Parathyroid hormone-related protein. Evidence for secretion of a novel mid-region fragment by three different cell types

Parathyroid hormone-related peptide and 8701-BC breast cancer cell growth and invasion in vitro: evidence for growth-inhibiting and invasion-promoting effects

It has been previously reported that 8701-BC cells, derived from a primary carcinoma of the breast, constitutively express parathyroid hormone-related peptide (PTHrP) gene and that N-terminal PTHrP immunoreactivity can be found in cell medium. Here we have firstly measured immunoreactive PTHrP in 8701-BC cell medium using antibodies raised against midregion and C-terminal fragments, and also demonstrated the expression of PTH/PTHrP receptor by 8701-BC cells. Secondly, we have examined the role, if any, elicited by diverse PTHrP domains on 8701-BC cell proliferation, and invasive behaviour in vitro related to production of extracellular proteolytic enzymes. Our data show that PTHrP [1-34], and, to a minor extent, [67-86] and [107-139], are anti-mitogenic but 'invadogenic' for 8701-BC cells, and suggest that diverse enzymatic activities may contribute to cell invasion in response to different PTHrP fragments. In light of the present data on a chemoattractive role for PTHrP in vitro, we hypothesize that this protein might intervene in local control of the invasive process in breast carcinoma.

Humoral hypercalcemia of malignancy: some enigmas on the clinical features

Humoral hypercalcemia of malignancy (HHM) is a common paraneoplastic syndrome mediated by tumor-derived parathyroid hormone-related peptide (PTHRP), which bears structural and functional similarities to PTH. Thus the clinical features of HHM are very similar to those of primary hyperparathyroidism (1 degree HPT), a prototype of humoral hypercalcemia caused by PTH. On the other hand, HHM syndrome differs from 1 degree HPT in several aspects, including serum 1,25(OH)2D levels, acid-base balance, and bone remodeling process, the reason of which remains largely unknown. We approached these questions using a unique animal model of HHM, nude rats implanted with PTHRP-overproducing human carcinomas. In this review we will summarize the results and discuss the implications in understanding the disease mechanism.

In vivo evidence for progressive activation of parathyroid hormone-related peptide gene transcription with tumor growth and stimulation of osteoblastic bone formation at an early stage of humoral hypercalcemia of cancer

The present study was undertaken to clarify in vivo the temporal profile of parathyroid hormone-related peptide (PTHRP) gene expression as well as bone histomorphometric features as a function of tumor growth, using an athymic rat model associated with humoral hypercalcemia of malignancy (HHM). Tumor-bearing animals exhibited hypercalcemia, hypophosphatemia, and increased circulating levels of PTHRP, and died within 3 weeks. Steady-state PTHRP mRNA levels and the transcription rate of PTHRP gene in the tumors were markedly increased with tumor growth. RNAse mapping analysis revealed that both upstream and downstream promoters of the human PTHRP gene were utilized in the tumors and became progressively activated with time. Bone histomorphometric analysis showed that osteoclastic bone resorption was progressively increased throughout the course, whereas osteoblastic bone formation was stimulated more than 2-fold at a very early stage (day 6 after tumor implantation) and then markedly suppressed thereafter on day 12 and day 18 compared with age-matched control animals. These results provide in vivo evidence that PTHRP gene transcription is progressively activated with tumor growth and that activation of osteoblasts does occur at a very early phase of HHM syndrome in contrast to the marked suppression of bone formation at later stages.

Effects of passive immunization against parathyroid hormone-related protein: PTHrP is the responsible factor in mediating hypercalcemia in the Walker carcinosarcoma 256 rat model

The Walker carcinosarcoma (WCS) 256 is a well-characterized rat model of humoral hypercalcemia of malignancy (HHM). We addressed the question of whether parathyroid hormone-related protein (PTHrP) is the factor responsible for mediating HHM in this model. WCS 256 cells were subcutaneously implanted in female rats. We examined the plasma at days 0, 2, 4, 6, and 8. The midregional PTHrP measured by radioimmunoassay (RIA) and the plasma calcium increased significantly. Measuring PTHrP by a two-site immunoradiometric assay (IRMA) showed comparable results. There was a strong positive correlation between plasma calcium and midregional PTHrP (r = 0.85, p < 0.0001). A strong positive correlation between tumor weight and both midregional PTHrP (r = 0.83, p < 0.0001) and plasma calcium (r = 0.87, p < 0.0001) was also found. After surgical removal of the tumor at day 5, both plasma calcium and plasma PTHrP levels fell to within the normal range. Ip administration of native polyclonal antiserum against PTHrP(53-84) led to a significant decrease of plasma calcium. Extracted WCS 256 tumor showed 5-fold increased levels of midregional PTHrP compared with liver. Immunohistochemistry and Western blot were positive for PTHrP. RNA from the WCS 256 tumor was positive for PTHrP whereas liver tissue RNA was negative. WCS 256 cells grown in vitro also secreted PTHrP into the medium. We conclude that PTHrP is synthesized and secreted by WCS 256 and that PTHrP is the factor responsible for mediating hypercalcemia in the WCS 256 rat model.

Parathyroid hormone-related protein (PTHrP) in breast cancer and benign breast tissue

Parathyroid hormone-related protein (PTHrP) 1-86 was quantified by immunoassay in extracts of 132 breast cancers, 27 samples of normal breast tissue and four fibroadenomas. PTHrP 1-86, was detected in 68% of primary tumours (range 40-302,000 fmol/g), 33% of normal breast tissues (range 100-1800 fmol/g), and all four fibroadenomas (range 110-11,600 fmol/g). PTHrP displayed molecular heterogeneity on gel filtration chromatography, and 1-86, 1-34 and 37-67 immunoreactivity eluted as 25-27 kDa together with a peak of 19-21 kDA containing only 37-67 activity. Tumour PTHrP 1-86 levels correlated inversely with age (P < 0.05) and were higher in premenopausal women (P = 0.05). The proportion of tumours containing PTHrP was higher in axillary node positive premenopausal women (P < 0.05). These data suggest that oestrogen may regulate expression of PTHrP in breast cancer and that production of PTHrP may be linked to development of axillary node metastases.

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.

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

Parathyroid hormone-related protein shares similarities in sequence and function with the endocrine hormone, parathyroid hormone. However, unlike parathyroid hormone, a product of the parathyroid glands, parathyroid hormone-related protein has a wide distribution in tissues, including the mammary gland. Although during pregnancy the expression of parathyroid hormone-related protein in the mammary gland is low, following birth, protein levels rise sharply in the gland in response to elevations in serum prolactin. Large amounts of parathyroid hormone-related protein are secreted into milk, suggesting a possible role in the neonate. Transient phosphaturia and elevations of parathyroid hormone-related protein in mammary vein plasma support a possible endocrine function for parathyroid hormone-related protein during lactation. Recent evidence suggests a local function for parathyroid hormone-related protein in the lactating mammary gland, and evidence exists that parathyroid hormone-related protein stimulates calcium secretion by the goat mammary gland. Parathyroid hormone-related protein, a putative vasodilator, is produced by the external nutrient vasculature of the mammary gland, and levels within this tissue are regulated during lactation. Infusion of parathyroid hormone-related protein into the ovine mammary artery increases gland blood flow, suggesting a role for the protein in modulation of mammary gland hemodynamics. Regulation of parathyroid hormone-related protein synthesis by the lactating gland, together with the protein's actions on regional blood flow and calcium secretion, support an important function in the mammary gland during lactogenesis.

Expression and processing of parathyroid hormone-related protein in a pancreatic endocrine cell tumour associated with hypercalcaemia

We describe a patient with a neuroendocrine tumour of the pancreas associated with hypercalcaemia which was attributed to production of parathyroid hormone-related protein (PTHrP) by the tumour. Plasma PTHrP 1-86 was significantly raised, and fell following surgical resection of the tumour. PTHrP mRNA and peptide were identified in tumour tissue by in-situ hybridization and immunohistochemistry respectively. PTHrP was quantitated in an extract of tumour tissue by three region-specific immunoassays (PTHrP 1-34 45.2 pmol/g, PTHrP 37-67 81.7 pmol/g, PTHrP 1-86 27.3 pmol/g) and suggested the presence of excess of amino-terminal and mid-region immunoreactivity. On chromatography of the tumour extract the first peak eluted as 22 kDa and comprised approximately equimolar 1-34, 37-67 and 1-86 activities. The second and major peak of 16 kDa contained only 37-67 activity, while the third peak of 6 kDa contained only 1-34 activity. This suggested that the tumour contained a native or intact form of PTHrP together with two major subfragments containing 37-67 and 1-34 activity respectively. Thus chromatographic separation and quantitation of PTHrP by region-specific immunoassays have provided new information on in-vivo proteolytic processing by tumour tissue by indicating that a site of cleavage is located between residues 17 and 61. Our findings are compatible with cleavage at residue 37, a site previously indicated from in-vitro studies.

Parathyroid hormone-related protein in normal and neoplastic canine tissues: immunohistochemical localization and biochemical extraction

Two polyclonal antibodies, directed against N-terminal amino acids (1-36) or the midregion (amino acids 34-53) of parathyroid hormone-related protein (PTHrP), were used to localize PTHrP in a variety of normal and neoplastic canine tissues. Parathyroid hormone (PTH) immunoreactivity was demonstrated using anti-bovine PTH (amino acids 14-34). The following tissues (among others) stained strongly positive for PTHrP: all layers of epidermal keratinocytes, with the most intense staining of the basal layer; hair follicle keratinocytes; myoepithelial cells of dermal apocrine glands, mammary glands, and apocrine glands of the anal sac; anal sac epithelium; mammary duct epithelium; and thyroid C cells. Adenocarcinomas of the anal sac stained moderately positive (5/22 dogs), weakly positive (11/22 dogs), or did not stain (6/22 dogs). Most parathyroid gland adenomas stained moderately (2/6 dogs) or weakly positive (3/6 dogs) for PTHrP. Squamous cell carcinomas (6/6 dogs) stained strongly positive. Lymphomas stained weakly positive (2/10 dogs) or did not stain (8/10 dogs). There was no consistent relationship between the staining intensity of the tumors and serum calcium concentrations of the dogs. The anti-PTH antibodies stained only parathyroid chief cells strongly positive. Concentrations of PTHrP were measured by radioimmunoassay in protein extracts from an adenocarcinoma derived from the apocrine glands of the anal sac, pancreas, kidney, liver, heart, thyroid, adrenal, and parathyroid glands. PTHrP concentrations varied from undetectable up to 150 pg/mg in normal tissues as compared with 2,000 pg/mg in apocrine adenocarcinoma of the anal sac.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypercalcaemia due to parathyroid hormone-related protein: long-term circulating levels may not reflect tumour activity

Parathyroid hormone-related protein is responsible for the hypercalcaemia caused by many tumours. Measurement of parathyroid hormone-related protein is becoming more accessible with the introduction of commercial assays. We report a case of hypercalcaemia of malignancy secondary to parathyroid hormone-related protein in a woman with renal carcinoma. The parathyroid hormone-related protein was assayed using a new immunoradiometric assay. We demonstrated an initial fall in parathyroid hormone-related protein and calcium levels after surgery and a rise in both before clinical relapse. However, the clinical relapse was itself associated with a fall in serum parathyroid hormone-related protein, nephrogenous cAMP and calcium, suggesting that the tumour had stopped producing parathyroid hormone-related protein or perhaps that post-translational processing had occurred as the tumour advanced. The tumour was investigated for parathyroid hormone-related protein mRNA content using reverse transcriptase polymerase chain reaction, both at diagnosis in surgically removed material, and using post-mortem specimens. The level of parathyroid hormone-related protein mRNA, while present, was much reduced in the recurrent tumour suggesting that active parathyroid hormone-related protein production fell substantially as the tumour advanced. This case suggests that, although demonstration of parathyroid hormone-related protein in hypercalcaemia is useful for diagnosis, tumoral secretion of this product may alter.

In situ localization of parathyroid hormone-like protein and mRNA in intraepithelial neoplasia and invasive carcinoma of the uterine cervix

Parathyroid-like protein (PLP), or parathyroid hormone-related peptide, is a well-recognized mediator of paraneoplastic hypercalcemia (humoral hypercalcemia of malignancy syndrome). In this study we examined the expression of PLP by 40 invasive squamous cell carcinomas (SCCs) of the cervix and selected carcinomas of nonsquamous histology. Using a polyclonal antibody to human PLP, 93% of SCCs, including two tumors from patients with humoral hypercalcemia of malignancy syndrome, showed moderate to strong cytoplasmic immunoperoxidase staining for PLP. The strongest staining often was observed in areas of invasion associated with stromal desmoplasia. The small number of weak or negatively stained SCCs were all poorly differentiated tumors. Although native uninvolved squamous epithelium showed weak to moderate staining of the superficial layers, there was variable or full-thickness immunostaining in areas of dysplasia. Normal endocervical glands and stroma as well as cervical adenocarcinomas and neuroendocrine carcinomas were negative. In situ hybridization studies showed abundant PLP mRNA within SCC in patients with hypercalcemia. However, PLP mRNA was of relatively low abundance in tumors of normocalcemic patients. Ultrastructural studies showed cytoplasmic, membrane-bound, granular inclusions in tumor cells from the hypercalcemic patients. Our data suggest that increased PLP gene transcription contributes to the increased production of PLP and the pathogenesis of humoral hypercalcemia of malignancy syndrome.

Parathyroid hormone-like peptide and parathyroid hormone are secreted from bovine parathyroid via different pathways

Parathyroid hormone-like peptide is a recently discovered protein which is thought to be responsible for the hypercalcemia of malignancy. Through the use of radioimmunoassay, Northern analysis and Western blot techniques this protein has been demonstrated to occur in a variety of tumor and normal cells. Its role in normal physiology is not established nor is there knowledge regarding its synthesis, secretion, and storage. We have investigated characteristics of the secretion of parathyroid hormone-like peptide in bovine parathyroid gland slices and cells to learn whether or not this protein is secreted in a manner similar to that of parathyroid hormone. We have used radioimmunoassays specific for PTH and PTH-rP to measure the secretion of each protein and have found that, unlike PTH, PTH-rP secretion was not influenced by the medium calcium concentration. Similarly, PTH-rP secretion was not influenced by other known PTH secretagogues such as c-AMP or isoproterenol. An examination of the subcellular distribution of PTH-rP revealed that 75-90% of it occurs in the soluble fraction of cell lysates. Analysis of isolated secretory granules demonstrated the presence of PTH while PTH-rP was undetectable in these organelles. We conclude that PTH-rP is not secreted from parathyroid cells via the regulated pathway utilizing PTH secretory granules.

Biosynthesis and processing of endogenous parathyroid hormone related peptide (PTHRP) by rat Leydig cell tumor H-500

We have examined in vitro the biosynthesis and processing of endogenous PTHRP in cultured rat H-500 Leydig tumor cells. Cells were grown to confluence and pulse labeled with [3H]Ile, 50 microCi/mL, in Ile free culture medium for 2 min to 6 h. In some experiments incubations were carried out in culture medium alone in the presence of 0.3 mM cycloheximide or 20 micrograms/mL unlabeled Ile. Cell extracts and culture media were analyzed by affinity chromatography employing an antibody directed against the bioactive NH2-terminal region, PTHRP(1-34), followed by gel-permeation or reversed-phase high-performance liquid chromatography. Incorporation of [3H]Ile into PTHRP in cell extracts increased over 20 min during pulse labeling and then remained constant throughout the incubation period up to 6 h. In contrast, the release of [3H]PTHRP into culture medium increased progressively over 6 h. Addition of cycloheximide or unlabeled Ile almost completely blocked incorporation of [3H]Ile into newly synthesised PTHRP. Three molecular forms of PTHRP were seen which comigrated with PTHRP(1-36), PTHRP(1-86), and PTHRP(1-141) standards in both chromatographic systems employed. After 20 min these species comprised approximately 63%, 30%, and 7% of newly synthesized PTHRP, respectively. These three molecular forms of PTHRP were observed both intra- and extracellularly, and no further metabolism of these species was seen after release into conditioned medium. Pulse-chase studies demonstrated a rapid decrease of newly synthesized PTHRP forms within cells after 20 min; there was, however, a progressive increase in [3H]PTHRP in conditioned culture medium.(ABSTRACT TRUNCATED AT 250 WORDS)