PTHrP(12-48) Modulates the Bone Marrow Microenvironment and Suppresses Human Osteoclast Differentiation and Lifespan

Re-Evaluating the Role of PTHrP in Breast Cancer

Parathyroid-hormone-related protein (PTHrP) is a protein with a long history of association with bone metastatic cancers. The paracrine signaling of PTHrP through the parathyroid hormone receptor (PTHR1) facilitates tumor-induced bone destruction, and PTHrP is known as the primary driver of humoral hypercalcemia of malignancy. In addition to paracrine signaling, PTHrP is capable of intracrine signaling independent of PTHR1 binding, which is essential for cytokine-like functions in normal physiological conditions in a variety of tissue types. Pre-clinical and clinical studies evaluating the role of PTHrP in breast cancer have yielded contradictory conclusions, in some cases indicating the protein is tumor suppressive, and in other studies, pro-growth. This review discusses the possible molecular basis for the disharmonious prognostic indications of these studies and highlights the implications of the paracrine, intracrine, and nuclear functions of the protein. This review also examines the current understanding of the functional domains of PTHrP and re-evaluates their role in the unique context of the breast cancer environment. This review will expand on the current understanding of PTHrP by attempting to reconcile the functional domains of the protein with its intracrine signaling in cancer.

Bone Metastasis of Breast Cancer: Molecular Mechanisms and Therapeutic Strategies

Bone metastasis is a common complication of many types of advanced cancer, including breast cancer. Bone metastasis may cause severe pain, fractures, and hypercalcemia, rendering clinical management challenging and substantially reducing the quality of life and overall survival (OS) time of breast cancer patients. Studies have revealed that bone metastasis is related to interactions between tumor cells and the bone microenvironment, and involves complex molecular biological mechanisms, including colonization, osteolytic destruction, and an immunosuppressive bone microenvironment. Agents inhibiting bone metastasis (such as bisphosphate and denosumab) alleviate bone destruction and improve the quality of life of breast cancer patients with bone metastasis. However, the prognosis of these patients remains poor, and the specific biological mechanism of bone metastasis is incompletely understood. Additional basic and clinical studies are urgently needed, to further explore the mechanism of bone metastasis and develop new therapeutic drugs. This review presents a summary of the molecular mechanisms and therapeutic strategies of bone metastasis of breast cancer, aiming to improve the quality of life and prognosis of breast cancer patients and provide a reference for future research directions.

Biological and Clinical Aspects of Metastatic Spinal Tumors

Simple Summary

Spine metastases are a common life-threatening complication of advanced-stage malignancies and often result in poor prognosis. Symptomatic spine metastases develop in the course of about 10% of malignant neoplasms. Therefore, it is essential for contemporary medicine to understand metastatic processes in order to find appropriate, targeted therapeutic options. Our literature review aimed to describe the up-to-date knowledge about the molecular pathways and biomarkers engaged in the spine’s metastatic processes. Moreover, we described current data regarding bone-targeted treatment, the emerging targeted therapies, radiotherapy, and immunotherapy used for the treatment of spine metastases. We hope that knowledge comprehensively presented in our review will contribute to the development of novel drugs targeting specific biomarkers and pathways. The more we learn about the molecular aspects of cancer metastasis, the easier it will be to look for treatment methods that will allow us to precisely kill tumor cells.

Abstract

Spine metastases are a common life-threatening complication of advanced-stage malignancies and often result in poor prognosis. Symptomatic spine metastases develop in the course of about 10% of malignant neoplasms. Therefore, it is essential for contemporary medicine to understand metastatic processes in order to find appropriate, targeted therapeutic options. Thanks to continuous research, there appears more and more detailed knowledge about cancer and metastasis, but these transformations are extremely complicated, e.g., due to the complexity of reactions, the variety of places where they occur, or the participation of both tumor cells and host cells in these transitions. The right target points in tumor metastasis mechanisms are still being researched; that will help us in the proper diagnosis as well as in finding the right treatment. In this literature review, we described the current knowledge about the molecular pathways and biomarkers engaged in metastatic processes involving the spine. We also presented a current bone-targeted treatment for spine metastases and the emerging therapies targeting the discussed molecular mechanisms.

Bone Health Management in the Continuum of Prostate Cancer Disease

Prostate cancer (PCa) is the second-leading cause of cancer-related deaths in men. PCa cells require androgen receptor (AR) signaling for their growth and survival. Androgen deprivation therapy (ADT) is the preferred treatment for patients with locally advanced and metastatic PCa disease. Despite their initial response to androgen blockade, most patients eventually will develop metastatic castration-resistant prostate cancer (mCRPC). Bone metastases are common in men with mCRPC, occurring in 30% of patients within 2 years of castration resistance and in >90% of patients over the course of the disease. Patients with mCRPC-induced bone metastasis develop lesions throughout their skeleton; the 5-year survival rate for these patients is 47%. Bone-metastasis-induced early changes in the bone that proceed the osteoblastic response in the bone matrix are monitored and detected via modern magnetic resonance and PET/CT imaging technologies. Various treatment options, such as targeting osteolytic metastasis with bisphosphonates, prednisone, dexamethasone, denosumab, immunotherapy, external beam radiation therapy, radiopharmaceuticals, surgery, and pain medications are employed to treat prostate-cancer-induced bone metastasis and manage bone health. However, these diagnostics and treatment options are not very accurate nor efficient enough to treat bone metastases and manage bone health. In this review, we present the pathogenesis of PCa-induced bone metastasis, its deleterious impacts on vital organs, the impact of metastatic PCa on bone health, treatment interventions for bone metastasis and management of bone- and skeletal-related events, and possible current and future therapeutic options for bone management in the continuum of prostate cancer disease.

RANKL biology

Since the receptor activator of nuclear factor-kappa B ligand (RANKL), its cognate receptor activator of nuclear factor-kappa B (RANK), and the decoy receptor osteoprotegerin (OPG) were discovered, a number of studies have uncovered the crucial role of the RANKL-RANK-OPG pathway in controlling the key aspect of bone homeostasis, the immune system, inflammation, cancer, and other systems under pathophysiological condition. These findings have expanded the understanding of the multifunctional biology of the RANKL-RANK-OPG pathway and led to the development of therapeutic potential targeting this pathway. The successful development and application of anti-RANKL antibody in treating diseases causing bone loss validates the utility of therapeutic approaches based on the modulation of this pathway. Moreover, recent studies have demonstrated the involvement of the RANKL-RANK pathway in osteoblast differentiation and bone formation, shedding light on the RANKL-RANK dual signaling in coupling bone resorption and bone formation. In this review, we will summarize the current understanding of the RANKL-RANK-OPG system in the context of the bone and the immune system as well as the impact of this pathway in disease conditions, including cancer development and metastasis.

Uncarboxylated osteocalcin promotes proliferation and metastasis of MDA-MB-231 cells through TGF-β/SMAD3 signaling pathway

BACKGROUND

Triple-negative breast cancer (TNBC) is the most severe type of breast cancer owing to its high heterogeneity, aggressiveness and lack of treatment. Studies have reported that uncarboxylated osteocalcin (GluOC) promotes the development of prostate and other cancers. Studies have also found elevated levels of serum osteocalcin in breast cancer patients with bone metastasis, and serum osteocalcin can be a marker of bone metastasis. However, whether GluOC promotes the development of TNBC and the related mechanisms need to be further clarified.

RESULTS

Our results revealed that GluOC is associated with the proliferation and metastasis of MDA-MB-231 cells. GluOC increased the viability and proliferation of MDA-MB-231 cells. In addition, GluOC enhanced the metastatic ability of MDA-MB-231 cells by promoting the expression of matrix metalloproteinase-2 (MMP2), matrix metalloproteinase-13 (MMP13), and vascular endothelial growth factor (VEGF) and inducing epithelial-mesenchymal transition (EMT). We also found that GluOC upregulated the expression of interleukin-8 (IL-8) and parathyroid hormone-related protein (PTHrP) genes in MDA-MB-231 breast cancer cells. Moreover, the promoting effect of GluOC was reversed in MDA-MB-231 breast cancer cells treated with specific inhibitor of SMAD3 (SIS3), a SMAD3 phosphorylation inhibitor.

CONCLUSION

Our research proved for the first time that GluOC facilitates the proliferation and metastasis of MDA-MB-231 cells by accelerating the transforming growth factor-β (TGF-β)/SMAD3 signaling pathway. Moreover, GluOC also promotes the gene expression of IL-8 and PTHrP. Both IL-8 and PTHrP can act as osteolytic factors in breast cancer cells. This study indicates that GluOC may be a useful target for preventing TNBC bone metastasis.

Targeting bone microenvironments for treatment and early detection of cancer bone metastatic niches

Bone tissues are the main metastatic sites of many cancers, and bone metastasis is an important cause of death. When bone metastasis occurs, dynamic interactions between tumor cells and bone tissues promote changes in the tumor-bone microenvironments that are conducive to tumor growth and progression, which also promote several related diseases, including pathological fracture, bone pain, and hypercalcemia. Accordingly, it has obvious clinical benefits for improving the cure rate and reducing the occurrence of related diseases through targeting bone microenvironments for the treatment and early detection of cancer bone metastasis niches. In this review, we briefly analyzed the relationship between bone microstructures and tumor metastasis, as well as microenvironmental changes in osteoblasts, osteoclasts, immune cells, and extracellular and bone matrixes caused when metastatic tumor cells colonize bones. We also discuss novel designs in nanodrugs for inhibiting tumor proliferation and migration through targeting to tumor bone metastases and abnormal bone-microenvironment components. In addition, related researches on the early detection of bone and multi-organ metastases by nanoprobes are also introduced. And we look forward to provide some useful proposals and enlightenments on nanotechnology-based drug delivery and probes for the treatment and early detection of bone metastasis.

Mechanisms of Bone Fragility: From Osteogenesis Imperfecta to Secondary Osteoporosis

Bone material strength is determined by several factors, such as bone mass, matrix composition, mineralization, architecture and shape. From a clinical perspective, bone fragility is classified as primary (i.e., genetic and rare) or secondary (i.e., acquired and common) osteoporosis. Understanding the mechanism of rare genetic bone fragility disorders not only advances medical knowledge on rare diseases, it may open doors for drug development for more common disorders (i.e., postmenopausal osteoporosis). In this review, we highlight the main disease mechanisms underlying the development of human bone fragility associated with low bone mass known to date. The pathways we focus on are type I collagen processing, WNT-signaling, TGF-ß signaling, the RANKL-RANK system and the osteocyte mechanosensing pathway. We demonstrate how the discovery of most of these pathways has led to targeted, pathway-specific treatments.

Extracellular Vesicles in Tumors: A Potential Mediator of Bone Metastasis

As one of the most common metastatic sites, bone has a unique microenvironment for the growth and prosperity of metastatic tumor cells. Bone metastasis is a common complication for tumor patients and accounts for 15-20% of systemic metastasis, which is only secondary to lung and liver metastasis. Cancers prone to bone metastasis include lung, breast, and prostate cancer. Extracellular vesicles (EVs) are lipid membrane vesicles released from different cell types. It is clear that EVs are associated with multiple biological phenomena and are crucial for intracellular communication by transporting intracellular substances. Recent studies have implicated EVs in the development of cancer. However, the potential roles of EVs in the pathological exchange of bone cells between tumors and the bone microenvironment remain an emerging area. This review is focused on the role of tumor-derived EVs in bone metastasis and possible regulatory mechanisms.

Integration of Molecular Docking and In Vitro Studies: A Powerful Approach for Drug Discovery in Breast Cancer

Molecular docking in the pharmaceutical industry is a powerful in silico approach for discovering novel therapies for unmet medical needs predicting drug–target interactions. It not only provides binding affinity between drugs and targets at the atomic level, but also elucidates the fundamental pharmacological properties of specific drugs. The purpose of this review was to illustrate newer and emergent uses of docking when combined with in vitro techniques for drug discovery in metastatic breast cancer. We grouped the selected articles into five main categories; namely, systematic repositioning of drugs, natural drugs, new synthesized molecules, combinations of drugs, and drug latentiation. We focused on new promising drugs that have a good affinity with their targets, thus inducing a favorable biological response. This review suggests that the integration of molecular docking and in vitro studies can accelerate cancer drug discovery showing a good consistency of the results between the two approaches.

Targeting the RANKL/RANK/OPG Axis for Cancer Therapy

RANKL and RANK are expressed in different cell types and tissues throughout the body. They were originally described for their essential roles in bone remodeling and the immune system but have subsequently been shown to provide essential signals from regulating mammary gland homeostasis during pregnancy to modulating tumorigenesis. The success of RANKL/RANK research serves as a paragon for translational research from the laboratory to the bedside. The case in point has been the development of Denosumab, a RANKL-blocking monoclonal antibody which has already helped millions of patients suffering from post-menopausal osteoporosis and skeletal related events in cancer. Here we will provide an overview of the pathway from its origins to its clinical relevance in disease, with a special focus on emerging evidence demonstrating the therapeutic value of targeting the RANKL/RANK/OPG axis not only in breast cancer but also as an addition to the cancer immunotherapy arsenal.

PTH and PTHrP Actions on Bone

Parathyroid hormone (PTH), PTH-related peptide (PTHrP), PTHR, and their cognate G protein-coupled receptor play defining roles in the regulation of extracellular calcium and phosphate metabolism and in controlling skeletal growth and repair. Acting through complex signaling mechanisms that in many instances proceed in a tissue-specific manner, precise control of these processes is achieved. A variety of direct and indirect disease processes, along with genetic anomalies, can cause these schemes to become dysfunctional. Here, we review the basic components of this regulatory network and present both the well-established elements and emerging findings and concepts with the overall objective to provide a framework for understanding the elementary aspects of how PTH and PTHrP behave and as a call to encourage further investigation that will yield more comprehensive understanding of the physiological and pathological steps at play, with a goal toward novel therapeutic interventions.

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.

Chitosan-Collagen 3D Matrix Mimics Trabecular Bone and Regulates RANKL-Mediated Paracrine Cues of Differentiated Osteoblast and Mesenchymal Stem Cells for Bone Marrow Macrophage-Derived Osteoclastogenesis

Recent studies have identified the regulatory mechanism of collagen in bone ossification and resorption. Due to its excellent bio-mimicry property, collagen is used for the treatment of several bone and joint disease such as arthritis, osteoporosis, and osteopenia. In bone, the biological action of collagen is highly influenced by the interactions of other bone materials such as glycosaminoglycan and minerals. In view of the above perceptions, collagen was crosslinked with chitosan, hydroxyapatite (H), and chondroitin sulfate (Cs), to produce a natural bone-like 3D structure and to evaluate its effect on bone homeostasis using bone marrow mesenchymal stem cells, osteoblast, and bone marrow macrophages. The XRD and micro-CT data confirmed the arrangement of H crystallites in the chitosan-collagen-H-Cs (CCHCs) three-dimensional (3D)-matrix and the three-dimensional structure of the matrix. The stimulatory osteoblastogenic and exploitive osteoclastogenic activity of 3D-matrices were identified using differentiated osteoblasts and osteoclasts, respectively. Besides, osteogenic progenitor’s paracrine cues for osteoclastogenesis showed that the differentiated osteoblast secreted higher levels of RANKL to support osteoclastogenesis, and the effect was downregulated by the CCHCs 3D-matrix. From that, it was hypothesized that the morphology of the CCHCs 3D-matrix resembles trabecular bone, which enhances bone growth, limits bone resorption, and could be a novel biomaterial for bone tissue engineering.

Hallmarks of Bone Metastasis

Breast cancer bone metastasis develops as the result of a series of complex interactions between tumor cells, bone marrow cells, and resident bone cells. The net effect of these interactions are the disruption of normal bone homeostasis, often with significantly increased osteoclast and osteoblast activity, which has provided a rational target for controlling tumor progression, with little or no emphasis on tumor eradication. Indeed, the clinical course of metastatic breast cancer is relatively long, with patients likely to experience sequential skeletal-related events (SREs), often over lengthy periods of time, even up to decades. These SREs include bone pain, fractures, and spinal cord compression, all of which may profoundly impair a patient's quality-of-life. Our understanding of the contributions of the host bone and bone marrow cells to the control of tumor progression has grown over the years, yet the focus of virtually all available treatments remains on the control of resident bone cells, primarily osteoclasts. In this perspective, our focus is to move away from the current emphasis on the control of bone cells and focus our attention on the hallmarks of bone metastatic tumor cells and how these differ from primary tumor cells and normal host cells. In our opinion, there remains a largely unmet medical need to develop and utilize therapies that impede metastatic tumor cells while sparing normal host bone and bone marrow cells. This perspective examines the impact of metastatic tumor cells on the bone microenvironment and proposes potential new directions for uncovering the important mechanisms driving metastatic progression in bone based on the hallmarks of bone metastasis.