Comparison of the effects of various lengths of synthetic human parathyroid hormone-related peptide (hPTHrP) of malignancy on bone resorption and formation in organ culture

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Breast cancer cells induce osteoclast formation by stimulating host IL-11 production and downregulating granulocyte/macrophage colony-stimulating factor

Breast cancer cells frequently metastasize to the skeleton, where they induce OCL formation and activity, resulting in extensive bone destruction. However, the mechanisms by which breast cancer cells mediate increased osteolysis remain unclear. To elucidate this point, we investigated how 3 human breast cancer cell lines, MDA-MB-231, MDA-MB-435 and MCF-7, induce OCL formation using a murine osteoblast-spleen cell coculture system and compared their effects with a human colorectal cancer cell line, HCT-15; a human lung cancer cell line, HT-1080; and a normal human breast cell line, HME. The breast cancer cell lines supported OCL formation only when osteoblasts were present in spleen cell cocultures, whilst the non-breast cancer cell lines and the normal breast cell line, HME, had no effect. Fractionation of BCCM by ultrafiltration established that osteoclastogenic activity was associated with factors having m.w. >3 kDa. Breast cancer cell lines produced primarily PTHrP, with lesser amounts of IL-6, IL-11 and TNF-alpha. The effect of BCCM on OCL formation in osteoblast-spleen cell cocultures was partially prevented by a neutralising antibody to human PTHrP and completely prevented by a neutralising antibody to either murine IL-11 or the murine IL-11 receptor; neutralising antibodies to human IL-6, IL-11 or TNF-alpha were without effect. BCCM or human PTHrP induced an increase in murine osteoblast IL-11 mRNA and protein production, effects that were prevented in the presence of a neutralising antibody to human PTHrP. The osteoclastogenic activity of IL-11 was mediated by enhancing osteoblast production of PGE(2) effects, which were abrogated by an inhibitor of cyclooxygenase. PGE(2) apparently enhanced OCL formation by downregulating GM-CSF production by spleen cells since recombinant murine GM-CSF inhibited OCL formation and a neutralising antibody to murine GM-CSF blocked these inhibitory effects. We conclude that breast cancer cells induce OCL formation by stimulating osteoblastic production of IL-11. The subsequent release of PGE(2) followed by inhibition of GM-CSF production by cells within the bone microenvironment plays an important part in mediating the effects of breast cancer cells on OCL formation and their resorptive activity.

Parathyroid hormone-related protein in epithelial lining fluid in humans negatively correlates with the severity of lung injury

OBJECTIVE

To determine the concentration of parathyroid hormone-related protein (PTHrP; an autocrine/paracrine regulator of type-2 alveolar epithelial cells proliferation and apoptosis) in the epithelial lining fluid (ELF) from patients without pulmonary disease and from patients with acute lung injury (ALI), and to evaluate whether PTHrP concentrations correlated with the intensity of lung injury.

DESIGN

Prospective study.

SETTING

An adult trauma/surgical ICU in an urban teaching hospital.

PATIENTS

A total of 20 patients with ALI receiving mechanical ventilation (patients), and 10 patients without pulmonary disease not receiving mechanical ventilation (control subjects).

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

PTHrP was detected in all BAL fluids, and ELF PTHrP concentrations (median; 25% to 75% percentiles) tended to be higher in patients (52.2 nmol/mL; 20.8 to 65.6 nmol/mL) than in control subjects (25.4 nmol/mL; 20.5 to 35.4 nmol/mL; p = 0.18). In patients, ELF PTHrP concentration correlated positively with the PaO(2)/fraction of inspired oxygen ratio (r = 0.53; p = 0.005), and negatively with lung injury score (r = - 0.44; p = 0.02), radiologic score (r = - 0.40; p = 0.04), and BAL albumin concentration (r = - 0.42; p = 0.02).

CONCLUSION

PTHrP is present in biologically significant concentrations in the alveolar milieu in humans. In patients with ALI, the PTHrP concentration correlates negatively with the degree of lung injury.

Human breast-cancer cells stimulate the fusion, migration and resorptive activity of osteoclasts in bone explants

A central event in bone resorption is the recruitment of osteoclasts to future resorption sites. Breast-cancer cells invariably metastasise to the skeleton and induce extensive bone destruction by osteoclasts. However, our understanding of the mechanisms by which cancer cells interact with osteoclasts remains unclear. Consequently, we compared the effects of conditioned medium (CM) from 2 human breast-cancer cell lines, MB-MDA-231 and MCF-7, with those of a normal human breast epithelial cell line, HME, on osteoclastic fusion, resorptive activity and migration from the periosteum to the developing marrow cavity of fetal mouse metatarsals in culture. Osteoclastic resorptive activity was assessed by pre-labelling 17-day-old fetal metatarsal explants with 45Ca, whilst fusion and migration were monitored by histomorphometry and osteoclasts were identified by their tartrate-resistant acid phosphatase activity. CM from TPA-stimulated breast-cancer cell lines produced a significant increase in osteoclastic resorptive activity, whilst the normal breast cell line produced a minimal increase. The breast-cancer cell lines also stimulated osteoclastic fusion and migration in the metatarsal explants, but the normal breast cell line was without effect. The stimulatory effect of CM from MDA-MB-231 cells on osteoclastic fusion, but not migration, was partially inhibited by preventing prostaglandin and leukotriene synthesis by cells within the bone explants. In contrast, a synthetic matrix metalloproteinase (MMP) inhibitor, but not a cysteine proteinase inhibitor, prevented the migration of osteoclasts to the calcified centre of the metatarsal explants in response to CM from MDA-MB-231 cells. MDA-MB-231 cells also induced an increase in the expression of MMP-9 by migrating osteoclasts. Fractionation of the TPA-stimulated breast cancer cell CM established that the resorptive activity was associated with factors of m.w. >3 kDa. We determined by immuno-assay that human breast-cancer cells secrete parathyroid hormone-related protein (PTH-rP), tumour necrosis factor-alpha (TNF-alpha) and interleukins (ILs) 6 and 11. Neutralizing experiments with human antibodies to these cytokines established that PTH-rP and TNF-alpha production by MDA-MB-231 cells were responsible for mediating their effects on osteoclastic migration and ultimately bone resorption in the metatarsal explants.

Cancer cells responsible for humoral hypercalcemia express mRNA encoding a secreted form of ODF/TRANCE that induces osteoclast formation

A novel cDNA encoding a secreted form of osteoclast differentiation factor/tumor necrosis factor-related activation-induced cytokine (sODF/TRANCE, GenBank Accession No. AB037599) was sequenced from 5' RACE cDNA clones of squamous cell carcinoma cell lines, SCC-4 and T3M-1 Cl.2, of which parental malignant tissues had caused severe humoral hypercalcemia. The sODF/TRANCE cDNA was composed of unknown 5' end sequence followed by the 100% identical sequence of the ODF/TRANCE extracellular domain-coding region. The longest open reading frame (ORF) of the novel cDNA completely matched the 3' end of the ORF of the ODF/TRANCE cDNA encoding C-terminal amino acid residues (74-318) in the extracellular region. The corresponding protein that reacted with the antibody specific for the extracellular domain of ODF/TRANCE was detected in the culture media conditioned by the cancer cells. Furthermore, human promyeloblastic leukemia cells, HL60, differentiated into osteoclast-like cells (OCLs) when cultured in the media conditioned by SCC-4 and T3M-1 Cl. 2 cells. The differentiation of HL60 cells into OCLs was inhibited by the anti-ODF/TRANCE antibody. These results strongly suggest that sODF/TRANCE plays an important role in enhanced bone-resorption in humoral hypercalcemia of malignancy.

[Parathyroid hormone related peptide (PTHrP) and bone metabolism]

The discovery of PTHrP was the result of research on the mechanisms by which some cancers cause hypercalcemia (humoral hypercalcemia of malignancy) without necessarily metastasizing to bone. PTHrP is also present in various normal adult and fetal tissues. Its concentration is normally very low (picomolar) in blood, but it is more abundant in milk (nanomolar concentration). PTHrP seems able to exert autocrine/paracrine as well as endocrine effects on bone metabolism. A major role for PTHrP in regulation of fetal bone metabolism has been demonstrated in mice. Homologous recombination has been used in these rodents to remove the major coding exon from one copy of the mouse PTHrP gene in embryonic stem cells. Subsequently generated chimeric mice transmit the mutant PTHrP allele through the germline. Homozygous mutants died immediately after birth and had a multitude of skeletal abnormalities. So PTHrP seems necessary to embryonic development of the skeleton. PTHrP (1-34), like PTH (1-34) fragments, might be responsible for both bone resorption and formation. Although the effects of the carboxyl-terminal fragments are still controversial, PTHrP (107-111) fragment seems able to inhibit osteoclast activity. PTHrP (1-34), whose 8 of the first 13 amino-acids are identical with those in PTH (1-34), acts through the same receptor as PTH on osteoblasts and renal cells membrane. The PTHrP/PTH receptor sequence is now well established. PTHrP-receptor coupling is mediated by cyclic AMP and/or inositols-phosphate. The consequent activation of protein kinase A and intracellular calcium or protein kinase C, respectively, locally induces growth factors or cytokines secretion, responsible for the observed effects. The role of PTHrP appears important during pregnancy and lactation, when it stimulates fetal bone growth by increasing calcium transport from the dam to its fetus and maternal bone resorption allowing calcium supply for milk production, respectively. Such a role would be particularly important in domestic ruminants, which are often simultaneously pregnant and lactating. The role of PTHrP during aging (especially in post-menopausal women in which bone loss may induce osteoporosis) remains unknown and might be of peculiar interest since PTHrP (1-34) and (107-111) are able to restore bone loss induced by ovariectomy in rats.

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.