Inhibitory effects of megakaryocytic cells in prostate cancer skeletal metastasis

Multiple influence of immune cells in the bone metastatic cancer microenvironment on tumors

Bone is a common organ for solid tumor metastasis. Malignant bone tumor becomes insensitive to systemic therapy after colonization, followed by poor prognosis and high relapse rate. Immune and bone cells in situ constitute a unique immune microenvironment, which plays a crucial role in the context of bone metastasis. This review firstly focuses on lymphatic cells in bone metastatic cancer, including their function in tumor dissemination, invasion, growth and possible cytotoxicity-induced eradication. Subsequently, we examine myeloid cells, namely macrophages, myeloid-derived suppressor cells, dendritic cells, and megakaryocytes, evaluating their interaction with cytotoxic T lymphocytes and contribution to bone metastasis. As important components of skeletal tissue, osteoclasts and osteoblasts derived from bone marrow stromal cells, engaging in 'vicious cycle' accelerate osteolytic bone metastasis. We also explain the concept tumor dormancy and investigate underlying role of immune microenvironment on it. Additionally, a thorough review of emerging treatments for bone metastatic malignancy in clinical research, especially immunotherapy, is presented, indicating current challenges and opportunities in research and development of bone metastasis therapies.

Real-Time Live Imaging of Osteoclast Activation via Cathepsin K Activity in Bone Diseases

Intravital fluorescence imaging of functional osteoclasts within their intact disease context provides valuable insights into the intricate biology at the microscopic level, facilitating the development of therapeutic approaches for osteoclast-associated bone diseases. However, there is a lack of studies investigating osteoclast activity within deep-seated bone lesions using appropriate fluorescent probes, despite the advantages offered by the multi-photon excitation system in enhancing deep tissue imaging resolution. In this study, we report on the intravital tracking of osteoclast activity in three distinct murine bone disease models. We utilized a cathepsin K (CatK)-responsive two-photon fluorogenic probe (CatKP1), which exhibited a notable fluorescence turn-on response in the presence of active CatK. By utilizing CatKP1, we successfully monitored a significant increase in osteoclast activity in hindlimb long bones and its attenuation through pharmacological intervention without sacrificing mice. Thus, our findings highlight the efficacy of CatKP1 as a valuable tool for unraveling pathological osteoclast behavior and exploring novel therapeutic strategies.

Role of Nitric Oxide in Megakaryocyte Function

Megakaryocytes are the main members of the hematopoietic system responsible for regulating vascular homeostasis through their progeny platelets, which are generally known for maintaining hemostasis. Megakaryocytes are characterized as large polyploid cells that reside in the bone marrow but may also circulate in the vasculature. They are generated directly or through a multi-lineage commitment step from the most primitive progenitor or Hematopoietic Stem Cells (HSCs) in a process called "megakaryopoiesis". Immature megakaryocytes enter a complicated development process defined as "thrombopoiesis" that ultimately results in the release of extended protrusions called proplatelets into bone marrow sinusoidal or lung microvessels. One of the main mediators that play an important modulatory role in hematopoiesis and hemostasis is nitric oxide (NO), a free radical gas produced by three isoforms of nitric oxide synthase within the mammalian cells. In this review, we summarize the effect of NO and its signaling on megakaryopoiesis and thrombopoiesis under both physiological and pathophysiological conditions.

S100 Calcium-Binding Protein P Secreted from Megakaryocytes Promotes Osteoclast Maturation

Megakaryocytes (MKs) differentiate from hematopoietic stem cells and produce platelets at the final stage of differentiation. MKs directly interact with bone cells during bone remodeling. However, whether MKs are involved in regulating bone metabolism through indirect regulatory effects on bone cells is unclear. Here, we observed increased osteoclast differentiation of bone marrow-derived macrophages (BMMs) cultured in MK-cultured conditioned medium (MK CM), suggesting that this medium contains factors secreted from MKs that affect osteoclastogenesis. To identify the MK-secreted factor, DNA microarray analysis of the human leukemia cell line K562 and MKs was performed, and S100 calcium-binding protein P (S100P) was selected as a candidate gene affecting osteoclast differentiation. S100P was more highly expressed in MKs than in K562 cells, and showed higher levels in MK CM than in K562-cultured conditioned medium. In BMMs cultured in the presence of recombinant human S100P protein, osteoclast differentiation was promoted and marker gene expression was increased. The resorption area was significantly larger in S100P protein-treated osteoclasts, demonstrating enhanced resorption activity. Overall, S100P secreted from MKs promotes osteoclast differentiation and resorption activity, suggesting that MKs indirectly regulate osteoclast differentiation and activity through the paracrine action of S100P.

Bone marrow niches in the regulation of bone metastasis

The bone marrow has been widely recognised to host a unique microenvironment that facilitates tumour colonisation. Bone metastasis frequently occurs in the late stages of malignant diseases such as breast, prostate and lung cancers. The biology of bone metastasis is determined by tumour-cell-intrinsic traits as well as their interaction with the microenvironment. The bone marrow is a dynamic organ in which various stages of haematopoiesis, osteogenesis, osteolysis and different kinds of immune response are precisely regulated. These different cellular components constitute specialised tissue microenvironments-niches-that play critical roles in controlling tumour cell colonisation, including initial seeding, dormancy and outgrowth. In this review, we will dissect the dynamic nature of the interactions between tumour cells and bone niches. By targeting certain steps of tumour progression and crosstalk with the bone niches, the development of potential therapeutic approaches for the clinical treatment of bone metastasis might be feasible.

Emerging roles for myeloid immune cells in bone metastasis

Metastasis, especially bone metastasis, is a major cause of cancer-related deaths, which is associated with long-term pain due to skeletal-related events and poor quality of life. Tumor cells alter the bone microenvironment through aberrant activation of osteoclasts and osteoblasts which induces bone osteolysis and release of growth factors leading to cancer growth. Though this phenomenon has been well characterized, bone-targeted therapies have shown little improvement in patient survival. Recent evidence indicates a growing appreciation for the complex bone environment, in addition to bone-remodeling stromal cells, which includes an abundance of myeloid immune cells that can either protect against or contribute to the progression of the disease within the bone cavity. Additionally, myeloid cells are recruited into primary tumor sites, where they promote development of the pre-metastatic niche and also can regulate tumor progression within the tumor-bone microenvironment through a milieu of complex mechanisms and involving heterogeneous myeloid populations. In this review, we have highlighted the complex roles of myeloid immunity in bone metastasis and hope to bring attention to the potential of novel immunotherapeutic interventions for the elimination of bone metastasis.

Platelets and Metastasis: New Implications of an Old Interplay

During the process of hematogenous metastasis, tumor cells interact with platelets and their precursors megakaryocytes, providing a selection driver for the metastatic phenotype. Cancer cells have evolved a plethora of mechanisms to engage platelet activation and aggregation. Platelet coating of tumor cells in the blood stream promotes the successful completion of multiple steps of the metastatic cascade. Along the same lines, clinical evidence suggests that anti-coagulant therapy might be associated with reduced risk of metastatic disease and better prognosis in cancer patients. Here, we review experimental and clinical literature concerning the contribution of platelets and megakaryocytes to cancer metastasis and provide insights into the clinical relevance of anti-coagulant therapy in cancer treatment.

Hepatic thrombopoietin gene silencing reduces platelet count and breast cancer progression in transgenic MMTV-PyMT mice

In humans, platelet count within the normal range is required for physiological hemostasis, but, adversely, platelets also support pathological thrombosis. Moreover, by releasing growth factors, they may enhance neoplastic proliferation. We hypothesize that platelet count correlates with platelet-dependent pathologies, even within the range of hemostatic competence. Because platelet production is promoted by thrombopoietin signaling through the myeloproliferative leukemia virus oncogene (cMPL), a receptor expressed on megakaryocytes, we evaluated the feasibility of selective targeting of hepatic thrombopoietin production to test this hypothesis. We synthesized murine- and primate-specific antisense oligonucleotides (THPO-ASO) that silence hepatic thrombopoietin gene (THPO) expression without blocking extrahepatic THPO. Repeated doses of THPO-ASO were administered to mice and a baboon, causing a sustained 50% decline in plasma thrombopoietin levels and platelet count within 4 weeks in both species. To investigate whether reducing platelet count within the translationally relevant hemostatic range could alter a neoplastic process, we administered THPO-ASO to 6-week-old transgenic MMTV-PyMT mice that develop early ductal atypia that progresses into cMPL-negative fatal metastatic breast cancer within 2 to 3 months. THPO-ASO treatment increased the average time to euthanasia (primary humane endpoint) at 2 cm3 combined palpable tumor volume. Our results show that THPO-ASO reduced blood platelet count, plasma platelet factor 4, vascular endothelial growth factor, thrombopoietin levels, bone marrow megakaryocyte density, tumor growth rate, proliferation index, vascularization, platelet and macrophage content, and pulmonary metastases vs untreated controls. These findings confirm that sustained and moderate pharmacological platelet count reduction is feasible with THPO-ASO administration and can delay progression of certain platelet-dependent pathological processes within a safe hemostatic platelet count range.

Improved outcomes after radiotherapy for prostate cancer: Anticoagulation, antiplatelet therapy, and platelet count as key factors in disease progression

BACKGROUND

Several studies have suggested that antiplatelet (AP) or anticoagulant (AC) therapy may improve outcome in men with prostate cancer. We evaluated the effects of AP/AC therapy and tested the hypothesis that platelet count may also be associated with outcomes.

METHODS

A total of 482 patients received primary radiotherapy (median dose 72 Gy) for nonmetastatic prostate cancer; 49% received androgen deprivation therapy. NCCN risk was low/intermediate/high risk in 39%/39%/22%. AP/AC therapy and platelet counts were analyzed with respect to freedom from biochemical failure (FFBF, nadir+2), distant metastasis (FFDM), and cause specific survival (CSS).

RESULTS

After a median follow-up of 103 months, 10-year FFBF, FFDM, and CSS were 77%, 92%, and 96%, respectively. The 10-year cumulative incidence of BF and DM (with death as a competing event) was 19% and 7.0%, respectively. The 32% of men on AP/AC therapy had a lower incidence of 10-year BF (P = .016) and a trend toward a lower incidence of DM (P = .084) and CSS (P = .091). In the entire cohort, lowest platelet quartile (platelet count <187) was associated with higher 10-year BF (31% vs 16%, P = .0042) but not DM (9.4% vs 5.2%, P = .22) nor CSS (P = .76) compared with those patients with platelet count ≥187. AP/AC therapy was associated with a larger absolute reduction in BF for men with lowest platelet quartile (10-year BF of 21% vs 38%, P = .092) vs platelet ≥187 (10-year BF of 10% vs 18%, P = .053). Lowest platelet quartile remained associated with higher BF and DM on multivariable analysis controlling for risk category, WBC, and Hg.

CONCLUSION

AP/AC was associated with improved FFBF. Low platelet count was associated with inferior FFBF and FFDM after prostate radiotherapy. This association was tempered when antiplatelet and anticoagulant therapy was administered.

Megakaryocytes in Bone Metastasis: Protection or Progression?

Bone is the primary site where some cancers develop secondary growth, particularly those derived from breast and prostate tissue. The spread of metastasis to distant sites relies on complex mechanisms by which only cells endowed with certain characteristics are able to reach secondary growth sites. Platelets play a pivotal role in tumour growth, by conferring resistance to shear stress to the circulating tumour cells and protection against natural killer cell attack. Mature polyploid megakaryocytes (MKs) reside in close proximity to the vascular sinusoids of bone marrow, where their primary function is to produce platelets. Emerging evidence has demonstrated that MKs are essential for skeletal homeostasis, due to the expression and production of the bone-related proteins osteocalcin, osteonectin, bone morphogenetic protein, osteopontin, bone sialoprotein, and osteoprotegerin. Debate surrounds the role that MKs play in the development of bone metastasis, which is the topic of this mini-review.

Bone Metastasis: Find Your Niche and Fit in

Metastasis to bones is determined by both intrinsic traits of metastatic tumor cells and properties appertaining to the bone microenvironment. Bone marrow niches are critical for all major steps of metastasis, including the seeding of disseminated tumor cells (DTCs) to bone, the survival of DTCs and microscopic metastases under dormancy, and the eventual outgrowth of overt metastases. In this review, we discuss the role of bone marrow niches in bone colonization. The emphasis is on complicated and dynamic nature of cancer cells-niche interaction, which may underpin the long-standing mystery of metastasis dormancy, and represent a therapeutic target for elimination of minimal residue diseases and prevention of life-taking, overt metastases.

Platelets, autotaxin and lysophosphatidic acid signalling: win-win factors for cancer metastasis

Platelets play a crucial role in the survival of metastatic cells in the blood circulation. The interaction of tumour cells with platelets leads to the production of plethoric factors among which our review will focus on lysophosphatidic acid (LPA), because platelets are the highest producers of this bioactive lysophospholipid in the organism. LPA promotes platelet aggregation, and blocking platelet function decreases LPA signalling and leads to inhibition of breast cancer cell metastasis. Autotaxin (ATX), a lysophospholipase D responsible for the basal concentration of LPA in blood, was detected in platelet α-granules. Functionally, active ATX is eventually released following tumour cell-induced platelet aggregation, thereby promoting metastasis. Megakaryocytes do not express ATX but respond to LPA stimulation. Whether LPA-primed megakaryocytes contribute to the recently reported negative action of megakaryocytes on cancer metastasis is not yet known. However, an understanding of the ATX/LPA signalling pathways in platelets, cancer cells and megakaryocytes opens up new approaches for fighting cancer metastasis.

The Novel Association of Circulating Tumor Cells and Circulating Megakaryocytes with Prostate Cancer Prognosis

Purpose: To develop an approach for the investigation of different subtypes of circulating tumor cells (CTC) and other cells to evaluate their potential prognostic value of prostate cancer.Experimental Design: Malignancy of CTCs undergoing epithelial-to-mesenchymal transition (EMT) was confirmed by repeated FISH. Subgroups of CTCs in 81 patients with prostate cancer (43 castration resistant and 38 untreated localized) were correlated to disease aggressiveness parameters. AUC analysis was applied to compare the performance for metastasis prediction between serum PSA level alone and a combined risk score using both PSA and EMTing CTC count. Circulating megakaryocytes and cancer patient survival association was performed using Cox model.Results: The majority of vimentin (VIM)⁺/CD45^- cells were malignant, with genomic alterations in several genomic regions. The number of cytokeratin (CK)^-/VIM⁺/CD45^- CTCs correlated with disease burden, tumor aggressiveness, and poorer survival. Meanwhile, CK⁺/VIM⁺/CD45^- CTCs were associated with metastases better than other subtypes of CTCs in these limited samples. Combination of PSA level and the number of CK⁺/VIM⁺/CD45^- CTCs enhanced the prediction of cancer metastases [AUC, 0.921; 95% confidence interval (CI), 0.858-0.985]. The number of circulating megakaryocytes was potentially associated with good patient survival in advanced prostate cancer (HR, 0.849; 95% CI, 0.628-1.146, per cell increase), and the difference between the number of mesenchymal CTCs and megakaryocytes strongly correlated to poor survival (HR, 10.17; 95% CI, 2.164-47.789, if score ≥2.0).Conclusions: This CTC analysis approach and the potential association of megakaryocytes with cancer prognosis may greatly enhance our ability to investigate the cancer metastasis process and to predict/monitor cancer progression. Clin Cancer Res; 23(17); 5112-22. ©2017 AACR.

Role of Megakaryocytes in Breast Cancer Metastasis to Bone

Little is known about how megakaryocytes may affect metastasis beyond serving as a source of platelets. In this study, we explored the functional implications of megakaryocyte accumulation in the femurs of mice after injection of metastatic or non-metastatic breast cancer cells in 4T1.2 BALB/cJ and MDA-MB-231 nude mouse models. At bone metastatic sites, but not primary growth sites, tumor growth was associated with increased megakaryopoiesis in both model systems. In the orthotopic BALB/cJ model, extramedullary hematopoiesis occurred in the spleen, resulting in a four-fold increase in megakaryocytes. In support of the hypothesis that reducing megakaryocytes may reduce metastasis, we found that thrombopoietin-deficient mice exhibited a 90% relative decrease in megakaryocytes, yet they developed more aggressive metastasis than wild-type hosts. In human clinical specimens, we observed an increase in megakaryocytes in the bone marrow of 6/8 patients with metastatic breast cancer compared with age- and gender-matched controls. Taken together, our results suggested that an increase in megakaryocytes occurring in response to metastatic cells entering the bone marrow confers some measure of protection against metastasis, challenging present views on the role of megakaryocytes in this setting. Cancer Res; 77(8); 1942-54. ©2017 AACR.

Osteoblasts Are the Centerpiece of the Metastatic Bone Microenvironment

The tumor microenvironment is comprised of diverse stromal cell populations in addition to tumor cells. Increasing evidence now clearly supports the role of microenvironment stromal cells in tumor progression and metastasis, yet the regulatory mechanisms and interactions among tumor and stromal cells remain to be elucidated. Bone metastasis is the major problem in many types of human malignancies including prostate, breast and lung cancers, and the biological basis of bone metastasis let alone curative approaches are largely undetermined. Among the many types of stromal cells in bone, osteoblasts are shown to be an important player. In this regard, osteoblasts are a key target cell type in the development of bone metastasis, but there are currently no drugs or therapeutic approaches are available that specifically target osteoblasts. This review paper summarizes the current knowledge on osteoblasts in the metastatic tumor microenvironment, aiming to provide clues and directions for future research endeavor.

Metastasis: new functional implications of platelets and megakaryocytes

Platelets are essential components of hemostasis. Due to a plethora of factors released on activation, platelet functions are also connected to tumor growth, notably by acting on angiogenesis. It is now well recognized that major roles of platelets in the poor outcome of cancer patients occurs during hematogenous dissemination of cancer cells. In this review, we describe recent insights into the molecular mechanisms supporting the prometastatic activity of platelets. Platelets have been shown to promote survival of circulating tumor cells (CTCs) in the bloodstream by conferring resistance to the shear stress and attack from natural killer cells. Recently, platelets were found to promote and/or maintain the state of epithelial to mesenchymal transition on CTCs through platelet secretion of transforming growth factor β in response to CTC activation. At a later stage in the metastatic process, platelets promote extravasation and establishment of metastatic cells in distant organs as observed in bone. This particular environment is also the site of hematopoiesis, megakaryocytopoiesis, and platelet production. Increasing the number of megakaryocytes (MKs) in the bone marrow results in a high bone mass phenotype and inhibits skeletal metastasis formation of prostate cancer cells. As a result of their specific location in vascular niches in the bone marrow, MK activity might contribute to the "seed and soil" suitability between CTCs and bone. In conclusion, recent findings have made a great advance in our knowledge on how platelets contribute to the metastatic dissemination of cancer cells and that may support the development of new antimetastasis therapies.

The role of platelets and megakaryocytes in bone metastasis

Blood platelets have been known for more than a century as important partners for successful metastatic dissemination of solid tumors. Cancer cell-induced platelet activation is a key event responsible for prometastatic activity of platelets. Blocking platelet aggregation inhibits the progression of skeletal metastases through mechanisms that are not fully understood. The establishment and progression of bone metastases are strongly influenced by the bone remodeling process. Growth factors and cytokines released upon platelet activation may contribute to both skeletal tumor growth and osteolytic lesions. Megakaryocytes are platelet precursors located in the bone marrow that control bone mass through direct stimulation of osteoblast functions and indirect inhibition of osteoclast activities. Considering growing evidence for their role in the metastatic cascade, platelets and/or megakaryocytes may provide new therapeutic opportunities to help limit bone metastases.

Metformin inhibits salivary adenocarcinoma growth through cell cycle arrest and apoptosis

The inhibitory effects of metformin have been observed in many types of cancer. However, its effect on human salivary gland carcinoma is unknown. The effect of metformin alone or in combination with pp242 (an mTOR inhibitor) on salivary adenocarcinoma cells growth were determined in vitro and in vivo. We found that metformin suppressed HSY cell growth in vitro in a time and dose dependent manner associated with a reduced expression of MYC onco-protein, and the same inhibitory effect of metformin was also confirmed in HSG cells. In association with the reduction of MYC onco-protein, metformin significantly restored p53 tumor suppressor gene expression. The distinctive effects of metformin and PP242 on MYC reduction and P53 restoration suggested that metformin inhibited cell growth through a different pathway from PP242 in salivary carcinoma cells. Furthermore, the anti-tumor efficacy of metformin was confirmed in vivo as indicated by the increases of tumor necrosis and reduced proliferation in xenograft tumors from metformin treated group. For the first time, the inhibitory effect of metformin on human salivary gland tumor cells was documented. Moreover, metformin inhibitory effects were enhanced by mTOR inhibitor suggesting that metformin and mTOR inhibitor utilize distinctive signaling pathways to suppress salivary tumor growth.

Integrating new discoveries into the "vicious cycle" paradigm of prostate to bone metastases

In prostate to bone metastases, the "vicious cycle" paradigm has been traditionally used to illustrate how metastases manipulate the bone forming osteoblasts and resorbing osteoclasts in order to yield factors that facilitate growth and establishment. However, recent advances have illustrated that the cycle is far more complex than this simple interpretation. In this review, we will discuss the role of exosomes and hematopoietic/mesenchymal stem/stromal cells (MSC) that facilitate the establishment and activation of prostate metastases and how cells including myeloid-derived suppressor cells, macrophages, T cells, and nerve cells contribute to the momentum of the vicious cycle. The increased complexity of the tumor-bone microenvironment requires a system level approach. The evolution of computational models to interrogate the tumor-bone microenvironment is also discussed, and the application of this integrated approach should allow for the development of effective therapies to treat and cure prostate to bone metastases.

Megakaryocytes are mechanically responsive and influence osteoblast proliferation and differentiation

Maintenance of bone mass and geometry is influenced by mechanical stimuli. Paradigms suggest that osteocytes embedded within the mineralized matrix and osteoblasts on the bone surfaces are the primary responders to physical forces. However, other cells within the bone marrow cavity, such as megakaryocytes (MKs), are also subject to mechanical forces. Recent studies have highlighted the potent effects of MKs on osteoblast proliferation as well as bone formation in vivo. We hypothesize that MKs are capable of responding to physical forces and that the interactions between these cells and osteoblasts can be influenced by mechanical stimulation. In this study, we demonstrate that two MK cell lines respond to fluid shear stress in culture. Furthermore, using laser capture microdissection, we isolated MKs from histologic sections of murine tibiae that were exposed to compressive loads in vivo. C-fos, a transcription factor shown to be upregulated in response to load in various tissue types, was increased in MKs from loaded relative to non-loaded limbs at a level comparable to that of osteocytes from the same limbs. We also developed a co-culture system to address whether mechanical stimulation of MKs in culture would impact osteoblast proliferation and differentiation. The presence of MKs in co-culture, but not conditioned media, had dramatic effects on proliferation of preosteoblast MC3T3-E1 cells in culture. Our data suggests a minimal decrease in proliferation as well as an increase in mineralization capacity of osteoblasts co-cultured with MKs exposed to shear compared to co-cultures with unstimulated MKs.

Dissecting the role of bone marrow stromal cells on bone metastases

Tumor-induced bone disease is a dynamic process that involves interactions with many cell types. Once metastatic cancer cells reach the bone, they are in contact with many different cell types that are present in the cell-rich bone marrow. These cells include the immune cells, myeloid cells, fibroblasts, osteoblasts, osteoclasts, and mesenchymal stem cells. Each of these cell populations can influence the behavior or gene expression of both the tumor cells and the bone microenvironment. Additionally, the tumor itself can alter the behavior of these bone marrow cells which further alters both the microenvironment and the tumor cells. While many groups focus on studying these interactions, much remains unknown. A better understanding of the interactions between the tumor cells and the bone microenvironment will improve our knowledge on how tumors establish in bone and may lead to improvements in diagnosing and treating bone metastases. This review details our current knowledge on the interactions between tumor cells that reside in bone and their microenvironment.

Blood clot formation does not affect metastasis formation or tumor growth in a murine model of breast cancer

Cancer is associated with increased fracture risk, due either to metastasis or associated osteoporosis. After a fracture, blood clots form. Because proteins of the coagulation cascade and activated platelets promote cancer development, a fracture in patients with cancer often raises the question whether it is a pathologic fracture or whether the fracture itself might promote the formation of metastatic lesions. We therefore examined whether blood clot formation results in increased metastasis in a murine model of experimental breast cancer metastasis. For this purpose, a clot was surgically induced in the bone marrow of the left tibia of immundeficient mice. Either one minute prior to or five minutes after clot induction, human cancer cells were introduced in the circulation by intracardiac injection. The number of cancer cells that homed to the intervention site was determined by quantitative real-time PCR and flow cytometry. Metastasis formation and longitudinal growth were evaluated by bioluminescence imaging. The number of cancer cells that homed to the intervention site after 24 hours was similar to the number of cells in the opposite tibia that did not undergo clot induction. This effect was confirmed using two more cancer cell lines. Furthermore, no difference in the number of macroscopic lesions or their growth could be detected. In the control group 72% developed a lesion in the left tibia. In the experimental groups with clot formation 79% and 65% developed lesions in the left tibia (p = ns when comparing each experimental group with the controls). Survival was similar too. In summary, the growth factors accumulating in a clot/hematoma are neither enough to promote cancer cell homing nor support growth in an experimental model of breast cancer bone metastasis. This suggests that blood clot formation, as occurs in traumatic fractures, surgical interventions, and bruises, does not increase the risk of metastasis formation.

Metformin targets c-MYC oncogene to prevent prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer-related death in American men and many PCa patients develop skeletal metastasis. Current treatment modalities for metastatic PCa are mostly palliative with poor prognosis. Epidemiological studies indicated that patients receiving the diabetic drug metformin have lower PCa risk and better prognosis, suggesting that metformin may have antineoplastic effects. The mechanism by which metformin acts as chemopreventive agent to impede PCa initiation and progression is unknown. The amplification of c-MYC oncogene plays a key role in early prostate epithelia cell transformation and PCa growth. The purpose of this study is to investigate the effect of metformin on c-myc expression and PCa progression. Our results demonstrated that (i) in Hi-Myc mice that display murine prostate neoplasia and highly resemble the progression of human prostate tumors, metformin attenuated the development of prostate intraepithelial neoplasia (PIN, the precancerous lesion of prostate) and PCa lesions. (ii) Metformin reduced c-myc protein levels in vivo and in vitro. In Myc-CaP mouse PCa cells, metformin decreased c-myc protein levels by at least 50%. (iii) Metformin selectively inhibited the growth of PCa cells by stimulating cell cycle arrest and apoptosis without affecting the growth of normal prostatic epithelial cells (RWPE-1). (iv) Reduced PIN formation by metformin was associated with reduced levels of androgen receptor and proliferation marker Ki-67 in Hi-Myc mouse prostate glands. Our novel findings suggest that by downregulating c-myc, metformin can act as a chemopreventive agent to restrict prostatic neoplasia initiation and transformation.

SUMMARY

Metformin, an old antidiabetes drug, may inhibit prostate intraepithelial neoplasia transforming to cancer lesion via reducing c-MYC, an 'old' overexpressed oncogene. This study explores chemopreventive efficacy of metformin in prostate cancer and its link to cMYC in vitro and in vivo.

Parathyroid hormone-related protein drives a CD11b+Gr1+ cell-mediated positive feedback loop to support prostate cancer growth

In the tumor microenvironment, CD11b(+)Gr1(+) bone marrow-derived cells are a predominant source of protumorigenic factors such as matrix metalloproteinases (MMP), but how distal tumors regulate these cells in the bone marrow is unclear. Here we addressed the hypothesis that the parathyroid hormone-related protein (PTHrP) potentiates CD11b(+)Gr1(+) cells in the bone marrow of prostate tumor hosts. In two xenograft models of prostate cancer, levels of tumor-derived PTHrP correlated with CD11b(+)Gr1(+) cell recruitment and microvessel density in the tumor tissue, with evidence for mediation of CD11b(+)Gr1(+) cell-derived MMP-9 but not tumor-derived VEGF-A. CD11b(+)Gr1(+) cells isolated from mice with PTHrP-overexpressing tumors exhibited relatively increased proangiogenic potential, suggesting that prostate tumor-derived PTHrP potentiates this activity of CD11b(+)Gr1(+) cells. Administration of neutralizing PTHrP monoclonal antibody reduced CD11b(+)Gr1(+) cells and MMP-9 in the tumors. Mechanistic investigations in vivo revealed that PTHrP elevated Y418 phosphorylation levels in Src family kinases in CD11b(+)Gr1(+) cells via osteoblast-derived interleukin-6 and VEGF-A, thereby upregulating MMP-9. Taken together, our results showed that prostate cancer-derived PTHrP acts in the bone marrow to potentiate CD11b(+)Gr1(+) cells, which are recruited to tumor tissue where they contribute to tumor angiogenesis and growth.

Nuclear localization of parathyroid hormone-related peptide confers resistance to anoikis in prostate cancer cells

Prostate cancer remains a leading cause of cancer-related death in men, largely attributable to distant metastases, most frequently to bones. Despite intensive investigations, molecular mechanisms underlying metastasis are not completely understood. Among prostate cancer-derived factors, parathyroid hormone-related peptide (PTHrP), first discovered as an etiologic factor for malignancy-induced hypercalcemia, regulates many cellular functions critical to tumor growth, angiogenesis, and metastasis. In this study, the role of PTHrP in tumor cell survival from detachment-induced apoptosis (i.e. anoikis) was investigated. Reduction of PTHLH (encoding PTHrP) gene expression in human prostate cancer cells (PC-3) increased the percentage of apoptotic cells when cultured in suspension. Conversely, overexpression of PTHrP protected prostate cancer cells (Ace-1 and LNCaP, both typically expressing low or undetectable basal PTHrP) from anoikis. Overexpression of nuclear localization signal (NLS)-defective PTHrP failed to protect cells from anoikis, suggesting that PTHrP-dependent protection from anoikis is an intracrine event. A PCR-based apoptosis-related gene array showed that detachment increased expression of the TNF gene (encoding the proapoptotic protein tumor necrosis factor-α) fourfold greater in PTHrP-knockdown PC-3 cells than in control PC-3 cells. In parallel, TNF gene expression was significantly reduced in PTHrP-overexpressing LNCaP cells, but not in NLS-defective PTHrP overexpressing LNCaP cells, when compared with control LNCaP cells. Subsequently, in a prostate cancer skeletal metastasis mouse model, PTHrP-knockdown PC-3 cells resulted in significantly fewer metastatic lesions compared to control PC-3 cells, suggesting that PTHrP mediated antianoikis events in the bloodstream. In conclusion, nuclear localization of PTHrP confers prostate cancer cell resistance to anoikis, potentially contributing to prostate cancer metastasis.

Roles of bone marrow cells in skeletal metastases: no longer bystanders

Bone serves one of the most congenial metastatic microenvironments for multiple types of solid tumors, but its role in this process remains under-explored. Among many cell populations constituting the bone and bone marrow microenvironment, osteoblasts (originated from mesenchymal stem cells) and osteoclasts (originated from hematopoietic stem cells) have been the main research focus for pro-tumorigenic roles. Recently, increasing evidence further elucidates that hematopoietic lineage cells as well as stromal cells in the bone marrow mediate distinct but critical functions in tumor growth, metastasis, angiogenesis and apoptosis in the bone microenvironment. This review article summarizes the key evidence describing differential roles of bone marrow cells, including hematopoietic stem cells (HSCs), megakaryocytes, macrophages and myeloid-derived suppressor cells in the development of metastatic bone lesions. HSCs promote tumor growth by switching on angiogenesis, but at the same time compete with metastatic tumor cells for occupancy of osteoblastic niche. Megakaryocytes negatively regulate the extravasating tumor cells by inducing apoptosis and suppressing proliferation. Macrophages and myeloid cells have pro-tumorigenic roles in general, suggesting a similar effect in the bone marrow. Hematopoietic and stromal cell populations in the bone marrow, previously considered as simple by-standers in the context of tumor metastasis, have distinct and active roles in promoting or suppressing tumor growth and metastasis in bone. Further investigation on the extended roles of bone marrow cells will help formulate better approaches to treatment through improved understanding of the metastatic bone microenvironment.

Cancer to bone: a fatal attraction

When cancer metastasizes to bone, considerable pain and deregulated bone remodelling occurs, greatly diminishing the possibility of cure. Metastasizing tumour cells mobilize and sculpt the bone microenvironment to enhance tumour growth and to promote bone invasion. Understanding the crucial components of the bone microenvironment that influence tumour localization, along with the tumour-derived factors that modulate cellular and protein matrix components of bone to favour tumour expansion and invasion, is central to the pathophysiology of bone metastases. Basic findings of tumour-bone interactions have uncovered numerous therapeutic opportunities that focus on the bone microenvironment to prevent and treat bone metastases.

Cancer to bone: a fatal attraction

When cancer metastasizes to bone, considerable pain and deregulated bone remodelling occurs, greatly diminishing the possibility of cure. Metastasizing tumour cells mobilize and sculpt the bone microenvironment to enhance tumour growth and to promote bone invasion. Understanding the crucial components of the bone microenvironment that influence tumour localization, along with the tumour-derived factors that modulate cellular and protein matrix components of bone to favour tumour expansion and invasion, is central to the pathophysiology of bone metastases. Basic findings of tumour-bone interactions have uncovered numerous therapeutic opportunities that focus on the bone microenvironment to prevent and treat bone metastases.

Integrins and bone metastasis: integrating tumor cell and stromal cell interactions

Integrins on both tumor cells and the supporting host stromal cells in bone (osteoclasts, new blood vessels, inflammatory cells, platelets and bone marrow stromal cells) play key roles in enhancing bone metastasis. Tumor cells localize to specific tissues through integrin-mediated contacts with extracellular matrix and stromal cells. Integrin expression and signaling are perturbed in cancer cells, allowing them to "escape" from cell-cell and cell-matrix tethers, invade, migrate and colonize within new tissues and matrices. Integrin signaling through αvβ3 and VLA-4 on tumor cells can promote tumor metastasis to and proliferation in the bone microenvironment. Osteoclast (OC) mediated bone resorption is a critical component of bone metastasis and can promote tumor growth in bone and αvβ3 integrins are critical to OC function and development. Tumors in the bone microenvironment can recruit new blood vessel formation, platelets, pro-tumor immune cells and bone marrow stromal cells that promote tumor growth and invasion in bone. Integrins and their ligands play critical roles in platelet aggregation (αvβ3 and αIIbβ3), hematopoietic cell mobilization (VLA-4 and osteopontin), neoangiogenesis (αvβ3, αvβ5, α6β4, and β1 integrin) and stromal function (osteopontin and VLA-4). Integrins are involved in the pathogenesis of bone metastasis at many levels and further study to define integrin dysregulation by cancer will yield new therapeutic targets for the prevention and treatment of bone metastasis.