone or parathyroid hormone-related peptide, and no archived tissue is available for further assessment.

Parathyroid hormone‐related peptide in pancreatic neuroendocrine tumours associated with hypercalcaemia

Hypercalcaemia is a common paraneoplastic syndrome. In the context of pancreatic neuroendocrine tumours, it is occasionally caused by secretion of parathyroid hormone-related peptide (PTH-rP). Two patients are reported in whom persistent hypercalcaemia was traced to a large neuroendocrine pancreatic tumour hypersecreting PTH-rP. Resection of the tumour reduced serum levels of calcium and PTH-rP transiently in each case until the patient developed bulky metastatic disease. A 33-year-old woman remained hypercalcaemic after the removal of all four hyperplastic parathyroid glands had rendered circulating parathormone levels undetectable. Radical distal pancreatectomy was followed over the next 4 years by operative debulking of liver metastases, multiple hepatic artery embolisations, octreotide injections and repeated admissions for intravenous fluid and biphosphonate therapy. A 41-year-old man presented with hypercalcaemia as well as features of somatostatinoma syndrome. Symptomatic improvement after radical distal pancreatectomy was short-lived, and hepatic artery embolisation failed to control his rapidly progressive disease. Malignant hypercalcaemia associated with a neuroendocrine pancreatic tumour hypersecreting PTH-rP is difficult to treat and can be life-threatening. Aggressive surgical treatment is recommended initially, while somatostatin analogues and hepatic artery embolisation are alternative therapeutic options for metastatic disease.

Extracellular Calcium-Sensing Receptor Expression and Its Potential Role in Regulating Parathyroid Hormone-Related Peptide Secretion in Human Breast Cancer Cell Lines

Extracellular Calcium-Sensing Receptor Expression and Its Potential Role in Regulating Parathyroid Hormone-Related Peptide Secretion in Human Breast Cancer Cell Lines

Extracellular calcium-sensing receptor expression and its potential role in regulating parathyroid hormone-related peptide secretion in human breast cancer cell lines.

Metastasis of breast cancer to bone occurs with advanced disease and produces substantial morbidity. Secretion of PTH-related peptide (PTHrP) from breast cancer cells is thought to play a key role in osteolytic metastases and is increased by transforming growth factor-beta (TGFbeta), which is released from resorbed bone. Elevated extracellular calcium (Ca2+(o)) also stimulates PTHrP secretion from various normal and malignant cells, an action that could potentially be mediated by the Ca2+(o)-sensing receptor (CaR) originally cloned from the parathyroid gland. Indeed, we previously showed that both normal breast ductal epithelial cells and primary breast cancers express the CaR. In this study we investigated whether the MCF-7 and MDA-MB-231 human breast cancer cell lines express the CaR and whether CaR agonists modulate PTHrP secretion. Northern blot analysis and RT-PCR revealed bona fide CaR transcripts, and immunocytochemistry and Western analysis with a specific anti-CaR antiserum demonstrated CaR protein expression in both breast cancer cell lines. Furthermore, elevated Ca2+(o) and the polycationic CaR agonists, neomycin and spermine, stimulated PTHrP secretion dose dependently, with maximal, 2.1- to 2.3-fold stimulation. In addition, pretreatment of MDA-MB-231 cells overnight with TGFbeta1 (0.2, 1, or 5 ng/ml) augmented both basal and high Ca2+-stimulated PTHrP secretion. Thus, in PTHrP-secreting breast cancers metastatic to bone, the CaR could potentially participate in a vicious cycle in which PTHrP-induced bone resorption raises the levels of Ca2+(o) and TGFbeta within the bony microenvironment, which then act in concert to evoke further PTHrP release and worsening osteolysis.

Anti-tumor effects of 1,25-dihydroxyvitamin D3 and vitamin D analogs.

The role of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) as a regulator of cell growth and differentiation is well recognized. Currently, 1, 25-(OH)2D3 and vitamin D analogs are being evaluated for their therapeutic potential in the treatment of hyperproliferative disorders like cancer. In the present review, we will discuss several processes that might be involved in 1,25-(OH)2D3- and vitamin D analog-mediated suppression of cancer cell growth. The effects on tumor cell proliferation, differentiation, apoptosis, angiogenesis, metastases, and parathyroid hormone-related peptide secretion will be highlighted. In addition, combination therapy with other tumor effec tive drugs will be addressed. Furtermore, we will focus on the potential drawbacks and the possible side effects of vitamin D compounds in the treatment of cancer.

Extracellular Calcium-Sensing Receptor (CaR) Expression and Its Potential Role in Parathyroid Hormone-Related Peptide (PTHrP) Secretion in the H-500 Rat Leydig Cell Model of Humoral Hypercalcemia of Malignancy

Humoral hypercalcemia of malignancy (HHM) occurs when secretion of parathyroid hormone-related peptide (PTHrP) by cancer cells causes hypercalcemia in the absence of skeletal metastases. High extracellular calcium (Ca2+o) increases secretion of PTH-like bioactivity by rat H-500 leydig cells, a transplantable model of HHM, an action potentially mediated by the Ca2+o-sensing receptor (CaR). In this study we investigated whether H-500 cells express the CaR and, if so, whether CaR agonists modulate PTHrP secretion. Northern blot analysis and RT-PCR revealed bona fide CaR transcript(s), and immunocytochemistry and Western analysis with a specific anti-CaR antiserum demonstrated CaR protein expression in H-500 cells. Furthermore, high Ca2+o and neomycin stimulated PTHrP secretion dose-dependently with maximal 2.7- and 3.3-fold increases at 5 mM Ca2+o and 300 μM neomycin, respectively. Thus in HHM caused by H-500 cells, the CaR could participate in a vicious cycle whereby PTHrP-induced increases in Ca2+o further stimulate PTHrP release and exacerbate hypercalcemia.