Free fatty acids in the presence of high glucose amplify monocyte inflammation via Toll-like receptors

Abstract

Type 2 diabetes (T2DM) is characterized by hyperglycemia, dyslipidemia, and increased inflammation. Previously, we showed that high glucose (HG) induces Toll-like receptor (TLR) expression, activity, and inflammation via NF-B followed by cytokine release in vitro and in vivo. Here, we determined how HG-induced inflammation is affected by free fatty acids (FFA) in human monocytes. THP-1 monocytic cells, CD14 ϩ human monocytes, and transiently transfected HEK293 cells were exposed to various FFA (0 -500 M) and glucose (5-20 mM) for evaluation of TLR2, TLR4, NF-B, IL-1␤, monocyte chemoattractant protein-1 (MCP-1), and superoxide release. In THP-1 cells, palmitate increased cellular TLR2 and TLR4 expression, generated reactive oxygen species (ROS), and increased NF-B activity, IL-1␤, and MCP-1 release in a dose-and time-dependent manner. Similar data were observed with stearate and FFA mixture but not with oleate. Conversely, NADPH oxidase inhibitor treatment repressed glucoseand palmitate-stimulated ROS generation and NF-B activity and decreased IL-1␤ and MCP-1 expression. Silencing TLR2, TLR4, and p47phox with small inhibitory RNAs (siRNAs) significantly reduced superoxide release, NF-B activity, IL-1␤, and MCP-1 secretion in HG and palmitate-treated THP-1 cells. Moreover, data from transient transfection experiments suggest that TLR6 is required for TLR2 and MD2 for TLR4 to augment inflammation in FFA-and glucoseexposed cells. These findings were confirmed with human monocytes. We conclude that FFA exacerbates HG-induced TLR expression and activity in monocytic cells with excess superoxide release, enhanced NF-B activity, and induced proinflammatory factor release.palmitate; hyperglycemia; Toll-like receptor 2; Toll-like receptor 4 THE LEVELS OF PLASMA free fatty acids (FFA) and glucose are elevated in obesity and type 2 diabetes (T2DM) and are implicated in the excess cardiovascular disease (CVD) risk associated with diabetes (15). Cell culture and animal studies have established chronic inflammation in the development and progression of insulin resistance (IR) and T2DM (29). In parallel, epidemiologic studies showed elevated plasma levels of C-reactive protein (CRP), serum amyloid A, cytokines, and chemokines as potential mediators of inflammation and predictors of CVD in T2DM patients (28). Monocytes and macrophages are pivotal cells orchestrating the development of IR and CVD (4). They are closely linked to the chronic inflammatory processes that underline IR, T2DM, and atherosclerosis (27). Proinflammatory factors derived from them have deleterious effects in the development and progression of IR and diabetes (27). Thus the mechanisms by which these metabolic abnormalities (dyslipidemia and hyperglycemia) contribute to systemic inflammation are essential for understanding of the pathophysiology of IR, T2DM, and CVD.Activation of the innate immune system via Toll-like receptors (TLRs) is implicated in a plethora of inflammatory diseases (8,33,36,41). TLRs are evolutionarily conserved pattern recognition recep...

Fatty acids from fat cell lipolysis do not activate an inflammatory response but are stored as triacylglycerols in adipose tissue macrophages

“…The concentration of palmitate required to elicit activation of macrophages is higher than that found during AT lipolysis [29,32]. Monounsaturated FAs do not stimulate TLR4-mediated activation of nuclear factor-κB (NF-κB) [29,33]. Unsaturated FAs have been shown to inhibit the LPS-induced NF-κB signalling pathway in macrophages [25,34].…”

Capsaicin attenuates palmitate-induced expression of macrophage inflammatory protein 1 and interleukin 8 by increasing palmitate oxidation and reducing c-Jun activation in THP-1 (human acute monocytic leukemia cell) cells

“…Other studies have shown that palmitate induces ROS generation in various cells such as skeletal muscle cells [13], THP-1 monocytes [12], pancreatic islet cells [38], and adipocytes [39] mainly through the activation of the NADPH (nicotinamide adenine dinucleotide phosphate) oxidase system [12]; they may also be formed as by-products of mitochondrial oxidative phosphorylation [37]. These ROS stimulate JNK activation [28] and induce mitochondrial dysfunction [15].…”

“…Saturated, but not unsaturated, fatty acids have been shown to increase the expression of the inflammatory genes interleukin 1β, MCP-1, IL-6, and IL-8 in adipose tissue and liver [11,12]. To determine the effect of saturated and unsaturated fatty acids on gene expression in THP-1 macrophages, the cells were treated with different concentrations of saturated (palmitate or stearate) or unsaturated fatty acid (oleate) for 8 hours; and MIP-1 and IL-8 mRNA levels were measured using RT-PCR (Fig.…”

“…Conversely, unsaturated free fatty acids (FFAs) such as oleate, linoleate, arachidonate, eicosapentaenate, and docosahexaenate decrease the inflammatory response and have a protective effect against saturated fatty acids [10]. The inflammatory response induced by saturated fatty acids can result from multiple mechanisms, including activation of stress signaling kinases (c-Jun N-terminal kinase [JNK], p38, IKK [IκB kinase]) via TLR-4 signaling; increases in lipid intermediates such as longchain fatty acyl coenzyme A (CoA), diacylglycerol (DAG), and ceramide via modulation of lipid metabolism; activation of endoplasmic reticulum (ER) stress pathways; increases in reactive oxygen species; and perturbations of mitochondrial functions [11][12][13][14][15]. Recent studies have shown that long-term exposure to saturated fatty acids causes mitochondrial dysfunction and inflammatory gene expression in cells [16].…”

Atopic Dermatitis: Update on Pathogenesis and Comorbidities

“…Multiple TLRs (TLR1-5, 7, and 9) are expressed on adipocytes, at least some of which appear to be functionally active [107][108][109][110]. High levels of free fatty acids from obesity and glucose from diabetes activate the innate immune system, with upregulated TLR expression that correlates with insulin resistance and increased inflammatory cytokine release in macrophages [111][112][113].…”