Oligomycin, an inhibitor of complex V of mitochondrial respiratory chain, induces an inflammatory response in rat knee joint

Abstract

Purpose: A decline of mitochondrial function has been described in OA chondrocytes and RA synoviocytes. Recent ex vivo findings support a connection between mitochondrial dysfunction and activation of inflammatory and destructive pathways in these cells. The aim of this study was to investigate in vivo articular model if the intraarticular injection of oligomycin, an inhibitor of mitochondrial function, induces a destructive and inflammatory response in rat knee joints. Methods: 24 female wistar rats (180-220g) was divided into three study groups: Healthy (no intraarticular injection); Lipopolysaccharide (LPS)-treated, positive control (left joint injected with: LPS 10ug and right joint with vehicle); and Oligomycin (OLI)-treated (left joint injected with Oligomycin 20ug and right joint with vehicle). Three intraarticular injections were carried out at days 0, 2 and 5. Rats were sacrificed at day 6, hind paws were collected and joint tissues were obtained. Measurement of joint diameters on stimulis- and control-injected paws was performed at days 0 and 6. Histopathologic lesions were evaluated by hematoxilyn-eosin (H&E) and masson trichromic stain sections in synovial tissue and by safranin O staining in cartilage. By RT-PCR, CINC-1, IL-1β, CCL-2 and TNF-α gene expression were analyzed in extracted cartilage. And by immunohistochemical staining, IL-8, equivalent of CINC-1, expression was localized in the joint tissue. Results: OLI-treated hind paws significantly increased the joint diameter (0.9±0.1 mm, n=8, p<0.05, vs vehicle-injected joints), similarly to LPS-treated (2±0.3 mm, n=8, p<0.05, vs vehicle-injected joints). In relation, histological evaluation of synovial tissue by H&E staining revealed that joints treated with mitochondrial inhibitor present greater synovial lining hyperplasia, proliferation of subsynovial tissue and infiltration of a marked number of inflammatory cells while the right control synovial only contained a moderate synovial proliferation and inflammation (3.3±0.1 vs. 2.1±0.2, respectively, n=8, p<0.001, vs vehicle-injected joints). Besides, immunohistochemical studies on IL-8 showed a greater expression in synovial tissue from OLI-injected joints versus those from vehicle-injected joints, coinciding with a strong neutrophils infiltration. In relation to cartilage, when the loss of matrix in the cartilage by safranin O staining was evaluated no differences were observed, neither when IL-8 immunoperoxidase staining was performed in cartilage. By contrast, when CINC-1 mRNA was analyzed in this tissue, it was detected a significant increment in OLI-injected joint (n=8, p<0.05 vs. vehicle-injected joint), similarly to LPS-treated. Conclusions: The data seems to support that a loss of mitochondrial function in the joint could participate in rheumatoid pathology through generating an inflammatory response in the articular tissue, contributing to the perpetuation of joint injury.