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.