Abstract
Obesity—in which free fatty acid (FFA) levels are chronically elevated—is a known risk factor for different rheumatic diseases, and obese patients are more likely to develop osteoarthritis (OA) also in non-weight-bearing joints. These findings suggest that FFA may also play a role in inflammation-related joint damage and bone loss in rheumatoid arthritis (RA) and OA. Therefore, the objective of this study was to analyze if and how FFA influence cells of bone metabolism in rheumatic diseases. When stimulated with FFA, osteoblasts from RA and OA patients secreted higher amounts of the proinflammatory cytokine interleukin (IL)-6 and the chemokines IL-8, growth-related oncogene α, and monocyte chemotactic protein 1. Receptor activator of nuclear factor kappa B ligand (RANKL), osteoprotegerin, and osteoblast differentiation markers were not influenced by FFA. Mineralization activity of osteoblasts correlated inversely with the level of FFA-induced IL-6 secretion. Expression of the Wnt signaling molecules, axin-2 and β-catenin, was not changed by palmitic acid (PA) or linoleic acid (LA), suggesting no involvement of the Wnt signaling pathway in FFA signaling for osteoblasts. On the other hand, Toll-like receptor 4 blockade significantly reduced PA-induced IL-8 secretion by osteoblasts, while blocking Toll-like receptor 2 had no effect. In osteoclasts, IL-8 secretion was enhanced by PA and LA particularly at the earliest time point of differentiation. Differences were observed between the responses of RA and OA osteoclasts. FFA might therefore represent a new molecular factor by which adipose tissue contributes to subchondral bone damage in RA and OA. In this context, their mechanisms of action appear to be dependent on inflammation and innate immune system rather than Wnt-RANKL pathways.
Highlights
Obesity is not just a matter of excess body weight, but it is associated with numerous inflammatory cardiovascular and metabolic diseases such as atherosclerosis, coronary heart disease, and type 2 diabetes [1]
Preliminary experiments with murine osteoblasts stimulated with palmitic acid (C16:0) and linoleic acid (C18:2) at 100 μM showed significant increases in IL-6 and monocyte chemotactic protein 1 (MCP-1) secretion (n = 6 each) (Figure S1)
Growthrelated oncogene α (GRO-α) secretion was induced from a nondetectable level (
Summary
Obesity is not just a matter of excess body weight, but it is associated with numerous inflammatory cardiovascular and metabolic diseases such as atherosclerosis, coronary heart disease, and type 2 diabetes [1] This link to inflammation can be attributed to adipose tissue being a source of various biologically active mediators including cytokines [e.g., tumor necrosis factor, interleukin (IL)-6] [2,3,4], adipokines (e.g., adiponectin, visfatin) [5, 6], complement factors [7, 8], and certain free fatty acids (FFA) [9]. Several animal models support the role of obesity or a highfat diet in OA: In mice, high fat diet-induced obesity caused OA and systemic inflammation in proportion to body fat, while OA symptoms were not deteriorated but instead alleviated by increased mechanical joint loading via intense long-term exercise [27]. We investigated whether selected FFA affect cells of bone
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