Background/Purpose: Previous findings indicated that autophagy is defective in Aging and Osteoarthritis (OA). Autophagy is essential to maintain chondrocyte homeostasis by regulating the intracellular macromolecule and organelle turnover. However, the specific targets that regulate this homeostatic mechanism are still unknown. The objective of study is to identify relevant targets regulating autophagy in Aging and OA.

Methods: Human chondrocytes were transfected with siRNA for Atg5 (100 nM, 72 hours), an important autophagy marker, to block the autophagy pathway. To identify the key proteins associated to defective autophagy, we performed quantitative proteomics analysis using iTRAQ labeling coupled with on-line 2D LC/MS/MS. Protein identification and quantification were performed using Protein Pilot Software v 4.0. Each MS/MS spectrum was searched in the Uniprot/Swissprot database for Homo sapiens. Human chondrocytes, human cartilage from healthy, aged and OA human patients and mouse knee joints from young and old mice were employed. To evaluate the role of Lamin A/C overexpression on autophagy pathway in human chondrocytes, CRISPR-Cas9 technology was used.

Results: 487 different proteins were identified in response to defective autophagy in human chondrocytes. 24 targets were significantly altered (p<0.05) between siCtrl and siAtg5 (11 were decreased and 13 increased). Cytoskeleton organization, collagen catabolism, oxidative stress, and aging pathways are associated with chondrocytes with deficient autophagy. Taking into account the key role of autophagy in the regulation of aging and OA, we focused on Prelamin A/C. Prelamin A/C is a nuclear protein implicated in premature cell senescence that was found upregulated (p < 0.05). Autophagy proteins Atg5 and LC3, as well as Lamin A/C were evaluated in human chondrocytes. The results indicated a reduction in autophagy expression, accompanied with an increase in Lamin A/C. Furthermore, in aging and OA human cartilage, autophagy markers were reduced while Lamin A/C expression was increased. Histological analysis of mouse knee joints from young and old mice revealed a reduction in LC3 expression, as well as an increase in Lamin A/C expression. Importantly, in vitro premature aging model by genetic deletion of Zinc Metalloproteinase 24 (Zmpste24) via CRISPR-Cas9, showed a Lamin A/C overexpression, accompanied by a reduction of LC3 in human chondrocytes, suggesting that autophagy loss-of-function is intimately correlated with increased premature senescence in human chondrocytes.

Conclusion: Proteomic analysis has revealed features of premature senescence when autophagy is disrupted in chondrocytes. Lamin A/C, a nuclear protein contributing to structural integrity to the nucleus and matrix was identified as candidate target for regulating cartilage function under defective autophagy, such as aging and OA. These results support the hypothesis that autophagy is decreased with aging. Therefore, targeting Lamin A/C a promising strategy to find novel therapeutics for cartilage aging and OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/deficient-autophagy-induces-premature-senescence-in-aging-and-osteoarthritis/