circEXOC5 promotes acute lung injury through the PTBP1/Skp2/Runx2 axis to activate autophagy

To understand the pathogenesis of acute lung injury (ALI), we focused on circEXOC5, a significantly up-regulated circular RNA in ALI. Using the in vivo cecal ligation and puncture (CLP)-induced ALI mouse model and in vitro LPS-challenged mouse pulmonary microvascular endothelial cell (MPVEC) model, we examined the impacts of knockdown circEXOC5 on lung injury, inflammation, and autophagy. The regulation between circEXOC5, polypyrimidine tract-binding protein 1 (PTBP1), S-phase kinase-associated protein 2 (Skp2), and Runt-related transcription factor 2 (Runx2) was investigated by combining RNA immunoprecipitation, qRT-PCR, mRNA stability, and ubiquitination assays. The significance of PTBP1 in circEXOC5-induced ALI phenotypes was examined both in vitro and in vivo. circEXOC5 was up-regulated and associated with increased inflammation and activated autophagy in cecal ligation and puncture-induced ALI lung tissues and LPS-challenged MPVECs. Through the interaction with PTBP1, circEXOC5 accelerated Skp2 mRNA decay, an E3 ubiquitin ligase for Runx2, and therefore increased Runx2 expression. Functionally, overexpressing PTBP1 reversed shcircEXOC5-inhibited ALI, inflammation, or autophagy. The signaling cascade circEXOC5/PTBP1/Skp2/Runx2, by essentially regulating inflammation and autophagy in MPVECs, aggravates sepsis-induced ALI.