Cellular senescence is broadly known as a stable cell cycle arrest accompanied by a senescence-associated secretory phenotype (SASP). In the past decades, calcium signaling has emerged as a key mediator of cellular senescence. However, the transcriptional regulation of calcium signaling during cellular senescence remains partially understood. We have previously identified the nuclear receptor RXRA as a key senescence repressor through inhibiting the endoplasmic reticulum (ER) calcium release channel inositol 1,4,5-trisphosphate receptor, type 2 (ITPR2) mediated intracellular calcium signaling. Nevertheless, as a transcriptional recruiter, the mechanism by which RXRA inhibits ITPR2 during cellular senescence remains unclear. Here we identified the zinc finger protein ZBTB17 can interact with RXRA. Interestingly, knockdown of ZBTB17 induces a cascade of RXRA-dependent intracellular calcium signaling, mitochondrial membrane potential (MMP), reactive oxygen species (ROS) accumulation, DNA damages, and ultimately cellular senescence. Moreover, the signaling and senescence phenotype induced by knocking down of ZBTB17 can also be abolished after silencing ITPR2. Altogether, our work provides a new mechanism controlling intracellular calcium signaling and cellular senescence and unveils novel insight toward the role of zinc finger proteins.
Introduction Osteoarthritis (OA) is the most prevalent degenerative joint disorder worldwide, characterized by impaired anabolic activity and excessive catabolic responses in chondrocytes. However, the precise mechanisms governing chondrocyte homeostasis during OA progression remain unclear. Objectives This study aimed to elucidate the role of polycomb protein BMI1 in maintaining chondrocyte homeostasis, identify its upstream transcriptional regulator and downstream signaling cascade, and evaluate its therapeutic potential in OA. Methods BMI1 expression was assessed in articular cartilage from OA patients and destabilization of the medial meniscus (DMM) mouse models. Transcriptional regulation of BMI1 by zinc finger and BTB domain–containing protein 17 (ZBTB17) was analyzed using Western blotting, chromatin immunoprecipitation and dual-luciferase reporter assays. In vivo effects of BMI1 were evaluated by intra-articular injection of rAAV6- Bmi1 . In vitro BMI1 overexpression and inhibition were achieved using Bmi1-overexpressing plasmid and PTC209, respectively. The BMI1-SMAD2 interaction was investigated using nuclear–cytoplasmic fractionation, co-immunoprecipitation, ubiquitination assays, cycloheximide chase assays, and immunofluorescence. Therapeutic efficacy of BMI1 was evaluated using a three-dimensional hydrogel-based delivery system in DMM mice. Results BMI1 expression was significantly downregulated in human and murine OA cartilage and negatively correlated with disease severity. ZBTB17 directly activated BMI1 transcription by binding to its promoter. BMI1 overexpression alleviated OA progression by enhancing chondrocyte anabolism and antioxidant capacity through upregulating SMAD2 and promoting its nuclear localization. Mechanistically, BMI1 interacted with SMAD2 to increase its stability by inhibiting SMURF2-mediated K48-linked ubiquitination while facilitating K63-linked ubiquitination at Lys157 to facilitate SMAD2 nuclear translocation. DKK1 was identified as a principal downstream target of the BMI1-SMAD2 axis. Finally, we confirmed that local delivery of BMI1 mdified stromal cells via a three-dimensional hydrogel significantly attenuated cartilage degeneration in vivo . Conclusion Our findings identify a novel ZBTB17–BMI1–SMAD2–DKK1 axis essential for chondrocyte homeostasis and highlight BMI1 as a promising therapeutic target for osteoarthritis.
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