Deacetylation of FOXO4 by Sirt1 stabilizes chondrocyte extracellular matrix upon activating SOX9

Area of Science:

• Cell Biology
• Biochemistry
• Molecular Biology

Background:

• FOXO4 plays a crucial role in cellular metabolism and in preventing cartilage degeneration in osteoarthritis (OA).
• Chondrocyte (CH) extracellular matrix (ECM) stability is vital for cartilage health, and its degeneration is a hallmark of OA.
• SOX9 is a key transcription factor involved in chondrogenesis and maintaining chondrocyte function.

Purpose of the Study:

• To investigate the role of deacetylated FOXO4 in stabilizing chondrocyte (CH) extracellular matrix (ECM) in osteoarthritis (OA).
• To provide evidence that FOXO4 stabilizes CH ECM through SOX9 activation.

Main Methods:

• Chromatin immunoprecipitation (ChIP) and Dual-Luciferase reporter assay to confirm FOXO4 binding to the SOX9 promoter.
• In vitro culture of human CHs with IL-1β to induce degeneration, with or without Sirt1 protein supplementation.
• siRNA-mediated silencing of FOXO4 and SOX9 in CHs to assess their functional roles.
• Western blot analysis for protein levels (Sirt1, SOX9, acetylated FOXO4) and RT-PCR for ECM-related gene expression (collagen I/II/X, aggrecan, MMP-13, ADAMTS-5).

Main Results:

• FOXO4 directly activates SOX9 transcription by binding to its promoter.
• IL-1β stimulation increased FOXO4 acetylation and decreased SOX9 expression, effects reversed by Sirt1 supplementation.
• Sirt1 overexpression upregulated ECM components (collagen II, aggrecan) and downregulated matrix-degrading enzymes (collagen I, collagen X, MMP-13, ADAMTS-5).
• Silencing FOXO4 abrogated Sirt1-induced SOX9 expression and ECM stabilization; SOX9 silencing also diminished Sirt1's protective effects on degenerated CHs.

Conclusions:

• FOXO4 acetylation increases during chondrocyte degeneration in OA.
• Deacetylation of FOXO4 by Sirt1 activates SOX9 expression.
• This Sirt1-mediated activation of SOX9 by FOXO4 is crucial for maintaining cartilage ECM stability.