OBJECTIVE: FOXO4 has essential roles in cellular metabolism and prevents cartilage degeneration in osteoarthritis (OA). Here we aim to provide evidence that deacetylated-FOXO4 stabilizes chondrocyte (CH) extracellular matrix (ECM) related to SOX9 activation. PATIENTS AND METHODS: We used Chromatin immunoprecipitation (ChIP) and Dual-Luciferase reporter assay to verify that the FOXO4 protein activates SOX9 by binding to its promoter. We cultured human CHs with IL-1 beta to cause degeneration and supplied Sirt1 protein to deacetylate FOXO4. To confirm the function of FOXO4 and SOX9 during CHs degeneration, we also used the FOXO4 and SOX9 silenced CHs by siRNA transfection as a comparison. Western blot assay was used to analyze the protein level of Sirt1, SOX9, and the acetylated condition of FOXO4. Besides, RT-PCR was used to measure the mRNA level of collagen I/II/X, aggrecan. MMP-13, and ADAMTS-5 for determining the ECM states. RESULTS: FOXO4 protein transcriptionally activates SOX9 expression by binding to its promoter. Under the IL-1 beta stimulation, FOXO4 acetyl-lysine rate increased, and the SOX9 protein expression decreased, which was alleviated after the supplement of exogenic Sirt1 protein. Meanwhile, Sirt1 overexpression increased the collagen II and aggrecan and reduced the collagen I, collagen X, MMP-13, and ADAMTS-5 mRNA expression. However, the silencing of FOXO4 abolished the Sirt1 induced SOX9 expression and weakened the ECM production stability. Additionally, SOX9 silencing also alleviated the effect of the Sirt1 supplement on the degenerated CHs, though the FOXO4 was highly deacetylated. CONCLUSIONS: FOXO4 acetylation aggravates during the degeneration of CHs, and the deacetylation of FOXO4 by Sirt1 could activate the SOX9 expression and result in maintaining the ECM stability of cartilage.