| Background:Osteoporosis(OP)is a progressive skeletal disease of low bone mineral density(BMD)often affecting aged population especially postmenopausal women and potentially leads to fragility fracture.To maintain proper strength and integrity,bone undergoes constant turnover that is controlled by a balance between osteoclast-maintained bone resorption(osteoclastosis)and osteoblast-incurred bone formation(osteoblastosis).OP is a result when osteoclastosis exceeds osteoblastosis and histomorphologically characterized by reduced BMD and trabecular microstructure deterioration.OP pathogenesis involves various endogenous and exogenous factors including aging,weight-related mechanic stimulation,abnormal mineral and hormone metabolisms,and stress of various natures that are mechanistically linked to the altered transcription of osteogenic genes.Recent studies have shown that epigenetic modification aberrations participate in the control of osteoclastogenesis and osteoblastogenesis.In particular,DNA methylation profiling investigations detect a large number of genomic loci/genes that are modified by DNA methylation alterations in trabeculae of OP patients,which represents another fundamental mechanism of OP pathogenesis.DNA methylation is one of the most stable epigenetic modifications of gene transcription and affects nearly 60-90%of genomic CpG dinucleotides,which is inversely regulated by two groups of enzymes of opposite catalytic functions.Three bioactive DNA methylation transferases(Dnmt),namely maintenance Dnmtl and de novo Dnmt3a and Dnmt3b,catalyze DNA methylation by adding a methyl group(CH3)to cytosine of CpG dinucleotides enriched in gene promoters/enhancers(CpG islands)that blocks the access of transcription factors and silences the gene transcription.On the contrary,DNA demethylation is mediated by ten-eleven translocation(TET)family enzymes via sequential oxidization of 5mC to 5hmC and its intermediates and completed by base excision process.DNA methylation is reversible and DNA demethylating drugs have shown therapeutic benefits against various chronic and aging-related disorders including OP in animal studies,however their long term uses might cause intolerable side effects in certain patients.Recent studies have discovered that proper physical exercises are capable of beneficially modulating DNA methylations in various physiological and pathological conditions including aging,cancer,type II diabetes.Physical exercises are feasible,side-effect free,and known to be beneficial for maintaining bone health and prophylactic against OP;however,whether its anti-OP effects involve epigenetic mechanisms and the critical genes mediating the protection are intriguing,but not experimentally established.It is well-known that weak oxidative stress responses and the associated low circulating antioxidants causally relate to lower BMD,suggesting that redox imbalance is a risking factor of OP.Nrf2(Nuclear factor erythroid derived 2-related factor-2)is a transcription factor essential for host cellular defense against oxidative stress and bone homeostasis.Nrf2 is ubiquitously expressed in almost all cell types including osteoblasts,osteocytes,and osteoclasts in an inactive form in cytoplasm.Upon stimulation by stress of various natures,Nrf2 is released from its inhibitor Keapl and translocates into cell nucleus where it binds to the antioxidant-response element(ARE)on target gene promoters and transactivates a number of antioxidant and detoxification enzymes such as catalase,superoxide dismutase(SOD)and glutathione peroxidase(GPX).Nrf2 genetic or pathological deficiencies correlate with increased susceptibility to OP in mice due to insufficient anti-oxidative stress capacity,while enhancing Nrf2 by various activators or agonists are protective against OP.Nrf2 promoter is featured by CpG island and Nrf2 down-regulations due to altered DNA methylation are observed in several aging-related pathological conditions such as aging,cancer and neurodegenerative disease,raising a possibility that epigenetic Nrf2 suppression might contribute to OP initiation or progression.Methods:1)To uncover the role of Nrf2 signaling in bone homeostasis,we adopted a well-known mouse OP model of ovariectomy in Nfe212 deficient mice and the wild-type(WT)littermates and assessed trabecular microstructure in femur.2)bone tissue expression of Nrf2 and the levels of bioactive DNA methytranferase as well as Nrf2 promoter methylation status were examined from Osteoporosis patients and Ovariectomy induced Osteoporosis mice by Western Blot,Immunohistochemistry and methylation specific PCR(MSP).3)Running exercise demethylates Nrf2 promoter and inhibits the aberrant femoral Dnmt expression.4)A strain of conditional Nrf2 knockout mice was created to test the critical role of running exercise restoration of Nrf2 for the anti-Osteoporosis functions in Ovariectomy mice.Results:1)Nrf2 is a critical anti-osteoporotic factor suppressed in osteoporotic femurs2)Nrf2 promoter is hypermethylated accompanied by Dnmt alterations in osteoporotic femurs.3)Running exercise demethylates Nrf2 promoter and inhibits the aberrant femoral Dnmt expression4)Running exercise derepresses Nrf2 and protects Ovx mice from osteoporosis5)Running exercise mitigates the suppression of Nrf2 downstream antioxidant enzymes6)Nrf2 is essential for the anti-osteoporosis effects of running exerciseConclusion:We reveal that Nfef212 deficient mice develop more severe OP after Ovx and Nrf2 is markedly repressed in osteoporotic femurs of both OP patients and Ovx mice.The Nrf2 repression is likely due to aberrant elevations of Dnmtl,Dnmt3a and Dnmt3b and the consequential Nrf2 promoter hypermethylation.We further show that one hour daily treadmill running of Ovx mice effectively reverses the epigenetic alterations and recovered the Nrf2 loss,and Nrf2 dependently protects against the femur BMD loss.These results uncover an important epigenetic feature of OP pathogenesis that links the epigenetic modulating capacity of physical exercise to its anti-OP function... |
PDF Full Text Request