Insulin-like growth factor-I and bone: lessons from mice and men

Impact of metabolic syndrome on bone mineral density in men over 50 and postmenopausal women according to U.S. survey results

Metabolic Syndrome (MetS) and bone mineral density (BMD) have shown a controversial link in some studies. This research aims to study their association in males over 50 and postmenopausal females using National Health and Nutrition Examination Survey (NHANES) data. Postmenopausal females and males over 50 were included in the study. MetS was defined by the National Cholesterol Education Program Adult Treatment Panel III guidelines. BMD values were measured at the thoracic spine, lumbar spine, and pelvis as the primary outcome. Weighted multivariate general linear models have been employed to explore the status of BMD in patients with MetS. Additionally, interaction tests and subgroup analyses were conducted. Utilizing the NHANES database from 2003 to 2006 and 2011-2018, we included 1924 participants, with 1029 males and 895 females. In postmenopausal women, after adjusting for covariates, we found a positive correlation between MetS and pelvic (β: 0.030 [95%CI 0.003, 0.06]) and thoracic (β: 0.030 [95%CI 0.01, 0.06]) BMD, though not for lumbar spine BMD (β: 0.020 [95%CI - 0.01, 0.05]). In males over 50 years old, MetS was positively correlated with BMD in both Model 1 (without adjusting for covariates) and Model 2 (considering age and ethnicity). Specifically, Model 2 revealed a positive correlation between MetS and BMD at the pelvis (β: 0.046 [95%CI 0.02, 0.07]), thoracic spine (β: 0.047 [95%CI 0.02, 0.07]), and lumbar spine (β: 0.040 [95%CI 0.02, 0.06]). Subgroup analysis demonstrated that the relationship between MetS and BMD remained consistent in all strata, underscoring the stability of the findings. In postmenopausal women, after adjusting for all covariates, a significant positive correlation was observed between MetS and BMD in the pelvis and thoracic spine, whereas this correlation was not significant for lumbar spine BMD. Conversely, in males, positive correlations between MetS and BMD at the lumbar spine, thoracic spine, and pelvis were identified in Model 2, which adjusted for age and ethnicity; however, these correlations disappeared after fully adjusting for all covariates. These findings highlight the potential moderating role of gender in the impact of MetS on BMD.

Circadian control of Nocturnin and its regulatory role in health and disease

Circadian rhythms are generated by intrinsic 24-h oscillations that anticipate the extrinsic changes associated with solar day. A conserved transcriptional-translational feedback loop generates these molecular oscillations of clock genes at the organismal and the cellular levels. One of the recently discovered outputs of circadian clock is Nocturnin (Noct) or Ccrn4l. In mice, Noct mRNA is broadly expressed in cells throughout the body, with a particularly high-amplitude rhythm in liver. NOCT belongs to the EEP family of proteins with the closest similarity to the CCR4 family of deadenylases. Multiple studies have investigated the role of Nocturnin in development, adipogenesis, lipid metabolism, inflammation, osteogenesis, and obesity. Further, mice lacking Noct (Noct KO or Noct-/-) are protected from high-fat diet-induced obesity and hepatic steatosis. Recent studies had provided new insights by investigating various aspects of Nocturnin, ranging from its sub-cellular localization to identification of its target transcripts. However, a profound understanding of its molecular function remains elusive. This review article seeks to integrate the available literature into our current understanding of the functions of Nocturnin, their regulatory roles in key tissues and to throw light on the existing scientific lacunae.

Activation of Focal Adhesion Kinase Restores Simulated Microgravity-Induced Inhibition of Osteoblast Differentiation via Wnt/Β-Catenin Pathway

Simulated microgravity (SMG) inhibits osteoblast differentiation (OBD) and induces bone loss via the inhibition of the Wnt/β-catenin pathway. However, the mechanism by which SMG alters the Wnt/β-catenin pathway is unknown. We previously demonstrated that SMG altered the focal adhesion kinase (FAK)-regulated mTORC1, AMPK and ERK1/2 pathways, leading to the inhibition of tumor cell proliferation/metastasis and promoting cell apoptosis. To examine whether FAK similarly mediates SMG-dependent changes to Wnt/β-catenin in osteoblasts, we characterized mouse MC3T3-E1 cells cultured under clinostat-modeled SMG (µg) conditions. Compared to cells cultured under ground (1 g) conditions, SMG reduces focal adhesions, alters cytoskeleton structures, and down-regulates FAK, Wnt/β-catenin and Wnt/β-catenin-regulated molecules. Consequently, protein-2 (BMP2), type-1 collagen (COL1), alkaline-phosphatase activity and matrix mineralization are all inhibited. In the mouse hindlimb unloading (HU) model, SMG-affected tibial trabecular bone loss is significantly reduced, according to histological and micro-computed tomography analyses. Interestingly, the FAK activator, cytotoxic necrotizing factor-1 (CNF1), significantly suppresses all of the SMG-induced alterations in MC3T3-E1 cells and the HU model. Therefore, our data demonstrate the critical role of FAK in the SMG-induced inhibition of OBD and bone loss via the Wnt/β-catenin pathway, offering FAK signaling as a new therapeutic target not only for astronauts at risk of OBD inhibition and bone loss, but also osteoporotic patients.

Efficient isolation of human gingival stem cells in a new serum-free medium supplemented with platelet lysate and growth hormone for osteogenic differentiation enhancement

BACKGROUND

The use of distant autografts to restore maxillary bone defects is clinically challenging and has unpredictable outcomes. This variation may be explained by the embryonic origin of long bone donor sites, which are derived from mesoderm, whereas maxillary bones derive from neural crest. Gingival stem cells share the same embryonic origin as maxillary bones. Their stemness potential and ease of access have been repeatedly shown. One limitation in human cell therapy is the use of foetal calf serum during cell isolation and culture. To overcome this problem, a new serum-free medium enriched with an alternative to foetal calf serum, i.e., platelet lysate, needs to be adapted to clinical grade protocols.

METHODS

Different serum-free media enriched with platelet lysate at various concentrations and supplemented with different growth factors were developed and compared to media containing foetal calf serum. Phenotypic markers, spontaneous DNA damage, and stem cell properties of gingival stem cells isolated in platelet lysate or in foetal calf serum were also compared, as were the immunomodulatory properties of the cells by co-culturing them with activated peripheral blood monocellular cells. T-cell proliferation and phenotype were also assessed by flow cytometry using cell proliferation dye and specific surface markers. Data were analysed with t-test for two-group comparisons, one-way ANOVA for multigroup comparisons and two-way ANOVA for repeated measures and multigroup comparisons.

RESULTS

Serum-free medium enriched with 10% platelet lysate and growth hormone yielded the highest expansion rate. Gingival stem cell isolation and thawing under these conditions were successful, and no significant DNA lesions were detected. Phenotypic markers of mesenchymal stem cells and differentiation capacities were conserved. Gingival stem cells isolated in this new serum-free medium showed higher osteogenic differentiation potential compared to cells isolated in foetal calf serum. The proportion of regulatory T cells obtained by co-culturing gingival stem cells with activated peripheral blood monocellular cells was similar between the two types of media.

CONCLUSIONS

This new serum-free medium is well suited for gingival stem cell isolation and proliferation, enhances osteogenic capacity and maintains immunomodulatory properties. It may allow the use of gingival stem cells in human cell therapy for bone regeneration in accordance with good manufacturing practice guidelines.

Cystic fibrosis bone disease treatment: Current knowledge and future directions

Bone disease is a frequent complication in adolescents and adults with cystic fibrosis (CF). Early detection and monitoring of bone mineral density and multidisciplinary preventive care are necessary from childhood through adolescence to minimize CF-related bone disease (CFBD) in adult CF patients. Approaches to optimizing bone health include ensuring adequate nutrition, particularly intake of calcium and vitamins D and K, addressing other secondary causes of low bone density such as hypogonadism, encouraging weight bearing exercise, and avoiding bone toxic medications. Of the currently available anti-resorptive or anabolic osteoporosis medications, only bisphosphonates have been studied in individuals with CF. Future studies are needed to better understand the optimal approach for managing CFBD.

miR-1827 inhibits osteogenic differentiation by targeting IGF1 in MSMSCs

We recently reported that maxillary sinus membrane stem cells (MSMSCs) have osteogenic potential. However, the biological mechanisms of bone formation remain unclear. In this study, we investigated the role and mechanisms of microRNAs (miRNAs) in the osteogenic differentiation of MSMSCs. The expression of miRNAs was determined in differentiated MSMSCs by comprehensive miRNA microarray analysis and quantitative RT-PCR (qRT-PCR). We selected miR-1827 for functional follow-up studies to explore its significance in MSMSCs. Here, miR-1827 was found to be up-regulated during osteogenic differentiation of MSMSCs. Over expression of miR-1827 inhibited osteogenic differentiation of MSMSCs in vitro, whereas the repression of miR-1827 greatly promoted cell differentiation. Further experiments confirmed that insulin-like growth factor 1 (IGF1) is a direct target of miR-1827. miR-1827 inhibited osteogenic differentiation partially via IGF1, which in turn is a positive regulator of osteogenic differentiation. Moreover, miR-1827 suppressed ectopic bone formation and silencing of miR-1827 led to increased bone formation in vivo. In summary, this study is the first to demonstrate that miR-1827 can regulate osteogenic differentiation. The increase in miR-1827 expression observed during osteogenesis is likely a negative feedback mechanism, thus offering a potential therapeutic target to address inadequate bone volume for dental implantation through inhibiting miR-1827.

Association Between Insulin Resistance and Bone Structure in Nondiabetic Postmenopausal Women

CONTEXT

The clinical consequences of insulin resistance and hyperinsulinemia on bone remain largely unknown.

OBJECTIVE

The objective of the study was to evaluate the effect of insulin resistance on peripheral bone geometry, volumetric bone mineral density (vBMD), bone microarchitecture, and estimated bone strength.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study included 146 postmenopausal, nondiabetic Caucasian women (mean age 60.3 ± 2.7 y) who were participating in the Study of Women's Health Across the Nation.

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

High-resolution peripheral quantitative computed tomography was used to assess bone density and microstructure at the distal radius and tibia. Fasting insulin and glucose were measured and insulin resistance was estimated using homeostasis model assessment of insulin resistance (HOMA-IR), with higher values indicating greater insulin resistance.

RESULTS

There was a negative association between HOMA-IR and bone size and a positive association between HOMA-IR and total vBMD, trabecular vBMD, trabecular thickness, and cortical thickness at the radius and tibia. These relationships remained, even after adjusting for body weight and other potential covariates (eg, time since menopause, cigarette smoking, physical activity, prior use of osteoporosis medications or glucocorticoids).

CONCLUSIONS

In nondiabetic, postmenopausal women, insulin resistance was associated with smaller bone size, greater volumetric bone mineral density, and generally favorable bone microarchitecture at weight-bearing and nonweight-bearing skeletal sites. These associations were independent of body weight and other potential covariates, suggesting that hyperinsulinemia directly affects bone structure independent of obesity and may explain, in part, the higher trabecular bone density and favorable trabecular microarchitecture seen in individuals with type 2 diabetes mellitus.

Proportionate Dwarfism in Mice Lacking Heterochromatin Protein 1 Binding Protein 3 (HP1BP3) Is Associated With Alterations in the Endocrine IGF-1 Pathway

Heterochromatin protein 1 binding protein 3 (HP1BP3) is a recently described histone H1-related protein with roles in chromatin structure and transcriptional regulation. To explore the potential physiological role of HP1BP3, we have previously described an Hp1bp3(-/-) mouse model with reduced postnatal viability and growth. We now find that these mice are proportionate dwarfs, with reduction in body weight, body length, and organ weight. In addition to their small size, microcomputed tomography analysis showed that Hp1bp3(-/-) mice present a dramatic impairment of their bone development and structure. By 3 weeks of age, mice of both sexes have severely impaired cortical and trabecular bone, and these defects persist into adulthood and beyond. Primary cultures of both osteoblasts and osteoclasts from Hp1bp3(-/-) bone marrow and splenocytes, respectively, showed normal differentiation and function, strongly suggesting that the impaired bone accrual is due to noncell autonomous systemic cues in vivo. One major endocrine pathway regulating both body growth and bone acquisition is the IGF regulatory system, composed of IGF-1, the IGF receptors, and the IGF-binding proteins (IGFBPs). At 3 weeks of age, Hp1bp3(-/-) mice exhibited a 60% reduction in circulating IGF-1 and a 4-fold increase in the levels of IGFBP-1 and IGFBP-2. These alterations were reflected in similar changes in the hepatic transcripts of the Igf1, Igfbp1, and Igfbp2 genes. Collectively, these results suggest that HP1BP3 plays a key role in normal growth and bone development by regulating transcription of endocrine IGF-1 components.

Adipose tissue and bone: role of PPARγ in adipogenesis and osteogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a critical factor for the reciprocal regulation of adipogenesis and osteogenesis. Because of their insulin-sensitizing effect, PPARγ agonists, the thiazolidinediones (TZDs), have been widely used for the treatment of type 2 diabetes mellitus; however, the use of TZDs has also been revealed to cause bone loss and bone fractures. The nodal point of regulation of skeletal metabolism by PPARγ activation may reside in its role in cell fate determination of mesenchymal stem cells toward adipogenesis at the expense of osteogenesis. In addition, accumulating evidence demonstrates that PPARγ possesses a circadian expression profile and plays an important role in the skeletal and adipose metabolism regulated by the circadian clock network. Recently, we have shown that nocturnin, a circadian-regulated gene, enhances PPARγ activity, resulting in the suppression of osteogenesis and enhancement of adipogenesis, thus providing additional evidence of the link between circadian networks and PPARγ. In this review, we will focus on the emerging concept of PPARγ as a regulator for skeletal metabolism and summarize recent findings about one of the mechanisms on how PPARγ is connected to the circadian-regulatory system, which involves nocturnin.

Diabetes, diabetic complications, and fracture risk

Diabetes and osteoporosis are both common diseases with increasing prevalences in the aging population. There is increasing evidence corroborating an association between diabetes mellitus and bone. This review will discuss the disease complications of diabetes on the skeleton, highlighting findings from epidemiological, molecular, and imaging studies in animal models and humans. Compared to control subjects, decreased bone mineral density (BMD) has been observed in type 1 diabetes mellitus, while on average, higher BMD has been found in type 2 diabetes; nonetheless, patients with both types of diabetes are seemingly at increased risk of fractures. Conventional diagnostics such as DXA measurements and the current fracture risk assessment tool (FRAX) risk prediction algorithm for estimating risk of osteoporotic fractures are not sufficient in the case of diabetes. A deterioration in bone microarchitecture and an inefficient distribution of bone mass with insufficiency of repair and adaptation mechanisms appear to be factors of relevance. A highly complex and heterogeneous molecular pathophysiology underlies diabetes-related bone disease, involving hormonal, immune, and perhaps genetic pathways. The detrimental effects of chronically elevated glucose levels on bone should be added to the more well-known complications of diabetes.

Combined treatment with GH and IGF-I: additive effect on cortical bone mass but not on linear bone growth in female rats

The growth-promoting effect of combined therapy with GH and IGF-I in normal rats is not known. We therefore investigated the efficacy of treatment with recombinant human (rh)GH and/or rhIGF-I on longitudinal bone growth and bone mass in intact, prepubertal, female Sprague-Dawley rats. rhGH was injected twice daily sc (5 mg/kg·d) and rhIGF-I continuously infused sc (2.2 or 4.4 mg/kg·d) for 28 days. Longitudinal bone growth was monitored by weekly x-rays of tibiae and nose-anus length measurements, and tibial growth plate histomorphology was analyzed. Bone mass was evaluated by peripheral quantitative computed tomography. In addition, serum levels of IGF-I, rat GH, acid labile subunit, IGF binding protein-3, 150-kDa ternary complex formation, and markers of bone formation and degradation were measured. Monotherapy with rhGH was more effective than rhIGF-I (4.4 mg/kg·d) to increase tibia and nose-anus length, whereas combined therapy did not further increase tibia, or nose-anus, lengths or growth plate height. In contrast, combined rhGH and rhIGF-I (4.4 mg/kg·d) therapy had an additive stimulatory effect on cortical bone mass vs rhGH alone. Combined treatment with rhGH and rhIGF-I resulted in markedly higher serum IGF-I concentrations vs rhGH alone but did not compromise the endogenous secretion of GH. We conclude that rhIGF-I treatment augments cortical bone mass but does not further improve bone growth in rhGH-treated young, intact, female rats.

Effect of whole-body vibration and insulin-like growth factor-I on muscle paralysis-induced bone degeneration after botulinum toxin injection in mice

Botulinum toxin A (BTX)-induced muscle paralysis results in pronounced bone degradation with substantial bone loss. We hypothesized that whole-body vibration (WBV) and insulin-like growth factor-I (IGF-I) treatment can counteract paralysis-induced bone degradation following BTX injections by activation of the protein kinase B (Akt) signaling pathway. Female C57BL/6 mice (n = 60, 16 weeks) were assigned into six groups (n = 10 each): SHAM, BTX, BTX+WBV, BTX+IGF-I, BTX+WBV+IGF-I, and a baseline group, which was killed at the beginning of the study. Mice received a BTX (1.0 U/0.1 mL) or saline (SHAM) injection in the right hind limb. The BTX+IGF-I and BTX+WBV+IGF-I groups obtained daily subcutaneous injections of human IGF-I (1 μg/day). The BTX+WBV and BTX+WBV+IGF-I groups underwent WBV (25 Hz, 2.1 g, 0.83 mm) for 30 min/day, 5 days/week for 4 weeks. Femora were scanned by pQCT, and mechanical properties were determined. On tibial sections TRAP staining, static histomorphometry, and immunohistochemical staining against Akt, phospho-Akt, IGF-IR (IGF-I receptor), and phospho-IGF-IR were conducted. BTX injection decreased trabecular and cortical bone mineral density. The WBV and WBV+IGF-I groups showed no difference in trabecular bone mineral density compared to the SHAM group. The phospho-IGF-IR and phospho-Akt stainings were not differentially altered in the injected hind limbs between groups. We found that high-frequency, low-magnitude WBV can counteract paralysis-induced bone loss following BTX injections, while we could not detect any effect of treatment with IGF-I.

New insights into IGF-1 signaling in the heart

Insulin-like growth factor 1 (IGF-1) signaling regulates contractility, metabolism, hypertrophy, autophagy, senescence, and apoptosis in the heart. IGF-1 deficiency is associated with an increased risk of cardiovascular disease, whereas cardiac activation of IGF-1 receptor (IGF-1R) protects from the detrimental effects of a high-fat diet and myocardial infarction. IGF-1R activates multiple pathways through its intrinsic tyrosine kinase activity and through coupling to heterotrimeric G protein. These pathways involve classic second messengers, phosphorylation cascades, lipid signaling, Ca(2+) transients, and gene expression. In addition, IGF-1R triggers signaling in different subcellular locations including the plasma membrane, perinuclear T tubules, and also in internalized vesicles. In this review, we provide a fresh and updated view of the complex IGF-1 scenario in the heart, including a critical focus on therapeutic strategies.

Wrist breadth and homeostasis model assessment of insulin resistance in youth: the Fels Longitudinal Study

OBJECTIVES

There is biological crosstalk between insulin signaling and bone remodeling pathways, and wrist circumference and bone area were recently found to associate with insulin resistance independent of body mass index (BMI) in overweight/obese children. We aimed to expand on this work by using more specific measures of adiposity for adjustment and examining children with broader range of BMI.

METHODS

We used serial data (1,051 total measures) on 313 non-Hispanic white youth (ages 8-18 y) from the Fels Longitudinal Study with homeostasis model assessment of insulin resistance (HOMA-IR) as the outcome. Internal standard deviation score (SDS) for wrist breadth was evaluated as a predictor of HOMA-IR (log-transformed) before and after adjusting for internal-sample SDSs for BMI, waist circumference (WC), and total body fat (TBF) from dual energy X-ray absorptiometry, in addition to age, sex, Tanner stage, and birth year, using generalized estimating equations.

RESULTS

Before additional adiposity adjustment, we found a significant positive association between wrist breadth SDS and log-transformed HOMA-IR (β = 0.13; 95%CI: 0.09-0.17), which remained significant after adjusting for TBF SDS (β = 0.09; 95%CI: 0.05-0.13; P < 0.001), BMI SDS (β = 0.06; 95%CI: 0.02-0.10; P = 0.007), and WC SDS (β = 0.06; 95%CI: 0.02-0.09; P = 0.005).

CONCLUSIONS

Further work is needed to determine whether simple frame size measures such as wrist breadth may be useful markers of metabolic risk.

Selective tyrosine kinase inhibition of insulin-like growth factor-1 receptor inhibits human and mouse breast cancer-induced bone cell activity, bone remodeling, and osteolysis

Insulin-like growth factor 1 (IGF-1) plays an important role in both bone metabolism and breast cancer. In this study, we investigated the effects of the novel IGF-1 receptor tyrosine kinase inhibitor cis-3-[3-(4-methyl-piperazin-l-yl)-cyclobutyl]-1-(2-phenyl-quinolin-7-yl)-imidazo[1,5-a]pyrazin-8-ylamine (PQIP) on osteolytic bone disease associated with breast cancer. Human MDA-MB-231 and mouse 4T1 breast cancer cells enhanced osteoclast formation in receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) stimulated bone marrow cultures, and these effects were significantly inhibited by PQIP. Functional studies in osteoclasts showed that PQIP inhibited both IGF-1 and conditioned medium-induced osteoclast formation by preventing phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) activation without interfering with RANKL or M-CSF signaling. Treatment of osteoblasts with PQIP significantly inhibited the increase in RANKL/osteoprotegerin (OPG) ratio by IGF-1 and conditioned medium and totally prevented conditioned medium-induced osteoclast formation in osteoblast-bone marrow (BM) cell cocultures, thereby suggesting an inhibitory effect on osteoblast-osteoclast coupling. PQIP also inhibited IGF-1-induced osteoblast differentiation, spreading, migration, and bone nodule formation. Treatment with PQIP significantly reduced MDA-MB-231 conditioned medium-induced osteolytic bone loss in a mouse calvarial organ culture system ex vivo and in adult mice in vivo. Moreover, once daily oral administration of PQIP significantly decreased trabecular bone loss and reduced the size of osteolytic bone lesions following 4T1 intratibial injection in mice. Quantitative histomorphometry showed a significant reduction in bone resorption and formation indices, indicative of a reduced rate of cancer-associated bone turnover. We conclude that inhibition of IGF-1 receptor tyrosine kinase activity by PQIP suppresses breast cancer-induced bone turnover and osteolysis. Therefore, PQIP, and its novel derivatives that are currently in advanced clinical development for the treatment of a number of solid tumors, may be of value in the treatment of osteolytic bone disease associated with breast cancer.

Insulin-like growth factor-I enhances proliferation and differentiation of human mesenchymal stromal cells in vitro

Human mesenchymal stromal cells (hMSCs) offer great potential for bone tissue engineering applications, but their in vivo performance remains limited. Preconditioning of these cells with small molecules to improve their differentiation before implantation, or incorporation of growth factors are possible solutions. Insulin-like growth factor-1 (IGF-1) is one of the most abundant growth factors in bone, involved in growth, development, and metabolism, but its effects on hMSCs are still subject of debate. Here we examined the effects of IGF-1 on proliferation and differentiation of hMSCs in vitro and we found that serum abolished the effects of IGF-1. Only in the absence of serum, IGF-1 increased proliferation, alkaline phosphatase expression, and osteogenic gene expression of hMSCs. Furthermore, we examined synergistic effects of bone morphogenetic protein-2 (BMP-2) and IGF-1 and, although IGF-1 enhanced BMP-2-induced mineralization, IGF-1 only slightly affected in vivo bone formation.

Review of Signaling Pathways Governing MSC Osteogenic and Adipogenic Differentiation

Mesenchymal stem cells (MSC) are multipotent cells, functioning as precursors to a variety of cell types including adipocytes, osteoblasts, and chondrocytes. Between osteogenic and adipogenic lineage commitment and differentiation, a theoretical inverse relationship exists, such that differentiation towards an osteoblast phenotype occurs at the expense of an adipocytic phenotype. This balance is regulated by numerous, intersecting signaling pathways that converge on the regulation of two main transcription factors: peroxisome proliferator-activated receptor- γ (PPAR γ ) and Runt-related transcription factor 2 (Runx2). These two transcription factors, PPAR γ and Runx2, are generally regarded as the master regulators of adipogenesis and osteogenesis. This review will summarize signaling pathways that govern MSC fate towards osteogenic or adipocytic differentiation. A number of signaling pathways follow the inverse balance between osteogenic and adipogenic differentiation and are generally proosteogenic/antiadipogenic stimuli. These include β -catenin dependent Wnt signaling, Hedgehog signaling, and NELL-1 signaling. However, other signaling pathways exhibit more context-dependent effects on adipogenic and osteogenic differentiation. These include bone morphogenic protein (BMP) signaling and insulin growth factor (IGF) signaling, which display both proosteogenic and proadipogenic effects. In summary, understanding those factors that govern osteogenic versus adipogenic MSC differentiation has significant implications in diverse areas of human health, from obesity to osteoporosis to regenerative medicine.

New insights into osteoporosis: the bone-fat connection

Osteoporosis and obesity are chronic disorders that are both increasing in prevalence. The pathophysiology of these conditions is multifactorial and includes genetic, environmental and hormonal determinants. Although it has long been considered that these are distinct disorders rarely found in the same individual, emerging evidence from basic and clinical studies support an important interaction between adipose tissue and the skeleton. It is proposed that adiposity may influence bone remodelling through three mechanisms: (i) secretion of cytokines that directly target bone, (ii) production of adipokines that influence the central nervous system thereby changing sympathetic impulses to bone and (iii) paracrine influences on adjacent skeletal cells. Here we focus on the current understanding of bone-fat interactions and the clinical implications of recent studies linking obesity to osteoporosis.

Association between bone mineral density and type 2 diabetes mellitus: a meta-analysis of observational studies

Type 2 diabetes mellitus (T2DM) influences bone metabolism, but the relation of T2DM with bone mineral density (BMD) remains inconsistent across studies. The objective of this study was to perform a meta-analysis and meta-regression of the literature to estimate the difference in BMD (g/cm²) between diabetic and non-diabetic populations, and to investigate potential underlying mechanisms. A literature search was performed in PubMed and Ovid extracting data from articles prior to May 2010. Eligible studies were those where the association between T2DM and BMD measured by dual energy X-ray absorptiometry was evaluated using a cross-sectional, cohort or case–control design, including both healthy controls and subjects with T2DM. The analysis was done on 15 observational studies (3,437 diabetics and 19,139 controls). Meta-analysis showed that BMD in diabetics was significantly higher, with pooled mean differences of 0.04 (95% CI: 0.02, 0.05) at the femoral neck, 0.06 (95% CI: 0.04, 0.08) at the hip and 0.06 (95% CI: 0.04, 0.07) at the spine. The differences for forearm BMD were not significantly different between diabetics and non-diabetics. Sex-stratified analyses showed similar results in both genders. Substantial heterogeneity was found to originate from differences in study design and possibly diabetes definition. Also, by applying meta-regression we could establish that younger age, male gender, higher body mass index and higher HbA_1C were positively associated with higher BMD levels in diabetic individuals. We conclude that individuals with T2DM from both genders have higher BMD levels, but that multiple factors influence BMD in individuals with T2DM.

Association between bone mineral density and type 2 diabetes mellitus: a meta-analysis of observational studies

Type 2 diabetes mellitus (T2DM) influences bone metabolism, but the relation of T2DM with bone mineral density (BMD) remains inconsistent across studies. The objective of this study was to perform a meta-analysis and meta-regression of the literature to estimate the difference in BMD (g/cm(2)) between diabetic and non-diabetic populations, and to investigate potential underlying mechanisms. A literature search was performed in PubMed and Ovid extracting data from articles prior to May 2010. Eligible studies were those where the association between T2DM and BMD measured by dual energy X-ray absorptiometry was evaluated using a cross-sectional, cohort or case-control design, including both healthy controls and subjects with T2DM. The analysis was done on 15 observational studies (3,437 diabetics and 19,139 controls). Meta-analysis showed that BMD in diabetics was significantly higher, with pooled mean differences of 0.04 (95% CI: 0.02, 0.05) at the femoral neck, 0.06 (95% CI: 0.04, 0.08) at the hip and 0.06 (95% CI: 0.04, 0.07) at the spine. The differences for forearm BMD were not significantly different between diabetics and non-diabetics. Sex-stratified analyses showed similar results in both genders. Substantial heterogeneity was found to originate from differences in study design and possibly diabetes definition. Also, by applying meta-regression we could establish that younger age, male gender, higher body mass index and higher HbA(1C) were positively associated with higher BMD levels in diabetic individuals. We conclude that individuals with T2DM from both genders have higher BMD levels, but that multiple factors influence BMD in individuals with T2DM.

Association between bone mineral density and type 2 diabetes mellitus: a meta-analysis of observational studies

Type 2 diabetes mellitus (T2DM) influences bone metabolism, but the relation of T2DM with bone mineral density (BMD) remains inconsistent across studies. The objective of this study was to perform a meta-analysis and meta-regression of the literature to estimate the difference in BMD (g/cm(2)) between diabetic and non-diabetic populations, and to investigate potential underlying mechanisms. A literature search was performed in PubMed and Ovid extracting data from articles prior to May 2010. Eligible studies were those where the association between T2DM and BMD measured by dual energy X-ray absorptiometry was evaluated using a cross-sectional, cohort or case-control design, including both healthy controls and subjects with T2DM. The analysis was done on 15 observational studies (3,437 diabetics and 19,139 controls). Meta-analysis showed that BMD in diabetics was significantly higher, with pooled mean differences of 0.04 (95% CI: 0.02, 0.05) at the femoral neck, 0.06 (95% CI: 0.04, 0.08) at the hip and 0.06 (95% CI: 0.04, 0.07) at the spine. The differences for forearm BMD were not significantly different between diabetics and non-diabetics. Sex-stratified analyses showed similar results in both genders. Substantial heterogeneity was found to originate from differences in study design and possibly diabetes definition. Also, by applying meta-regression we could establish that younger age, male gender, higher body mass index and higher HbA(1C) were positively associated with higher BMD levels in diabetic individuals. We conclude that individuals with T2DM from both genders have higher BMD levels, but that multiple factors influence BMD in individuals with T2DM.

Administration of BMP2/7 in utero partially reverses Rubinstein-Taybi syndrome-like skeletal defects induced by Pdk1 or Cbp mutations in mice

Mutations in the coactivator CREB-binding protein (CBP) are a major cause of the human skeletal dysplasia Rubinstein-Taybi syndrome (RTS); however, the mechanism by which these mutations affect skeletal mineralization and patterning is unknown. Here, we report the identification of 3-phosphoinositide-dependent kinase 1 (PDK1) as a key regulator of CBP activity and demonstrate that its functions map to both osteoprogenitor cells and mature osteoblasts. In osteoblasts, PDK1 activated the CREB/CBP complex, which in turn controlled runt-related transcription factor 2 (RUNX2) activation and expression of bone morphogenetic protein 2 (BMP2). These pathways also operated in vivo, as evidenced by recapitulation of RTS spectrum phenotypes with osteoblast-specific Pdk1 deletion in mice (Pdk1osx mice) and by the genetic interactions observed in mice heterozygous for both osteoblast-specific Pdk1 deletion and either Runx2 or Creb deletion. Finally, treatment of Pdk1osx and Cbp+/- embryos with BMPs in utero partially reversed their skeletal anomalies at birth. These findings illustrate the in vivo function of the PDK1-AKT-CREB/CBP pathway in bone formation and provide proof of principle for in utero growth factor supplementation as a potential therapy for skeletal dysplasias.

Stem cell antigen-1 positive cell-based systemic human growth hormone gene transfer strategy increases endosteal bone resorption and bone loss in mice

BACKGROUND

The present study assesses the effect of the stem cell antigen-1 positive (Sca-1(+) ) cell-based human growth hormone (hGH) ex vivo gene transfer strategy on endosteal bone mass in the mouse.

METHODS

Sublethally irradiated recipient mice were transplanted with Sca-1(+) cells transduced with lentiviral vectors expressing hGH or β-galactosidase control genes. Bone parameters were assessed by micro-computed tomography and histomorphometry.

RESULTS

This hGH strategy drastically increased hGH mRNA levels in bone marrow cells and serum insulin-like growth factor-I (IGF-I) (by nearly 50%, p < 0.002) in hGH recipient mice. Femoral trabecular bone volume of the hGH mice was significantly reduced by 35% (p < 0.002). The hGH mice also had decreased trabecular number (by 26%; p < 0.0001), increased trabecular separation (by 38%; p < 0.0002) and reduced trabecular connectivity density (by 64%; p < 0.001), as well as significantly more osteoclasts (2.5-fold; p < 0.05) and greater osteoclastic surface per bone surface (2.6-fold; p < 0.01).

CONCLUSIONS

Targeted expression of hGH in cells of marrow cavity through the Sca-1(+) cell-based gene transfer strategy increased circulating IGF-I and decreased endosteal bone mass through an increase in resorption in recipient mice. These results indicate that high local levels of hGH or IGF-I in the bone marrow microenvironment enhanced resorption, which is consistent with previous findings in transgenic mice with targeted bone IGF-I expression showing that high local IGF-I expression increased bone remodeling, favoring a net bone loss. Thus, GH and/or IGF-I would not be an appropriate transgene for use in this Sca-1(+) cell-based gene transfer strategy to promote endosteal bone formation. Published 2011 John Wiley & Sons, Ltd.

Older men with low serum IGF-1 have an increased risk of incident fractures: the MrOS Sweden study

Osteoporosis-related fractures constitute a major health concern not only in women but also in men. Insulin-like growth factor 1 (IGF-1) is a key determinant of bone mass, but the association between serum IGF-1 and incident fractures in men remains unclear. To determine the predictive value of serum IGF-1 for fracture risk in men, older men (n = 2902, mean age of 75 years) participating in the prospective, population-based Osteoporotic Fractures in Men (MrOS) Sweden study were followed for a mean of 3.3 years. Serum IGF-1 was measured at baseline by radioimmunoassay. Fractures occurring after the baseline visit were validated. In age-adjusted hazards regression analyses, serum IGF-1 associated inversely with risk of all fractures [hazard ratio (HR) per SD decrease = 1.23, 95% confidence interval (CI) 1.07-1.41], hip fractures (HR per SD decrease = 1.45, 95% CI 1.07-1.97), and clinical vertebral fractures (HR per SD decrease = 1.40, 95% CI 1.10-1-78). The predictive role of serum IGF-1 for fracture risk was unaffected by adjustment for height, weight, prevalent fractures, falls, and major prevalent diseases. Further adjustment for bone mineral density (BMD) resulted in an attenuated but still significant association between serum IGF-1 and fracture risk. Serum IGF-1 below but not above the median was inversely related to fracture incidence. The population-attributable risk proportion was 7.5% for all fractures and 22.9% for hip fractures. Taken together, older men with low serum IGF-1 have an increased fracture risk, especially for the two most important fracture types, hip and vertebral fractures. The association between serum IGF-1 and fracture risk is partly mediated via BMD.

The heparin-binding domain of IGFBP-2 has insulin-like growth factor binding-independent biologic activity in the growing skeleton

Insulin-like growth factor-binding protein 2 (IGFBP-2) is a member of a family of six highly conserved IGFBPs that are carriers for the insulin-like growth factors (IGFs). IGFBP-2 levels rise during rapid neonatal growth and at the time of peak bone acquisition. In contrast, Igfbp2(-/-) mice have low bone mass accompanied by reduced osteoblast numbers, low bone formation rates, and increased PTEN expression. In the current study, we postulated that IGFBP-2 increased bone mass partly through the activity of its heparin-binding domain (HBD). We synthesized a HBD peptide specific for IGFBP-2 and demonstrated in vitro that it rescued the mineralization phenotype of Igfbp2(-/-) bone marrow stromal cells and calvarial osteoblasts. Consistent with its cellular actions, the HBD peptide ex vivo stimulated metacarpal periosteal expansion. Furthermore, administration of HBD peptide to Igfbp2(-/-) mice increased osteoblast number, suppressed marrow adipogenesis, restored trabecular bone mass, and reduced bone resorption. Skeletal rescue in the Igfbp2(-/-) mice was characterized by reduced PTEN expression followed by enhanced Akt phosphorylation in response to IGF-I and increased β-catenin signaling through two mechanisms: 1) stimulation of its cytosolic accumulation and 2) increased phosphorylation of serine 552. We conclude that the HBD peptide of IGFBP-2 has anabolic activity by activating IGF-I/Akt and β-catenin signaling pathways. These data support a growing body of evidence that IGFBP-2 is not just a transport protein but rather that it functions coordinately with IGF-I to stimulate growth and skeletal acquisition.

The IGF-I regulatory system and its impact on skeletal and energy homeostasis

Insulin-like growth factor (IGF)-I is important in the acquisition and maintenance of both soft and hard tissues. Skeletal remodeling requires energy and recent work has demonstrated that bone can influence insulin sensitivity and thereby regulate metabolic processes. New insights from mouse models into the role of IGF-binding proteins (IGFBPs) as more than mere depots for the IGFs has reignited investigations into the metabolic targets influenced by the IGF regulatory system and the pathways that link bone to adipose tissue. Although there remains continued uncertainty about the relative balance between the effects of circulating versus tissue IGF-I actions, the role of the IGFBPs has been redefined both as modulators of IGF-I action and as independent signaling factors. This review highlights several recent findings that shed new light on the physiologic role of the IGF regulatory system and its influence on skeletal and fat metabolism.

Emerging therapeutic opportunities for skeletal restoration

Osteoporosis, a syndrome characterized by thin bones and fractures, has become more prevalent in both women and men. Established therapies for treating this disorder consist primarily of drugs that prevent bone loss, such as the bisphosphonates and selective oestrogen receptor modulators. Although these drugs have been shown to reduce fractures in randomized trials, there is an urgent need for treatments that could lower fracture risk further without additional adverse effects. The introduction of parathyroid hormone (teriparatide), which significantly increases bone mineral density, albeit for a relatively short duration, raised expectations that drugs that stimulate bone formation might cure osteoporosis. After outlining current approaches for treating osteoporosis, this Review focuses on emerging therapeutic opportunities for osteoporosis that are based on recent insights into skeletal physiology. Such novel strategies offer promise not only for reducing age-related bone loss and the associated risk of fractures but also for restoring bone mineral density to healthy levels.

25-hydroxyvitamin D, insulin-like growth factor-I, and bone mineral accrual during growth

CONTEXT

The extent to which 25-hydroxyvitamin D [25(OH)D] and IGF-I influence bone mineral content (BMC) accrual from early to mid-puberty is unclear. OBJECTIVE, SETTING, AND PARTICIPANTS: This study sought to determine relationships among 25(OH)D, IGF-I, and BMC in community-dwelling prepubertal females (n = 76; aged 4-8 yr at baseline) over a period of up to 9 yr.

DESIGN

The hypothesis that changes in IGF-I vs. 25(OH)D are more strongly associated with BMC accrual was formulated after data collection. 25(OH)D and IGF-I were log-transformed and further adjusted using two-way ANOVA for differences in season and race. Linear mixed modeling (including a random subject-specific intercept and a random subject-specific slope on age) was employed to analyze the proportion of variance the transformed 25(OH)D and IGF-I variables explained for the bone outcomes.

RESULTS

IGF-I was more strongly associated with BMC accrual than 25(OH)D at the total body (R(2) = 0.874 vs. 0.809), proximal femur (R(2) = 0.847 vs. 0.771), radius (R(2) = 0.812 vs. 0.759), and lumbar spine (R(2) = 0.759 vs. 0.698). The rate of BMC accrual was positively associated with changes in IGF-I but negatively associated with 25(OH)D. When IGF-I and 25(OH)D were included in the same regression equation, 25(OH)D did not have a significant predictive effect on BMC accrual above and beyond that of IGF-I.

CONCLUSIONS

These prospective data in early adolescent females indicate that both 25(OH)D and IGF-I have a significant impact on bone mineral accrual; however, the positive association of IGF-I and BMC accrual is greater than the negative association of 25(OH)D and BMC accrual.

Impaired mesenchymal stem cell differentiation and osteoclastogenesis in mice deficient for Igf2-P2 transcripts

During embryonic development, Igf2 gene transcription is highly regulated through the use of several promoters whose specific roles are not defined. Here, we show that loss-of-function of one of these promoters, Igf2-P2, results in growth defects that are temporally and quantitatively different from those seen in Igf2-null mutants. In particular, Igf2-P2 mutants exhibit skeletal abnormalities characterized by thin and short bones with reduced mineralization and medullar cavity and with altered bone remodeling. These abnormalities are associated with decreased numbers of embryonic mesenchymal chondroprogenitors, adult mesenchymal stem cells and osteoprogenitors. Differentiation of osteoprogenitors into osteoblasts is impaired in the Igf2-P2 mutant mice in a cell-autonomous manner, and osteopontin is a target of the IGF2 signaling pathway during this differentiation. Igf2-P2 mutant mice also display impaired formation of giant osteoclasts owing to a defective micro-environment. These results support a model wherein transcriptional activity of the Igf2-P2 promoter regulates the fate of mesenchymal progenitors during bone development and remodeling in the adult, and regulates osteogenesis in a cell-autonomous and non-autonomous manner.

PPARγ: a circadian transcription factor in adipogenesis and osteogenesis

Peroxisome proliferator-activated receptor γ (PPARγ) is a critical factor for adipogenesis and glucose metabolism, but accumulating evidence demonstrates the involvement of PPARγ in skeletal metabolism as well. PPARγ agonists, the thiazolidinediones, have been widely used for the treatment of type 2 diabetes mellitus owing to their effectiveness in lowering blood glucose levels. However, the use of thiazolidinediones has been associated with bone loss and fractures. Thiazolidinedione-induced alterations in the bone marrow milieu-that is, increased bone marrow adiposity with suppression of osteogenesis-could partially explain the pathogenesis of drug-induced bone loss. Furthermore, several lines of evidence place PPARγ at the center of a regulatory loop between circadian networks and metabolic output. PPARγ exhibits a circadian expression pattern that is magnified by consumption of a high-fat diet. One gene with circadian regulation in peripheral tissues, nocturnin, has been shown to enhance PPARγ activity. Importantly, mice deficient in nocturnin are protected from diet-induced obesity, exhibit impaired circadian expression of PPARγ and have increased bone mass. This Review focuses on new findings regarding the role of PPARγ in adipose tissue and skeletal metabolism and summarizes the emerging role of PPARγ as an integral part of a complex circadian regulatory system that modulates food storage, energy consumption and skeletal metabolism.

Nocturnin suppresses igf1 expression in bone by targeting the 3' untranslated region of igf1 mRNA

IGF-I is an anabolic factor that mediates GH and PTH actions in bone. Expression of skeletal Igf1 differs for inbred strains of mice, and Igf expression levels correlate directly with bone mass. Previously we reported that peroxisome proliferator-activated receptor-γ2 activation in bone marrow suppressed Igf1 expression and that peroxisome proliferator-activated receptor-γ2 activation-induced Nocturnin (Noc) expression, a circadian gene with peak expression at light offset, which functions as a deadenylase. In 24-h studies we found that Igf1 mRNA exhibited a circadian rhythm in femur with the lowest Igf1 transcript levels at night when Noc transcripts were highest. Immunoprecipitation/RT-PCR analysis revealed a physical interaction between Noc protein and Igf1 transcripts. To clarify which portions of the Igf1 3' untranslated region (UTR) were necessary for regulation by Noc, we generated luciferase constructs containing various lengths of the Igf1 3'UTR. Noc did not affect the 170-bp short-form 3'UTR, but suppressed luciferase activity in constructs bearing the longer-form 3'UTR, which contains a number of potential regulatory motifs involved in mRNA degradation. C57BL/6J mice have low skeletal Igf1 mRNA compared with C3H/HeJ mice, and the Igf1 3' UTR is polymorphic between these strains. Interestingly, the activity of luciferase constructs bearing the long-form 3'UTR from C57BL/6J mice were repressed by Noc overexpression, whereas those bearing the corresponding region from C3H/HeJ were not. In summary, Noc interacts with Igf1 in a strain- and tissue-specific manner and reduces Igf1 expression by targeting the longer form of the Igf1 3'UTR. Posttranscriptional regulation of Igf1 may be critically important during skeletal acquisition and maintenance.

Skeletal aging and the adipocyte program: New insights from an "old" molecule

Aging is associated with profound changes in bone mass and body composition. Emerging evidence supports the hypothesis that alterations in mesenchymal stromal cell fate are a critical etiologic factor. In addition, time-keeping at the cellular level is affected as aging progresses, particularly in the adipocyte. In this Extra View we discuss the interactive role of three molecules, PPARγ, nocturnin and IGF-I in regulating stem cell fate in the marrow and the potential implications of this network for understanding cellular aging.

Caloric restriction leads to high marrow adiposity and low bone mass in growing mice

The effects of caloric restriction (CR) on the skeleton are well studied in adult rodents and include lower cortical bone mass but higher trabecular bone volume. Much less is known about how CR affects bone mass in young, rapidly growing animals. This is an important problem because low caloric intake during skeletal acquisition in humans, as in anorexia nervosa, is associated with low bone mass, increased fracture risk, and osteoporosis in adulthood. To explore this question, we tested the effect of caloric restriction on bone mass and microarchitecture during rapid skeletal growth in young mice. At 3 weeks of age, we weaned male C57Bl/6J mice onto 30% caloric restriction (10% kcal/fat) or normal diet (10% kcal/fat). Outcomes at 6 (n = 4/group) and 12 weeks of age (n = 8/group) included body mass, femur length, serum leptin and insulin-like growth factor 1 (IGF-1) values, whole-body bone mineral density (WBBMD, g/cm(2)), cortical and trabecular bone architecture at the midshaft and distal femur, bone formation and cellularity, and marrow fat measurement. Compared with the normal diet, CR mice had 52% and 88% lower serum leptin and 33% and 39% lower serum IGF-1 at 6 and 12 weeks of age (p < .05 for all). CR mice were smaller, with lower bone mineral density, trabecular, and cortical bone properties. Bone-formation indices were lower, whereas bone-resorption indices were higher (p < .01 for all) in CR versus normal diet mice. Despite having lower percent of body fat, bone marrow adiposity was elevated dramatically in CR versus normal diet mice (p < .05). Thus we conclude that caloric restriction in young, growing mice is associated with impaired skeletal acquisition, low leptin and IGF-1 levels, and high marrow adiposity. These results support the hypothesis that caloric restriction during rapid skeletal growth is deleterious to cortical and trabecular bone mass and architecture, in contrast to potential skeletal benefits of CR in aging animals.

Signaling through the M(3) muscarinic receptor favors bone mass accrual by decreasing sympathetic activity

Bone remodeling is regulated by various neuronal inputs, including sympathetic tone, which is known to inhibit bone mass accrual. This aspect of sympathetic nervous system function raises the prospect that the other arm of the autonomic nervous system, the parasympathetic nervous system, may also affect bone remodeling. Here, we use various mutant mouse strains, each lacking one of the muscarinic receptors that mediate parasympathetic activity, to show that the parasympathetic nervous system acting through the M(3) muscarinic receptor is a positive regulator of bone mass accrual, increasing bone formation and decreasing bone resorption. Gene expression studies, cell-specific gene deletion experiments, and analysis of compound mutant mice showed that the parasympathetic nervous system favors bone mass accrual by acting centrally and by decreasing the sympathetic tone. By showing that both arms of the autonomic nervous system affect bone remodeling, this study further underscores the importance of neuronal regulation of bone.

Restoration of peri-implant defects in immediate implant installations by Choukroun platelet-rich fibrin and silk fibroin powder combination graft

OBJECTIVE

The objective of this study was to determine the capability of silk fibroin powder as a biomaterial template for the restoration of peri-implant defects when mixed with Choukroun platelet-rich fibrin (PRF) in vivo.

STUDY DESIGN

Ten New Zealand white rabbits were used for this study. Using a trephine bur (diameter 7.0 mm), 2 monocortical defects were prepared. Subsequently, 2 dental implants were installed into the tibia (diameter 3.0 mm, length 10.0 mm). In the experimental group, the peri-implant defect was filled with a combination graft of silk fibroin powder and Choukroun PRF. The control was left in an unfilled state. The animals were killed at 8 weeks. Subsequently, a removal torque test and a histomorphometric analysis were done.

RESULTS

The removal torque for the experimental group was 30.34 +/- 5.06 N.cm, whereas it was 21.86 +/- 3.39 N.cm for the control. The difference between the 2 groups was statistically significant (P = .010). Mean new bone formation was 51.93 +/- 27.90% in the experimental group and 11.67 +/- 15.12% in the control (P = .003). Mean bone-to-implant contact was 43.07 +/- 21.96% in the experimental group and 15.37 +/- 23.84% in the control (P = .002).

CONCLUSION

A peri-implant defect can be successfully repaired by the application of Choukroun PRF and silk fibroin powder.

Selective signaling by Akt2 promotes bone morphogenetic protein 2-mediated osteoblast differentiation

Mesenchymal stem cells are essential for repair of bone and other supporting tissues. Bone morphogenetic proteins (BMPs) promote commitment of these progenitors toward an osteoblast fate via functional interactions with osteogenic transcription factors, including Dlx3, Dlx5, and Runx2, and also can direct their differentiation into bone-forming cells. BMP-2-stimulated osteoblast differentiation additionally requires continual signaling from insulin-like growth factor (IGF)-activated pathways. Here we identify Akt2 as a critical mediator of IGF-regulated osteogenesis. Targeted knockdown of Akt2 in mouse primary bone marrow stromal cells or in a mesenchymal stem cell line, or genetic knockout of Akt2, did not interfere with BMP-2-mediated signaling but resulted in inhibition of osteoblast differentiation at an early step that preceded production of Runx2. In contrast, Akt1-deficient cells differentiated normally. Complete biochemical and morphological osteoblast differentiation was restored in cells lacking Akt2 by adenoviral delivery of Runx2 or by a recombinant lentivirus encoding wild-type Akt2. In contrast, lentiviral Akt1 was ineffective. Taken together, these observations define a specific role for Akt2 as a gatekeeper of osteogenic differentiation through regulation of Runx2 gene expression and indicate that the closely related Akt1 and Akt2 exert distinct effects on the differentiation of mesenchymal precursors.