Background/Purpose: Malondialdehyde (MDA) is a highly reactive compound generated during lipid-peroxidation in conditions associated with oxidative stress. MDA can irreversibly modify proteins (e.g. lysine, arginine and histidine residues). In addition, acetaldehyde can further react with the MDA adducts to form malondialdehyde-acetaldehyde (MAA) modification. Such protein modifications can lead to immunogenic neo-epitopes that are recognized by autoantibodies. In fact, anti-MDA/MAA IgG levels are increased in the serum of patients with autoimmune diseases, such as rheumatoid arthritis (RA).

Very little is known about the pathogenic pathways induced by different types of autoantibodies in RA. Interestingly, anti-MDA/MAA antibodies have been shown to promote osteoclast (OC) differentiation in cell culture, suggesting a potential role for these antibodies in bone damage associated with RA. Here we elucidated the pathways specifically triggered by anti-MDA/MAA autoantibodies in developing osteoclasts.

Methods: Recombinant human monoclonal anti-MDA/MAA antibodies, which were previously cloned from single synovial B cells of RA patients, or control antibodies were added to different OC assays. OCs were generated from monocyte-derived macrophages in the presence of RANK-L and M-CSF. OC development was monitored by light microscopy following tartrate-resistant acid phosphatase (TRAP) staining and in erosion assays using calcium phosphate-coated plates. Bone morphometrics were analyzed in anti-MDA/MAA-injected mice using X-ray microscopy. Cellular metabolism was analyzed by mass spectrometry, Seahorse XF Analyzer and a colorimetric L-Lactate assay.

Results: Anti-MDA/MAA antibodies induced a robust OC differentiation in vitro and bone loss in vivo. The anti-MDA/MAA antibodies acted on developing OCs by increasing glycolysis through an Fcγ receptor I-mediated pathway and the upregulation of the transcription factors HIF-1α and Myc. Such regulation of cellular metabolism was exclusively observed in the presence of the osteoclastogenic anti-MDA/MAA antibodies, but not in the presence of other autoantibodies (e.g. ACPA or other anti-MDA/MAA). The anti-MDA/MAA treatment induced a shift in the tricarboxylic acid (TCA) cycle activity in developing OCs, increasing citrate production. Interestingly, osteoclast differentiation was in general associated with the accumulation of citrate/aconitate in the cells, which suggested intense lipid biosynthesis. Indeed, we detected a profound shift in cellular lipid compositions in developing OCs and we showed that phosphoglyceride/triacylglyceride biosynthesis was essential for osteoclast development.

Conclusion: We described a novel type of autoantibody-induced pathway in RA, which could contribute to increased OC activation and a consequent bone loss. Our results showed that anti-MDA/MAA antibodies promoted osteoclast development by increasing glycolysis and modulating the TCA cycle through a signaling pathway that included Fcγ receptor I and transcription factors acting on glycolysis. A TCA cycle bias towards citrate production suggested that anti-MDA/MAA antibodies might stimulate osteoclastogenesis via increasing lipid biosynthesis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/autoantibodies-against-malondialdehyde-modifications-promote-osteoclast-development-by-reprogramming-cellular-metabolism/