Synergistic induction of TNF-α by palmitate and oxidative stress in human monocytic cells involves the NF-κB/p38 MAPK signaling

Abstract

Abstract Tumor necrosis factor (TNF)-α is a key proinflammatory cytokine expressed in obesity/ type-2 diabetes. In metabolic disease conditions, TNF-α is co-expressed with increased circulatory levels of free fatty acids, such as long-chain unsaturated fatty acid palmitate. The hypoxic milieu in the adipose tissue is known to exert oxidative stress. Although it is known that palmitate may induce TNF-α in different cell types, it still remains unclear whether the oxidative stress could augment TNF-α expression in human monocytic cells, and thus exacerbate metabolic inflammation. To test the hypothesis, THP-1 monocytic cells were stimulated with palmitate, in presence or absence of hydrogen peroxide (H2O2) or 1% hypoxia. TNF-α, CHOP, ERN1, and HIF-1α mRNA expression was assessed by qRT-PCR; TNF-α protein expression was detected by flow cytometry/ immunohistochemistry; reactive oxygen species (ROS) expression was measured by DCFH-DA assay; and NF-κB/p38 MAPK phosphorylation was detected by western blotting. We found that TNF-α mRNA and protein expression were significantly upregulated in THP-1 cells co-stimulated with palmitate and H2O2/hypoxia. TNF-α expression associated positively with intracellular ROS, transcripts of CHOP, ERN1, HIF-1α, and phosphorylation of NF-κB/p38 MAPK. Synergistic TNF-α expression co-induced by palmitate and H2O2/hypoxia was diminished by cell treatments with curcumin, apocynin, or N-acetyl cysteine (P<0.01). In conclusion, palmitate and oxidative stress promote the TNF-α expression in human monocytic cells via the mechanisms involving ER stress, HIF-1α stabilization, and NF-κB/p38 MAPK signaling. These findings may have pathophysiological significance in metabolic inflammatory conditions.