Background Synovial macrophages have previously shown to play a significant role in the etiopathology of experimental collagenase-induced osteoarthritis (OA)1. In addition to production of the pro-inflammatory protein S100A8/9 in early OA2, synovial lining macrophages have also shown to play a crucial role in promotion of transforming growth factor-beta (TGFβ) mediated osteophyte formation3. In an inflammatory milieu such as OA, accumulated low density lipoprotein (LDL) is oxidized, resulting in high intra-articular levels of oxidized LDL (oxLDL). OxLDL is taken up by macrophages via scavenger receptors, resulting in an more aggressive phenotype4.
Objectives To investigate whether LDL accumulation by either LDL-receptor (LDLr) deficiency or a cholesterol-rich diet leads to increased oxLDL uptake by synovial macrophages and affects synovial activation and osteophyte formation.
Methods LDLr deficient mice and their wild type (WT) controls received either a high cholesterol or control diet for 120 days. Experimental OA was induced by intra-articular injection of collagenase on day 84 and 86. Paraffin sections of OA knee joints were analyzed for cartilage destruction and osteophyte formation using the Pritzker score and image analysis, respectively. ApoB and S100A8 were detected using immunohistochemistry. Murine bone marrow derived macrophages were stimulated with 50 μg/mL oxLDL, after which supernatant was functionally tested for active TGFβ presence using a TGFβ reporter gene assay.
Results Mice receiving a cholesterol-rich diet not only developed increased serum LDL cholesterol levels, but also showed enhanced ApoB expression in synovial lining macrophages. In line with that, LDLr deficient mice, which already had systemically high basal levels of LDL, showed a much higher accumulation of ApoB in the synovial lining after receiving a cholesterol-rich diet. Although increased LDL levels did not enhance thickening of the synovium, S100A8 expression within macrophages was markedly increased, reflecting an elevated activation status. Even though no effect of LDL accumulation on cartilage destruction was found, both a cholesterol-rich diet and LDLr deficiency stongly increased bone formation in ligaments with a fold change of 6.7 and 6.1, respectively. Moreover, an increase in osteophyte size was found at the margins of the tibial plateau (4.4 fold increase after a cholesterol-rich diet and 5.3 fold increase in LDLr deficient mice compared to WT mice). To elucidate the mechanism, we finally studied the presence of active TGFβ, which is crucial in driving osteophyte formation, in culture supernatant of oxLDL stimulated macrophages. Using a TGFβ reporter assay, stimulation of macrophages with oxLDL showed an increased presence of functional TGFβ in supernatant compared to that of unstimulated macrophages (fold change of 2.9).
Conclusions LDL cholesterol accumulation by either LDLr deficiency or a cholesterol-rich diet leads to increased synovial activation and osteophyte formation in experimental OA. Uptake of oxLDL by synovial macrophages may activate TGFβ.
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Disclosure of Interest None Declared