BNIP3 Mediates the Different Adaptive Responses of Fibroblast-like Synovial Cells to Hypoxia in Patients With Osteoarthritis and Rheumatoid Arthritis

Abstract Background: Hypoxia is one of the important characteristics of synovial microenvironment in rheumatoid arthritis (RA), and it is very important in the process of synovial hyperplasia. Fibroblast-like synovial cells (FLSs) are relatively affected by hypoxia injury in cell survival, while FLSs from patients with RA (RA-FLSs) are particularly resistant to hypoxia-induced cell death. The purpose of this study was to evaluate whether FLSs in patients with osteoarthritis (OA) and RA-FLSs have the same adaptation to hypoxia. Methods: CCK-8, flow cytometry and BrdU were used to detect the proliferation of OA-FLSs and RA-FLSs under different oxygen concentrations. Apoptosis was detected by AV/PI, TUNEL and Western blot, mitophagy was observed by electron microscope and Western blot, mitochondrial state was detected by reactive oxygen species (ROS) and mitochondrial membrane potential by flow cytometry, BNIP3 and HIF-1α were detected by Western blot and RT-qPCR. The silencing of BNIP3 is achieved by stealth RNA system technology. Results: After hypoxia, the survival rate of OA-FLSs was reduced, and the proliferation activity of RA-FLSs was further increased. Hypoxia induced increased apoptosis and inhibited autophagy of OA-FLSs, but not in RA-FLSs. Hypoxia treatment led to a more lasting adaptive response. RA-FLSs showed a more significant increase in gene expression regulated by HIF-1α transcription. Interestingly, they showed higher BNIP3 expression than OA-FLSs, and showed stronger mitophagy and proliferation activities. The BNIP3 siRNA experiment in RA-FLSs confirmed the potential role of BNIP3 in the survival of FLSs. The inhibition of BNIP3 resulted in the decrease of cell proliferation and the decrease of mitophagy and the increase of apoptosis. Conclusion: In summary, RA-FLSs maintained redox balance through mitophagy to promote cell survival under hypoxia. The mitophagy of OA-FLSs was too little to maintain the redox balance of mitochondria, leading to apoptosis. The difference of mitophagy between OA-FLSs and RA-FLSs under hypoxia is mediated by the expression of BNIP3.