Osteoarthritis (OA), a chronic degenerative joint disease, is the most common form of arthritis. OA affects 242 million individuals worldwide, but that number will grow due to increasing life expectancies (1). This statistic is alarming, considering the disability, the loss of quality of life, and the costs to the health system generated by OA. Currently, there are pharmacological treatments available to manage OA symptoms such as pain (2–4) as well as surgical joint replacement at the end stage of disease (5,6). Unfortunately however, there is no cure for OA. Progressive understanding of the pathophysiology of OA suggests that the disease is a heterogeneous condition, so further research is needed to direct the clinical approaches to disease management (7).
Recent studies have shown that OA is a multifactorial disease of the whole joint, however its pathogenesis remains still poorly understood (8). Genetic, environmental, and biomechanical factors can accelerate the onset of OA (9). Articular cartilage is a highly specialized tissue that forms the smooth gliding surface of synovial joints, with chondrocytes as the only cellular component of cartilage (10). The homeostasis of the cartilage extracellular matrix (ECM) involves a dynamic equilibrium between anabolic and catabolic pathways controlled by chondrocytes (11). The progression of OA is associated with dramatic alteration in the integrity of the cartilage ECM network formed by a large number of proteoglycans (mostly aggrecan), collagen II, and other non-collagenous matrix proteins (12). In addition, ECM synthesis is regulated by a number of transcriptional regulators involved in chondrogenesis, specifically Sex- determining- region-Y Box 9 (SOX9), L-SOX 5 and SOX6 that regulate type II collagen (Col2a1) and Aggrecan (Acan) gene expression (13). On the other hand, catabolic events are dominant in OA and cells are exposed to degenerative enzymes such as aggrecanases (e.g. ADAMTS-4, -5) (12,14), collagenases (e.g. MMP-1,-3, -8, -13) (15), and gelatinases (e.g.MMP-2, and MMP-9), all of which have implications in articular cartilage degeneration (16). A number of growth factors (17) play a role in OA pathology, such as transforming growth factor-β (18), BMP-2 (19), Insulin growth factor 1 (IGF-1) (20) fibroblast growth factor (FGF) and others, but the exact regulation of chondrocyte physiology is still not completely understood.
Recent studies in our laboratory (21,22) have identified the epidermal growth factor receptor (EGFR) and its ligand transforming growth factor alpha (TGFα) as possible mediators of cartilage degeneration (23–25). The human TGFA gene locus was also strongly linked to hip OA and cartilage thickness in genome-wide association studies (26,27). TGFα stimulates EGFR signaling and activates various cell-signaling pathways in chondrocytes, including extracellular signal-regulated kinase 1 and 2 (ERK1/2) and phosphoinositide 3-kinase (P13K) (28). EGFR signaling plays important roles in endochondral ossification (29,30), growth plate development (29) and cartilage maintenance and homeostasis (31–33), but many aspects of its action in cartilage are still not well understood. However, both protective and catabolic effects of EGFR signaling in OA have been reported, suggesting context-specific roles of this pathway (34).
Mitogen-inducible gene 6 (Mig-6) is also known as Gene 33, ErbB receptor feedback inhibitor 1 (ERRFI1), or RALT, and is found in the cytosol (35). Mig-6 protein binds to and inhibits EGFR signaling through a two-tiered mechanism: suppression of EGFR catalytic activity and receptor down-regulation (36). Interestingly, various studies have reported that loss of Mig-6 induces the onset of OA-like symptoms in mice (35,37–39). Cartilage-specific (Col2-Cre) knockout of Mig-6 mice results in formation of chondro-osseous nodules in the knee, but also increased thickness of articular cartilage in the knee, ankle, and elbow (40). Prx1-Cre-mediated knockout of Mig-6 in the limb mesenchyme results in a similar phenotype as that observed in cartilage-specific knockout mice (32). These phenotypes appeared to be caused by an increase in chondrocyte proliferation in articular cartilage, supported by increased expression of Sox9 and EGFR activation in cartilage (32). Since our studies suggest dosage- and/or context-specific roles of EGFR signaling in the process of cartilage degeneration in OA, in this study we used a cartilage-specific (Col2-Cre) to examine effects of Mig-6 overexpression specifically in articular cartilage.