The parathyroid hormone-related protein (PTHrP) gene: use of downstream TATA promotor and PTHrP 1-139 coding pathways in primary breast cancers vary with the occurrence of bone metastasis

Parathyroid hormone-related protein in breast cancer bone metastasis

Parathyroid hormone-related protein (PTHrP) was discovered as the tumor product causing the humoral hypercalcemia of malignancy. Its structural similarity to the hormone, PTH, with 8 of the first 13 amino acids identical, was sufficient to explain the sharing by PTHrP and PTH of a common receptor, PTH1R, although the remainder of the sequences are unique. PTHrP has important roles in development of several organs, including breast and bone, and functions as a paracrine factor postnatally in these and other tissues. In addition to its hormonal role in cancer, PTHrP is produced by two thirds of primary breast cancers and 90% of bone metastases from breast cancer, leading to the concept that its production in bone by breast cancer cells promotes bone resorption, thus favoring tumor establishment and expansion, and an exit from tumor dormancy in bone through downregulation of leukemia inducing factor receptor (LIFR). Cancer production of PTHrP is increased by bone-derived growth factors, with particular attention paid to TGFβ, as well as by promoter-driven transcriptional effects, such as the hedgehog signaling factor, GLI2, and microenvironment effects including changes in underlying stiffness of substrates for cells. Although interest has been focused on PTHrP-induced bone resorption in bone metastasis, a mechanistically separate, protective effect against tumor progression has been proposed. Although there is conflicting mouse data, there are clinical studies suggesting that increased production of PTHrP by breast cancers confers upon them a less invasive phenotype, an effect distinct from the bone resorption-stimulating action that favors bone metastasis.

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.

Epidermal growth-factor-induced transcript isoform variation drives mammary cell migration

Signal-induced transcript isoform variation (TIV) includes alternative promoter usage as well as alternative splicing and alternative polyadenylation of mRNA. To assess the phenotypic relevance of signal-induced TIV, we employed exon arrays and breast epithelial cells, which migrate in response to the epidermal growth factor (EGF). We show that EGF rapidly – within one hour – induces widespread TIV in a significant fraction of the transcriptome. Importantly, TIV characterizes many genes that display no differential expression upon stimulus. In addition, similar EGF-dependent changes are shared by a panel of mammary cell lines. A functional screen, which utilized isoform-specific siRNA oligonucleotides, indicated that several isoforms play essential, non-redundant roles in EGF-induced mammary cell migration. Taken together, our findings highlight the importance of TIV in the rapid evolvement of a phenotypic response to extracellular signals.

PTHrP in differentiating human mesenchymal stem cells: transcript isoform expression, promoter methylation, and protein accumulation

Human PTHrP gene displays a complex organization with nine exons producing diverse mRNA variants due to alternative splicing at 5' and 3' ends and the existence of three different transcriptional promoters (P1, P2 and P3), two of which (P2 and P3) contain CpG islands. It is known that the expression of PTHrP isoforms may be differentially regulated in a developmental stage- and tissue-specific manner. To search for novel molecular markers of stemness/differentiation, here we have examined isoform expression in fat-derived mesenchymal stem cells both maintained in stem conditions and induced toward adipo- and osteogenesis. In addition, the expression of the splicing isoforms derived from P2 and P3 promoters was correlated to the state of methylation of the latter. Moreover, we also performed a quantitative evaluation of intracellular and secreted PTHrP protein product in undifferentiated stem cells and in parallel cultures at various differentiation stages. The data obtained indicate that from the stemness condition to that of osteo- and adipo-genic differentiated cells, the expression of isoforms becomes increasingly selective, thereby being a potential gene signature for the monitoring of cell stem or committed/differentiating state and that the switching-off of PTHrP isoform expression is mostly promoter methylation-dependent. Moreover, PTHrP intracellular retention is down-regulated in osteo-differentiating cells whereas the secretion of the protein in the extracellular medium is up-regulated with respect to stem cells, thereby suggesting that these variations of the intracellular and extracellular levels of PTHrP could potentially be enclosed in the list of the available protein signature of osteogenic differentiation.

Twenty-five years of PTHrP progress: from cancer hormone to multifunctional cytokine

Twenty-five years ago a "new" protein was identified from cancers that caused hypercalcemia. It was credited for its ability to mimic parathyroid hormone (PTH), and hence was termed parathyroid hormone-related protein (PTHrP). Today it is recognized for its widespread distribution, its endocrine, paracrine, and intracrine modes of action driving numerous physiologic and pathologic conditions, and its central role in organogenesis. The multiple biological activities within a complex molecule with paracrine modulation of adjacent target cells present boundless possibilities. The protein structure of PTHrP has been traced, dissected, and deleted comprehensively and conditionally, yet numerous questions lurk in its past that will carry into the future. Issues of the variable segments of the protein, including the enigmatic nuclear localization sequence, are only recently being clarified. Aspects of PTHrP production and action in the menacing condition of cancer are emerging as dichotomies that may represent intended temporal actions of PTHrP. Relative to PTH, the hormone regulating calcium homeostasis, PTHrP "controls the show" locally at the PTH/PTHrP receptor throughout the body. Great strides have been made in our understanding of PTHrP actions, yet years of exciting investigation and discovery are imminent. © 2012 American Society for Bone and Mineral Research.

Parathyroid hormone related protein (PTHrP) in tumor progression

Parathyroid hormone-related protein (PTHrP) is widely expressed in fetal and adult tissues and is a key regulator for cellular calcium transport and smooth muscle cell contractility, as well as a crucial control factor in cell proliferation, development and differentiation. PTHrP stimulates or inhibits apoptosis in an autocrine/paracrine and intracrine fashion, and is particularly important for hair follicle and bone development, mammary epithelial development and tooth eruption. PTHrP's dysregulated expression has traditionally been associated with oncogenic pathologies as the major causative agent of malignancy-associated hypercalcemia, but recent evidence revealed a driving role in skeletal metastasis progression. Here, we demonstrate that PTHrP is also closely involved in breast cancer initiation, growth and metastasis through mechanisms separate from its bone turnover action, and we suggest that PTHrP as a facilitator of oncogenes would be a novel target for therapeutic purposes.

Advances in the biology of bone metastasis: how the skeleton affects tumor behavior

It is increasingly evident that the microenvironment of bone can influence the cancer phenotype in many ways that favor growth in bone. The ability of cancer cells to adhere to bone matrix and to promote osteoclast formation are key requirements for the establishment and growth of bone metastases. Several cytokine products of breast cancers (e.g. PTHrP, IL-11, IL-8) have been shown to act upon host cells of the bone microenvironment to promote osteoclast formation, allowing for excessive bone resorption. The increased release of matrix-derived growth factors, especially TGF-β, acts back upon the tumor to facilitate further tumor expansion and enhance cytokine production, and also upon osteoblasts to suppress bone formation. This provides a self-perpetuating cycle of bone loss and tumor growth within the skeleton. Other contributing factors favoring tumor metastasis and colonization in bone include the unique structure and stiffness of skeletal tissue, along with the diverse cellular composition of the marrow environment (e.g. bone cells, stromal fibroblasts, immune cells), any of which can contribute to the phenotypic changes that can take place in metastatic deposits that favor their survival. Additionally, it is also apparent that breast cancer cells begin to express different bone specific proteins as well as proteins important for normal breast development and lactation that allow them to grow in bone and stimulate bone destruction. Taken together, these continually emerging areas of study suggest new potential pathways important in the pathogenesis of bone metastasis and potential areas for targeting therapeutics.

Targeting chemokine (C-C motif) ligand 2 (CCL2) as an example of translation of cancer molecular biology to the clinic

Chemokines are a family of small and secreted proteins that play pleiotropic roles in inflammation-related pathological diseases, including cancer. Among the identified 50 human chemokines, chemokine (C-C motif) ligand 2 (CCL2) is of particular importance in cancer development since it serves as one of the key mediators of interactions between tumor and host cells. CCL2 is produced by cancer cells and multiple different host cells within the tumor microenvironment. CCL2 mediates tumorigenesis in many different cancer types. For example, CCL2 has been reported to promote prostate cancer cell proliferation, migration, invasion, and survival, via binding to its functional receptor CCR2. Furthermore, CCL2 induces the recruitment of macrophages and induces angiogenesis and matrix remodeling. Targeting CCL2 has been demonstrated as an effective therapeutic approach in preclinical prostate cancer models, and currently, neutralizing monoclonal antibody against CCL2 has entered into clinical trials in prostate cancer. In this chapter, targeting CCL2 in prostate cancer will be used as an example to show translation of laboratory findings from cancer molecular biology to the clinic.

von Hippel-Lindau tumor suppressor gene-dependent mRNA stabilization of the survival factor parathyroid hormone-related protein in human renal cell carcinoma by the RNA-binding protein HuR

We have shown that parathyroid hormone-related protein (PTHrP) is a survival factor for human renal cell carcinoma (RCC) and that its expression is negatively regulated by the von Hippel-Lindau (VHL) tumor suppressor gene at the level of messenger RNA (mRNA) stability, as observed for tumor growth factors (TGFs). Our goals were to analyze the alternative splicing of PTHrP mRNA in human RCC and from these results to identify VHL/hypoxia-induced factor (HIF) system-regulated mRNA-binding proteins involved in PTHrP mRNA stability. We used: (i) a panel of human RCC cells expressing or not VHL; (ii) VHL-deficient 786-0 cells transfected with active or inactive VHL and (iii) human RCC samples and corresponding normal tissues. By quantitative real-time reverse transcription-polymerase chain reaction analysis, the 141 PTHrP mRNA isoform was found to be predominant in all cells and tumors (80%). In cells transfected with VHL, the expressions of all isoforms were decreased by 50%. Eight proteins with molecular weights ranging from 20 to 75 kDa were found to bind to biotinylated transcripts spanning the 141 PTHrP mRNA AU-rich 3'-untranslated region whose abundancy was dependent on VHL expression. The protein having an apparent molecular weight of 30 kDa was identified by western blot as HuR, a RNA-binding protein with stabilizing functions on various mRNA coding for proteins important in malignant transformation including vascular endothelial growth factor and TGF-beta. PTHrP expression studies confirmed the involvement of HuR in PTHrP upregulation in this disease. Common mRNA-binding proteins regulated by the VHL/HIF system may constitute new therapeutic opportunities against human RCC that remains refractory to therapies.

Amphiregulin-EGFR signaling regulates PTHrP gene expression in breast cancer cells

Parathyroid hormone-related protein (PTHrP) is an autocrine/paracrine factor produced by breast cancer cells that is speculated to play a major role in permitting breast cancer cells to grow into the bone microenvironment by stimulating the bone resorption axis. It has been previously shown that EGFR signaling induces the production of PTHrP in several primary and transformed epithelial cell types. Therefore, we investigated the relationship between EGFR and PTHrP gene expression in human breast cancer cells. Of a panel of 7 breast epithelial and cancer cell lines, the osteolytic, EGFR- positive lines (MDA-MB-231 and NS2T2A1) exhibited higher levels of PTHrP transcript expression. Amphiregulin mRNA levels in all lines were approximately 2 orders of magnitude higher than those of TGFalpha or HB-EGF. In the EGFR bearing lines, the receptor was phosphorylated at tyrosine 992 under basal conditions, and the addition of 100 nM amphiregulin did not lead to the phosphorylation of other tyrosine residues typically phosphorylated by the prototypical ligand EGF. Treatment of the EGFR positive lines with the EGFR inhibitor PD153035 and amphiregulin-neutralizing antibodies reduced PTHrP mRNA levels by 50-70%. Stable EGFR expression in the MCF7 line failed to increase basal PTHrP mRNA levels; however, treatment of this cell line with exogenous EGF or amphiregulin increased PTHrP transcription 3-fold. Transient transfection analysis suggests that the MAPK pathway and ETS transcription factors mediate EGFR coupling to PTHrP gene expression. Taken together, it appears that autocrine stimulation of EGFR signaling by amphiregulin is coupled to PTHrP gene expression via EGFR Tyr992 and MAPK, and that this pathway may contribute to PTHrP expression by breast tumor cells.

PTHrP-induced MCP-1 production by human bone marrow endothelial cells and osteoblasts promotes osteoclast differentiation and prostate cancer cell proliferation and invasion in vitro

Prostate cancer (PCa) preferentially metastasizes to bone resulting in osteoblastic lesions with underlying osteolytic activities. The mechanisms through which PCa cells promote osteolytic activities and subsequent osteoblastic bone formation remain poorly understood. Parathyroid hormone-related protein (PTHrP), produced by bone cells and PCa, binds to receptors on osteoblasts and stimulates bone formation and resorption. We have previously reported that MCP-1 acts as a paracrine and autocrine factor for PCa progression. However, the role of PTHrP in regulating MCP-1 expression in bone microenvironment, specifically by human bone marrow endothelial cells (HBME) and osteoblasts (hFOB), as well as by PCa cells, has not been studied. Accordingly, we first determined the effect of PTHrP on MCP-1 expression by bone cells and PCa cells. PTHrP induced both MCP-1 protein and mRNA expression by HBME and hFOB cells, but not by PCa LNCaP and PC3 cells. To further determine the mechanisms of PTHrP-induced MCP-1 transcription, analysis of the MCP-1 promoter was performed. MCP-1 promoter activity was induced by PTHrP. Both C/EBPbeta and NF-kappaB binding elements are required for PTHrP-induced MCP-1 transcription. Finally, when a constitutively-active PTH receptor construct was transfected into HBME and hFOB cells, MCP-1 production was increased. The conditioned media collected from these cells induced osteoclast differentiation and PC3 proliferation and invasion in vitro. These inductions were partially inhibited by MCP-1 neutralizing antibody. We conclude that PTHrP-induced MCP-1 production by HBME and hFOB cells promotes osteoclast differentiation in vitro and such induction may play a critical role in PCa development in the bone microenvironment.

Transcriptional regulation of parathyroid hormone-related protein promoter P2 by NF-kappaB in adult T-cell leukemia/lymphoma

Parathyroid hormone-related protein (PTHrP) plays a primary role in the development of humoral hypercalcemia of malignancy (HHM) that occurs in the majority of patients with adult T-cell leukemia/lymphoma (ATLL) due to human T-cell lymphotropic virus type-1 (HTLV-1) infection. We previously showed that ATLL cells constitutively express high levels of PTHrP via activation of promoters P2 and P3, resulting in HHM. In this study, we characterized a nuclear factor-kappaB (NF-kappaB) binding site in the P2 promoter of human PTHrP. Using electrophoretic mobility shift assays, we detected a specific complex in Tax-expressing human T cells composed of p50/c-Rel, and two distinct complexes in ATLL cells consisting of p50/p50 homodimers and a second unidentified protein(s). Chromatin immunoprecipitation assays confirmed in vivo binding of p50 and c-Rel on the PTHrP P2 promoter. Using transient co-transfection with NF-kappaB expression plasmids and PTHrP P2 luciferase reporter-plasmid, we showed that NF-kappaB p50/p50 alone and p50/c-Rel or p50/Bcl-3 cooperatively upregulated the PTHrP P2 promoter. Furthermore, inhibition of NF-kappaB activity by Bay 11-7082 reduced PTHrP P2 promoter-initiated transcripts in HTLV-1-infected T cells. In summary, the data demonstrated that transcriptional regulation of PTHrP in ATLL cells can be controlled by NF-kappaB activation and also suggest a Tax-independent mechanism of activation of PTHrP in ATLL.

PTHrP, calcitonin and calcitriol in a case of severe, protracted and refractory hypercalcemia due to a pancreatic neuroendocrine tumor

A patient with a primary neuroendocrine tumor of the pancreas, presented with severe hypercalcemia. This hypercalcemia of malignancy (HCM) failed to respond to intensive bisphosphonate treatment and needed continuous enhanced diuresis. Only after successful antitumor therapy did the hypercalcemia subside. Hypercalcemia was associated with increased concentrations of plasma PTHrP, calcitonin and 1,25-(OH)(2)D(3). Bone mineral density was markedly increased. We demonstrated the presence of both PTHrP and calcitonin in the tumor at the mRNA and protein level, using RT-PCR, immunohistochemistry and Western blotting. The high levels of plasma PTHrP and the demonstrated predominant renal mechanism in this case of HCM are suspected to be the cause for its refractoriness to bone resorption inhibitors. Our findings furthermore suggest that the tumoral production of calcitonin and PTHrP might have contributed to the increased bone mineral storage of calcium and thus probably attenuated the development of frank hypercalcemia.

PTHrP P3 promoter activity in breast cancer cell lines: role of Ets1 and CBP (CREB binding protein)

Parathyroid hormone-related protein (PTHrP) is produced by many tumors including breast cancer. We have reported that Ets1 factor activates P3 PTHrP promoter in our model of tumorigenic breast cancer cell and not in pre- or non-tumorigenic cell lines, thus contributing to an increased PTHrP production. In this study, gel retardation assays revealed that Etsl and its promoter binding site (EBS) specifically formed complexes whose abundance correlates with Ets1 levels in the three cell lines. Coexpression of Etsl and CBP induced a synergistic activation of the P3 promoter only in the tumorigenic cell line. This synergism required the integrity of the EBS and was abrogated by E1A. All breast cancer cell lines showed high basal concentrations of phosphorylated CREB. Moreover a CRE-like sequence was also required for Ets1/CBP synergy and, finally, CREB expression was found to enhance the PTHrP P3 promoter activity. Thus a multipartite complex of transcription factors and coactivators seems to regulate PTHrP transcription and contribute to the alterations that promote tumorigenic behavior in breast epithelial cells.

Parathyroid hormone-related protein and ezrin are up-regulated in human lung cancer bone metastases

Lung cancer often metastasizes to bone in patients with advanced disease. Identification of the factors involved in the interactions between lung cancer cells and bone will improve the prevention and treatment of bone metastases. We identified changes in metastasis-related gene expression of human HARA lung squamous carcinoma cells co-cultured with neonatal mouse calvariae using a pathway-specific microarray analysis. Nine genes were up-regulated and two genes down-regulated in HARA cells co-cultured with mouse calvariae. Five of the nine up-regulated genes, including caveolin 1, CD44, EphB2, ezrin, and Parathyroid hormone-related protein (PTHrP), and one down-regulated gene, SLPI, were further confirmed by Reverse transcription-polymerase chain reaction (RT-PCR). A mouse model was subsequently used to study the role of PTHrP and ezrin in bone metastasis in vivo. PTHrP (all three isoforms) and ezrin were up-regulated in HARA cells at sites of bone metastasis as detected by RT-PCR and immunohistochemistry. The PTHrP 141 mRNA isoform was increased by the greatest extent (13.9-fold) in bone metastases compared to PTHrP 139 and PTHrP 173 mRNA. We then generated a HARA cell line in which PTHrP expression was inducibly silenced by RNA interference. Silencing of PTHrP expression caused significant reduction of submembranous F-actin and decreased HARA cell invasion. Ezrin up-regulation was confirmed by Western blots on HARA cells co-cultured with adult mouse long bones. Further, Transforming growth factor beta (TGF-beta) was identified as one of the factors in the bone microenvironment that was responsible for the up-regulation of ezrin. The identification of PTHrP and ezrin as important regulators of lung cancer bone metastasis offers new mechanistic insights into the metastasis of lung cancer and provides potential targets for the prevention and treatment of lung cancer metastasis.

Skeletal metastasis: Established and emerging roles of parathyroid hormone related protein (PTHrP)

Parathyroid hormone related protein (PTHrP) is a well characterized tumor derived product that also has integral functions in normal development and homeostasis. PTHrP is produced by virtually all tumor types that metastasize to bone and numerous studies have demonstrated a correlation between PTHrP expression and skeletal localization of tumors. PTHrP has prominent effects in bone via its interaction with the PTH-1 receptor on osteoblastic cells. Through indirect means, PTHrP supports osteoclastogenesis by upregulating the receptor activator of NFkappaB ligand (RANKL) in osteoblasts. PTHrP also regulates osteoblast proliferation and differentiation in manners that are temporal and dose dependent. Bone turnover has been implicated in the localization of tumors to bone and PTHrP increases bone turnover. Bone turnover results in the release of growth factors such as TGFbeta and minerals such as calcium, both of which impact tumor cell growth and contribute to continued PTHrP production. PTHrP also has anabolic properties and could be in part responsible for osteoblastic type reactions in prostate cancer. Finally, emerging roles of PTH and PTHrP in the support of hematopoietic stem cell development in the bone marrow microenvironment suggest that an interaction between hematopoietic cells and tumor cells warrants further investigation.

Hypercalcemia induced by ovarian clear cell carcinoma producing all transcriptional variants of parathyroid hormone-related peptide gene during pregnancy

BACKGROUND

Parathyroid hormone-related peptide (PTHrP) appears as the major causative agent responsible for the humoral hypercalcemia of malignancy (HHM). However, the use of promoters and splicing patterns of PTHrP gene in HHM have not been reported yet.

CASE

A 35-year-old woman was diagnosed as an ovarian clear cell carcinoma with HHM caused by elevated serum PTHrP after delivery. An immunohistochemical study showed PTHrP expression in the tumor tissue. The Southern blot analysis following RT-PCR confirmed the presence of all types of PTHrP mRNA transcripts produced by a combination of three promoters, one 5' alternative splicing and three alternative 3' splicing events.

CONCLUSION

An ovarian clear cell carcinoma induced PTHrP-related HHM, which resulted from the high expression of all isoforms for PTHrP gene.

Parathyroid Hormone–Related Protein Localization in Breast Cancers Predict Improved Prognosis

In a prospective study of 526 consecutive patients with operable breast cancer, the significance of positive parathyroid hormone-related protein (PTHrP) staining by immunohistology has been evaluated for a median of 10-year follow-up. Improved survival was observed for the 79% of tumors which stained positively for PTHrP [estimated univariate hazard ratio, 0.43; 95% confidence interval (95% CI), 0.30-0.62; P < 0.001]. Adjustments for N stage, progesterone receptor status, and log tumor size changed this estimate only slightly to 0.47 (95% CI, 0.63-0.69; P = 0.001). Patients with PTHrP-positive primary tumors were less likely to develop bone metastases (hazard ratio, 0.63; 95% CI, 0.41-0.98; P = 0.04). PTHrP status was associated with estrogen receptor (P = 0.01), progesterone receptor (P = 0.03), and menopausal status (P = 0.006) but was not significantly associated with tumor size, vascular invasion, tumor grade, or patient age. Of 19 patients requiring surgery for bone metastases, the primary cancers were PTHrP negative in seven, all but one of whom had PTHrP-positive bone metastases. All 12 patients with PTHrP-positive primary cancers also had positive bone metastases. We conclude that increased production of PTHrP by breast cancers confers on them a less invasive phenotype, an effect distinct from the bone resorption-stimulating action that favors bone metastasis. It is likely that the latter property is influenced by factors in the bone microenvironment.

Parathyroid hormone-related protein in human renal cell carcinoma

Parathyroid hormone-related protein (PTHrP), a polyprotein discovered in 1987, plays crucial roles not only in development and in various physiological events associated with normal life, but also in a number of pathological conditions such as cancer. PTHrP appears as the major causative agent in humoral hypercalcemia of malignancy (HHM) associated to a broad range of tumors. However, this is only one aspect of the multiple facets of PTHrP in cancer biology. Indeed, the complex growth factor-like properties of PTHrP has shed new light onto potential roles of this peptide in the regulation of tumor growth and invasion. Initial studies in breast, prostate and lung cancer and recent results in renal cell carcinoma (RCC) suggest such roles and highlight the therapeutic potential of PTHrP-targeting strategies in human cancer including RCC. In this review, the role of PTHrP in RCC tumorigenesis and its potential as a therapeutic target will be discussed.

Alternative splicing of parathyroid hormone-related protein mRNA: expression and stability

Parathyroid hormone-related protein (PTHrP) is a multifunctional protein that is often dysregulated in cancer. The human PTHrP gene is alternatively spliced into three isoforms, each with a unique 3'-untranslated region (3'-UTR), encoding 139, 173 and 141 amino acid proteins. The regulation of PTHrP mRNA isoform expression has not been completely elucidated, but it may be affected by transforming growth factor-beta1 (TGF-beta1). In this study, we examined differences in the PTHrP mRNA isoform expression in two squamous carcinoma cell lines (SCC2/88 and HARA), an immortalized keratinocyte cell line (HaCaT), and spontaneous human lung cancer with adjacent normal tissue. In addition, the effect of TGF-beta1 on PTHrP mRNA isoform expression and stability was examined. Cell-type specific expression of PTHrP mRNA isoforms occurred between the various cell lines, normal human lung, and immortalized human keratinocytes (HaCaT). PTHrP isoform expression pattern was significantly altered between normal lung tissue and the adjacent lung cancer. In vitro studies revealed that TGF-beta1 differentially altered the mRNA steady-state levels and mRNA stability of the PTHrP isoforms. Protein-RNA binding studies identified different proteins binding to the 3'-UTR of the PTHrP isoforms (139) and (141), which may be important in the differential mRNA stability and response to cytokines between the PTHrP isoforms. The data demonstrate that there is cell-type specific expression of PTHrP mRNA isoforms, and disruption of the normal regulation during cancer progression may in part be associated with TGF-beta1-induced changes in PTHrP mRNA isoform expression and stability.

PTH-related protein enhances MCF-7 breast cancer cell adhesion, migration, and invasion via an intracrine pathway

Breast cancer is the most common carcinoma that metastasizes to the bone. Parathyroid hormone-related protein (PTHrP), a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process in breast cancer. PTHrP overexpression increases mitogenesis and decreases apoptosis in the human breast cancer cell line MCF-7. In this study, MCF-7 cells were used as a model system to study the effects of PTHrP on breast cancer cell adhesion, migration, and invasion. Clones of MCF-7 cells were established that overexpress wild-type PTHrP or PTHrP mutated in the nuclear localization sequence (NLS). Wild-type PTHrP-overexpressing cells showed significantly higher laminin adhesion and migration, and Matrigel invasion than empty vector-transfectants or cells overexpressing NLS-mutated PTHrP. Wild-type PTHrP also increased the cell surface expression of the pro-invasive integrins alpha6 and beta4; deletion of the NLS negated these effects. Exogenous PTHrP (1-34), (67-86), (107-139), and (140-173) had no effect on integrin expression, or on cell adhesion, migration, and invasion. These results indicate that PTHrP exerts its effects on cell adhesion, migration, invasion, and integrin expression via an intracrine pathway. PTHrP may play a role in breast cancer metastasis by upregulating proinvasive integrin expression, and controlling PTHrP production in breast cancer may provide therapeutic benefit.

THE EFFECT OF THE BISPHOSPHONATE ALENDRONATE ON VIABILITY OF CANINE OSTEOSARCOMA CELLS IN VITRO

The objective of this study was to determine the effect of alendronate on the viability of canine osteosarcoma cells and nonneoplastic canine cells. The sample population was composed of canine osteosarcoma tumor cells. Osteosarcoma cells and canine fibroblasts were maintained in culture under standard conditions. The MTT assay for cell viability was performed after 24, 48, and 72 h of incubation with alendronate (0.001 to 1000 microM) or no drug (control). Plates were set up so that each concentration and the control had a sample number of 8. The optical density (OD) of each well was measured at 540 nm using an enzyme-linked immunosorbent assay microplate reader. The percent viability was determined for each concentration and for each incubation time. After 24 h of incubation of POS (parent osteosarcoma) and HMPOS cells with alendronate, there was no significant difference in mean OD at any drug concentration when compared with control samples. A significant concentration- and time-dependent reduction in mean OD of osteosarcoma cells was observed after 48 and 72 h of incubation, with alendronate concentrations ranging from 10 to 1000 microM. The lowest percent cell viability observed in treated cells was 35%. Conversely, alendronate did not significantly affect mean OD in fibroblasts, and the lowest percent cell viability observed was 76%. Our data indicate that alendronate may have the potential to inhibit canine osteosarcoma tumor growth. It will be important to determine the clinical relevance of these in vitro findings. If similar findings are observed in vivo, use of alendronate may also be indicated as an adjuvant to existing chemotherapeutic protocols.

Ets-1 activates parathyroid hormone-related protein gene expression in tumorigenic breast epithelial cells

Parathyroid hormone-related protein (PTHrP) is produced by many tumors not associated with humoral hypercalcemia, including breast cancers. In this study, we used three human immortalized mammary epithelial cell lines that differ in tumorigenicity and PTHrP expression. Using RT-PCR we investigated 5' and 3' alternative splicing of PTHrP transcripts and promoter usage in the lines. Increased levels of P3-derived transcripts and the 1-139 mRNA isoform were observed in the most tumorigenic cell line. Transient transfection experiments identified elements close to P3 promoter that appeared to account for a portion of differential PTHrP expression among the three cell lines. Using site-directed mutagenesis, a previously described Ets-1/Sp1 binding site upstream of P3 was determined to be crucial for full activity of this promoter. RT-PCR and western blot evaluation of Ets family member expression found that Ese-1 was present in all three lines, but that appreciable levels of Ets-1 protein were present exclusively in the most tumorigenic line. Cotransfection of Ets-1 expression vectors activated PTHrP reporter constructs in the most tumorigenic line but not in the other cell lines. These findings suggest a potential mechanism by which PTHrP transcription may be regulated as a consequence of events that promote tumorigenic behavior in breast epithelial cells.

Parathyroid hormone-related protein (PTHrP)-(1-139) isoform is efficiently secreted in vitro and enhances breast cancer metastasis to bone in vivo

Parathyroid hormone-related peptide (PTHrP) is a mediator of local osteolysis due to breast cancer. Three isoforms of PTHrP, (1-139), (1-141), and (1-173), are products of alternative splicing in humans, but the specific contribution of each of these isoforms to osteolytic metastasis caused by breast cancer has not been evaluated. To determine the role of PTHrP isoforms in breast cancer metastasis to bone, the human breast cancer cell line MDA-MB-231 (MDA-231) was stably transfected with cDNAs for human prepro PTHrP-(1-139), -(1-141), or -(1-173). Stable MDA/PTHrP-(1-139) clones expressed more PTHrP mRNA and secreted more PTHrP protein, compared with MDA/PTHrP-(1-141), -(1-173), or parental MDA-231. Parental MDA-231 cells and clones expressing each isoform had similar growth rates in vitro. In a mouse model of bone metastases, the osteolytic lesion area of radiographs was greatest in mice bearing MDA/PTHrP-(1-139) compared with those bearing MDA/PTHrP-(1-141), -(1-173), or parental MDA-231. Ca(++) and plasma PTHrP concentrations were significantly higher in the MDA/PTHrP-(1-139) compared with the MDA/PTHrP-(1-141), -(1-173), or parental MDA-231 groups. These data demonstrate that the PTHrP-(1-139) isoform was produced to a greater extent than PTHrP-(1-141) or -(1-173), and in vivo enhanced osteolysis with increased plasma PTHrP concentrations and hypercalcemia compared with overexpression of PTHrP-(1-141) or -(1-173).

Cloning and sequencing of the 3'-region of the canine parathyroid hormone-related protein gene and analysis of alternate mRNA splicing in two canine carcinomas

A canine genomic library in Lambda FIX II vector was screened with a 281-base pair canine PTHrP cDNA to the prepro- and coding regions. Two genomic clones were isolated and mapped to the 3'-end of the PTHrP gene by polymerase chain reaction (PCR) amplification of exons in this region. One clone (3.5 kb) was amplified by PCR, partially sequenced, and compared to the human PTHrP gene. Regions were identified with a high degree of homology to exons 6, 7, and 8 of the human PTHrP gene. A polyadenylation site was present 3' to the exon 8-like region. Reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated that exon 7 of the PTHrP gene was transcribed in two canine carcinomas (SCC 2/88 cells and CAC-8 tumor line) which produce PTHrP. This confirmed that the 3'-region of the canine PTHrP gene is alternately spliced with splicing of exon 6 to exons 7 or 9. Transcription of exon 8 was not demonstrated by RT-PCR and suggests that the exon 8-like region of the dog PTHrP gene is not utilized. The exon 8-like region contained an early stop codon that was not present in exon 8 of the human PTHrP gene.

Transforming growth factor beta regulates parathyroid hormone-related protein expression in MDA-MB-231 breast cancer cells through a novel Smad/Ets synergism

The majority of breast cancers metastasizing to bone secrete parathyroid hormone-related protein (PTHrP). PTHrP induces local osteolysis that leads to activation of bone matrix-borne transforming growth factor beta (TGF beta). In turn, TGF beta stimulates PTHrP expression and, thereby, accelerates bone destruction. We studied the mechanism by which TGF beta activates PTHrP in invasive MDA-MB-231 breast cancer cells. We demonstrate that TGF beta 1 up-regulates specifically the level of PTHrP P3 promoter-derived RNA in an actinomycin D-sensitive fashion. Transient transfection studies revealed that TGF beta 1 and its effector Smad3 are able to activate the P3 promoter. This effect depended upon an AGAC box and a previously described Ets binding site. Addition of Ets1 greatly enhanced the Smad3/TGF beta-mediated activation. Ets2 had also some effect, whereas other Ets proteins, Elf-1, Ese-1, and Erf-1, failed to cooperate with Smad3. In comparison, Ets1 did not increase Smad3/TGF beta-induced stimulation of the TGF beta-responsive plasminogen activator inhibitor 1 (PAI-1) promoter. Smad3 and Smad4 were able to specifically interact with the PTHrP P3-AGAC box and to bind to the P3 promoter together with Ets1. Inhibition of endogenous Ets1 expression by calphostin C abrogated TGF beta-induced up-regulation of the P3 transcript, whereas it did not affect the TGF beta effect on PAI expression. In TGF beta receptor II- and Ets1-deficient, noninvasive MCF-7 breast cancer cells, TGF beta 1 neither influenced endogenous PTHrP expression nor stimulated the PTHrP P3 promoter. These data suggest that TGF beta activates PTHrP expression by specifically up-regulating transcription from the PTHrP P3 promoter through a novel Smad3/Ets1 synergism.

Parathyroid hormone-related peptide stimulates proliferation of highly tumorigenic human SV40-immortalized breast epithelial cells

Parathyroid hormone-related protein (PTHrP) is the main mediator of humoral hypercalcemia of malignancy (HHM) and it is produced by many tumors, including breast cancers. Breast epithelial cells as well as breast cancer tumors and cell lines have been reported as expressing PTHrP and the PTH/PTHrP receptor, suggesting that PTHrP may act as an autocrine factor influencing proliferation or differentiation of these cell types. We investigated PTHrP gene expression, PTH/PTHrP receptor signaling, and PTHrP-induced mitogenesis in three immortalized human mammary epithelial cell lines that exhibit differential tumorigenicity. The most tumorigenic cells expressed the highest levels of PTHrP messenger RNA (mRNA) and protein. We used reverse-transcription polymerase chain reaction (RT-PCR) and immunoblotting to detect the PTH/PTHrP receptor transcripts and proteins in all of the three cell lines. Treatment with human PTHrP(1-34) [hPTHrP(1-34)] and hPTH(1-34) increased intracellular cyclic adenosine monophosphate (cAMP) but not free Ca2+ in the nontumorigenic line. These agonists increased both cAMP and free Ca2+ levels in the moderately tumorigenic line, but only increased free Ca2+ in the highly tumorigenic line. Application of the PTH/PTHrP receptor antagonist [Asn10,Leu11,D Trp12]PTHrP(7-34) or PTHrP antibodies reduced [3H]thymidine incorporation in a dose-dependent fashion in the highly tumorigenic cell line but did not affect the other lines. Thus, treatment with a PTH/PTHrP receptor antagonist reduced cell proliferation, suggesting that PTHrP signaling mediated by the phospholipase C (PLC) pathway stimulates proliferation of a highly tumorigenic immortalized breast epithelial cell line.

Pathophysiologic interactions in skeletal metastasis

BACKGROUND

This review summarizes evidence that the formation of bone metastases is the result of multiple synergistic cellular and molecular interactions between metastatic cells and the unique microenvironment in bone.

METHODS

Molecular technologies have been used to detect cancer cells in bone and to define their genotypic and phenotypic properties. Bone organ cultures have been employed to analyze the ability of tumor cells to modulate bone resorption and to study the effects of resorption products on the phenotypic properties of cancer cells. Experimental models of bone metastasis provide the ability to examine the effects of modulating specific host or tumor properties in vivo by quantifying their effects on the formation of bone tumors.

RESULTS

By means of the blood stream, cells from many common neoplasms seed bone marrow as an early clinical event. The subsequent growth of these cells into clinically significant metastatic lesions is associated with their ability to stimulate bone resorption through osteoclasts and macrophages or through a direct action on bone. In turn, the products of bone resorption, which include matrix-derived growth factors, act on the tumor cells to stimulate the expression of properties that promote their metastatic competence. These include the induction of integrin adhesion molecules, the stimulation of cell motility and chemotaxis, the enhanced expression of matrix metalloproteinases, and the stimulation of tumor cell growth.

CONCLUSIONS

The interdependency of tumor cells and bone was recognized by Steven Paget over 100 years ago, and it provides a rational basis for the development of current therapeutic strategies against bone metastasis.

Regulation of parathyroid hormone-related protein gene expression in murine keratinocytes by E1A isoforms: a role for basal promoter and Ets-1 site

PTHrP gene expression was evaluated in a murine keratinocyte line, Pam 212K, transformed with E1A and ras. We found that the 12S-E1A oncogene, with or without ras transformation, markedly reduced PTHrP mRNA expression. Using transient transfection assays, we found that the 12S isoform repressed activity from a 5'PTHrP-driven reporter gene. E1A-induced repression of PTHrP reporter constructs appears to be mediated by sequences within minimal promoter region. The 13S-E1A isoform did not repress PTHrP reporter gene activity, and a 13S-deletion mutant that lacked the repressor domains activated a subset of reporter constructs. Mutation of an Ets-1 binding site upstream of the basal promoter substantially decreased activation of reporter constructs by this 13S-deletion mutant. These findings suggest that the E1A oncoprotein may serve as a model for both activation and repression of PTHrP gene expression.