Abstract
Milk fat globule-epidermal growth factor 8 (MFG-E8) is an anti-inflammatory glycoprotein that mediates the clearance of apoptotic cells and is implicated in the pathogenesis of autoimmune and inflammatory diseases. Because MFG-E8 also controls bone metabolism, we investigated its role in rheumatoid arthritis (RA), focusing on inflammation and joint destruction. The regulation of MFG-E8 by inflammation was assessed in vitro using osteoblasts, in arthritic mice and in patients with RA. K/BxN serum transfer arthritis (STA) was applied to MFG-E8 knock-out mice to assess its role in the pathogenesis of arthritis. Stimulation of osteoblasts with lipopolysaccharide (LPS) and tumor necrosis factor (TNF)-α downregulated the expression of MFG-E8 by 30% to 35%. MFG-E8-deficient osteoblasts responded to LPS with a stronger production of pro-inflammatory cytokines. In vivo, MFG-E8 mRNA levels were 52% lower in the paws of collagen-induced arthritic (CIA) mice and 24% to 42% lower in the serum of arthritic mice using two different arthritis models (CIA and STA). Similarly, patients with RA (n = 93) had lower serum concentrations of MFG-E8 (-17%) compared with healthy controls (n = 140). In a subgroup of patients who had a moderate to high disease activity (n = 21), serum concentrations of MFG-E8 rose after complete or partial remission had been achieved (+67%). Finally, MFG-E8-deficient mice subjected to STA exhibited a stronger disease burden, an increased number of neutrophils in the joints, and a more extensive local and systemic bone loss. This was accompanied by an increased activation of osteoclasts and a suppression of osteoblast function in MFG-E8-deficient mice. Thus, MFG-E8 is a protective factor in the pathogenesis of RA and subsequent bone loss. Whether MFG-E8 qualifies as a novel biomarker or therapeutic target for the treatment of RA is worth addressing in further studies.