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Vanadate prevents glucocorticoid-induced apoptosis of osteoblasts in vitro and osteocytes in vivo.

ABSTRACT: Skeletal mass is maintained by a balance between formation and resorption, cell proliferation and apoptosis. In vitro, glucocorticoids (GCs) decrease extracellular signal-regulated kinases (ERK) activation by mitogens, thus inhibiting osteoblast proliferation. Both ERK activity and proliferation are restored by co-treatment with the protein tyrosine phosphatase inhibitor, vanadate. Since ERK signalling may also be anti-apoptotic, we explored the effects of vanadate on GC-induced apoptosis in vitro and in vivo. Apoptosis in MBA-15.4 pre-osteoblasts increased from 6 h and remained up to eightfold higher through 6 days of 10(- 6) M dexamethasone (Dex) treatment. Co-incubation with 10(- 7) M vanadate markedly reduced apoptosis at all time points. Vanadate also prevented GC-induced poly-ADP-ribose polymerase cleavage. We assessed the transcriptional profiles of seven anti-apoptotic proteins (Bcl-2, Bcl-X(L), inhibitors of apoptosis protein-1 (IAP-1), IAP-2, X-linked IAP (XIAP), Fas-associated death-domain-like IL-1beta-converting enzyme-inhibitory protein (FLIP(Long)) and FLIP(Short)) in osteoblasts subjected to various stimuli using real-time quantitative PCR. Although these anti-apoptotic genes responded to different mitogenic conditions, Dex failed to repress their expression, and in fact significantly up-regulated Bcl-X(L), IAP-2 and XIAP. Dex may therefore induce apoptosis by up-regulating pro-apoptotic gene expression. We have previously demonstrated that rats treated with GC develop low formation osteoporosis (bone histomorphometry and DEXA) and skeletal fragility (breaking strength) that were largely prevented by co-treatment with vanadate. We report here that vertebrae from rats treated with 3.5 mg/kg per day methylprednisolone for 9 weeks showed increased incidence of terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick end-labelling-positive apoptotic osteocytes, which was reduced by vanadate co-treatment. We conclude that vanadate prevents GC-induced apoptosis of pre-osteoblasts in vitro and osteocytes in vivo, and this may contribute to its bone-sparing effects in vivo.

SUBMITTER: Conradie MM

PROVIDER: S-EPMC2173947 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Methylglyoxal (MGO) is an active metabolite of glucose and plays a prominent role in the pathogenesis of diabetic vascular complications, including endothelial cell apoptosis induced by oxidative stress. Metformin (MET), a widely prescribed antidiabetic agent, appears to reduce excessive reactive oxygen species (ROS) generation and limit cell apoptosis. However, the molecular mechanisms underlying this process are still not fully elucidated. We reported here that MET prevents MGO-induced apoptosis by suppressing oxidative stress in vitro and in vivo. Protein expression and protein phosphorylation were investigated using western blotting, ELISA, and immunohistochemical staining, respectively. Cell viability and apoptosis were assessed by the MTT assay, TUNEL staining, and Annexin V-FITC and propidium iodide double staining. ROS generation and mitochondrial membrane potential (MMP) were measured with fluorescent probes. Our results revealed that MET prevented MGO-induced HUVEC apoptosis, inhibited apoptosis-associated biochemical changes such as loss of MMP, the elevation of the Bax/Bcl-2 ratio, and activation of cleaved caspase-3, and attenuated MGO-induced mitochondrial morphological alterations in a dose-dependent manner. MET pretreatment also significantly suppressed MGO-stimulated ROS production, increased signaling through the ROS-mediated PI3K/Akt and Nrf2/HO-1 pathways, and markedly elevated the levels of its downstream antioxidants. Finally, similar results were obtained in vivo, and we demonstrated that MET prevented MGO-induced oxidative damage, apoptosis, and inflammation. As expected, MET reversed MGO-induced downregulation of Nrf2 and p-Akt. In addition, a PI3K inhibitor (LY-294002) and a Nrf2 inhibitor (ML385) observably attenuated the protective effects of MET on MGO-induced apoptosis and ROS generation by inhibiting the Nrf2/HO-1 pathways, while a ROS scavenger (NAC) and a permeability transition pores inhibitor (CsA) completely reversed these effects. Collectively, these findings broaden our understanding of the mechanism by which MET regulates apoptosis induced by MGO under oxidative stress conditions, with important implications regarding the potential application of MET for the treatment of diabetic vascular complications.

Project description:BackgroundGlucocorticoid (GC) is one of frequently used anti-inflammatory agents, but its administration is unfortunately accompanied with bone loss. Although sporadic studies indicated that osteocytes are subject to a series of pathological changes under GC stress, including overexpression of cathepsin K, the definite role of osteocytes in GC-induced bone loss remains largely unclear.MethodsGene expression of Ctsk and protein levels of cathepsin K were assessed in MLO-Y4 cell lines exposed to dexamethasone (Dex) of different time (0, 12, 24 hours) and dose (0, 10-8 and 10-6 M) courses by RT-qPCR and western blotting, respectively. Confocal imaging and immunostaining were then performed to evaluate the effects of osteocyte-derived cathepsin K on type I collagen in a primary osteocyte ex vivo culture system. MitoTracker Red was used to stain mitochondria for mitochondria morphology assessment and JC-1 assay was employed to evaluate the mitochondria membrane potential in MLO-Y4 cells following Dex treatment. Activation of PINK1-mediated mitophagy was evaluated by immunostaining of the PINK1 protein and CytoID assay. Mdivi-1 was used to inhibit mitophagy and siRNAs were used for the inhibition of Pink1 and Atg5.ResultsGC triggered osteocytes to produce excessive cathepsin K which in turn led to the degradation of type I collagen in the extracellular matrix in a primary osteocyte ex vivo culture system. Meanwhile, GC administration increased mitochondrial fission and membrane depolarization in osteocytes. Further, the activation of PINK1-mediated mitophagy was demonstrated to be responsible for the diminishment of dysfunctional mitochondria in osteocytes. Examination of relationship between mitophagy and cathepsin K production revealed that inhibition of mitophagy via knocking down Pink1 gene abolished the GC-triggered cathepsin K production. Interestingly, GC's activation effect towards cathepsin K via mitophagy was found to be independent on the canonical autophagy as this effect was not impeded when inhibiting the canonical autophagy via Atg5 suppression.ConclusionGC-induced PINK1-mediated mitophagy substantially modulates the production of cathepsin K in osteocytes, which could be an underlying mechanism by which osteocytes contribute to the extracellular matrix degradation during bone loss.The translational potential of this articleFindings of the current study indicate a possible role of osteocyte mitophagy in GC-induced bone loss, which provides a potential therapeutic approach to alleviate GC-induced osteoporosis by targeting PINK1-mediated osteocytic mitophagy.

Project description:Osteocytes represent the most abundant cellular component of mammalian bones with important functions in bone mass maintenance and remodeling. To elucidate the differential gene expression between osteoblasts and osteocytes we completed a comprehensive analysis of their gene profiles. Selective identification of these two mature populations was achieved by utilization of visual markers of bone lineage cells. We have utilized dual GFP reporter mice in which osteocytes are expressing GFP (topaz) directed by the DMP1 promoter, while osteoblasts are identified by expression of GFP (cyan) driven by 2.3 kb of the Col1a1 promoter. Histological analysis of 7-day-old neonatal calvaria confirmed the expression pattern of DMP1GFP in osteocytes and Col2.3 in osteoblasts and osteocytes. To isolate distinct populations of cells we utilized fluorescent activated cell sorting (FACS). Cell suspensions were subjected to RNA extraction, in vitro transcription and labeling of cDNA and gene expression was analyzed using the Illumina WG-6v1 BeadChip. Following normalization of raw data from four biological replicates, 3444 genes were called present in all three sorted cell populations: GFP negative, Col2.3cyan(+) (osteoblasts), and DMP1topaz(+) (preosteocytes and osteocytes). We present the genes that showed in excess of a 2-fold change for gene expression between DMP1topaz(+) and Col2.3cyan(+) cells. The selected genes were classified and grouped according to their associated gene ontology terms. Genes clustered to osteogenesis and skeletal development such as Bmp4, Bmp8a, Dmp1, Enpp1, Phex and Ank were highly expressed in DMP1topaz(+)cells. Most of the genes encoding extracellular matrix components and secreted proteins had lower expression in DMP1topaz(+) cells, while most of the genes encoding plasma membrane proteins were increased. Interestingly a large number of genes associated with muscle development and function and with neuronal phenotype were increased in DMP1topaz(+) cells, indicating some new aspects of osteocyte biology. Although a large number of genes differentially expressed in DMP1topaz(+) and Col2.3cyan(+) cells in our study have already been assigned to bone development and physiology, for most of them we still lack any substantial data. Therefore, isolation of osteocyte and osteoblast cell populations and their subsequent microarray analysis allowed us to identify a number or genes and pathways with potential roles in regulation of bone mass.

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Vanadate prevents glucocorticoid-induced apoptosis of osteoblasts in vitro and osteocytes in vivo.

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