Metformin attenuates trauma‐induced heterotopic ossification via inhibition of Bone Morphogenetic Protein signalling

AMP‐activated protein kinase (AMPK) is an intracellular sensor of energy homoeostasis that is activated under energy stress and suppressed in energy surplus. AMPK activation leads to inhibition of anabolic processes that consume ATP. Osteogenic differentiation is a process that highly demands ATP during which AMPK is inhibited. The bone morphogenetic proteins (BMPs) signalling pathway plays an essential role in osteogenic differentiation. The present study examines the inhibitory effect of metformin on BMP signalling, osteogenic differentiation and trauma‐induced heterotopic ossification. Our results showed that metformin inhibited Smad1/5 phosphorylation induced by BMP6 in osteoblast MC3T3‐E1 cells, concurrent with up‐regulation of Smad6, and this effect was attenuated by knockdown of Smad6. Furthermore, we found that metformin suppressed ALP activity and mineralization of the cells, an event that was attenuated by the dominant negative mutant of AMPK and mimicked by its constitutively active mutant. Finally, administration of metformin prevented the trauma‐induced heterotopic ossification in mice. In conjuncture, AMPK activity and Smad6 and Smurf1 expression were enhanced by metformin treatment in the muscle of injured area, concurrently with the reduction of ALK2. Collectively, our study suggests that metformin prevents heterotopic ossification via activation of AMPK and subsequent up‐regulation of Smad6. Therefore, metformin could be a potential therapeutic drug for heterotopic ossification induced by traumatic injury.     

Abnormal metabolism flexibility in response to high palmitate concentrations in myotubes derived from obese type 2 diabetic patients

Insulin resistance in type 2 diabetes (T2D) is associated with intramuscular lipid (IMCL) accumulation. To determine whether impaired lipid oxidation is involved in IMCL accumulation, we measured expression of genes involved in mitochondrial oxidative metabolism or biogenesis, mitochondrial content and palmitate beta-oxidation before and after palmitate overload (600 μM for 16 h), in myotubes derived from healthy subjects and obese T2D patients. Mitochondrial gene expression, content and network were not different between groups. Basal palmitate beta-oxidation was not affected in T2D myotubes, whereas after 16 h of palmitate pre-treatment, T2D myotubes in contrast to control myotubes, showed an inability to increase palmitate beta-oxidation (p < 0.05). Interestingly, acetyl-CoA carboxylase (ACC) phosphorylation was increased with a tendency for statistical significance after palmitate pre-treatment in control myotubes (p = 0.06) but not in T2D myotubes which can explain their inability to increase palmitate beta-oxidation after palmitate overload. To determine whether the activation of the AMP activated protein kinase (AMPK)-ACC pathway was able to decrease lipid content in T2D myotubes, cells were treated with AICAR and metformin. These AMPK activators had no effect on ACC and AMPK phosphorylation in T2D myotubes as well as on lipid content, whereas AICAR, but not metformin, increased AMPK phosphorylation in control myotubes. Interestingly, metformin treatment and mitochondrial inhibition by antimycin induced increased lipid content in control myotubes. We conclude that T2D myotubes display an impaired capacity to respond to metabolic stimuli.     

Activation of the AMP-activated protein kinase by the anti-diabetic drug metformin in vivo. Role of mitochondrial reactive nitrogen species

Metformin, one of the most commonly used drugs for the treatment of type II diabetes, was recently found to exert its therapeutic effects, at least in part, by activating the AMP-activated protein kinase (AMPK). However, the site of its action, as well as the mechanism to activate AMPK, remains elusive. Here we report how metformin activates AMPK. In cultured bovine aortic endothelial cells, metformin dose-dependently activated AMPK in parallel with increased detection of reactive nitrogen species (RNS). Further, either depletion of mitochondria or adenoviral overexpression of superoxide dismutases, as well as inhibition of nitric-oxide synthase, abolished the metformin-enhanced phosphorylations and activities of AMPK, implicating that activation of AMPK by metformin might be mediated by the mitochondria-derived RNS. Furthermore, administration of metformin, which increased 3-nitrotyrosine staining in hearts of C57BL6, resulted in parallel activation of AMPK in the aorta and hearts of C57BL6 mice but not in those of endothelial nitric-oxide synthase (eNOS) knockout mice in which metformin had no effect on 3-nitrotyrosine staining. Because the eNOS knockout mice expressed normal levels of AMPK-alpha that was activated by 5-aminoimidazole-4-carboxamide riboside, an AMPK agonist, these data indicate that RNS generated by metformin is required for AMPK activation in vivo. In addition, metformin significantly increased the co-immunoprecipitation of AMPK and its upstream kinase, LKB1, in C57BL6 mice administered to metformin in vivo. Using pharmacological and genetic inhibitors, we found that inhibition of either c-Src or PI3K abolished AMPK that was enhanced by metformin. We conclude that activation of AMPK by metformin might be mediated by mitochondria-derived RNS, and activation of the c-Src/PI3K pathway might generate a metabolite or other molecule inside the cell to promote AMPK activation by the LKB1 complex.     

Crosstalk between AMPK activation and angiotensin II‐induced hypertrophy in cardiomyocytes: the role of mitochondria

AMP‐kinase (AMPK) activation reduces cardiac hypertrophy, although underlying molecular mechanisms remain unclear. In this study, we elucidated the anti‐hypertrophic action of metformin, specifically, the role of the AMPK/eNOS/p53 pathway. H9c2 rat cardiomyocytes were treated with angiotensin II (AngII) for 24 hrs in the presence or absence of metformin (AMPK agonist), losartan [AngII type 1 receptor (AT1R) blocker], Nω‐nitro‐L‐arginine methyl ester (L‐NAME, pan‐NOS inhibitor), splitomicin (SIRT1 inhibitor) or pifithrin‐α (p53 inhibitor). Results showed that treatment with metformin significantly attenuated AngII‐induced cell hypertrophy and death. Metformin attenuated AngII‐induced activation (cleavage) of caspase 3, Bcl‐2 down‐regulation and p53 up‐regulation. It also reduced AngII‐induced AT1R up‐regulation by 30% (P < 0.05) and enhanced AMPK phosphorylation by 99% (P < 0.01) and P‐eNOS levels by 3.3‐fold (P < 0.01). Likewise, losartan reduced AT1R up‐regulation and enhanced AMPK phosphorylation by 54% (P < 0.05). The AMPK inhibitor, compound C, prevented AT1R down‐regulation, indicating that metformin mediated its effects via AMPK activation. Beneficial effects of metformin and losartan converged on mitochondria that demonstrated high membrane potential (Δψ_m) and low permeability transition pore opening. Thus, this study demonstrates that the anti‐hypertrophic effects of metformin are associated with AMPK‐induced AT1R down‐regulation and prevention of mitochondrial dysfunction through the SIRT1/eNOS/p53 pathway.     

Co-stimulation with bone morphogenetic protein-9 and FK506 induces remarkable osteoblastic differentiation in rat dedifferentiated fat cells

Dedifferentiated fat (DFAT) cells, which are isolated from mature adipocytes using the ceiling culture method, exhibit similar characteristics to mesenchymal stem cells, and possess adipogenic, osteogenic, chondrogenic, and myogenic potentials. Bone morphogenetic protein (BMP)-2 and -9, members of the transforming growth factor-β superfamily, exhibit the most potent osteogenic activity of this growth factor family. However, the effects of BMP-2 and BMP-9 on the osteogenic differentiation of DFAT remain unknown. Here, we examined the effects of BMP-2 and BMP-9 on osteoblastic differentiation of rat DFAT (rDFAT) cells in the presence or absence of FK506, an immunosuppressive agent. Co-stimulation with BMP-9 and FK506 induced gene expression of runx2, osterix, and bone sialoprotein, and ALP activity compared with BMP-9 alone, BMP-2 alone and BMP-2 + FK506 in rDFAT cells. Furthermore, it caused mineralization of cultures and phosphorylation of smad1/5/8, compared with BMP-9 alone. The ALP activity induced by BMP-9 + FK506 was not influenced by addition of noggin, a BMP antagonist. Our data suggest that the combination of BMP-9 and FK506 potently induces osteoblastic differentiation of rDFAT cells.     

Effects of BMP-2 and vitamin D3 on the osteogenic differentiation of adipose stem cells

We studied the effect of bone morphogenetic protein-2 (BMP-2) and vitamin D₃ on the osteogenic differentiation of adipose stem cells (ASCs). ASCs were treated with 10, 50, and 100 ng/ml of BMP-2, and 10⁻⁸, 10⁻⁷, 10⁻⁶ M vitamin D₃. Then, to investigate the effects of combined treatment, ASCs were treated with BMP-2 and vitamin D₃ dose-dependently and time-dependently. The osteogenic differentiation was assessed by alkaline phosphatase (ALP) activities/staining and the mineralization was evaluated by Alizarin red S staining. ALP activity and mineralization dose-dependently increased in early stages (ALP on 7th day and mineralization on the 14th day) while all three doses of BMP-2 or vitamin D₃ showed comparable effects in late stages (ALP on the 14th day and mineralization on the 21st day) in ASCs. BMP-2 and vitamin D₃ had synergistic effect on the osteogenic differentiation of ASCs. While all three doses of BMP-2 acted similarly in reinforcing the effect of vitamin D₃, vitamin D₃ dose-dependently augmented the osteogenic effect of BMP-2. When BMP-2 was constantly treated, vitamin D₃ effect did not differ depending on the period of vitamin D₃ treatment. However, when vitamin D₃ was constantly treated, the BMP was more effective when treated for the last 7 days than when treated for the first 7 days. In conclusion, BMP-2 and vitamin D₃ promote osteogenic differentiation of ASCs, and can work synergistically. These results can be used to induce effective and economical osteogenic induction of ASCs for bone tissue engineering.     

eNOS activation mediated by AMPK after stimulation of endothelial cells with histamine or thrombin is dependent on LKB1

Reports on the role of AMP-activated protein kinase (AMPK) in thrombin-mediated activation of endothelial nitric-oxide synthase (eNOS) in endothelial cells have been conflicting. Previously, we have shown that under culture conditions that allow reduction of ATP-levels after stimulation, activation of AMPK contributes to eNOS phosphorylation and activation in endothelial cells after treatment with thrombin. In this paper we examined the signaling pathways mediating phosphorylation and activation of eNOS after stimulation of cultured human umbilical vein endothelial cells (HUVEC) with histamine and the role of LKB1-AMPK in the signaling. In Morgan's medium 199 intracellular ATP was lowered by treatment with histamine or the ionophore A23187 while in medium RMPI 1640 ATP was unchanged after identical treatment. In medium 199 inhibition of Ca^+ 2/CaM kinase kinase (CaMKK) by STO-609 only partially inhibited AMPK phosphorylation but after gene silencing of LKB1 with siRNA there was a total inhibition of AMPK phosphorylation by STO-609 after treatment with either histamine or thrombin, demonstrating phosphorylation of AMPK by both upstream kinases, LKB1 and CaMKK. Downregulation of AMPK with siRNA partially inhibited eNOS phosphorylation caused by histamine in cells maintained in medium 199. Downregulation of LKB1 by siRNA inhibited both phosphorylation and activity of eNOS and addition of the AMPK inhibitor Compound C had no further effect on eNOS phosphorylation. When experiments were carried out in medium 1640, STO-609 totally prevented the phosphorylation of AMPK without affecting eNOS phosphorylation. AMPKα2 downregulation resulted in a loss of the integrity of the endothelial monolayer and increased expression of GRP78, indicative of endoplasmic reticular (ER) stress. Downregulation of AMPKα1 had no such effect. The results show that culture conditions affect endothelial signal transduction pathways after histamine stimulation. Under conditions where intracellular ATP is lowered by histamine, AMPK is activated by both LKB1 and CaMKK and, in turn, mediates eNOS phosphorylation in an LKB1 dependent manner. Both AMPKα1 and − α2 are involved in the signaling. Under conditions where intracellular ATP is unchanged after histamine treatment, CaMKK alone activates AMPK and eNOS is phosphorylated and activated independent of AMPK.     

Metformin induces suppression of NAD(P)H oxidase activity in podocytes

Hyperglycemia increases the production of reactive oxygen species (ROS). NAD(P)H oxidase, producing superoxide anion, is the main source of ROS in diabetic podocytes and their production contributes to the development of diabetic nephropathy. We have investigated the effect of an antidiabetic drug, metformin on the production of superoxide anion in cultured podocytes and attempted to elucidate underlying mechanisms.The experiments were performed in normal (NG, 5.6 mM) and high (HG, 30 mM) glucose concentration. Overall ROS production was measured by fluorescence of a DCF probe. Activity of NAD(P)H oxidase was measured by chemiluminescence method. The AMP-dependent kinase (AMPK) activity was determined by immunobloting, measuring the ratio of phosphorylated AMPK to total AMPK. Glucose accumulation was measured using 2-deoxy-[1,2-³H]-glucose.ROS production increased by about 27% (187 ± 8 vs. 238 ± 9 arbitrary units AU, P < 0.01) in HG. Metformin (2 mM, 2 h) markedly reduced ROS production by 45% in NG and 60% in HG. Metformin decreased NAD(P)H oxidase activity in NG (36%) and HG (86%). AMPK activity was increased by metformin in NG and HG (from 0.58 ± 0.07 to. 0.99 ± 0.06, and from 0.53 ± 0.03 to 0.64 ± 0.03; P < 0.05). The effects of metformin on the activities of NAD(P)H oxidase and AMPK were abolished in the presence of AMPK inhibitor, compound C.We have shown that metformin decreases production of ROS through reduction of NAD(P)H oxidase activity. We also have demonstrated relationship between activity of NAD(P)H oxidase and AMPK.     

Metformin stimulates osteoprotegerin and reduces RANKL expression in osteoblasts and ovariectomized rats

Anti-diabetic drug metformin has been shown to enhance osteoblasts differentiation and inhibit osteoclast differentiation in vitro and prevent bone loss in ovariectomized (OVX) rats. But the mechanisms through which metformin regulates osteoclastogensis are not known. Osteoprotegerin (OPG) and receptor activator of nuclear factor κB ligand (RANKL) are cytokines predominantly secreted by osteoblasts and play critical roles in the differentiation and function of osteoclasts. In this study, we demonstrated that metformin dose-dependently stimulated OPG and reduced RANKL mRNA and protein expression in mouse calvarial osteoblasts and osteoblastic cell line MC3T3-E1. Inhibition of AMP-activated protein kinase (AMPK) and CaM kinase kinase (CaMKK), two targets of metformin, suppressed endogenous and metformin-induced OPG secretion in osteoblasts. Moreover, supernatant of osteoblasts treated with metformin reduced formation of tartrate resistant acid phosphatase (TRAP)-positive multi-nucleated cells in Raw264.7 cells. Most importantly, metformin significantly increased total body bone mineral density, prevented bone loss and decreased TRAP-positive cells in OVX rats proximal tibiae, accompanied with an increase of OPG and decrease of RANKL expression. These in vivo and in vitro studies suggest that metformin reduces RANKL and stimulates OPG expression in osteoblasts, further inhibits osteoclast differentiation and prevents bone loss in OVX rats.     

AMP-activated protein kinase (AMPK) positively regulates osteoblast differentiation via induction of Dlx5-dependent Runx2 expression in MC3T3E1 cells

This study examined the role of AMPK activation in osteoblast differentiation and the underlining mechanism. An AMPK activator (AICAR or metformin) stimulated osteoblast differentiation with increases in ALP and OC protein production as well as the induction of AMPK phosphorylation in MC3T3E1 cells. In addition, metformin induced the phosphorylation of Smad1/5/8 and expression of Dlx5 and Runx2, whereas compound C or dominant negative AMPK inhibited these effects. Transient transfection studies also showed that metformin increased the BRE-Luc and Runx2-Luc activities, which were inhibited by DN-AMPK or compound C. Down-regulation of Dlx5 expression by siRNA suppressed metformin-induced Runx2 expression. These results suggest that the activation of AMPK stimulates osteoblast differentiation via the regulation of Smad1/5/8-Dlx5-Runx2 signaling pathway.     

Analysis of microRNA expression profile induced by AICAR in mouse hepatocytes

AMP-activated protein kinase (AMPK) has been proposed to act as a key energy sensor mediating the metabolism of glucose and lipids, and pharmacological activation of AMPK may provide a new strategy for the management of type 2 diabetes. MicroRNAs (miRNAs) are a group of endogenous noncoding RNA that play important roles in many biological processes including energy metabolism. Whether miRNAs mediate AMPK action in regulating metabolic process is not clear. In this study, 0.5 mM 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) was added to increase activation of AMPK in 8 week old C57BL/6 mice primary hepatocytes. MiRNA microarray was performed to compare the miRNA expression profiles of hepatocytes treated with or without AICAR. We discovered that 41 miRNAs were significantly altered in AICAR-treated sample (fold change: > 2) compared with untreated control sample. Among them, 19 miRNAs were upregulated. MiRNA targets were predicted by TargetScan. Further bioinformatic analysis indicated that these predicted targets might be mainly involved in pathways of cellular metabolism and tumor pathogenesis. FUNDO analysis suggested that these predicted targets were enriched in cancer, diabetes mellitus, hypertension, obesity and heart failure (P < 0.01). A series of miRNAs could be regulated by the activation of AMPK and might mediate the action of AMPK during metabolic processes and tumor pathogenesis. Predicted target genes discovered in this study and pathway analysis provide new insights into hepatic metabolism and tumor pathogenesis regulated by AMPK signaling and clues to the possible molecular mechanism underlying the effect of AMPK.     

BMP-13 Emerges as a Potential Inhibitor of Bone Formation

Bone morphogenetic protein-13 (BMP-13) plays an important role in skeletal development. In the light of a recent report that mutations in the BMP-13 gene are associated with spine vertebral fusion in Klippel-Feil syndrome, we hypothesized that BMP-13 signaling is crucial for regulating embryonic endochondral ossification. In this study, we found that BMP-13 inhibited the osteogenic differentiation of human bone marrow multipotent mesenchymal stromal cells (BM MSCs) in vitro. The endogenous BMP-13 gene expression in MSCs was examined under expansion conditions. The MSCs were then induced to differentiate into osteoblasts in osteo-inductive medium containing exogenous BMP-13. Gene expression was analysed by real-time PCR. Alkaline phosphatase (ALP) expression and activity, proteoglycan (PG) synthesis and matrix mineralization were assessed by cytological staining or ALP assay. Results showed that endogenous BMP-13 mRNA expression was higher than BMP-2 or -7 during MSC growth. BMP-13 supplementation strongly inhibited matrix mineralization and ALP activity of osteogenic differentiated MSCs, yet increased PG synthesis under the same conditions. In conclusion, BMP-13 inhibited osteogenic differentiation of MSCs, implying that functional mutations or deficiency of BMP-13 may allow excess bone formation. Our finding provides an insight into the molecular mechanisms and the therapeutic potential of BMP-13 in restricting pathological bone formation.     

miR-542-3p suppresses osteoblast cell proliferation and differentiation,targets BMP-7 signaling and inhibits bone formation

MicroRNAs (miRNAs) are short non-coding RNAs that interfere with translation of specific target mRNAs and thereby regulate diverse biological processes. Recent studies have suggested that miRNAs might have a role in osteoblast differentiation and bone formation. Here, we show that miR-542-3p, a well-characterized tumor suppressor whose downregulation is tightly associated with tumor progression via C-src-related oncogenic pathways, inhibits osteoblast proliferation and differentiation. miRNA array profiling in Medicarpin (a pterocarpan with proven bone-forming effects) induced mice calvarial osteoblast cells and further validation by quantitative real-time PCR revealed that miR-542-3p was downregulated during osteoblast differentiation. Over-expression of miR-542-3p inhibited osteoblast differentiation, whereas inhibition of miR-542-3p function by anti-miR-542-3p promoted expression of osteoblast-specific genes, alkaline phosphatase activity and matrix mineralization. Target prediction analysis tools and experimental validation by luciferase 3′ UTR reporter assay identified BMP-7 (bone morphogenetic protein 7) as a direct target of miR-542-3p. It was seen that over-expression of miR-542-3p leads to repression of BMP-7 and inhibition of BMP-7/PI3K- survivin signaling. This strongly suggests that miR-542-3p suppresses osteogenic differentiation and promotes osteoblast apoptosis by repressing BMP-7 and its downstream signaling. Furthermore, silencing of miR-542-3p led to increased bone formation, bone strength and improved trabecular microarchitecture in sham and ovariectomized (Ovx) mice. Although miR-542-3p is known to be a tumor repressor, we have identified second complementary function of miR-542-3p where it inhibits BMP-7-mediated osteogenesis. Our findings suggest that pharmacological inhibition of miR-542-3p by anti-miR-542-3p could represent a therapeutic strategy for enhancing bone formation in vivo.