Expression and action of parathyroid hormone-related peptide in human cervical epithelial cells

Parathyroid hormone-related protein (PTHrP) and malignancy

PTHrP (parathyroid hormone related protein) is an important mediator of malignancy-related tumor progression and hypercalcemia that shares considerable homology and functionality with parathyroid hormone. In this chapter, we review what has been elucidated to date regarding PTHrP's role in malignancies. Starting with a review of calcium metabolism and regulation, we then summarize the discovery and structure of PTHrP and development of sensitive immunoassays for specific measurement. Subsequently, we explore its role in tumor progression, with emphasis on the primary tumor as well as skeletal and non-osseus metastases. We then consider the clinical implications of PTHrP in cancer before concluding with a discussion of both established and potential treatments for malignancy associated hypercalcemia and bone metastases.

Asparagine improves intestinal integrity, inhibits TLR4 and NOD signaling, and differently regulates p38 and ERK1/2 signaling in weanling piglets after LPS challenge

Asparagine (Asn), an activator of ornithine decarboxylase (ODC), stimulates cell proliferation in intestinal epithelial cells. We hypothesized that Asn can mitigate LPS-induced injury of intestinal structure and barrier function by regulating inflammatory signaling pathways. We executed the following experiment using weanling pigs for each of the groups: (1) non-challenged control; (2) LPS-challenged control; (3) LPS + 0.5% Asn; (4) LPS + 1.0% Asn. After 21-d feeding, pigs received an i.p. injection of either saline or LPS. Four h after injection, the mid-jejunum and mid-ileum samples were collected. We found that Asn restored ODC expression that was decreased by LPS treatment. Asn also restored intestinal morphology and barrier function that were impaired by LPS treatment. In addition, Asn down-regulated intestinal caspase-3 protein expression and TNF-α concentration, and decreased the mRNA expression of intestinal TLR4, TLR4 downstream signals (myeloid differentiation factor 88, IL-1 receptor-associated kinase 1 and TNF-α receptor-associated factor 6 and NOD1, NOD2 and their adaptor molecule (receptor-interacting serine/threonine-protein kinase 2). Moreover, Asn decreased p38 phosphorylation but increased ERK1/2 phosphorylation. Our results suggest that Asn improves intestinal integrity during an inflammatory insult, which appears to be related to the decrease of intestinal pro-inflammatory cytokine (via TLR4, NODs and p38) and of enterocyte apoptosis (via p38 and ERK1/2).

Cancer Diagnostics via Ultrasensitive Multiplexed Detection of Parathyroid Hormone-Related Peptides with a Microfluidic Immunoarray

Parathyroid hormone-related peptide (PTHrP) is recognized as the major causative agent of humoral hypercalcemia of malignancy (HHM). The paraneoplastic PTHrP has also been implicated in tumor progression and metastasis of many human cancers. Conventional PTHrP detection methods like immunoradiometric assay (IRMA) lack the sensitivity required to measure target peptide levels prior to the development of hypercalcemia. In general, sensitive, multiplexed peptide measurement by immunoassay represents challenges that we address in this paper. We describe here the first ultrasensitive multiplexed peptide assay to measure intact PTHrP 1-173 as well as circulating N-terminal and C-terminal peptide fragments. This versatile approach should apply to almost any collection of peptides that are long enough to present binding sites for two antibodies. To target PTHrP, we employed a microfluidic immunoarray featuring a chamber for online capture of the peptides from serum onto magnetic beads decorated with massive numbers of peptide-specific antibodies and enzyme labels. Magnetic bead-peptide conjugates were then washed and sent to a detection chamber housing an antibody-modified 8-electrode array fabricated by inkjet printing of gold nanoparticles. Limits of detection (LODs) of 150 aM (∼1000-fold lower than IRMA) in 5 μL of serum were achieved for simultaneous detection of PTHrP isoforms and peptide fragments in 30 min. Good correlation for patient samples was found with IRMA (n = 57); r² = 0.99 assaying PTHrP 1-86 equiv fragments. Analysis by a receiver operating characteristic (ROC) plot gave an area under the curve of 0.96, 80–83% clinical sensitivity, and 96–100% clinical specificity. Results suggest that PTHrP1-173 isoform and its short C-terminal fragments are the predominant circulating forms of PTHrP. This new ultrasensitive, multiplexed assay for PTHrP and fragments is promising for clinical diagnosis, prognosis, and therapeutic monitoring from early to advanced stage cancer patients and to examine underlying mechanisms of PTHrP overproduction.

The paracrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and PTHrP in prehypertrophic chondrocytes

The endocrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and parathyroid hormone (PTH) plays a central role in skeletal development. PTH-related protein (PTHrP) shares homology and its receptor (PTHR1) with PTH. The aim of this study was to investigate whether there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate, in parallel with the endocrine feedback loop between 1,25(OH)₂D₃ and PTH. This was investigated in ATDC5 cells treated with 10⁻⁸ M 1,25(OH)₂D₃ or PTHrP, Col2-pd2EGFP transgenic mice, and primary Col2-pd2EGFP growth plate chondrocytes isolated by FACS, using RT-qPCR, Western blot, PTHrP ELISA, chromatin immunoprecipitation (ChIP) assay, silencing of the 1,25(OH)₂D₃ receptor (VDR), immunofluorescent staining, immunohistochemistry, and histomorphometric analysis of the growth plate. The ChIP assay confirmed functional binding of the VDR to the PTHrP promoter, but not to the PTHR1 promoter. Treatment with 1,25(OH)₂D₃ decreased PTHrP protein production, an effect which was prevented by silencing of the VDR. Treatment with PTHrP significantly induced VDR production, but did not affect 1α- and 24-hydroxylase expression. Hypertrophic differentiation was inhibited by PTHrP and 1,25(OH)₂D₃ treatment. Taken together, these findings indicate that there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate. 1,25(OH)₂D₃ decreases PTHrP production, while PTHrP increases chondrocyte sensitivity to 1,25(OH)₂D₃ by increasing VDR production. In light of the role of 1,25(OH)₂D₃ and PTHrP in modulating chondrocyte differentiation, 1,25(OH)₂D₃ in addition to PTHrP could potentially be used to prevent undesirable hypertrophic chondrocyte differentiation during cartilage repair or regeneration.

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.

Prostate cancer cell type-specific involvement of the VDR and RXR in regulation of the human PTHrP gene via a negative VDRE

Parathyroid hormone-related protein (PTHrP) increases the growth and osteolytic potential of prostate cancer cells, making it important to control PTHrP expression in these cells. We show that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its non-hypercalcemic analog, EB1089, decrease PTHrP mRNA and cellular protein levels in the androgen-dependent human prostate cancer cell line LNCaP and its androgen-independent derivative, the C4-2 cell line. This effect is mediated via a negative Vitamin D response element (nVDREhPTHrP) within the human PTHrP gene and involves an interaction between nVDREhPTHrP and the Vitamin D receptor (VDR). The retinoid X receptor (RXR) is a frequent heterodimeric partner of the VDR. We show that RXRalpha forms part of the nuclear protein complex that interacts with nVDREhPTHrP along with the VDR in LNCaP and C4-2 cells. We also show that the RXR ligand, 9-cis-retinoic acid, downregulates PTHrP mRNA levels; this decrease is more pronounced in LNCaP than in C4-2 cells. In addition, 9-cis-retinoic acid enhances the 1,25(OH)2D3-mediated downregulation of PTHrP expression in both cell lines; this effect also is more pronounced in LNCaP cells. Proliferation of LNCaP, but not C4-2, cells is decreased by 9-cis-retinoic acid. Promoter activity driven by nVDREhPTHrP cloned upstream of the SV40 promoter and transiently transfected into LNCaP and C4-2 cells is downregulated in response to 1,25(OH)2D3 and EB1089 in both cell lines. Co-treatment with these compounds and 9-cis-retinoic acid further decreases CAT activity in LNCaP, but not C4-2, cells. These results indicate that PTHrP gene expression is regulated by 1,25(OH)2D3 in a cell type-specific manner in prostate cancer cells.

Immunohistochemical localization of parathyroid hormone-related protein (PTHrP) and serum PTHrP in normocalcemic patients with oral squamous cell carcinoma

Cancer cells produce parathyroid hormone-related protein (PTHrP) in the early phase of malignancy development, before hypercalcemia occurs. The relationship between PTHrP and the clinicopathologic features of oral squamous cell carcinoma is poorly understood. We studied 60 patients (43 men, 17 women; mean age, 64.8 +/- 11.2 years) with primary oral squamous cell carcinoma, from whom pretreatment biopsy specimens were obtained. We examined the relationship among immunohistochemical PTHrP expression, serum PTHrP levels, clinical characteristics of the tumor, and histopathologic aspects of the tumor. The mean calcium concentration for the 60 patients was 9.1 +/- 0.4 mg/dl. No patients had laboratory evidence of hypercalcemia before treatment. Six patients had serum levels of C-terminal (C)-PTHrP higher than the normal level of 55.3 pmol/l. There were no significant differences in serum C-PTHrP levels according to TNM stages. Abundant positive immunoreactivity for anti-PTHrP (1-34) antibody was recognized diffusely in the whole cytoplasm of many tumor cells. Anti-PTHrP (38-64) antibody staining tended to localize as small granules in the cytoplasm, especially close to the nuclear periphery. There was no correlation between the serum C-PTHrP concentration and the intensity of either immunostain. The intensity of PTHrP was proportionally related to the degree of differentiation or extent of keratinization (P < 0.05) and the histologic malignancy grade of the tumor (P < 0.05), when using antibody against PTHrP (1-34), but not when using antibody against PTHrP (38-64). Serum C-PTHrP levels did not correlate with the intensity of cellular PTHrP expression and characteristics of the tumor at the initial patient visit. The fragment that includes PTHrP (1-34) may be involved in the differentiation of oral squamous cell carcinoma. The differences between immunoreactivities may have been due to differing tissue malignancies and the use of different antibodies. The results suggest the need for caution when interpreting immunoreactivities of PTHrP in malignancies.

PTHrP stimulated by the calcium-sensing receptor requires MAP kinase activation

Increases in extracellular calcium concentration ([Ca(2+)](o)) stimulate from normal and malignant cells secretion of parathroid hormone-related protein (PTHrP), a major mediator of humoral hypercalcemia of malignancy. Because the calcium-sensing receptor (CaR) is a determinant of calcium-regulated hormone secretion, we examined whether HEK cells stably transfected with human CaR secreted PTHrP in response to CaR stimulation. Increases in [Ca(2+)](o) or neomycin and Gd(3+) all substantially increased PTHrP secretion in CaR-HEK cells but had no effect on nontransfected cells. CaR activation likewise increased PTHrP transcripts. PD-098059 and U-0126, inhibitors of the mitogen-activated protein kinase kinase MEK1/2, abolished CaR-stimulated secretion but had no effect on basal secretion. An inhibitor of p38 MAP kinase, SB-203580, also attenuated CaR-stimulated secretion. Western analysis revealed that CaR activation caused a robust increase in MEK1/2 and p38 MAP kinase phosphorylation. A Src family kinase inhibitor, PP2, blocked both basal and CaR-stimulated secretion. We conclude that CaR specifically mediates the effect of increasing [Ca(2+)](o) on PTHrP synthesis and secretion and that activated MEK1/2 and p38 MAP kinases are determinants of the CaR's stimulation of PTHrP secretion.

Signal transduction in the parathyroid

PURPOSE OF REVIEW

To review recent developments regarding the mechanisms underlying the regulation of various aspects of parathyroid function.

RECENT FINDINGS

New studies published during the past year focused on calcium sensing via the calcium-sensing receptor, signal transduction within parathyroid cells, regulation of parathyroid hormone secretion, and the role of caveolae in calcium-sensing receptor-mediated signal transduction. In recent years, in-vitro and in-vivo studies have suggested a dominant role for the calcium-sensing receptor in the regulation of not only parathyroid hormone secretion but also parathyroid cellular proliferation by extracellular calcium. The development of a mouse model for primary hyperparathyroidism that over expresses cyclin D1 in the parathyroid gland provides an experimental system for studying the molecular basis for reduced calcium receptor expression and its role in the pathophysiology of primary hyperparathyroidism. There is also increasing evidence for the importance of vitamin D and the level of inorganic phosphate in regulating parathyroid function.

SUMMARY

Important advances are being made in understanding extracellular calcium- and calcium-sensing receptor-regulated signal transduction in the parathyroid but the subsequent steps coupling the calcium-sensing receptor to the control of parathyroid hormone secretion and parathyroid cellular proliferation remain to be fully elucidated.

A new strategy for modulating chemotherapy-induced alopecia, using PTH/PTHrP receptor agonist and antagonist

Parathyroid hormone (PTH) related peptide (PTHrP) and the PTH/PTHrP receptor (PTH/PTHrP-R) show prominent cutaneous expression, where this signaling system may exert important paracrine and/or autocrine functions, such as in hair growth control. Chemotherapy-induced alopecia - one of the fundamental unsolved problems of clinical oncology - is driven in part by defined abnormalities in hair follicle cycling. We have therefore explored the therapeutic potential of a PTH/PTHrP-R agonist and two PTH/PTHrP-R antagonists in a mouse model of cyclophosphamide-induced alopecia. Intraperitoneal administration of the agonist PTH(1-34) or the antagonists PTH(7-34) and PTHrP(7-34) significantly altered the follicular response to cyclophosphamide in vivo. PTH(7-34) and PTHrP(7-34) shifted it towards a mild form of "dystrophic anagen", associated with a significant reduction in apoptotic (TUNEL+) hair bulb cells, thus mitigating the degree of follicle damage and retarding the onset of cyclophosphamide-induced alopecia. PTH(1-34), in contrast, forced hair follicles into "dystrophic catagen", associated with enhanced intrafollicular apoptosis. We had previously shown that an induced shift in the follicular damage-response towards "dystrophic catagen" mitigates cyclophosphamide-induced alopecia, whereas a shift towards "dystrophic catagen" initially enhanced the hair loss, yet subsequently promoted accelerated hair follicle recovery. Therefore, this study in an established animal model of chemotherapy-induced alopecia, which closely mimics human chemotherapy-induced alopecia, strongly encourages the exploration of PTH/PTHrP-R agonists and antagonists as novel therapeutic agents in chemotherapy-induced alopecia.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

A cancer modifier role for parathyroid hormone-related protein

The parathyroid hormone-related protein (PTHrP) gene (Pthlh) maps in the distal region of mouse chromosome 6 that contains a quantitative trait locus associated with genetic predisposition to skin tumorigenesis. Here, we report a genetic polymorphism located in the osteostatin encoding region of the Pthlh gene and that produces Thr/ Pro PTHrP variants. PthlhThr and PthlhPro alleles were significantly linked with resistance and susceptibility to skin carcinogenesis in phenotypically selected Car-R and Car-S outbred mice. Transfection of human NCI-H520 squamous cell carcinoma cells with the PthlhPro allele resulted in cells growing in clusters, tending to pile up, and growing at a significantly faster rate in nude mice than non-transfected and PthlhThr-transfected cells. These results point to the role of the Pthlh gene as a cancer modifier gene in skin tumorigenesis.

Expression and regulation of parathyroid hormone-related peptide in normal and malignant melanocytes

We examined parathyroid hormone-related peptide (PTHrP) production and regulation in both normal human melanocytes and in a human amelanotic melanoma cell line (A375). Northern blot and immunocytochemical analysis demonstrated that both cultured A375 cells and normal human melanocytes express PTHrP, but A375 cells expressed much higher levels of the peptide. PTHrP secretory rate increased at least 10-fold after treatment with 10% fetal bovine serum (100.2 +/- 2.8 pmol/10(6) cells vs. basal <15 pmol/10(6) cells) in proliferating A375 cells but only twofold in confluent cells. Treatment of A375 cells with increasing concentrations of 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] or its low-calcemic analog EB-1089 revealed that EB-1089 was 10-fold more potent than 1, 25-(OH)(2)D(3) on inhibition of both cell proliferation and PTHrP expression. Furthermore, inoculation of A375 cells into the mammary fat pad of female severe combined immunodeficiency mice resulted in the development of hypercalcemia and elevated concentrations of plasma immunoreactive PTHrP in the absence of detectable skeletal metastases. Our study, therefore, demonstrates a stepwise increase in PTHrP expression when cells progress from normal to malignant phenotype and suggests that EB-1089 should be further evaluated as a therapeutic agent in human melanoma.

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

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

Proliferation of parathyroid cells negatively correlates with expression of parathyroid hormone-related protein in secondary parathyroid hyperplasia

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is now suspected to act as an autocrine or paracrine regulator of cell growth or differentiation, although it was originally reported as a hypercalcemic substance in malignancies. This study was performed to assess the relationship between PTHrP expression and cell proliferation in human parathyroid glands.

METHODS

The localization of PTH and PTHrP was studied in 42 samples of hyperplastic parathyroid from 14 long-term hemodialysis cases with immunohistochemistry and in situ hybridization. Results were compared with proliferative activity (proliferating cell nuclear antigen index: counts of proliferating cell nuclear antigen-positive cells/100 cells). The localization of the PTH/PTHrP receptor was also examined. Ten normal glands were studied as controls.

RESULTS

In hyperplasia, cells positive for PTH, PTHrP, or both were observed immunohistochemically. The areas expressing PTHrP mRNA completely coincided with those positive for PTHrP immunohistochemically. Oxyphilic or transitional oxyphilic cells were consistently positive for PTHrP. PTH/PTHrP receptors were located in the cytoplasmic membrane in most parathyroid cells. Proliferating cell nuclear antigen-positive cells were rare in normal glands with an index of 0. 22 +/- 0.09 (mean +/- sem). They were significantly increased in hyperplastic cases but less for PTHrP-positive than for -negative cells (1.25 +/- 0.16 as compared with 7.80 +/- 0.52; P < 0.0001).

CONCLUSION

The observed low level of proliferation of PTHrP-positive cells suggests a functional role for PTHrP as a possible growth suppressor in the human parathyroid.