Abnormal proliferation and differentiation of osteoblasts from girls with adolescent idiopathic scoliosis to melatonin

Physiological consequences of space flight, including abnormal bone metabolism, space radiation injury, and circadian clock dysregulation: Implications of melatonin use and regulation as a countermeasure

Exposure to the space environment induces a number of pathophysiological outcomes in astronauts, including bone demineralization, sleep disorders, circadian clock dysregulation, cardiovascular and metabolic dysfunction, and reduced immune system function. A recent report describing experiments aboard the Space Shuttle mission, STS-132, showed that the level of melatonin, a hormone that provides the biochemical signal of darkness, was decreased during microgravity in an in vitro culture model. Additionally, abnormal lighting conditions in outer space, such as low light intensity in orbital spacecraft and the altered 24-h light-dark cycles, may result in the dysregulation of melatonin rhythms and the misalignment of the circadian clock from sleep and work schedules in astronauts. Studies on Earth have demonstrated that melatonin regulates various physiological functions including bone metabolism. These data suggest that the abnormal regulation of melatonin in outer space may contribute to pathophysiological conditions of astronauts. In addition, experiments with high-linear energy transfer radiation, a ground-based model of space radiation, showed that melatonin may serve as a protectant against space radiation. Gene expression profiling using an in vitro culture model exposed to space flight during the STS-132 mission, showed that space radiation alters the expression of DNA repair and oxidative stress response genes, indicating that melatonin counteracts the expression of these genes responsive to space radiation to promote cell survival. These findings implicate the use of exogenous melatonin and the regulation of endogenous melatonin as countermeasures for the physiological consequences of space flight.

The role of endocrine hormones in the pathogenesis of adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a common spinal deformity characterized by changes in the three-dimensional structure of the spine. It usually initiates during puberty, the peak period of human growth when the secretion of numerous hormones is changing, and it is more common in females than in males. Accumulating evidence shows that the abnormal levels of many hormones including estrogen, melatonin, growth hormone, leptin, adiponectin and ghrelin, may be related to the occurrence and development of AIS. The purpose of this review is to provide a summary and critique of the research published on each hormone over the past 20 years, and to highlight areas for future study. It is hoped that the presentation will help provide a better understanding of the role of endocrine hormones in the pathogenesis of AIS.

A comparison of foot posture and walking performance in patients with mild, moderate, and severe adolescent idiopathic scoliosis

Background

Adolescent idiopathic scoliosis (AIS) is the most common form of scoliosis. However, the underlying mechanisms linking spinal curvature in AIS to foot characteristics and walking performance remain unclear.

Objective

This study aimed to compare walking performance between adolescents with mild, moderate, and severe scoliosis and matched healthy peers with foot posture as covariates.

Methods

This cross-sectional study of 96 adolescents was conducted between April 2020 to October 2020 in China, with 32 healthy peers in the control group and 64 patients in the AIS group. Foot posture and morphology, plantar pressure distribution, and gait characteristics were analyzed. One-way analysis of variance with Bonferroni correction and a post hoc comparison of the mean differences between the different groups was performed. Multiple analyses of covariance adjusted for age, sex, body mass index, foot posture index (FPI), arch index (AI), and walking speed were performed.

Results

Of the 64 adolescents with scoliosis, 18 had mild AIS, 32 had moderate AIS, and 14 had severe AIS. The AI and FPI were much higher in the moderate and severe AIS groups (p = 0.018) and the severe AIS group (p<0.001), respectively, than in the control group. The severe AIS group had advanced and longer midstance (p = 0.014) and delayed propulsion phase (p = 0.013) than the control group. Patients with moderate and severe AIS had asymmetrical gait periods in the left and right limbs (p<0.05). Significant differences in the center-of-pressure excursion index (CPEI) were found between the moderate and severe AIS and control groups (p = 0.003).

Conclusion

Moderate and severe AIS significantly influenced walking performance; however, no significant differences were observed between adolescents with mild AIS and healthy controls. Thus, early intervention could target the prevention of specific functional deficits and prevent it from progressing to a severe state.

Septin4 regulates endoplasmic reticulum stress and apoptosis in melatonin‑induced osteoblasts

Idiopathic scoliosis (IS) is a spinal 3-dimensional deformity with an unknown cause. Melatonin is secreted by the pineal body and contributes to the occurrence and progression of IS. In our previous preliminary study, it was reported that high concentrations of melatonin can induce osteoblast apoptosis, thus acting as an IS treatment, but the mechanism of action is unknown. Therefore, the present study was performed to further investigate the possible mechanism underlying the efficacy of melatonin as a treatment for IS. The present results indicated that high concentrations of melatonin mediate endoplasmic reticulum stress (ERS)-induced apoptosis in hFOB 1.19 cells, and this resulted in a significant and dose-dependent increase in the expression of Septin4, as well as the expression levels of glucose-regulated protein (GRP)78, GRP94 and cleaved caspase-3. Furthermore, osteoblasts were overexpressed with Septin4 and the mechanism via which melatonin induces osteoblast ERS was demonstrated to be via the regulation of Septin4. In addition, it was indicated that cytoskeleton destruction, cell morphology changes and the decrease in the number of cells were aggravated after osteoblasts were overexpressed with Septin4, as indicated by phalloidin and DAPI staining. Collectively, the present results suggest that the Septin4 protein may be a target of ERS in melatonin-induced osteoblast apoptosis, which is involved in bone metabolism diseases, thus providing novel evidence for clinical melatonin treatment of IS.

Melatonin up-regulates bone marrow mesenchymal stem cells osteogenic action but suppresses their mediated osteoclastogenesis via MT2 -inactivated NF-κB pathway

BACKGROUND AND PURPOSE

Melatonin is a neurohormone involved in bone homeostasis. Melatonin directs bone remodelling and the role of bone marrow mesenchymal stem cells (BMMSCs) in the regulating melatonin-mediated bone formation-resorption balance remains undefined.

EXPERIMENTAL APPROACH

Osteoporosis models were established and bone tissue and serum were collected to test the effects of melatonin on bone homeostasis. Melatonin receptors were knocked down, the NF-κB signalling pathway and receptor activator of NF-κB ligand (RANKL) expression were investigated. Communication between bone marrow mesenchymal stem cells and osteoclasts was detected with direct-contact or indirect-contact system.

KEY RESULTS

Bone loss and microstructure disorder in mice were reversed after melatonin treatment, as a result of anabolic and anti-resorptive effects. In vitro, a physiological (low) concentration of melatonin promoted the bone marrow mesenchymal stem cells, osteogenic lineage commitment and extracellular mineralization but had no impact on extracellular matrix synthesis. After MT knockdown, especially MT₂ , the positive effects of melatonin on osteogenesis were attenuated. The canonical NF-κB signalling pathway was the first discovered downstream signalling pathway after MT receptor activation and was found to be down-regulated by melatonin during osteogenesis. Melatonin suppressed BMMSC-mediated osteoclastogenesis by inhibiting RANKL production in BMMSCs and this effect only occurred when BMMSCs and osteoclast precursors were co-cultured in an indirect-contact manner.

CONCLUSION AND IMPLICATIONS

Our work suggests that melatonin plays a crucial role in bone balance, significantly accelerates the osteogenic differentiation of bone marrow mesenchymal stem cells by suppressing the MT₂ -dependent NF-κB signalling pathway, and down-regulates osteoclastogenesis via RANKL paracrine secretion.

A validated composite model to predict risk of curve progression in adolescent idiopathic scoliosis

BACKGROUND

In adolescent idiopathic scoliosis (AIS), the continuous search for effective prognostication of significant curve progression at the initial clinical consultation to inform decision for timely treatment and to avoid unnecessary overtreatment remains a big challenge as evidence of the multifactorial etiopathogenic nature is increasingly reported. This study aimed to formulate a composite model composed of clinical parameters and circulating markers in the prediction of curve progression.

METHOD

This is a two-phase study consisting of an exploration cohort (120 AIS, mean Cobb angle of 25°± 8.5 at their first clinical visit) and a validation cohort (51 AIS, mean Cobb angle of 23° ± 5.0° at the first visit). Patients with AIS were followed-up for a minimum of six years to formulate a composite model for prediction. At the first visit, clinical parameters were collected from routine clinical practice, and circulating markers were assayed from blood.

FINDING

We constructed the composite predictive model for curve progression to severe Cobb angle > 40° with a high HR of 27.9 (95% CI of 6.55 to 119.16). The area under curve of the composite model is higher than that of individual parameters used in current clinical practice. The model was validated by an independent cohort and achieved a sensitivity of 72.7% and a specificity of 90%.

INTERPRETATION

This is the first study proposing and validating a prognostic composite model consisting of clinical and circulating parameters which could quantitatively evaluate the probability of curve progression to a severe curvature in AIS at the initial consultation. Further validation in clinic will facilitate application of composite model in assisting objective clinical decision.

Abnormal Osteoblastic Response to Leptin in Patients with Adolescent Idiopathic Scoliosis

Adolescent idiopathic scoliosis (AIS) is a complex three-dimensional structural deformity of the spine with unknown etiology. Although leptin has been postulated as one of the etiologic factors in AIS, its effects on osteoblastic activity remain unknown. Herein, we conducted this study to investigate whether there are abnormal functional responses to leptin and abnormal expression of leptin receptor in AIS osteoblasts. In vitro assays were performed with osteoblasts isolated from 12 severe AIS girls and 6 non-AIS controls. The osteoblasts were exposed to different concentrations of leptin (0, 10, 100, 1000 ng/mL). The effects of leptin on cell proliferation, differentiation and mineralization were determined. Protein expressions of leptin receptor (LEP-R) under basal and osteogenic conditions were also evaluated by Western blot. Our results showed that leptin significantly stimulated osteoblasts from non-AIS subjects to proliferate, differentiate and mineralized. However, in the AIS group, the stimulatory effects of leptin on cell proliferation, differentiation, and mineralization were not observed. In addition, no statistically significant difference in the expression of leptin receptor under both basal and osteogenic conditions was found between AIS and control group. In conclusion, these findings might help to explain the low bone mass and deranged bone quality that is clinically associated with AIS girls.

SPRY4 is responsible for pathogenesis of adolescent idiopathic scoliosis by contributing to osteogenic differentiation and melatonin response of bone marrow-derived mesenchymal stem cells

Adolescent idiopathic scoliosis (AIS) is a complex, three-dimensional deformity of the spine that commonly occurs in pubescent girls. Decreased osteogenic differentiation and aberrant melatonin signalling have been demonstrated in mesenchymal stem cells (MSCs) from AIS patients and are implicated in the pathogenesis of AIS. However, the molecular mechanisms underlying these abnormal cellular features remain largely unknown. Our previous work comparing gene expression profiles between MSCs from AIS patients and healthy controls identified 1027 differentially expressed genes. In the present study, we focused on one of the most downregulated genes, SPRY4, in the MAPK signalling pathway and examined its role in osteogenic differentiation. We found that SPRY4 is markedly downregulated in AIS MSCs. Knockdown of SPRY4 impaired differentiation of healthy MSCs to osteoblasts, while SPRY4 overexpression in AIS MSCs enhanced osteogenic differentiation. Furthermore, melatonin treatment boosted osteogenic differentiation, whereas SPRY4 ablation ablated the promotional effects of melatonin. Moreover, SPRY4 was upregulated by melatonin exposure and contributed to osteogenic differentiation and melatonin response in a MEK-ERK1/2 dependent manner. Thus, loss of SPRY4 in bone marrow derived-MSCs results in reduced osteogenic differentiation, and these defects are further aggravated under the influence of melatonin. Our findings provide new insights for understanding the role of melatonin in AIS aetiology and highlight the importance of MSCs in AIS pathogenesis.

Melatonin is a potential drug for the prevention of bone loss during space flight

Astronauts experience osteoporosis-like loss of bone mass because of microgravity conditions during space flight. To prevent bone loss, they need a riskless and antiresorptive drug. Melatonin is reported to suppress osteoclast function. However, no studies have examined the effects of melatonin on bone metabolism under microgravity conditions. We used goldfish scales as a bone model of coexisting osteoclasts and osteoblasts and demonstrated that mRNA expression level of acetylserotonin O-methyltransferase, an enzyme essential for melatonin synthesis, decreased significantly under microgravity. During space flight, microgravity stimulated osteoclastic activity and significantly increased gene expression for osteoclast differentiation and activation. Melatonin treatment significantly stimulated Calcitonin (an osteoclast-inhibiting hormone) mRNA expression and decreased the mRNA expression of receptor activator of nuclear factor κB ligand (a promoter of osteoclastogenesis), which coincided with suppressed gene expression levels for osteoclast functions. This is the first study to report the inhibitory effect of melatonin on osteoclastic activation by microgravity. We also observed a novel action pathway of melatonin on osteoclasts via an increase in CALCITONIN secretion. Melatonin could be the source of a potential novel drug to prevent bone loss during space flight.

The effect of exogenous melatonin on reducing scoliotic curvature and improving bone quality in melatonin-deficient C57BL/6J mice

It is well-documented that melatonin deficiency has been linked to the etiopathogenesis of adolescent idiopathic scoliosis. In this study, we intended to apply melatonin in melatonin-deficient mice to ascertain whether melatonin could reduce the incidence/severity of scoliosis, and investigate the role of melatonin on bone mineral density in scoliosis. A total of 80 mice were divided into 4 groups: 20 quadrupedal mice and 20 bipedal mice served as controls; 20 quadrupedal and 20 bipedal mice received oral melatonin (8 mg/kg BW) daily. After 5th, 10th, 15th and 20th weeks of treatment, radiographs and in vivo micro-CT were used to determine the incidence of scoliosis and bone qualities, respectively. Upon sacrifice, the levels of melatonin were measured in each group. At 20th week, the occurrence of scoliosis was 80%, 30%, 22% and 5% in bipedal, quadrupedal, bipedal + melatonin and quadrupedal + melatonin group, respectively. The trabecular bone quality of the vertebral body was significantly ameliorated in the melatonin-treated bipedal models. Likewise, the number of osteoclasts was significantly less in those treated with melatonin. Our results indicated that melatonin deficiency may be crucial for scoliotic development, and restoration of melatonin levels can prevent scoliotic development with the improvement in bone density.

Abnormal osteogenic and chondrogenic differentiation of human mesenchymal stem cells from patients with adolescent idiopathic scoliosis in response to melatonin

Abnormalities of membranous and endochondral ossification in patients with adolescent idiopathic scoliosis (AIS) remain incompletely understood. To investigate abnormalities in the melatonin signaling pathway and cellular response to melatonin in AIS, a case-control study of osteogenic and chondrogenic differentiation was performed using human mesenchymal stem cells (hMSCs). AIS was diagnosed by physical and radiographic examination. hMSCs were isolated from the bone marrow of patients with AIS and control subjects (n=12 each), and purified by density gradient centrifugation. The expression levels of melatonin receptors (MTs) 1 and 2 were detected by western blotting. Osteogenic and chondrogenic differentiation was induced by culturing hMSCs in osteogenic and chondrogenic media containing vehicle or 50 nM melatonin. Alkaline phosphatase (ALP) activity assays, quantitative glycosaminoglycan (GAG) analysis, and reverse transcription-quantitative polymerase chain reaction analysis were performed. Compared with controls, MT2 demonstrated low expression in the AIS group. Melatonin increased ALP activity, GAG synthesis and upregulated the expression of genes involved in osteogenic and chondrogenic differentiation including, ALP, osteopontin, osteocalcin, runt-related transcription factor 2, collagen type II, collagen type X, aggrecan and sex-determining region Y-box 9 in the normal control hMSCs, but did not affect the AIS groups. Thus, AIS hMSCs exhibit abnormal cellular responses to melatonin during osteogenic and chondrogenic differentiation, which may be associated with abnormal membranous and endochondral ossification, and skeletal growth. These results indicate a potential modulating role of melatonin via the MT2 receptor on abnormal osteogenic and chondrogenic differentiaation in patients with AIS.

Adolescent idiopathic scoliosis: evidence for intrinsic factors driving aetiology and progression

Adolescent idiopathic scoliosis (AIS) is now considered to be a multifactorial heterogeneous disease, with recent genomic studies supporting the role of intrinsic factors in contributing to the onset of disease pathology and curve progression. Understanding the key molecular signalling pathways by which these intrinsic factors mediate AIS pathology may facilitate the development of pharmacological therapeutics and the identification of predictive markers of progression. The heterogenic nature of AIS has implicated multiple tissue types in the disease pathophysiology, including spinal bone, intervertebral disc and paraspinal muscles. In this review, we highlight some of the mechanisms and intrinsic molecular regulators within these different tissue types and review the evidence for their involvement in AIS pathology.

Relationship between bone density and bone metabolism in adolescent idiopathic scoliosis

Several authors have confirmed that 27 to 38 % of AIS patients had osteopenia. But few studies have assessed bone metabolism in AIS. This study assessed bone mineral density and bone metabolism in AIS patients using the bone metabolism markers, BAP and TRAP5b. The subjects were 49 consecutive adolescent AIS patients seen at our institutes between March 2012 and September 2013. Sixty-five percent of AIS patients had osteopenia or osteoporosis and 59 % of AIS patients had high values for TRAP5b. The AIS patients with high values of TRAP5b had lower Z scores than those with normal values of TRAP5b. Higher rates of bone resorption are associated with low bone density in AIS patients.

Effect of Melatonin on the Extracellular-Regulated Kinase Signal Pathway Activation and Human Osteoblastic Cell Line hFOB 1.19 Proliferation

It has been shown that melatonin may affect bone metabolism. However, it is controversial whether melatonin could promote osteoblast proliferation, and the precise molecular mechanism of melatonin on osteoblast proliferation is still obscure. In this study, the results of the CCK-8 assay showed that melatonin significantly promoted human osteoblastic cell line hFOB 1.19 cell proliferation at 1, 2.5, 5, 10, 25, 50 and 100 µM concentrations for 24 h, but there were no significant differences among the groups. Western blot demonstrated that 10 µM melatonin significantly promoted ERK1/2 phosphorylation. Furthermore, we also detected the phosphorylation of c-Raf, MEK1/2, p90RSK and MSK1, and all of them increased with 10 µM melatonin. U0126 (a selective inhibitor of MEK that disrupts downstream activation of ERK1/2) downregulated the phosphorylation of ERK1/2, p90RSK and MSK1. U0126 also attenuated the proliferation of osteoblasts stimulated by melatonin. In conclusion, this study for the first time indicates that melatonin (10 nM–100 µM) promotes the proliferation of a human osteoblastic cell line hFOB 1.19 through activation of c-Raf, MEK1/2, ERK1/2, p90RSK and MSK1.

Participation of sex hormones in multifactorial pathogenesis of adolescent idiopathic scoliosis

PURPOSE

In order to verify the potential association between the aetiopathogenesis of adolescent idiopathic scoliosis (AIS) and the process of sexual maturation, we determined the concentrations of oestrogens in pre- and postmenarcheal girls affected by this condition. AIS, occurring mostly in pubescent girls, is one of the most frequent forms of faulty posture. Therefore, it was assumed that the multifactorial pathomechanism of AIS involves significant deficiency of oestrogens.

METHODS

The diagnosis of AIS was established on the basis of physical examination and analyses of radiograms. Concentrations of FSH, LH, oestrogens, progesterone, osteocalcin and RANKL were determined by ELISA. The activity of alkaline phosphatase (AP) was measured by kinetic method. The study included pre- and postmenarcheal girls with AIS and corresponding groups of scoliosis-free controls.

RESULTS

In premenarcheal scoliotic girls, the levels of FSH, LH and oestradiol were lower; the levels of progesterone, oestrone and oestriol were higher; and the concentrations of oestrone and oestriol were similar compared to premenarcheal controls. Higher levels of RANKL, osteocalcin and AP were observed in premenarcheal adolescents with AIS compared to controls. The concentrations of FSH, LH, oestradiol, and progesterone in postmenarcheal girls with scoliosis were lower, oestrone were slightly lower and oestriol did not differ compared with the control group. Significantly higher levels of RANKL, osteocalcin and AP were observed in postmenarcheal scoliotic adolescents compared with controls.

CONCLUSIONS

There is an interdependence between the concentration of oestradiol and development of scoliosis. Determination of estradiol may have diagnostic value in the screening of spinal pathologies associated with AIS.

Relationship between bone density and bone metabolism in adolescent idiopathic scoliosis

Several authors have confirmed that 27 to 38% of AIS patients had osteopenia. But few studies have assessed bone metabolism in AIS. This study assessed bone mineral density and bone metabolism in AIS patients using the bone metabolism markers, BAP and TRAP5b. The subjects were 49 consecutive adolescent AIS patients seen at our institutes between March 2012 and September 2013. Sixty-five percent of AIS patients had osteopenia or osteoporosis and 59% of AIS patients had high values for TRAP5b. The AIS patients with high values of TRAP5b had lower Z scores than those with normal values of TRAP5b. Higher rates of bone resorption are associated with low bone density in AIS patients.

Abnormal response of the proliferation and differentiation of growth plate chondrocytes to melatonin in adolescent idiopathic scoliosis

Abnormalities in the melatonin signaling pathway and the involvement of melatonin receptor MT2 have been reported in patients with adolescent idiopathic scoliosis (AIS). Whether these abnormalities were involved in the systemic abnormal skeletal growth in AIS during the peripubertal period remain unknown. In this cross-sectional case-control study, growth plate chondrocytes (GPCs) were cultured from twenty AIS and ten normal control subjects. Although the MT2 receptor was identified in GPCs from both AIS and controls, its mRNA expression was significantly lower in AIS patients than the controls. GPCs were cultured in the presence of either the vehicle or various concentrations of melatonin, with or without the selective MT2 melatonin receptor antagonist 4-P-PDOT (10 µM). Then the cell viability and the mRNA expression of collagen type X (COLX) and alkaline phosphatase (ALP) were assessed by MTT and qPCR, respectively. In the control GPCs, melatonin at the concentrations of 1, 100 nM and 10 µM significantly reduced the population of viable cells, and the mRNA level of COLX and ALP compared to the vehicle. Similar changes were not observed in the presence of 4-P-PDOT. Further, neither proliferation nor differentiation of GPCs from AIS patients was affected by the melatonin treatment. These findings support the presence of a functional abnormality of the melatonin signaling pathway in AIS GPCs, which might be associated with the abnormal endochondral ossification in AIS patients.

A review of pinealectomy-induced melatonin-deficient animal models for the study of etiopathogenesis of adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a common orthopedic disorder of unknown etiology and pathogenesis. Melatonin and melatonin pathway dysfunction has been widely suspected to play an important role in the pathogenesis. Many different types of animal models have been developed to induce experimental scoliosis mimicking the pathoanatomical features of idiopathic scoliosis in human. The scoliosis deformity was believed to be induced by pinealectomy and mediated through the resulting melatonin-deficiency. However, the lack of upright mechanical spinal loading and inherent rotational instability of the curvature render the similarity of these models to the human counterparts questionable. Different concerns have been raised challenging the scientific validity and limitations of each model. The objectives of this review follow the logical need to re-examine and compare the relevance and appropriateness of each of the animal models that have been used for studying the etiopathogenesis of adolescent idiopathic scoliosis in human in the past 15 to 20 years.

Abnormal Skeletal Growth in Adolescent Idiopathic Scoliosis Is Associated with Abnormal Quantitative Expression of Melatonin Receptor, MT2

The defect of the melatonin signaling pathway has been proposed to be one of the key etiopathogenic factors in adolescent idiopathic scoliosis (AIS). A previous report showed that melatonin receptor, MT2, was undetectable in some AIS girls. The present study aimed to investigate whether the abnormal MT2 expression in AIS is quantitative or qualitative. Cultured osteoblasts were obtained from 41 AIS girls and nine normal controls. Semi-quantification of protein expression by Western blot and mRNA expression by TaqMan real-time PCR for both MT1 and MT2 were performed. Anthropometric parameters were also compared and correlated with the protein expression and mRNA expression of the receptors. The results showed significantly lower protein and mRNA expression of MT2 in AIS girls compared with that in normal controls (p = 0.02 and p = 0.019, respectively). No differences were found in the expression of MT1. When dichotomizing the AIS girls according to their MT2 expression, the group with low expression was found to have a significantly longer arm span (p = 0.036). The results of this study showed for the first time a quantitative change of MT2 in AIS that was also correlated with abnormal arm span as part of abnormal systemic skeletal growth.

Melatonin inhibits adipogenesis and enhances osteogenesis of human mesenchymal stem cells by suppressing PPARγ expression and enhancing Runx2 expression

Adipogenesis and osteogenesis, a reciprocal relationship in bone marrow, are complex processes including proliferation of precursor cells, commitment to the specific lineage, and terminal differentiation. Accumulating evidence from in vitro and in vivo studies suggests that melatonin affects terminal differentiation of osteoblasts and adipocytes, but little is known about the effect of melatonin on the process of adipogenesis and osteogenesis, especially adipogenesis. This study was performed to determine the effect of melatonin on adipogenesis and osteogenesis in human mesenchymal stem cells (hMSCs). Cell proliferation assays demonstrated that melatonin had no apparent effect on the proliferation of hMSCs. When melatonin was added to the adipogenic/osteogenic medium, it directly inhibited adipogenesis and simultaneously promoted osteogenesis of hMSCs in a dose-dependent manner. Furthermore, quantitative RT-PCR demonstrated that melatonin significantly suppressed peroxisome proliferator-activated receptor gamma (PPARγ) expression (day 3, 25% decrease; day 6, 47% decrease), but promoted Runx2 expression (day 3, 87% increase; day 6, 56% increase) in the early stages of adipogenesis and osteogenesis of hMSCs. Moreover, melatonin down-regulated several markers of terminal adipocyte differentiation, including leptin (30%), lipoprotein lipase (LPL, 41%), adiponectin (51%), and adipocyte protein 2 (αP2, 45%). Meanwhile, melatonin up-regulated several markers of osteoblast differentiation, including alkaline phosphatase (110%), osteopontin (218%), and osteocalcin (310%). These results suggest that melatonin directly inhibits hMSCs adipogenic differentiation and significantly enhances hMSCs osteogenic differentiation by suppressing PPARγ expression and enhancing Runx2 expression; this provides further evidence for melatonin as an anti-osteoporosis drug.