INTRODUCTION
Dysregulated adaptive immunity can precede the clinical manifestation of affected joints in rheumatoid arthritis (RA) and the repeated activation of innate immunity can contribute to the hyper-inflammation and disease progression of RA (O’Neil & Kaplan, 2019). Identification and characterization of novel, safer and more cost-effective drug, in particular antagonists with different inhibitory properties, are of great importance. Of the important cellular participants in the joints of RA, synovial lining fibroblasts [also called fibroblast-like synoviocytes (FLSs)] exhibit an aggressive and apoptosis resistant phenotype (Bartok & Firestein, 2010) and produce excessive cytokines, small molecule mediators of inflammation(Weijun et al., 2018). and proteolytic enzymes (Ai et al., 2018). Therefore, inhibiting the ensuing inflammatory cycle of FLSs might improve clinical outcomes in RA patients.
Azithromycin (AZM) is a broad-spectrum antibiotic against respiratory, urogenital, dermal and other bacterial infections (Parnham, Haber, Giamarellos-Bourboulis, Perletti, & Vos, 2014). It has recently received increasing attention because of additional effects on chronic inflammatory disorders such as diffuse panbronchiolits, bronchiolitis obliterans and rosacea (Spagnolo, Fabbri, & Bush, 2013). The anti-inflammatory effects of AZM could be ascribed to prevent oxidative stress, cytokine productions, fibrosis occurrence and the consequent tissue destruction (Vanaudenaerde, Wuyts, Geudens, Dupont, & Verleden, 2010). The immunomodulation of AZM is associated with the inhibition of nuclear factor kappa-B (NF-κB) mediators, inhibitor kappa B kinase β (IKKβ) (Haydar et al., 2019). Importantly, AZM could inhibit inflammation and liposomal enzyme release in arthritic rats (Liu, Pu, Li, Zhou, & Wan, 2017), even though its exact effects and the molecular mechanism remain obscure (Nujić, Banjanac, Munić, Polančec, & Eraković Haber, 2012). Importantly, fibroblasts, due to their wide distribution, have been proposed as a potential reservoir for AZM, slowly releasing or passing it to nearby phagocytes for transport to the site of infection (Cory et al., 2013; Ozsvari, Nuttall, Sotgia, & Lisanti, 2018). While RA FLSs contribute greatly to the pathological progression of RA, characterizing the effects of AZM on RA FLSs would provide novel evidence for AZM in RA treatment (Zhang et al., 2015).
The endoplasmic reticulum (ER) is the site of biosynthesis for all secreted and membrane proteins and the accumulation of unfolded proteins in the ER leads to ER stress (Lu et al., 2020). Unfolded protein response (UPR) represents as an adaptive mechanism to keep homeostasis of ER in response to ER stress (Hetz & Saxena, 2017). Especially, unlike other cell types, RA FLSs are resistant against ER stress-induced apoptosis (Rahmati, Moosavi, & McDermott, 2018). As inflammation and the ER stress pathways are interconnected and concurrently regulated (Xu, Yang, Berezowska, Gao, & Peng, 2019), a more effective approach being able to modify integrated biological outcomes by simultaneously targeting both ER stress and inflammation pathways can be of major therapeutic benefit (Reverendo, Mendes, Argüello, Gatti, & Pierre, 2019). Herein, we provide genetic and biochemical evidence that AZM can serve as a promising approach to treat RA and identified GRP78-mediated UPR activation as a critical anti-arthritis function of AZM.