RESUMEN
Endothelial progenitors found among the peripheral blood (PB) mononuclear cells (MNCs) are interesting cells for their angiogenic properties. Mesenchymal stromal cells (MSCs) in turn can produce proangiogenic factors as well as differentiate into mural pericytes, making MSCs and MNCs an attractive coculture setup for regenerative medicine. In this study, human bone marrow-derived MSCs and PB-derived MNCs were cocultured in basal or osteoblastic medium without exogenously supplied growth factors to demonstrate endothelial cell, pericyte and osteoblastic differentiation. The expression levels of various proangiogenic factors, as well as endothelial cell, pericyte and osteoblast markers in cocultures were determined by quantitative polymerase chain reaction. Immunocytochemistry for vascular endothelial growth factor receptor-1 and α-smooth muscle actin as well as staining for alkaline phosphatase were performed after 10 and 14 days. Messenger ribonucleic acid expression of endothelial cell markers was highly upregulated in both basal and osteoblastic conditions after 5 days of coculture, indicating an endothelial cell differentiation, which was supported by immunocytochemistry for vascular endothelial growth factor receptor-1. Stromal derived factor-1 and vascular endothelial growth factor were highly expressed in MSC-MNC coculture in basal medium but not in osteoblastic medium. On the contrary, the expression levels of bone morphogenetic protein-2 and angiopoietin-1 were significantly higher in osteoblastic medium. Pericyte markers were highly expressed in both cocultures after 5 days. In conclusion, it was demonstrated endothelial cell and pericyte differentiation in MSC-MNC cocultures both in basal and osteoblastic medium indicating a potential for neovascularization for tissue engineering applications.
Asunto(s)
Inductores de la Angiogénesis/metabolismo , Diferenciación Celular , Células Endoteliales/citología , Leucocitos Mononucleares/metabolismo , Células Madre Mesenquimatosas/metabolismo , Neovascularización Fisiológica , Pericitos/citología , Biomarcadores/metabolismo , Diferenciación Celular/efectos de los fármacos , Forma de la Célula/efectos de los fármacos , Células Cultivadas , Técnicas de Cocultivo , Medios de Cultivo , Células Endoteliales/efectos de los fármacos , Regulación de la Expresión Génica/efectos de los fármacos , Humanos , Leucocitos Mononucleares/efectos de los fármacos , Células Madre Mesenquimatosas/efectos de los fármacos , Neovascularización Fisiológica/efectos de los fármacos , Osteoblastos/citología , Osteoblastos/efectos de los fármacos , Osteoblastos/metabolismo , Pericitos/efectos de los fármacos , ARN Mensajero/genética , ARN Mensajero/metabolismo , Receptor beta de Factor de Crecimiento Derivado de Plaquetas/metabolismo , Factor de Crecimiento Transformador beta/metabolismo , Factor A de Crecimiento Endotelial Vascular/farmacología , Receptor 1 de Factores de Crecimiento Endotelial Vascular/genética , Receptor 1 de Factores de Crecimiento Endotelial Vascular/metabolismo , Factor de von Willebrand/genética , Factor de von Willebrand/metabolismoRESUMEN
The development of in vitro culturing techniques for osteoblastic differentiation of human mesenchymal stem cells (hMSC) is important for cell biology research and the development of tissue-engineering applications. Dexamethasone (Dex) is a commonly used supplement, but the optimal use of Dex treatment is still unclear. By adjusting the timing of Dex supplementation, the negative effects of long-term Dex treatment could be overcome. Transient Dex treatment could contribute toward minimizing broad donor variation, which is a major challenge. We compared the two most widely used Dex concentrations of 10 and 100 nM as transient or continuous treatment and studied inter- and intraindividual variations in osteoblastic differentiation of hMSC. Characterized bone marrow-derived hMSC from 17 female donors of different age groups were used. During osteoblastic induction, the cells were treated with 10 or 100 nM Dex either transiently for different time periods or continuously. Differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and staining for ALP, von Kossa, collagen type I, and osteocalcin. Cell proliferation, cell viability, and apoptosis were also monitored. The strongest osteoblastic differentiation was observed when 100 nM Dex was present for the first week. In terms of inter- and intraindividual coefficients of variations, transient treatment with 100 nM Dex was superior to the other culture conditions and showed the lowest variations in all age groups. This study demonstrates that the temporary presence of 100 nM Dex during the first week of induction culture promotes hMSC osteoblastic differentiation and reduces inter- and intraindividual variations. With this protocol, we can reproducibly produce functional osteoblasts in vitro from the hMSC of different donor populations.
Asunto(s)
Células de la Médula Ósea/citología , Técnicas de Cultivo de Célula/métodos , Dexametasona/farmacología , Células Madre Mesenquimatosas/citología , Osteoblastos/citología , Adulto , Fosfatasa Alcalina/metabolismo , Apoptosis , Diferenciación Celular , Proliferación Celular , Supervivencia Celular , Colágeno Tipo I/metabolismo , Femenino , Glucocorticoides/farmacología , Humanos , Persona de Mediana Edad , Osteoblastos/metabolismo , Osteocalcina/biosíntesis , Ingeniería de Tejidos/métodosRESUMEN
We examined the presence of circulating plastic adherent multipotent mesenchymal stem cells (MSCs) in fracture patients. Three patient groups (n = 10-18) were evaluated, including elderly females with a femoral neck fracture treated with cemented hemiarthroplasty, an age- and sex-matched group with hip osteoarthritis (OA) treated with cemented total hip arthroplasty (THA), and younger adults with surgically treated lower extremity fractures. The presence of circulating MSCs pre- and postoperatively was compared to bone marrow (BM) MSCs from the same subjects. Criteria for identifying MSCs included cell surface markers (CD105+, CD73+, CD90+, CD45-, CD14-), proliferation through several passages as well as osteogenic, chondrogenic, and adipogenic differentiation. Plastic adherent MSCs were found in peripheral blood (PB) from 22% of hip fracture patients, 46% of younger fracture patients, and in none of 63 pre- and postmenopausal women with hip OA. When detectable, circulating MSCs appeared between 39 and 101 h after fracture. PB derived MSCs did not differ from BM derived MSCs, except for a small population (<15%) of CD34+ cells among PB derived MSCs. This initial study indicates mobilization of MSCs into the circulation in response to fracture, even in very old patients, while circulating MSCs were not detectable before or after elective THA.
Asunto(s)
Fracturas del Cuello Femoral/sangre , Curación de Fractura/fisiología , Fracturas Óseas/sangre , Células Madre Mesenquimatosas/citología , Osteoartritis de la Cadera/sangre , Adulto , Anciano , Anciano de 80 o más Años , Artroplastia de Reemplazo de Cadera , Diferenciación Celular , Femenino , Fracturas del Cuello Femoral/patología , Fracturas Óseas/patología , Humanos , Masculino , Persona de Mediana Edad , Osteoartritis de la Cadera/patologíaRESUMEN
Bone is an elementary component in the human skeleton. It protects vital organs, regulates calcium levels and allows mobility. As a result of daily activities, bones are cyclically strained causing microdamage. This damage, in the form of numerous microcracks, can cause bones to fracture and therefore poses a threat to mechanical integrity. Bone is able to repair the microcracks through a process called remodelling which is tightly regulated by bone forming and resorbing cells. However, the manner by which microcracks are detected, and repair initiated, has not been elucidated until now. Here we show that microcrack accumulation causes damage to the network of cellular processes, resulting in the release of RANKL which stimulates the differentiation of cells specialising in repair.
Asunto(s)
Remodelación Ósea/fisiología , Huesos/patología , Curación de Fractura/fisiología , Animales , Resorción Ósea/metabolismo , Resorción Ósea/patología , Huesos/citología , Huesos/metabolismo , Huesos/fisiología , Diferenciación Celular/fisiología , Fracturas por Estrés/metabolismo , Fracturas por Estrés/patología , Humanos , Osteogénesis/fisiología , Osteopetrosis/metabolismo , Osteopetrosis/patología , Osteoprotegerina/metabolismo , Ligando RANK/biosíntesis , Ligando RANK/metabolismo , Transducción de Señal , Regulación hacia ArribaRESUMEN
BACKGROUND: Macrophage inhibitory cytokine-1 (MIC-1) belongs to the bone morphogenic protein/transforming growth factor-beta (BMP/TGF-beta) superfamily. Serum MIC-1 concentrations are elevated in patients with advanced prostate cancer. The effects of MIC-1 on prostate cancer bone metastases are unknown. METHODS: In vitro effects of MIC-1 on osteoblast differentiation and activity were analyzed with alkaline phosphatase and mineralization assays; osteoclast numbers were counted microscopically. MIC-1 effects on TLR9 expression were studied with Western blotting. Human Du-145 prostate cancer cells were stably transfected with a cDNA encoding for mature MIC-1 or with an empty vector. The in vivo growth characteristics of the characterized cells were studied with the intra-tibial model of bone metastasis. Tumor associated bone changes were viewed with X-rays, histology, and histomorphometry. Bone formation was assayed by measuring serum PINP. RESULTS: MIC-1 induced osteoblast differentiation and activity and osteoclast formation in vitro. These effects were independent of TLR9 expression, which was promoted by MIC-1. Both MIC-1 and control tumors induced mixed sclerotic/lytic bone lesions, but MIC-1 increased the osteolytic component of tumors. Osteoclast formation at the tumor-bone interface was significantly higher in the MIC-1 tumors, whereas bone formation was significantly higher in the control mice. At sacrifice, the mice bearing MIC-1 tumors were significantly lighter with significantly smaller tumors. CONCLUSIONS: MIC-1 up-regulates TLR9 expression in various cells. MIC-1 stimulates both osteoblast and osteoclast differentiation in vitro, independently of TLR9. MIC-1 over-expressing prostate cancer cells that grow in bone induce osteoclast formation and cachexia.
Asunto(s)
Neoplasias Óseas/secundario , Regulación Neoplásica de la Expresión Génica , Factor 15 de Diferenciación de Crecimiento/genética , Metástasis de la Neoplasia/genética , Osteoclastos/fisiología , Neoplasias de la Próstata/patología , Pérdida de Peso/genética , Animales , Neoplasias Óseas/genética , Neoplasias Óseas/patología , Línea Celular , Línea Celular Tumoral , Humanos , Masculino , Ratones , Ratones Noqueados , Osteoblastos/fisiología , Receptor Toll-Like 9/deficiencia , Receptor Toll-Like 9/genéticaRESUMEN
Mesenchymal stem cells (MSCs) are multipotent cells that arise from the mesenchyme during development. They reside in the bone marrow close to hematopoietic stem cell niches allowing them to maintain bone marrow homeostasis and to regulate the maturation of both hematopoietic and non-hematopoietic cells. MSCs possess an extensive potential to proliferate and differentiate e.g. into osteoblasts, osteocytes, adipocytes and chondrocytes. Nevertheless, there still are some open questions about the complex process of MSC differentiation involving different transcription factors and signaling pathways, which will be discussed in this review. We also shortly introduce the characteristics and function of bone-forming osteoblasts and their role in angiogenesis. MSCs are of interest in clinical applications, since they can be easily isolated from bone marrow aspirates and expanded in vitro. When the source of osteoprogenitors is compromised, cell-based therapies could provide a novel way to repair bone defects. Indeed, there is an increasing interest in the use of MSCs and more differentiated cells in clinical applications for bone repair, which will be introduced in this review. A major section of the review is dedicated to the functions of osteocytes in the regulation of bone remodeling. Finally, we present an original hypothesis about the possible role of osteocytes in future bone tissue engineering.
Asunto(s)
Diferenciación Celular , Células Madre Mesenquimatosas/citología , Osteoblastos/citología , Osteocitos/citología , Animales , Huesos/irrigación sanguínea , Huesos/citología , Humanos , Ingeniería de TejidosRESUMEN
The osteocyte is the most abundant cell type in bone and is embedded in mineralized bone matrix. Osteocytes are still poorly characterized because of their location and the lack of primary osteocyte isolation methods. Data on the cell biology of osteocytes is especially limited. We have isolated primary osteocytes from rat cortical bone by applying repeated enzymatic digestion and decalcification. The isolated osteocytes expressed typical osteocytic morphology with cell-cell contacts via long protrusions after a 1-day culture. These cells were negative or faintly positive for alkaline phosphatase but expressed high levels of osteocalcin, PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome), and DMP1 (dentin matrix protein 1). These cells also revealed patchy membrane staining for connexin43. For studying the function of gap junctions in isolated osteocytes, we microinjected rhodamine-labeled dextran (MW: 10,000) and Lucifer yellow (MW: 457) and found that Lucifer yellow was rapidly transmitted to several surrounding cells, whereas dextran remained in the injected cells. Heptanol and 18alpha-glycyrrhetinic acid inhibited the transfer of Lucifer yellow. This clearly showed the existence of functional gap junctions in cultured osteocytes. Enveloped viruses, such as vesicular stomatitis virus and influenza A virus, were used for studying cell polarity. We were unable to demonstrate plasma membrane polarization with enveloped viruses in isolated primary osteocytes in culture. Our results suggest that osteocytes do not possess apical and basolateral plasma membrane domains as do osteoblasts, which are their precursors.
Asunto(s)
Uniones Comunicantes/metabolismo , Osteocitos/citología , Osteocitos/metabolismo , Fosfatasa Alcalina/metabolismo , Animales , Animales Recién Nacidos , Membrana Celular/metabolismo , Polaridad Celular , Separación Celular , Células Cultivadas , Conexina 43/metabolismo , Proteínas de la Matriz Extracelular/metabolismo , Ácido Glicirretínico/farmacología , Heptanol/farmacología , Isoquinolinas/metabolismo , Glicoproteínas de Membrana/metabolismo , Metaloendopeptidasas/metabolismo , Osteocalcina/metabolismo , Osteocitos/virología , Endopeptidasa Neutra Reguladora de Fosfato PHEX , Fosfoproteínas/metabolismo , Estructura Terciaria de Proteína , Ratas , Ratas Sprague-Dawley , Proteínas Virales de Fusión/metabolismoRESUMEN
UNLABELLED: Using human peripheral blood CD14(+) osteoclast precursors, we show that testosterone directly inhibits osteoclast formation and bone resorption at physiological concentrations. Instead, estrogen has no direct effects, whereas its action seems to be mediated through osteoblasts by producing osteoprotegerin. Both estrogen and testosterone acts through their cognate receptors. INTRODUCTION: Estrogen (E2) deficiency is associated with both the development of postmenopausal and senile form of osteoporosis in elderly women. Testosterone (Te) deficiency, on the other hand, may cause osteoporosis in men. In both sexes, osteoporosis is associated with disturbed bone turnover, including increased bone resorption caused by enhanced osteoclast formation and increased osteoclast activity. However, the mechanisms by which E2 or Te act on bone are not fully understood, and one of the central questions is whether these hormones act directly on osteoclast precursors or whether their action is mediated through osteoblastic cells. MATERIALS AND METHODS: We cultured human peripheral blood CD14(+) osteoclast precursors in the presence of RANKL, macrophage-colony stimulating factor (M-CSF), TNF-alpha, and dexamethasone to induce them to differentiate into osteoclasts. To study the possible osteoblast-mediated effects, osteoclast precursors were also co-cultured either with human MG-63 or SaOS-2 osteoblast-derived osteosarcoma cells. These cultures were treated with 10(-8)-10(-12) M of E2 or Te for 7 days. RESULTS: E2 did not have any direct effect on osteoclast formation, whereas testosterone inhibited osteoclast formation and bone resorption in a dose-dependent manner. In co-cultures, where MG-63 or SaOS-2 cells were present, E2 and Te inhibited osteoclast formation in a dose-dependent manner. At the same time, E2 and Te treatment in MG-63 or SaOS-2 cell-containing cultures stimulated significantly the formation of osteoprotegerin (OPG) compared with untreated cultures measured by ELISA assay from the culture medium. The effects of E2 and Te on osteoclast formation and bone resorption were completely antagonized by an E2 receptor (ER) antagonist, ICI 182,780, and an androgen receptor (AR) antagonist, flutamide, suggesting ER- and AR-mediated mechanisms, respectively, in these cultures. CONCLUSIONS: Te is likely to have direct and indirect inhibitory effects on human osteoclast formation and bone resorption, whereas the effect of E2 on osteoclast precursors and osteoclasts seems to be mediated by osteoblastic cells. Inhibitory effect of E2 is associated with the stimulated secretion of OPG by osteoblast-derived osteosarcoma cells. Mechanism of action of E2 and Te is mediated by ER and AR, respectively.
Asunto(s)
Diferenciación Celular/efectos de los fármacos , Estrógenos/farmacología , Osteoclastos/efectos de los fármacos , Testosterona/farmacología , Fosfatasa Ácida/análisis , Adulto , Antagonistas de Andrógenos/farmacología , Animales , Resorción Ósea/metabolismo , Resorción Ósea/patología , Resorción Ósea/prevención & control , Proteínas Portadoras/farmacología , Bovinos , Diferenciación Celular/fisiología , Línea Celular Tumoral , Técnicas de Cocultivo , Dexametasona/farmacología , Relación Dosis-Respuesta a Droga , Estradiol/análogos & derivados , Estradiol/farmacología , Antagonistas de Estrógenos/farmacología , Flutamida/farmacología , Fulvestrant , Glicoproteínas/metabolismo , Humanos , Isoenzimas/análisis , Receptores de Lipopolisacáridos/análisis , Factor Estimulante de Colonias de Macrófagos/farmacología , Masculino , Glicoproteínas de Membrana/farmacología , Monocitos/química , Monocitos/citología , Monocitos/efectos de los fármacos , Osteoclastos/citología , Osteoclastos/metabolismo , Osteoprotegerina , Ligando RANK , Receptor Activador del Factor Nuclear kappa-B , Receptores Citoplasmáticos y Nucleares/metabolismo , Receptores del Factor de Necrosis Tumoral/metabolismo , Fosfatasa Ácida Tartratorresistente , Factor de Necrosis Tumoral alfa/farmacologíaRESUMEN
Osteocytes have been suggested to play a role in the regulation of bone resorption, although their effect on bone turnover has remained controversial. In order to study this open question, we developed an organ culture system based on isolated rat calvaria, where the osteocyte viability and its effect on osteoclastic bone resorption can be monitored. Our results suggest that osteocytes are constitutively negative regulators of osteoclastic activity. Osteoclasts, which were cultured on calvarial slices with living osteocytes inside, failed to form actin rings which are the hallmarks of resorbing cells. A similar inhibitory effect was also achieved by the conditioned medium obtained from calvarial organ culture, suggesting that living osteocytes produce yet unrecognized osteoclast inhibitors. On the contrary, when osteocyte apoptosis was induced, this inhibitory effect disappeared and strong osteoclastic bone resorption activity was observed. Thus, local apoptosis of osteocytes may play a major role in triggering local bone remodeling.
Asunto(s)
Apoptosis , Resorción Ósea/patología , Resorción Ósea/fisiopatología , Osteoclastos/patología , Osteocitos/patología , Cráneo/patología , Cráneo/fisiopatología , Animales , Comunicación Celular , Células Cultivadas , Técnicas de Cocultivo/métodos , Ratas , Ratas Sprague-Dawley , MujeresRESUMEN
Osteocytes are the most abundant cells in bone and there is increasing evidence that they control bone remodeling via direct cell-to-cell contacts and by soluble factors. In the present study, we have used the MLO-Y4 cell line to study the effect of osteocytes on the proliferation, differentiation and bone-forming capacity of bone marrow mesenchymal stem cells (MSC). Conditioned media (CM) from osteocytic MLO-Y4 and osteoblastic MC3T3-E1 cell lines were collected and added on mouse bone marrow cultures, in which MSC were induced to osteoblasts. There was a significant increase in alkaline phosphatase activity and osteocalcin expression in the presence of MLO-Y4 CM. No such stimulus could be observed with MC3T3-E1 CM. There was almost 4-fold increase in bone formation and up to 2-fold increase in the proliferation of MSC with MLO-Y4 CM. The highly proliferating bone marrow cells were negative for ALP and OCN, suggesting that they could represent early osteoblast precursors. MLO-Y4 CM did not enhance the viability of mature osteoblasts nor protected them of apoptosis. This is the first study to describe soluble signals between osteocytes and osteoblasts and there most likely are several still unidentified or unknown factors in osteocyte CM. We conclude that osteocytes have an active stimulatory role in controlling bone formation.
Asunto(s)
Diferenciación Celular/fisiología , Sustancias de Crecimiento/metabolismo , Células Madre Mesenquimatosas/metabolismo , Osteoblastos/metabolismo , Osteocitos/metabolismo , Osteogénesis/fisiología , Fosfatasa Alcalina/efectos de los fármacos , Fosfatasa Alcalina/metabolismo , Animales , Células de la Médula Ósea/citología , Células de la Médula Ósea/efectos de los fármacos , Células de la Médula Ósea/metabolismo , Diferenciación Celular/efectos de los fármacos , División Celular/efectos de los fármacos , División Celular/fisiología , Linaje de la Célula/efectos de los fármacos , Linaje de la Célula/fisiología , Supervivencia Celular/efectos de los fármacos , Supervivencia Celular/fisiología , Células Cultivadas , Medios de Cultivo Condicionados/farmacología , Medio de Cultivo Libre de Suero/farmacología , Sustancias de Crecimiento/farmacología , Células Madre Mesenquimatosas/citología , Células Madre Mesenquimatosas/efectos de los fármacos , Ratones , Modelos Biológicos , Osteoblastos/citología , Osteoblastos/efectos de los fármacos , Osteocalcina/efectos de los fármacos , Osteocalcina/metabolismo , Osteocitos/citología , Osteogénesis/efectos de los fármacos , Regulación hacia Arriba/efectos de los fármacos , Regulación hacia Arriba/fisiologíaRESUMEN
Osteocalcin detected from serum samples is considered a specific marker of osteoblast activity and bone formation rate. However, osteocalcin embedded in bone matrix must also be released during bone resorption. To understand the contribution of each type of bone cell in circulating osteocalcin levels, we used immunoassays detecting different molecular forms of osteocalcin to monitor bone resorption in vitro. Osteoclasts were obtained from rat long bones and cultured on bovine bone slices using osteocalcin-depleted fetal bovine serum. In addition, human osteoclasts differentiated from peripheral blood mononuclear cells were used. Both rat and human osteoclasts released osteocalcin from bovine bone into medium. The amount of osteocalcin increased in the presence of parathyroid hormone, a stimulator of resorption, and decreased in the presence of bafilomycin A1, an inhibitor of resorption. The amount of osteocalcin in the medium correlated with a well characterized marker of bone resorption, the C-terminal telopeptide of type I collagen (r > 0.9, p < 0.0001). The heterogeneity of released osteocalcin was determined using reverse phase high performance liquid chromatography, and several molecular forms of osteocalcin, including intact molecule, were identified in the culture medium. In conclusion, osteocalcin is released from the bone matrix during bone resorption as intact molecules and fragments. In addition to the conventional use as a marker of bone formation, osteocalcin can be used as a marker of bone resorption in vitro. Furthermore, bone matrix-derived osteocalcin may contribute to circulating osteocalcin levels, suggesting that serum osteocalcin should be considered as a marker of bone turnover rather than bone formation.
Asunto(s)
Matriz Ósea/metabolismo , Resorción Ósea/metabolismo , Osteocalcina/análisis , Animales , Biomarcadores/análisis , Huesos/citología , Huesos/metabolismo , Bovinos , Técnicas de Cocultivo , Humanos , Inmunoensayo/normas , Macrólidos/farmacología , Osteocalcina/metabolismo , Osteoclastos/citología , Osteoclastos/fisiología , Hormona Paratiroidea/farmacología , Fragmentos de Péptidos/análisis , Fragmentos de Péptidos/metabolismo , Ratas , Ratas Sprague-DawleyRESUMEN
Osteocytes are the most abundant cells in bone and distributed throughout the bone matrix. They are connected to the each other and to the cells on the bone surface. Thus, they may also secrete some regulatory factors controlling bone remodeling. Using a newly established osteocyte-like cell line MLO-Y4, we have studied the interactions between osteocytes and osteoclasts. We collected the conditioned medium (CM) from MLO-Y4 cells, and added it into the rat osteoclast cultures. The conditioned medium had no effect on osteoclast number in 24-h cultures, but it dramatically inhibited resorption. With 5, 10, and 20% CM, there was 25, 39, and 42% inhibition of resorption, respectively. Interestingly, the inhibitory effect was even more pronounced, when MLO-Y4 cells were pretreated with 10(-8) M 17-beta-estradiol. With 5, 10, and 20% CM, there was 46, 51, and 58% of inhibition. When the conditioned medium was treated with neutralizing antibody against transforming growth factor-beta (TGF-beta), the inhibitory effect was abolished. This suggests that osteocytes secrete significant amounts of TGF-beta, which inhibits bone resorption and is modulated by estrogen. RT-PCR and Western blot analysis show that in MLO-Y4 cells, the prevalent TGF-beta isoform is TGF-beta3. We conclude that osteocytes have an active, inhibitory role in the regulation of bone resorption. Our results further suggest a novel role for TGF-beta in the regulation of communication between different bone cells and suggest that at least part of the antiresorptive effect of estrogen in bone could be mediated via osteocytes.