A2.9 High Serum-Cholesterol Levels by Either Low Density Lipoprotein Receptor Deficiency or a Cholesterol-Rich Diet Result in Synovial Activation and Osteophyte Formation During Experimental Osteoarthritis

Abstract

Purpose: Recently, we have shown that synovial activation plays an important role in etiopathology of osteoarthritis (OA). Synovial macrophages are involved in cartilage damage and are crucial for osteophyte formation in experimental OA models. Increased cholesterol levels have been correlated to OA pathology suggesting a metabolic mechanism. Atherosclerotic studies show that scavenger receptors on macrophages are capable of taking up oxidized low density lipoprotein (oxLDL), resulting in increased inflammatory properties of the macrophage. Accumulated LDL can be oxidized in an inflammatory milieu such as OA, possibly resulting in oxLDL uptake of synovial macrophages. We investigated whether increased LDL levels could lead to more severe OA pathology in experimental induced OA. Methods: LDL receptor deficient (LDLr-/-) mice and their wild type (WT) controls received either a cholesterol-rich or control diet for 120 days (n=10 mice per group). Experimental OAwas induced by intra-articular injection of collagenase on day 84 and 86. 36 days after OA induction, mice were sacrificed and total knee joints (for histological analysis) and serum were collected. Cartilage damage, synovial thickening and immuno-histological staining was analyzed using an arbitrary scale. Osteophyte formation was measured using digital image analysis. Bone marrow derived cells were differentiated into macrophages and preincubated with oxLDL or LDL for 24 hours, after which they were stimulated with the endogenous toll-like receptor 4 ligand S100A8. RNA was analyzed for gene expression. Data are depicted as mean SEM. Results: WT mice receiving a normal diet developed moderate cartilage destruction (6.1 1.5), synovial thickening (1.4 0.2) and osteophyte formation (32.4 mm2-14.6). SerumLDL levelswere significantly higher in LDLr-/-mice compared toWTmice (7.33mmol/L-0.46and0.54mmol/L 0.04 respectively; p<0.05), which was additionally increased by a cholesterol-rich diet (38.73 mmol/L 3.11; p<0.0001). Despite differences in serum LDL levels, no significant differences between the four groups were found regarding synovial thickening and cartilage destruction. Expression of S100A8 by the synovial lining, however,was increased after receiving a cholesterol-rich diet, suggesting synovial activation. Furthermore, a cholesterol-rich diet increased ApoB accumulation in synovial lining macrophages of LDLr-/- mice. Interestingly, at the tibial plateau, LDLr-/- mice showed almost a fourfold increase of osteophyte formation compared to WT mice (206.3 mm2 36.3; p<0.05). When receivinga cholesterol-richdiet, osteophyte formationat the lateral sideof the tibial plateauinLDLr-/-mice further increasedfrom107.0 mm2-49.3 to 309.4 mm2 41.7 (p<0.05). In vitro stimulation of oxLDL-laden macrophages with S100A8 showed a significant decrease of anti-inflammatory cytokine IL-10 expression and an increase of BMP6 expression compared to macrophages that were not pre-incubated with oxLDL. Conclusions: Increased serum cholesterol levels by either LDL receptor deficiency or as a result of a cholesterol-rich diet stimulate oxLDL uptake by synovial lining macrophages, resulting in synovial activation. In accordance with in vitro data, synovial activation elicited by oxLDL leads to an inflammatory milieu reflected by increased S100A8 levels and may stimulate osteophyte formation at the margins of the tibial plateau.