C-terminal fragments of parathyroid hormone-related protein, PTHrP-(107-111) and (107-139), and the N-terminal PTHrP-(1-40) fragment stimulate membrane-associated protein kinase C activity in rat spleen lymphocytes

Osteostatin, a peptide for the future treatment of musculoskeletal diseases

Nowadays, the treatment of musculoskeletal diseases represents a major challenge in the developed world. Diseases such as osteoporosis, osteoarthritis and arthritis have a high incidence and prevalence as a consequence of population aging, and they are also associated with a socioeconomic burden. Many efforts have been made to find a treatment for these diseases with various levels of success, but new approaches are still needed to deal with these pathologies. In this context, one peptide derived for the C-terminal extreme of the Parathormone related Peptide (PTHrP) called Osteostatin can be useful to treat musculoskeletal diseases. This pentapeptide (TRSAW) has demonstrated both in different in vitro and in vivo models, its role as a molecule with anti-resorptive, anabolic, anti-inflammatory, and anti-antioxidant properties. Our aim with this work is to review the Osteostatin main features, the knowledge of its mechanisms of action as well as its possible use for the treatment of osteoporosis, bone regeneration and fractures and against arthritis given its anti-inflammatory properties.

Multiple actions of parathyroid hormone-related protein in breast cancer bone metastasis

The sequence similarity within the amino-terminal regions of parathyroid hormone (PTH) and PTH-related protein (PTHrP) allows the two to share actions at a common site, the PTH1 receptor. A number of biological activities have been ascribed to actions of other domains within PTHrP. PTHrP production by late stage breast cancer has been shown to contribute to bone metastasis formation through promotion of osteoclast formation and bone resorption by action through PTH1 receptors. There is evidence also for a role for PTHrP early in breast cancer that is protective against tumour progression. No signalling pathway has been identified for this effect. PTHrP has also been identified as a factor promoting the emergence of breast cancer cells from dormancy in bone. In that case, PTHrP does not function through activation of PTH1 receptors, despite having very substantial effects on transcriptional activity of the breast cancer cells. This indicates actions of PTHrP that are non-canonical, that is, mediated through domains other than the amino-terminal. It is concluded that PTHrP has several distinct paracrine, autocrine, and intracrine actions in the course of breast cancer pathophysiology. Some are mediated through action at PTH1 receptors and others are controlled by other domains within PTHrP. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Proteolytic Regulation of Parathyroid Hormone-Related Protein: Functional Implications for Skeletal Malignancy

Parathyroid hormone-related protein (PTHrP), with isoforms ranging from 139 to 173 amino acids, has long been implicated in the development and regulation of multiple tissues, including that of the skeleton, via paracrine and autocrine signaling. PTHrP is also known as a potent mediator of cancer-induced bone disease, contributing to a vicious cycle between tumor cells and the bone microenvironment that drives the formation and progression of metastatic lesions. The abundance of roles ascribed to PTHrP have largely been attributed to the N-terminal 1-36 amino acid region, however, activities for mid-region and C-terminal products as well as additional shorter N-terminal species have also been described. Studies of the protein sequence have indicated that PTHrP is susceptible to post-translational proteolytic cleavage by multiple classes of proteases with emerging evidence pointing to novel functional roles for these PTHrP products in regulating cell behavior in homeostatic and pathological contexts. As a consequence, PTHrP products are also being explored as potential biomarkers of disease. Taken together, our enhanced understanding of the post-translational regulation of PTHrP bioactivity could assist in developing new therapeutic approaches that can effectively treat skeletal malignancies.

Lung carcinoma progression and survival versus amino- and carboxyl-parathyroid hormone-related protein expression

PURPOSE

Expression of the carboxyl PTHrP region of parathyroid hormone-related protein (PTHrP) is a positive prognostic indicator in women with lung cancer, but amino PTHrP is a negative indicator in other lung cancer patients. This project investigated whether PTHrP could be expressed as predominantly amino PTHrP or carboxyl PTHrP in individual lung carcinomas. It also assessed domain-specific effects on cancer progression and patient survival.

METHODS

PTHrP immunoreactivities were analyzed versus survival in a human lung cancer tissue microarray (TMA). Growth was compared in athymic mice for isogenic lung carcinoma xenografts differing in expression of amino and carboxyl PTHrP domains.

RESULTS

In the TMA, 33 of 99 patient tumors expressed only one PTHrP domain, while 54 expressed both. By Cox regression, the hazard ratio for cancer-specific mortality (95% confidence interval) was 2.6 (1.28-5.44) for amino PTHrP (P = 0.008) and 0.6 (0-2.58) for carboxyl PTHrP (P = 0.092). Xenografts of H358 lung adenocarcinoma cells that overexpressed amino PTHrP grew twice as fast as isogenic low PTHrP tumors in athymic mice, but growth of tumors expressing amino plus carboxyl PTHrP was not significantly different than growth of the control tumors. In summary, the presence of amino PTHrP signifies worse prognosis in lung cancer patients. In mouse xenografts, this effect was abrogated if carboxyl PTHrP was also present.

CONCLUSION

Amino PTHrP and carboxyl PTHrP can vary independently in different lung carcinomas. Carboxyl PTHrP may temper the stimulatory effect of amino PTHrP on cancer progression.

Parathyroid Hormone-related Peptide (hPTHrP) and Parathyroid Hormone-related Peptide Receptor Type 1 (PTHR1) Expression in Human Thymus

Parathyroid hormone-related peptide (hPTHrP) is expressed in human tissues and regulates cellular proliferation, differentiation, and apoptosis by an autocrine/paracrine loop. In rodent thymus, both parathormone and parathyroid hormone-related peptide (PTHrP) are expressed by thymic epithelial cells (TECs). The present study demonstrated by RT-PCR and immunohistochemistry that hPTHrP and parathyroid hormone-related peptide receptor type 1 (PTHR1) were expressed in human thymus at both RNA and protein levels. hPTHrP was expressed mainly in the thymic medulla by epithelial (cytokeratin-positive), mature dendritic (CD40+/86+) and plasmacytoid interleukin (IL)-3Rα+ cells. This protein was also present in some cells forming Hassall's bodies and a few subcapsular and cortical TECs. PTHR1 was expressed by scattered subcapsular and cortical TECs and by rare TECs in the medulla. Thymocytes did not express either hPTHrP or PTHR1. Primary cultures of human TECs revealed the presence of both hPTHrP and PTHR1 mRNAs, confirming the capacity of TECs to synthesize both peptides. Moreover, synthetic (1–39) hPTHrP peptide administered on cultured TECs induced the expression of IL-6 mRNA, suggesting that hPTHrP can regulate thymic functions by inducing in TECs the expression of IL-6, which is involved in the development and maturation of thymocytes.

Investigational parathyroid hormone receptor analogs for the treatment of osteoporosis

INTRODUCTION

Intermittent parathyroid hormone (PTH) administration, acting through multiple signaling pathways, exerts an osteoanabolic effect on the skeleton that surpasses the effect of other antiosteoporotic agents. However, its efficacy is limited by the coupling effect and relatively common adverse events. Thus, the development of more sophisticated PTH receptor analogs seems imperative.

AREAS COVERED

In this review, the authors summarize the role of PTH signaling pathway in bone remodeling. The authors also summarize investigational analogs targeting this pathway, which may be potential treatments for osteoporosis.

EXPERT OPINION

β-arrestins are multifunctional cytoplasmic molecules that are decisive for regulating intracellular PTH signaling. Recently, in preclinical studies, arrestin analogs have achieved the anabolic bone effect of PTH without an accompanying increase in bone resorption. However, it is not yet known whether these analogs have adverse effects and there are no clinical data for their efficacy to date. On the other hand, several molecules derived either from PTH and PTH-related protein (PTHrP) molecules have been developed. Alternative routes of PTH 1 - 34 delivery (oral, transdermal), the PTH analog ostabolin and the N-terminal PTHrP analogs PTHrP 1 - 36 and abaloparatide, have recently been or are currently being tested in clinical trials and are more likely to become available for use in the near future.

Both N- and C-terminal domains of parathyroid hormone-related protein increase interleukin-6 by nuclear factor-kappa B activation in osteoblastic cells

Parathyroid hormone (PTH)-related protein (PTHrP) seems to affect bone resorption by interaction with bone cytokines, among them interleukin-6 (IL-6). Recent studies suggest that nuclear factor (NF)-kappaB activation has an important role in bone resorption. We assessed whether the N-terminal fragment of PTHrP, and its C-terminal region, unrelated to PTH, can activate NF-kappaB, and its relationship with IL-6 gene induction in different rat and human osteoblastic cell preparations. Here we present molecular data demonstrating that both PTHrP (1-36) and PTHrP (107-139) activate NF-kappaB, leading to an increase in IL-6 mRNA, in these cells. Using anti-p65 and anti-p50 antibodies, we detected the presence of both proteins in the activated NF-kappaB complex. This effect induced by either the N- or C-terminal PTHrP domain in osteoblastic cells appears to occur by different intracellular mechanisms, involving protein kinase A or intracellular Ca(2+)/protein kinase C activation, respectively. However, the effect of each peptide alone did not increase further when added together. Our findings lend support to the hypothesis that the C-terminal domain of PTHrP, in a manner similar to its N-terminal fragment, might stimulate bone resorption. These studies also provide further insights into the putative role of PTHrP as a modulator of bone remodeling.

C-terminal parathyroid hormone-related protein (PTHrP) (107-139) stimulates intracellular Ca(2+) through a receptor different from the type 1 PTH/PTHrP receptor in osteoblastic osteosarcoma UMR 106 cells

Studies were undertaken to determine whether PTH-related protein (PTHrP) (107-139) mobilizes [Ca(2+)](i) in osteoblastic osteosarcoma UMR 106 cells. PTHrP (107-139), in a manner similar to PTHrP (107-111), induced a rapid [Ca(2+)](i) response in these cells that was dose dependent (EC(50) of approximately 0.1 pM) and more efficient than that of PTHrP (1-36) (EC(50) of approximately 1 nM). This effect of PTHrP (107-139) was abrogated by micromolar doses of verapamil or nifedipine. However, it was unaffected by 10 microM U73122 (a phospholipase C inhibitor), 100 microg/ml heparin (an inositol 1,4,5-trisphosphate receptor inhibitor), or 400 ng/ml pertussis toxin (a G(i) inhibitor), which inhibited the [Ca(2+)](i) response to PTHrP (1-36), or by either 25 nM bisindolylmaleimide I (BIM), a protein kinase (PK) C inhibitor, or 1 microM phorbol-12-myristate-13-acetate preincubation (22 h). PTHrP (107-139) and PTHrP (1-36), at 100 nM, desensitized the [Ca(2+)](i) response to a second challenge with the same peptide, but not with the other peptide in these cells. PTHrP (7-34), a type 1 PTH/PTHrP receptor (PTH1R) antagonist, decreased the effect of PTHrP (1-36) on [Ca(2+)](i). In contrast, PTHrP (107-111), but neither PTHrP (109-138) nor PTHrP (7-34), abolished this effect of PTHrP (107-139). Both PTHrP (107-139) and PTHrP (1-36), added together at submaximal doses, induced a higher [Ca(2+)](i) response. Moreover, PTHrP (107-139) increased the efficacy of PTHrP (1-36) on [Ca(2+)](i), but decreased its induced increase in PKA activity in these cells. Verapamil or nifedipine (at 50 microM) or 25 nM BIM, but not 25 microM adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, a PKA inhibitor, abolished the PTHrP (107-139)-induced increase in interleukin 6 messenger RNA (assessed by RT, followed by PCR) in UMR 106 cells. This peptide also increased c-fos messenger RNA in these cells; an effect inhibited by BIM, but unaffected by either verapamil or EGTA. These findings support the existence of high-affinity receptors for PTHrP (107-139), associated with an induced Ca(2+) influx, different from the PTH1R in UMR 106 cells. The present results suggest that PTHrP could affect bone turnover by interacting with the PTH1R and other yet unknown receptors in bone cells through complex mechanisms.

C-terminal parathyroid hormone-related protein increases vascular endothelial growth factor in human osteoblastic cells

The N-terminal region of parathyroid hormone (PTH) and PTH-related protein (PTHrP) interacts with a common PTH/PTHrP receptor in osteoblasts. These cells synthesize PTHrP, but its role in bone turnover is unclear. Intermittent treatment with N-terminal PTHrP or PTH stimulates bone growth in vivo, possibly by increasing local bone factors. In addition, C-terminal PTHrP (107-139), which does not bind to the PTH/PTHrP receptor, appears to affect bone resorption in vivo and in vitro, although its effect on bone formation in vivo remains controversial. Bone angiogenesis is an often overlooked but critical event in the process of bone remodeling. Recently, PTH (1-34) has been shown to induce gene expression of vascular endothelial growth factor (VEGF), a potent angiogenic factor, by osteoblastic cells. However, no data are available on the effect of PTHrP (107-139) on VEGF expression in these cells. Using semiquantitative reverse transcription followed by PCR, we found that PTHrP (107-139), between 10 nM and 1 pM, increased VEGF mRNA in human osteoblastic (hOB) cells from trabecular bone. This effect of this agonist, at 10 nM, was maximal (fivefold for VEGF(165), and twofold for VEGF(121), compared to control) within 1 to 4 h. This effect was similar to that induced by PTHrP (1-34) in these cells, as well as in human osteosarcoma MG-63 cells, using Northern blot analysis. Moreover, the effect of both peptides, added together at 100 pM, was not higher than that observed with each peptide alone in hOB cells. The effects of PTHrP (107-139) and that of PTHrP (1-34) were abolished by actinomycin D in hOB cells. In these cells, the protein kinase C inhibitor staurosporine, but not the protein kinase A inhibitor H89, inhibited the increase in VEGF mRNA induced by 10 nM PTHrP (107-139). PTHrP (107-139), at 10 nM, also stimulated cytosolic VEGF immunostaining in hOB cells, and VEGF secretion into the medium conditioned by hOB or MG-63 cells for 24 h, which was (ng/mg protein): 10 +/- 1 or 5 +/- 3 (control), respectively, and 21 +/- 1 or 11 +/- 2 (PTHrP [107-139]-stimulated), respectively. Furthermore, medium conditioned by these cells for 24 h in the presence of 10 nM PTHrP (107-139), with or without 10 nM PTHrP (1-34), increased about 30% bovine aortic endothelial cell (BAEC) growth at 48 h. This effect was inhibited by adding a specific anti-VEGF antibody to the BAEC incubation medium. These findings demonstrate that the C-terminal domain of PTHrP induces expression and secretion of VEGF, a main angiogenic factor, in hOB cells and MG-63 cells. This relationship between PTHrP and VEGF has potential implications for both bone vascularization and bone formation, and neoangiogenesis in PTHrP-producing tumors.

Endogenous parathyroid hormone-related peptide enhances proliferation and inhibits differentiation in the osteoblast-like cell line ROS 17/2.8

To investigate potential effects of endogenous parathyroid hormone-related peptide (PTHrP) on osteoblast function, ROS 17/2.8 cells were transfected with full-length PTHrP cDNA in a sense or antisense orientation to alter PTHrP production. Compared with vector-transfected control cells, PTHrP-overproducing (sense-transfected) cells showed increased DNA synthesis ([(3)H]-thymidine incorporation) and increased growth (cell number). The extent of apoptosis was compared for the different clones using the terminal deoxynucleotide-mediated dUTP nick-end-labeling assay (TUNEL) and Hoechst staining. No differences in percentages of apoptotic cells were found under basal culture conditions or after 3 days of serum deprivation, which, itself, markedly increased numbers of apoptotic cells. The effect of PTHrP on osteoblast differentiation was assessed by examining two protein markers of differentiation, alkaline phosphatase, and bone morphogenetic protein (BMP)-2. Alkaline phosphatase activity was decreased in sense-transfected cells and increased in antisense-transfected cells, compared with cells transfected with empty vector. PTHrP-overproducing cells also showed decreased numbers of BMP-2-positive cells, whereas antisense-transfected cells showed no difference compared with vector control. The results indicate that: (a) endogenously produced PTHrP can increase growth of these osteoblastic cells by stimulating proliferation while not affecting apoptosis; and (b) the increased cell proliferation produced by PTHrP was accompanied by a reduction in activity or amount of two proteins normally expressed by differentiated osteoblasts.

Structure study of osteostatin PTHrP[Thr107](107-139)

The structure of chicken osteostatin or parathyroid hormone-related protein (PTHrP) (residues 107-139) containing an Ala/Thr substitution at the N-terminus was studied using two-dimensional proton NMR spectroscopy in an aqueous environment. Osteostatin is a separate circulating domain responsible for a range of activities related to the modulation of bone formation as well as keratinocyte proliferation. Anti-mitogenic properties of osteostatin have been detected in breast cancer cells and cytosolic calcium is used by osteostatin to signal in some neurons through a non-PTH receptor, unlike the separate circulating N-terminal domain. A structural basis for the activity is presented with particular emphasis given to the conformation of the bioactive segment 107-111, forming part of a finger-like projection capable of binding to the non-PTH receptor both in the presence and absence of the remainder of the molecule which appears simply to act as a largely globular carrier.

Parathyroid hormone-related protein (107-139) stimulates interleukin-6 expression in human osteoblastic cells

The N-terminal region of both parathyroid hormone (PTH) and PTH-related protein (PTHrP) binds to the same PTH/PTHrP receptor in osteoblasts. However, C-terminal PTHrP (107-139) inhibits growth and various functions of osteoblasts and osteoclasts apparently through PTHrP-specific receptors. PTH (1-34) and PTHrP (1-34) rapidly induce interleukin-6 (IL-6) expression by osteoblasts. The aim of the present study was to assess the effects of PTHrP (107-139) on IL-6 gene expression and secretion by osteoblastic cells from human trabecular bone (hOB). Using reverse transcription followed by PCR, it was found that IL-6 mRNA was twofold maximally increased by either PTHrP (1-34) or PTHrP (107-139), at 10 nM, over basal within 1 to 2 h in hOB cells. This effect of PTHrP (107-139), and that of PTHrP (1-34), were abolished by the transcription inhibitor actinomycin D. Meanwhile, puromycin, a protein synthesis inhibitor, superinduced IL-6 expression in the presence or absence of each PTHrP peptide. Both PTHrP (1-34) and PTHrP (107-139), but not PTHrP (38-64), stimulated IL-6 secretion to the hOB cell-conditioned medium at 24 h, dose dependently. In addition, this maximal stimulatory effect (twofold over basal) was similar with each PTHrP peptide alone, and not additive when added together. PTHrP (107-139) stimulation of mRNA and protein in hOB cells was abolished by bisindolylmaleimide I, a protein kinase C inhibitor, but not by either adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS), or N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H89), two protein kinase A inhibitors. These results indicate that C-terminal PTHrP, like its N-terminal domain, induces IL-6 production by human osteoblastic cells. This effect of both PTHrP regions could provide a mechanism to modulate bone turnover.

Stimulation of membrane-associated protein kinase-C activity in spleen lymphocytes by hPTH-(1-31)NH2, its lactam derivative, [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2, and hPTH-(1-30)NH2

Human parathyroid hormone, hPTH-(1-34), stimulates adenylyl cyclase and phosphatidylinositol-bisphosphate-specific phospholipase-C (PIP2-PLC), as indicated by increased membrane-associated protein kinase C (PKC) activity in ROS 17/2 rat osteosarcoma cells. The C-terminally truncated hPTH-(1-31)NH2 stimulates adenylyl cyclase as strongly as hPTH-(1-34) in these cells, but it does not stimulate PKC activity. Even [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2, a 6-fold stronger adenylyl cyclase stimulator than hPTH-(1-34), cannot stimulate PKC activity in ROS cells. Therefore PTH required its 32-34 region to stimulate PIP2-PLC/PKCs in this osteosarcoma line. In contrast, hPTH-(1-31)NH2 [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2 and even hPTH-(1-30)NH2 can stimulate PKC activity in freshly isolated rat spleen lymphocytes as strongly as hPTH-(1-34)NH2. The difference in the ability of membrane-associated PKC activity in spleen lymphocytes, but not in ROS cells, to be stimulated by C-terminally truncated PTH fragments might be due to different receptor densities or to the lymphocyte's atypical PTH/PTHrP receptor.

The structure of human parathyroid hormone-related protein(1-34) in near-physiological solution

Parathyroid hormone-related protein plays a major role in the pathogenesis of humoral hypercalcemia of malignancy. Under normal physiological conditions, parathyroid hormone-related protein is produced in a wide variety of tissues and acts in an autocrine or paracrine fashion. Parathyroid hormone-related protein and parathyroid hormone bind to and activate the same G-protein-coupled receptor. Here we present the structure of the biologically active NH2-terminal domain of human parathyroid hormone-related protein(1-34) in near-physiological solution in the absence of crowding reagents as determined by two-dimensional proton magnetic resonance spectroscopy. An improved strategy for structure calculation revealed the presence of two helices, His-5-Leu-8 and Gln-16-Leu-27, connected by a flexible linker. The parathyroid hormone-related protein(1-34) structure and the structure of human parathyroid hormone(1-37) as well as human parathyroid hormone(1-34) are highly similar, except for the well defined turn, His-14-Ser-17, present in parathyroid hormone. Thus, the similarity of the binding affinities of parathyroid hormone and parathyroid hormone-related protein to their common receptor may be based on their structural similarity.

Parathyroid hormone-related proteins is abundant in osteoarthritic cartilage, and the parathyroid hormone-related protein 1-173 isoform is selectively induced by transforming growth factor beta in articular chondrocytes and suppresses generation of extracellular inorganic pyrophosphate

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a major, locally expressed regulator of growth cartilage chondrocyte proliferation, differentiation, synthetic function, and mineralization. Because mechanisms that limit cartilage chondrocytes from maturing and mineralizing are diminished in osteoarthritis (OA), we studied PTHrP expression by articular chondrocytes.

METHODS

PTHrP was studied in normal knee cartilage samples and cultured articular chondrocytes, and in cartilage specimens from knees with advanced OA, obtained at the time of joint replacement.

RESULTS

PTHrP was more abundant in OA than in normal human knee articular cartilage. Both demonstrated PTH/PTHrP receptor expression. PTHrP 1-173, one of three alternatively spliced PTHrP isoforms, was exclusively expressed and induced by transforming growth factor beta in cultured chondrocytes. Chondrocytes mainly used the GC-rich P2 alternative promoter to express PTHrP messenger RNA. Inhibition by PTHrP 1-173, but not by PTHrP 1-146 or PTHrP 1-87, of inorganic pyrophosphate (PPi) elaboration suggested selective functional properties of the 1-173 isoform. Exposure to a neutralizing antibody to PTHrP increased PPi elaboration by articular chondrocytes.

CONCLUSION

Increased expression of PTHrP, including the 1-173 isoform, has the potential to contribute to the pathologic differentiated functions of chondrocytes, including mineralization, in OA.

C-terminal parathyroid hormone-related protein inhibits proliferation and differentiation of human osteoblast-like cells

Parathyroid hormone-related protein (PTHrP) is synthesized by osteoblasts, although its local role in bone is not completely understood. The C-terminal (107-111) region of PTHrP seems to be a potent inhibitor of osteoblastic bone resorption. We studied the effect of this PTHrP domain on the proliferation and synthesis of osteoblastic markers in osteoblast-like cells from adult human bone. We found that the human (h)PTHrP(107-139) fragment, between 10 fM and 10 nM, inhibited 3H-thymidine incorporation into these cells. The antiproliferative effect of the latter fragment, or that of hPTHrP(107-111), was similar to that induced by [Tyr34] hPTHrP(1-34) amide, bovine PTH(1-34), and hPTHrP(1-141), while hPTHrP(38-64) amide was ineffective. Human PTHrP(7-34) amide, at 10 nM, and 1 microM phorbol-12-myristate-13-acetate also significantly decreased DNA synthesis in human osteoblast-like cells. Neither hPTHrP(7-34) amide nor hPTHrP(107-139), at 10 nM, stimulated protein kinase A (PKA) activity in these cells. Moreover, 100 nM H-89, a PKA inhibitor, did not eliminate the inhibitory effect of hPTHrP(107-139) on these cells' growth. However 100 nM calphostin C, a PKC inhibitor, blunted this effect of PTHrP(107-139). In addition to their antimitogenic effect, hPTHrP(107-139) and hPTHrP(107-111) inhibited basal and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-stimulated alkaline phosphatase activity in these cells. Both fragments, like 1,25(OH)2D3, decreased C-terminal type I procollagen secretion into the cell-conditioned medium, but osteocalcin secretion by these cells was unaffected by the C-terminal PTHrP fragments. These findings suggest that PTHrP may act as a local regulator of bone formation.

Antiproliferative effect of the C-terminal fragments of parathyroid hormone-related protein, PTHrP-(107-111) and (107-139), on osteoblastic osteosarcoma cells

The C-terminal region of parathyroid hormone-related protein (PTHrP) containing the sequence (107-111) appears to be a potent inhibitor of osteoclastic bone resorption. In the present study, we have investigated the effect of human (h)PTHrP (107-139) and hPTHrP (107-111)NH2 on the proliferation of osteoblastic rat osteosarcoma UMR 106 cells. We found that both C-terminal PTHrP peptides, like hPTHrP (1-141), were antimitogenic for these cells, between 1 pM and 10 nM. [Tyr34]hPTHrP (1-34)NH2 was as potent as these peptides but less effective as growth inhibitor in these cells. UMR 106 cells were found to produce and secrete immunoreactive PTHrP. Addition of anti-PTHrP neutralizing antibodies to C- and N-terminal epitopes of PTHrP increased the growth of these cells. Our data suggest that the antiproliferative effect of these C-terminal PTHrP analogs may be independent of cyclic adenosine 3':5'-monophosphate (cAMP) and mediated by protein kinase C. These findings support an autocrine role of PTHrP in bone metabolism.

Inositol 1-,4-,5-trisphosphate-dependent Ca2+ signaling by the recombinant human PTH/PTHrP receptor stably expressed in a human kidney cell line

We previously reported the preparation and partial characterization of a series of human embryonic kidney cell lines (HEK-293) stably expressing various numbers of the recombinant human (h) parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (Rc). Using this expression system we examined ligand (PTH or PTHrP) binding characteristics and cyclic AMP responsiveness. We have now extended these studies to investigate the calcium signal transduction pathways activated by the hPTH/PTHrP Rc. In parental HEK-293 cells, which lack endogenous PTH/PTHrP Rc, incubation with hPTH(1-34) had no effect on cytosolic free Ca2+ concentration [Ca2+]i. In HEK-293 clone C-21, stably expressing approximately 400,000 Rc/cell, PTH stimulated an increase in [Ca2+]i by Ca2+ release from intracellular stores; PTH released Ca2+ exclusively from the IP3 sensitive Ca2+ pool. Unlike previous studies, the ability of PTH to elicit both cAMP responses and [Ca2+]i transients occurred over a wide range of Rc numbers (between 400,000 and 3000 Rc/cell); both responses were always observed at PTH concentrations in the same dose range although the magnitude of the responses decrease with Rc number. Pretreatment of C-21 cells with pertussis toxin for 24 h, which significantly enhanced PTH-stimulated cAMP accumulation, did not modulate PTH-stimulated [Ca2+]i transients. At each PTH concentration tested which resulted in increased cAMP levels, there was also an increase in [Ca2+]i transients. Treatment of C-21 cells with a battery of midregion and C-terminal PTH or PTHrP peptides showed no effect on either [Ca2+]i transients or cAMP accumulation, indicating a lack of functional interactions between these peptides and the form of the hPTH/PTHrP Rc stably expressed in these cells. Immunological analysis of G-protein expression demonstrated the presence of Gs, Gi, and Gq in all parental and transfected cells lines examined. Taken together, these data demonstrate that the hPTH/PTHrP Rc, stably expressed in HEK-293 cells, elicits responses in both the cAMP and IP3-dependent [Ca2+]i pathways and is responsive only to N-terminal PTH/PTHrP peptides.

Distribution and functions of parathyroid hormone-related protein in vertebrate cells

Parathyroid hormone-related protein (PTHrP) was isolated from tumors and identified as the agent of humoral hypercalcemia of malignancy (HHM) in 1987. Since then its gene structure in several mammalian and an avian species has been analyzed and its gene expression demonstrated in many adult and embryonic tissues derived from all three germ layers. The composition and structure of PTHrP peptide depends on both differential gene splicing and posttranslational processing, which result in a range of peptides of potentially diverse functions. This chapter describes the distribution of PTHrP in both normal and neoplastic adult and embryonic tissues. PTHrP is of fundamental importance to cell survival because the absence of the gene is fatal; this aspect of PTHrP function in cell physiology becomes overwhelmingly important in neoplasia. Intracrine or paracrine actions for PTHrP seem to be most likely in mammalian and avian physiology, but in fishes high circulating levels suggest classic endocrine functions as well. Much remains to be learned of the biology of this fascinating protein.

C-terminal fragment of parathyroid hormone-related protein, PTHrP-(107-111), stimulates membrane-associated protein kinase C activity and modulates the proliferation of human and murine skin keratinocytes

Low concentrations of the C-terminal parathyroid hormone-related protein (PTHrP) fragments, PTHrP-(107-111) and PTHrP-(107-139), stimulated membrane-associated protein kinase Cs (PKCs), but not adenylyl cyclase or an internal Ca2+ surge, in early passage human skin keratinocytes and BALB/MK-2 murine skin keratinocytes. The fragment maximally stimulated membrane-associated PKCs in BALB/MK-2 cells at 5 x 10(-9) to 10(-8) M. The maximally PKC-stimulating concentrations of PTHrP-(107-111) also stopped or stimulated BALB/MK-2 keratinocyte proliferation depending on whether the cells were, respectively, cycling or quiescent at the time of exposure. Thus, just one brief (30-minute) pulse of 10(-8) M PTHrP-(107-111) stopped the proliferation of BALB/MK-2 keratinocytes for at least 5 days. On the other hand, daily 30-minute pulses of 10(-8) M PTHrP-(107-111) started and then maintained the proliferation of initially quiescent BALB/MK-2 cells. Similarly PTHrP-(107-111) inhibited DNA synthesis by cycling primary adult human keratinocytes, but it stimulated DNA synthesis by quiescent human keratinocytes.

Signaling by N- and C-terminal sequences of parathyroid hormone-related protein in hippocampal neurons

Parathyroid hormone-related protein (PTHrP) is synthesized in the brain, and a single type of cloned receptor for the N-terminal portion of PTHrP and PTH is present in the central nervous system. Nothing is known about the physiological actions or signaling pathways used by PTHrP in the brain. Using cultured rat hippocampal neurons, we demonstrate that N-terminal PTHrP[1-34] and PTH[1-34] signal via cAMP and cytosolic calcium transients. The cAMP response showed strong acute (< or = 6 h) homologous and heterologous desensitization after preincubation with PTHrP or PTH. In contrast, the acute calcium response did not desensitize after preincubation with PTHrP; in fact, preincubation dramatically recruited additional responsive neurons. Unexpectedly, C-terminal PTHrP[107-139], which does not bind or activate the cloned PTH/PTHrP receptor, signaled in neurons via cytosolic calcium but not cAMP. Although some neurons responded to both PTHrP[1-34] and PTHrP[107-139], others responded only to PTHrP[1-34]. We conclude that certain hippocampal neurons exhibit dual signaling in response to PTHrP[1-34] and that some neurons have a receptor for C-terminal PTHrP that signals only via cytosolic calcium.

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.

Parathyroid hormone fragments may stimulate bone growth in ovariectomized rats by activating adenylyl cyclase

PTH is regarded conventionally as a catabolic hormone that stimulates osteoclastic resorption of bone. However, it has been known since 1932 that intermittent pulses of PTH stimulate bone formation in animals and humans. PTH independently activates two signal mechanisms: one that stimulates adenylyl cyclase and one that stimulates protein kinase C (PKC). The goal of this study was to use the 3- to 5-month-old ovariectomized (OVX) rat model to determine which of the two signal mechanisms is responsible for the anabolic action of PTH on bone. OVX triggered a large loss of trabecular bone without significantly affecting the normal slow growth of cortical bone in the distal halves of the femora. Daily injections of human hPTH(1-34) fragment (1 nmol/100 g body weight), which stimulated both adenylyl cyclase and membrane-associated PKC activity in osteoblast-like ROS 17/2 rat osteosarcoma cells, stimulated the growth of both cortical and trabecular bone in the OVX rats. Daily injections of the same dose of hPTH(1-31), which stimulated adenylyl cyclase but not PKC in ROS 17/2 cells, stimulated trabecular bone growth in the OVX rats less effectively than hPTH(1-34), but it stimulated cortical bone growth as rapidly and as dramatically as hPTH(1-34). Injections of equimolar amounts of desamino-hPTH(1-34) [N-propionyl(2-3)hPTH-amide], which stimulated PKC as strongly as hPTH(1-34) in ROS 17/2 cells but had a drastically reduced ability to stimulate adenylyl cyclase, or injections of recombinant hPTH(8-84) which stimulated PKC only in the ROS 17/2 cells, did not stimulate cortical or trabecular bone growth in the OVX animals. Thus, cyclic AMP and cyclic AMP-dependent protein kinases may be the primary mediators of the anabolic action of intermittent pulses of PTH on bone in OVX rats.