CBX3 aggravates nucleus pulposus cell apoptosis and extracellular matrix degradation in vitro via repressing SESN2-mediated mitophagy in an H3K9me3-dependent manner.

BACKGROUND: Mitophagy is crucial for maintaining mitochondrial quality and is implicated in intervertebral disc degeneration (IVDD). This study investigated the role and mechanism of Chromobox protein homolog 3 (CBX3) in IVDD, focusing on mitophagy regulation. METHODS: An in vitro IVDD model was established using interleukin (IL)-1β-treated nucleus pulposus cells (NPCs). RT-qPCR and western blot were applied to detect mRNA and protein expressions. Cell viability and apoptosis were examined with CCK-8 and flow cytometry, respectively. Immunofluorescence was conducted to measure colocalization of Mitotracker Red and LC3. Cellular ATP content was analyzed with the ATP detection kit. The molecular mechanism was explored using chromatin immunoprecipitation, peptide pull-down assay, and luciferase reporter assay. RESULTS: CBX3 was upregulated in IL-1β-stimulated NPCs. Its knockdown alleviated NPC apoptosis and extracellular matrix degradation, evidenced by reduced MMP3 and increased collagen II/aggrecan. Furthermore, CBX3 depletion restored PINK1/Parkin-mediated mitophagy, indicated by elevated PINK1/Parkin level, enhanced mitochondrial-LC3 colocalization and LC3II/I ratio, as well as decreased p62 protein level, and increased ATP levels. The mitophagy inhibitor Mdivi-1 reversed these protective effects of CBX3 knockdown. Mechanistically, CBX3 transcriptionally repressed sestrin2 (SESN2) in an H3K9me3-dependent manner. Silencing SESN2 abolished the benefits of CBX3 knockdown. CONCLUSION: CBX3 repressed SESN2 transcription via an H3K9me3-dependent mechanism, leading to impaired PINK1/Parkin-mediated mitophagy, which in turn promoted NPC apoptosis and matrix degradation. This study identified a promising therapeutic target for IVDD.