Induction of TLR Tolerance in Human Macrophages by Adiponectin: Does LPS Play a Role?

Abstract

Obesity is regarded as a pro‐inflammatory state. It is associated with low circulating levels of the adipokine, adiponectin, which is considered to be an anti‐inflammatory. However, adiponectin knockout mice do not consistently demonstrate pro‐inflammatory phenotypes, suggesting more complexity in the in vivo immunomodulatory effects of adiponectin than originally anticipated. Moreover, adiponectin exerts pro‐inflammatory effects in some experimental systems. This contradiction has been resolved by hypothesizing that adiponectin induces tolerance to inflammatory stimuli, notably Toll‐like receptor (TLR) ligands. We noticed that this effect resembled lipopolysaccharide (LPS) tolerance and therefore tested adiponectin from a variety of sources for LPS contamination. All adiponectin tested carried low levels of LPS in the range of 1–30 pg/μg of adiponectin, sufficient to produce final LPS concentrations in the pg/ml range under experimental conditions. We found that induction of tolerance to TLR ligands by adiponectin in human monocyte‐derived macrophages could be reproduced by such LPS concentrations. Moreover, the LPS antagonist, polymixin B, substantially inhibited induction of tolerance by adiponectin. Furthermore, polymixin B and a naturally occurring antagonist LPS were able to partially attenuate induction of tumour necrosis factor‐α and interleukin‐6 in human monocyte‐derived macrophages by adiponectin. Polymixin B also inhibited nuclear factor‐κB and mitogen‐activated protein kinase signalling elicited by adiponectin. We therefore propose that some of adiponectin’s immunomodulatory effects, in particular, its TLR‐tolerising actions in human monocyte‐derived macrophages, may be confounded by induction of tolerance by contaminating LPS.

Interactions of Adiponectin and Lipopolysaccharide from Porphyromonas gingivalis on Human Oral Epithelial Cells

“…It has even been speculated that an initial up-regulation of pro-inflammatory mediators by adiponectin might also be the cause for the subsequent anti-inflammatory effects of this adipokine [43], [44]. Moreover, this initial pro-inflammatory effect might result from contamination of adiponectin with LPS during the manufacture process [45]. Interestingly, such an initial adiponectin-induced increase of pro-inflammatory cytokines was not observed in our study.…”

The Activity of Adiponectin in Bone

“…Low levels of adiponectin were found to be expressed in both osteoblastic and osteoclastic cells, suggesting a contribution of autocrine/paracrine mechanisms of action in addition to the endocrine activity of adipocyte-secreted adiponectin in bone [26,28]. It is important to note that adiponectin binds lipopolysaccharide (LPS) [51] and that most commercially available preparations of adiponectin appear to be contaminated with LPS [52,53]. Thus, studies of adiponectin that do not include appropriate controls for the specificity of the effects could be measuring the combined activity of adiponectin and LPS in the experimental system.…”

Cytokines and Hormones That Contribute to the Positive Association between Fat and Bone

“…Two groups reported a direct inhibitory effect of adiponectin on osteoclast formation in CD14 positive cells and in RAW264.7 cells (47, 50), whereas two studies that used similar experimental systems showed no effects (48, 49). The inconsistency of some of the in vitro data could be explained by the finding that commercially available preparations of adiponectin are contaminated with lipopolysaccharide, and that this contaminant is responsible for some of the reported in vitro effects observed (51, 52). …”