DDIT3/CHOP promotes LPS/ATP-induced pyroptosis in osteoblasts via mitophagy inhibition

Inflammatory environments can trigger endoplasmic reticulum (ER) stress and lead to pyroptosis in various tissues and cells, including liver, brain, and immune cells. As a key factor of ER stress, DNA damage-inducible transcript 3 (DDIT3)/CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) is upregulated in osteoblasts during inflammatory stimulation. DDIT3/CHOP may therefore regulate osteoblast pyroptosis in inflammatory conditions. During this investigation, we found that lipopolysaccharides (LPS)/adenosine 5'-triphosphate (ATP) stimulation in vitro induced osteoblasts to undergo pyroptosis, and the expression of DDIT3/CHOP was increased during this process. The overexpression of DDIT3/CHOP further promoted osteoblast pyroptosis as evidenced by the increased expression of the inflammasome NLR family pyrin domain containing 3 (NLRP3) and ratios of caspase-1 p20/caspase-1 and cleaved gasdermin D (GSDMD)/GSDMD. To explore the specific mechanism of this effect, we found through fluorescence imaging and Western blot analysis that LPS/ATP stimulation promoted PTEN-induced kinase 1 (PINK1)/E3 ubiquitin-protein ligase parkin (Parkin)-mediated mitophagy in osteoblasts, and this alteration was suppressed by the DDIT3/CHOP overexpression, resulting in increased ratio of pyroptosis compared with the control groups. The impact of DDIT3/CHOP on pyroptosis in osteoblasts was reversed by the application of carbonyl cyanide 3-chlorophenylhydrazone (CCCP), a specific mitophagy agonist. Therefore, our data demonstrated that DDIT3/CHOP promotes osteoblast pyroptosis by inhibiting PINK1/Parkin-mediated mitophagy in an inflammatory environment.