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.
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Handling editor Josef S Smolen
GB and JE contributed equally.
Contributors ER, LK, AH, JS, VL, JHE, YEH, AM and MD-G designed research. GB, OQ, ER, LB-A, JE, HZ, LK, and AH performed research. LK, AH, JS, VL, JHE, HZ, YEH, AM and MD-G contributed new reagents/analytic tools and clinical samples. All authors analysed the data and performed statistical analysis. GB, JE and MD-G wrote the paper. All authors approved the final manuscript.
Funding This work was funded by European Commission Framework 7 programme (EU FP7; HEALTH.2012.2.4.5-2, project number 305815, Novel Diagnostics and Biomarkers for Early Identification of Chronic Inflammatory Joint Diseases); Israel Science Foundation (grant no. 121/12, 370/17), US-Israeli Binational foundation Grant 2013145 (to VL and MDG) and Rosetrees Trust (grant no. A770).
Competing interests JHE disclosed that she is an equity holder and receives patent royalties from UNITY Biotechnology; serves as a scientific advisory board member at Camden Nexus Fund, Acell and Histogenics. JHE is a consultant at Vericel and a founder and equity holder of Aegeria Soft Tissue. AM disclosed that he has consulted for Unity Biotechnology, Kolon TissueGene and Gordian Biotechnology. Note COI forms cannot be merged, but are available with corresponding author and can be sent via email to the editorial staff upon request.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available in a public, open access repository. Data sharing not applicable as no datasets generated and/or analysed for this study. Data are available upon reasonable request. All data relevant to the study are included in the article or uploaded as supplementary information.
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