Endothelial-to-Osteoblast Conversion Generates Osteoblastic Metastasis of Prostate Cancer

Mechanical Loading of Osteocytes via Oscillatory Fluid Flow Regulates Early-Stage PC-3 Prostate Cancer Metastasis to Bone

Bone metastasis is a devastating complication for advanced-stage prostate cancer patients. Osteocytes, as the primary mechanosensors in bone, have been recently investigated for their role in prostate cancer bone metastasis. In vivo findings show potential benefits of exercise as a preventative intervention strategy for bone metastasis. In contrast, in vitro studies indicate direct prostate cancer-osteocyte interactions under mechanical loading promote prostate cancer growth and migration. These findings are not consistent with in vivo results and may be more reflective of late-stage metastatic colonization. Here, the role of flow-stimulated osteocytes during early-stage bone metastasis, particularly prostate cancer-endothelial interactions, is examined. Flow-stimulated osteocytes reduce PC-3 prostate cancer cell adhesion and trans-endothelial migration by 32.3% and 40% compared to static controls. Both MLO-Y4 and primary murine osteocytes under mechanical loading regulate the extravasation distance and frequency of PC-3 cells in a microfluidic tissue model. Application of vascular cellular adhesion molecule 1 (VCAM-1) neutralizing antibody abolishes the difference in cancer cell adhesion, extravasation frequency, and number of extravasated PC-3 cells between static and flow-stimulated groups. Taken together, the role of osteocytes in early-stage bone metastasis using PC-3 cells as a model is demonstrated here, bridging the gap between in vitro and in vivo findings.

The p53-miR17 family-Rankl axis bridges liver-bone communication

Our study elucidates the crucial role of the liver in bone homeostasis through the p53-miR17 family (miR17-miR20/miR20-miR106/miR93-miR106)-Rankl axis. We demonstrate the enhanced hepatocyte Rankl expression in inflammaging conditions, such as aging, ovariectomized (OVX) mice, and elderly humans. Mice with hepatocyte-specific Rankl deletion exhibit significant resistance to bone mass loss associated with aging, lipopolysaccharide (LPS)-induced inflammation, or estrogen deficiency, compared with controls. Our study highlights hepatocytes as the primary source of Rankl in the liver and serum under these conditions. We identify the p53-miR17 family axis as a crucial regulator for hepatocyte Rankl expression, with p53 inhibiting the miR17 family transcription. Through bioinformatics analysis and in vitro validation, we identify Rankl mRNA as a direct target of the miR17 family. Targeting this axis via CasRx-mediated mRNA editing or miRNA interference significantly attenuates bone mass loss in mice. Our investigation underscores the pivotal significance and therapeutic potential of modulating the p53-miR17 family-Rankl axis in the treatment of inflammaging-associated osteoporosis.

Prostate cancer-induced endothelial-cell-to-osteoblast transition drives immunosuppression in the bone-tumor microenvironment through Wnt pathway-induced M2 macrophage polarization

Immune checkpoint therapy has limited efficacy for patients with bone-metastatic castration-resistant prostate cancer (bmCRPC). To improve immunotherapy for bmCRPC, we aimed to identify the mechanism of bmCRPC-induced changes in the immune microenvironment. Among bmCRPC patients, higher levels of a 32-gene M2-like macrophage signature in bone metastasis samples correlated with shorter overall survival. Immunohistochemistry showed that CD206-positive (CD206+) macrophages were enriched in bmCRPC bone biopsy specimens compared with primary tumors or lymph node metastases. In preclinical osteogenic prostate cancer (Pca) xenograft models, CD206+ macrophages were recruited to areas with tumor-induced bone. RNA sequencing (RNAseq) analysis showed higher expression of an M2-like gene signature, with activated canonical and noncanonical Wnt pathways, in tumor-associated macrophages isolated from osteogenic tumors (bone-TAMs) than in TAMs isolated from nonosteogenic tumors (ctrl-TAMs). Mechanistic studies showed that endothelial cells (ECs) that had undergone EC-to-osteoblast (EC-to-OSB) transition, the precursors of tumor-induced OSBs, produced paracrine factors, including Wnts, CXCL14, and lysyl oxidase, which induced M2 polarization and recruited M2-like TAMs to the bone-tumor microenvironment (bone-TME). Bone-TAMs suppressed CD8+ T cells' proliferation and cytolytic activity, and these effects were partially reversed by treating bone-TAMs with Wnt inhibitors. Genetic or pharmacological inhibition of Pca-induced EC-to-OSB transition reduced the levels of M2-like macrophages in osteogenic tumors. Our study demonstrates that Pca-induced EC-to-OSB transition drives immunosuppression in the bone-TME, suggesting that therapies that reduce Pca-induced bone formation may improve immunotherapeutic outcomes for bmCRPC.

Skeletal stem and progenitor cells in bone development and repair

Bone development, growth, and repair are complex processes involving various cell types and interactions, with central roles played by skeletal stem and progenitor cells. Recent research brought new insights into the skeletal precursor populations that mediate intramembranous and endochondral bone development. Later in life, many of the cellular and molecular mechanisms determining development are reactivated upon fracture, with powerful trauma-induced signaling cues triggering a variety of postnatal skeletal stem/progenitor cells (SSPCs) residing near the bone defect. Interestingly, in this injury context, the current evidence suggests that the fates of both SSPCs and differentiated skeletal cells can be considerably flexible and dynamic, and that multiple cell sources can be activated to operate as functional progenitors generating chondrocytes and/or osteoblasts. The combined implementation of in vivo lineage tracing, cell surface marker-based cell selection, single-cell molecular analyses, and high-resolution in situ imaging has strongly improved our insights into the diversity and roles of developmental and reparative stem/progenitor subsets, while also unveiling the complexity of their dynamics, hierarchies, and relationships. Albeit incompletely understood at present, findings supporting lineage flexibility and possibly plasticity among sources of osteogenic cells challenge the classical dogma of a single primitive, self-renewing, multipotent stem cell driving bone tissue formation and regeneration from the apex of a hierarchical and strictly unidirectional differentiation tree. We here review the state of the field and the newest discoveries in the origin, identity, and fates of skeletal progenitor cells during bone development and growth, discuss the contributions of adult SSPC populations to fracture repair, and reflect on the dynamism and relationships among skeletal precursors and differentiated cell lineages. Further research directed at unraveling the heterogeneity and capacities of SSPCs, as well as the regulatory cues determining their fate and functioning, will offer vital new options for clinical translation toward compromised fracture healing and bone regenerative medicine.

Radium-223 Treatment Produces Prolonged Suppression of Resident Osteoblasts and Decreased Bone Mineral Density in Trabecular Bone in Osteoblast Reporter Mice

Radium 223 (Ra-223) is an α-emitting bone-homing radiopharmaceutical that targets tumor-induced osteoblasts and is used to reduce bone pain and prolong overall survival in men with bone-metastatic, castrate-resistant prostate cancer. However, increased fracture risk in skeletal sites with no bone metastasis has been observed in patients treated with Ra-223. Both luciferase- or green fluorescence protein (GFP)-labeled osteoblast reporter mice were used to monitor the effect of Ra-223 on resident osteoblasts and normal bone structure. Upon Ra-223 treatment, 70% of resident osteoblasts were reduced within 2 days, and the osteoblast reduction lasted for at least 18 weeks without detectable recovery, as measured by in vivo bioluminescent imaging. In GFP-labeled osteoblast reporter mice, Ra-223 mainly reduced osteoblasts localized in the trabecular bone areas; the osteoblasts in the growth plates were less affected. Micro-computed tomography analyses showed that Ra-223 significantly reduced bone mineral density and bone microstructure in the trabecular area of femurs but not in the cortical bone. Tumor-induced bone was generated by inoculating osteogenic TRAMP-BMP4 prostate cancer cells into the mouse femurs; Ra-223 treatment significantly reduced tumor-induced osteoblasts. Our study shows that Ra-223 affects bone structures that are not involved in bone metastasis. Strategies that improve bone health may reduce fracture risk in patients receiving Ra-223.

Deciphering the impact of aging on splenic endothelial cell heterogeneity and immunosenescence through single-cell RNA sequencing analysis

BACKGROUND

Aging is associated with significant structural and functional changes in the spleen, leading to immunosenescence, yet the detailed effects on splenic vascular endothelial cells (ECs) and their immunomodulatory roles are not fully understood. In this study, a single-cell RNA (scRNA) atlas of EC transcriptomes from young and aged mouse spleens was constructed to reveal age-related molecular changes, including increased inflammation and reduced vascular development and also the potential interaction between splenic endothelial cells and immune cells.

RESULTS

Ten clusters of splenic endothelial cells were identified. DEGs analysis across different EC clusters revealed the molecular changes with aging, showing the increase in the overall inflammatory microenvironment and the loss in vascular development function of aged ECs. Notably, four EC clusters with immunological functions were identified, suggesting an Endothelial-to-Immune-like Cell Transition (EndICLT) potentially driven by aging. Pseudotime analysis of the Immunology4 cluster further indicated a possible aging-induced transitional state, potentially initiated by Ctss gene activation. Finally, the effects of aging on cell signaling communication between different EC clusters and immune cells were analyzed.

CONCLUSIONS

This comprehensive atlas elucidates the complex interplay between ECs and immune cells in the aging spleen, offering new insights into endothelial heterogeneity, reprogramming, and the mechanisms of immunosenescence.

Mouse Models to Evaluate the Functional Role of the Tumor Microenvironment in Cancer Progression and Therapy Responses

The tumor microenvironment (TME) is a complex ecosystem of both cellular and noncellular components that functions to impact the evolution of cancer. Various aspects of the TME have been targeted for the control of cancer; however, TME composition is dynamic, with the overall abundance of immune cells, endothelial cells (ECs), fibroblasts, and extracellular matrix (ECM) as well as subsets of TME components changing at different stages of progression and in response to therapy. To effectively treat cancer, an understanding of the functional role of the TME is needed. Genetically engineered mouse models have enabled comprehensive insight into the complex interactions within the TME ecosystem that regulate disease progression. Here, we review recent advances in mouse models that have been employed to understand how the TME regulates cancer initiation, progression, metastasis, and response to therapy.

Molecular landscape of prostate cancer bone metastasis

Prostate cancer (PC) has a high propensity to develop bone metastases, causing severe pain and pathological fractures that profoundly impact a patients' normal functions. Current clinical intervention is mainly palliative focused on pain management, and tumor progression is refractory to standard therapeutic regimens. This limited treatment efficacy is at least partially due to a lack of comprehensive understanding of the molecular landscape of the disease pathology, along with the intensive overlapping of physiological and pathological molecular signaling. The niche is overwhelmed with diverse cell types with inter- and intra-heterogeneity, along with growth factor-enriched cells that are supportive of invading cell proliferation, providing an additional layer of complexity. This review seeks to provide molecular insights into mechanisms underlying PC bone metastasis development and progression.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

Prognostic analysis of percutaneous vertebroplasty (PVP) combined with 125I implantation on lumbosacral vertebral osteoblastic metastases

OBJECTIVE

Lumbosacral vertebral osteoblastic metastasis is treated with percutaneous vertebroplasty (PVP) combined with 125I seed implantation and PVP alone. Compared to PVP alone, we evaluated the effects of combination therapy with PVP and 125I seed implantation on pain, physical condition, and survival and evaluated the clinical value of PVP combined with 125I particle implantation.

METHODS

We retrospectively analyzed 62 patients with lumbosacral vertebral osseous metastases treated at our hospital between 2016 and 2019. All the patients met the inclusion criteria for 125I implantation, and they were randomly divided into a combined treatment group and a pure PVP surgery group. The visual analog pain scale (VAS), Karnofsky Performance Status (KPS), and survival time were recorded at different time points, including preoperative, postoperative 1 day, 1 month, 3 months, 6 months, 12 months, and 36 months in each group. The variation in clinical indicators and differences between the groups were analyzed using SPSS version 20.0. Correlations between different variables were analyzed using the nonparametric Spearman's rank test. The Kaplan-Meier method was used to estimate the relationship between survival time and KPS score, VAS score, or primary tumor progression, and survival differences were analyzed using the log-rank test. Multivariate analyses were performed using a stepwise Cox proportional hazards model to identify independent prognostic factors.

RESULTS

Compared to the PVP treatment group, the pain level in the combined treatment group was significantly reduced (P = 0.000), and the patient's physical condition in the combination treatment group significantly improved. Kaplan-Meier analysis showed that the survival rate of the PVP group was significantly lower than that of the combination group (P = 0.038). We also found that the median survival of patients in both groups significantly increased with an increase in the KPS score (14 months vs. 33 months) (P = 0.020). Patients with more than three transfer sections had significantly lower survival rates than those with one or two segments of the section (P = 0.001). Further, Cox regression analysis showed that age (P = 0.002), the spinal segment for spinal metastasis (P = 0.000), and primary tumor growth rate (P = 0.005) were independent factors that affected the long-term survival of patients with lumbosacral vertebral osseous metastases.

CONCLUSIONS

PVP combined 125I seeds implantation surgery demonstrated superior effectiveness compared to PVP surgery alone in treating lumbosacral vertebral osseous metastases, which had feasibility in the clinical operation. Preoperative KPS score, spine transfer section, and primary tumor growth rate were closely related to the survival of patients with lumbosacral vertebral osteoblastic metastasis. Age, spinal segment for spinal metastasis, and primary tumor growth can serve as prognostic indicators and guide clinical treatment.

Essential role of MFSD1-GLMP-GIMAP5 in lymphocyte survival and liver homeostasis

We detected ENU-induced alleles of Mfsd1 (encoding the major facilitator superfamily domain containing 1 protein) that caused lymphopenia, splenomegaly, progressive liver pathology, and extramedullary hematopoiesis (EMH). MFSD1 is a lysosomal membrane-bound solute carrier protein with no previously described function in immunity. By proteomic analysis, we identified association between MFSD1 and both GLMP (glycosylated lysosomal membrane protein) and GIMAP5 (GTPase of immunity-associated protein 5). Germline knockout alleles of Mfsd1, Glmp, and Gimap5 each caused lymphopenia, liver pathology, EMH, and lipid deposition in the bone marrow and liver. We found that the interactions of MFSD1 and GLMP with GIMAP5 are essential to maintain normal GIMAP5 expression, which in turn is critical to support lymphocyte development and liver homeostasis that suppresses EMH. These findings identify the protein complex MFSD1-GLMP-GIMAP5 operating in hematopoietic and extrahematopoietic tissues to regulate immunity and liver homeostasis.

Prostate cancer-induced endothelial-to-osteoblast transition generates an immunosuppressive bone tumor microenvironment

Immune checkpoint therapy has limited efficacy for patients with bone metastatic castrate-resistant prostate cancer (bmCRPC). In this study, we revealed a novel mechanism that may account for the relative resistance of bmCRPC to immune checkpoint therapy. We found that prostate cancer (PCa)-induced bone via endothelial-to-osteoblast (EC-to-OSB) transition causes an ingress of M2-like macrophages, leading to an immunosuppressive bone tumor microenvironment (bone-TME). Analysis of a bmCRPC RNA-seq dataset revealed shorter overall survival in patients with an M2-high versus M2-low signature. Immunohistochemical (IHC) analysis showed CD206 + M2-like macrophages were enriched in bmCRPC specimens compared with primary tumors or lymph node metastasis. In osteogenic PCa xenografts, CD206 + macrophages were enriched adjacent to tumor-induced bone. FACS analysis showed an increase in CD206 + cells in osteogenic tumors compared to non-osteogenic tumors. Genetic or pharmacological inhibition of the EC-to-OSB transition reduced aberrant bone and M2-like macrophages in osteogenic tumors. RNAseq analysis of tumor-associated macrophages from osteogenic (bone-TAMs) versus non-osteogenic (ctrl-TAMs) tumors showed high expression of an M2-like gene signature, canonical and non-canonical Wnt pathways, and a decrease in an M1-like gene signature. Isolated bone-TAMs suppressed T-cell proliferation while ctrl-TAMs did not. Mechanistically, EC-OSB hybrid cells produced paracrine factors, including Wnts, CXCL14 and LOX, which induced M2 polarization and recruited M2-like TAMs to bone-TME. Our study thus links the unique EC-to-OSB transition as an "upstream" event that drives "downstream" immunosuppression in the bone-TME. These studies suggest that therapeutic strategies that inhibit PCa-induced EC-to-OSB transition may reverse immunosuppression to promote immunotherapeutic outcomes in bmCRPC.

Significance

The insight that prostate cancer-induced bone generates an immunosuppressive bone tumor microenvironment offers a strategy to improve responses to immunotherapy approaches in patients with bone metastatic castrate-resistant prostate cancer.

Crosstalk between Endothelial Cells and Tumor Cells: A New Era in Prostate Cancer Progression

Prostate cancer stands as one of the most prevalent malignancies afflicting men worldwide. The tumor microenvironment plays a pivotal role in tumor progression, comprising various cell types including endothelial cells, tumor-associated fibroblasts, and macrophages. Recent accumulating evidence underscores the indispensable contribution of endothelial cells to prostate cancer development. Both endothelial cells and tumor cells release a multitude of factors that instigate angiogenesis, metastasis, and even drug resistance in prostate cancer. These factors serve as regulators within the tumor microenvironment and represent potential therapeutic targets for managing prostate cancer. In this review, we provide an overview of the crucial functions of endothelial cells in angiogenesis, metastasis, and drug resistance, and their prospective therapeutic applications in combating this disease.

The functional roles of m6A modification in prostate cancer

Prostate cancer (PCa) is the most prevalent malignancy of the male genitourinary system, and its etiology suggests that genetics is an essential risk factor for its development and progression, while exogenous factors may have an significant impact on this risk. Initial diagnosis of advanced PCa is relatively frequent, and androgen deprivation therapy (ADT) is the predominant standard of care for PCa and the basis for various novel combination therapy regimens, and is often required throughout the patient's subsequent treatment. Although diagnostic modalities and treatment options are evolving, some patients suffer from complications, including biochemical relapse, metastasis and treatment resistance. Mechanisms of PCa pathogenesis and progression have been the focus of research. N6-methyladenosine (m6A) is an RNA modification involved in cell physiology and tumor metabolism. It has been observed to affect the evolution of diverse cancers through the regulation of gene expression. Genes associated with m6A are prominent in PCa and are involved in multiple aspects of desmoresistant PCa occurrence, progression, PCa bone metastasis (BM), and treatment resistance. Here, we explore the role of m6A modifications in promoting PCa.

The role of prostate-specific antigen in the osteoblastic bone metastasis of prostate cancer: a literature review

Prostate cancer is the only human malignancy that generates predominantly osteoblastic bone metastases, and osteoblastic bone metastases account for more than 90% of osseous metastases of prostate cancer. Prostate-specific antigen (PSA) plays an important role in the osteoblastic bone metastasis of prostate cancer, which can promote osteomimicry of prostate cancer cells, suppress osteoclast differentiation, and facilitate osteoblast proliferation and activation at metastatic sites. In the meantime, it can activate osteogenic factors, including insulin-like growth factor, transforming growth factor β2 and urokinase-type plasminogen activator, and meanwhile suppress osteolytic factors such as parathyroid hormone-related protein. To recapitulate, PSA plays a significant role in the osteoblastic predominance of prostate cancer bone metastasis and bone remodeling by regulating multiple cells and factors involved in osseous metastasis.

Exploring the crosstalk between endothelial cells, immune cells, and immune checkpoints in the tumor microenvironment: new insights and therapeutic implications

The tumor microenvironment (TME) is a highly intricate milieu, comprising a multitude of components, including immune cells and stromal cells, that exert a profound influence on tumor initiation and progression. Within the TME, angiogenesis is predominantly orchestrated by endothelial cells (ECs), which foster the proliferation and metastasis of malignant cells. The interplay between tumor and immune cells with ECs is complex and can either bolster or hinder the immune system. Thus, a comprehensive understanding of the intricate crosstalk between ECs and immune cells is essential to advance the development of immunotherapeutic interventions. Despite recent progress, the underlying molecular mechanisms that govern the interplay between ECs and immune cells remain elusive. Nevertheless, the immunomodulatory function of ECs has emerged as a pivotal determinant of the immune response. In light of this, the study of the relationship between ECs and immune checkpoints has garnered considerable attention in the field of immunotherapy. By targeting specific molecular pathways and signaling molecules associated with ECs in the TME, novel immunotherapeutic strategies may be devised to enhance the efficacy of current treatments. In this vein, we sought to elucidate the relationship between ECs, immune cells, and immune checkpoints in the TME, with the ultimate goal of identifying novel therapeutic targets and charting new avenues for immunotherapy.

The epithelial-mesenchymal plasticity landscape: principles of design and mechanisms of regulation

Epithelial-mesenchymal plasticity (EMP) enables cells to interconvert between several states across the epithelial-mesenchymal landscape, thereby acquiring hybrid epithelial/mesenchymal phenotypic features. This plasticity is crucial for embryonic development and wound healing, but also underlies the acquisition of several malignant traits during cancer progression. Recent research using systems biology and single-cell profiling methods has provided novel insights into the main forces that shape EMP, which include the microenvironment, lineage specification and cell identity, and the genome. Additionally, key roles have emerged for hysteresis (cell memory) and cellular noise, which can drive stochastic transitions between cell states. Here, we review these forces and the distinct but interwoven layers of regulatory control that stabilize EMP states or facilitate epithelial-mesenchymal transitions (EMTs) and discuss the therapeutic potential of manipulating the EMP landscape.

Pathological angiogenesis: mechanisms and therapeutic strategies

In multicellular organisms, angiogenesis, the formation of new blood vessels from pre-existing ones, is an essential process for growth and development. Different mechanisms such as vasculogenesis, sprouting, intussusceptive, and coalescent angiogenesis, as well as vessel co-option, vasculogenic mimicry and lymphangiogenesis, underlie the formation of new vasculature. In many pathological conditions, such as cancer, atherosclerosis, arthritis, psoriasis, endometriosis, obesity and SARS-CoV-2(COVID-19), developmental angiogenic processes are recapitulated, but are often done so without the normal feedback mechanisms that regulate the ordinary spatial and temporal patterns of blood vessel formation. Thus, pathological angiogenesis presents new challenges yet new opportunities for the design of vascular-directed therapies. Here, we provide an overview of recent insights into blood vessel development and highlight novel therapeutic strategies that promote or inhibit the process of angiogenesis to stabilize, reverse, or even halt disease progression. In our review, we will also explore several additional aspects (the angiogenic switch, hypoxia, angiocrine signals, endothelial plasticity, vessel normalization, and endothelial cell anergy) that operate in parallel to canonical angiogenesis mechanisms and speculate how these processes may also be targeted with anti-angiogenic or vascular-directed therapies.

Alendronate conjugate for targeted delivery to bone-forming prostate cancer

Bone is the primary metastasis site for lethal prostate cancer, often resulting in poor prognosis, crippling pain, and diminished functioning that drastically reduce both quality of life and survivability Uniquely, prostate cancer bone metastasis induces aberrant bone overgrowth, due to an increase of osteoblasts induced by tumor-secreted bone morphogenetic protein 4 (BMP4). Conjugating drugs to substances that target the tumor-induced bone area within the metastatic tumor foci would be a promising strategy for drug delivery. To develop such a strategy, we conjugated a near infrared (NIR) fluorescent probe, the dye Cy5.5, to serve as a surrogate for drugs, with alendronate, which targets bone. Characterization, such as infrared spectroscopy, confirmed the synthesis of the Cy5.5-ALN conjugate. The maximum absorbance of free Cy5.5, which was at 675 nm, did not change upon conjugation. Alendronate targeted the bone component hydroxyapatite in a dose-dependent manner up to 2.5 μM, with a maximum of 85% of Cy5.5-ALN bound to hydroxyapatite, while free Cy5.5 alone had 6% binding. In in vitro cell binding studies, Cy5.5-ALN bound specifically with mineralized bone matrix of differentiated MC3T3-E1 cells or 2H11 endothelial cells that were induced to become osteoblasts through endothelial-to-osteoblast transition, the underlying mechanism of prostate-cancer-induced bone formation. Neither Cy5.5-ALN nor free Cy5.5 bound to undifferentiated MC3T3-E1 or 2H11 cells. Bone-targeting efficiency studies in non-tumor-bearing mice revealed accumulation over time in the spine, jaw, knees, and paws injected with Cy5.5-ALN, and quantification showed higher accumulation in femurs than in muscle at up to 28 days, while the free Cy5.5 dye was observed circulating without preferential accumulation and decreased over time. There was a linear relationship with fluorescence when the injected concentration of Cy5.5-ALN was between 0.313 and 1.25 nmol/27 g of mouse, as quantified in mouse femurs both in vivo and ex vivo. Ex vivo evaluation of bone-targeting efficiency in nude mice was 3 times higher for bone-forming C4-2b-BMP4 tumors compared to non-bone-forming C4-2b tumors (p-value <0.001). Fluorescence microscopy imaging of the tumors showed that Cy5.5-ALN co-localized with the bone matrix surrounding tumor-induced bone, but not with the viable tumor cells. Together, these results suggest that a drug-ALN conjugate is a promising approach for targeted delivery of drug to the tumor-induced bone area in the metastatic foci of prostate cancer.

Endothelial-to-osteoblast transition in normal mouse bone development

Metastatic prostate cancer (PCa) in bone induces bone-forming lesions. We have previously shown that PCa-induced bone originates from endothelial cells (ECs) that have undergone EC-to-osteoblast (OSB) transition. Here, we investigated whether EC-to-OSB transition also occurs during normal bone formation. We developed an EC and OSB dual-color reporter mouse (DRM) model that marks EC-OSB hybrid cells with red and green fluorescent proteins. We observed EC-to-OSB transition (RFP and GFP co-expression) in both endochondral and intramembranous bone formation during embryonic development and in adults. Co-expression was confirmed in cells isolated from DRM. Bone marrow- and lung-derived ECs underwent transition to OSBs and mineralization in osteogenic medium. RNA-sequencing revealed GATA family transcription factors were upregulated in EC-OSB hybrid cells and knockdown of GATA3 inhibited BMP4-induced mineralization. Our findings support that EC-to-OSB transition occurs during normal bone development and suggest a new paradigm regarding the endothelial origin of OSBs.

Bone Metastasis Initiation Is Coupled with Bone Remodeling through Osteogenic Differentiation of NG2+ Cells

The bone microenvironment is dynamic and undergoes remodeling in normal and pathologic conditions. Whether such remodeling affects disseminated tumor cells (DTC) and bone metastasis remains poorly understood. Here, we demonstrated that pathologic fractures increase metastatic colonization around the injury. NG2+ cells are a common participant in bone metastasis initiation and bone remodeling in both homeostatic and fractured conditions. NG2+ bone mesenchymal stem/stromal cells (BMSC) often colocalize with DTCs in the perivascular niche. Both DTCs and NG2+ BMSCs are recruited to remodeling sites. Ablation of NG2+ lineage impaired bone remodeling and concurrently diminished metastatic colonization. In cocultures, NG2+ BMSCs, especially when undergoing osteodifferentiation, enhanced cancer cell proliferation and migration. Knockout of N-cadherin in NG2+ cells abolished these effects in vitro and phenocopied NG2+ lineage depletion in vivo. These findings uncover dual roles of NG2+ cells in metastasis and remodeling and indicate that osteodifferentiation of BMSCs promotes metastasis initiation via N-cadherin-mediated cell-cell interaction.

SIGNIFICANCE

The bone colonization of cancer cells occurs in an environment that undergoes constant remodeling. Our study provides mechanistic insights into how bone homeostasis and pathologic repair lead to the outgrowth of disseminated cancer cells, thereby opening new directions for further etiologic and epidemiologic studies of tumor recurrences. This article is highlighted in the In This Issue feature, p. 247.

The road of NSCLC stem cells toward bone metastases

Despite advancement in therapeutic options, Non-Small Cell lung cancer (NSCLC) remains a lethal disease mostly due to late diagnosis at metastatic phase and drug resistance. Bone is one of the more frequent sites for NSCLC metastatization. A defined subset of cancer stem cells (CSCs) that possess motile properties, mesenchymal features and tumor initiation potential are defined as metastasis initiating cells (MICs). A better understanding of the mechanisms supporting MIC dissemination and interaction with bone microenvironment is fundamental to design novel rational therapeutic option for long lasting efficient treatment of NSCLC. In this review we will summarize findings about bone metastatic process initiated by NSCLC MICs. We will review how MICs can reach bone and interact with its microenvironment that supports their extravasation, seeding, dormancy/proliferation. The role of different cell types inside the bone metastatic niche, such as endothelial cells, bone cells, hematopoietic stem cells and immune cells will be discussed in regards of their impact in dictating the success of metastasis establishment by MICs. Finally, novel therapeutic options to target NSCLC MIC-induced bone metastases, increasing the survival of patients, will be presented.

Retinoic Acid Receptor Activation Reduces Metastatic Prostate Cancer Bone Lesions by Blocking the Endothelial-to-Osteoblast Transition

Metastatic prostate cancer in the bone induces bone-forming lesions that contribute to progression and therapy resistance. Prostate cancer-induced bone formation originates from endothelial cells (EC) that have undergone endothelial-to-osteoblast (EC-to-OSB) transition in response to tumor-secreted BMP4. Current strategies targeting prostate cancer-induced bone formation are lacking. Here, we show that activation of retinoic acid receptor (RAR) inhibits EC-to-OSB transition and reduces prostate cancer-induced bone formation. Treatment with palovarotene, an RARγ agonist being tested for heterotopic ossification in fibrodysplasia ossificans progressiva, inhibited EC-to-OSB transition and osteoblast mineralization in vitro and decreased tumor-induced bone formation and tumor growth in several osteogenic prostate cancer models, and similar effects were observed with the pan-RAR agonist all-trans-retinoic acid (ATRA). Knockdown of RARα, β, or γ isoforms in ECs blocked BMP4-induced EC-to-OSB transition and osteoblast mineralization, indicating a role for all three isoforms in prostate cancer-induced bone formation. Furthermore, treatment with palovarotene or ATRA reduced plasma Tenascin C, a factor secreted from EC-OSB cells, which may be used to monitor treatment response. Mechanistically, BMP4-activated pSmad1 formed a complex with RAR in the nucleus of ECs to activate EC-to-OSB transition. RAR activation by palovarotene or ATRA caused pSmad1 degradation by recruiting the E3-ubiquitin ligase Smad ubiquitination regulatory factor1 (Smurf1) to the nuclear pSmad1/RARγ complex, thus blocking EC-to-OSB transition. Collectively, these findings suggest that palovarotene can be repurposed to target prostate cancer-induced bone formation to improve clinical outcomes for patients with bone metastasis.

SIGNIFICANCE

This study provides mechanistic insights into how RAR agonists suppress prostate cancer-induced bone formation and offers a rationale for developing RAR agonists for prostate cancer bone metastasis therapy. See related commentary by Bhowmick and Bhowmick, p. 2975.

Bone Marrow Endothelial Cells Increase Prostate Cancer Cell Apoptosis in 3D Triculture Model of Reactive Stroma

Simple Summary

Prostate cancer (PCa) metastasizes preferentially to the bone marrow where it becomes difficult to treat. PCa cells in the bone marrow may survive, dormant and undetected for many years before patients eventually relapse with metastatic disease. Bone marrow is a complex tissue that initially is hostile to the PCa cells, Understanding how cancer cells survive in the bone marrow and what changes to the bone microenvironment permit them to switch to an actively growing state could offer new therapeutic strategies to combat metastatic PCa. In this study, we describe a method to culture PCa cells with two other cell types from the bone marrow, stromal cells and endothelial cells, as a way to study the interactions among these cell types. We found that factors produced by bone marrow endothelial cells, but not endothelial cells from other tissues, trigger PCa cells to either die or enter a dormant state, similar to what has been observed in patients when PCa cells initially colonize the bone marrow. Further analysis of the cell interactions within the culture model described in this study will offer increased understanding of PCa interaction with the bone marrow environment.

Abstract

The bone marrow tumor microenvironment (BMTE) is a complex network of cells, extracellular matrix, and sequestered signaling factors that initially act as a hostile environment for disseminating tumor cells (DTCs) from the cancerous prostate. Three-dimensional (3D) culture systems offer an opportunity to better model these complex interactions in reactive stroma, providing contextual behaviors for cancer cells, stromal cells, and endothelial cells. Using a new system designed for the triculture of osteoblastic prostate cancer (PCa) cells, stromal cells, and microvascular endothelial cells, we uncovered a context-specific pro-apoptotic effect of endothelial cells of the bone marrow different from those derived from the lung or dermis. The paracrine nature of this effect was demonstrated by observations that conditioned medium from bone marrow endothelial cells, but not from dermal or lung endothelial cells, led to PCa cell death in microtumors grown in 3D BMTE-simulating hydrogels. Analysis of the phosphoproteome by reverse phase protein analysis (RPPA) of PCa cells treated with conditioned media from different endothelial cells identified the differential regulation of pathways involved in proliferation, cell cycle regulation, and apoptosis. The findings from the RPPA were validated by western blotting for representative signaling factors identified, including forkhead box M1 (FOXM1; proliferation factor), pRb (cell cycle regulator), and Smac/DIABLO (pro-apoptosis) among treatment conditions. The 3D model presented here thus presents an accurate model to study the influence of the reactive BMTE, including stromal and endothelial cells, on the adaptive behaviors of cancer cells modeling DTCs at sites of bone metastasis. These findings in 3D culture systems can lead to a better understanding of the real-time interactions among cells present in reactive stroma than is possible using animal models.

Cellular plasticity in bone metastasis

Metastasis is responsible for a large majority of death from malignant solid tumors. Bone is one of the most frequently affected organs in cancer metastasis, especially in breast and prostate cancer. Development of bone metastasis requires cancer cells to successfully complete a number of challenging steps, including local invasion and intravasation, survival in circulation, extravasation and initial seeding, and finally, formation of metastatic colonies after a period of dormancy or indolent growth. During this process, cancer cells often undergo a series of cellular and molecular changes to gain cellular plasticity that helps them adapt to various environments they encounter along the journey of metastasis. Understanding the mechanisms behind cellular plasticity and adaptation during the formation of bone metastasis is crucial for the development of novel therapies.

Fundamentals of bone vasculature: Specialization, interactions and functions

Angiogenesis, hematopoiesis and osteogenesis are fundamental processes mediating complex and essential biological functions. In the bone marrow, endothelial cells (ECs) are a principal mediator of regulatory signals that govern hematopoietic and mesenchymal stem cells. EC and osteoblast interactions and niche functions of ECs are fundamental in maintaining bone health and coordinating repair and regeneration following injury. These cellular interactions are subject to dysregulation and deterioration under stress, aging, chronic disease states and malignancy. Thus, the prospect of manipulating the bone vasculature has tremendous potential to advance therapeutic interventions for the management of bone diseases. This review discusses the current state of vascular-skeletal tissue interactions focusing on osteoblast and hematopoietic stem cells interaction with ECs.

Prostate tumor-induced stromal reprogramming generates Tenascin C that promotes prostate cancer metastasis through YAP/TAZ inhibition

Metastatic prostate cancer (PCa) in bone induces bone-forming lesions that enhance PCa progression. How tumor-induced bone formation enhances PCa progression is not known. We have previously shown that PCa-induced bone originates from endothelial cells (ECs) that have undergone endothelial-to-osteoblast (EC-to-OSB) transition by tumor-secreted bone morphogenetic protein 4 (BMP4). Here, we show that EC-to-OSB transition leads to changes in the tumor microenvironment that increases the metastatic potential of PCa cells. We found that conditioned medium (CM) from EC-OSB hybrid cells increases the migration, invasion, and survival of PC3-mm2 and C4-2B4 PCa cells. Quantitative mass spectrometry (Isobaric Tags for Relative and Absolute Quantitation) identified Tenascin C (TNC) as one of the major proteins secreted from EC-OSB hybrid cells. TNC expression in tumor-induced OSBs was confirmed by immunohistochemistry of MDA PCa-118b xenograft and human bone metastasis specimens. Mechanistically, BMP4 increases TNC expression in EC-OSB cells through the Smad1-Notch/Hey1 pathway. How TNC promotes PCa metastasis was next interrogated by in vitro and in vivo studies. In vitro studies showed that a TNC-neutralizing antibody inhibits EC-OSB-CM-mediated PCa cell migration and survival. TNC knockdown decreased, while the addition of recombinant TNC or TNC overexpression increased migration and anchorage-independent growth of PC3 or C4-2b cells. When injected orthotopically, PC3-mm2-shTNC clones decreased metastasis to bone, while C4-2b-TNC-overexpressing cells increased metastasis to lymph nodes. TNC enhances PCa cell migration through α5β1 integrin-mediated YAP/TAZ inhibition. These studies elucidate that tumor-induced stromal reprogramming generates TNC that enhances PCa metastasis and suggest that TNC may be a target for PCa therapy.

Evolving cancer-niche interactions and therapeutic targets during bone metastasis

Many cancer types metastasize to bone. This propensity may be a product of genetic traits of the primary tumour in some cancers. Upon arrival, cancer cells establish interactions with various bone-resident cells during the process of colonization. These interactions, to a large degree, dictate cancer cell fates at multiple steps of the metastatic cascade, from single cells to overt metastases. The bone microenvironment may even influence cancer cells to subsequently spread to multiple other organs. Therefore, it is imperative to spatiotemporally delineate the evolving cancer-bone crosstalk during bone colonization. In this Review, we provide a summary of the bone microenvironment and its impact on bone metastasis. On the basis of the microscopic anatomy, we tentatively define a roadmap of the journey of cancer cells through bone relative to various microenvironment components, including the potential of bone to function as a launch pad for secondary metastasis. Finally, we examine common and distinct features of bone metastasis from various cancer types. Our goal is to stimulate future studies leading to the development of a broader scope of potent therapies.

Targeting the Wnt signaling pathway for breast cancer bone metastasis therapy

Osteolytic bone destruction is found in approximately 60% of advanced breast cancer patients. With the pathogenesis of bone metastasis being unclear, traditional antiresorptive therapeutic strategies might not be ideal for treatment. The Wnt pathway is a highly organized cascade involved in multiple stages of cancer bone metastasis, and Wnt-targeted therapeutic strategies have shown promise in achieving favorable outcomes. In this review, we summarize the current progress of pharmacological Wnt modulators against breast cancer bone metastasis, discuss emerging therapeutic strategies based on Wnt pathway-related targets for bone therapy, and highlight opportunities to better harness the Wnt pathway for bone metastasis therapeutics to further reveal the implications of the Wnt pathway in bone metastasis pathology and provide new ideas for the development of Wnt-based intervention strategies against breast cancer bone metastasis.

Mediterranean Diet Food Components as Possible Adjuvant Therapies to Counteract Breast and Prostate Cancer Progression to Bone Metastasis

Bone metastasis is a serious and often lethal complication of particularly frequent carcinomas, such as breast and prostate cancers, which not only reduces survival but also worsens the patients’ quality of life. Therefore, it is important to find new and/or additional therapeutic possibilities that can counteract the colonization of bone tissue. High adherence to the Mediterranean diet (MD) is effective in the prevention of cancer and improves cancer patients’ health, thus, here, we considered its impact on bone metastasis. We highlighted some molecular events relevant for the development of a metastatic phenotype in cancer cells and the alterations of physiological bone remodeling, which occur during skeleton colonization. We then considered those natural compounds present in MD foods with a recognized role to inhibit or reverse the metastatic process both in in vivo and in vitro systems, and we reported the identified mechanisms of action. The knowledge of this bioactivity by the dietary components of the MD, together with its wide access to all people, could help not only to maintain healthy status but also to improve the quality of life of patients with bone metastases.

Multiple pathways coordinating reprogramming of endothelial cells into osteoblasts by BMP4

Cell type transition occurs during normal development and under pathological conditions. In prostate cancer bone metastasis, prostate cancer-secreted BMP4 induces endothelial cell-to-osteoblast (EC-to-OSB) transition. Such tumor-induced stromal reprogramming supports prostate cancer progression. We delineate signaling pathways mediating EC-to-OSB transition using EC lines 2H11 and SVR. We found that BMP4-activated pSmad1-Notch-Hey1 pathway inhibits EC migration and tube formation. BMP4-activated GSK3β-βcatenin-Slug pathway stimulates Osx expression. In addition, pSmad1-regulated Dlx2 converges with the Smad1 and β-catenin pathways to stimulate osteocalcin expression. By co-expressing Osx, Dlx2, Slug and Hey1, we were able to achieve EC-to-OSB transition, leading to bone matrix mineralization in the absence of BMP4. In human prostate cancer bone metastasis specimens and MDA-PCa-118b and C4-2b-BMP4 osteogenic xenografts, immunohistochemical analysis showed that β-catenin and pSmad1 are detected in activated osteoblasts rimming the tumor-induced bone. Our results elucidated the pathways and key molecules coordinating prostate cancer-induced stromal programming and provide potential targets for therapeutic intervention.

Radium-223 Treatment Increases Immune Checkpoint Expression in Extracellular Vesicles from the Metastatic Prostate Cancer Bone Microenvironment

PURPOSE

Radium-223 prolongs survival in a fraction of men with bone metastatic prostate cancer (PCa). However, there are no markers for monitoring response and resistance to Radium-223 treatment. Exosomes are mediators of intercellular communication and may reflect response of the bone microenvironment to Radium-223 treatment. We performed molecular profiling of exosomes and compared the molecular profile in patients with favorable and unfavorable overall survival.

EXPERIMENTAL DESIGN

We performed exosomal transcriptome analysis in plasma derived from our preclinical models (MDA-PCa 118b tumors, TRAMP-C2/BMP4 PCa) and from the plasma of 25 patients (paired baseline and end of treatment) treated with Radium-223. All samples were run in duplicate, and array data analyzed with fold changes +2 to -2 and P < 0.05.

RESULTS

We utilized the preclinical models to establish that genes derived from the tumor and the tumor-associated bone microenvironment (bTME) are differentially enriched in plasma exosomes upon Radium-223 treatment. The mouse transcriptome analysis revealed changes in bone-related and DNA damage repair-related pathways. Similar findings were observed in plasma-derived exosomes from patients treated with Radium-223 detected changes. In addition, exosomal transcripts detected immune-suppressors (e.g., PD-L1) that were associated with shorter survival to Radium-223. Treatment of the Myc-CaP mouse model with a combination of Radium-223 and immune checkpoint therapy (ICT) resulted in greater efficacy than monotherapy.

CONCLUSIONS

These clinical and coclinical analyses showed that RNA profiling of plasma exosomes may be used for monitoring the bTME in response to treatment and that ICT may be used to increase the efficacy of Radium-223.

Exploiting bone niches: progression of disseminated tumor cells to metastasis

Many solid cancers metastasize to the bone and bone marrow (BM). This process may occur even before the diagnosis of primary tumors, as evidenced by the discovery of disseminated tumor cells (DTCs) in patients without occult malignancies. The cellular fates and metastatic progression of DTCs are determined by complicated interactions between cancer cells and BM niches. Not surprisingly, these niches also play important roles in normal biology, including homeostasis and turnover of skeletal and hematopoiesis systems. In this Review, we summarize recent findings on functions of BM niches in bone metastasis (BoMet), particularly during the early stage of colonization. In light of the rich knowledge of hematopoiesis and osteogenesis, we highlight how DTCs may progress into overt BoMet by taking advantage of niche cells and their activities in tissue turnover, especially those related to immunomodulation and bone repair.

Application of "omics" sciences to the prediction of bone metastases from breast cancer: State of the art

Breast cancer (BC) is the most frequent malignancy and the first cause of cancer-related death in women. The majority of patients with advanced BC develop skeletal metastases which may ultimately lead to serious complications, termed skeletal-related events, that often dramatically impact on quality of life and survival. Therefore, the identification of biomarkers able to stratify BC patient risk to develop bone metastases (BM) is fundamental to define personalized diagnostic and therapeutic strategies, possibly at the earliest stages of the disease. In this regard, the advent of "omics" sciences boosted the investigation of several putative biomarkers of BC osteotropism, including deregulated genes, proteins and microRNAs. The present review revisits the current knowledge on BM development in BC and the most recent studies exploring potential BM-predicting biomarkers, based on the application of omics sciences to the study of primary breast malignancies.

Recent Advances in Nanomedicine for the Diagnosis and Treatment of Prostate Cancer Bone Metastasis

Patients with advanced prostate cancer can develop painful and debilitating bone metastases. Currently available interventions for prostate cancer bone metastases, including chemotherapy, bisphosphonates, and radiopharmaceuticals, are only palliative. They can relieve pain, reduce complications (e.g., bone fractures), and improve quality of life, but they do not significantly improve survival times. Therefore, additional strategies to enhance the diagnosis and treatment of prostate cancer bone metastases are needed. Nanotechnology is a versatile platform that has been used to increase the specificity and therapeutic efficacy of various treatments for prostate cancer bone metastases. In this review, we summarize preclinical research that utilizes nanotechnology to develop novel diagnostic imaging tools, translational models, and therapies to combat prostate cancer bone metastases.

Combinatorial effect of radium-223 and irreversible electroporation on prostate cancer bone metastasis in mice

BACKGROUND

Metastatic prostate cancer in bone is difficult to treat as the tumor cells are relatively resistant to hormonal or chemotherapies when compared to primary prostate cancer. Irreversible electroporation (IRE) is a minimally invasive ablation procedure that has potential applications in the management of prostate cancer in bone. However, a common limitation of IRE is tumor recurrence, which arises from incomplete ablation that allows remaining cancer cells to proliferate. In this study, we combined IRE with radium-223 (Ra-223), a bone-seeking radionuclide that emits short track length alpha particles and thus is associated with reduced damage to the bone marrow and evaluated the impact of the combination treatment on bone-forming prostate cancer tumors.

METHODS

The antitumor activity of IRE and Ra-223 as single agents and in combination was tested in vitro against three bone morphogenetic protein 4 (BMP4)-expressing prostate cancer cell lines (C4-2B-BMP4, Myc-CaP-BMP4, and TRAMP-C2-BMP4). Similar evaluation was performed in vivo using a bone-forming C4-2B-BMP4 tumor model in nude mice.

RESULTS

IRE and Ra-223 as monotherapy inhibited prostate cancer cell proliferation in vitro, and their combination resulted in significant reduction in cell viability compared to monotherapy. In vivo evaluation revealed that IRE with single-dose administration of Ra-233, compared to IRE alone, reduced the rate of tumor recurrence by 40% following initial apparent complete ablation and decreased the rate of proliferation of incompletely ablated tumor as quantified in Ki-67 staining (53.58 ± 16.0% for IRE vs. 20.12 ± 1.63%; for IRE plus Ra-223; p = 0.004). Histological analysis qualitatively showed the enhanced killing of tumor cells adjacent to bone by Ra-223 compared to those treated with IRE alone.

CONCLUSION

IRE in combination with Ra-223, which enhanced the destruction of cancer cells that are adjacent to bone, resulted in reduction of tumor recurrence through improved clearance of proliferative cells in the tumor region.

Bone metastasis: mechanisms, therapies, and biomarkers

Skeletal metastases are frequent complications of many cancers, causing bone complications (fractures, bone pain, disability) that negatively affect the patient's quality of life. Here, we first discuss the burden of skeletal complications in cancer bone metastasis. We then describe the pathophysiology of bone metastasis. Bone metastasis is a multistage process: long before the development of clinically detectable metastases, circulating tumor cells settle and enter a dormant state in normal vascular and endosteal niches present in the bone marrow, which provide immediate attachment and shelter, and only become active years later as they proliferate and alter the functions of bone-resorbing (osteoclasts) and bone-forming (osteoblasts) cells, promoting skeletal destruction. The molecular mechanisms involved in mediating each of these steps are described, and we also explain how tumor cells interact with a myriad of interconnected cell populations in the bone marrow, including a rich vascular network, immune cells, adipocytes, and nerves. We discuss metabolic programs that tumor cells could engage with to specifically grow in bone. We also describe the progress and future directions of existing bone-targeted agents and report emerging therapies that have arisen from recent advances in our understanding of the pathophysiology of bone metastases. Finally, we discuss the value of bone turnover biomarkers in detection and monitoring of progression and therapeutic effects in patients with bone metastasis.

Endothelial Reprogramming by Disturbed Flow Revealed by Single-Cell RNA and Chromatin Accessibility Study

Disturbed flow (d-flow) induces atherosclerosis by regulating gene expression in endothelial cells (ECs). For further mechanistic understanding, we carried out a single-cell RNA sequencing (scRNA-seq) and scATAC-seq study using endothelial-enriched single cells from the left- and right carotid artery exposed to d-flow (LCA) and stable-flow (s-flow in RCA) using the mouse partial carotid ligation (PCL) model. We find eight EC clusters along with immune cells, fibroblasts, and smooth muscle cells. Analyses of marker genes, pathways, and pseudotime reveal that ECs are highly heterogeneous and plastic. D-flow induces a dramatic transition of ECs from atheroprotective phenotypes to pro-inflammatory cells, mesenchymal (EndMT) cells, hematopoietic stem cells, endothelial stem/progenitor cells, and an unexpected immune cell-like (EndICLT) phenotypes. While confirming KLF4/KLF2 as an s-flow-sensitive transcription factor binding site, we also find those sensitive to d-flow (RELA, AP1, STAT1, and TEAD1). D-flow reprograms ECs from atheroprotective to proatherogenic phenotypes, including EndMT and potentially EndICLT.

Bone metastases

Bone is the most frequent site for metastasis for many cancers, notably for tumours originating in the breast and the prostate. Tumour cells can escape from the primary tumour site and colonize the bone microenvironment. Within the bone, these disseminated tumour cells, as well as those arising in the context of multiple myeloma, may assume a state of dormancy, remaining quiescent for years before resuming proliferation and causing overt metastasis, which causes bone destruction via activation of osteoclast-mediated osteolysis. This structural damage can lead to considerable morbidity, including pain, fractures and impaired quality of life. Although treatment of bone metastases and myeloma bone disease is rarely curative, disease control is often possible for many years through the use of systemic anticancer treatments on a background of multidisciplinary supportive care. This care should include bone-targeted agents to inhibit tumour-associated osteolysis and prevent skeletal morbidity as well as use of appropriate local treatments such as radiation therapy, orthopaedic surgery and specialist palliative care to minimize the impact of metastatic bone disease on physical functioning. In this Primer, we provide an overview of the clinical features, the pathophysiology and the specific treatment approaches to prevent and treat bone metastases from solid tumours as well as myeloma bone disease.

Innovative Options for Bone Metastasis Treatment: An Extensive Analysis on Biomaterials-Based Strategies for Orthopedic Surgeons

Bone is the third most frequent site of metastasis, with a particular incidence in breast and prostate cancer patients. For example, almost 70% of breast cancer patients develop several bone metastases in the late stage of the disease. Bone metastases are a challenge for clinicians and a burden for patients because they frequently cause pain and can lead to fractures. Unfortunately, current therapeutic options are in most cases only palliative and, although not curative, surgery remains the gold standard for bone metastasis treatment. Surgical intervention mostly provides the replacement of the affected bone with a bioimplant, which can be made by materials of different origins and designed through several techniques that have evolved throughout the years simultaneously with clinical needs. Several scientists and clinicians have worked to develop biomaterials with potentially successful biological and mechanical features, however, only a few of them have actually reached the scope. In this review, we extensively analyze currently available biomaterials-based strategies focusing on the newest and most innovative ideas while aiming to highlight what should be considered both a reliable choice for orthopedic surgeons and a future definitive and curative option for bone metastasis and cancer patients.

Bone Tropism in Cancer Metastases

Bone is a frequent site of metastases in many cancers. Both bone properties and the tumor-intrinsic traits are associated with the metastatic propensity to bone (i.e., the bone tropism). Whereas an increasing body of mechanistic studies expanded our understanding on bone tropism, they also revealed complexity across the bone lesions originated from different cancer types. In this review, we will discuss the physical, chemical, and biological properties of bone microenvironment, identify potential players in every stage of bone metastases, and introduce some of the known mechanisms regulating the bone colonization. Our objectives are to integrate the knowledge established in different biological contexts and highlight the determinants of bone tropism.

Ephrin-B2-EphB4 communication mediates tumor-endothelial cell interactions during hematogenous spread to spinal bone in a melanoma metastasis model

Metastases account for the majority of cancer deaths. Bone represents one of the most common sites of distant metastases, and spinal bone metastasis is the most common source of neurological morbidity in cancer patients. During metastatic seeding of cancer cells, endothelial-tumor cell interactions govern extravasation to the bone and potentially represent one of the first points of action for antimetastatic treatment. The ephrin-B2-EphB4 pathway controls cellular interactions by inducing repulsive or adhesive properties, depending on forward or reverse signaling. Here, we report that in an in vivo metastatic melanoma model, ephrin-B2-mediated activation of EphB4 induces tumor cell repulsion from bone endothelium, translating in reduced spinal bone metastatic loci and improved neurological function. Selective ephrin-B2 depletion in endothelial cells or EphB4 inhibition increases bone metastasis and shortens the time window to hind-limb locomotion deficit from spinal cord compression. EphB4 overexpression in melanoma cells ameliorates the metastatic phenotype and improves neurological outcome. Timely harvesting of bone tissue after tumor cell injection and intravital bone microscopy revealed less tumor cells attached to ephrin-B2-positive endothelial cells. These results suggest that ephrin-B2-EphB4 communication influences bone metastasis formation by altering melanoma cell repulsion/adhesion to bone endothelial cells, and represents a molecular target for therapeutic intervention.

Switching Homes: How Cancer Moves to Bone

Bone metastases (BM) are a very common complication of the most prevalent human cancers. BM are extremely painful and may be life-threatening when associated with hypercalcaemia. BM can lead to kidney failure and cardiac arrhythmias and arrest, but why and how do cancer cells decide to "switch homes" and move to bone? In this review, we will present what answers science has provided so far, with focus on the molecular mechanisms and cellular aspects of well-established findings, such as the concept of "vicious cycle" and "osteolytic" vs. "osteosclerotic" bone metastases; as well as on novel concepts, such as cellular dormancy and extracellular vesicles. At the molecular level, we will focus on hypoxia-associated factors and angiogenesis, the Wnt pathway, parathyroid hormone-related peptide (PTHrP) and chemokines. At the supramolecular/cellular level, we will discuss tumour dormancy, id est the mechanisms through which a small contingent of tumour cells coming from the primary site may be kept dormant in the endosteal niche for many years. Finally, we will present a potential role for the multimolecular mediators known as extracellular vesicles in determining bone-tropism and establishing a premetastatic niche by influencing the bone microenvironment.

Cabozantinib Reverses Renal Cell Carcinoma-mediated Osteoblast Inhibition in Three-dimensional Coculture In Vitro and Reduces Bone Osteolysis In Vivo

Renal cell carcinoma bone metastases (RCCBM) are typically osteolytic. We previously showed that BIGH3 (beta Ig-h3/TGFBI), secreted by 786-O renal cell carcinoma, plays a role in osteolytic bone lesion in RCCBM through inhibition of osteoblast (OSB) differentiation. To study this interaction, we employed three-dimensional (3D) hydrogels to coculture bone-derived 786-O (Bo-786) renal cell carcinoma cells with MC3T3-E1 pre-OSBs. Culturing pre-OSBs in the 3D hydrogels preserved their ability to differentiate into mature OSB; however, this process was decreased when pre-OSBs were cocultured with Bo-786 cells. Knockdown of BIGH3 in Bo-786 cells recovered OSB differentiation. Furthermore, treatment with bone morphogenetic protein 4, which stimulates OSB differentiation, or cabozantinib (CBZ), which inhibits VEGFR1 and MET tyrosine kinase activities, also increased OSB differentiation in the coculture. CBZ also inhibited pre-osteoclast RAW264.7 cell differentiation. Using RCCBM mouse models, we showed that CBZ inhibited Bo-786 tumor growth in bone. CBZ treatment also increased bone volume and OSB number, and decreased osteoclast number and blood vessel density. When tested in SN12PM6 renal cell carcinoma cells that have been transduced to overexpress BIGH3, CBZ also inhibited SN12PM6 tumor growth in bone. These observations suggest that enhancing OSB differentiation could be one of the therapeutic strategies for treating RCCBM that exhibit OSB inhibition characteristics, and that this 3D coculture system is an effective tool for screening osteoanabolic agents for further in vivo studies.

Prostate-specific antigen modulates the osteogenic differentiation of MSCs via the cadherin 11-Akt axis

BACKGROUND

A high prevalence of osteoblastic bone metastases is characteristic of prostate cancer. Prostate-specific antigen (PSA) is a serine protease uniquely produced by prostate cancer cells and is an important serological marker for prostate cancer. However, whether PSA modulates the osteogenic process remains largely unknown. In this study, we explored the effect of PSA on modulating the osteoblastic differentiation of mesenchymal stem cells (MSCs). In this study, we used flow cytometry, CCK-8 assay, Alizarin red S (ARS) staining and quantification, alkaline phosphatase (ALP) activity and staining, Western blotting, and quantitative real-time PCR (qRT-PCR) to explore the effect of PSA on osteogenic differentiation of MSCs.

RESULTS

We first demonstrated that although PSA did not affect the proliferation, morphology, or phenotype of MSCs, it significantly promoted the osteogenic differentiation of MSCs in a concentration-dependent manner. Furthermore, we demonstrated that PSA promoted the osteogenic differentiation of MSCs by elevating the expression of Cadherin 11 in MSCs and, thus, activating the Akt signaling pathway.

CONCLUSIONS

In conclusion, we demonstrated that PSA could promote the osteogenesis of MSCs through Akt signaling pathway activation by elevating the expression of cadherin-11 in MSCs. These findings imply a possible role of PSA in osteoblastic bone metastases in prostate cancer.

Quantitative bone SPECT/CT: high specificity for identification of prostate cancer bone metastases

Purpose

Bone scintigraphy with ^99mTc-labeled diphosphonates can identify prostate cancer bone metastases with high sensitivity, but relatively low specificity, because benign conditions such as osteoarthritis can also trigger osteoblastic reactions. We aimed to investigate the diagnostic performance of ^99mTc-2,3-dicarboxy propane-1,1-diphosphonate (^99mTc-DPD) uptake quantification by single-photon emission computed tomography coupled with computed tomography (SPECT/CT) for distinguishing prostate cancer bone metastases from spinal and pelvic osteoarthritic lesions.

Methods

We retrospectively assessed 26 bone scans from 26 patients with known prostate cancer bone metastases and 13 control patients with benign spinal and pelvic osteoarthritic changes without known neoplastic disease. Quantitative SPECT/CT (xSPECT, Siemens Symbia Intevo, Erlangen, Germany) was performed and standardized uptake values (SUVs) were quantified with measurements of SUV_max and SUV_mean (g/mL) in all bone metastases for the prostate cancer group and in spinal and pelvic osteoarthritic changes for the control group. We used receiver operating characteristics (ROC) curves to determine the optimum SUV_max cutoff value to distinguish between bone metastases and benign spinal and pelvic lesions.

Results

In total, 264 prostate cancer bone metastases were analyzed, showing a mean SUV_max and SUV_mean of 34.6 ± 24.6 and 20.8 ± 14.7 g/mL, respectively. In 24 spinal and pelvic osteoarthritic lesions, mean SUV_max and SUV_mean were 14.2 ± 3.8 and 8.9 ± 2.2 g/mL, respectively. SUV_max and SUV_mean were both significantly different between the bone metastases and osteoarthritic groups (p ≤ 0.0001). Using a SUV_max cutoff of 19.5 g/mL for prostate cancer bone metastases in the spine and pelvis, sensitivity, specificity, positive and negative predictive values were 87, 92, 99 and 49%, respectively.

Conclusion

This study showed significant differences in quantitative ^99mTc-DPD uptake on bone SPECT/CT between prostate cancer bone metastases and spinal and pelvic osteoarthritic changes, with higher SUV_max and SUV_mean in metastases. Using a SUV_max cutoff of 19.5 g/mL, high specificity and positive predictive value for metastases identification in the spine and pelvis were found, thus increasing accuracy of bone scintigraphy.

Bone secreted factors induce cellular quiescence in prostate cancer cells

Disseminated tumor cells (DTCs) undergo a dormant state in the distant metastatic site(s) before becoming overt metastatic diseases. In prostate cancer (PCa), bone metastasis can occur years after prostatectomy, suggesting that bone may provide dormancy-inducing factors. To search for these factors, we prepared conditioned media (CM) from calvariae. Using live-cell imaging, we found that Calvarial-CM treatment increased cellular quiescence in C4-2B4 PCa cells. Mass spectrometry analysis of Calvarial-CM identified 132 secreted factors. Western blot and ELISA analyses confirmed the presence of several factors, including DKK3, BMP1, neogenin and vasorin in the Calvarial-CM. qRT-PCR analysis of total calvariae versus isolated osteoblasts showed that DKK3, BMP1, vasorin and neogenin are mainly expressed by osteoblasts, while MIA, LECT1, NGAL and PEDF are expressed by other calvarial cells. Recombinant human DKK3, BMP1, vasorin, neogenin, MIA and NGAL treatment increased cellular quiescence in both C4-2b and C4-2B4 PCa cells. Mechanistically, DKK3, vasorin and neogenin, but not BMP1, increased dormancy through activating the p38MAPK signaling pathway. Consistently, DKK3, vasorin and neogenin failed to induce dormancy in cells expressing dominant-negative p38αMAPK while BMP1 remained active, suggesting that BMP1 uses an alternative dormancy signaling pathway. Thus, bone secretes multiple dormancy-inducing factors that employ distinct signaling pathways to induce DTC dormancy in bone.

Exosomes isolated from CAPS1‑overexpressing colorectal cancer cells promote cell migration

Calcium‑dependent activator protein for secretion 1 (CAPS1) has been reported to promote metastasis in colorectal cancer (CRC), however, the underlying mechanisms have not yet been elucidated. The present study revealed that exosomes derived from CAPS1‑overexpressing CRC cells could enhance the migration of normal colonic epithelial FHC cells. GW4869, an inhibitor of exosomes, could attenuate the migration of FHC cells. Furthermore, liquid chromatography‑mass spectrometry (LC‑MS) and bioinformatics analysis demonstrated that overexpression of CAPS1 could alter the expression pattern of exosomal proteins involved in cell migration. Bone morphogenetic protein 4, which may serve vital roles in the process of CAPS1‑induced cell migration, was downregulated in the exosomes. In summary, the present results demonstrated that CAPS1 promotes cell migration by regulating exosomes. Inhibiting the secretion of exosomes may be helpful for the treatment of patients with metastatic CRC.