Adiponectin Inhibits Pro-inflammatory Signaling in Human Macrophages Independent of Interleukin-10

Macrophages participate pivotally in the pathogenesis of many chronic inflammatory diseases including atherosclerosis. Adiponectin, a vasculoprotective molecule with insulin-sensitizing and anti-atherogenic properties, suppresses pro-inflammatory gene expression in macrophages by mechanisms that remain incompletely understood. This study investigated the effects of adiponectin on major pro-inflammatory signaling pathways in human macrophages. We demonstrate that pretreatment of these cells with adiponectin inhibits phosphorylation of nuclear factor κB inhibitor (IκB), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK), induced by either lipopolysaccharide (LPS) or tumor necrosis factor (TNF) α, as well as STAT3 phosphorylation induced by interleukin-6 (IL6). Antagonism of IL10 by either neutralizing antibodies or siRNA-mediated silencing did not abrogate the anti-inflammatory actions of adiponectin, indicating that the ability of adiponectin to render human macrophages tolerant to various pro-inflammatory stimuli does not require this cytokine. A systematic search for adiponectin-inducible genes with established anti-inflammatory properties revealed that adiponectin augmented the expression of A20, suppressor of cytokine signaling (SOCS) 3, B-cell CLL/lymphoma (BCL) 3, TNF receptor-associated factor (TRAF) 1, and TNFAIP3-interacting protein (TNIP) 3. These results suggest that adiponectin triggers a multifaceted response in human macrophages by inducing the expression of various anti-inflammatory proteins that act at different levels in concert to suppress macrophage activation.Adipose tissue, long considered a lipid storage depot, has now gained recognition as an endocrine organ that produces various bioactive molecules with local and systemic functions, collectively known as adipokines (1, 2). Among them, adiponectin has emerged as a key vasculoprotective molecule with insulin-sensitizing, anti-inflammatory, and anti-atherogenic properties (3–5). Numerous (but not all) clinical studies have correlated hypoadiponectinemia with incidence of coronary artery disease, insulin resistance, type 2 diabetes, and hypertension. Experimental studies have demonstrated anti-inflammatory and anti-atherogenic properties of adiponectin by showing that its in vivo overexpression reversed abnormal neointimal thickening in adiponectin-deficient mice, alleviated atherosclerotic lesions in apolipoprotein E-deficient mice, and improved endothelial vasodilator dysfunction and hypertension in obese mice. Cell-based studies demonstrated various potentially anti-atherogenic functions of adiponectin in the major cell types found in atheroma: endothelial cells, smooth muscle cells, and macrophages (3–5).Adiponectin circulates in the plasma at concentrations of 3–30 μg/ml, forming three major oligomeric complexes with distinct biological functions: trimer, hexamer, and high molecular mass form (3–5). A bioactive proteolytic product that includes the adiponectin C1q-like globular domain also exists in plasma, albeit at very low concentrations (6), and in cell culture medium conditioned by THP-1 or U937 cells stimulated with phorbol esters (7).Macrophages contribute critically to the pathogenesis of many chronic inflammatory processes including atherogenesis, and thus comprise key targets for the anti-inflammatory action of adiponectin. Adiponectin inhibits lipopolysaccharide (LPS)²-induced pro-inflammatory gene expression in pig and human macrophages, rat Kupffer cells, and RAW264.7 cells by mechanisms that remain incompletely understood but that involve suppression of LPS-induced nuclear factor κB (NFκB) activation (8–11). Adiponectin induces expression of interleukin-10 (IL10), an immunomodulatory cytokine with potent anti-inflammatory activity, in leukocytes (12, 13). Park et al. (14) recently showed that IL10 generated after treating RAW 264.7 cells with globular adiponectin figures essentially in rendering macrophages tolerant to LPS.We have recently reported that full-length adiponectin inhibits expression of T-lymphocyte-active CXC chemokine receptor 3 (CXCR3) chemokine ligands in human macrophages stimulated by LPS, a process that involves inhibition of interferon (IFN) regulatory factor 3 (IRF3) activation (15). The present study investigated in detail the effects of adiponectin on signaling pathways elicited by the potent pro-inflammatory stimulants LPS, TNFα, and IL6 in human macrophages, and addressed in particular the role of IL10 as a potential mediator of adiponectin function. Our results indicate that adiponectin-induced anti-inflammation in primary human macrophages occurs primarily independently of IL10 and likely involves the concerted action of a group of adiponectin-induced anti-inflammatory molecules that include A20, suppressor of cytokine signaling (SOCS) 3, B-cell CLL/lymphoma (BCL) 3, and TNF receptor-associated factor (TRAF) 1.