TLR4 counteracts BVRA signaling in human leukocytes via differential regulation of AMPK, mTORC1 and mTORC2

Sci Rep. 2019 May 7;9(1):7020. doi: 10.1038/s41598-019-43347-8.

Abstract

TLR4 is implicated in diseases associated with chronic low-grade inflammation, yet homeostatic signaling mechanisms that prevent and/or are affected by chronic TLR4 activation are largely uncharacterized. We recently reported that LPS/TLR4 activates in human leukocytes signaling intermediates (SI), abbreviated TLR4-SI, which include mTORC1-specific effectors and targets, and that leukocytes of patients with T2D or after cardiopulmonary bypass (CPB) expressed similar SI. Extending these previous findings, here we show that TLR4-SI expression post-CPB was associated with low serum bilirubin and reduced preoperative expression of biliverdin reductase A (BVRA), the enzyme that converts biliverdin to bilirubin, in patient's leukocytes. Biliverdin inhibited TLR4 signaling in leukocytes and triggered phosphorylation of mTORC2-specific targets, including Akt, PKCζ, AMPKα-LKB1-TSC1/2, and their association with BVRA. Torin, PP242, and a PKCζ inhibitory peptide, but not rapamycin, prevented these biliverdin-induced responses and TLR4 inhibition. In contrast, LPS/TLR4 triggered decreases in BVRA, AMPKα and PKCζ expression, and an increase in haptoglobin, a heme binding protein, in leukocytes in vivo and in vitro, indicating that activated TLR4 may suppress biliverdin/BVRA signaling. Significantly, compared to non-diabetics, BVRA and PKCζ expression was low and haptoglobin was high in T2D patients leukocytes. Sustained TLR4 activation may deregulate homeostatic anti-inflammatory BVRA/mTORC2 signaling and thereby contribute to chronic inflammatory diseases.

MeSH terms

  • AMP-Activated Protein Kinases / metabolism
  • Aged
  • Bilirubin / blood
  • Cardiopulmonary Bypass / adverse effects*
  • Diabetes Mellitus, Type 2 / metabolism*
  • Female
  • Gene Expression Regulation
  • Humans
  • Leukocytes / metabolism*
  • Male
  • Mechanistic Target of Rapamycin Complex 1 / metabolism
  • Mechanistic Target of Rapamycin Complex 2 / metabolism
  • Oxidoreductases Acting on CH-CH Group Donors / metabolism*
  • Phosphorylation
  • Signal Transduction
  • Toll-Like Receptor 4 / metabolism*

Substances

  • TLR4 protein, human
  • Toll-Like Receptor 4
  • Oxidoreductases Acting on CH-CH Group Donors
  • BLVRA protein, human
  • Mechanistic Target of Rapamycin Complex 1
  • Mechanistic Target of Rapamycin Complex 2
  • AMP-Activated Protein Kinases
  • Bilirubin