Paracrine and endocrine actions of bone-the functions of secretory proteins from osteoblasts, osteocytes, and osteoclasts.
ABSTRACT: The skeleton is a dynamic organ that is constantly remodeled. Proteins secreted from bone cells, namely osteoblasts, osteocytes, and osteoclasts exert regulation on osteoblastogenesis, osteclastogenesis, and angiogenesis in a paracrine manner. Osteoblasts secrete a range of different molecules including RANKL/OPG, M-CSF, SEMA3A, WNT5A, and WNT16 that regulate osteoclastogenesis. Osteoblasts also produce VEGFA that stimulates osteoblastogenesis and angiogenesis. Osteocytes produce sclerostin (SOST) that inhibits osteoblast differentiation and promotes osteoclast differentiation. Osteoclasts secrete factors including BMP6, CTHRC1, EFNB2, S1P, WNT10B, SEMA4D, and CT-1 that act on osteoblasts and osteocytes, and thereby influenceaA osteogenesis. Osteoclast precursors produce the angiogenic factor PDGF-BB to promote the formation of Type H vessels, which then stimulate osteoblastogenesis. Besides, the evidences over the past decades show that at least three hormones or "osteokines" from bone cells have endocrine functions. FGF23 is produced by osteoblasts and osteocytes and can regulate phosphate metabolism. Osteocalcin (OCN) secreted by osteoblasts regulates systemic glucose and energy metabolism, reproduction, and cognition. Lipocalin-2 (LCN2) is secreted by osteoblasts and can influence energy metabolism by suppressing appetite in the brain. We review the recent progresses in the paracrine and endocrine functions of the secretory proteins of osteoblasts, osteocytes, and osteoclasts, revealing connections of the skeleton with other tissues and providing added insights into the pathogenesis of degenerative diseases affecting multiple organs and the drug discovery process.
Project description:The role of myeloid cell-specific TGF-? signaling in non-small-cell lung cancer (NSCLC)-induced osteolytic bone lesion development is unknown. We used a genetically engineered mouse model, Tgfbr2LysMCre knockout (KO), which has a loss of TGF-? signaling specifically in myeloid lineage cells, and we found that the area of H1993 cell-induced osteolytic bone lesions was decreased in Tgfbr2LysMCre KO mice, relative to the area in control littermates. The bone lesion areas were correlated with tumor cell proliferation, angiogenesis, and osteoclastogenesis in the microenvironment. The smaller bone lesion area was partially rescued by bFGF, which was expressed by osteoblasts. Interestingly, bFGF was able to rescue the osteoclastogenesis, but not the tumor cell proliferation or angiogenesis. We then focused on identifying osteoclast factors that regulate bFGF expression in osteoblasts. We found that the expression and secretion of CTHRC1 was downregulated in osteoclasts from Tgfbr2LysMCre KO mice; CTHRC1 was able to promote bFGF expression in osteoblasts, possibly through the Wnt/?-catenin pathway. Functionally, bFGF stimulated osteoclastogenesis and inhibited osteoblastogenesis, but had no effect on H1993 cell proliferation. On the other hand, CTHRC1 promoted osteoblastogenesis and H1993 cell proliferation. Together, our data show that myeloid-specific TGF-? signaling promoted osteolytic bone lesion development and bFGF expression in osteoblasts; that osteoclast-secreted CTHRC1 stimulated bFGF expression in osteoblasts in a paracrine manner; and that CTHRC1 and bFGF had different cell-specific functions that contributed to bone lesion development.
Project description:Heightened activity of osteoclast is considered to be the culprit in breaking the balance during bone remodeling in pathological conditions, such as osteoporosis. As a "foe" of skeletal health, many antiosteoporosis therapies aim to inhibit osteoclastogenesis. However, bone remodeling is a dynamic process that requires the subtle coordination of osteoclasts and osteoblasts. Severe suppression of osteoclast differentiation will impair bone formation because of the coupling effect. Thus, understanding the complex roles of osteoclast in maintaining proper bone remodeling is highly warranted to develop better management of osteoporosis. This review aimed to determine the varied roles of osteoclasts in maintaining skeletal health and to highlight the positive roles of osteoclasts in maintaining normal bone remodeling. Generally, osteoclasts interact with osteocytes to initiate targeted bone remodeling and have crosstalk with mesenchymal stem cells and osteoblasts via secreted factors or cell-cell contact to promote bone formation. We believe that a better outcome of bone remodeling disorders will be achieved when proper strategies are made to coordinate osteoclasts and osteoblasts in managing such disorders.
Project description:Osteocytes, >90% of the cells in bone, lie embedded within the mineralized matrix and coordinate osteoclast and osteoblast activity on bone surfaces by mechanisms still unclear. Bone anabolic stimuli activate Wnt signaling, and human mutations of components along this pathway underscore its crucial role in bone accrual and maintenance. However, the cell responsible for orchestrating Wnt anabolic actions has remained elusive. We show herein that activation of canonical Wnt signaling exclusively in osteocytes [dominant active (da)?cat(Ot) mice] induces bone anabolism and triggers Notch signaling without affecting survival. These features contrast with those of mice expressing the same daß-catenin in osteoblasts, which exhibit decreased resorption and perinatal death from leukemia. daßcat(Ot) mice exhibit increased bone mineral density in the axial and appendicular skeleton, and marked increase in bone volume in cancellous/trabecular and cortical compartments compared with littermate controls. daßcat(Ot) mice display increased resorption and formation markers, high number of osteoclasts and osteoblasts in cancellous and cortical bone, increased bone matrix production, and markedly elevated periosteal bone formation rate. Wnt and Notch signaling target genes, osteoblast and osteocyte markers, and proosteoclastogenic and antiosteoclastogenic cytokines are elevated in bones of daßcat(Ot) mice. Further, the increase in RANKL depends on Sost/sclerostin. Thus, activation of osteocytic ?-catenin signaling increases both osteoclasts and osteoblasts, leading to bone gain, and is sufficient to activate the Notch pathway. These findings demonstrate disparate outcomes of ?-catenin activation in osteocytes versus osteoblasts and identify osteocytes as central target cells of the anabolic actions of canonical Wnt/?-catenin signaling in bone.
Project description:Estrogen deficiency in menopause is a major cause of osteoporosis in women. Estrogen acts to maintain the appropriate ratio between bone-forming osteoblasts and bone-resorbing osteoclasts in part through the induction of osteoclast apoptosis. Recent studies have suggested a role for Fas ligand (FasL) in estrogen-induced osteoclast apoptosis by an autocrine mechanism involving osteoclasts alone. In contrast, we describe a paracrine mechanism in which estrogen affects osteoclast survival through the upregulation of FasL in osteoblasts (and not osteoclasts) leading to the apoptosis of pre-osteoclasts. We have characterized a cell-type-specific hormone-inducible enhancer located 86 kb downstream of the FasL gene as the target of estrogen receptor-alpha induction of FasL expression in osteoblasts. In addition, tamoxifen and raloxifene, two selective estrogen receptor modulators that have protective effects in bone, induce apoptosis in pre-osteoclasts by the same osteoblast-dependent mechanism. These results demonstrate that estrogen protects bone by inducing a paracrine signal originating in osteoblasts leading to the death of pre-osteoclasts and offer an important new target for the prevention and treatment of osteoporosis.
Project description:Bone remodeling in health and disease is carried out by osteoblasts and osteoclasts, which respectively produce bone matrix and resorb it. Endocrine and paracrine control of these cells can be direct, but they are also exerted indirectly, either by influencing progenitor cell differentiation or by stimulating paracrine signals from local accessory cells including osteocytes (which form a critical communication and regulation network within the bone matrix), macrophages and T lymphocytes. Here we review the osteotropic actions of the interleukin-6 family member cytokine oncostatin M (OSM), which is of particular interest because of its ability to stimulate bone accrual. OSM is produced within the bone microenvironment by cells of both mesenchymal and hematopoietic origin, including osteocytes, osteoblasts, macrophages and T lymphocytes, and can act via two receptor complexes: OSM receptor:gp130 and leukemia inhibitory factor receptor (LIFR):gp130. Although OSM can directly stimulate osteoblast mineralization activity and differentiation, it can also stimulate mesenchymal stem cell osteoblastic commitment at the expense of adipogenesis. In osteocytes, OSM can suppress the production of the bone formation inhibitor sclerostin, an action that is mediated by LIFR:gp130. OSM also stimulates the production of receptor activator of nuclear factor κB ligand by osteoblasts and thereby drives the formation of osteoclasts particularly in pathological conditions. Thus, cellular effects of OSM on bone metabolism include direct and indirect actions mediated by two related receptor/ligand complexes. OSM therefore provides an example of paracrine and endocrine control mechanisms that regulate bone mass by controlling both bone formation and resorption.
Project description:Collagen triple helix repeat-containing1 (Cthrc1) has previously been implicated in osteogenic differentiation and positive regulation of bone mass, however, the underlying mechanisms remain unclear. Here we characterized the bone phenotype of a novel Cthrc1 null mouse strain using bone histomorphometry, ?CT analysis and functional readouts for bone strength. In male Cthrc1 null mice both trabecular bone as well as cortical bone formation was impaired, whereas in female Cthrc1 null mice only trabecular bone parameters were altered. Novel and highly specific monoclonal antibodies revealed that CTHRC1 is expressed by osteocytes and osteoblasts, but not osteoclasts. Furthermore, Cthrc1 null mice exhibited increased bone resorption with increased number of osteoclast and increased osteoclast activity together with enhanced expression of osteoclastogenic genes such as c-Fos, Rankl, Trap, and Nfatc1. Differentiation of bone marrow-derived monocytes isolated from Cthrc1 null mice differentiated into osteoclasts as effectively as those from wildtype mice. In the presence of CTHRC1 osteoclastogenic differentiation of bone marrow-derived monocytes was dramatically inhibited as was functional bone resorption by osteoclasts. This process was accompanied by downregulation of osteoclastogenic marker genes, indicating that extrinsically derived CTHRC1 is required for such activity. In vitro, CTHRC1 had no effect on osteogenic differentiation of bone marrow stromal cells, however, calvarial osteoblasts from Cthrc1 null mice exhibited reduced osteogenic differentiation compared to osteoblasts from wildtypes. In a collagen antibody-induced arthritis model Cthrc1 null mice suffered significantly more severe inflammation and joint destruction than wildtypes, suggesting that CTHRC1 expressed by the activated synoviocytes has anti-inflammatory effects. Mechanistically, we found that CTHRC1 inhibited NF?B activation by preventing I?B? degradation while also inhibiting ERK1/2 activation. Collectively our studies demonstrate that CTHRC1 secreted from osteocytes and osteoblasts functions as an inhibitor of osteoclast differentiation via inhibition of NF?B-dependent signaling. Furthermore, our data suggest that CTHRC1 has potent anti-inflammatory properties that limit arthritic joint destruction.
Project description:Bone integrity is maintained by a dynamic equilibrium between the activities of bone-forming osteoblasts and bone-resorbing osteoclasts. Osteolytic lesions are a painful consequence of metastasis of breast cancer cells to bone in an overwhelming majority of breast cancer patients. Factors secreted by breast cancer cells propel a cascade of events that trigger osteoclastogenesis and elevated bone resorption. In the present study, we show that the Hedgehog (Hh) ligands secreted by breast cancer cells promote osteoclast differentiation and potentiate the activity of mature osteoclasts. Paracrine Hh signaling induced by breast cancer cells mediates a detrimental chain of events by the up-regulation of osteopontin (OPN), which in turn enhances osteoclastic activity by up-regulating cathepsin K and MMP9. Hh signaling is essential for osteoclasts because blocking the Hh pathway using the pharmacological Hh inhibitor, cyclopamine, results in an overall decrease in osteoclastogenesis and resorptive activity. Our studies suggest that inhibiting Hh signaling interferes with the ability of pre-osteoclasts to respond to the stimulatory effects of the breast cancer cells, indicating that Hh signaling is vital to osteoclast activity.
Project description:The ongoing research on biomaterials that support bone regeneration led to the quest for materials or material modifications that can actively influence the activity or balance of bone tissue cells. The bone biocompatibility of porous chitosan scaffolds was modified in the present study by the addition of calcium phosphates or hemocyanin. The first strategy comprised the incorporation of calcium phosphates into chitosan to create a biomimetic chitosan-mineral phase composite. The second strategy comprised dip-coating of chitosan scaffolds with hemocyanin extracted from crayfish hemolymph. The cytocompatibility was assessed in a mono-culture of human bone marrow stromal cells (hBMSCs) and their differentiation to osteoblasts; in a mono-culture of human monocytes (hMs) and their maturation to osteoclasts; and in a co-culture of hBMSC/osteoblasts-hM/osteoclasts. Mineral incorporation caused an increase in scaffold bioactivity, as shown by reduced calcium concentration in the cell culture medium, delayed differentiation of hBMSCs, and reduced osteoclastic maturation of hMs in mono-culture. Dip-coating with hemocyanin led to increased proliferation of hBMSCs and equivalent osteoclast maturation in mono-culture, while in co-culture, both an inhibitory effect of mineral incorporation on osteoblastogenesis and stimulatory effects of hemocyanin were observed. It was concluded that highly bioactive scaffolds (containing mineral phases) restrain osteoblast and osteoclast development, while hemocyanin coating significantly supports osteoblastogenesis. These influences on the osteoblasts/osteoclasts activity ratio may support scaffold-driven bone healing in the future.
Project description:Joint pain is the defining symptom of osteoarthritis (OA) but its origin and mechanisms remain unclear. Here, we investigated an unprecedented role of osteoclast-initiated subchondral bone remodeling in sensory innervation for OA pain. We show that osteoclasts secrete netrin-1 to induce sensory nerve axonal growth in subchondral bone. Reduction of osteoclast formation by knockout of receptor activator of nuclear factor kappa-B ligand (Rankl) in osteocytes inhibited the growth of sensory nerves into subchondral bone, dorsal root ganglion neuron hyperexcitability, and behavioral measures of pain hypersensitivity in OA mice. Moreover, we demonstrated a possible role for netrin-1 secreted by osteoclasts during aberrant subchondral bone remodeling in inducing sensory innervation and OA pain through its receptor DCC (deleted in colorectal cancer). Importantly, knockout of Netrin1 in tartrate-resistant acid phosphatase-positive (TRAP-positive) osteoclasts or knockdown of Dcc reduces OA pain behavior. In particular, inhibition of osteoclast activity by alendronate modifies aberrant subchondral bone remodeling and reduces innervation and pain behavior at the early stage of OA. These results suggest that intervention of the axonal guidance molecules (e.g., netrin-1) derived from aberrant subchondral bone remodeling may have therapeutic potential for OA pain.
Project description:During osteoporosis bone formation by osteoblasts is reduced and/or bone resorption by osteoclasts is enhanced. Currently, only a few factors have been identified in the regulation of bone integrity by osteoblast-derived osteocytes. In this study, we show that specific disruption of menin, encoded by multiple endocrine neoplasia type 1 (Men1), in osteoblasts and osteocytes caused osteoporosis despite the preservation of osteoblast differentiation and the bone formation rate. Instead, an increase in osteoclast numbers and bone resorption was detected that persisted even when the deletion of Men1 was restricted to osteocytes. We demonstrate that isolated Men1-deficient osteocytes expressed numerous soluble mediators, such as C-X-C motif chemokine 10 (CXCL10), and that CXCL10-mediated osteoclastogenesis was reduced by CXCL10-neutralizing antibodies. Collectively, our data reveal a novel role for Men1 in osteocyte-osteoclast crosstalk by controlling osteoclastogenesis through the action of soluble factors. A role for Men1 in maintaining bone integrity and thereby preventing osteoporosis is proposed.