Lipopolysaccharide and palmitic acid synergistically induced MCP-1 production via MAPK-meditated TLR4 signaling pathway in RAW264.7 cells

BACKGROUND: Obesity increases the risk of developing diabetes mellitus. Clinical studies suggest that risk factors like palmitic acid (PA) and lipopolysaccharide (LPS) exist simultaneously in diabetes with obesity. Combination of PA and LPS even at low concentration can induce strong inflammatory reaction. Monocyte chemoattractant protein-1 (MCP-1) is an important inflammatory chemokine related to insulin resistance and type II diabetes. Our previous study using PCR array revealed that LPS and PA synergistically induce MCP-1 mRNA expression in macrophage cells RAW264.7, while the protein expression of MCP-1 in this case was not investigated. Moreover, the underling mechanism in the synergistic effect of MCP-1 expression or production induced by treatment of LPS and PA combination remains unclear. METHODS: Protein secretion of MCP-1 was measured by the enzyme-linked immunosorbent assay (ELISA) and mRNA levels of MCP-1 and Toll-like receptor 4 (TLR4) were measured by real-time PCR. Statistical analysis was conducted using SPSS software. RESULTS: LPS could increase MCP-1 transcription as well as secretion in RAW264.7, and PA amplified this effect obviously. Meanwhile, combination of LPS with PA increased TLR4 mRNA expression while LPS alone or PA alone could not, TLR4 knockdown inhibited MCP-1 transcription/secretion induced by LPS plus PA. Moreover, not NF-κB inhibitor but inhibitors of mitogen-activated protein kinase (MAPK) signaling pathways, including c-Jun NH2-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p38 MAPK were found to block MCP-1 generation stimulated by LPS plus PA. CONCLUSION: LPS and PA synergistically induced MCP-1 secretion in RAW264.7 macrophage cells, in which MCP-1 transcription mediated by MAPK/TLR4 signaling pathways was involved. Combined treatment of PA and LPS in RAW264.7 cells mimics the situation of diabetes with obesity that has higher level of PA and LPS, MAPK/TLR4/ MCP-1 might be potential therapeutic targets for diabetes with obesity.