Background/Purpose: Recently we have shown that both malondialdehyde-acetaldehyde adducts (MAA) and citrullinated proteins (CIT) are co-localized in joint and lung tissues of patients with rheumatoid arthritis (RA). Therefore, to investigate the possibility that these post-translational modifications pose a synergistic pathogenic role in RA, human synovial fibroblasts and lung epithelial cells were exposed to MAA, CIT or the combination of MAA-CIT and the expression of extracellular matrix proteins (ECMP) were determined. 

Methods: Human fibroblast-like synovial cells (HFLS) and lung epithelial cells (A549) were grown to confluency in 24-well plates.  Following serum starvation, the cells were incubated with 25 µg/ml of human serum albumin (HSA), HSA-MAA, HSA-CIT, or HSA-MAA-CIT for 4 hours (n=5 each group).  RNA was extracted from the cells and subjected to RT-PCR for expression of select ECMPs including fibronectin, collagen, and elastin. 

Results: Stimulation with either HSA-MAA or HSA-MAA-CIT significantly increased fibronectin expression (mRNA) compared to stimulation with HSA or HSA-CIT in both the HFLS (Panel A) and A549 (Panel B) cell lines (Figure 1).  Stimulation with HSA-MAA, HSA-CIT, and HSA-MAA-CIT did not significantly alter collagen expression in either cell line (p >0.05; Panels C and D), although collagen expression was quantitatively increased with HSA-MAA, HSA-CIT, and HSA-MAA-CIT stimulation (vs. HSA alone) in the A549 lung cell line (Panel D). HSA-MAA stimulation significantly (p< 0.001) increased elastin expression in the A549 lung cell line as compared to all other groups (Panel E).  Stimulation with HSA-MAA, HSA-CIT, and HSA-MAA-CIT (vs. HSA) did not significantly alter elastin expression in the HFLS cell line (Panel F).

Conclusion: Proteins that are MAA modified or MAA modified and citrullinated induce fibronectin expression in both lung epithelial cells as well as synovial fibroblasts. This effect appears to be particularly striking in lung epithelial cells where dually modified protein induced a ~15-fold increase in fibronectin exposure. While the impact of these protein modifications on collagen and elastin expression were more variable, these data suggests MAA and/or CIT modifications may expert relevant pathogenic (pro-fibrotic effects) in RA that could be both protein and site-dependent. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/increased-expression-of-extracellular-matrix-proteins-in-human-fibroblast-synoviocytes-and-lung-epithelial-cells-following-malondialdehyde-acetaldehyde-adduct-maa-citrullinated-protein-stimulation/