75-kd sirtuin 1 blocks tumor necrosis factor α-mediated apoptosis in human osteoarthritic chondrocytes

Two-year post-distraction cartilage-related structural improvement is accompanied by increased serum full-length SIRT1

BACKGROUND

Previously, fragments from Sirtuin 1 (SIRT1) were identified in preclinical and clinical samples to display an increase in serum levels for N-terminal (NT) SIRT1 vs. C-terminal (CT) SIRT1, indicative of early signs of OA. Here we tested NT/CT SIRT1 levels as well as a novel formulated sandwich assay to simultaneously detect both domains of SIRT1 in a manner that may inform us about the levels of full-length SIRT1 in the circulation (flSIRT1) of clinical cohorts undergoing knee joint distraction (KJD).

METHODS

We employed an indirect ELISA assay to test NT- and CT-SIRT1 levels and calculated their ratio. Further, to test flSIRT1 we utilized novel antibodies (Ab), which were validated for site specificity and used in a sandwich ELISA method, wherein the CT-reactive served as capture Ab, and its NT-reactive served as primary detection Ab. This method was employed in human serum samples derived from a two-year longitudinal study of KJD patients. Two-year clinical and structural outcomes were correlated with serum levels of flSIRT1 compared to baseline.

RESULTS

Assessing the cohort, exhibited a significant increase of NT/CT SIRT1 serum levels with increased osteophytes and PIIANP/CTX-II at baseline, while a contradictory increase in NT/CT SIRT1 was associated with less denuded bone, post-KJD. On the other hand, flSIRT1 exhibited an upward trend in serum level, accompanied by reduced denuded bone for 2-year adjusted values. Moreover, 2 year-adjusted flSIRT1 levels displayed a steeper linear regression for cartilage and bone-related structural improvement than those observed for NT/CT SIRT1.

CONCLUSIONS

Our data support that increased flSIRT1 serum levels are a potential molecular endotype for cartilage-related structural improvement post-KJD, while NT/CT SIRT1 appears to correlate with osteophyte and PIIANP/CTX-II reduction at baseline, to potentially indicate baseline OA severity.

Sirtuin 1 in osteoarthritis: Perspectives on regulating glucose metabolism

Osteoarthritis (OA) is a degenerative disease characterised by articular cartilage destruction, and its complex aetiology contributes to suboptimal clinical treatment outcomes. A close association exists between glucose metabolism dysregulation and OA pathogenesis. Owing to the unique environment of low oxygen and glucose concentrations, chondrocytes rely heavily on their glycolytic capacity, exhibiting distinct spatiotemporal differences. However, under pathological stimulation, chondrocytes undergo excessive glycolytic activity while mitochondrial respiration and other branches of glucose metabolism are compromised. This metabolic change induces cartilage degeneration by reprogramming the inflammatory responses. Sirtuins, a highly conserved family of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases, regulate glucose metabolism in response to energy fluctuations in different cellular compartments，alleviating metabolic stress. SIRT1, the most extensively studied sirtuin, participates in maintaining glucose homeostasis in almost all key metabolic tissues. While actively contributing to the OA progression and displaying diverse biological effects in cartilage protection, SIRT1's role in regulating glucose metabolism in chondrocytes has not received sufficient attention. This review focuses on discussing the beneficial role of SIRT1 in OA progression from a metabolic regulation perspective based on elucidating the primary characteristics of chondrocyte glucose metabolism. We also summarise the potential mechanisms and therapeutic strategies targeting SIRT1 in chondrocytes to guide clinical practice and explore novel therapeutic directions.

Vitamin D induces SIRT1 activation through K610 deacetylation in colon cancer

Posttranslational modifications of epigenetic modifiers provide a flexible and timely mechanism for rapid adaptations to the dynamic environment of cancer cells. SIRT1 is an NAD+-dependent epigenetic modifier whose activity is classically associated with healthy aging and longevity, but its function in cancer is not well understood. Here, we reveal that 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3, calcitriol), the active metabolite of vitamin D (VD), promotes SIRT1 activation through auto-deacetylation in human colon carcinoma cells, and identify lysine 610 as an essential driver of SIRT1 activity. Remarkably, our data show that the post-translational control of SIRT1 activity mediates the antiproliferative action of 1,25(OH)2D3. This effect is reproduced by the SIRT1 activator SRT1720, suggesting that SIRT1 activators may offer new therapeutic possibilities for colon cancer patients who are VD deficient or unresponsive. Moreover, this might be extrapolated to inflammation and other VD deficiency-associated and highly prevalent diseases in which SIRT1 plays a prominent role.

Transcriptomic analyses of joint tissues during osteoarthritis development in a rat model reveal dysregulated mechanotransduction and extracellular matrix pathways

OBJECTIVE

Transcriptomic changes in joint tissues during the development of osteoarthritis (OA) are of interest for the discovery of biomarkers and mechanisms of disease. The objective of this study was to use the rat medial meniscus transection (MMT) model to discover stage and tissue-specific transcriptomic changes.

DESIGN

Sham or MMT surgeries were performed in mature rats. Cartilage, menisci and synovium were scored for histopathological changes at 2, 4 and 6 weeks post-surgery and processed for RNA-sequencing. Differentially expressed genes (DEG) were used to identify pathways and mechanisms. Published transcriptomic datasets from animal models and human OA were used to confirm and extend present findings.

RESULTS

The total number of DEGs was already high at 2 weeks (723 in meniscus), followed by cartilage (259) and synovium (42) and declined to varying degrees in meniscus and synovium but increased in cartilage at 6 weeks. The most upregulated genes included tenascins. The 'response to mechanical stimulus' and extracellular matrix-related pathways were enriched in both cartilage and meniscus. Pathways that were enriched in synovium at 4 weeks indicate processes related to synovial hyperplasia and fibrosis. Synovium also showed upregulation of IL-11 and several MMPs. The mechanical stimulus pathway included upregulation of the mechanoreceptors PIEZO1, PIEZO2 and TRPV4 and nerve growth factor. Analysis of data from prior RNA-sequencing studies of animal models and human OA support these findings.

CONCLUSION

These results indicate several shared pathways that are affected during OA in cartilage and meniscus and support the role of mechanotransduction and other pathways in OA pathogenesis.

The epigenetic players and the chromatin marks involved in the articular cartilage during osteoarthritis

Epigenetics defines the modifications of the genome that do not involve a change in the nucleotide sequence of DNA. These modifications constitute a mechanism of gene regulation poorly explored in the context of cartilage physiology. They are now intensively studied by the scientific community working on articular cartilage and its related pathology such as osteoarthritis. Indeed, epigenetic regulations can control the expression of crucial gene in the chondrocytes, the only resident cells of cartilage. Some epigenetic changes are considered as a possible cause of the abnormal gene expression and the subsequent alteration of the chondrocyte phenotype (hypertrophy, proliferation, senescence…) as observed in osteoarthritic cartilage. Osteoarthritis is a joint pathology, which results in impaired extracellular matrix homeostasis and leads ultimately to the progressive destruction of cartilage. To date, there is no pharmacological treatment and the exact causes have yet to be defined. Given that the epigenetic modifying enzymes can be controlled by pharmacological inhibitors, it is thus crucial to describe the epigenetic marks that enable the normal expression of extracellular matrix encoding genes, and those associated with the abnormal gene expression such as degradative enzyme or inflammatory cytokines encoding genes. In this review, only the DNA methylation and histone modifications will be detailed with regard to normal and osteoarthritic cartilage. Although frequently referred as epigenetic mechanisms, the regulatory mechanisms involving microRNAs will not be discussed. Altogether, this review will show how this nascent field influences our understanding of the pathogenesis of OA in terms of diagnosis and how controlling the epigenetic marks can help defining epigenetic therapies.

Overview of Anti-Inflammatory and Anti-Nociceptive Effects of Polyphenols to Halt Osteoarthritis: From Preclinical Studies to New Clinical Insights

Knee osteoarthritis (OA) is one of the most multifactorial joint disorders in adults. It is characterized by degenerative and inflammatory processes that are responsible for joint destruction, pain and stiffness. Despite therapeutic advances, the search for alternative strategies to target inflammation and pain is still very challenging. In this regard, there is a growing body of evidence for the role of several bioactive dietary molecules (BDMs) in targeting inflammation and pain, with promising clinical results. BDMs may be valuable non-pharmaceutical solutions to treat and prevent the evolution of early OA to more severe phenotypes, overcoming the side effects of anti-inflammatory drugs. Among BDMs, polyphenols (PPs) are widely studied due to their abundance in several plants, together with their benefits in halting inflammation and pain. Despite their biological relevance, there are still many questionable aspects (biosafety, bioavailability, etc.) that hinder their clinical application. This review highlights the mechanisms of action and biological targets modulated by PPs, summarizes the data on their anti-inflammatory and anti-nociceptive effects in different preclinical in vitro and in vivo models of OA and underlines the gaps in the knowledge. Furthermore, this work reports the preliminary promising results of clinical studies on OA patients treated with PPs and discusses new perspectives to accelerate the translation of PPs treatment into the clinics.

Ginsenoside Rg3 Attenuates TNF-α-Induced Damage in Chondrocytes through Regulating SIRT1-Mediated Anti-Apoptotic and Anti-Inflammatory Mechanisms

The upregulation of tumor necrosis factor-alpha (TNF-α) is a common event in arthritis, and the subsequent signaling cascade that leads to tissue damage has become the research focus. To explore a potential therapeutic strategy to prevent cartilage degradation, we tested the effect of ginsenoside Rg3, a bioactive component of Panax ginseng, on TNF-α-stimulated chondrocytes.TC28a2 Human Chondrocytes were treated with TNF-α to induce damage of chondrocytes. SIRT1 and PGC-1a expression levels were investigated by Western blotting assay. Mitochondrial SIRT3 and acetylated Cyclophilin D (CypD) were investigated using mitochondrial isolation. The mitochondrial mass number and mitochondrial DNA copy were studied for mitochondrial biogenesis. MitoSOX and JC-1 were used for the investigation of mitochondrial ROS and membrane potential. Apoptotic markers, pro-inflammatory events were also tested to prove the protective effects of Rg3. We showed Rg3 reversed the TNF-α-inhibited SIRT1 expression. Moreover, the activation of the SIRT1/PGC-1α/SIRT3 pathway by Rg3 suppressed the TNF-α-induced acetylation of CypD, resulting in less mitochondrial dysfunction and accumulation of reactive oxygen species (ROS). Additionally, we demonstrated that the reduction of ROS ameliorated the TNF-α-elicited apoptosis. Furthermore, the Rg3-reverted SIRT1/PGC-1α/SIRT3 activation mediated the repression of p38 MAPK, which downregulated the NF-κB translocation in the TNF-α-treated cells. Our results revealed that administration of Rg3 diminished the production of interleukin 8 (IL-8) and matrix metallopeptidase 9 (MMP-9) in chondrocytes via SIRT1/PGC-1α/SIRT3/p38 MAPK/NF-κB signaling in response to TNF-α stimulation. Taken together, we showed that Rg3 may serve as an adjunct therapy for patients with arthritis.

Low-Frequency Vibration Promotes Tumor Necrosis Factor-α Production to Increase Cartilage Degeneration in Knee Osteoarthritis

OBJECTIVE

Low-frequency vibration accelerates cartilage degeneration in knee osteoarthritis (KOA) rat model. In this article, we investigated whether whole-body vibration (WBV) increases cartilage degeneration by regulating tumor necrosis factor-α (TNF-α) in KOA.

DESIGN

Proteomics analysis was used to filter candidate protein from synovial fluid (SF) in KOA people after WBV. Enzyme-linked immunosorbent assay (ELISA) was used to estimate changes in TNF-α levels in SF. The C57 mice and TNF-α knock-out mice were sacrificed for the KOA model and WBV intervention. The cartilage was tested by ELISA, histology, terminal-deoxynucleotidyl transferase mediated nick end labeling (TUNEL), immunohistochemistry, and reverse transcriptase polymerase chain reaction. Luciferase activity test in vitro study was conducted to confirm the relationship between TNF-α and the candidate protein.

RESULTS

Differentially expressed proteins were enriched in the glycolytic process, glucose catabolic, and regulation of interleukin-8 (IL-8) secretion processes. Phosphoglycerate kinase, triosephosphate isomerase 1, T cell immunoglobulin- and mucin-domain-containing molecules 2, fumarylacetoacetate hydrolase (FAH), and TNF were the hub node. TNF-α expression increased in SF after WBV (P < 0.05). The cartilage was more degenerated in the TNF-α^-/- mice group compared to controls. A significant change was observed in collagen II and FAH (P < 0.05). TNF-α expression improved in C57 mice (P < 0.05). Apoptosis of chondrocytes was inhibited in TNF-α^-/- mice by the TUNEL test. Luciferase activity significantly increased in TNF-α + FAH-Luc cells (P < 0.05).

CONCLUSION

A novel mechanism underlying WBV-triggered cartilage degeneration was found in KOA that demonstrated the critical regulatory function of TNF-α and FAH during WBV.

The selective degradation of sirtuins via macroautophagy in the MPP+ model of Parkinson's disease is promoted by conserved oxidation sites

The sirtuin (SIRT) protein family has been of major research interest over the last decades because of their involvement in aging, cancer, and cell death. SIRTs have been implicated in gene and metabolic regulation through their capacity to remove acyl groups from lysine residues in proteins in an NAD⁺-dependent manner, which may alter individual protein properties as well as the histone–DNA interaction. Since SIRTs regulate a wide range of different signaling cascades, a fine-tuned homeostasis of these proteins is imperative to guarantee the function and survival of the cell. So far, however, how exactly this homeostasis is established has remained unknown. Here, we provide evidence that neuronal SIRT degradation in Parkinson’s disease (PD) models is executed by autophagy rather than the proteasome. In neuronal Lund human mesencephalic (LUHMES) cells, all seven SIRTs were substrates for autophagy and showed an accelerated autophagy-dependent degradation upon 1-methyl-4-phenylpyridinium (MPP⁺) mediated oxidative insults in vitro, whereas the proteasome did not contribute to the removal of oxidized SIRTs. Through blockade of endogenous H₂O₂ generation and supplementation with the selective radical scavenger phenothiazine (PHT), we could identify H₂O₂-derived species as the responsible SIRT-oxidizing agents. Analysis of all human SIRTs suggested a conserved regulatory motif based on cysteine oxidation, which may have triggered their degradation via autophagy. High amounts of H₂O₂, however, rapidly carbonylated selectively SIRT2, SIRT6, and SIRT7, which were found to accumulate carbonylation-prone amino acids. Our data may help in finding new strategies to maintain and modify SIRT bioavailability in neurodegenerative disorders.

Knee Osteoarthritis Progression Is Delayed in Silent Information Regulator 2 Ortholog 1 Knock-in Mice

Overexpression of silent information regulator 2 ortholog 1 (SIRT1) is associated with beneficial roles in aging-related diseases; however, the effects of SIRT1 overexpression on osteoarthritis (OA) progression have not yet been studied. The aim of this study was to investigate OA progression in SIRT1-KI mice using a mouse OA model. OA was induced via destabilization of the medial meniscus using 12-week-old SIRT1-KI and wild type (control) mice. OA progression was evaluated histologically based on the Osteoarthritis Research Society International (OARSI) score at 4, 8, 12, and 16 weeks after surgery. The production of SIRT1, type II collagen, MMP-13, ADAMTS-5, cleaved caspase 3, Poly (ADP-ribose) polymerase (PARP) p85, acetylated NF-κB p65, interleukin 1 beta (IL-1β), and IL-6 was examined via immunostaining. The OARSI scores were significantly lower in SIRT1-KI mice than those in control mice at 8, 12, and 16 weeks after surgery. The proportion of SIRT1 and type II collagen-positive-chondrocytes was significantly higher in SIRT1-KI mice than that in control mice. Moreover, the proportion of MMP-13-, ADAMTS-5-, cleaved caspase 3-, PARP p85-, acetylated NF-κB p65-, IL-1β-, and IL-6-positive chondrocytes was significantly lower in SIRT1-KI mice than that in control mice. The mechanically induced OA progression was delayed in SIRT1-KI mice compared to that in control mice. Therefore, overexpression of SIRT1 may represent a mechanism for delaying OA progression.

Adaptive responses to air pollution in human dermal fibroblasts and their potential roles in aging

The damaging effects of air pollution on the skin are becoming increasingly researched and the outcomes of this research are now a major influence in the selection and development of protective ingredients for skincare formulations. However, extensive research has not yet been conducted into the specific cellular defense systems that are being affected after exposure to such pollutants. Research investigating the affected systems is integral to the development of suitable interventions that are capable of augmenting the systems most impacted by air pollutant exposure. The following studies involved exposing primary human dermal fibroblasts to different concentrations of particulate matter and analyzing its effects on mitochondrial complex activity, nuclear factor erythroid 2-related factor 2 localization using immunocytochemistry and protein expression of electron transport chain complex proteins, sirtuin-1 (SIRT1), and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) using western blotting. Particulate matter-induced alterations in both mitochondrial complex protein and activity, indicating oxidative stress, which was also complimented by increased expression of antioxidant proteins GSTP1/2 and SOD2. Particulate matter also seemed to modify expression of the proteins SIRT1 and PGC-1α which are heavily involved in the regulation of mitochondrial biogenesis and energy metabolism. Given the reported results indicating that particulate matter induces damage through oxidative stress and has a profound effect on mitochondrial homeostasis, interventions involving targeted mitochondrial antioxidants may help to minimize the damaging downstream effects of pollutant-induced oxidative stress originating from the mitochondria.

Lysosomal dysfunction in osteoarthritis and aged cartilage triggers apoptosis in chondrocytes through BAX mediated release of Cytochrome c

OBJECTIVE

Lysosomes are the major catabolic organelle of the cell and regulate the macromolecular and organelle turnover and programmed cell death. Here, we investigated the lysosome dysfunction in cartilage and its role in chondrocytes apoptosis and the associated mechanism.

DESIGN

Lysosomal acidification in Osteoarthritis (OA) and aged cartilage was determined by LysoSensor staining. Lysosomal function in chondrocytes was blocked by siRNA mediated depletion of Lysosomal Associated Membrane Protein 2 (LAMP2) or with lysosome inhibitors. Chondrocyte apoptosis was determined by LDH release, Caspase-3/7 activation, TUNEL and PI uptake assays. Loss of mitochondrial membrane potential (MMP/ΔΨM) and mitochondrial superoxide level was determined by JC-1 and MitoSOX staining, respectively. Colocalization of mitochondria with BCL2 associated X (BAX) and Cytochrome c was determined by immunostaining. Destabilization of medial meniscus (DMM) was performed to induce OA in mice.

RESULTS

Lysosomal acidification was found to be significantly decreased in aged mouse and human and mouse OA cartilage which also showed increased chondrocyte apoptosis. Inhibition of lysosomal function resulted in increased oxidative stress, accumulation of dysfunctional mitochondria and apoptosis in chondrocytes in monolayer and in cartilage explant cultures. Depletion of LAMP2 expression or treatment of chondrocytes with lysosomal function inhibitors increased the expression and mitochondrial translocation of BAX leading to Cytochrome c release. Lysosomal dysfunction-induced apoptosis in chondrocytes was not blocked by antioxidants MitoTempo or Diphenyleneiodonium (DPI) but was abrogated by inhibiting BAX.

CONCLUSION

Lysosomal dysfunction induce apoptosis in chondrocytes through BAX-mediated mitochondrial damage and release of Cytochrome c. Our data points to lysosomal function restoration and/or BAX inhibition in chondrocytes as a therapeutic approach for OA.

Mitochondrial Targeting of Probes and Therapeutics to the Powerhouse of the Cell

Mitochondria, colloquially known as "the powerhouse of the cell", play important roles in production, but also in processes critical for cellular fate such as cell death, differentiation, signaling, metabolic homeostasis, and innate immunity. Due to its many functions in the cell, the mitochondria have been linked to a variety of human illnesses such as diabetes, cancer, and neurodegenerative diseases. In order to further our understanding and pharmaceutical targeting of this critical organelle, effective strategies must be employed to breach the complex barriers and microenvironment of mitochondria. Here, we summarize advancements in mitochondria-targeted probes and therapeutics.

Early-onset subclinical cardiovascular damage assessed by non-invasive methods in children with Juvenile Idiopathic Arthritis: analytical cross-sectional study

Chronic inflammation starting early in life and continuing into adulthood may predispose children with Juvenile Idiopathic Arthritis (JIA) to cardiovascular (CV) complications. To compare non-invasive CV risk markers- left ventricular mass index (LVMi), brachial artery flow mediated dilatation (FMD) and carotid artery intima-media thickness (CIMT) between patients with JIA and healthy controls. Measurements of LVMi, CIMT and FMD and lipid profile were compared between 4 and 18 year old 81 patients with JIA and 78 age and sex matched healthy controls. Among 81, 20 had systemic onset, 19 enthesitis related arthritis, 9 polyarticular rheumatoid factor (RF) + ve, 19 polyarticular RF -ve, 11 oligo-articular, and 3 un-differentiated JIA. FMD was significantly lower (p < 0.001), CIMT and LVMi significantly higher in patients (p ≤  0.001). CIMT showed positive correlation with blood pressure (p = 0.001), disease duration (p  ≤  0.001) and negative correlation with high density lipoprotein (HDL) (p ≤  0.001). FMD correlated positively with HDL (p = 0.006) and negatively with disease duration (p ≤  0.001). CIMT (p = 0.017) and FMD (p = 0.04) were significantly worse in active than inactive disease. Children with JIA have worse lipid profile, increased LVMi, CIMT, and reduced brachial artery FMD, suggestive of early cardiovascular dysfunction.

Serum NT/CT SIRT1 ratio reflects early osteoarthritis and chondrosenescence

Objective

Previous work has established that the deacetylase sirtuin-1 (SIRT1) is cleaved by cathepsin B in chondrocytes subjected to proinflammatory stress, yielding a stable but inactive N-terminal (NT) polypeptide (75SIRT1) and a C-terminal (CT) fragment. The present work examined if chondrocyte-derived NT-SIRT1 is detected in serum and may serve as an investigative and exploratory biomarker of osteoarthritis (OA).

Methods

We developed a novel ELISA assay to measure the ratio of NT to CT of SIRT1 in the serum of human individuals and mice subjected to post-traumatic OA (PTOA) or age-dependent OA (ADOA). We additionally monitored NT/CT SIRT1 in mice subject to ADOA/PTOA followed by senolytic clearance. Human chondrosenescent and non-senescent chondrocytes were exposed to cytokines and analysed for apoptosis and NT/CT SIRT1 ratio in conditioned medium.

Results

Wild-type mice with PTOA or ADOA of moderate severity exhibited increased serum NT/CT SIRT1 ratio. In contrast, this ratio remained low in cartilage-specific Sirt1 knockout mice despite similar or increased PTOA and ADOA severity. Local clearance of senescent chondrocytes from old mice with post-traumatic injury resulted in a lower NT/CT ratio and reduced OA severity. While primary chondrocytes exhibited NT/CT ratio increased in conditioned media after prolonged cytokine stimulation, this increase was not evident in cytokine-stimulated chondrosenescent cells. Finally, serum NT/CT ratio was elevated in humans with early-stage OA.

Conclusions

Increased levels of serum NT/CT SIRT1 ratio correlated with moderate OA in both mice and humans, stemming at least in part from non-senescent chondrocyte apoptosis, possibly a result of prolonged inflammatory insult.

CMH-Small Molecule Docks into SIRT1, Elicits Human IPF-Lung Fibroblast Cell Death, Inhibits Ku70-deacetylation, FLIP and Experimental Pulmonary Fibrosis

Regenerative capacity in vital organs is limited by fibrosis propensity. Idiopathic pulmonary fibrosis (IPF), a progressive lung disease linked with aging, is a classic example. In this study, we show that in flow cytometry, immunoblots (IB) and in lung sections, FLIP levels can be regulated, in vivo and in vitro, through SIRT1 activity inhibition by CMH (4-(4-Chloro-2-methylphenoxy)-N-hydroxybutanamide), a small molecule that, as we determined here by structural biology calculations, docked into its nonhistone substrate Ku70-binding site. Ku70 immunoprecipitations and immunoblots confirmed our theory that Ku70-deacetylation, Ku70/FLIP complex, myofibroblast resistance to apoptosis, cell survival, and lung fibrosis in bleomycin-treated mice, are reduced and regulated by CMH. Thus, small molecules associated with SIRT1-mediated regulation of Ku70 deacetylation, affecting FLIP stabilization in fibrotic-lung myofibroblasts, may be a useful strategy, enabling tissue regeneration.

SIRT1 directly activates autophagy in human chondrocytes

Osteoarthritis (OA) is the most common form of arthritis worldwide with no effective treatment. Ageing is the primary risk factor for OA. We sought to investigate if there is a distinct and functional convergence of ageing-related mechanisms SIRT1 and autophagy in chondrocytes. Our results show that, levels of SIRT1 are decreased in human normal aged and OA cartilage compared with young cartilage. Moreover, silencing SIRT1 in chondrocytes lead to decreased expression of chondrogenic markers but did not alter the expression of catabolic proteases. In contrast, activation of SIRT1 increased autophagy in chondrocytes by the deacetylation of lysine residues on crucial autophagy proteins (Beclin1, ATG5, ATG7, LC3). This activation was shown to be mTOR/ULK1 independent. Our results indicate that maintenance of autophagy in chondrocytes by SIRT1 is essential for preserving cartilage integrity throughout life and therefore is a target for drug intervention to protect against OA.

Relevance of SIRT1-NF-κB Axis as Therapeutic Target to Ameliorate Inflammation in Liver Disease

Inflammation is an adaptive response in pursuit of homeostasis reestablishment triggered by harmful conditions or stimuli, such as an infection or tissue damage. Liver diseases cause approximately 2 million deaths per year worldwide and hepatic inflammation is a common factor to all of them, being the main driver of hepatic tissue damage and causing progression from non-alcoholic fatty liver disease (NAFLD) to non-alcoholic steatohepatitis (NASH), cirrhosis and, ultimately, hepatocellular carcinoma (HCC). The metabolic sensor SIRT1, a class III histone deacetylase with strong expression in metabolic tissues such as the liver, and transcription factor NF-κB, a master regulator of inflammatory response, show an antagonistic relationship in controlling inflammation. For this reason, SIRT1 targeting is emerging as a potential strategy to improve different metabolic and/or inflammatory pathologies. In this review, we explore diverse upstream regulators and some natural/synthetic activators of SIRT1 as possible therapeutic treatment for liver diseases.

Cartilage Ablation of Sirt1 Causes Inhibition of Growth Plate Chondrogenesis by Hyperactivation of mTORC1 Signaling

A growing body of evidence implies a pivotal role of sirtuin-1 (Sirt1) in chondrocyte function and homeostasis; however, its underlying mechanisms mediating chondrogenesis, which is an essential process for physiological skeletal growth, are still poorly understood. In the current study, we generated TamCartSirt1-/- [Sirt1 conditional knockout (cKO)] mice to explore the role of Sirt1 during postnatal endochondral ossification. Compared with control mice, cKO mice exhibited growth retardation associated with inhibited chondrocyte proliferation and hypertrophy, as well as activated apoptosis. These effects were regulated by hyperactivation of mammalian target of rapamycin complex 1 (mTORC1) signaling, and thereby inhibition of autophagy and induction of endoplasmic reticulum stress in growth plate chondrocytes. IP injection of the mTORC1 inhibitor rapamycin to mice with Sirt1 deletion partially neutralized such inhibitory effects of Sirt1 ablation on longitudinal bone growth, indicating the causative link between SIRT1 and mTORC1 signaling in the growth plate. Mechanistically, SIRT1 interacted with tuberous sclerosis complex 2 (TSC2), a key upstream negative regulator of mTORC1 signaling, and loss of Sirt1 inhibited TSC2 expression, resulting in hyperactivated mTORC1 signaling in chondrocytes. In conclusion, our findings suggest that loss of Sirt1 may trigger mTORC1 signaling in growth plate chondrocytes and contributes to growth retardation, thus indicating that SIRT1 is an important regulator during chondrogenesis and providing new insights into the clinical potential of SIRT1 in bone development.

Epigenetic and non-epigenetic mode of SIRT1 action during oocyte meiosis progression

Background

SIRT1 histone deacetylase acts on many epigenetic and non-epigenetic targets. It is thought that SIRT1 is involved in oocyte maturation; therefore, the importance of the ooplasmic SIRT1 pool for the further fate of mature oocytes has been strongly suggested. We hypothesised that SIRT1 plays the role of a signalling molecule in mature oocytes through selected epigenetic and non-epigenetic regulation.

Results

We observed SIRT1 re-localisation in mature oocytes and its association with spindle microtubules. In mature oocytes, SIRT1 distribution shows a spindle-like pattern, and spindle-specific SIRT1 action decreases α-tubulin acetylation. Based on the observation of the histone code in immature and mature oocytes, we suggest that SIRT1 is mostly predestined for an epigenetic mode of action in the germinal vesicles (GVs) of immature oocytes. Accordingly, BML-278-driven trimethylation of lysine K9 in histone H3 in mature oocytes is considered to be a result of GV epigenetic transformation.

Conclusions

Taken together, our observations point out the dual spatiotemporal SIRT1 action in oocytes, which can be readily switched from the epigenetic to non-epigenetic mode of action depending on the progress of meiosis.

Electronic supplementary material

The online version of this article (10.1186/s40104-019-0372-3) contains supplementary material, which is available to authorized users.

Sirtuin 1 Regulates Mitochondrial Biogenesis and Provides an Endogenous Neuroprotective Mechanism Against Seizure-Induced Neuronal Cell Death in the Hippocampus Following Status Epilepticus

Status epilepticus may decrease mitochondrial biogenesis, resulting in neuronal cell death occurring in the hippocampus. Sirtuin 1 (SIRT1) functionally interacts with peroxisome proliferator-activated receptors and γ coactivator 1α (PGC-1α), which play a crucial role in the regulation of mitochondrial biogenesis. In Sprague-Dawley rats, kainic acid was microinjected unilaterally into the hippocampal CA3 subfield to induce bilateral seizure activity. SIRT1, PGC-1α, and other key proteins involving mitochondrial biogenesis and the amount of mitochondrial DNA were investigated. SIRT1 antisense oligodeoxynucleotide was used to evaluate the relationship between SIRT1 and mitochondrial biogenesis, as well as the mitochondrial function, oxidative stress, and neuronal cell survival. Increased SIRT1, PGC-1α, and mitochondrial biogenesis machinery were found in the hippocampus following experimental status epilepticus. Downregulation of SIRT1 decreased PGC-1α expression and mitochondrial biogenesis machinery, increased Complex I dysfunction, augmented the level of oxidized proteins, raised activated caspase-3 expression, and promoted neuronal cell damage in the hippocampus. The results suggest that the SIRT1 signaling pathway may play a pivotal role in mitochondrial biogenesis, and could be considered an endogenous neuroprotective mechanism counteracting seizure-induced neuronal cell damage following status epilepticus.

Cytoplasmic SIRT1 inhibits cell migration and invasion by impeding epithelial-mesenchymal transition in ovarian carcinoma

Sirtuin1 (SIRT1) is a mammalian NAD⁺-dependent type III deacetylase that plays paramount roles in diverse cellular processes. The nucleocytoplasmic shuttling of SIRT1 was discovered more than a decade ago, but the roles of subcellular SIRT1 localization in tumor progression remain unclear. Here, we report that cytoplasmic SIRT1 acts as a tumor suppressor in ovarian carcinoma. By creating ovarian carcinoma cell lines overexpressing wild-type SIRT1 and nuclear localization signals (NLSs) mutated SIRT1 together with both unbiased proteomic and acetylomic approaches and Transwell assays, we identified that mutations in the NLS sequences prevented SIRT1 from entering the nucleus, resulting in the predominant cytoplasmic localization of SIRT1; the cytoplasmic localization of SIRT1 suppressed the mesenchymal program, activated the epithelial program, and inhibited the migration and invasion of tumor cells, thus providing experimental evidence that SIRT1 functions as a tumor suppressor or oncogene may depend on its subcellular localization. Altogether, our findings may highlight a novel role of cytoplasmic SIRT1 in ovarian carcinoma, providing new possible insights for studies investigating the role of SIRT1 in tumor progression.

Role of NGF-TrkA signaling in calcification of articular chondrocytes

Nerve growth factor (NGF) is a key regulator of chronic osteoarthritic pain, but the exact targets of NGF action on human articular cartilage is unknown. This study aimed to test the hypothesis that the NGF-tropomyosin receptor kinase A (TrkA) (high-affinity NGF receptor) pathway plays a role in the calcification process of human articular chondrocytes (hACs). A 14-aa small peptide of NGF (Nsp) previously shown to activate NGF signaling in rat PC12 cells was used as an NGF signaling agonist, and recombinant NGF and the pan-Trk inhibitor GNF-5837 were employed as signaling modulating agents. The functional consequences of NGF-TrkA signaling were examined in human healthy articular chondrocytes maintained under conditions supportive of osteogenesis in vitro. The NGF-mimetic bioactivity of Nsp was first confirmed on the basis of maintenance of neurite outgrowth in PC12 cells. Primary human chondrocytes responded to Nsp in vitro. Perturbation of NGF signaling with NGF, Nsp, and GNF-5837 resulted in a strong induction of chondrocyte calcification, and gene expression data suggested that the Indian Hedgehog-parathyroid hormone-related protein signaling axis was involved. These findings suggest functional involvement of NGF signaling in calcification of hACs and the importance of NGF signaling in articular cartilage homeostasis.-Jiang, Y., Tuan, R. S. Role of NGF-TrkA signaling in calcification of articular chondrocytes.

The role of sirtuin 1 and its activator, resveratrol in osteoarthritis

Osteoarthitis (OA) is the most common aging-related joint pathology; the aging process results in changes to joint tissues that ultimately contribute to the development of OA. Articular chondrocytes exhibit an aging-related decline in their proliferative and synthetic capacity. Sirtuin 1 (SIRT 1), a longevity gene related to many diseases associated with aging, is a nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacetylase and master metabolic regulator. Along with its natural activator resveratrol, SIRT 1 actively participates in the OA pathological progress. SIRT 1 expression in osteoarthritic cartilage decreases in the disease progression of OA; it appears to play a predominantly regulatory role in OA. SIRT 1 can regulate the expression of extracellular matrix (ECM)-related proteins; promote mesenchymal stem cell differentiation; play anti-catabolic, anti-inflammatory, anti-oxidative stress, and anti-apoptosis roles; participate in the autophagic process; and regulate bone homeostasis in OA. Resveratrol can activate SIRT 1 in order to inhibit OA disease progression. In the future, activating SIRT 1 via resveratrol with improved bioavailability may be an appropriate therapeutic approach for OA.

Epigenetic dynamic during endochondral ossification and articular cartilage development

Within the last decade epigenetics has emerged as fundamental regulator of numerous cellular processes, including those orchestrating embryonic and fetal development. As such, epigenetic factors play especially crucial roles in endochondral ossification, the process by which bone tissue is created, as well during articular cartilage formation. In this review, we summarize the recent discoveries that characterize how DNA methylation, histone post-translational modifications and non-coding RNA (e.g., miRNA and lcnRNA) epigenetically regulate endochondral ossification and chondrogenesis.

Protective effect of astaxanthin against contrast-induced acute kidney injury via SIRT1-p53 pathway in rats

PURPOSE

The present study was designed to further investigate the protective effect of astaxanthin (AST) on contrast-induced acute kidney injury (CI-AKI) in rats and the relationship between SIRT1-p53 pathway and astaxanthin.

METHODS

40 adult male Sprague Dawley (SD) rats were randomly divided into five groups (n = 8/group): control (CON), normal rats treated with AST (AST), CM-treated (CM), CM rats treated with isoform of nitric oxide synthase (iNOS) inhibitor (iNOS + CM), and CM rats treated with AST (AST + CM). Serum creatinine (Scr) and blood urea nitrogen (BUN) values were measured at 72 h following the procedure. Hematoxylin and eosin (H-E) staining was used to observe the pathologic changes of kidney. Tunel staining was used to test apoptosis of kidney tubules. Oxidative stress, SIRT1 activity, nitric oxide (NO), and 3-nitrotyrosine (3-NT) content were individually measured with the commercial available kits.

RESULTS

Compared with the CON group, Scr and BUN levels significantly increased in the CM group (P < 0.05), and the values in two pre-treatment groups (iNOS + CM and AST + CM) had significantly decreased (P < 0.05). H-E and Tunel staining had shown that renal tubular injury was severe in CM group. The renal injury score and apoptosis index in the two pre-treatment groups also decreased (P < 0.05). The present study showed that in CM group the levels of oxidative stress indicators significantly increased, and the activities of antioxidant stress indicators significantly decreased. These indicators in two pre-treatment groups significantly improved (P < 0.05). In the CM group the expression levels of SITR1 significantly increased, and the ac-p53/p53 significantly increased (P < 0.05). Compared with the CM group, in AST + CM group the expression levels of SIRT1 increased, the expression levels of p53 and ac-p53/p53 decreased (P < 0.05).The levels of NO and 3-NT in CM group significantly increased (P < 0.05). Compared the CM group, the levels in the two pre-treatment groups significantly decreased (P < 0.05).

CONCLUSIONS

Astaxanthin has a protective effect on CI-AKI, the mechanism may be related to the SIRT1-p53 pathway. Astaxanthin can reduce the content of NO and 3-NT in renal tissue of CI-AKI, and alleviate the renal injury induced by contrast agents.

A predicted unstructured C-terminal loop domain in SIRT1 is required for cathepsin B cleavage

The C-terminus of SIRT1 can be cleaved by cathepsin B at amino acid H533 to generate a lower-functioning, N-terminally intact 75 kDa polypeptide (75SIRT1) that might be involved in age-related pathologies. However, the mechanisms underlying cathepsin B docking to and cleavage of SIRT1 are unclear. Here, we first identified several 75SIRT1 variants that are augmented with aging correlatively with increased cathepsin B levels in various mouse tissues, highlighting the possible role of this cleavage event in age-related pathologies. Then, based on H533 point mutation and structural modeling, we generated a functionally intact ΔSIRT1 mutant, lacking the internal amino acids 528-543 (a predicted C-terminus loop domain), which exhibits resistance to cathepsin B cleavage in vitro and in cell cultures. Finally, we showed that cells expressing ΔSIRT1 under pro-inflammatory stress are more likely to undergo caspase 9- dependent apoptosis than those expressing 75SIRT1. Thus, our data suggest that the 15-amino acid predicted loop motif embedded in the C-terminus of SIRT1 is susceptible to proteolytic cleavage by cathepsin B, leading to the formation of several N-terminally intact SIRT1 truncated variants in various aging mouse tissues.This article has an associated First Person interview with the first author of the paper.

Paracrine effects of human adipose-derived mesenchymal stem cells in inflammatory stress-induced senescence features of osteoarthritic chondrocytes

Aging and exposure to stress would determine the chondrocyte phenotype in osteoarthritis (OA). In particular, chronic inflammation may contribute to stress-induced senescence of chondrocytes and cartilage degeneration during OA progression. Recent studies have shown that adipose-derived mesenchymal stem cells exert paracrine effects protecting against degenerative changes in chondrocytes. We have investigated whether the conditioned medium (CM) from adipose-derived mesenchymal stem cells may regulate senescence features induced by inflammatory stress in OA chondrocytes. Our results indicate that CM down-regulated senescence markers induced by interleukin-1β including senescence-associated β-galactosidase activity, accumulation of γH2AX foci and morphological changes with enhanced formation of actin stress fibers. Treatment of chondrocytes with CM also decreased the production of oxidative stress, the activation of mitogen-activated protein kinases, and the expression of caveolin-1 and p21. The effects of CM were related to the reduction in p53 acetylation which would be dependent on the enhancement of Sirtuin 1 expression. Therefore, CM may exert protective effects in degenerative joint conditions by countering the premature senescence of OA chondrocytes induced by inflammatory stress.

Decreased Serum Sirtuin-1 in COPD

Background

The protein deacetylase sirtuin-1 (SIRT1) is an antiaging molecule that is decreased in the lung in patients with COPD. Recently, SIRT1 was reported to be detectable in serum, but serum SIRT1 (s120S) levels have not yet been reported in patients with COPD.

Methods

Serum SIRT1 protein of all samples was measured by Western blot, and the SIRT1 protein band densities were calculated and compared with clinical parameters.

Results

Several molecular sizes of SIRT1, including 120 kDa (actual size) and fragments (102 and 75 kDa) were quantified by Western blot. Among them, only the 120-kDa s120S was significantly decreased in patients with COPD compared with the control subjects without COPD (s120S ratio in healthy subjects = 0.90 ± 0.34 vs those with COPD = 0.68 ± 0.24; P = .014) and was positively correlated with airway obstruction (FEV₁/FVC, r = 0.31; P = .020); its severity measured by FEV₁ % predicted (r = 0.29; P = .029). s120S also showed a positive correlation with BMI (r = 0.36; P = .0077) and diffusing capacity of the lung per unit volume (the carbon monoxide transfer coefficient: K_CO%) (r = 0.32; P = .025). It was also significantly decreased with increasing severity of lung emphysema (r = –0.40; P = .027) and with a clinical history of frequent COPD exacerbations (infrequent vs frequent, 0.76 ± 0.20 vs 0.56 ± 0.26; P = .027). SIRT1 was not detected in supernatant of A549 and primary epithelial cells in normal culture conditions.

Conclusions

s120S was decreased in the patients with COPD, potentially as reflected by the reduced SIRT1 within cells as a result of oxidative stress, and might be a potential biomarker for certain disease characteristics of COPD.

Mitochondria as a Signaling Hub and Target for Phenoptosis Shutdown

Mitochondria have long been studied as the main energy source and one of the most important generators of reactive oxygen species in the eukaryotic cell. Yet, new data suggest mitochondria serve as a powerful cellular regulator, pathway trigger, and signal hub. Some of these crucial mitochondrial functions appear to be associated with RNP-granules. Deep and versatile involvement of mitochondria in general cellular regulation may be the legacy of parasitic behavior of the ancestors of mitochondria in the host cells. In this regard, we also discuss here the perspectives of using mitochondria-targeted compounds for systemic correction of phenoptotic shifts.

Inflammation and epigenetic regulation in osteoarthritis

Osteoarthritis (OA) was once defined as a non-inflammatory arthropathy, but it is now well-recognized that there is a major inflammatory component to this disease. In addition to synovial cells, articular chondrocytes and other cells of diarthrodial joints are also known to express inflammatory mediators. It has been proposed that targeting inflammation pathways could be a promising strategy to treat OA. There have been many reports of cross-talk between inflammation and epigenetic factors in cartilage. Specifically, inflammatory mediators have been shown to regulate levels of enzymes that catalyze changes in DNA methylation and histone structure, as well as alter levels of non-coding RNAs. In addition, expression levels of a number of these epigenetic factors have been shown to be altered in OA, thereby suggesting potential interplay between inflammation and epigenetics in this disease. This review provides information on inflammatory pathways in arthritis and summarizes published research on how epigenetic regulators are affected by inflammation in chondrocytes. Furthermore, we discuss data showing how altered expression of some of these epigenetic factors can induce either catabolic or anti-catabolic effects in response to inflammatory signals. A better understanding of how inflammation affects epigenetic factors in OA may provide us with novel therapeutic strategies to treat this condition.

The expression of SIRT1 in articular cartilage of patients with knee osteoarthritis and its correlation with disease severity

Background

The study aims to investigate the expression of SIRT1 in articular cartilage of patients with primary knee osteoarthritis (OA) and its relationship with disease severity.

Methods

Cartilage tissue samples were collected from 38 knee OA patients and 9 normal healthy controls and then ascribed to normal, mild, moderate, and severe groups on the basis of the improved Mankin grading system. The expression of SIRT1 in articular cartilage was detected by immunohistochemistry and western blots. The expression of p53 and acetylated p53 (Ac-p53) was also measured by western blots.

Results

The mutual comparisons of the SIRT1 expression levels in all groups have statistical significance except the one between the mild and moderate groups. Moreover, western blot results showed that the SIRT1 was decreased and p53/Ac-p53 were increased in the OA group. The average gray level of SIRT1 increases with the improving grade of the improved Mankin grading system scorers.

Conclusions

The expression of SIRT1 in articular cartilage is negatively associated with severity of knee OA, indicating that SIRT1 may act as a monitoring indicator for determining development and progression of knee OA.

Cysteine cathepsins control hepatic NF-κB-dependent inflammation via sirtuin-1 regulation

Sirtuin-1 (SIRT1) regulates hepatic metabolism but its contribution to NF-κB-dependent inflammation has been overlooked. Cysteine cathepsins (Cathepsin B or S, CTSB/S) execute specific functions in physiological processes, such as protein degradation, having SIRT1 as a substrate. We investigated the roles of CTSB/S and SIRT1 in the regulation of hepatic inflammation using primary parenchymal and non-parenchymal hepatic cell types and cell lines. In all cells analyzed, CTSB/S inhibition reduces nuclear p65-NF-κB and κB-dependent gene expression after LPS or TNF through enhanced SIRT1 expression. Accordingly, SIRT1 silencing was sufficient to enhance inflammatory gene expression. Importantly, in a dietary mouse model of non-alcoholic steatohepatitis, or in healthy and fibrotic mice after LPS challenge, cathepsins as well as NF-κB-dependent gene expression are activated. Consistent with the prominent role of cathepsin/SIRT1, cysteine cathepsin inhibition limits NF-κB-dependent hepatic inflammation through the regulation of SIRT1 in all in vivo settings, providing a novel anti-inflammatory therapeutic target in liver disease.

Acetylation reduces SOX9 nuclear entry and ACAN gene transactivation in human chondrocytes

Changes in the content of aggrecan, an essential proteoglycan of articular cartilage, have been implicated in the pathophysiology of osteoarthritis (OA), a prevalent age-related, degenerative joint disease. Here, we examined the effect of SOX9 acetylation on ACAN transactivation in the context of osteoarthritis. Primary chondrocytes freshly isolated from degenerated OA cartilage displayed lower levels of ACAN mRNA and higher levels of acetylated SOX9 compared with cells from intact regions of OA cartilage. Degenerated OA cartilage presented chondrocyte clusters bearing diffused immunostaining for SOX9 compared with intact cartilage regions. Primary human chondrocytes freshly isolated from OA knee joints were cultured in monolayer or in three-dimensional alginate microbeads (3D). SOX9 was hypo-acetylated in 3D cultures and displayed enhanced binding to a -10 kb ACAN enhancer, a result consistent with higher ACAN mRNA levels than in monolayer cultures. It also co-immunoprecipitated with SIRT1, a major deacetylase responsible for SOX9 deacetylation. Finally, immunofluorescence assays revealed increased nuclear localization of SOX9 in primary chondrocytes treated with the NAD SIRT1 cofactor, than in cells treated with a SIRT1 inhibitor. Inhibition of importin β by importazole maintained SOX9 in the cytoplasm, even in the presence of NAD. Based on these data, we conclude that deacetylation promotes SOX9 nuclear translocation and hence its ability to activate ACAN.

Reciprocal regulation by hypoxia-inducible factor-2α and the NAMPT-NAD(+)-SIRT axis in articular chondrocytes is involved in osteoarthritis

OBJECTIVE

Hypoxia-inducible factor-2α (HIF-2α) transcriptionally upregulates Nampt in articular chondrocytes. NAMPT, which exhibits nicotinamide phosphoribosyltransferase activity, in turn causes osteoarthritis (OA) in mice by stimulating the expression of matrix-degrading enzymes. Here, we sought to elucidate whether HIF-2α activates the NAMPT-NAD(+)-SIRT axis in chondrocytes and thereby contributes to the pathogenesis of OA.

METHODS

Assays of NAD levels, SIRT activity, reporter gene activity, mRNA, and protein levels were conducted in primary cultured mouse articular chondrocytes. Experimental OA in mice was induced by intra-articular (IA) injection of adenovirus expressing HIF-2α (Ad-Epas1) or NAMPT (Ad-Nampt). The functions of SIRT in OA were examined by IA co-injection of SIRT inhibitors or adenovirus expressing individual SIRT isoforms or shRNA targeting specific SIRT isoforms.

RESULTS

HIF-2α activated the NAMPT-NAD(+)-SIRT axis in chondrocytes by upregulating NAMPT, which stimulated NAD(+) synthesis and thereby activated SIRT family members. The activated NAMPT-SIRT pathway, in turn, promoted HIF-2α protein stability by negatively regulating its hydroxylation and 26S proteasome-mediated degradation, resulting in increased HIF-2α transcriptional activity. Among SIRT family members (SIRT1-7), SIRT2 and SIRT4 were positively associated with HIF-2α stability and transcriptional activity in chondrocytes. This reciprocal regulation was required for the expression of catabolic matrix metalloproteinases (MMP3, MMP12, and MMP13) and OA cartilage destruction caused by IA injection of Ad-Epas1 Ad-Nampt.

CONCLUSION

The reciprocal regulation of HIF-2α and the NAMPT-NAD(+)-SIRT axis in articular chondrocytes is involved in OA cartilage destruction caused by HIF-2α or NAMPT.

HDAC4 mediates IFN-γ induced disruption of energy expenditure-related gene expression by repressing SIRT1 transcription in skeletal muscle cells

Metabolic homeostasis is achieved through balanced energy storage and output. Impairment of energy expenditure is a hallmark event in patients with obesity and type 2 diabetes. Previously we have shown that the pro-inflammatory cytokine interferon gamma (IFN-γ) disrupts energy expenditure in skeletal muscle cells via hypermethylated in cancer 1 (HIC1)-class II transactivator (CIITA) dependent repression of SIRT1 transcription. Here we report that repression of SIRT1 transcription by IFN-γ paralleled loss of histone acetylation on the SIRT1 promoter region with simultaneous recruitment of histone deacetylase 4 (HDAC4). IFN-γ activated HDAC4 in vitro and in vivo by up-regulating its expression and stimulating its nuclear accumulation. HIC1 and CIITA recruited HDAC4 to the SIRT1 promoter and cooperated with HDAC4 to repress SIRT1 transcription. HDAC4 depletion by small interfering RNA or pharmaceutical inhibition normalized histone acetylation on the SIRT1 promoter and restored SIRT1 expression in the presence of IFN-γ. Over-expression of HDAC4 suppressed the transcription of genes involved in energy expenditure in a SIRT1-dependent manner. In contrast, HDAC4 knockdown/inhibition neutralized the effect of IFN-γ on cellular metabolism by normalizing SIRT1 expression. Therefore, our data reveal a role for HDAC4 in regulating cellular energy output and as such provide insights into rationalized design of novel anti-diabetic therapeutics.

Mitochondrial biogenesis is impaired in osteoarthritis chondrocytes but reversible via peroxisome proliferator-activated receptor γ coactivator 1α

OBJECTIVE

The etiology of chondrocyte mitochondrial dysfunction in osteoarthritis (OA) is not completely understood. OA chondrocytes are deficient in the metabolic biosensors active AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT-1), which modulate the mitochondrial biogenesis "master regulator" peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α). Moreover, PGC-1α critically mediates AMPK anticatabolic activity in chondrocytes. The aim of this study was to test the hypothesis that mitochondrial biogenesis is deficient in human OA chondrocytes and that this deficiency functionally increases chondrocyte procatabolic responses, which are reversed by activation of the AMPK/SIRT-1/PGC-1α pathway.

METHODS

We assessed the expression and activity (phosphorylation) of AMPKα, SIRT-1, and PGC-1α in human knee chondrocytes and human and mouse knee cartilage, and we defined and compared the content and function of mitochondria, including oxidative phosphorylation and expression of mitochondrial biogenesis factors (mitochondrial transcriptional factor A [TFAM] and nuclear respiratory factors [NRFs]).

RESULTS

Human knee OA chondrocytes had a decreased mitochondrial biogenesis capacity, which was linked to reduced AMPKα activity and decreased expression of SIRT-1, PGC-1α, TFAM, NRF-1, and NRF-2. Human knee OA and aging mouse knee cartilage had decreased expression of TFAM and ubiquinol-cytochrome c reductase core protein, a subunit of mitochondrial complex III, in situ. Chondrocyte TFAM knockdown inhibited mitochondrial biogenesis and enhanced procatabolic responses to interleukin-1β. Finally, activation of AMPK by A-769662 increased PGC-1α expression via SIRT-1 and reversed impairments in mitochondrial biogenesis, oxidative phosphorylation, and intracellular ATP in human knee OA chondrocytes.

CONCLUSION

Mitochondrial biogenesis is deficient in human OA chondrocytes, and this deficiency promotes chondrocyte procatabolic responses. TFAM-mediated activation of the AMPK/SIRT-1/PGC-1α pathway reverses these effects, suggesting translational potential of pharmacologic AMPK activators to limit OA progression.

Epigenetic Regulation of Chondrocyte Catabolism and Anabolism in Osteoarthritis

Osteoarthritis (OA) is one of the most prevalent forms of joint disorder, associated with a tremendous socioeconomic burden worldwide. Various non-genetic and lifestyle-related factors such as aging and obesity have been recognized as major risk factors for OA, underscoring the potential role for epigenetic regulation in the pathogenesis of the disease. OA-associated epigenetic aberrations have been noted at the level of DNA methylation and histone modification in chondrocytes. These epigenetic regulations are implicated in driving an imbalance between the expression of catabolic and anabolic factors, leading eventually to osteoarthritic cartilage destruction. Cellular senescence and metabolic abnormalities driven by OA-associated risk factors appear to accompany epigenetic drifts in chondrocytes. Notably, molecular events associated with metabolic disorders influence epigenetic regulation in chondrocytes, supporting the notion that OA is a metabolic disease. Here, we review accumulating evidence supporting a role for epigenetics in the regulation of cartilage homeostasis and OA pathogenesis.

Intra-articular resveratrol injection prevents osteoarthritis progression in a mouse model by activating SIRT1 and thereby silencing HIF-2α

We investigated the feasibility of the intra-articular injection of resveratrol for preventing the progression of existing cartilage degeneration in a mouse model of osteoarthritis (OA). The effects of resveratrol on the expression of silent information regulator 2 type 1 (SIRT1), hypoxia-inducible factor-2α (HIF-2α) and catabolic factors in OA cartilage was explored. OA was induced in the mouse knee via destabilization of the medial meniscus (DMM). Resveratrol was injected weekly into the operated knee beginning 4 weeks after surgery. The OA phenotype was evaluated via histological and immunohistochemical analyses at 8 weeks after DMM. Western blot analysis was performed to identify whether resveratrol modulated the interleukin (IL)-1β-induced expression of HIF-2α in human chondrocytes. Histologically, resveratrol treatment preserved the structural homeostasis of the articular cartilage and the subchondral bone. Following resveratrol injection, the expression of collagen type II was retained, but the expression of inducible nitric oxide synthase and matrix metalloproteinase-13 was reduced in OA cartilage. Moreover, the administration of resveratrol significantly induced the activation of SIRT1 and the inhibition of HIF-2α expression in mouse OA cartilage and in IL-1β-treated human chondrocytes. These findings indicate that the intra-articular injection of resveratrol significantly prevents the destruction of OA cartilage by activating SIRT1 and thereby suppressing the expression of HIF-2α and catabolic factors.

Detecting cathepsin activity in human osteoarthritis via activity-based probes

Introduction

Lysosomal cathepsins have been reported to contribute to Osteoarthritis (OA) pathophysiology due to their increase in pro-inflammatory conditions. Given the causal role of cathepsins in OA, monitoring their specific activity could provide means for assessing OA severity. To this end, we herein sought to assess a cathepsin activity-based probe (ABP), GB123, in vitro and in vivo.

Methods

Protein levels and activity of cathepsins B and S were monitored by immunoblot analysis and GB123 labeling in cultured primary chondrocytes and conditioned media, following stimuli with tumor necrosis factor alpha (TNFα) and/or Interleukin 1 beta (IL-1β). Similarly, cathepsin activity was examined in sections of intact cartilage (IC) and degraded cartilage (DC) regions of OA. Finally, synovial fluid (SF) and serum from donors with no signs of diseases, early OA, late OA and rheumatoid arthritis (RA) patients were analyzed with GB123 to detect distinct activity levels of cathepsin B and S.

Results

Cathepsin activity in cell lysates, conditioned media explants and DC sections showed enhanced enzymatic activity of cathepsins B and S. Further histological analysis revealed that cathepsin activity was found higher in superficial zones of DC than in IC. Examining serum and SF revealed that cathepsin B is significantly elevated with OA severity in serum and SF, yet levels of cathepsin S are more correlated with synovitis and RA.

Conclusions

Based on our data, cathepsin activity monitored by ABPs correlated well with OA severity and joint inflammation, directing towards a novel etiological target for OA, which possesses significant translational potential in developing means for non-invasive detection of early signs of OA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0586-5) contains supplementary material, which is available to authorized users.

Chopping off the chondrocyte proteome

The progressive nature of osteoarthritis is manifested by the dynamic increase of degenerated articular cartilage, which is one of the major characteristics of this debilitating disease. As articular chondrocytes become exposed to inflammatory stress they enter a pro-catabolic state, which leads to the secretion and activation of a plethora of proteases. In aim to detect the disease before massive areas of cartilage are destroyed, various protein and non-protein biomarkers have been examined in bodily fluids and correlated with disease severity. This review will discuss the widely research extracellular degraded products as well as products generated by affected cellular pathways upon increased protease activity. While extracellular components could be more abundant, cleaved cellular proteins are less abundant and are suggested to possess a significant effect on cell metabolism and cartilage secretome. Subtle changes in cell secretome could potentially act as indicators of the chondrocyte metabolic and biological state. Therefore, it is envisioned that combined biomarkers composed of both cell and extracellular-degraded secretome could provide a valuable platform for testing drug efficacy to halt disease progression at its early stages.

Effects of acute doxorubicin treatment on hepatic proteome lysine acetylation status and the apoptotic environment

AIM: To determine if doxorubicin (Dox) alters hepatic proteome acetylation status and if acetylation status was associated with an apoptotic environment.

METHODS: Doxorubicin (20 mg/kg; Sigma, Saint Louis, MO; n = 8) or NaCl (0.9%; n = 7) was administered as an intraperitoneal injection to male F344 rats, 6-wk of age. Once animals were treated with Dox or saline, all animals were fasted until sacrifice 24 h later.

RESULTS: Dox treatment decreased proteome lysine acetylation likely due to a decrease in histone acetyltransferase activity. Proteome deacetylation may likely not be associated with a proapoptotic environment. Dox did not increase caspase-9, -8, or -3 activation nor poly (adenosine diphosphate-ribose) polymerase-1 cleavage. Dox did stimulate caspase-12 activation, however, it likely did not play a role in apoptosis induction.

CONCLUSION: Early effects of Dox involve hepatic proteome lysine deacetylation and caspase-12 activation under these experimental conditions.

Set7/9 impacts COL2A1 expression through binding and repression of SirT1 histone deacetylation

Type II collagen is a key cartilaginous extracellular protein required for normal endochondral development and cartilage homeostasis. COL2A1 gene expression is positively regulated by the NAD-dependent protein deacetylase Sirtuin 1 (SirT1), through its ability to bind chromatin regions of the COL2A1 promoter and enhancer. Although SirT1/Sox9 binding on the enhancer site of COL2A1 was previously demonstrated, little is known about its functional role on the gene promoter site. Here, we examined the mechanism by which promoter-associated SirT1 governs COL2A1 expression. Human chondrocytes were encapsulated in three-dimensional (3D) alginate beads where they exhibited upregulated COL2A1 mRNA expression and increased levels of SirT1 occupancy on the promoter and enhancer regions, when compared to monolayer controls. Chromatin immunoprecipitation (ChIP) analyses of 3D cultures showed augmented levels of the DNA-binding transcription factor SP1, and the histone methyltransferase Set7/9, on the COL2A1 promoter site. ChIP reChIP assays revealed that SirT1 and Set7/9 form a protein complex on the COL2A1 promoter region of 3D-cultured chondrocytes, which also demonstrated elevated trimethylated lysine 4 on histone 3 (3MeH3K4), a hallmark of Set7/9 methyltransferase activity. Advanced passaging of chondrocytes yielded a decrease in 3MeH3K4 and Set7/9 levels on the COL2A1 promoter and reduced COL2A1 expression, suggesting that the SirT1/Set7/9 complex is preferentially formed on the COL2A1 promoter and required for gene activation. Interestingly, despite SirT1 occupancy, its deacetylation targets (ie, H3K9/14 and H4K16) were found acetylated on the COL2A1 promoter of 3D-cultured chondrocytes. A possible explanation for this phenotype is the enrichment of the histone acetyltransferases P300 and GCN5 on the COL2A1 promoter of3 D-cultured chondrocytes. Our study indicates that Set7/9 prevents the histone deacetylase activity of SirT1, potentiating euchromatin formation on the promoter site of COL2A1 and resulting in morphology-dependent COL2A1 gene transactivation.