Transforming growth factor beta2 inhibits adipocyte differentiation induced by skeletal unloading in rat bone marrow stroma

PPAR Gamma Activity and Control of Bone Mass in Skeletal Unloading

Bone loss occuring with unloading is associated with decreased osteoblastogenesis and increased bone marrow adipogenesis, resulting in bone loss and decreased bone formation. Here, we review the present knowledge on the role of PPARγ in the control of osteoblastogenesis and bone mass in skeletal unloading. We showed that PPARγ positively promotes adipogenesis and negatively regulates osteoblast differentiation of bone marrow stromal cells in unloading, resulting in bone loss. Manipulation of PPARγ2 expression by exogenous TGF-β2 inhibits the exaggerated adipogenesis and corrects the balance between osteoblastogenesis and adipogenesis induced by unloading, leading to prevention of bone loss. This shows that PPARγ plays an important role in the control of bone mass in unloaded bone. Moreover, this opens the possibility that manipulation of PPARγ may correct the balance between osteoblastogenesis and adipogenesis and prevent bone loss, which may have potential implications in the treatment of bone loss in clinical conditions.

CCAAT/enhancer binding protein β-deficiency enhances type 1 diabetic bone phenotype by increasing marrow adiposity and bone resorption

Bone loss in type 1 diabetes is accompanied by increased marrow fat, which could directly reduce osteoblast activity or result from altered bone marrow mesenchymal cell lineage selection (adipocyte vs. osteoblast). CCAAT/enhancer binding protein beta (C/EBPβ) is an important regulator of both adipocyte and osteoblast differentiation. C/EBPβ-null mice have delayed bone formation and defective lipid accumulation in brown adipose tissue. To examine the balance of C/EBPβ functions in the diabetic context, we induced type 1 diabetes in C/EBPβ-null (knockout, KO) mice. We found that C/EBPβ deficiency actually enhanced the diabetic bone phenotype. While KO mice had reduced peripheral fat mass compared with wild-type mice, they had 5-fold more marrow adipocytes than diabetic wild-type mice. The enhanced marrow adiposity may be attributed to compensation by C/EBPδ, peroxisome proliferator-activated receptor-γ2, and C/EBPα. Concurrently, we observed reduced bone density. Relative to genotype controls, trabecular bone volume fraction loss was escalated in diabetic KO mice (-48%) compared with changes in diabetic wild-type mice (-22%). Despite greater bone loss, osteoblast markers were not further suppressed in diabetic KO mice. Instead, osteoclast markers were increased in the KO diabetic mice. Thus, C/EBPβ deficiency increases diabetes-induced bone marrow (not peripheral) adipose depot mass, and promotes additional bone loss through stimulating bone resorption. C/EBPβ-deficiency also reduced bone stiffness and diabetes exacerbated this (two-way ANOVA P < 0.02). We conclude that C/EBPβ alone is not responsible for the bone vs. fat phenotype switch observed in T1 diabetes and that suppression of CEBPβ levels may further bone loss and decrease bone stiffness by increasing bone resorption.

Effect of prior treatment with resveratrol on density and structure of rat long bones under tail-suspension

Physical inactivity during space flight or prolonged bed rest causes rapid and marked loss of bone mass in humans. Resveratrol, a red wine polyphenol that is currently under study for its therapeutic antioxidant properties, has been shown to significantly modulate biomarkers of bone metabolism, i.e., to promote osteoblast differentiation and to prevent bone loss induced by estrogen deficiency. However, there is no direct evidence supporting its inhibitory effect toward bone loss during physical inactivity. In the present study, effects of resveratrol on bone mineral density (BMD), bone mineral content, and bone structure were examined in the femora and tibiae of tail-suspended and unsuspended rats using X-ray micro-computed tomography (micro-CT). Rats were treated with 400 mg/kg/day of resveratrol for 45 days and half of them were suspended during the last 2 weeks of treatment. Suspension caused a decrease in tibial and femoral BMD and deterioration of trabecular and cortical bone. Bone deterioration during suspension was paralleled by increased bone marrow area, which could be caused by an increase in stromal cells with osteoclastogenic potential or in adipocytes. Resveratrol had a preventive effect against bone loss induced by hindlimb immobilization. In particular, trabecular bone in the proximal tibial metaphysis was totally preserved in rats treated with resveratrol before tail-suspension.

Zinc finger protein 467 is a novel regulator of osteoblast and adipocyte commitment

Osteoblasts and adipocytes are derived from common mesenchymal progenitor cells. The bone loss of osteoporosis is associated with altered progenitor differentiation from an osteoblastic to an adipocytic lineage. cDNA microarrays and quantitative real-time PCR (Q-PCR) were carried out in a differentiating mouse stromal osteoblastic cell line, Kusa 4b10, to identify gene targets of factors that stimulate osteoblast differentiation including parathyroid hormone (PTH) and gp130-binding cytokines, oncostatin M (OSM) and cardiotrophin-1 (CT-1). Zinc finger protein 467 (Zfp467) was rapidly down-regulated by PTH, OSM, and CT-1. Retroviral overexpression and RNA interference for Zfp467 in mouse stromal cells showed that this factor stimulated adipocyte formation and inhibited osteoblast commitment compared with controls. Regulation of adipocyte markers, including peroxisome proliferator-activated receptor (PPAR) γ, C/EBPα, adiponectin, and resistin, and late osteoblast/osteocyte markers (osteocalcin and sclerostin) by Zfp467 was confirmed by Q-PCR. Intra-tibial injection of calvarial cells transduced with retroviral Zfp467 doubled the number of marrow adipocytes in C57Bl/6 mice compared with vector control-transduced cells, providing in vivo confirmation of a pro-adipogenic role of Zfp467. Furthermore, Zfp467 transactivated a PPAR-response element reporter construct and recruited a histone deacetylase complex. Thus Zfp467 is a novel co-factor that promotes adipocyte differentiation and suppresses osteoblast differentiation. This has relevance to therapeutic interventions in osteoporosis, including PTH-based therapies currently available, and may be of relevance for the use of adipose-derived stem cells for tissue engineering.

Divergent modulation of adipose-derived stromal cell differentiation by TGF-beta1 based on species of derivation

BACKGROUND

Adipose-derived stromal cells hold promise for skeletal tissue engineering. However, various studies have observed that adipose-derived stromal cells differ significantly in their biology depending on species of derivation. In the following study, the authors sought to determine the species-specific response of adipose-derived stromal cells to recombinant TGF-beta1 (rTGF-beta1).

METHODS

Adipose-derived stromal cells were derived from mouse and human sources. Recombinant TGF-beta1 was added to culture medium (2.5 to 10 ng/ml); proliferation and osteogenic and adipogenic differentiation were assessed by standardized parameters, including cell counting, alkaline phosphatase, alizarin red, oil red O staining, and quantitative real-time polymerase chain reaction.

RESULTS

Recombinant TGF-beta1 was found to significantly repress cellular proliferation in both mouse and human adipose-derived stromal cells (p < 0.01). Recombinant TGF-beta1 was found to significantly repress osteogenic differentiation in mouse adipose-derived stromal cells. In contrast, osteogenic differentiation of human adipose-derived stromal cells proceeded unimpeded in either the presence or the absence of rTGF-beta1. Interestingly, rTGF-beta1 induced expression of a number of osteogenic genes in human adipose-derived stromal cells, including BMP2 and BMP4.

CONCLUSIONS

The authors' results further detail an important facet in which mouse and human adipose-derived stromal cells differ. Mouse adipose-derived stromal cell osteogenesis is completely inhibited by rTGF-beta1, whereas human adipose-derived stromal cell osteogenesis progresses in the presence of rTGF-beta1. These data highlight the importance of species of derivation in basic adipose-derived stromal cell biology. Future studies will examine in more detail the species-specific differences among adipose-derived stromal cell populations.

Low-level vibrations retain bone marrow's osteogenic potential and augment recovery of trabecular bone during reambulation

Mechanical disuse will bias bone marrow stromal cells towards adipogenesis, ultimately compromising the regenerative capacity of the stem cell pool and impeding the rapid and full recovery of bone morphology. Here, it was tested whether brief daily exposure to high-frequency, low-magnitude vibrations can preserve the marrow environment during disuse and enhance the initiation of tissue recovery upon reambulation. Male C57BL/6J mice were subjected to hindlimb unloading (HU, n = 24), HU interrupted by weight-bearing for 15 min/d (HU+SHAM, n = 24), HU interrupted by low-level whole body vibrations (0.2 g, 90 Hz) for 15 min/d (HU+VIB, n = 24), or served as age-matched controls (AC, n = 24). Following 3 w of disuse, half of the mice in each group were released for 3 w of reambulation (RA), while the others were sacrificed. RA+VIB mice continued to receive vibrations for 15 min/d while RA+SHAM continued to receive sham loading. After disuse, HU+VIB mice had a 30% greater osteogenic marrow stromal cell population, 30% smaller osteoclast surface, 76% greater osteoblast surface but similar trabecular bone volume fraction compared to HU. After 3 w of reambulation, trabecular bone of RA+VIB mice had a 30% greater bone volume fraction, 51% greater marrow osteoprogenitor population, 83% greater osteoblast surfaces, 59% greater bone formation rates, and a 235% greater ratio of bone lining osteoblasts to marrow adipocytes than RA mice. A subsequent experiment indicated that receiving the mechanical intervention only during disuse, rather than only during reambulation, was more effective in altering trabecular morphology. These data indicate that the osteogenic potential of bone marrow cells is retained by low-magnitude vibrations during disuse, an attribute which may have contributed to an enhanced recovery of bone morphology during reambulation.

Signaling networks and transcription factors regulating mechanotransduction in bone

Mechanical stimulation has a critical role in the development and maintenance of the skeleton. This function requires the perception of extracellular stimuli as well as their conversion into intracellular biochemical responses. This process is called mechanotransduction and is mediated by a plethora of molecular events that regulate bone metabolism. Indeed, mechanoreceptors, such as integrins, G protein-coupled receptors, receptor protein tyrosine kinases, and stretch-activated Ca(2+) channels, together with their downstream effectors coordinate the transmission of load-induced signals to the nucleus and the expression of bone-related genes. During the past decade, scientists have gained increasing insight into the molecular networks implicated in bone mechanotransduction. In the present paper, we consider the major signaling cascades and transcription factors that control bone and cartilage mechanobiology and discuss the influence of the mechanical microenvironment on the determination of skeletal morphology.

Inflammatory bowel disease causes reversible suppression of osteoblast and chondrocyte function in mice

Decreased bone density and stature can occur in pediatric patients with inflammatory bowel disease (IBD). Little is known about how IBD broadly impacts the skeleton. To evaluate the influence of an acute episode of IBD on growing bone, 4-wk-old mice were administered 5% dextran sodium sulfate (DSS) for 5 days to induce colitis and their recovery was monitored. During active disease and early recovery, trabecular bone mineral density, bone volume, and thickness were decreased. Cortical bone thickness, outer perimeter, and density were also decreased, whereas inner perimeter and marrow area were increased. These changes appear to maintain bone strength since measures of moments of inertia were similar between DSS-treated and control mice. Histological (static and dynamic), serum, and RNA analyses indicate that a decrease in osteoblast maturation and function account for changes in bone density. Unlike some conditions of bone loss, marrow adiposity did not increase. Similar to reports in humans, bone length decreased and correlated with decreases in growth plate thickness and chondrocyte marker expression. During disease recovery, mice experienced a growth spurt that led to their achieving final body weights and bone length, density, and gene expression similar to healthy controls. Increased TNF-alpha and decreased IGF-I serum levels were observed with active disease and returned to normal with recovery. Changes in serum TNF-alpha (increased) and IGF-I (decreased) paralleled changes in bone parameters and returned to normal values with recovery, suggesting a potential role in the skeletal response.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.

Leptin treatment prevents type I diabetic marrow adiposity but not bone loss in mice

Leptin is a hormone secreted by adipocytes that is implicated in the regulation of bone density. Serum leptin levels are decreased in rodent models of type 1 (T1-) diabetes and in diabetic patients. Whether leptin mediates diabetic bone changes is unclear. Therefore, we treated control and T1-diabetic mice with chronic (28 days) subcutaneous infusion of leptin or saline to elucidate the therapeutic potential of leptin for diabetic osteoporosis. Leptin prevented the increase of marrow adipocytes and the increased aP2 expression that we observed in vehicle-treated diabetic mice. However, leptin did not prevent T1-diabetic decreases in trabecular bone volume fraction or bone mineral density in tibia or vertebrae. Consistent with this finding, markers of bone formation (osteocalcin RNA and serum levels) in diabetic mice were not restored to normal levels with leptin treatment. Interestingly, markers of bone resorption (TRAP5 RNA and serum levels) were decreased in diabetic mice by leptin treatment. In summary, we have demonstrated a link between low leptin levels in T1-diabetes and marrow adiposity. However, leptin treatment alone was not successful in preventing bone loss.

Cyclic stretch-induced TGFbeta1/Smad signaling inhibits adipogenesis in umbilical cord progenitor cells

Human umbilical cord perivascular cells (HUCPVCs) can differentiate along numerous lineages making them a favourable cell source for tissue regeneration. However, how these cells respond to biomechanical forces is unclear. This study aimed to determine whether cyclic stretch could regulate adipogenic differentiation of HUCPVCs, and to elucidate the mechanism of this regulation. In adipogenic culture, HUCPVCs expressed the adipocyte-specific transcription factors PPARgamma and C/EBPalpha and accumulated cytoplasmic lipid droplets. Exposure of these cells to equibiaxial cyclic stretch (10%, 0.5 Hz) in the presence of adipogenic medium, increased Smad2 phosphorylation compared to static samples and inhibited the expression of adipocyte markers; ERK1/2 phosphorylation was not changed. Inhibiting TGFbeta1 signaling decreased Smad2 phosphorylation and prevented the inhibition of adipogenic differentiation by cyclic stretch. These results demonstrate that cyclic equibiaxial stretch regulates HUCPVC differentiation even in the presence of an adipogenic milieu and should be an important consideration in developing future progenitor cell therapies.

Transforming growth factor-beta prevents osteoblast apoptosis induced by skeletal unloading via PI3K/Akt, Bcl-2, and phospho-Bad signaling

Loss of mechanical loading induces rapid bone loss resulting from reduced osteoblastogenesis and decreased bone formation. The signaling mechanisms involved in this deleterious effect on skeletal metabolism remain poorly understood. We have previously shown that hindlimb suspension in rats increases osteoblast apoptosis associated with decreased phosphatidylinositol 3-kinase (PI3K) signaling. In this study, we investigated whether transforming growth factor (TGF)-beta2 may prevent the altered signaling and osteoblast apoptosis induced by skeletal unloading in vivo. Hindlimb suspension-induced decreased bone volume was associated with reduced alpha(5)beta(1)-integrin protein levels and PI3K/Akt signaling in unloaded bone. Continuous administration of TGF-beta2 using osmotic minipumps prevented the decreased alpha(5)beta(1)-integrin expression and the reduced PI3K/Akt signaling in unloaded bone, resulting in the prevention of osteoblast apoptosis. We also show that TGF-beta2 prevented the decreased Bcl-2 levels induced by unloading, which suggests that TGF-beta2 targets Bcl-2 via PI3K/Akt to prevent osteoblast apoptosis in unloaded bone. Furthermore, we show that TGF-beta2 prevented the decrease in phosphorylated Bad, the inactive form of the proapoptotic protein Bad, induced by unloading. These results identify a protective role for TGF-beta2 in osteoblast apoptosis induced by mechanical unloading via the alpha(5)beta(1)/PI3K/Akt signaling cascade and downstream Bcl-2 and phospho-Bad survival proteins. We thus propose a novel role for TGF-beta2 in protection from unloading-induced apoptosis in vivo.

Understanding the pathology and mechanisms of type I diabetic bone loss

Type I (T1) diabetes, also called insulin dependent diabetes mellitus (IDDM), is characterized by little or no insulin production and hyperglycemia. One of the less well known complications of T1-diabetes is bone loss which occurs in humans and animal models. This complication is receiving increased attention because T1-diabetics are living longer due to better therapeutics, and are faced with their existing health concerns being compounded by complications associated with aging, such as osteoporosis. Both male and female, endochondrial and intra-membranous, and axial and appendicular bones are susceptible to T1-diabetic bone loss. Exact mechanisms accounting for T1-diabetic bone loss are not known. Existing data indicate that the bone defect in T1-diabetes is anabolic rather than catabolic, suggesting that anabolic therapeutics may be more effective in preventing bone loss. Potential contributors to T1-diabetic suppression of bone formation are discussed in this review and include: increased marrow adiposity, hyperlipidemia, reduced insulin signaling, hyperglycemia, inflammation, altered adipokine and endocrine factors, increased cell death, and altered metabolism. Differences between T1-diabetic- and age-associated bone loss underlie the importance of condition specific, individualized treatments for osteoporosis. Optimizing therapies that prevent bone loss or restore bone density will allow T1-diabetic patients to live longer with strong healthy bones.

Transcription factors controlling osteoblastogenesis

The recent development of molecular biology and mouse genetics and the analysis of the skeletal phenotype induced by genetic mutations in humans led to a better understanding of the role of transcription factors that govern bone formation. This review summarizes the role of transcription factors in osteoblastogenesis and provides an integrated perspective on how the activities of multiple classes of factors are coordinated for the complex process of developing the osteoblast phenotype. The roles of Runx2, the principal transcriptional regulator of osteoblast differentiation, Osterix, beta-Catenin and ATF which act downstream of Runx2, and other transcription factors that contribute to the control of osteoblastogenesis including the AP1, C/EBPs, PPARgamma and homeodomain, helix-loop-helix proteins are discussed. This review also updates the regulation of transcription factor expression by signaling factors and hormones that control osteoblastogenesis.

Effects of cyclic mechanical stretching on the mRNA expression of tendon/ligament-related and osteoblast-specific genes in human mesenchymal stem cells

The purpose of this study was to explore the influences of cyclic mechanical stretching on the mRNA expressions of tendon/ligament-related and osteoblast-specific marker genes in human MSCs seeded onto a collagen type I-coated surface. The stretch-induced mRNA expressions of mesenchymal stem cell protein (MSCP), matrix metalloproteinase-3 (MMP-3), and marker genes related to tendon/ligament cells (type I collagen, type III collagen, and tenascin-C) and those typical of osteoblasts (core binding factor alpha 1 (Cbfa1), alkaline phosphatase (ALP), and osteocalcin (OCN)) were analyzed by quantitative real-time PCR. The results revealed significant downregulation of MSCP and upregulation of MMP-3 genes in MSCs subjected to mechanical loading, regardless of the magnitude of the stretching (high or low). Moreover, the typical marker genes of the osteoblast lineage were upregulated by low-magnitude stretching, whereas tendon/ligament-related genes were upregulated by high-magnitude stretching for a long period. Cbfa1 and ALP were upregulated starting as early at 8 hr, followed by a downward trend and no significant change in expression at the other time points. The mRNA expressions of type I collagen, type III collagen, and tenascin-C significantly increased in MSCs subjected to 10% stretching for 48 hr, and this effect still existed after the stretched cells had rested for 48 hr. This study demonstrated the effect of cyclic mechanical stretching on differential transcription of marker genes related to different cell lineages. Low-magnitude stretching increased mRNA expressions of Cbfa1 and ALP and was possibly involved in the early osteoblastic differentiation of MSCs, whereas high-magnitude stretching upregulated the mRNA expressions of tendon/ligament-related genes.

Is adiposity advantageous for bone strength? A peripheral quantitative computed tomography study in late adolescent females

BACKGROUND

Whereas excess adiposity is presumed to be advantageous for the skeleton, studies investigating relations between bone strength and fat during youth have been equivocal.

OBJECTIVES

Relations of percentage body fat (BF) and bone strength indexes were assessed in late adolescent females, taking into consideration surrogates of muscle force [ie, muscle cross-sectional area (MCSA) and bone length]. Bone measurements in the normal- and high-fat groups were also compared.

DESIGN

Late adolescent females (n = 115; aged 18.2 +/- 0.4 y) participated in this cross-sectional study. Fat-free soft tissue mass, fat mass, and percentage BF were measured with the use of dual-energy X-ray absorptiometry. Tibial and radial peripheral quantitative computed tomography measurements were taken at the 4% (trabecular bone), 20% (cortical bone), and 66% (for measurement of MCSA) sites from the distal metaphyses.

RESULTS

Percentage BF was inversely related to radial cortical bone area, total bone cross-sectional area (CSA), cortical bone mineral content (BMC), periosteal circumference, and strength-strain index (SSI) (20% site; all P < 0.05). After control for MCSA and limb length, negative relations remained between percentage BF and radial measurements and were also observed at the tibia (20% site). Unadjusted bone measures were not different between groups. After control for MCSA, the high- compared with the normal-fat group had lower bone measures at the 20% site (cortical bone area and cortical BMC at the tibia, total bone CSA at the radius, and SSI at both the tibia and radius; P < 0.05 for all).

CONCLUSION

Excess weight in the form of fat mass does not provide additional benefits, and may potentially be negative, for adolescent bone.

Type I diabetic bone phenotype is location but not gender dependent

Bone is highly dynamic and responsive. Bone location, bone type and gender can influence bone responses (positive, negative or none) and magnitude. Type I diabetes induces bone loss and increased marrow adiposity in the tibia. We tested if this response exhibits gender and location dependency by examining femur, vertebrae and calvaria of male and female, control and diabetic BALB/c mice. Non-diabetic male mice exhibited larger body, muscle, and fat mass, and increased femur BMD compared to female mice, while vertebrae and calvarial bone parameters did not exhibit gender differences. Streptozotocin-induced diabetes caused a reduction in BMD at all sites examined irrespective of gender. Increased marrow adiposity was evident in diabetic femurs and calvaria (endochondrial and intramembranous formed bones, respectively), but not in vertebrae. Leptin-deficient mice also exhibit location dependent bone responses and we found that serum leptin levels were significantly lower in diabetic compared to control mice. However, in contrast to leptin-deficient mice, the vertebrae of T1-diabetic mice exhibit bone loss, not gain. Taken together, our findings indicate that TI-diabetic bone loss in mice is not gender, bone location or bone type dependent, while increased marrow adiposity is location dependent.

Opposite effects of leptin on bone metabolism: a dose-dependent balance related to energy intake and insulin-like growth factor-I pathway

Published data describing leptin effects on bone are at variance with both positive and negative consequences reported. These findings are consistent with a bimodal threshold response to serum leptin levels. To test this theory, two groups of female rats (tail-suspended and unsuspended) were treated with ip leptin at two different doses or vehicle for 14 d. In tail-suspended rats, low-dose leptin compensated the decrease in serum leptin levels observed with suspension and was able to prevent the induced bone loss at both the trabecular and cortical level (assessed by three-dimensional microtomography). In contrast, high-dose leptin inhibited femoral bone growth and reduced bone mass by decreasing bone formation rate and increasing bone resorption in both tail-suspended and unsuspended groups. High- and low-dose leptin administration resulted in a reduced medullar adipocytic volume in all groups. High-dose leptin (but not low) induced a decrease in body-weight abdominal fat mass and serum IGF-I levels. Thus, the observed bone changes at high-dose leptin are at least partly mediated by a leptin-induced energy imbalance. In conclusion, a balance between negative and positive leptin effects on bone is dependent on a bimodal threshold that is triggered by leptin serum concentration. Also, the negative effects of high leptin levels are likely induced by reduced energy intake and related hormonal changes. The respective part of each pathway will be unraveled by additional studies.

Loss of myostatin (GDF8) function increases osteogenic differentiation of bone marrow-derived mesenchymal stem cells but the osteogenic effect is ablated with unloading

Myostatin (GDF8) is a negative regulator of skeletal muscle growth and mice lacking myostatin show a significant increase in muscle mass and bone density compared to normal mice. In order to further define the role of myostatin in regulating bone mass we sought to determine if loss of myostatin function significantly altered the potential for osteogenic differentiation in bone marrow-derived mesenchymal stem cells in vitro and in vivo. We first examined expression of the myostatin receptor, the type IIB activin receptor (AcvrIIB), in bone marrow-derived mesenchymal stem cells (BMSCs) isolated from mouse long bones. This receptor was found to be expressed at high levels in BMSCs, and we were also able to detect AcvrIIB protein in BMSCs in situ using immunofluorescence. BMSCs isolated from myostatin-deficient mice showed increased osteogenic differentiation compared to wild-type mice; however, treatment of BMSCs from myostatin-deficient mice with recombinant myostatin did not attenuate the osteogenic differentiation of these cells. Loading of BMSCs in vitro increased the expression of osteogenic factors such as BMP-2 and IGF-1, but treatment of BMSCs with recombinant myostatin was found to decrease the expression of these factors. We investigated the effects of myostatin loss-of-function on the differentiation of BMSCs in vivo using hindlimb unloading (7-day tail suspension). Unloading caused a greater increase in marrow adipocyte number, and a greater decrease in osteoblast number, in myostatin-deficient mice than in normal mice. These data suggest that the increased osteogenic differentiation of BMSCs from mice lacking myostatin is load-dependent, and that myostatin may alter the mechanosensitivity of BMSCs by suppressing the expression of osteogenic factors during mechanical stimulation. Furthermore, although myostatin deficiency increases muscle mass and bone strength, it does not prevent muscle and bone catabolism with unloading.

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.

Mesenchymal stem cells prevent progressive experimental renal failure but maldifferentiate into glomerular adipocytes

Glomerulonephritis (GN) is a major cause of renal failure. This study sought to determine whether intrarenal injection of rat mesenchymal stem cells (MSC) can preserve renal function in a progressive rat model of GN. Early in GN (day 10), fluorescently labeled rat MSC localized to more than 70% of glomeruli, ameliorated acute renal failure, and reduced glomerular adhesions. Fifty days later, proteinuria had progressed in controls to 40 +/- 25 mg/d but stayed low in MSC-treated rats (13 +/- 4 mg/d; P < 0.01). Renal function on day 60 in the MSC group was better than in medium controls. Kidneys of the MSC group as compared with controls on day 60 contained 11% more glomeruli per 1-mm(2) section of cortex but also significantly more collagen types I, III, and IV and alpha-smooth muscle actin. Approximately 20% of the glomeruli of MSC-treated rats contained single or clusters of large adipocytes with pronounced surrounding fibrosis. Adipocytes exhibited fluorescence in their cytoplasm and/or intracellular lipid droplets. Lipid composition in these adipocytes in vivo mirrored that of MSC that underwent adipogenic differentiation in vitro. Thus, in this GN model, the early beneficial effect of MSC of preserving damaged glomeruli and maintaining renal function was offset by a long-term partial maldifferentiation of intraglomerular MSC into adipocytes accompanied by glomerular sclerosis. These data suggest that MSC treatment can be a valuable therapeutic approach only if adipogenic maldifferentiation is prevented.

Anemia of immobility: caused by adipocyte accumulation in bone marrow

Anemia of chronic disease has long been used to classify a non-regenerative, low-grade, chronic, normocytic, normochromic anemia that presents with no obvious etiology. Within this group, some patients have a chronic inflammatory condition that limits erythrocyte generation or access to iron stores. This specific type of anemia has been termed anemia of chronic inflammation. However, a substantial remainder of patients diagnosed with anemia of chronic disease present with no active inflammation. These include many clinical populations with reduced limb loading, such as spinal cord injured patients, astronauts, elderly people with limited mobility and experimental bed-rest subjects. In some populations with decreased mobility, accumulation of fat in the bone marrow has been demonstrated. We hypothesize that adipocyte accumulation in bone marrow both passively and actively impairs erythropoiesis and thus defines a new type of anemia called anemia of immobility. The non-specific umbrella term anemia of chronic disease thus becomes obsolete in favour of either the diagnosis of anemia of immobility or anemia of chronic inflammation according to the distinct mechanism involved.

Gene expression profiling of bone marrow stromal cells from juvenile, adult, aged and osteoporotic rats: with an emphasis on osteoporosis

PURPOSE

Osteoporosis is a multi-factorial, age-related disease with a complex etiology and mode of regulation involving a large numbers of genes. To better understand the possible relationships among genes, we fingerprinted genes in a rat model induced by ovariectomy to determine differences among osteoporotic, non-osteoporotic, aged and juvenile rats.

METHODS

We applied genome wide cDNA microarray technology to analyze genes expressed in bone marrow mesenchymal stromal cells (BMSC) and compared non-osteoporotic adult vs. osteoporotic, non-osteoporotic adult vs. aged, and non-osteoporotic adult vs. juvenile. Rigorous statistical analysis of functional annotation (EASE program) identified over-represented biological and molecular functions with significant group wide changes (p< or =0.05). Some of the expressed genes were further confirmed by quantitative RT-PCR (reverse transcription-polymerase chain reaction).

RESULTS

Differences in gene expression were observed by identifying transcripts selected by t-test that were consistently changed by a minimum of two-fold. There were 195 transcripts that showed an increased expression and 109 transcripts that showed decreased expression relative to the osteoporotic condition. Of these, 75% transcripts were unknown gene products or ESTs (expressed sequence tag). A number of genes found in the aged and juvenile groups were not present in the osteoporotic rats. Functional clustering of the genes using the EASE bioinformatics program revealed that transcripts in osteoporosis were associated with signal transduction, lipid metabolism, protein metabolism, ionic and protein transport, neuropeptide and G protein signaling pathways. Although some of the genes have previously been shown to play a key role in osteoporosis, several genes were uniquely identified in this study and likely play a role in developing aged related osteoporosis that could have compelling implications in the development of new diagnostic strategies and therapeutics for osteoporosis.

CONCLUSIONS

These data suggest that osteoporosis is associated with changes of multiple novel gene expression and that numerous pathways could play important roles in osteoporosis pathogenesis.

Inhibition of PPARgamma prevents type I diabetic bone marrow adiposity but not bone loss

Diabetes type I is associated with bone loss and increased bone adiposity. Osteoblasts and adipocytes are both derived from mesenchymal stem cells located in the bone marrow, therefore we hypothesized that if we could block adipocyte differentiation we might prevent bone loss in diabetic mice. Control and insulin-deficient diabetic BALB/c mice were chronically treated with a peroxisomal proliferator-activated receptor gamma (PPARgamma) antagonist, bisphenol-A-diglycidyl ether (BADGE), to block adipocyte differentiation. Effects on bone density, adiposity, and gene expression were measured. BADGE treatment did not prevent diabetes-associated hyperglycemia or weight loss, but did prevent diabetes-induced hyperlipidemia and effectively blocked diabetes type I-induced bone adiposity. Despite this, BADGE treatment did not prevent diabetes type I suppression of osteoblast markers (runx2 and osteocalcin) and bone loss (as determined by micro-computed tomography). BADGE did not suppress osteoblast gene expression or bone mineral density in control mice, however, chronic (but not acute) BADGE treatment did suppress osteocalcin expression in osteoblasts in vitro. Taken together, our findings suggest that BADGE treatment is an effective approach to reduce serum triglyceride and free fatty acid levels as well as bone adiposity associated with type I diabetes. The inability of BADGE treatment to prevent bone loss in diabetic mice suggests that marrow adiposity is not linked to bone density status in type I diabetes, but we cannot exclude the possibility of additional BADGE effects on osteoblasts or other bone cells, which could contribute to preventing the rescue of the bone phenotype.

Osteogenic growth peptide C-terminal pentapeptide [OGP(10-14)] acts on rat bone marrow mesenchymal stem cells to promote differentiation to osteoblasts and to inhibit differentiation to adipocytes

Cumulative evidence indicates that bone marrow mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating to osteogenic and adipogenic lineages when stimulated under appropriate conditions. Whether OGP(10-14) directly regulates the progenitor cells differentiating into osteoblasts or adipocytes remains unknown. In the present study, we investigated the roles of OGP(10-14) in differentiation along these separate lineages using rat bone marrow MSCs. Our results showed that OGP(10-14) promoted osteogenic differentiation of the stem cells and concurrently inhibited adipocyte formation. OGP(10-14) increased alkaline phosphatase (ALP) activity and mineralized nodule formation, and stimulated osteoblast-specific mRNA expression of core-binding factor 1 (cbfa1). In contrast, OGP(10-14) decreased adipocyte numbers and inhibited adipocyte-specific mRNA expression of peroxisome proliferator-activated receptor-gamma 2 (PPARgamma2). These observations suggest that commitment of MSCs into osteogenic or adipogenic lineages is regulated by OGP(10-14).

B cells and T cells are critical for the preservation of bone homeostasis and attainment of peak bone mass in vivo

Bone homeostasis is regulated by a delicate balance between osteoblastic bone formation and osteoclastic bone resorption. Osteoclastogenesis is controlled by the ratio of receptor activator of NF-kappaB ligand (RANKL) relative to its decoy receptor, osteoprotegerin (OPG). The source of OPG has historically been attributed to osteoblasts (OBs). While activated lymphocytes play established roles in pathological bone destruction, no role for lymphocytes in basal bone homeostasis in vivo has been described. Using immunomagnetic isolation of bone marrow (BM) B cells and B-cell precursor populations and quantitation of their OPG production by enzyme-linked immunosorbent assay (ELISA) and real-time reverse transcriptase-polymerase chain reaction (RT-PCR), cells of the B lineage were found to be responsible for 64% of total BM OPG production, with 45% derived from mature B cells. Consistently B-cell knockout (KO) mice were found to be osteoporotic and deficient in BM OPG, phenomena rescued by B-cell reconstitution. Furthermore, T cells, through CD40 ligand (CD40L) to CD40 costimulation, promote OPG production by B cells in vivo. Consequently, T-cell-deficient nude mice, CD40 KO mice, and CD40L KO mice display osteoporosis and diminished BM OPG production. Our data suggest that lymphocytes are essential stabilizers of basal bone turnover and critical regulators of peak bone mass in vivo.

Skeletal unloading induces osteoblast apoptosis and targets α5β1-PI3K-Bcl-2 signaling in rat bone

The mechanisms underlying the altered osteoblastogenesis and bone loss in response to disuse are incompletely understood. Using the rat tail suspension model, we studied the effect of skeletal unloading on osteoblast and osteocyte apoptosis. Tail suspension for 2 to 7 days decreased tibial bone mass and induced early apoptotic loss of osteoblasts and delayed apoptotic loss of osteocytes. Surrenal gland weight and plasma corticosterone levels did not differ in loaded and unloaded rats at any time point, indicating that osteoblast/osteocyte apoptosis occurred independently of endogenous glucocorticoids. The mechanistic basis for the disuse-induced osteoblast/osteocyte apoptosis was examined. We found that alpha5beta1 integrin and phosphorylated phosphatidyl-inositol-3 kinase (p-PI3K) protein levels were transiently decreased in unloaded metaphyseal long bone compared to loaded bones. In contrast, p-FAK and p-ERK p42/44 levels were not significantly altered. Interestingly, the reduced p-PI3K levels in unloaded long bone was associated with decreased levels of the survival protein Bcl-2 with unaltered Bax levels, causing increased Bax/Bcl-2 levels. The results indicate that skeletal unloading in rats induces a glucocorticoid-independent, immediate increase in osteoblast apoptosis associated with decreased alpha5beta1-PI3K-Bcl-2 survival pathway in rat bone, which may contribute to the altered osteoblastogenesis and osteopenia induced by unloading.

Bone loss and increased bone adiposity in spontaneous and pharmacologically induced diabetic mice

Insulin-dependent diabetes mellitus (IDDM) is associated with increased risk of osteopenia/osteoporosis in humans. The mechanisms accounting for diabetic bone loss remain unclear. Pharmacologic inducers of IDDM, such as streptozotocin, mimic key aspects of diabetes in rodents, allow analysis at the onset of diabetes, and induce diabetes in genetically modified mice. However, side effects of streptozotocin, unrelated to diabetes, can complicate data interpretation. The nonobese diabetic (NOD) mouse model develops diabetes spontaneously without external influences, negating side effects of inducing agents. Unfortunately, in this model the onset of diabetes is unpredictable, occurs in a minority of male mice, and can only be studied in a single mouse strain. To validate the relevance of the more flexible streptozotocin-induced diabetes model for studying diabetes-associated bone loss, we compared its phenotype to the spontaneously diabetic NOD model. Both models exhibited hyperglycemia and loss of body, fat pad, and muscle weight. Furthermore, these genetically different and distinct models of diabetes induction demonstrated similar bone phenotypes marked by significant trabecular bone loss and increased bone marrow adiposity. Correspondingly, both diabetic models exhibited decreased osteocalcin mRNA and increased adipocyte fatty acid-binding protein 2 mRNA levels in isolated tibias and calvaria. Taken together, multiple streptozotocin injection-induced diabetes is a valid model for understanding the acute and chronic pathophysiologic responses to diabetes and their mechanisms in bone.

Mechanisms of disease: is osteoporosis the obesity of bone?

Osteoporosis and obesity, two disorders of body composition, are growing in prevalence. Interestingly, these diseases share several features including a genetic predisposition and a common progenitor cell. With aging, the composition of bone marrow shifts to favor the presence of adipocytes, osteoclast activity increases, and osteoblast function declines, resulting in osteoporosis. Secondary causes of osteoporosis, including diabetes mellitus, glucocorticoids and immobility, are associated with bone-marrow adiposity. In this review, we ask a provocative question: does fat infiltration in the bone marrow cause low bone mass or is it a result of bone loss? Unraveling the interface between bone and fat at a molecular and cellular level is likely to lead to a better understanding of several diseases, and to the development of drugs for both osteoporosis and obesity.

Injections of leptin into rat ventromedial hypothalamus increase adipocyte apoptosis in peripheral fat and in bone marrow

The accumulation of fat cells (adipocytes) in bone marrow is now thought to be a factor contributing to age-related bone loss. Women with osteoporosis have higher numbers of marrow adipocytes than women with healthy bone, and bone formation rate is inversely correlated with adipocyte number in bone tissue biopsies from both men and women. Adipogenic differentiation of bone marrow stromal cells increases with age, but the factors regulating populations of mature adipocytes are not well understood. Leptin is thought to regulate adipose tissue mass via its receptors in the ventromedial hypothalamus (VMH). We have therefore tested the hypothesis that stimulation of leptin receptors in the VMH regulates adipocyte number in bone marrow. Results indicate that unilateral twice-daily injections of leptin into the rat VMH for only 4 or 5 days cause a significant reduction in the number of adipocytes in peripheral fat pads and bone marrow and indeed eliminate adipocytes almost entirely from bone marrow of the proximal tibia. Osteoblast surface is not affected with leptin treatment. Apoptosis assays performed on bone marrow samples from control and treated rats have revealed a significant increase in protein concentration of the apoptosis marker caspase-3 with leptin treatment. We conclude that stimulation of leptin receptors in the VMH significantly decreases the adipocyte population in bone marrow, primarily through apoptosis of marrow adipocytes. Elimination of marrow adipocytes via this central pathway may represent a useful strategy for the treatment and prevention of osteoporosis.

Dose effects of propranolol on cancellous and cortical bone in ovariectomized adult rats

Animal studies suggest that bone remodeling is under beta-adrenergic control via the sympathetic nervous system. The purpose of this study was to examine the preventive effect of different doses of nonspecific beta-blockers (propranolol) on trabecular and cortical bone envelopes in ovariectomized rats. Six-month-old female Wistar rats were ovariectomized (OVX, n = 60) or sham-operated (n = 15). Then, OVX rats were subcutaneously injected with 0.1 (n = 15), 5 (n = 15), or 20 (n = 15) mg/kg propranolol or vehicle (n = 15) for 10 weeks. Tibial and femoral bone mineral density (BMD) were analyzed longitudinally by dual-energy X-ray absorptiometry. At death, the left tibial metaphysis and L(4) vertebrae were removed, and microcomputed tomography (Skyscan 1072; Skyscan, Aartselaar, Belgium) was performed for trabecular bone structure investigation. Histomorphometry analysis was performed on the right proximal tibia to assess bone cell activities. After 10 weeks, OVX rats had decreased BMD and trabecular parameters and increased bone turnover, as well as cortical porosity compared with the sham group (p < 0.001). Bone architecture alteration was preserved by 0.1 mg/kg propranolol due to higher trabecular number and thickness (+50.35 and +6.81%, respectively, than OVX; p < 0.001) and lower cortical pore number (-52.38% than OVX; p < 0.001). Animals treated by 0.1 mg/kg propranolol had a lower osteoclast surface and a higher osteoblast activity compared with OVX. Animals treated by 20 mg of propranolol did not significantly differ from OVX rats. Animals treated by 5 mg of propranolol have been partially preserved from the ovariectomy. These results showed a dose effect of beta-blockers. The lower the dose of propranolol breeding, the better the preventive effect against ovariectomy.

Effects of different magnitudes of mechanical strain on Osteoblasts in vitro

In addition to systemic and local factors, mechanical strain plays a crucial role in bone remodeling during growth, development, and fracture healing, and especially in orthodontic tooth movement. Although many papers have been published on the effects of mechanical stress on osteoblasts or osteoblastic cells, little is known about the effects of different magnitudes of mechanical strain on such cells. In the present study, we investigated how different magnitudes of cyclic tensile strain affected osteoblasts. MC3T3-E1 osteoblastic cells were subjected to 0%, 6%, 12% or 18% elongation for 24h using a Flexercell Strain Unit, and then the mRNA and protein expressions of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL) were examined. The results showed that cyclic tensile strain induced a magnitude-dependent increase (0%, 6%, 12%, and 18%) in OPG synthesis and a concomitant decrease in RANKL mRNA expression and sRANKL release from the osteoblasts. Furthermore, the induction of OPG mRNA expression by stretching was inhibited by indomethacin or genistein, and the stretch-induced reduction of RANKL mRNA was inhibited by PD098059. These results indicate that different magnitudes of cyclic tensile strain influence the biological behavior of osteoblasts, which profoundly affects bone remodeling.

Resolving the Two "Bony" Faces of PPAR-gamma

Bone loss with aging results from attenuated and unbalanced bone turnover that has been associated with a decreased number of bone forming osteoblasts, an increased number of bone resorbing osteoclasts, and an increased number of adipocytes (fat cells) in the bone marrow. Osteoblasts and adipocytes are derived from marrow mesenchymal stroma/stem cells (MSC). The milieu of intracellular and extracellular signals that controls MSC lineage allocation is diverse. The adipocyte-specific transcription factor peroxisome proliferator-activated receptor-gamma (PPAR-gamma) acts as a critical positive regulator of marrow adipocyte formation and as a negative regulator of osteoblast development. In vivo, increased PPAR-gamma activity leads to bone loss, similar to the bone loss observed with aging, whereas decreased PPAR-gamma activity results in increased bone mass. Emerging evidence suggests that the pro-adipocytic and the anti-osteoblastic properties of PPAR-gamma are ligand-selective, suggesting the existence of multiple mechanisms by which PPAR-gamma controls bone mass and fat mass in bone.

Playing with bone and fat

The relationship between bone and fat formation within the bone marrow microenvironment is complex and remains an area of active investigation. Classical in vitro and in vivo studies strongly support an inverse relationship between the commitment of bone marrow-derived mesenchymal stem cells or stromal cells to the adipocyte and osteoblast lineage pathways. In this review, we focus on the recent literature exploring the mechanisms underlying these differentiation events and discuss their implications relevant to osteoporosis and regenerative medicine.

Analysis of fatty acid composition in human bone marrow aspirates

In the present study, the fatty acid composition of bone marrow aspirates and serum phospholipids in nine patients with hematologic diseases was investigated, and the effect of fatty acids on osteoblast differentiation in ST2 cells was examined. The concentrations of oleic acid and palmitic acid were significantly higher in bone marrow aspirates than in serum phospholipids, but the concentrations of other fatty acids did not differ. The rate of alkaline phosphatase positive ST2 cells induced by BMP2 was significantly increased by oleic acid, but was unaffected by the presence or absence of palmitic acid. We conclude that the fatty acid composition of bone marrow aspirates differs from that of serum phospholipids. This difference may affect osteoblast differentiation in the bone marrow microenvironment.

Mechanical modulation of molecular signals which regulate anabolic and catabolic activity in bone tissue

Identifying the molecular mechanisms that regulate bone's adaptive response to alterations in load bearing may potentiate the discovery of interventions to curb osteoporosis. Adult female mice (BALB/cByJ) were subjected to catabolic (disuse) and anabolic (45 Hz, 0.3g vibration for 10 min/day) signals, and changes in the mRNA levels of thirteen genes were compared to altered indices of bone formation. Age-matched mice served as controls. Following 4 days of disuse, significant (P = 0.05) decreases in mRNA levels were measured for several genes, including collagen type I (-55%), osteonectin (-44%), osterix (-36%), and MMP-2 (-36%) all of which, after 21 days, had normalized to control levels. In contrast, expression of several genes in the vibrated group, which failed to show significant changes at 4 days, demonstrated significant increases after 21 days, including inducible nitric oxide synthase (iNOS) (39%, P = 0.07), MMP-2 (54%), and receptor activator of the nuclear factor kB ligand (RANKL) (32%). Correlations of gene expression patterns across experimental conditions and time points allowed the functional clustering of responsive genes into two distinct groups. Each cluster's specific regulatory role (formation vs. resorption) was reinforced by the 60% suppression of formation rates caused by disuse, and the 55% increase in formation rates stimulated by mechanical signals (P < 0.05). These data confirm the complexity of the bone remodeling process, both in terms of the number of genes involved, their interaction and coordination of resorptive and formative activity, and the temporal sensitivity of the processes. More detailed spatial and temporal correlations between altered mRNA levels and tissue plasticity may further delineate the molecules responsible for the control of bone mass and morphology.

Increased bone adiposity and peroxisomal proliferator-activated receptor-gamma2 expression in type I diabetic mice

Decreased bone mass, osteoporosis, and increased fracture rates are common skeletal complications in patients with insulin-dependent diabetes mellitus (IDDM; type I diabetes). IDDM develops from little or no insulin production and is marked by elevated blood glucose levels and weight loss. In this study we use a streptozotocin-induced diabetic mouse model to examine the effect of type I diabetes on bone. Histology and microcomputed tomography demonstrate that adult diabetic mice, exhibiting increased plasma glucose and osmolality, have decreased trabecular bone mineral content compared with controls. Bone resorption could not completely account for this effect, because resorption markers (tartrate-resistant acid phosphatase 5b, urinary deoxypyridinoline excretion, and tartrate-resistant acid phosphatase 5 mRNA) are unchanged or reduced at 2 and/or 4 wk after diabetes induction. However, osteocalcin mRNA (a marker of late-stage osteoblast differentiation) and dynamic parameters of bone formation were decreased in diabetic tibias, whereas osteoblast number and runx2 and alkaline phosphatase mRNA levels did not differ. These findings suggest that the final stages of osteoblast maturation and function are suppressed. We also propose a second mechanism contributing to diabetic bone loss: increased marrow adiposity. This is supported by increased expression of adipocyte markers [peroxisome proliferator-activated receptor gamma2, resistin, and adipocyte fatty acid binding protein (alphaP2)] and the appearance of lipid-dense adipocytes in diabetic tibias. In contrast to bone marrow, adipose stores at other sites are depleted in diabetic mice, as indicated by decreased body, liver, and peripheral adipose tissue weights. These findings suggest that IDDM contributes to bone loss through changes in marrow composition resulting in decreased mature osteoblasts and increased adipose accumulation.

Leptin modulates both resorption and formation while preventing disuse-induced bone loss in tail-suspended female rats

In vitro studies have demonstrated leptin-positive effects on the osteoblast lineage and negative effects on osteoclastogenesis. Therefore, we tested the hypothesis that leptin may prevent tail-suspension-induced bone loss characterized by an uncoupling pattern of bone remodeling, through both mechanisms. Female rats were randomly tail-suspended or not and treated either with ip administration of leptin or vehicle for 3, 7, and 14 d. As measured by dual energy x-ray absorptiometry, tail-suspension induced a progressive decrease in tibia-metaphysis bone mineral density, which was prevented by leptin. Histomorphometry showed that this was related to the prevention of the transient increase in osteoclast number observed with suspension at d 7. These effects could be mediated by the receptor activator of nuclear factor kappaB-ligand (RANKL)/osteoprotegerin (OPG) pathway since we observed using direct RT-PCR, a suspension-induced increase in RANKL gene expression in proximal tibia at d 3, which was counterbalanced by leptin administration with a similar 3-fold increase in OPG expression and a RANKL to OPG ratio close to nonsuspended conditions. In addition, leptin prevented the decrease in bone formation rate induced by tail-suspension at d 14. The latter could be related to the role of leptin in mediating the reciprocal differentiation between adipocytes and osteoblasts, because leptin concurrently blunted the disuse-induced increase in bone marrow adipogenesis. In summary, these data suggest that peripheral administration of leptin could prevent disuse-induced bone loss through, first, a major inhibitory effect on bone resorption and, second, a delayed effect preventing the decrease in bone formation.

Differential regulation of cadherin expression by osteotropic hormones and growth factors in vitro in human osteoprogenitor cells

AIM

To examine if cadherins are expressed constitutively in human bone marrow stromal cells (hBMSC) and investigate the regulation of cadherin expression by various osteotropic hormones and local factors.

METHODS

Cadherin expression was examined in first passaged (secondary) hBMSC as well as in the conditionally-immortalized human osteoprogenitor cell line (hOP-7). Using a monoclonal antibody (MoAb C-1821) to a cytoplasmic domain common to all known cadherins (pan-cadherin MoAb), cadherins were immunolocalized in first passaged hBMSC as well as in hOP-7 cells. In addition, intense immunostaining for cadherin expression was associated with alkaline phosphatase (ALP) in nodules formed in the high density cultures of hOP-7 cells. Human E-cadherin (HECD) was specifically detected by Western blotting in extracts of untreated hBMSC using an anti-HECD MoAb 004FD.

RESULTS

Differential regulation of cadherin expression by various osteotropic hormones and local factors (parathyroid hormone, dexamethasone, estradiol, prostaglandin E2, basic fibroblast growth factor, and tumor necrosis factor-beta) was also observed. In addition, blocking cadherins with the MoAb C-1821 increased basal ALP activity and had an additive effect on 1, 25(OH) 2D3-induced ALP activity.

CONCLUSION

Cadherins are expressed in human osteoprogenitor cells and are involved in the osteogenic differentiation. The differential modulation of cadherin expression by osteotropic agents indicates that these agents may regulate osteoprogenitor cells through different cadherins and these cadherins may play different roles.

Skeletal unloading alleviates the anabolic action of intermittent PTH(1-34) in mouse tibia in association with inhibition of PTH-induced increase in c-fos mRNA in bone marrow cells

We analyzed the effect of unloading by tail suspension on the anabolic action of intermittent PTH in the tibia of growing mice. Unloading alleviated the PTH-induced increase of bone formation and accelerated bone resorption, consequently reducing bone mass. Reduction of the PTH-induced anabolic actions on bone was associated with unloading, which was apparently related to suppression of c-fos mRNA expression in bone marrow.

INTRODUCTION

The effects of intermittent parathyroid hormone (PTH) administration on unloading bone have not been well elucidated at the cellular and molecular levels. We tested the effects of PTH on unloaded tibias of tail-suspended mice.

MATERIALS AND METHODS

Eighty male C57BL/6J mice, 8 weeks of age, were divided into four groups with loading or unloading and administration of PTH (40 microg/kg body weight) or vehicle five times per week. Mice were killed at 8 or 15 days, and both tibias were obtained. Bone histomorphometry of the trabecular bone in the proximal tibia, development of osteogenic cells, and mRNA expression of osteogenic molecules in bone marrow cells were assessed.

RESULTS AND CONCLUSIONS

At 15 days of unloading, bone volume decreased in PTH-treated mice. The increase in the bone formation rate by PTH was depressed, and the osteoclast surface was thoroughly increased. The increase in alkaline phosphatase-positive colony-forming units-fibroblastic (CFU-f) colonies induced by PTH was maintained and that of TRACP+ multinucleated cells enhanced. The PTH-induced increase in c-fos mRNA was depressed, but the increases in Osterix and RANKL mRNA were maintained. Unloading promoted the PTH-associated osteoclastogenesis and seemed to delay the progression of osteogenic differentiation in association with reduction of the PTH-dependent increase of c-fos mRNA in bone marrow cells.

Identification of mesenchymal progenitor cells in normal and osteoarthritic human articular cartilage

OBJECTIVE

To determine the presence of mesenchymal progenitor cells (MPCs) in human articular cartilage.

METHODS

Primary cell cultures established from normal and osteoarthritic (OA) human knee articular cartilage were analyzed for the expression of CD105 and CD166, cell surface markers whose coexpression defines mesenchymal stem cells (MSCs) in bone marrow and perichondrium. The potential of cartilage cells to differentiate to adipogenic, osteogenic, and chondrogenic lineages was analyzed after immunomagnetic selection for CD105+/CD166+ cells and was compared with bone marrow-derived MSCs (BM-MSCs).

RESULTS

Up to 95% of isolated cartilage cells were CD105+ and approximately 5% were CD166+. The mean +/- SEM percentage of CD105+/CD166+ cells in normal cartilage was 3.49 +/- 1.93%. Primary cell cultures from OA cartilage contained significantly increased numbers of CD105+/CD166+ cells. Confocal microscopy confirmed the coexpression of both markers in the majority of BM-MSCs and a subpopulation of cartilage cells. Differentiation to adipocytes occurred in cartilage-derived cell cultures, as indicated by characteristic cell morphology and oil red O staining of lipid vacuoles. Osteogenesis was observed in isolated CD105+/CD166+ cells as well as in primary chondrocytes cultured in the presence of osteogenic supplements. Purified cartilage-derived CD105+/CD166+ cells did not express markers of differentiated chondrocytes. However, the cells were capable of chondrocytic differentiation and formed cartilage tissue in micromass pellet cultures.

CONCLUSION

These findings indicate that multipotential MPCs are present in adult human articular cartilage and that their frequency is increased in OA cartilage. This observation has implications for understanding the intrinsic repair capacity of articular cartilage and raises the possibility that these progenitor cells might be involved in the pathogenesis of arthritis.

Modeled microgravity inhibits osteogenic differentiation of human mesenchymal stem cells and increases adipogenesis

Space flight-induced bone loss has been attributed to a decrease in osteoblast function, without a significant change in bone resorption. To determine the effect of microgravity (MG) on bone, we used the Rotary Cell Culture System [developed by the National Aeronautics and Space Administration (NASA)] to model MG. Cultured mouse calvariae demonstrated a 3-fold decrease in alkaline phosphatase (ALP) activity and failed to mineralize after 7 d of MG. ALP and osteocalcin gene expression were also decreased. To determine the effects of MG on osteoblastogenesis, we cultured human mesenchymal stem cells (hMSC) on plastic microcarriers, and osteogenic differentiation was induced immediately before the initiation of modeled MG. A marked suppression of hMSC differentiation into osteoblasts was observed because the cells failed to express ALP, collagen 1, and osteonectin. The expression of runt-related transcription factor 2 was also inhibited. Interestingly, we found that peroxisome proliferator-activated receptor gamma (PPARgamma2), which is known to be important for adipocyte differentiation, adipsin, leptin, and glucose transporter-4 are highly expressed in response to MG. These changes were not corrected after 35 d of readaptation to normal gravity. In addition, MG decreased ERK- and increased p38-phosphorylation. These pathways are known to regulate the activity of runt-related transcription factor 2 and PPARgamma2, respectively. Taken together, our findings indicate that modeled MG inhibits the osteoblastic differentiation of hMSC and induces the development of an adipocytic lineage phenotype. This work will increase understanding and aid in the prevention of bone loss, not only in MG but also potentially in age-and disuse-related osteoporosis.

Vitamin D treatment of senescence accelerated mice (SAM-P/6) induces several regulators of stromal cell plasticity

In an attempt to understand the regulation of bone marrow multipotential cells plasticity in vivo, we treated 4-month-old SAM-P/6 mice with a constant infusion of either 18 pmol/24 h of 1,25(OH)2D3 or vehicle alone for 6 weeks. In vehicle treated animals 78% +/- 4 adipose volume vs. total volume was stained positive with oil red O as compared to only 32 +/- 3% in 1,25(OH)2D3 treated animals (P < 0.001). Furthermore, we aimed to identify the changes in gene expression induced by 1,25(OH)2D3 in bone marrow cells by analyzing a set of 5440 genes in the NIA 15K Mouse cDNA microarray. Overall, a coordinated regulation of genes which both stimulate osteoblastogenesis and inhibit adipogenesis was observed in 1,25(OH)2D3-treated mice when compared to vehicle treated mice. In summary, this study illustrates the anti-adipogenic effect of 1,25(OH)2D3 in bone cells and identifies some of the possible key signals involved in bone cell plasticity.

Thy-1 expression in human fibroblast subsets defines myofibroblastic or lipofibroblastic phenotypes

Fibroblasts represent a dynamic population of cells, exhibiting functional heterogeneity within and among tissues. Fibroblast heterogeneity also results from phenotypic differences and may arise from activation or differentiation processes taking place in the cells. We previously reported that human fibroblasts were heterogeneous with respect to surface Thy-1 expression and that separation into Thy-1(+) and Thy-1(-) subsets resulted in functionally distinct subpopulations, leading to the concept of fibroblast subset specialization. In this report we investigated whether Thy-1(+) and/or Thy-1(-) fibroblasts were capable of differentiating into myofibroblasts or lipofibroblasts. Fibroblast subsets were used from human myometrium and orbit to test this hypothesis. Only Thy-1(+) human myometrial and orbital fibroblasts were capable of myofibroblast differentiation after treatment with TGFbeta or platelet concentrate supernatant, assessed by alpha smooth muscle actin expression. Interestingly, only Thy-1(-), but not Thy-1(+) subsets differentiated to lipofibroblasts, as determined by the accumulation of cytoplasmic lipid droplets after treatment with 15-deoxy-Delta(12, 14)-PGJ(2) or ciglitazone. We propose that fibroblast Thy-1 display pre-determines lineage to a contractile or lipid-like phenotype in the human myometrium and orbit. This additional distinction between Thy-1(+) and Thy-1(-) human fibroblast subtypes has important consequences in normal tissue homeostasis and in pathogenesis of orbital and myometrial diseases characterized by persistent myofibroblasts or fat accumulation, such as occurs in Graves' ophthalmopathy, tissue fibrosis, abnormal wound healing, and scarring.

Angiogenesis and marrow stromal cell fates: roles in bone strength

In vivo osteogenic responses to anabolic stimuli--whether pharmacological or mechanical--are invariably accompanied by neovascularization. Microvascular endothelial-mesenchymal interactions have emerged that provide mechanistic insight into the roles of angiogenesis in the osteoanabolic response; these interactions resemble the epithelial-mesenchymal signaling that controls tissue morphogenesis during prenatal development. Microvascular smooth muscle cells called pericytes function as multipotent mesenchymal progenitors that contribute to bone, fat, cartilage and smooth muscle formation throughout life. This abbreviated overview recounts progress made in the past decade that highlights the physiological contributions of angiogenesis to bone formation and bone strength. It highlights the need to support research that details the mechanisms whereby angiogenesis, metabolic milieu and mechanical stimuli interact to control marrow stromal cell fate during the postnatal developmental process of aging and the disease processes of musculoskeletal frailty.

Intravenous versus intrastriatal cord blood administration in a rodent model of stroke

Human umbilical cord blood (hUCB) is a rich source of hematopoietic stem cells that have been used to reconstitute immune cells and blood lineages. Cells from another hematopoietic source, bone marrow, have been found to differentiate into neural cells and are effective in the treatment of stroke. In this study, we administered hUCB cells intravenously into the femoral vein or directly into the striatum and assessed which route of cell administration produced the greatest behavioral recovery in rats with permanent middle cerebral artery occlusion (MCAO). All animals were immunosuppressed with cyclosporine (CSA). When spontaneous activity was measured using the Digiscan automated system, it was found to be significantly less when hUCB was transplanted 24 hr after stroke compared with nontransplanted, stroked animals (P < 0.01). Furthermore, behavioral recovery was similar with both striatal and femoral hUCB delivery. This is in contrast to the step test, in which significant improvements were found only after femoral delivery of the hUCB cells. In the passive avoidance test, transplanted animals learned the task faster than nontransplanted animals (P < 0.05). Together, these results suggest that hUCB transplantation may be an effective treatment for brain injuries, such as stroke, or neurodegenerative disorders. In addition, intravenous delivery may be more effective than striatal delivery in producing long-term functional benefits to the stroked animal.