Hormonal changes affect the bone and bone marrow cells in a rat model

Estrogens as regulator of hematopoietic stem cell, immune cells and bone biology

Hematopoietic stem cells provide continuous supply of all the immune cells, through proliferation and differentiation decisions. These decisions are controlled by local bone marrow environment as well as by long-range signals for example endocrine system. Sex dependent differential immunological responses have been described under homeostasis and disease conditions. Females show higher longevity than male counterpart that seems to depend on major female sex hormone, estrogen. There are four estrogens - Estrone (E1), estradiol (E2), Estriol (E3) and Estetrol (E4) that spatially and temporarily present during different female reproductive phases. In this review, we discussed recent updates describing the effects of estrogen on HSC, immune cells and in bone biology. Estradiol (E2) being a major/abundant estrogen is extensively investigated, while effects of other estrogens E1, E3 and E4 are started to unravel recently. Furthermore, clinical effect of estrogen as hormone therapy is discussed in HSC and immune cells perspectives. The data presented in this review is compiled by searches of PubMed, database of American Cancer Society (ACS). We have included article from September 1994 to March 2020 as covering all article in chronological order is not fissile so we included relevant article with substantial information in this specific area of research by using the search term (alone or in combination) estrogen, hematopoietic stem cell, immune cells, gender difference, estrone, estriol, estetrol, therapeutic application, pregnancy, effect on bone.

Effects of bazedoxifene and low-intensity aerobic exercise on bone and fat parameters in ovariectomized rats

INTRODUCTION

Postmenopausal osteoporosis and dyslipidemia are well-known skeletal and metabolic changes in middle-aged women. We investigated the effects of combined treatments with a selective estrogen receptor modulator (SERM) and exercise on bone and fat parameters in ovariectomized (OVX) rats.

MATERIALS AND METHODS

Sixteen-week-old female Sprague-Dawley rats underwent bilateral ovariectomy, and rats were randomized to BZA (bazedoxifene at 0.3 mg/kg/day), Exe (treadmill exercise at 12-15 m/min, 60 min/day, 5 days/week), Comb (BZA and Exe), and Cont (control treated with vehicle and no exercise) groups 8 weeks after ovariectomy. After 4 or 8 weeks of treatment, bone mineral density (BMD) of the total femur and lumbar spine and whole-body percentage fat mass were determined by dual-energy X-ray absorptiometry, and mechanical testing of the femoral shaft, and bone and fat histomorphometric analyses of the proximal tibia were performed.

RESULTS

Treadmill exercise had decreased bone marrow adipocytes from 4 weeks of treatment and whole-body percentage fat mass at 8 weeks. BZA increased BMD at the lumbar spine and decreased the whole-body percentage fat mass from 4 weeks and bone marrow adipocytes at 8 weeks. Combination therapy increased BMD for the lumbar spine and decreased bone marrow adipocytes and whole-body percentage fat mass from 4 weeks.

CONCLUSION

Combination therapy with BZA and exercise appears effective to improve bone and fat parameters in OVX rats.

Erucic acid derived from rosemary regulates differentiation of mesenchymal stem cells into osteoblasts/adipocytes via suppression of peroxisome proliferator-activated receptor γ transcriptional activity

Osteoporosis is associated with increase in fat tissue in bone marrow in humans. Mesenchymal stem cells in bone marrow are induced to differentiate into osteoblasts rather than adipocytes by the stimulation of peroxisome proliferator-activated receptor (PPAR) γ antagonists. PPARγ antagonists are expected to be useful to prevent osteoporosis by regulating the lineages of mesenchymal stem cells in bone marrow, as well as the prevention of obesity. In this study, we explored natural components suppressing PPARγ transcriptional activity in rosemary. Separation of active fraction of rosemary extract by repeated high performance liquid chromatograph and PPARγ luciferase reporter assay identified erucic acid, one of the monounsaturated fatty acids, as an active component. Twenty-five-micrometer erucic acid significantly decreased PPARγ luciferase activity and enhanced the differentiation of mouse-delivered C3H10T1/2 cells into osteoblasts rather than adipocytes. Furthermore, 25-μM erucic acid significantly decreased the expression of adipocyte marker genes, while accelerating osteoblast marker genes. In conclusion, erucic acid is a novel natural component derived from rosemary regulating mesenchymal stem cell differentiation via suppression of PPARγ transcriptional activity.

Mechanical signals protect stem cell lineage selection, preserving the bone and muscle phenotypes in obesity

The incidence of obesity is rapidly rising, increasing morbidity and mortality rates worldwide. Associated comorbidities include type 2 diabetes, heart disease, fatty liver disease, and cancer. The impact of excess fat on musculoskeletal health is still unclear, although it is associated with increased fracture risk and a decline in muscular function. The complexity of obesity makes understanding the etiology of bone and muscle abnormalities difficult. Exercise is an effective and commonly prescribed nonpharmacological treatment option, but it can be difficult or unsafe for the frail, elderly, and morbidly obese. Exercise alternatives, such as low-intensity vibration (LIV), have potential for improving musculoskeletal health, particularly in conditions with excess fat. LIV has been shown to influence bone marrow mesenchymal stem cell differentiation toward higher-order tissues (i.e., bone) and away from fat. While the exact mechanisms are not fully understood, recent studies utilizing LIV both at the bench and in the clinic have demonstrated some efficacy. Here, we discuss the current literature investigating the effects of obesity on bone, muscle, and bone marrow and how exercise and LIV can be used as effective treatments for combating the negative effects in the presence of excess fat.

Extracorporeal shock waves alone or combined with raloxifene promote bone formation and suppress resorption in ovariectomized rats

Osteoporosis is a metabolic skeletal disease characterized by an imbalance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. We examined the beneficial effect of shock waves (SW) alone or in combination with raloxifene (RAL) on bone loss in ovariectomized rats (OVX). Sixteen weeks after surgery, OVX were treated for five weeks with SW at the antero-lateral side of the right hind leg, one session weekly, at 3 Hz (EFD of 0.33 mJ/mm²), or with RAL (5 mg/kg/die, per os) or with SW+RAL. Sera, femurs, tibiae and vertebrae were sampled for following biochemical and histological analysis. SW, alone or combined with RAL, prevented femur weight reduction and the deterioration of trabecular microarchitecture both in femur and vertebrae. All treatments increased Speed of Sound (SoS) values, improving bone mineral density, altered by OVX. Serum parameters involved in bone remodeling (alkaline phosphatase, receptor activator of nuclear factor kappa-B ligand, osteoprotegerin) and osteoblast proliferation (PTH), altered by ovariectomy, were restored by SW and RAL alone or in combination. In tibiae, SW+RAL significantly reduced cathepsin k and TNF-α levels, indicating the inhibition of osteoclast activity, while all treatments significantly increased runt-related transcription factor 2 and bone morphogenetic-2 expression, suggesting an increase in osteoblastogenic activity. Finally, in bone marrow from tibiae, SW or RAL reduced PPARγ and adiponectin transcription, indicating a shift of mesenchymal cells toward osteoblastogenesis, without showing a synergistic effect. Our data indicate SW therapy, alone and in combination with raloxifene, as an innovative strategy to limit the hypoestrogenic bone loss, restoring the balance between bone formation and resorption.

INF-γ encoding plasmid administration triggers bone loss and disrupts bone marrow microenvironment

IFN-γ is a pleotropic cytokine produced in the bone microenvironment. Although IFN-γ is known to play a critical role on bone remodeling, its function is not fully elucidated. Consistently, outcomes on the effects of IFN-γ recombinant protein on bone loss are contradictory among reports. In our work we explored, for the first time, the role of IFN-γ encoding plasmid (pIFN-γ) in a mouse model of osteopenia induced by ovariectomy and in the sham-operated counterpart to estimate its effects in skeletal homeostasis. Ovariectomy produced a dramatic decrease of bone mineral density (BMD). pINF-γ injected mice showed a pathologic bone and bone marrow phenotype; the disrupted cortical and trabecular bone microarchitecture was accompanied by an increased release of pro-inflammatory cytokine by bone marrow cells. Moreover, mesenchymal stem cells' (MSCs) commitment to osteoblast was found impaired, as evidenced by the decline of osterix-positive (Osx⁺) cells within the mid-diaphyseal area of femurs. For instance, a reduction and redistribution of CXCL12 cells have been found, in accordance with bone marrow morphological alterations. As similar effects were observed both in sham-operated and in ovariectomized mice, our studies proved that an increased IFN-γ synthesis in bone marrow might be sufficient to induce inflammatory and catabolic responses even in the absence of pathologic predisposing substrates. In addition, the obtained data might raise questions about pIFN-γ's safety when it is used as vaccine adjuvant.

Marrow adipogenesis and bone loss that parallels estrogen deficiency is slowed by low-intensity mechanical signals

Ovariectomized mice were used to assess the ability of low-intensity vibrations to protect bone microarchitecture and marrow composition. Results indicate that low-intensity vibrations (LIV), introduced 2 weeks postsurgery, slows marrow adipogenesis in OVX mice but does not restore the bone within the period studied. However, immediate application of LIV partially protects quality.

INTRODUCTION

The aim of this study was to evaluate consequences of estrogen depletion on bone marrow (BM) phenotype and bone microarchitecture, and effects of mechanical signals delivered as LIV on modulating these changes.

METHODS

LIV (0.3 g, 90 Hz) was applied to C57BL/6 mice immediately following ovariectomy or 2 weeks postestrogen withdrawal for 2 (ST-LIV) or 6 weeks (LT-LIV), respectively. Sham-operated age-matched controls (ST-AC, LT-AC) and ovariectomized controls (ST-OVX, LT-OVX) received sham LIV treatment. Bone microstructure was evaluated through μCT and BM adipogenesis through histomorphometry, serum markers, and genes expression analysis.

RESULTS

LT-OVX increased BM adipogenesis relative to LT-AC (+136 %, p ≤ 0.05), while LT-LIV introduced for 6w suppressed this adipose encroachment (-55 %, p ≤ 0.05). In parallel with the fatty marrow, LT-OVX showed a marked loss of trabecular bone, -40 % (p ≤ 0.05) in the first 2 weeks following ovariectomy compared to LT-AC. Application of LT-LIV for 6w following this initial 2w bone loss failed to restore the lost trabeculae but did initiate an anabolic response as indicated by increased serum alkaline phosphatase (+26 %, p ≤ 0.05). In contrast, application of LIV immediately following ovariectomy was more efficacious in the protection of trabecular bone, with a +29 % (p > 0.05) greater BV/TV compared to ST-OVX at the 2w time period.

CONCLUSIONS

LIV can mitigate adipocyte accumulation in OVX marrow and protect it by favoring osteoblastogenesis over adipogenesis. These data also emphasize the rapidity of bone loss with OVX and provide perspective in the timing of treatments for postmenopausal osteoporosis where sooner is better than later.

The alliance of mesenchymal stem cells, bone, and diabetes

Bone fragility has emerged as a new complication of diabetes. Several mechanisms in diabetes may influence bone homeostasis by impairing the action between osteoblasts, osteoclasts, and osteocytes and/or changing the structural properties of the bone tissue. Some of these mechanisms can potentially alter the fate of mesenchymal stem cells, the initial precursor of the osteoblast. In this review, we describe the main factors that impair bone health in diabetic patients and their clinical impact.

Determinants of bone marrow adiposity: the modulation of peroxisome proliferator-activated receptor-γ2 activity as a central mechanism

Although the presence of adipocytes in the bone marrow is a normal physiological phenomenon, the role of these cells in bone homeostasis and during pathological states has not yet been fully delineated. As osteoblasts and adipocytes originate from a common progenitor, with an inverse relationship existing between osteoblastogenesis and adipogenesis, bone marrow adiposity often negatively correlates with osteoblast number and bone mineral density. Bone adiposity can be affected by several physiological and pathophysiological factors, with abnormal, elevated marrow fat resulting in a pathological state. This review focuses on the regulation of bone adiposity by physiological factors, including aging, mechanical loading and growth factor expression, as well as the pathophysiological factors, including diseases such as anorexia nervosa and dyslipidemia, and pharmacological agents such as thiazolidinediones and statins. Although these factors regulate bone marrow adiposity via a plethora of different intracellular signaling pathways, these diverse pathways often converge on the modulation of the expression and/or activity of the pro-adipogenic transcription factor peroxisome proliferator-activated receptor (PPAR)-γ2, suggesting that any factor that affects PPAR-γ2 may have an impact on the fat content of bone.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

Failure to generate bone marrow adipocytes does not protect mice from ovariectomy-induced osteopenia

A reciprocal association between bone marrow fat and bone mass has been reported in ovariectomized rodents, suggesting that bone marrow adipogenesis has a negative effect on bone growth and turnover balance. Mice with loss of function mutations in kit receptor (kit(W/W-v)) have no bone marrow adipocytes in tibia or lumbar vertebra. We therefore tested the hypothesis that marrow fat contributes to the development of osteopenia by comparing the skeletal response to ovariectomy (ovx) in growing wild type (WT) and bone marrow adipocyte-deficient kit(W/W-v) mice. Mice were ovx at 4 weeks of age and sacrificed 4 or 10 weeks post-surgery. Body composition was measured at necropsy by dual-energy X-ray absorptiometry. Cortical (tibia) and cancellous (tibia and lumbar vertebra) bone architecture were evaluated by microcomputed tomography. Bone marrow adipocyte size and density, osteoblast- and osteoclast-lined bone perimeters, and bone formation were determined by histomorphometry. Ovx resulted in an increase in total body fat mass at 10 weeks post-ovx in both genotypes, but the response was attenuated in the in kit(W/W-v) mice. Adipocytes were present in bone marrow of tibia and lumbar vertebra in WT mice and bone marrow adiposity increased following ovx. In contrast, marrow adipocytes were not detected in either intact or ovx kit(W/W-v) mice. However, ovx in WT and kit(W/W-v) mice resulted in statistically indistinguishable changes in cortical and cancellous bone mass, cortical and cancellous bone formation rate, and cancellous osteoblast and osteoclast-lined bone perimeters. In conclusion, our findings do not support a causal role for increased bone marrow fat as a mediator of ovx-induced osteopenia in mice.

Blocking β-adrenergic signaling attenuates reductions in circulating leptin, cancellous bone mass, and marrow adiposity seen with dietary energy restriction

We tested whether β-adrenergic blockade attenuates bone loss and increased marrow adiposity during energy restriction (ER) and whether such an effect is associated with changes in serum leptin and leptin expression in bone and marrow tissues. Female 4-mo-old Sprague-Dawley rats were assigned into four groups (n = 10 each): two groups of 40% ER treated with vehicle (ERVEH; saline) or β-blocker (ERBB; DL-propranolol; 250 μg · kg(-1) · h(-1)) during 12 wk, and two groups of ad libitum-fed controls treated with the same two agents (CONVEH, CONBB, respectively). Over 84 days, CONVEH and CONBB rats gained but ERVEH and ERBB rats lost body fat mass; lean mass did not change in any group. Reduction in serum leptin in ERVEH rats was mitigated in ERBB rats (-5.32 vs. -1.15 ng/ml, respectively). The decline in proximal tibia cancellous vBMD observed in ERVEH rats was attenuated in ERBB rats (-85.24 vs. -53.94 mg/cm(3), respectively). Adipocyte number in ERVEH rats was dramatically higher vs. CON rats at week 12, but this increment was abolished by β-blockade in ERBB animals. The number of osteoblastic cells and marrow adipocytes staining positively for leptin in ERVEH rats tended to be lower vs. that of both CON groups, but β-blockade appears to reverse this effect in ERBB rats. In summary, β-adrenergic blockade mitigated metaphyseal bone loss and bone marrow adiposity during energy restriction and attenuated reductions in serum leptin. These data suggest an important role for β-adrenoreceptor signaling pathway in the cancellous bone and marrow fat response to energy restriction.

The influence of therapeutic radiation on the patterns of bone marrow in ovary-intact and ovariectomized mice

Background

The functional components of bone marrow (i.e., the hematopoietic and stromal populations) and the adjacent bone have traditionally been evaluated incompletely as distinct entities rather than the integrated system. We perturbed this system in vivo using a medically relevant radiation model in the presence or absence of ovarian function to understand integrated tissue interaction.

Methodology/Principal Findings

Ovary-intact and ovariectomized mice underwent either no radiation or single fractional 16 Gy radiation to the caudal skeleton (I±R, OVX±R). Marrow fat, hematopoietic cellularity, and cancellous bone volume fraction (BV/TV %) were assessed. Ovariectomy alone did not significantly reduce marrow cellularity in non-irradiated mice (OVX−R vs. I−R, p = 0.8445) after 30 days; however it impaired the hematopoietic recovery of marrow following radiation exposure (OVX+R vs. I+R, p = 0.0092). The combination of radiation and OVX dramatically increases marrow fat compared to either factor alone (p = 0.0062). The synergistic effect was also apparent in the reduction of hematopoietic marrow cellularity (p = 0.0661); however it was absent in BV/TV% changes (p = 0.2520). The expected inverse relationship between marrow adiposity vs. hematopoietic cellularity and bone volume was observed. Interestingly compared with OVX mice, intact mice demonstrated double the reduction in hematopoietic cellularity and a tenfold greater degree of bone loss for a given unit of expansion in marrow fat.

Conclusions/Significance

Ovariectomy prior to delivery of a clinically-relevant focal radiation exposure in mice, exacerbated post-radiation adipose accumulation in the marrow space but blunted bone loss and hematopoietic suppression. In the normally coupled homeostatic relationship between the bone and marrow domains, OVX appears to alter feedback mechanisms. Confirmation of this non-linear phenomenon (presumably due to differential radiosensitivity) and demonstration of the mechanism of action is needed to provide strategies to diminish the effect of radiation on exposed tissues.

Reduction by strontium of the bone marrow adiposity in mice and repression of the adipogenic commitment of multipotent C3H10T1/2 cells

Multipotent mesenchymal cells (MMCs) differentiate into osteoblasts or adipocytes through RUNX2 and PPARγ2, respectively. Strontium ranelate has been shown to promote osteoblastogenesis and prevent adipogenesis in long-term experiments using MMCs. The present study involved in-vitro and in-vivo investigations of whether Sr might first be an inhibitor of adipogenesis, thus explaining late osteoblastogenesis. It was established in vivo that Sr reduces adipogenesis in mice treated only for 3 weeks with a 6 mmol/kg/day dose of Sr while the trabecular bone volume is increased. In order to decipher molecular mechanisms during inhibition of adipogenesis, we used murine MMCs C3H10T1/2 cultured under adipogenic conditions (AD) and treated Sr of a concentration up to 3 mM. It was shown that early on (day 1), Sr dose-dependently reduced PPARγ2 and CEBPα mRNA without affecting the RUNX2 gene expression whereas it repressed ALP mRNA. Later (day 5), PPARγ2 and CEBPα mRNA remained inhibited by Sr, preventing adipocyte lipid accumulation, while Runx2 and ALP mRNA were increased. Moreover, under the mentioned conditions, Sr was able to quickly induce the Cyclin D1 gene expression, proliferation and fibronectin fibrillogenesis, both involved in the inhibition of adipogenesis. The inhibition of the ERK pathway by U0126 blunted the Sr-induced PPARγ2 repression while restoring the lipid accumulation. These results demonstrated that Sr was capable of rapidly reducing adipogenesis by a selective PPARγ2 repression that can be explained by its ability to promote MMC proliferation.

TGF-β mediates suppression of adipogenesis by estradiol through connective tissue growth factor induction

In the bone marrow cavity, adipocyte numbers increase, whereas osteoblast progenitor numbers decrease with aging. Because adipocytes and osteoblasts share a common progenitor, it is possible that this shift is due to an increase in adipocyte-lineage cells at the expense of osteoblast-lineage commitment. Estrogens inhibit adipocyte differentiation, and in both men and women, circulating estrogens correlate with bone loss with aging. In bone cells, estrogens stimulate expression of TGF-β and suppress mesenchymal cell adipogenesis. Using a tripotential mesenchymal cell line, we have examined whether estradiol suppression of adipocyte differentiation is due to stimulation of TGF-β and the mechanism by which TGF-β suppresses adipogenesis. We observed that estradiol-mediated suppression of adipogenic gene expression required at least 48 h treatment. TGF-β expression increased within 24 h of estradiol treatment, and TGF-β inhibition reversed estradiol influences on adipogenesis and adipocyte gene expression. Connective tissue growth factor (CTGF) mediates TGF-β suppression of adipogenesis in mouse 3T3-L1 cells. CTGF expression was induced within 24 h of TGF-β treatment, whereas estradiol-mediated induction required 48 h treatment. Moreover, estradiol-mediated induction of CTGF was abrogated by TGF-β inhibition. These data support that estradiol effects on adipogenesis involves TGF-β induction, which then induces CTGF to suppress adipogenesis.

Estrogen promotes B cell activation in vitro through down-regulating CD80 molecule expression

Estrogen is the main female hormone of women. It has diverse effects on cell growth, differentiation and homeostatic functions. Accumulated evidence has indicated that estrogen may regulate multiple immune functions and the immune status of women. However, there is little report on the effect of estrogen on mature B cell functions. In this study, we observed the effect of 17β-estradiol (E2) on the proliferation, apoptosis, antibody production and differentiation of splenic B cells of mice in vitro. Splenocytes of female BALB/c mice were isolated and cultured with E2. E2 treatments decreased the expression of CD80 molecule on splenic B cells but enhanced the total IgG antibody production of splenocyte, without promoting the differentiation of B cells to plasma cells. E2 protected splenic B cells from the serum-deficiency-induced apoptosis but had no influence on the proliferation of B cells. These results suggest that estrogen may promote the activity of B cells through down-regulating the expression of CD80 molecule on B cells.

Dietary supplementation with dried plum prevents ovariectomy-induced bone loss while modulating the immune response in C57BL/6J mice

This study was designed to investigate the effects of dried plum on the changes in bone metabolism and the immune response associated with ovarian hormone deficiency. Adult female C57BL/6J mice were either sham-operated (Sham) and fed AIN-93 diet (control) or ovariectomized (OVX) and fed a control diet with 0%, 5%, 15% or 25% dried plum (w/w), corresponding to control, low- (LDP), medium- (MDP) and high (HDP)-dose dried plum. Four weeks of HDP supplementation prevented the decrease in spine bone mineral density and content induced by OVX. The OVX compromise in trabecular bone of the vertebra and proximal tibia was prevented by the higher doses of dried plum, and in the vertebra these effects resulted in greater (P<.05) bone strength and stiffness. In the bone marrow, OVX suppressed granulocyte and committed monocyte populations and increased the lymphoblast population, but the MDP and HDP restored these myeloid and lymphoid populations to the level of the Sham. Dried plum also suppressed lymphocyte tumor necrosis factor (TNF)-α production ex vivo by splenocytes, in response to concanavalin (Con) A stimulation. These data indicate that dried plum's positive effects on bone structural and biomechanical properties coincide with the restoration of certain bone marrow myeloid and lymphoid populations, and suppressed splenocyte activation occurring with ovarian hormone deficiency.

The effects of native and synthetic estrogenic compounds as well as vitamin D less-calcemic analogs on adipocytes content in rat bone marrow

BACKGROUND

We demonstrated previously that phytoestrogens and vitamin D analogs like estradiol-17β (E2) modulate bone morphology in rat female model.

AIM

We now analyze the effects of phytoestrogens, E2, selective E2 re ceptor modulators, and the less-calcemic analogs of vitamin D: JKF1624F2-2 (JKF) or QW1624F2-2 (QW) on fat content in bone marrow (BM) from long bones in ovariectomized female rats (OVX).

MATERIALS AND METHODS

OVX rats were injected with treatments known to affect bone formation, 5 days per week for 2.5 month for analysis of fat content in BM.

RESULTS

In OVX young adults there is a decreased bone formation and a 10-fold increase in fat cells content in BM. Treatment with E2, raloxifene (Ral) or DT56a resulted in almost completely abolishment of fat cells content. Daidzein (D) decreased fat cells content by 80%, genistein (G) or biochainin A (BA) did not change fat cells content and carboxy BA (cBA) had a small but significant effect. JKF or QW did not affect fat cells content, whereas combined treatment of JKF or QW with E2 resulted in complete abolishment of fat cells content. These changes in fat cells content are inversely correlated with changes in bone formation.

CONCLUSIONS

Our results demonstrate that adipogenesis induced by OVX is a reversible process which can be corrected by hormonal treatments. The awareness of a relationship between fat and bone at the marrow level might provide a better understanding of the pathophysiology of bone loss as well as a novel approach to diagnosis and treatment of postmenopausal osteoporosis.

Effect of BMP-9 and its derived peptide on the differentiation of human white preadipocytes

Several studies have shown that bone morphogenetic proteins (BMPs) can influence adipogenic and osteogenic cell lineages. We have shown that a peptide derived from BMP-9 (pBMP-9) at 400 ng/ml inhibits the proliferation of preosteoblasts and induces differentiation. We have now determined the effects of pBMP-9 (400 ng/ml) and equimolar concentrations of BMP-2 (100 ng/ml), BMP-9 (84.6 ng/ml) and pBMP-9 (9.04 ng/ml) on human white preadipocytes (HWP). pBMP-9 dose dependently reduced the proliferation of HWP without affecting the number of apoptotic cells. Incubation of the cells for 1 h with BMP-2, BMP-9 or pBMP-9 activated the Smad1/5/8 pathway, while incubation for 7 days in adipocyte differentiation (AD) serum-free medium containing ciglitazone and equimolar concentrations of BMP-2, BMP-9 or pBMP-9 enhanced the levels of mRNA of the adipogenic markers aP2 and adipoQ and increased the number of lipid vesicles. Thus, pBMP-9, like BMP-9, can increase the AD of HWP in AD serum-free medium.

Growth hormone regulates the balance between bone formation and bone marrow adiposity

Cancellous bone decreases and bone marrow fat content increases with age. Osteoblasts and adipocytes are derived from a common precursor, and growth hormone (GH), a key hormone in integration of energy metabolism, regulates the differentiation and function of both cell lineages. Since an age-related decline in GH is associated with bone loss, we investigated the relationship between GH and bone marrow adiposity in hypophysectomized (HYPOX) rats and in mice with defects in GH signaling. HYPOX dramatically reduced body weight gain, bone growth and mineralizing perimeter, serum insulin-like growth factor 1 (IGF-1) levels, and mRNA levels for IGF-1 in liver and bone. Despite reduced body mass and adipocyte precursor pool size, HYPOX resulted in a dramatic increase in bone lipid levels, as reflected by increased bone marrow adiposity and bone triglyceride and cholesterol content. GH replacement normalized bone marrow adiposity and precursor pool size, as well as mineralizing perimeter in HYPOX rats. In contrast, 17beta -estradiol, IGF-1, thyroxine, and cortisone were ineffective. Parathyroid hormone (PTH) reversed the inhibitory effects of HYPOX on mineralizing perimeter but had no effect on adiposity. Finally, bone marrow adiposity was increased in mice deficient in GH and IGF-1 but not in mice deficient in serum IGF-1. Taken together, our findings indicate that the reciprocal changes in bone and fat mass in GH signaling-deficient rodents are not directly coupled with one another. Rather, GH enhances adipocyte as well as osteoblast precursor pool size. However, GH increases osteoblast differentiation while suppressing bone marrow lipid accumulation.

The regulation of MS-KIF18A expression and cross talk with estrogen receptor

This study provides a novel view on the interactions between the MS-KIF18A, a kinesin protein, and estrogen receptor alpha (ERα) which were studied in vivo and in vitro. Additionally, the regulation of MS-KIF18A expression by estrogen was investigated at the gene and protein levels. An association between recombinant proteins; ERα and MS-KIF18A was demonstrated in vitro in a pull down assay. Such interactions were proven also for endogenous proteins in MBA-15 cells were detected prominently in the cytoplasm and are up-regulated by estrogen. Additionally, an association between these proteins and the transcription factor NF-κB was identified. MS-KIF18A mRNA expression was measured in vivo in relation to age and estrogen level in mice and rats models. A decrease in MS-KIF18A mRNA level was measured in old and in OVX-estrogen depleted rats as compared to young animals. The low MS-KIF18A mRNA expression in OVX rats was restored by estrogen treatment. We studied the regulation of MS-KIF18A transcription by estrogen using the luciferase reporter gene and chromatin immuno-percipitation (ChIP) assays. The luciferase reporter gene assay demonstrated an increase in MS-KIF18A promoter activity in response to 10⁻⁸ M estrogen and 10⁻⁷M ICI-182,780. Complimentary, the ChIP assay quantified the binding of ERα and pcJun to the MS-KIF18A promoter that was enhanced in cells treated by estrogen and ICI-182,780. In addition, cells treated by estrogen expressed higher levels of MS-KIF18A mRNA and protein and the protein turnover in MBA-15 cells was accelerated. Presented data demonstrated that ERα is a defined cargo of MS-KIF18A and added novel insight on the role of estrogen in regulation of MS-KIF18A expression both in vivo and in vitro.

Insights into the transcriptional and chromatin regulation of mesenchymal stem cells in musculo-skeletal tissues

Utilizing adult stem cells for regenerative medicine of skeletal tissues requires the development of molecular and biochemical tools that will allow isolation of these cells and direction of their differentiation towards a desired lineage and tissue formation. Stem cell commitment and fate decision into specialized functional cells involve coordinated activation and silencing of lineage-specific genes. Transcription factors and chromatin-remodeling proteins are key players in the control process of lineage commitment and differentiation during embryogenesis and adulthood. Transcription factors act in cooperation with co-regulator proteins to generate tissue-specific responses that elicits the tissue specific gene expression. Consequently, one of the main challenges of today's research is to characterize molecular pathways that coordinate the lineage-specific differentiation. Epigenetic regulation includes chromatin remodeling that control structural changes of DNA required for the binding of transcription factors to promoter regions. Revealing the mechanisms of action of such factors will provide understanding of how transcription and chromatin regulatory factors function together to regulate stem cell lineage fate decision.

Effects of estrogen therapy on bone marrow adipocytes in postmenopausal osteoporotic women

One-year treatment of osteoporotic postmenopausal women with transdermal estrogen resulted in significant decreases in bone marrow adipocyte volume and prevented increases in adipocyte number as compared to placebo-treated controls. Estrogen treatment also prevented increases in mean adipocyte size over 1 year.

INTRODUCTION

Aging is associated not only with bone loss but also with increases in bone marrow adipocytes. Since osteoblasts and adipocytes are derived from a common precursor, it is possible that with aging, there is a preferential "switch" in commitment of this precursor to the adipocyte over the osteoblast lineage. We tested the hypothesis that the apparent "age-related" increase in marrow adipocytes is due, at least in part, to estrogen (E) deficiency.

METHODS

Reanalysis of bone biopsies from a randomized, placebo-controlled trial involving 56 postmenopausal osteoporotic women (mean age, 64 years) treated either with placebo (PL, n = 27) or transdermal estradiol (0.1 mg/d, n = 29) for 1 year.

RESULTS

Adipocyte volume/tissue volume (AV/TV) and adipocyte number (Ad#) increased (by 20%, P < 0.05) in the PL group, but were unchanged (Ad#) or decreased (AV/TV, by -24%, P < 0.001) in the E group. E treatment also prevented increases in mean adipocyte size over 1 year.

CONCLUSIONS

These findings represent the first in vivo demonstration in humans that not only ongoing bone loss, but also the increase in bone marrow adipocyte number and size in postmenopausal osteoporotic women may be due, at least in part, to E deficiency.

Application of the laser capture microdissection technique for molecular definition of skeletal cell differentiation in vivo

Laser capture microdissection (LCM) method allows selection of individual or clustered cells from intact tissues. This technology enables one to pick cells from tissues that are difficult to study individually, sort the anatomical complexity of these tissues, and make the cells available for molecular analyses. Following the cells' extraction, the nucleic acids and proteins can be isolated and used for multiple applications that provide an opportunity to uncover the molecular control of cellular fate in the natural microenvironment. Utilization of LCM for the molecular analysis of cells from skeletal tissues will enable one to study differential patterns of gene expression in the native intact skeletal tissue with reliable interpretation of function for known genes as well as to discover novel genes. Variability between samples may be caused either by differences in the tissue samples (different areas isolated from the same section) or some variances in sample handling. LCM is a multi-task technology that combines histology, microscopy work, and dedicated molecular biology. The LCM application will provide results that will pave the way toward high throughput profiling of tissue-specific gene expression using Gene Chip arrays. Detailed description of in vivo molecular pathways will make it possible to elaborate on control systems to apply for the repair of genetic or metabolic diseases of skeletal tissues.

Mechanical loading down-regulates peroxisome proliferator-activated receptor gamma in bone marrow stromal cells and favors osteoblastogenesis at the expense of adipogenesis

Because a lack of mechanical information favors the development of adipocytes at the expense of osteoblasts, we hypothesized that the peroxisome proliferator-activated receptor gamma (PPARgamma)-dependent balance between osteoblasts and adipocytes is affected by mechanical stimuli. We tested the robustness of this hypothesis in in vivo rodent osteogenic exercise, in vitro cyclic loading of cancellous haversian bone samples, and cyclic stretching of primary stromal and C3H10T1/2 cells. We found that running rats exhibit a decreased marrow fat volume associated with an increased bone formation, presumably through recruitment of osteoprogenitors. In the tissue culture model and primary stromal cells, cyclic loading induced higher Runx2 and lower PPARgamma2 protein levels. Given the proadipocytic and antiosteoblastic activities of PPARgamma, we studied the effects of cyclic stretching in C3H10T1/2 cells, treated either with the PPARgamma activator, Rosiglitazone, or with GW9662, a potent antagonist of PPARgamma. We found, through both cytochemistry and analysis of lineage marker expression, that under Roziglitazone cyclic stretch partially overcomes the induction of adipogenesis and is still able to favor osteoblast differentiation. Conversely, cyclic stretch has additive effects with GW9662 in inducing osteoblastogenesis. In conclusion, we provide evidence that mechanical stimuli are potential PPARgamma modulators counteracting adipocyte differentiation and inhibition of osteoblastogenesis.

SVEP1 expression is regulated in estrogen-dependent manner

The SVEP1 protein comprises modules related to the selectin super family and other motifs found in cell surface molecules. Earlier, we demonstrated that SVEP1 is expressed in osteogenic cells both in vivo and in vitro; in the current study we elaborate on the regulation of SVEP1 by 17beta-Estradiol (17betaE2). SVEP1 message is expressed in vivo by bone marrow cells of sham-operated rats, but not in estrogen-depleted ovariectomized (OVX) rats. We demonstrated that 17betaE2 treatment increases the level of the SVEP1 expression in cultured osteoblasts. SVEP1 was identified also in breast carcinoma (BC) cells known to reside in bone when metastasized from the primary tumor. SVEP1 expression was demonstrated by immunohistochemistry and fluorescence-activated cell sorting (FACS) on various BC cell lines. The chromatin immunoprecipitation (ChIP) assay was applied to analyze the estrogen receptor (ER) binding to the putative SVEP1 promoter. We demonstrated that treatment with 17betaE2 or ICI 182,780 affects this binding and regulates the mRNA and protein levels of SVEP1 in BC cells. We propose that SVEP1 may serve as a useful biomarker for studying the mechanism of cells interactions within the local microenvironment affected by estrogen.

Dynamic interactions of chromatin-related mesenchymal modulator, a chromodomain helicase-DNA-binding protein, with promoters in osteoprogenitors

The newly identified protein chromatin-related mesenchymal modulator (CReMM) is expressed by marrow stromal progenitors in vivo and ex vivo. CReMM belongs to a recently identified subgroup of chromodomain helicase-DNA-binding proteins composed of multiple domains including chromodomains, SNF2/ATPase, helicase-C domain, SANT, and A/T-hook-DNA binding domain. Chromatin immunoprecipitation assay was applied to follow the dynamics of CReMM binding to A/T-rich regions on promoters of genes that play a role in osteoblast maturation. CReMM interaction with BMP4 and biglycan promoters in the marrow stromal cells was challenged with transforming growth factor-beta. Treatment with 17beta-estradiol enhanced the binding to estrogen receptor and abolished binding to the prolactin receptor promoters; CReMM interaction with osteocalcin promoter was identified constantly. CReMM binding to the analyzed endogenous promoters suggests its direct role in the transcriptional program activated during osteogenic cell differentiation, which may be a useful tool for following the molecular mechanism of the "stemness" of mesenchymal cells.

Normal structure, function, and histology of the bone marrow

While a complete blood count provides information regarding possible treatment-related effects reflected in the peripheral blood, morphological evaluation of bone marrow cytology and paraffin sections provides information about bone marrow tissue architecture that otherwise would be missed by examination of peripheral blood alone. In decalcified, paraffin-embedded, hematoxylin and eosin (H&E)-stained sections of bone marrow, the more mature stages of the erythroid and myeloid cells, adipocytes, mast cells, and megakaryocytes can be identified, but lymphoid cells as well as immature progenitor cells can not be reliably identified. The quality of the marrow sections is governed by numerous variables related to specimen collection and processing and must be considered. In addition to discussing normal structure, function, and histology of bone marrow, methods for preparation and evaluation of bone marrow are presented.

Expression and regulation of CReMM, a chromodomain helicase-DNA-binding (CHD), in marrow stroma derived osteoprogenitors

This study follows the expression of CReMM, a new CHD family member, in osteoprogenitors. CReMM expression was analyzed in primary cultured mesnchymal cells from rat and human. Analysis in ex vivo cultured marrow stromal cells (MSC) from rats revealed higher level of CReMM in cells from young (3 months), when compared to cells from old (15 months) rats. CReMM level was higher in human MSC then in mature trabecular bone cells (TBC). Within the MSC population, osteogenic clones showed higher levels of CReMM then non-osteogenic ones. We used bone marrow derived osteogenic cell line (MBA-15) to elaborate on the regulation of CReMM expression in correlation with cell proliferation and co-expression with alkaline phosphatase (ALK). CReMM is highly expressed in proliferating cells and is inversely related to expression of ALK. MBA-15 cells were challenged with dexamethasone (Dex) or 17beta-estradiol and quantification of CReMM at the protein (ELISA) and mRNA (RT-PCR) levels had shown that Dex upregulated CReMM levels. Since CReMM is regulated by Dex, we analyzed the interaction of CReMM with the glucocorticoid receptor (GR), which mediates Dex action. Co-immunopercipitation (Co-IP) demonstrated an association between CReMM and GR. In summary, CReMM is a CHD protein expressed by osteoprogenitors, and we suggest it plays a role in mediating transcriptional response to hormones that coordinate osteoblast function.

Expression and regulation of CReMM, a chromodomain helicase-DNA-binding (CHD), in marrow stroma derived osteoprogenitors

This study follows the expression of CReMM, a new CHD family member, in osteoprogenitors. CReMM expression was analyzed in primary cultured mesnchymal cells from rat and human. Analysis in ex vivo cultured marrow stromal cells (MSC) from rats revealed higher level of CReMM in cells from young (3 months), when compared to cells from old (15 months) rats. CReMM level was higher in human MSC then in mature trabecular bone cells (TBC). Within the MSC population, osteogenic clones showed higher levels of CReMM then non-osteogenic ones. We used bone marrow derived osteogenic cell line (MBA-15) to elaborate on the regulation of CReMM expression in correlation with cell proliferation and co-expression with alkaline phosphatase (ALK). CReMM is highly expressed in proliferating cells and is inversely related to expression of ALK. MBA-15 cells were challenged with dexamethasone (Dex) or 17beta-estradiol and quantification of CReMM at the protein (ELISA) and mRNA (RT-PCR) levels had shown that Dex upregulated CReMM levels. Since CReMM is regulated by Dex, we analyzed the interaction of CReMM with the glucocorticoid receptor (GR), which mediates Dex action. Co-immunopercipitation (Co-IP) demonstrated an association between CReMM and GR. In summary, CReMM is a CHD protein expressed by osteoprogenitors, and we suggest it plays a role in mediating transcriptional response to hormones that coordinate osteoblast function.

Conversion of adipogenic to osteogenic phenotype using crystalline porous biomatrices of marine origin

Adipogenic and osteogenic cells share part of the early differentiation cascade of mesenchymal stem cells (MSCs). The choice of a mesenchymal precursor cell to differentiate into a particular cell type is dictated by many spatial and temporal cues, including growth factors, neighboring mature cells, and the extracellular matrix (ECM), which plays an important role in bone formation. Whether adipocytes that have initiated differentiation along one lineage can convert into osteogenic lineage by merely interacting with materials having specific surface parameters is unknown. Using crystalline three-dimensional (3D) biomatrices of marine origin (CaCO(3)), we explored whether preadipocytes can convert into osteoblasts. Cells (3T3F442A) were seeded on 3D biomatrices of marine origin (Porites lutea). Analyses were made at different time intervals-1, 2, 5, 7, 14, 21, and 28 days post-seeding. Cell characterizations were done using morphological (light microscopy and scanning electron microscopy), histological (Alizarin red, von Kossa and Oil red O staining), enzymatic (alkaline phosphatase activity, and quantitative PCR testing transcript levels of osteocalcin, alkaline phosphatase, core binding factor- 1 (Cbfa1), and fatty acid binding protein (aP2). We demonstrated 3T3F442A preadipocyte modulation and differentiation into bone-forming cells when grown on biomatrix of marine origin without addition of other bone morphogenesis inducers. We found an active ossification process typical of osteogenic phenotype as early as 2 days after seeding. It is suggested that this crystalline biomatrix having a particular 3D topology or surface parameters supports fast cellular adhesion, proliferation, and differentiation of preadipocytes to osteogenic phenotype.

Transcriptional profiling of mesenchymal stromal cells from young and old rats in response to Dexamethasone

Background

Marrow-derived stromal cells (MSCs) maintain the capability of self-renewal and differentiation into multiple lineages in adult life. Age-related changes are recognized by a decline in the stemness potential that result in reduced regeneration potential of the skeleton. To explore the molecular events that underline skeletal physiology during aging we catalogued the profile of gene expression in ex vivo cultured MSCs derived from 3 and 15 month old rats. The ex vivo cultured cells were analyzed following challenge with or without Dexamethasone (Dex). RNA retrieved from these cells was analyzed using Affymetrix Gene Chips to compare the effect of Dex on gene expression in both age groups.

Results

The molecular mechanisms that underline skeletal senescence were studied by gene expression analysis of RNA harvested from MSCs. The analysis resulted in complex profiles of gene expression of various differentiation pathways. We revealed changes of lineage-specific gene expression; in general the pattern of expression included repression of proliferation and induction of differentiation. The functional analysis of genes clustered were related to major pathways; an increase in bone remodeling, osteogenesis and muscle formation, coupled with a decrease in adipogenesis. We demonstrated a Dex-related decrease in immune response and in genes that regulate bone resorption and an increase in osteoblastic differentiation. Myogenic-related genes and genes that regulate cell cycle were induced by Dex. While Dex repressed genes related to adipogenesis and catabolism, this decrease was complementary to an increase in expression of genes related to osteogenesis.

Conclusion

This study summarizes the genes expressed in the ex vivo cultured mesenchymal cells and their response to Dex. Functional clustering highlights the complexity of gene expression in MSCs and will advance the understanding of major pathways that trigger the natural changes underlining physiological aging. The high throughput analysis shed light on the anabolic effect of Dex and the relationship between osteogenesis, myogenesis and adipogenesis in the bone marrow cells.

Enhanced marrow adipogenesis and bone resorption in estrogen-deprived rats treated with the PPARgamma agonist BRL49653 (rosiglitazone)

Thiazolidinediones are insulin-sensitizing agents and in clinical use for the treatment of type II diabetes. Under specific experimental conditions, these molecules induce adipogenic differentiation of mesenchymal precursor cells at the expense of osteoblasts in vitro, suggesting possible negative effects on the skeleton. We measured effects of the thiazolidinedione BRL49653 on bone tissue of intact and estrogen-deprived skeletally mature adult female Wistar rats (6-9 months old). Weight gain and decreased plasma triglyceride levels confirmed the effectiveness of the treatment. However, no change in bone mass or fat marrow volume was observed in intact rats treated for 8 weeks with 5, 10, or 20 mg/kg of BRL49653. Study of marrow cultures established at necropsy revealed a higher responsiveness to adipogenic differentiation protocols of cultures established from the 10-mg/kg group compared to vehicle control. In a second study, the effects of thiazolidinedione treatment on the skeleton of estrogen-deprived rats were investigated. Application of 10 mg/kg of BRL49653 for 12 weeks resulted in enhanced bone loss (+31%; pQCT) and increased fat marrow volume (+117%; histomorphometry) compared to vehicle-treated OVX control. Interestingly, osteoblast number was comparable in both cases. Bone resorption parameters were significantly increased in the treatment group (+27% osteoclast number, +30% eroded surface). Enhanced bone loss due to treatment was consistently observed in the tibia, femur, and the lumbar spine. Our data indicate that thiazolidinediones may enhance bone loss induced by estrogen deprivation.

Differential Effects of Steroids and Retinoids on Bovine Myelopoiesis in Vitro

Pregnancy and parturition are associated with physiological changes caused by steroid hormones. Alterations in number, maturity, and function of polymorphonuclear leukocytes observed in dairy cows at parturition suggest a common causative relationship with steroid hormones. This study was designed to investigate the effects of progesterone, 17-beta-estradiol, and hydrocortisone on the proliferation of bovine progenitor cells. An in vitro culturing system was used, and colonies were scored after 7 d of incubation. At low concentrations, 17-beta-estradiol inhibited proliferation of granulocyte progenitor cells. Hydrocortisone reduced growth of granulocyte and monocyte colonies, whereas myelopoiesis was not altered by progesterone. Furthermore, we studied the effect of retinoids on colony formation of bovine bone marrow cells. All-trans- and 9-cis-retinoic acid stimulated growth of granulocyte colonies and inhibited proliferation of the monocyte lineage. The addition of the 13-cis-isomer also increased numbers of granulocyte colony-forming units. This study indicates that steroid hormones may be responsible for alterations in the bovine hematopoietic profiles observed in circulation during the postpartum period. White blood cells, especially polymorphonuclear leukocytes, which are derived from bone marrow, are an important first line defense against mastitis. Therefore, these effects of steroids might contribute to the increased susceptibility of dairy cows to Escherichia coli mastitis. We furthermore hypothesize that an important role might be attributed to retinoic acid in its regulation of bovine myelopoiesis. Modulation of myelopoiesis in favor of the granulocyte lineage during the acute-phase reaction may be an adaptive mechanism designed to increase the capacity of first-line defense to intramammary infections.

Identification of cultured progenitor cells from human marrow stroma

The marrow stromal cells (MSC) are essential for regulation of bone remodeling and hematopoiesis. It is of prime importance to isolate MSC and to expand the proliferating cells ex vivo. In this study, we analyzed cultured MSC for various cellular parameters, including cell morphology, cell cycle, and expression of cell surface antigens by flow cytometry. MSC were divided based on cell size to small (S-cells) and large (L-cells) and were visualized by light and electron microscope. The S-cells were proliferating cells correlated with G0/G1 phase of cell cycle, and expressed cFOS. The expression of surface markers CD-34, -44, -51, -61, -62E, -62P, -62L was quantified using flow cytometry. CD-44 was ubiquitously expressed by S and L cells, CD-51 and -61 were expressed by 30%-38% of S-cells. CD-34 and -62 expressed 20% positive of the analyzed cells that were of the proliferating progenitors (S-cells). This study enables the identification of subpopulations from MSC with special attention paid to the proliferating cells from ex vivo cultures of marrow stroma.

Bone morphogenetic protein and retinoic acid signaling cooperate to induce osteoblast differentiation of preadipocytes

Mesenchymal cells can differentiate into osteoblasts, adipocytes, myoblasts, or chondroblasts. Whether mesenchymal cells that have initiated differentiation along one lineage can transdifferentiate into another is largely unknown. Using 3T3-F442A preadipocytes, we explored whether extracellular signals could redirect their differentiation from adipocyte into osteoblast. 3T3-F442A cells expressed receptors and Smads required for bone morphogenetic protein (BMP) signaling. BMP-2 increased proliferation and induced the early osteoblast differentiation marker alkaline phosphatase, yet only mildly affected adipogenic differentiation. Retinoic acid inhibited adipose conversion and cooperated with BMP-2 to enhance proliferation, inhibit adipogenesis, and promote early osteoblastic differentiation. Expression of BMP-RII together with BMP-RIA or BMP-RIB suppressed adipogenesis of 3T3-F442A cells and promoted full osteoblastic differentiation in response to retinoic acid. Osteoblastic differentiation was characterized by induction of cbfa1, osteocalcin, and collagen I expression, and extracellular matrix calcification. These results indicate that 3T3-F442A preadipocytes can be converted into fully differentiated osteoblasts in response to extracellular signaling cues. Furthermore, BMP and retinoic acid signaling cooperate to stimulate cell proliferation, repress adipogenesis, and promote osteoblast differentiation. Finally, BMP-RIA and BMP-RIB induced osteoblast differentiation and repressed adipocytic differentiation to a similar extent.