Galectin 3 Induces a Distinctive Pattern of Cytokine and Chemokine Production in Rheumatoid Synovial Fibroblasts via Selective Signaling Pathways

Rheumatoid arthritis (RA) is a persistent systemic inflammatory disease characterized by inflammation involving multiple cell types, with the progressive destruction of involved joints (1). An essential component of the switch to persistence that underlies joint destruction is the production of chemokines, which recruit mononuclear cells, such as lymphocytes and monocytes, to the inflamed joint (2). Galectins, an evolutionarily conserved family of animal lectins, have diverse roles in cellular homeostasis and have been shown to modulate inflammatory responses, functioning as either proinflammatory or antiinflammatory regulators, in part through their ability to cluster and modulate signaling through glycan receptors associated with multiple ligands (3). This ability to influence immune responses has been demonstrated in animal models of a number of diseases, including arthritis (4). Galectin 3, a chimera-type member of the galectin family, has a C-terminal carbohydrate recognition domain responsible for carbohydrate binding but exhibits an N-terminal domain that is responsible for interactions between subunits facilitating its oligomerization (5,6). The biologic functions attributed to this lectin are likely to depend on both ligand crosslinking and oligomerization (6,7). Galectin 3 has been associated with a proinflammatory role in models of fibrotic disease affecting the lung and liver (8,9) and has been shown to promote monocyte chemotaxis and macrophage activation (10-12) in addition to neutrophil activation, degranulation, and superoxide production (13-15), suggesting a critical role in the development of innate immune responses. Furthermore, a key role of galectin 3 has also been shown in the survival and progression of tumor metastases by modulating different processes, including homotypic and heterotypic cell adhesion, migration, angiogenesis, and tumor-immune escape (16). In the context of synovitis, Ohshima and colleagues (17) have demonstrated increased levels of galectin 3 and its binding protein in RA synovial tissue compared with osteoarthritis (OA) synovial tissue. Furthermore, galectin 3 levels are increased in RA in both synovial fluid and peripheral blood compartments, where levels correlate with C-reactive protein (17). Interestingly, up-regulated expression of galectin 3 correlates with abnormal cell apoptosis in synovial tissue from patients with juvenile RA (18). In contrast, galectin 1, a prototype member of the galectin family composed of 1 conserved carbohydrate recognition domain that can dimerize, has a predominantly antiinflammatory role, suppressing experimental models of inflammatory diseases, such as hepatitis, experimental autoimmune encephalomyelitis, uveitis, colitis, and arthritis (19-22). Furthermore, this glycan-binding protein appears to play an important role in the mechanisms involved in Treg cell–mediated suppression of immune responses (23), inhibition of T cell receptor–mediated signal transduction (24), and differential regulation of T helper cell viability (25). Intriguingly, synovial fibroblasts engineered to overexpress galectin 1 ameliorated collagen-induced arthritis and induced a bias toward a Th2-mediated cytokine profile in vivo (22). Synovial fibroblasts have an established role as sentinel cells for immune cell activation in the joint (2), and in RA, these cells are responsible for secreting significant quantities of inflammatory cytokines (26). RA synovial fibroblasts actively contribute to destruction of cartilage and bone via secretion of matrix metalloproteinases (MMPs) and cathepsins, and via expression of RANKL, resulting in promotion of monocyte-to-osteoclast differentiation (27). The expanded population of synovial fibroblasts in RA is also a prolific source of chemokines responsible for the recruitment and retention of cells within the joint (2). It is clear that synovial fibroblasts are an important source of galectin 3 within the joint, as shown by messenger RNA (mRNA) and proteomic analyses (17,28). Following stimulation by the products of cartilage degradation, synovial fibroblasts also produce galectin 3 (29). However, there are likely to be many other sources of galectin 3 within the joint, including macrophages, which synthesize this glycan-binding protein in significant amounts (30). Although considerable information is available on the cellular sources of galectins in the synovium, the downstream effects of galectin 3 on different cell types in rheumatoid synovium remain largely unexplored. We therefore examined the effects of exogenous galectin 3 on RA synovial fibroblasts, comparing them with genetically matched control skin fibroblasts. We show that inflammatory cytokines, such as interleukin-6 (IL-6), and neutrophil-attracting chemokines, such as IL-8, are produced equally by galectin 3– and tumor necrosis factor α (TNFα)–stimulated synovial and skin fibroblasts. However, in response to galectin 3, synovial fibroblasts, but not skin fibroblasts, secrete mononuclear cell–attracting chemokines such as CCL2, CCL3, and CCL5. The molecular basis for this selectivity is due to the differential activation of MAPK and phosphatidylinositol 3-kinase (PI 3-kinase) signaling pathways in response to galectin 3. The increased expression of galectin 3 found in fibroblast-rich areas of the synovium may therefore have significant functional consequences in terms of recruitment of monocyte and lymphocyte infiltrates.