Regulation of parathyroid hormone-related protein expression in MCF-7 breast carcinoma cells by estrogen and antiestrogens

Osteolytic effects of tumoral estrogen signaling in an estrogen receptor-positive breast cancer bone metastasis model

AIM

Estrogen receptor α-positive (ER+) subtypes of breast cancer have the greatest predilection for forming osteolytic bone metastases (BMETs). Because tumor-derived factors mediate osteolysis, a possible role for tumoral ERα signaling in driving ER+ BMET osteolysis was queried using an estrogen (E2)-dependent ER+ breast cancer BMET model.

METHODS

Female athymic Foxn1nu mice were inoculated with human ER+ MCF-7 breast cancer cells via the left cardiac ventricle post-E2 pellet placement, and age- and dose-dependent E2 effects on osteolytic ER+ BMET progression, as well as direct bone effects of E2, were determined.

RESULTS

Osteolytic BMETs, which did not form in the absence of E2 supplementation, occurred with the same frequency in young (5-week-old) vs. skeletally mature (16-week-old) E2 (0.72 mg)-treated mice, but were larger in young mice where anabolic bone effects of E2 were greater. However, in mice of a single age and across a range of E2 doses, anabolic E2 bone effects were constant, while osteolytic ER+ BMET lesion incidence and size increased in an E2-dose-dependent fashion. Osteoclasts in ER+ tumor-bearing (but not tumor-naive) mice increased in an E2-dose dependent fashion at the bone-tumor interface, while histologic tumor size and proliferation did not vary with E2 dose. E2-inducible tumoral secretion of the osteolytic factor parathyroid hormone-related protein (PTHrP) was dose-dependent and mediated by ERα, with significantly greater levels of secretion from ER+ BMET-derived tumor cells.

CONCLUSION

These results suggest that tumoral ERα signaling may contribute to ER+ BMET-associated osteolysis, potentially explaining the greater predilection for ER+ tumors to form clinically-evident osteolytic BMETs.

From Good to Bad: The Opposing Effects of PTHrP on Tumor Growth, Dormancy, and Metastasis Throughout Cancer Progression

Parathyroid hormone related protein (PTHrP) is a multifaceted protein with several biologically active domains that regulate its many roles in normal physiology and human disease. PTHrP causes humoral hypercalcemia of malignancy (HHM) through its endocrine actions and tumor-induced bone destruction through its paracrine actions. PTHrP has more recently been investigated as a regulator of tumor dormancy owing to its roles in regulating tumor cell proliferation, apoptosis, and survival through autocrine/paracrine and intracrine signaling. Tumor expression of PTHrP in late stages of cancer progression has been shown to promote distant metastasis formation, especially in bone by promoting tumor-induced osteolysis and exit from dormancy. In contrast, PTHrP may protect against further tumor progression and improve patient survival in early disease stages. This review highlights current knowledge from preclinical and clinical studies examining the role of PTHrP in promoting tumor progression as well as skeletal and soft tissue metastasis, especially with regards to the protein as a regulator of tumor dormancy. The discussion will also provide perspectives on PTHrP as a prognostic factor and therapeutic target to inhibit tumor progression, prevent tumor recurrence, and improve patient survival.

A Low-Density DNA Microarray for Analysis of Markers in Breast Cancer

Breast cancer remains a major cause of death in women from Western countries. In the near future, advances in both nucleic acids technology and tumor biology should be widely exploited to improve the diagnosis, prognosis, and outcome prediction of this disease. The DNA microarray, also called biochip, is a promising tool for performing massive, simultaneous, fast, and standardized analyses of multiple molecular markers in tumor samples. However, most currently available microarrays are expensive, which is mainly due to the amount (several thousands) of different DNA capture sequences that they carry. While these high-density microarrays are best suited for basic studies, their introduction into the clinical routine remains hypothetical. We describe here the principles of a low-density microarray, carrying only a few hundreds of capture sequences specific to markers whose importance in breast cancer is generally recognized or suggested by the current medical literature. We provide a list of about 250 of these markers. We also examine some potential difficulties (homologies between marker and/or variant sequences, size of sequences, etc.) associated with the production of such a low-cost microarray.

Exposures to synthetic estrogens at different times during the life, and their effect on breast cancer risk

Women are using estrogens for many purposes, such as to prevent pregnancy or miscarriage, or to treat menopausal symptoms. Estrogens also have been used to treat breast cancer which seems puzzling, since there is convincing evidence to support a link between high lifetime estrogen exposure and increased breast cancer risk. In this review, we discuss the findings that maternal exposure to the synthetic estrogen diethylstilbestrol during pregnancy increases breast cancer risk in both exposed mothers and their daughters. In addition, we review data regarding the use of estrogens in oral contraceptives and as postmenopausal hormone therapy and discuss the opposing effects on breast cancer risk based upon timing of exposure. We place particular emphasis on studies investigating how maternal estrogenic exposures during pregnancy increase breast cancer risk among daughters. New data suggest that these exposures induce epigenetic modifications in the mammary gland and germ cells, thereby causing an inheritable increase in breast cancer risk for multiple generations.

Gender difference in bone metastasis of human small cell lung cancer, SBC-5 cells in natural killer-cell depleted severe combined immunodeficient mice

Lung cancer frequently develops multiple organ metastases, which thus makes this disease a leading cause of malignancy-related death worldwide. A gender difference is reported to affect the incidence and mortality of lung cancer; however, whether and how the gender difference is involved in lung cancer metastasis is unclear. This study evaluated the gender difference in multiple organ metastases in human small cell lung cancer (SBC-5) cells by using natural killer cell-depleted severe combined immunodeficient mice. Among multiple organ metastases, only bone metastasis formation significantly increased in female mice in comparison to males, while no significant difference was observed in the metastases to the liver and lungs. The suppression of androgen by castration or androgen receptor antagonist treatment in male mice also induced a significant increase of bone metastases. The number of osteoclasts in the bone metastatic lesions was greater in female mice and in mice with androgen suppression than in control male. However, there was no significant difference in the serum concentration of parathyroid hormone-related protein (PTHrP) associated with gender or androgen suppression. An in vitro study also indicated that sex steroid treatment had no effect on the proliferation or PTHrP production in SBC-5 cells. These results indicate that the balance of sex steroids therefore plays an important role in the formation of bone metastasis in small cell lung cancer, and suggests diverse mechanisms of interaction between cancer cells and host cells in the bone microenvironment.

Negative regulation of the osteoblast function in multiple myeloma through the repressor gene E4BP4 activated by malignant plasma cells

PURPOSE

To explore the pathogenetic mechanisms that suppress the osteoblast function in multiple myeloma because osteogenesis results in defective new bone formation and repair.

EXPERIMENTAL DESIGN

Microarray gene analysis revealed the overexpression of E4BP4, a transcriptional repressor gene, in normal osteoblasts cocultured with myeloma cells that were releasing the parathyroid hormone-related protein (PTHrP). Thus, the effect of E4BP4 was assessed in PTHrP-stimulated osteoblasts by measuring the RNA levels of both Runx2 and Osterix as major osteoblast transcriptional activators. Because E4BP4 is a negative regulator of the cyclooxygenase-2 (COX-2) pathway that drives the expression of both Runx2 and Osterix, these factors were investigated after prostaglandin E(2) treatment to overcome the COX-2 defect as well as in E4BP4-silenced osteoblasts. Finally, E4BP4, PTHrP, Osterix, and osteocalcin levels were measured in vivo in patients with bone disease together with the E4BP4 protein in bone biopsies.

RESULTS

E4BP4 was specifically induced by PTHrP and inhibited both Runx2 and Osterix, whereas E4BP4-silenced osteoblasts expressed functional levels of both factors. The prostaglandin E(2) treatment of E4BP4-up-regulated osteoblasts promptly restored Runx2 and Osterix activities, suggesting that integrity of COX-2 pathway is essential for their transcription. Down-regulation of Osterix by E4BP4 was confirmed in vivo by its inverse levels in osteoblasts from myeloma patients with increased serum PTHrP, whose bone biopsies expressed the E4BP4 protein.

CONCLUSIONS

Our data support the role of E4BP4 as osteoblast transcriptional repressor in inhibiting both Runx2 and Osterix in myeloma bone disease and correlate its effect with the increased PTHrP activity.

Parathyroid hormone-related protein varies with sex and androgen status in nonsmall cell lung cancer

BACKGROUND

In nonsmall cell lung cancer, tumor parathyroid hormone-related protein (PTHrP) expression predicts longer survival in women but not in men. To explain the sex-dependent survival effect, the authors proposed that hormonal influences decrease PTHrP in men versus women, that PTHrP inhibits tumor growth, and that the effect is greater in women than in men. The objectives of this study were to compare lung carcinoma PTHrP expression and carcinoma growth in male and female mice and to determine whether gonadal steroids regulate PTHrP in lung cancer cells.

METHODS

Tumor PTHrP content was measured by immunoassay, and tumor burden was assessed with multiple measures in BEN squamous cell orthotopic lung carcinomas in athymic mice. In addition, lung adenocarcinoma PTHrP messenger RNA (mRNA) values determined by microarray analyses were compared between men and women. Cultured lung cancer cells were assayed for PTHrP after treatment with estradiol or R1881, a synthetic androgen.

RESULTS

Lung carcinomas contained approximately 3 times more PTHrP in female mice than in male mice. Similarly, levels of PTHrP mRNA were significantly greater in adenocarcinomas from patients who were women than from patients who were men. Male mice had greater tumor burden than female mice. Androgen treatment reduced PTHrP in 3 lung cancer lines. Estradiol had no effect. Testosterone treatment also reduced lung carcinoma PTHrP in female mice.

CONCLUSIONS

Lung carcinomas in females expressed more PTHrP than in males possibly because of negative regulation by androgens in males. Female mice with higher tumor PTHrP content had significantly less tumor burden than male mice, supporting the hypothesis that PTHrP inhibits tumor growth.

A PTH/PTHrP receptor antagonist blocks the hypercalcemic response to estradiol-17β

Estradiol (E2) increases circulating calcium and phosphate levels in fish, thus acting as a hypercalcemic and hyperphosphatemic factor during periods of high calcium requirements, such as during vitellogenesis. Since parathyroid hormone (PTH)-related protein (PTHrP) has been shown to be calciotropic in fish, we hypothesized that the two hormones could be mediating the same process. Sea bream ( Sparus auratus) juveniles receiving a single intraperitoneal injection of piscine PTHrP(1-34) showed an elevation in calcium plasma levels within 24 h. In contrast, injections of the PTH/PTHrP receptor antagonist PTHrP(7-34) decreased circulating levels of calcium in the same period. Intraperitoneal implants of estradiol-17β (E2; 10 μg/g) evoked significant increases of circulating plasma levels of calcium and phosphorus and a sustained increases of circulating plasma levels of PTHrP. However, a combined treatment of E2 and PTHrP(7-34) evoked a markedly lower calcium response compared with E2 alone. We conclude that PTHrP or a related peptide that binds the PTH/PTHrP receptor mediates, at least in part, the hypercalcemic effect of E2 in calcium and phosphate balance in fish.

Characterisation of ligand binding to the parathyroid hormone/parathyroid hormone-related peptide receptor in MCF7 breast cancer cells and SaOS-2 osteosarcoma cells

Parathyroid hormone-related peptide (PTHrP) and parathyroid hormone (PTH)/PTHrP-receptor, PTH/PTHrP-R, are frequently expressed in mammary carcinomas as well as in bone cells. In this study we compared the ligand binding characteristics of the PTH/PTHrP-R in SaOS-2 human osteosarcoma cells with those in MCF7 breast cancer cells. We used both Scatchard analysis of saturation kinetics for iodinated ligand and the level of expressed receptor protein by visualising the single radio-labelled receptor-ligand complex from isolated membrane preparations from the two cell lines. In MCF7 cells, ligand binding, (receptor number) was increased by prior exposure of the cultured cells to epidermal growth factor (EGF), estradiol (E2), or dexamethasone (DEX), and decreased following calcitriol (1,25 DHCC). In contrast in the SaOS-2 cells, PTH/PTHrP-R number was increased by exposure to E2 and 1,25DHCC and decreased by DEX while EGF had no effect. These data were confirmed when the PTH/PTHrP-R was cross linked with (125)I-PTHrP-1-34(Tyr), and extended by visualising the intensity of the isolated radiolabelled receptor complex by autoradiography following SDS-PAGE at several time points during the treatment.

The parathyroid hormone family of peptides: structure, tissue distribution, regulation, and potential functional roles in calcium and phosphate balance in fish

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are two factors that share amino acid sequence homology and act via a common receptor. In tetrapods, PTH is the main endocrine factor acting in bone and kidney to regulate calcium and phosphate. PTHrP is an essential paracrine developmental factor present in many tissues and is involved in the regulation of ossification, mammary gland development, muscle relaxation, and other functions. Fish apparently lack an equivalent of the parathyroid gland and were long thought to be devoid of PTH. Only in recent years has the existence of PTH-like peptides and their receptors in fish been firmly established. Two forms of PTH, two of PTHrP, and a protein with intermediate characteristics designated PTH-L are encoded by separate genes in teleost fish. Three receptors encoded by separate genes in fish mediate PTH/PTHrP actions, whereas only two receptors have so far been found in terrestrial vertebrates. PTHrP has been more intensively studied than PTH, from lampreys to advanced teleosts. It is expressed in many tissues and is present in high concentration in fish blood. Administration of this peptide alters calcium metabolism and has marked effects on associated gene expression and enzyme activity in vivo and in vitro. This review provides a comprehensive overview of the physiological roles, distribution, and molecular relationships of the piscine PTH-like peptides.

Breast cancer metastasis to bone: mechanisms of osteolysis and implications for therapy

The most common skeletal complication of breast cancer is osteolytic bone metastasis. Bone metastases are present in 80% of patients with advanced disease and cause significant morbidity. They are most often osteolytic, but can be osteoblastic or mixed. Tumor cells, osteoblasts, osteoclasts and bone matrix are the four components of a vicious cycle necessary for the initiation and development of bone metastases. Tumor cell gene expression is modified by interaction with bone-derived factors. For example, parathyroid hormone related protein (PTHrP), a tumor cell factor, is upregulated by bone-derived transforming growth factor beta (TGFbeta). Tumor cell factors, in turn, act upon bone cells to cause dysregulated bone destruction and formation. PTHrP increases osteoblast expression of RANK (receptor activator of NFkappaB) ligand which, in turn, activates osteoclasts. PTHrP-independent osteolytic factors, such as interleukin [IL]-11 and IL-8, also contribute to the vicious cycle. Other tumor-bone interactions, such as stimulation of tumor-homing through the CXCR4 chemokine receptor by its bone-derived ligand stromal-derived factor-1 (SDF-1), may be responsible for the site-specific predilection of breast cancer for bone. These factors and their roles in fueling the vicious cycle may identify novel targets for therapies to prevent metastasis.

Expression of parathyroid hormone-related peptide (PthrP) and its receptor (PTH1R) during the histogenesis of cartilage and bone in the chicken mandibular process

The purpose of this study was to examine the expression and actions of parathyroid hormone-related protein (PTHrP) when skeletal histogenesis occurs in the chicken mandible. Prior to the appearance of skeletal tissues, PTHrP and PTH1R were co-expressed by cells in the ectoderm, skeletal muscle, peripheral nerve and mesenchyme. Hyaline cartilage was first observed at HH stage 27 when many but not all chondroblasts expressed PTHrP and PTH1R. By stage 34, PTHrP and PTH1R were not detected in chondrocytes but were expressed in the perichondrium. Alkaline phosphatase (AP)-positive preosteoblasts and woven bone appeared at stages 31 and 34, respectively. Preosteoblasts, osteoblasts and osteocytes co-expressed PTHrP and PTH1R. Treatment with chicken PTHrP (1-36) increased cAMP in mesenchyme from stage 26 embryos. Continuous exposure to chicken PTHrP (1-36) for 14 days increased cartilage nodule number and decreased AP while intermittent exposure did not affect cartilage nodule number and increased AP in cultures of stage 26 mesenchymal cells. Adding a neutralizing anti-PTHrP antibody to the cultures reduced cartilage nodule number and did not affect AP. These findings show that PTHrP and PTH1R are co-expressed by extraskeletal and skeletal cells before and during skeletal tissue histogenesis, and that PTHrP may influence skeletal tissue histogenesis by affecting the differentiation of mandibular mesenchymal cells into chondroblasts and osteoblasts.

Bone phenotype of the aromatase deficient mouse

Estrogens are important for normal bone growth and metabolism. The mechanisms are incompletely understood. Thus, we have undertaken characterization of the skeletal phenotype of aromatase (ArKO) deficient mice. No abnormalities have been noted in skeletal patterning in newborns. Adult ArKO mice show decreased femur length and decreased peak Bone Mineral Density (BMD) with accelerated bone loss by 7 months of age in females. Magnetic resonance microscopy (MR) and microCT (microCT) imaging disclosed decreased cancellous connectivity and reduced cancellous bone volume in ArKO females. Bone formation rate (BFR) is increased in ArKO females and decreased in ArKO males. Estradiol therapy reverses these changes. This anabolic effect of estradiol in the male skeleton is supported by 18-F- Positron Emission Tomography (PET) imaging, which clearly demonstrates decreased spinal uptake, but marked increase after estradiol therapy. Serum IGF-1 levels are high in young female ArKO mice but low in young ArKO males. The reduced BMD in ArKO females, despite the presence of elevated serum IGF 1, suggests that other mechanism(s) are operative. There is increased B-cell lymphopoiesis in adult female ArKO bone marrow cells. These results show that ArKO mice show the effects of estrogen deficiency on bone growth, mass, metabolism, microarchitecture and the hematopoietic microenvironment.

A role for parathyroid hormone-related protein in the pathogenesis of inflammatory/autoimmune diseases

Our increased understanding of the critical role of cytokines in chronic inflammatory/autoimmune diseases has led to the recent development of effective anti-cytokine treatments. In particular, agents blocking the function of TNF-alpha, a cytokine first identified as an endotoxin-inducible mediator of tumor cell necrosis, are now licensed for the treatment of rheumatoid arthritis (RA) and inflammatory bowel disease. However, TNF-alpha is but one member of a cytokine network that is responsible for mediating these inflammatory disorders. Therefore, as our understanding of the pathophysiologic role of other members of this inflammatory network increases, other cytokines may similarly be identified as effective targets for treatment. In this article, we will review evidence which suggests that parathyroid hormone-related protein (PTHrP), a peptide which, like TNF-alpha, was first identified because of its effects in the setting of malignancy, may in fact serve an important non-neoplastic, physiologic function by mediating the inflammatory/autoimmune host response. Data identifying PTHrP as a member of the cytokine network induced in multi-organ inflammation and rheumatoid arthritis will be summarized, initial evidence comparing the therapeutic efficacy of PTHrP- vs. TNF-alpha-blockade in the treatment of endotoxemia will be reviewed, and potential future areas of research, including assessment of the effects of PTHrP blockade in the treatment of RA, will be discussed.

Molecular mechanisms of osteolytic bone metastases

BACKGROUND

Breast carcinoma commonly metastasizes to the skeleton in patients with advanced disease to cause bone destruction and the associated pain, hypercalcemia, fracture, and nerve-compression syndromes. In this scenario, the bone destruction is mediated by the osteoclast. Tumor-produced parathyroid hormone-related protein (PTHrP), a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process. Transforming growth factor beta (TGFbeta), which is abundant in bone matrix and is released as a consequence of osteoclastic bone resorption, may promote breast carcinoma osteolysis by stimulating PTHrP production by tumor cells.

METHODS

Stable breast carcinoma MDA-MB-231 cell lines were constructed that expressed mutant TGFbeta receptors, Smad proteins, or estrogen receptor (ER)-alpha and were used to determine the role of TGFbeta in modulating tumor production of PTHrP. These stable cell lines were applied to a mouse model of human breast carcinoma metastases to the bone to dissect the molecular mechanisms responsible for osteolytic bone metastases.

RESULTS

TGFbeta promoted the development and progression of osteolytic bone metastases by inducing tumor production of PTHrP, the effect of which was mediated through the Smad signaling pathway. PTHrP stimulated osteoclastic bone resorption by increasing osteoblast production of the receptor activator of nuclear factor K B (RANK) ligand and decreasing osteoblast production of osteoprotegerin (OPG). A constitutively active ER-alpha mutation (Tyr537Asn), identified from a human bone metastases, when it was expressed in human breast carcinoma cells, caused increased production of PTHrP. TGFbeta significantly enhanced the ER-alpha-mediated transcriptional activity induced by ER-alpha (Tyr537Asn), and this resulted in further stimulation of PTHrP production.

CONCLUSIONS

These data indicate a central role for TGFbeta in the pathogenesis of osteolytic bone metastases from breast carcinoma by 1) the induction of PTHrP through the Smad signaling pathway and 2) the potentiation of ER-alpha-mediated transcription induced by a constitutively active ER-alpha. Understanding the mechanisms of osteolysis at a molecular level will generate more effective therapeutic agents for patients with this devastating complication of cancer.

Regulation of PTHrP and PTH/PTHrP receptor by extracellular Ca2+ concentration and hormones in the breast cancer cell line 8701-BC

It was previously reported that 8701-BC breast tumour cells express the gene for parathyroid hormone-related peptide (PTHrP) and PTH/PTHrP receptor (PTHrP-R) and release immunoreactive PTHrP (iPTHrP) into the extracellular medium. Since the regulation of PTHrP and PTHrP-R by breast cancer cells has been poorly investigated so far, we have chosen the 8701-BC cell line as a model system to investigate whether alterations in the extracellular Ca2+ concentration ([Ca2+]e) and treatment with some well-known differentiation agents for breast cells, such as dimethyl sulfoxide, hydrocortisone, progesterone, prolactin, all-trans retinoic acid and transforming growth factor-beta1 might (i) modulate quantitatively the release of iPTHrP, (ii) affect the PTHrP promoter usage and mRNA splicing patterns, and (iii) modify the expression of PTHrP-R. The data obtained indicate that 8701-BC cells are potentially able to utilise different start sites and mRNA splicing patterns for PTHrP transcription, and respond to variations of [Ca2+]e and to the addition of two hormones, hydrocortisone and progesterone, with modifications in the extracellular amount of iPTHrP. Moreover, expression of PTHrP-R is also modulated by changes of [Ca2+]e or treatment with hydrocortisone. This indicates that the 8701 -BC cell line is a suitable in vitro model for further studies on the complex molecular regulation of the PTHrP/PTHrP-R pair in breast cancer.

Extracellular matrix regulation of PTHrP and PTH/PTHrP receptor in a human breast cancer cell line

It was previously reported that 8701-BC breast cancer cells express the gene for parathyroid hormone-related peptide (PTHrP) and its cognate receptor (PTHrP-R), and release immunoreactive PTHrP in the extracellular medium; it was also found that PTHrP, in turn, exerts a role on the proliferative and invasive behavior in vitro of the same cell line. On the other hand, evidence has been produced that adhesion of 8701-BC cells onto different collagen substrates influences in various ways a number of phenotypic expressions, such as cell growth, motility, invasion of reconstituted basement membrane and production of lytic enzymes of the extracellular matrix (ECM). In light of these previous data, we have examined whether substrates of either reconstituted basement membrane or representative collagen components of the breast tumor stroma (type I, V and OF/LB) might (i) regulate the PTHrP promoter usage and mRNA splicing patterns, (ii) modulate quantitatively the extracellular release of immunoreactive PTHrP (iPTHrP), and (iii) affect the expression of PTHrP-R. The results obtained give evidence that (i) 8701-BC cells are able to utilize different start sites and mRNA splicing patterns for PTHrP transcription; (ii) 'structural' components of the stroma, such as collagens, are by themselves capable of controlling both the expression pattern of the PTHrP gene and the extent of extracellular release of iPTHrP, and (iii) PTHrP-R expression can be up- or down-regulated in response to the ECM substrate present. These data demonstrate that PTHrP and PTHrP-R expression by 8701-BC neoplastic cells can be modulated by ECM molecules, indirectly supporting the active participation of stromal collagen composition in the regulation of PTHrP-controlled circuits which may play a role in carcinogenesis.