Yong Pan

Sirtuin-3 (Sirt3) is an essential metabolic regulatory enzyme that plays an important role in mitochondrial metabolism, but its role in bone marrow and skeletal homeostasis remains largely unknown. In this study, we hypothesize that increased expression of Sirt3 plays a role in skeletal aging. Using mice that overexpress Sirt3 (Sirt3Tg), we show that Sirt3 is a positive regulator of adipogenesis and osteoclastogenesis, and a negative regulator of skeletal homeostasis. Sirt3Tg mice exhibited an increased number of adipocytes in the tibia compared to control mice. Bone marrow stromal cells (BMSCs) from Sirt3Tg mice displayed an enhanced ability to differentiate into adipocytes compared to control BMSCs. We found a 2.5 fold increase in the number of osteoclasts/bone surface in Sirt3Tg mice compared with control mice (P< 0.03), and increased osteoclastogenesis in vitro. Importantly, Sirt3 activates the mTOR pathway to regulate osteoclastogenesis. Sirt3Tg male mice exhibited a significant reduction in cortical thickness at the tibio-fibular junction (P in vivo, and identify Sirt3 as a potential target for the treatment of osteoporosis.

Tissue fibrosis is a major cause of organ dysfunction during chronic diseases and aging. A critical step in this process is transforming growth factor β1 (TGF-β1)-mediated transformation of fibroblasts into myofibroblasts, cells capable of synthesizing extracellular matrix. Here, we show that SIRT3 controls transformation of fibroblasts into myofibroblasts via suppressing the profibrotic TGF-β1 signaling. We found that Sirt3 knockout (KO) mice with age develop tissue fibrosis of multiple organs, including heart, liver, kidney, and lungs but not whole-body SIRT3-overexpressing mice. SIRT3 deficiency caused induction of TGF-β1 expression and hyperacetylation of glycogen synthase kinase 3β (GSK3β) at residue K15, which negatively regulated GSK3β activity to phosphorylate the substrates Smad3 and β-catenin. Reduced phosphorylation led to stabilization and activation of these transcription factors regulating expression of the profibrotic genes. SIRT3 deacetylated and activated GSK3β and thereby blocked TGF-β1 signaling and tissue fibrosis. These data reveal a new role of SIRT3 to negatively regulate aging-associated tissue fibrosis and discloses a novel phosphorylation-independent mechanism controlling the catalytic activity of GSK3β.

Objective: To quantify functional age‐related changes in the cartilage antioxidant network in order to discover novel mediators of cartilage oxidative stress and osteoarthritis (OA) pathophysiology.Methods: We evaluated histopathologic changes of knee OA in 10‐, 20‐, and 30‐month‐old male F344BN rats and analyzed cartilage oxidation according to the ratio of reduced to oxidized glutathione. Antioxidant gene expression and protein abundance were analyzed by quantitative reverse transcription–polymerase chain reaction and selected reaction–monitoring mass spectrometry, respectively. Superoxide dismutase 2 (SOD2) activity and acetylation were analyzed by colorimetric enzyme assays and Western blotting, respectively. We examined human OA cartilage to evaluate the clinical relevance of SOD2 acetylation, and we tested age‐related changes in the mitochondrial deacetylase sirtuin 3 (SIRT‐3) in rats and mice.Results: Cartilage oxidation and OA severity in F344BN rats increased with age and were associated with an increase in SOD2 expression and protein abundance. However, SOD2‐specific activity decreased with age due to elevated posttranslational lysine acetylation. Consistent with these findings, SIRT‐3 levels decreased substantially with age, and treatment with SIRT‐3 increased SOD2 activity in an age‐dependent manner. SOD2 was also acetylated in human OA cartilage, and activity was increased with SIRT‐3 treatment. Moreover, in C57BL/6J mice, cartilage SIRT‐3 expression decreased with age, and whole‐body deletion of SIRT‐3 accelerated the development of knee OA.Conclusion: Our results show that SIRT‐3 mediates age‐related changes in cartilage redox regulation and protects against early‐stage OA. These findings suggest that mitochondrial acetylation promotes OA and that restoration of SIRT‐3 in aging cartilage may improve cartilage resistance to oxidative stress by rescuing acetylation‐dependent inhibition of SOD2 activity.

The fungicide rapamycin increases lifespan in eukaryotes by interfering with the activity of a serine/threonine kinase called TOR (target of rapamycin). TOR complex 1 (TORC1) is an essential integrator of cellular nutrient cues, growth signals and cellular metabolism. Here, we review major components of TORC1, its downstream effectors and lifespan studies in various organisms involving these signaling components. In particular, we focus on the role of rapamycin in mitochondrial biogenesis, in metabolic regulation and in the control of reactive oxygen species production.

The TGF-beta signaling pathway is important in the development and invasion of cancers. Int7G24A is an intronic variant of TGF-beta receptor type 1 and has been shown to be associated with the occurrence of some kinds of cancers. Nevertheless, the association of this polymorphism with osteosarcoma is unknown. In this study, we evaluated Int7G24A variant frequencies in osteosarcoma cases. The case–control study involved 168 osteosarcoma patients and 168 age- and gender-matched controls. The blood samples were obtained, and Int7G24A variant was determined by PCR amplification and DNA sequencing. The odds ratio (OR) and 95% confidence interval (95% CI) for the Int7G24A polymorphism were calculated using unconditional logistic regression adjusted for age and gender. Three analysis models, which are the dominant model, additive model and recessive model, were used to analyze the contribution of Int7G24A variant to osteosarcoma susceptibility. Heterozygotic and homozygotic Int7G24A variants were 33.93 and 6.55% in total 168 cases, while they were 28.57 and 2.98%, respectively, in total 168 controls. The ORs for homozygosity and heterozygosity of Int7G24A allele were 1.56 [95% CI, 0.98–1.83] and 2.89 [95% CI, 1.46–4.92] in additive model. The ORs of Int7G24A genotypes in dominant model and in recessive model were 1.75 [95% CI, 1.21–2.68] and 2.21 [95% CI, 1.34–4.72], respectively. There were significant increases in Int7G24A variants in osteosarcoma cases when compared to control in every three models. Further analysis showed that Int7G24A genotypes were not associated with gender and osteosarcoma location of the cases. However, Int7G24A was significantly increased in the cases less than 20 years old. Moreover, Int7G24A was significantly associated with increased distant metastasis of osteosarcoma. It is concluded that Int7G24A is a polymorphism of TGFBR1 that is associated with the susceptibility and distant metastasis of osteosarcoma.

Comment on: Extension of chronological life span by reduced TOR signaling requires down-regulation of Sch9p and involves increased mitochondrial OXPHOS complex density. Yong Pan, Gerald S. Shadel. Aging 2009; 1:131-145