MIP-1α Induction by Palmitate in the Human Monocytic Cells Implicates TLR4 Signaling Mechanism

Cell Physiol Biochem. 2019;52(2):212-224. doi: 10.33594/000000015. Epub 2019 Feb 28.

Abstract

Background/aims: MIP-1α (macrophage inflammatory protein 1α)/CCL3 chemokine is associated with the adipose tissue inflammation in obesity. Both MIP-1α and free fatty acids are elevated in obesity/T2D. We asked if free fatty acid palmitate could modulate MIP1α expression in the human monocytic cells.

Methods: Human monocytic THP-1 cells and macrophages were stimulated with palmitate and TNF-α (positive control). MIP-1α expression was measured with real time RT-PCR, Flow Cytometry and ELISA. Signaling pathways were identified by using THP-1-XBlue™ cells, THP-1-XBlue™-defMyD cells, anti-TLR4 mAb and TLR4 siRNA.

Results: Our data show that palmitate induced significant increase in MIP1α production in monocytic THP-1 cells/macrophages. MIP-1α induction was significantly suppressed when cells were treated with anti-TLR4 antibody prior stimulation with palmitate. Using TLR4 siRNA, we further demonstrate that palmitate-induced MIP-1α expression in monocytic cells requires TLR4. Moreover, THP1 cells defective in MyD88, a major adaptor protein involved in TLR4 signaling, were unable to induce MIP-1α production in response to palmitate. Palmitate-induced MIP-1α expression was suppressed by inhibition of MAPK, NFkB and PI3K signaling pathways. In addition, palmitate-induced NF-κB/AP-1 activation was observed while production of MIP-1α. However, this activation of NF-κB/AP-1 was abrogated in MyD88 deficient cells.

Conclusion: Overall, these results show that palmitate induces TLR4dependent MIP-1α expression requiring the MyD88 recruitment and activation of MAPK, NF-κB/AP-1 and PI3K signaling. It implies that the increased systemic levels of free fatty acid palmitate in obesity/T2D may contribute to metabolic inflammation through excessive production of MIP-1a.

Keywords: MIP-1α/CCL3; Palmitate; TLR4.

MeSH terms

  • Adaptor Proteins, Signal Transducing / genetics
  • Adaptor Proteins, Signal Transducing / metabolism*
  • Diabetes Mellitus, Type 2 / genetics
  • Diabetes Mellitus, Type 2 / metabolism
  • Diabetes Mellitus, Type 2 / pathology
  • Humans
  • MAP Kinase Signaling System / drug effects*
  • Macrophages / metabolism*
  • Macrophages / pathology
  • Monocytes / metabolism*
  • Monocytes / pathology
  • Myeloid Differentiation Factor 88 / genetics
  • Myeloid Differentiation Factor 88 / metabolism
  • Obesity / genetics
  • Obesity / metabolism
  • Obesity / pathology
  • Palmitic Acid / pharmacology*
  • THP-1 Cells
  • Toll-Like Receptor 4 / genetics
  • Toll-Like Receptor 4 / metabolism*

Substances

  • Adaptor Proteins, Signal Transducing
  • MAPKAP1 protein, human
  • MYD88 protein, human
  • Myeloid Differentiation Factor 88
  • TLR4 protein, human
  • Toll-Like Receptor 4
  • Palmitic Acid