SMILE inhibits BMP-2-induced expression of osteocalcin by suppressing the activity of the RUNX2 transcription factor in MC3T3E1 cells

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

eIF2α-ATF4 Pathway Activated by a Change in the Calcium Environment Participates in BCP-Mediated Bone Regeneration

Biphasic calcium phosphate (BCP) ceramic is a classic bone void filler and a common basis of new materials for bone defect repair. However, the specific mechanism of BCP in osteogenesis has not been fully elucidated. Endoplasmic reticulum stress (ERs) and the subsequent PERK-eIF2α-ATF4 pathway can be activated by various factors, including trauma and intracellular calcium changes, and therefore worth exploring as a potential mechanism in BCP-mediated bone repair. Herein, a rat lateral femoral epicondyle defect model in vivo and a simulated BCP-mediated calcium environment in vitro were constructed for the analysis of BCP-related osteogenesis and the activation of ERs and the eIF2α-ATF4 pathway. An inhibitor of eIF2α dephosphorylation (salubrinal) was also used to explore the effect of the eIF2α-ATF4 pathway on BCP-mediated bone regeneration. The results showed that the ERs and eIF2α-ATF4 pathway activation were observed during 4 weeks of bone repair, with a rapid but brief increase immediately after artificial defect surgery and a re-increase after 4 weeks with the resorption of BCP materials. Mild ERs and the activated eIF2α induced by the calcium changes mediated by BCP regulated the expression of osteogenic-related proteins and had an important role during the defect repair. In conclusion, the eIF2α-ATF4 pathway activated by a change in the calcium environment participates in BCP-mediated bone regeneration. eIF2α-ATF4 and ERs could provide new directions for further studies on new materials in bone tissue engineering.

Fat Mass and Obesity-Associated (FTO) Stimulates Osteogenic Differentiation of C3H10T1/2 Cells by Inducing Mild Endoplasmic Reticulum Stress via a Positive Feedback Loop with p-AMPK

Fat mass and obesity-associated (FTO) gene helps to regulate energy homeostasis in mammals by controlling energy expenditure. In addition, FTO functions in the regulation of obesity and adipogenic differentiation; however, a role in osteogenic differentiation is unknown. This study investigated the effects of FTO on osteogenic differentiation of C3H10T1/2 cells and the underlying mechanism. Expression of osteogenic and endoplasmic reticulum (ER) stress markers were characterized by reverse-transcriptase polymerase chain reaction and western blotting. Alkaline phosphatase (ALP) staining was performed to assess ALP activity. BMP2 treatment increased mRNA expression of osteogenic genes and FTO. Overexpression of FTO increased expression of the osteogenic genes distal-less homeobox5 (Dlx5) and runt-related transcription factor 2 (Runx2). Activation of adenosine monophosphate-activated protein kinase (AMPK) increased FTO expression, and there was a positive feedback loop between FTO and p-AMPK. p-AMPK and FTO induced mild ER stress; however, tunicamycin-induced severe ER stress suppressed FTO expression and AMPK activation. In summary, FTO induces osteogenic differentiation of C3H10T1/2 cells upon BMP2 treatment by inducing mild ER stress via a positive feedback loop with p-AMPK. FTO expression and AMPK activation induce mild ER stress. By contrast, severe ER stress inhibits osteogenic differentiation by suppressing FTO expression and AMPK activation.

Differential Expression of Cyclic AMP-Response Element Binding Protein Zhangfei (CREBZF) in the Mouse Testis during Postnatal Development

Cyclic AMP-response element binding protein zhangfei (CREBZF), a member of ATF/CREB (activating transcription factor/ cAMP response element binding protein) family, regulates numerous cellular functions and development of cells by interacting transcription factors. This study discovered the expression pattern of CREBZF in seminiferous tubule of testes during the postnatal development of mice. In testis, CREBZF mRNA expression was the highest among other organs. Immunofluorescence analyses showed that the CREBZF was specifically expressed on spermatocyte but not in spermatogonia and Sertoli cells in seminiferous epithelium of mouse testis. Semi-quantitative polymerase chain reaction (PCR) analysis showed that CREBZF transcript level was significantly elevated during postnatal development of mouse testis. Confocal imaging analysis indicated that the protein expression of CREBZF in seminiferous tubule remained low until postnatal day (PD) 14, and was dramatically increased in PD 21. Interestingly, only one type of the spermatocyte expressed CREBZF specifically among SCP3-positive spermatocytes. Taken together, these results suggest that CREBZF may be novel putative marker of the spermatocyte and regulate meiosis during postnatal development of mice.

Regulation of Cyclic AMP-Response Element Binding Protein Zhangfei (CREBZF) Expression by Estrogen in Mouse Uterus

CREBZF (cAMP-response element binding protein zhangfei) is a member of ATF/CREB family, and which regulates various cellular functions by suppressing major factors with direct interaction. In this study, we have examined the expression of CREBZF on mouse endometrium during uterus estrous cycles and estrogen (E2) treatment. In uterus, CREBZF mRNA expression was higher than other organs and mRNA and protein of CREBZF was increased in proestrus phase and decreased in estrus phase. The expression of CREBZF in 3-weeks old mouse uterus was reduced by E2 injection in endometrium. In addition, the expression of progesterone receptor, a marker of E2 in ovariectomized mice was found to be strongly expressed in stroma, while CREBZF was only expressed in epithelium. Also, we conformed that E2-suppressed CREBZF was restored by co-injection of ICI 182,780, an estrogen receptor antagonist. Overall, these results suggest that CREBZF is regulated by estrogen and involved in ER signaling pathway in mouse uterus.

Curcumin induces osteoblast differentiation through mild-endoplasmic reticulum stress-mediated such as BMP2 on osteoblast cells

AIMS

Curcumin (diferuloylmethane or [1E,6E]-1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6heptadiene-3,5-dione) is a phenolic natural product derived from the rhizomes of the turmeric plant, Curcuma longa. It is reported to have various biological actions such as anti-oxidative, anti-inflammatory, and anti-cancer effects. However, the molecular mechanism of osteoblast differentiation by curcumin has not yet been reported.

MAIN METHODS

The cytotoxicity of curcumin was identified using the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Expression of osteogenic markers and endoplasmic reticulum (ER) stress markers in C3H1-T1/2 cells were measured using reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blotting. Alkaline phosphatase (ALP) staining was performed to assess ALP activity in C3H10T1/2 cells. Transcriptional activity was detected using a luciferase reporter assay.

KEY FINDINGS

Curcumin increased the expression of genes such as distal-less homeobox 5 (Dlx5), runt-related transcription factor 2 (Runx2), ALP, and osteocalcin (OC), which subsequently induced osteoblast differentiation in C3H10T1/2 cells. In addition, ALP activity and mineralization was found to be increased by curcumin treatment. Curcumin also induced mild ER stress similar to bone morphogenetic protein 2 (BMP2) function in osteoblast cells. Next, we confirmed that curcumin increased mild ER stress and osteoblast differentiation similar to BMP2 in C3H10T1/2 mesenchymal stem cells. Transient transfection studies also showed that curcumin increased ATF6-Luc activity, while decreasing the activities of CREBH-Luc and SMILE-Luc. In addition, similar to BMP2, curcumin induced the phosphorylation of Smad 1/5/9.

SIGNIFICANCE

Overall, these results demonstrate that curcumin-induced mild ER stress increases osteoblast differentiation via ATF6 expression in C3H10T1/2 cells.

The Effects of Cerium Valence States at Cerium Oxide Coatings on the Responses of Bone Mesenchymal Stem Cells and Macrophages

Ideal orthopedic coatings should trigger good osteogenic response and limited inflammatory response. The cerium valence states in ceria are associated with their anti-oxidative activity and anti-inflammatory property. In the study, we prepared two kinds of plasma sprayed CeO₂ coatings with different Ce⁴⁺ concentrations to investigate the effects of Ce valence states on the response of bone mesenchymal stem cells (BMSCs) and macrophage RAW264.7. Both the coatings (CeO₂-A and CeO₂-B) were characterized via XRD, SEM, and X-ray photoelectron spectroscopy. The CeO₂ coatings enhanced osteogenic behaviors of BMSCs in terms of cellular proliferation, alkaline phosphatase (ALP) activity and calcium deposition activity in comparison with the Ti substrate. In particular, the CeO₂-B coating (higher Ce⁴⁺ concentration) elicited greater effects than the CeO₂-A coating (higher Ce³⁺ concentration). RT-PCR and western blot results suggested that the CeO₂-B coating promoted BMSCs osteogenic differentiation through the SMAD-dependent BMP signaling pathway, which activated Runx2 expression and subsequently enhanced the expression of ALP and OCN. With respect to either CeO₂-A coating or Ti substrate, the CeO₂-B coating exerted greater effects on the macrophages, increasing the anti-inflammatory cytokines (IL-10 and IL-1ra) expression and suppressing the expression of the pro-inflammatory cytokines (TNF-α and IL-6) and ROS production. Furthermore, it also upregulated the expression of osteoinductive molecules (TGF-β1 and BMP2) in the macrophages. The regulation of cerium valence states at plasma sprayed ceria coatings can be a valuable strategy to improve osteogenic properties and alleviate inflammatory response.

Increased microRNA-93-5p inhibits osteogenic differentiation by targeting bone morphogenetic protein-2

BACKGROUND AND PURPOSE

Trauma-induced osteonecrosis of the femoral head (TIONFH) is a major complication of femoral neck fractures. Degeneration and necrosis of subchondral bone can cause collapse, which results in hip joint dysfunction in patients. The destruction of bone metabolism homeostasis is an important factor for osteonecrosis. MicroRNAs (miRNAs) have an important role in regulating osteogenic differentiation, but the mechanisms underlying abnormal bone metabolism of TIONFH are poorly understood. In this study, we screened specific miRNAs in TIONFH by microarray and further explored the mechanism of osteogenic differentiation.

DESIGN

Blood samples from patients with TIONFH and patients without necrosis after trauma were compared by microarray, and bone collapse of necrotic bone tissue was evaluated by micro-CT and immunohistochemistry. To confirm the relationship between miRNA and osteogenic differentiation, we conducted cell culture experiments. We found that many miRNAs were significantly different, including miR-93-5p; the increase in this miRNA was verified by Q-PCR. Comparison of the tissue samples showed that miR-93-5p expression increased, and alkaline phosphatase (ALP) and osteopontin (OPN) levels decreased, suggesting miR-93-5p may be involved in osteogenic differentiation. Further bioinformatics analysis indicated that miR-93-5p can target bone morphogenetic protein 2 (BMP-2). A luciferase gene reporter assay was performed to confirm these findings. By simulating and/or inhibiting miR-93-5p expression in human bone marrow mesenchymal stem cells, we confirmed that osteogenic differentiation-related indictors, including BMP-2, Osterix, Runt-related transcription factor, ALP and OPN, were decreased by miR-93-5p.

CONCLUSION

Our study showed that increased miR-93-5p in TIONFH patients inhibited osteogenic differentiation, which may be associated with BMP-2 reduction. Therefore, miR-93-5p may be a potential target for prevention of TIONFH.

The effect of atorvastatin, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (HMG-CoA), on the prevention of osteoporosis in ovariectomized rabbits

Osteoporosis is a most frequent systemic skeletal disease characterized as low bone mineral density and microarchitectural deterioration of bone tissue, resulting in increased bone fragility and fracture risk. Although several drugs such as bisphosphonates, estrogen replacement treatment, and selective estrogen receptor modulators have been used to treat osteoporosis, all these are not the ideal drugs because of insufficient curative ability and adverse side effects. Recently, atorvastatin has ordinarily been prescribed as an anti-hyperlipidemia drug, not as an anti-osteoporosis drug. However, its clinical outcome and potential treatment mechanism are still unclear. In this study, the bilateral ovariectomy of rabbits was duplicated to develop osteoporosis animal model. The effect of atorvastatin on in vivo was determined, and the functional mechanism was studied in vitro after the curative effect was explored. Atorvastatin was observed to significantly increase the mechanical parameters such as maximum load, stiffness, and energy-absorbing capacity, and it improved the microarchitecture. The anti-osteoporosis activity of atorvastatin may be the result of the promotion of differentiation of osteoblasts by inducing synthesis of vascular endothelial growth factor, bone morphogenetic protein 2 (BMP2), core-binding factor alpha 1 (CBFα1), and inhibition of osteoclast formation through the osteoprotegerin (OPG)-receptor activator for the nuclear factor κB ligand (RANKL) system. Our study observations give reliable experimental evidence for clinical application of atorvastatin to treat the disorder of osteoporosis.

Curculactones A and B induced the differentiation of C3H10T1/2 and MC3T3-E1 cells to osteoblasts

Curculactones A and B are rare γ-lactone derivatives obtained from yellow, natural curcumin following γ-irradiation, and are a type of small molecules with a moderate anti-obesity effect. However, the exact role of curculactones A and B in osteoblast differentiation is unknown. In this study, the effects of curculactones A and B on the differentiation of the mesenchymal cell line C3H10T1/2 and pre-osteoblast cell line MC3T3-E1 to osteoblasts were examined. Curculactones A or B could markedly increase the mRNA levels of osteogenic marker genes and alkaline phosphatase (ALP) activity. Collectively, our findings indicate that curculactones A or B induced osteoblast differentiation through osteogenic expression of genes such as distal-less homeobox 5 (Dlx5), runt-related transcription factor 2 (Runx2), ALP, and osteocalcin (OC).

FTY720 enhances osteogenic differentiation of bone marrow mesenchymal stem cells in ovariectomized rats

Sphingosine-1-phosphate and its structural analog FTY720 (fingolimod) are important in the inhibition of osteoclast differentiation and bone resorption, however, it remains unknown whether they enhance osteogenic differentiation of the bone marrow mesenchymal stem cells (BM‑MSCs). The present study investigated the effect of FTY720 on the osteogenic differentiation of BM‑MSCs from the femurs of the ovariectomized (OVX) rats. Three different concentrations (1, 10 and 100 nM) of FTY720 were demonstrated to markedly upregulate mRNA expression levels of Runt‑related transcription factor 2 (Runx2) and Sp7 transcription factor (Sp7) at 2 weeks, and alkaline phosphatase (ALP) at 3 weeks. The osteocalcin (OCN) expression was similar at weeks 2 and 3. The protein expression levels of Runx2, Sp7, OCN and ALP induced by three different concentrations of FTY720 were higher than those in the control groups at 3 weeks in the OVX and sham groups. The findings of the current study suggested a beneficial effect of FTY720 on bone formation in OVX rats, and provided a potential therapeutic method of FTY720 to prevent alveolar bone resorption in patients with post‑menopausal osteoporosis.

Strontium attenuates rhBMP-2-induced osteogenic differentiation via formation of Sr-rhBMP-2 complex and suppression of Smad-dependent signaling pathway

Strontium (Sr(2+)) has pronounced effects on stimulating bone formation and inhibiting bone resorption in bone regeneration. In this current study, the effect and the underlying mechanism involved of Sr(2+) on the biological activity of bone morphogenetic protein-2 (BMP-2) were studied in detail with pluripotent skeletal muscle myogenic progenitor C2C12 model cell line. The results indicated that Sr(2+) could bind recombinant human BMP-2 (rhBMP-2) rapidly, even in the presence of Ca(2+) and Mg(2+), and inhibited rhBMP-2-induced osteogenic differentiation in vitro and osteogenetic efficiency in vivo. Further studies demonstrated that Sr(2+) treatment undermined the binding capacity of rhBMP-2 with its receptor BMPRIA and thus attenuated Smad 1/5/8 phosphorylation without affecting their dephosphorylation in C2C12 cells. Furthermore, circular dichroism spectroscopy, fluorescence spectroscopy and X-ray photoelectron spectroscopy all revealed that the inhibitory effect of Sr(2+) on the rhBMP-2 osteogenic activity was associated with the formation of Sr-rhBMP-2 complex and ensuing enhancement of β-sheet structure. Our work suggests the activity of rhBMP-2 to induce osteogenic differentiation was decreased by directly interaction with free Sr ions in solution, which should provide guide and assist for development of BMP-2-based materials for bone regeneration.

STATEMENT OF SIGNIFICANCE

Due to easy denaturation and ensuing the reduced activity of rhBMP-2, preserving/enhancing the capacity of rhBMP-2 to induce osteogenic differentiation is of critical importance in developing the protein-based therapy. Cations as effective elements influence the conformation and thereby the bioactivity of protein. Strontium (Sr(2+)), stimulating bone formation and inhibiting bone resorption, has been incorporated into biomaterials/scaffold to improve the bioactivity for bone-regeneration applications. However, Sr(2+)-induced changes in the conformation and bioactivity of BMP-2 have never been investigated. In this study, the formation of Sr-rhBMP-2 complex inhibited the osteogenic differentiation in vitro and osteogenetic efficiency in vivo through the inhibition of BMP/Smad signaling pathway, providing guidance for development of Sr-containing BMP-2-based bone scaffold/matrice and other Sr-dopped protein therapy.

Calcium ion-induced formation of β-sheet/-turn structure leading to alteration of osteogenic activity of bone morphogenetic protein-2

Preserving bioactivity of bone morphogenetic protein 2 (BMP-2) still remains a challenge in protein-based therapy. It is not known how Ca²⁺ released from extracellular matrix or existing in physiological environment influences bioactivity in situ till now. Here, effects of extracellular Ca²⁺ on conformation and osteogenic bioactivity of recombinant human BMP-2 (rhBMP-2) were investigated systematically. In vitro results indicated that Ca²⁺ could bind rhBMP-2 rapidly and had no obvious effect on cell behaviors. Low concentration of Ca²⁺ (0.18 mM) enhanced rhBMP-2-induced osteogenic differentiation, while high Ca²⁺ concentration (>1.80 mM) exerted negative effect. In vivo ectopic bone formation exhibited similar trend. Further studies by circular dichroism spectroscopy, fluorescence spectroscopy, together with cell culture experiments revealed at low concentration, weak interaction of Ca²⁺ and rhBMP^-2 slightly increased β-sheet/-turn content and facilitated recognition of BMP-2 and BMPRIA. But, high Ca²⁺ concentration (>1.8 mM) induced formation of Ca-rhBMP-2 complex and markedly increased content of β-sheet/-turn, which led to inhibition binding of rhBMP-2 and BMPRIA and thus suppression of downstream Smad1/5/8, ERK1/2 and p38 mitogen-associated protein kinase signaling pathways. Our work suggests osteogenic bioactivity of BMP-2 can be adjusted via extracellular Ca²⁺, which should provide guide and assist for development of BMP-2-based materials for bone regeneration.

Small heterodimer partner-interacting leucine zipper protein inhibits adipogenesis by regulating peroxisome proliferator-activated receptor γ activity

AIMS

Adipocytes play a critical role in energy balance. Growth of fat tissue is achieved via an increase in adipocyte mass and the formation of newly differentiated adipocytes from precursor cells. Understanding the cellular and molecular mechanisms of adipocyte differentiation is crucial for the study of obesity- and fat-related diseases. The present study was designed to study whether small heterodimer partner-interacting leucine zipper protein (SMILE), a novel co-repressor, could regulate differentiation of adipocyte in 3T3-L1 cells.

MATERIALS AND METHODS

Treatment of endoplasmic stress inducers, thapsigargin and tunicamycin, inhibited adipocyte differentiation, stimulated Smile mRNA expression, and repressed the expression of adiponectin (Adipoq) in 3T3-L1 pre-adipocyte. Overexpression of SMILE in 3T3-L1 cells decreased the expression of the mRNA encoding Adipoq, a major marker of adipocytes, significantly. Furthermore, knockdown of SMILE recovered the thapsigargin-mediated repression of Adipoq transcription. Co-immunoprecipitation experiments revealed that SMILE interacted physically with PPARγ in 3T3-L1 cells. In addition, chromatin immunoprecipitation experiments revealed that SMILE suppressed the binding affinity of PPARγ for the Adipoq promoter.

KEY FINDINGS

We demonstrate that SMILE controls adipocyte differentiation by regulating the transactivity of peroxisome proliferator-activated receptor γ (PPARγ).

SIGNIFICANCE

These findings demonstrate that SMILE represses adipocyte differentiation by regulating PPARγ transactivity; hence, SMILE is a potential regulator of PPARγ-related diseases.