Abstract
Chronic low-grade inflammation, also known as metabolic inflammation, is a hallmark of obesity and parallels with the presence of elevated circulatory levels of free fatty acids and inflammatory cytokines/chemokines. CCL4/MIP-1β chemokine plays a key role in the adipose tissue monocyte recruitment. Increased circulatory levels of TNF-α, palmitate and CCL4 are co-expressed in obesity. We asked if the TNF-α/palmitate could interact cooperatively to augment the CCL4 production in human monocytic cells and macrophages. THP-1 cells/primary macrophages were co-treated with TNF-α/palmitate and CCL4 mRNA/protein expression was assessed using qRT-PCR/ELISA. TLR4 siRNA, a TLR4 receptor-blocking antibody, XBlue™-defMyD cells and pathway inhibitors were used to decipher the signaling mechanisms. We found that TNF-α/palmitate co-stimulation augmented the CCL4 expression in monocytic cells and macrophages compared to controls (p < 0.05). TLR4 suppression or neutralization abrogated the CCL4 expression in monocytic cells. Notably, CCL4 cooperative induction in monocytic cells was: (1) Markedly less in MyD88-deficient cells, (2) IRF3 independent, (3) clathrin dependent and (4) associated with the signaling mechanism involving ERK1/2, c-Jun, JNK and NF-κB. In conclusion, TNF-α/palmitate co-stimulation promotes the CCL4 expression in human monocytic cells through the mechanism involving a TLR4-MyD88 axis and MAPK/NF-κB pathways. These findings unravel a novel mechanism of the cooperative induction of CCL4 by TNF-α and palmitate which could be relevant to metabolic inflammation.
Highlights
Chronic low-grade inflammation, which is called metabolic inflammation, plays a critical role in obesity-related insulin resistance and type-2 diabetes (T2D), as well as metabolic syndrome
To further see if this cooperativeness was reproducible in primary macrophages, we treated the human macrophages and the data show that CCL4 mRNA and protein expressions were remarkably upregulated by tumor necrosis factor (TNF)-α/palmitate co-stimulation as compared to TNF-α treatment alone (CCL4 mRNA: p = 0.022; CCL4 protein: p = 0.018) (Figure 1C,D)
In this study we show, for the first time to our knowledge, that the cooperative interaction between an inflammatory cytokine (TNF-α) and a saturated fatty acid triggers CCL4 expression in the human monocytic cells
Summary
Chronic low-grade inflammation, which is called metabolic inflammation, plays a critical role in obesity-related insulin resistance and type-2 diabetes (T2D), as well as metabolic syndrome. Colonization of the adipose tissue by proinflammatory M1 macrophages is a striking feature of progressive obesity and involves the increased expression of tumor necrosis factor (TNF)-α, IL-1β, IL-6, IFN-γ, IL-8, IP-10, CCL2, CCL3, CCL4, CCL5, Eotaxins (CCL-11, 24, 26), CX3CL1 and several other chemokines These inflammatory proteins may contribute, at various levels, to extravasation of circulatory monocytes into the adipose tissue and sustain metabolic inflammation which is pivotal to the development of insulin resistance. In addition to metabolic diseases, CCL4 is involved in the pathogenesis of systemic lupus erythematosus [6], multiple sclerosis [7], multiple myeloma [8], psoriasis [9], cystic fibrosis [10] and sarcoidosis [11] Both immune and non-immune cells can express chemokines in response to TNF-α [12], IL-1β [13], leukotriene-B4 [14], complement fraction C5a [15], viruses [16], bacterial LPS [17,18] and free fatty acids [18,19]
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