TNF-α in Combination with Palmitate Enhances IL-8 Production via TLR4-Dependent Signaling Pathway—Potential Relevance to Metabolic Inflammation

Several chemokines produced by the adipose tissue in obesity are involved in metabolic inflammation. IL-8 or CXCL-8, a chemokine, is produced by immune cells and adipocytes and the increased levels of IL-8 are associated with several inflammatory disorders. Elevated IL-8 plasma levels have also been linked to insulin resistance (IR) and associated complications. It is still an open debate how does the obesity lead to an overwhelming production of IL-8. Not only the saturated free fatty acids and TNF-α are found to be elevated in obese humans but both have also been implicated with IR. Therefore, we sought to determine how these two obesity-associated factors could influence the production of IL-8 in human monocytic cells. Our data show that human monocytic cells/macrophages that were co-stimulated with palmitate and TNF-α synergistically increased IL-8 production. Of note, the synergistic production of IL-8 by TNF-α/palmitate was suppressed either by neutralizing TLR4 or by genetic silencing of TLR4. Moreover, inhibiting the clathrin-dependent endocytosis of TLR4 significantly suppressed the synergistic production of IL-8. This synergy between TNF-α and palmitate for IL-8 production was partly blocked by IRF3 knock down or TRIF inhibition. Both MyD88-deficient and MyD88-competent cells responded comparably to TNF-α/Palmitate co-stimulation. Interestingly, increased NF-κB/AP-1 activity was detected in the TNF-α/palmitate co-stimulated cells. Our human data show that increased adipose tissue TNF-α expression correlated positively with IL-8 expression (r=0.44, p=0.003). IL-8 also correlated positively with the fasting blood glucose (r=0.32, p=0.012), HBA1C (r=0.36, p=0.01 and HOMA-IR (r=0.30, p=0.05). These findings suggest that the cross-talk between saturated fatty acid palmitate and TNF-α signaling pathways is a key driver of the inflammatory response in obesity via an excessive production of IL-8.