Osteoclasts eat stem cells out of house and home

LCN2 overexpression in bone enhances the hematopoietic compartment via modulation of the bone marrow microenvironment

Lipocalin-2 (LCN2) is a member of the lipocalin family whose expression is modulated in several conditions, including cell differentiation, innate immunity, stress, and cancer. Although it is known that it is expressed in bone, its function in this tissue remains poorly studied. To this end, we took advantage of transgenic mice lines that expressed LCN2 driven by a bone specific type I collagen (LCN2-Tg). In the bone marrow (BM) of LCN2-Tg mice we observed an increased number of phenotypically long-term hematopoietic stem cells (LT-HSC) that also displayed a higher proliferation rate compared to wild-type controls (Wt). Furthermore, hematopoietic progenitor cells, obtained from LCN2-Tg BM showed an increased clonogenic capacity compared to those obtained from LCN2-Tg spleen, a higher concentration of serum erythropoietin and a higher number of mature erythrocytes in the peripheral blood of old LCN2-Tg animals compared to aged-matched wt. The findings of a combined increase in the BM of the LCN2-Tg mice of SDF-1, SCF, and TIMP-1 levels along with the reduction of both MMP-9 activity and cathepsin K concentration may explain the observed effects on the HSC compartment. This study shows that LCN2 overexpression in bones modifies the BM microenvironment via modulation of the expression of key secreted factors and cytokines, which in turn regulate the HSC niche behavior enhancing both HSC homing in young mice and erythrocytes production in older mice.

Low intensity vibration mitigates tumor progression and protects bone quantity and quality in a murine model of myeloma

Myeloma facilitates destruction of bone and marrow. Since physical activity encourages musculoskeletal preservation we evaluated whether low-intensity vibration (LIV), a means to deliver mechanical signals, could protect bone and marrow during myeloma progression. Immunocompromised-mice (n=25) were injected with human-myeloma cells, while 8 (AC) were saline-injected. Myeloma-injected mice (LIV; n=13) were subjected to daily-mechanical loading (15min/d; 0.3g @ 90Hz) while 12 (MM) were sham-handled. At 8w, femurs had 86% less trabecular bone volume fraction (BV/TV) in MM than in AC, yet only a 21% decrease in LIV was observed in comparison to AC, reflecting a 76% increase versus MM. Cortical BV was 21% and 15% lower in MM and LIV, respectively, than in AC; LIV showing 30% improvement over MM. Similar outcomes were observed in the axial skeleton, showing a 35% loss in MM with a 27% improved retention of bone in the L5 of LIV-treated mice as compared to MM. Transcortical-perforations in the femur from myeloma-induced osteolysis were 9× higher in MM versus AC, reduced by 57% in LIV. Serum-TRACP5b, 61% greater in MM versus AC, rose by 33% in LIV compared to AC, a 45% reduction in activity when compared to MM. Histomorphometric analyses of femoral trabecular bone demonstrated a 70% elevation in eroded surfaces of MM versus AC, while measures in LIV were 58% below those in MM. 72% of marrow in the femur of MM mice contained tumor, contrasted by a 31% lower burden in LIV. MM mice (42%) presented advanced-stage necrosis of tibial marrow while present in just 8% of LIV. Myeloma infiltration inversely correlated to measures of bone quality, while LIV slowed the systemic, myeloma-associated decline in bone quality and inhibited tumor progression through the hindlimbs.

Rescue of a primary myelofibrosis model by retinoid-antagonist therapy

Molecular targeting of the two receptor interaction domains of the epigenetic repressor silencing mediator of retinoid and thyroid hormone receptors (SMRT(mRID)) produced a transplantable skeletal syndrome that reduced radial bone growth, increased numbers of bone-resorbing periosteal osteoclasts, and increased bone fracture risk. Furthermore, SMRT(mRID) mice develop spontaneous primary myelofibrosis, a chronic, usually idiopathic disorder characterized by progressive bone marrow fibrosis. Frequently linked to polycythemia vera and chronic myeloid leukemia, myelofibrosis displays high patient morbidity and mortality, and current treatment is mostly palliative. To decipher the etiology of this disease, we identified the thrombopoietin (Tpo) gene as a target of the SMRT-retinoic acid receptor signaling pathway in bone marrow stromal cells. Chronic induction of Tpo in SMRT(mRID) mice results in up-regulation of TGF-β and PDGF in megakaryocytes, uncontrolled proliferation of bone marrow reticular cells, and fibrosis of the marrow compartment. Of therapeutic relevance, we show that this syndrome can be rescued by retinoid antagonists, demonstrating that the physical interface between SMRT and retinoic acid receptor can be a potential therapeutic target to block primary myelofibrosis disease progression.

Role of osteoclasts in regulating hematopoietic stem and progenitor cells

Bone marrow (BM) cavities are utilized for hematopoiesis and to maintain hematopoietic stem cells (HSCs). HSCs have the ability to self-renew as well as to differentiate into multiple different hematopoietic lineage cells. HSCs produce their daughter cells throughout the lifespan of individuals and thus, maintaining HSCs is crucial for individual life. BM cavities provide a specialized microenvironment termed “niche” to support HSCs. Niches are composed of various types of cells such as osteoblasts, endothelial cells and reticular cells. Osteoclasts are unique cells which resorb bones and are required for BM cavity formation. Loss of osteoclast function or differentiation results in inhibition of BM cavity formation, an osteopetrotic phenotype. Osteoclasts are also reportedly required for hematopoietic stem and progenitor cell (HSPC) mobilization to the periphery from BM cavities. Thus, lack of osteoclasts likely results in inhibition of HSC maintenance and HSPC mobilization. However, we found that osteoclasts are dispensable for hematopoietic stem cell maintenance and mobilization by using three independent osteoclast-less animal models. In this review, I will discuss the roles of osteoclasts in hematopoietic stem cell maintenance and mobilization.

The nucleotide sugar UDP-glucose mobilizes long-term repopulating primitive hematopoietic cells

Hematopoietic stem progenitor cells (HSPCs) are present in very small numbers in the circulating blood in steady-state conditions. In response to stress or injury, HSPCs are primed to migrate out of their niche to peripheral blood. Mobilized HSPCs are now commonly used as stem cell sources due to faster engraftment and reduced risk of posttransplant infection. In this study, we demonstrated that a nucleotide sugar, UDP-glucose, which is released into extracellular fluids in response to stress, mediates HSPC mobilization. UDP-glucose-mobilized cells possessed the capacity to achieve long-term repopulation in lethally irradiated animals and the ability to differentiate into multi-lineage blood cells. Compared with G-CSF-mobilized cells, UDP-glucose-mobilized cells preferentially supported long-term repopulation and exhibited lymphoid-biased differentiation, suggesting that UDP-glucose triggers the mobilization of functionally distinct subsets of HSPCs. Furthermore, co-administration of UDP-glucose and G-CSF led to greater HSPC mobilization than G-CSF alone. Administration of the antioxidant agent NAC significantly reduced UDP-glucose-induced mobilization, coinciding with a reduction in RANKL and osteoclastogenesis. These findings provide direct evidence demonstrating a potential role for UDP-glucose in HSPC mobilization and may provide an attractive strategy to improve the yield of stem cells in poor-mobilizing allogeneic or autologous donors.

Cell autonomous and nonautonomous mechanisms drive hematopoietic stem/progenitor cell loss in the absence of DNA repair

Daily, cells incur tens of thousands of DNA lesions caused by endogenous processes. Due to their long-lived nature, adult stem cells may be particularly susceptible to the negative impact of this constant genotoxic stress. Indeed, in murine models of DNA repair deficiencies, there is accumulation of DNA damage in hematopoietic stem cells and premature loss of function. Herein, we demonstrate that mice expressing reduced levels of ERCC1-XPF DNA repair endonuclease (Ercc1-/Δ mice) spontaneously display a progressive decline in the number and function of hematopoietic stem/progenitor cells (HSPCs). This was accompanied by increased cell death, expression of senescence markers, reactive oxygen species, and DNA damage in HSPC populations, illustrating cell autonomous mechanisms that contribute to loss of function. In addition, the bone marrow microenvironment of Ercc1-/Δ mice was not permissive for the engraftment of transplanted normal stem cells. Bones from Ercc1-/Δ mice displayed excessive osteoclastic activity, which alters the microenvironment in a way that is unfavorable to HSPC maintenance. This was accompanied by increased proinflammatory cytokines in the bone marrow of Ercc1-/Δ mice. These data provide novel evidence that spontaneous, endogenous DNA damage, if not repaired, promotes progressive attrition of adult stem cells via both cell autonomous and nonautonomous mechanisms.

Bone metastases: molecular mechanisms and novel therapeutic interventions

It has been long recognized that skeleton represents one of the most favored metastatic sites for common cancers like breast and prostate. During the last decade the molecular mechanisms that are responsible for the development of bone metastasis have been gradually illuminated. It appears that the bone microenvironment has a pivotal role in this process. Metastatic tumor cells interact with bone triggering a cascade of molecular events that produce osteolytic and/or osteoblastic phenomena. In this review, we summarize and discuss the most significant factors and signaling pathways implicated in bone colonization. Moreover, based on the recent literature and data, we foresee the need for designing novel agents that will efficiently disrupt these interactions among cancer cells and bone microenvironment, bringing hope for more effective treatments.

Alterations in the self-renewal and differentiation ability of bone marrow mesenchymal stem cells in a mouse model of rheumatoid arthritis

Introduction

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease primarily involving the synovium. Evidence in recent years has suggested that the bone marrow (BM) may be involved, and may even be the initiating site of the disease. Abnormalities in haemopoietic stem cells' (HSC) survival, proliferation and aging have been described in patients affected by RA and ascribed to abnormal support by the BM microenvironment. Mesenchymal stem cells (MSC) and their progeny constitute important components of the BM niche. In this study we test the hypothesis that the onset of inflammatory arthritis is associated with altered self-renewal and differentiation of bone marrow MSC, which alters the composition of the BM microenvironment.

Methods

We have used Balb/C Interleukin-1 receptor antagonist knock-out mice, which spontaneously develop RA-like disease in 100% of mice by 20 weeks of age to determine the number of mesenchymal progenitors and their differentiated progeny before, at the start and with progression of the disease.

Results

We showed a decrease in the number of mesenchymal progenitors with adipogenic potential and decreased bone marrow adipogenesis before disease onset. This is associated with a decrease in osteoclastogenesis. Moreover, at the onset of disease a significant increase in all mesenchymal progenitors is observed together with a block in their differentiation to osteoblasts. This is associated with accelerated bone loss.

Conclusions

Significant changes occur in the BM niche with the establishment and progression of RA-like disease. Those changes may be responsible for aspects of the disease, including the advance of osteoporosis. An understanding of the molecular mechanisms leading to those changes may lead to new strategies for therapeutic intervention.

Tumor necrosis factor-alpha alters the modulatory effects of mesenchymal stem cells on osteoclast formation and function

Mesenchymal stem cells (MSCs) are characterized by their hematopoiesis-supporting and immunosuppressive capacity, while osteoclasts are main cell components in the endosteal hematopoietic stem cell niche and pivotal players in osteoimmunology. To clarify the association of these 2 kinds of cells, mouse CD11b(+) monocytes were cultured onto MSC layers in the presence or absence of macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL). The results showed that MSCs independently supported osteoclast development and this effect was enhanced by M-CSF and RANKL. Interestingly, tumor necrosis factor-alpha (TNF-alpha)-stimulated MSCs turned to inhibit osteoclast formation and protect tusk slices from osteoclastic resorption. Real-time PCR and ELISA assays demonstrated that osteoprotegerin expression at both mRNA and protein levels in TNF-alpha-stimulated MSCs was up-regulated, at least partially by activating the mitogen-activated protein kinase pathway. Furthermore, TNF-alpha-stimulated MSCs maintained their immunophenotypic, multipotential differentiation and immunosuppressive characteristics. Moreover, MSCs treated with synovial fluid from rheumatoid arthritis patients modulated osteoclast generation in close relation with the TNF-alpha levels. This study suggests that MSCs exhibit dual modulatory function on osteoclasts and the result might shed light on understanding the involvement of MSCs in the inflammatory diseases.

Removal of the spleen in mice alters the cytokine expression profile of the marrow micro-environment and increases bone formation

Splenectomized mice express progressively increased numbers of platelets in the blood and reduced numbers of megakaryocytes in the marrow with age. The megakaryocytes in the marrow of these animals express reduced levels of Gata1, a transcription factor necessary for their maturation. In addition, the marrow from these animals expresses greater levels of cytokines (TGF-beta, PDGF-alpha, and VEGF) known to be produced at high levels by megakaryocytes expressing reduced levels of Gata1. This high level of cytokine expression is in turn associated with active osteoblast proliferation localized to areas of the femur, where megakaryocytes expressing reduced Gata1 levels are also found. These results confirm the role of megakaryocytes as regulator of bone formation in mice and suggest that a cross-talk between the spleen and marrow may regulate the total numbers of hemopoietic niches present in an animal.

Defects in osteoblast function but no changes in long-term repopulating potential of hematopoietic stem cells in a mouse chronic inflammatory arthritis model

Recent studies support the notion that there is an intricate relationship between hematopoiesis and bone homeostasis in normal steady states. Using mice undergoing chronic inflammatory arthritis, we investigated the relationship between hematopoiesis and bone homeostasis in pathologic conditions. We demonstrate that mice undergoing chronic inflammatory arthritis displayed osteoporosis resulting from a severe defect in osteoblast function. Despite the defective osteoblast function, however, the hematopoietic stem cells from these mice exhibited normal properties in either long-term repopulation or cell cycling. Therefore, the bone-forming capacity of osteoblasts is distinct from their ability to maintain hematopoietic stem cells in chronic inflammatory conditions.

CD34 human hematopoietic progenitor cell line, MUTZ-3, differentiates into functional osteoclasts

OBJECTIVE

CD14(+) monocyte cell lines can differentiate into an osteoclast (OC)-like lineage. However, the identification of human cell lines with stem cell characteristics, capable of differentiating into OCs, would provide a tool for the study of the molecular mechanisms regulating their commitment, differentiation, and function. Since the human acute myeloid leukemia cell line MUTZ-3 contains both CD34(+) stem cell and CD14(+) cell populations, we investigated the capacity of the stem/progenitor CD34(+) population to differentiate into functional OCs.

MATERIALS AND METHODS

Sorted MUTZ-3-CD34(+) and MUTZ-3-CD14(+) cells were cultured in presence of M-CSF, RANK-L, and TNF-alpha to generate OCs. Differentiation was evaluated by TRAP staining and RT-PCR, which assessed the expression of c-fms, RANK, MMP-9, CATK, TRAP, and CTR in -CD34(+)OC and -CD14(+)OC cells. Resorption pit formation was also evaluated. CD34, CD14, M-CSF-R, RANK, and CTR expression was assessed by FACS analysis.

RESULTS

MUTZ-3-CD34(+) differentiated into OCs, displaying the full range of differentiation markers; MMP-9, CATK, TRAP, and RANK mRNA were detected from day 3 of culture, whereas CTR from day 12. Stimulated MUTZ-3-CD34(+) generated functional osteoclasts that formed extensive resorption lacunae on both mineralized surface and bone slices. Surprisingly, in both sorted populations we identified a population M-CSF-R(+)/RANK(+) that at the same time co-expressed CD14 and CD34.

CONCLUSIONS

These findings demonstrate that MUTZ-3 cells constitute an invaluable model to study the expression pattern in different developmental stages of commitment and differentiation. Importantly, the data indicate that the CD14(+)CD34(+)M-CSF-R(+)RANK(+) population represents an intermediate stage of differentiation from CD34 precursors and monocytes to osteoclast.

Donor cell leukemia: insight into cancer stem cells and the stem cell niche

Donor cell leukemia (DCL) is a rare complication of hematopoietic cell transplantation (HCT). Its incidence has been reported between 0.12% and 5%, although the majority of cases are anecdotal. The mechanisms of leukemogenesis in DCL may be distinct from other types of leukemia. Possible causes of DCL include oncogenic alteration or premature aging of transplanted donor cells in an immunosuppressed person. Although many studies have recently better characterized leukemic stem cells, it is important to also consider that both intrinsic cell factors and external signals from the hematopoietic microenvironment govern the developmental fate of hematopoietic stem cells (HSCs). Therefore, in cases of DCL, alteration of the microenvironment after HCT may increase the likelihood that some progeny of normal HSCs become leukemic. This complex intercommunication between cells, growth factors, and cytokines in the hematopoietic microenvironment are critical to balance HSC self-renewal, proliferation, and differentiation. However, this homeostasis is likely perturbed in the development of DCL, allowing unique insight into the stimuli that regulate normal and potentially abnormal hematopoietic development. In this article, we discuss the possible pathogenesis of DCL, its association with stem cells, and its likely dependence on a less-supportive stem cell niche.