Protective role of calcitriol in PM2.5-induced apoptosis and inflammation in bronchial epithelial cells in vitro via vitamin D receptor-mediated Nrf2 signaling

Abstract

PM2.5 contributes to lung injury by inducing oxidative stress, inflammatory responses, and apoptosis. Calcitriol exerts a protective role against lung injury by regulating the vitamin D receptor (VDR) and Nrf2 signaling pathways, mitigating PM2.5-induced oxidative stress and inflammation. In this study, we investigated the protective effects of calcitriol against PM2.5-induced apoptosis, oxidative damage, and inflammation in human bronchial epithelial BEAS-2B cells, with a specific focus on the crosstalk between VDR and Nrf2 signaling. BEAS-2B cells were pre-treated with calcitriol (1, 10, 100 nM) for 24 h before PM2.5 exposure (100 µg/mL). Apoptosis, DNA damage, and inflammation were assessed by flow cytometry, ELISA, qRT-PCR, and Western blot analysis. Chromatin immunoprecipitation (ChIP) was performed to evaluate VDR–antioxidant response element (ARE) binding. Calcitriol suppressed apoptotic signaling by reducing p53 phosphorylation and downregulating the mRNA expression of p53 and caspase-3, while also mitigating oxidative DNA damage, as indicated by decreased levels of 8-hydroxy-2′-deoxyguanosine (8-OHdG) in BEAS-2B cells. Additionally, calcitriol suppressed inflammatory responses by downregulating NF-κB activity and the mRNA expression of NF-κB p65 and its downstream pro-inflammatory genes, including IκB-α, TNF-α, and IL-6. Moreover, calcitriol treatment increased VDR protein expression and enhanced Nrf2 activity. ChIP assays demonstrated that calcitriol enhanced VDR binding to AREs, thereby promoting the transcription of key Nrf2-regulated cytoprotective genes, including heme oxygenase-1 (HO-1) and NADPH quinone dehydrogenase 1 (NQO1). These findings provide mechanistic insight into the pharmacological effects of calcitriol, underscoring its potential to alleviate PM2.5-induced cellular injury through VDR-mediated activation of Nrf2 redox signaling.