Induction of hypoxia inducible factor 1α by Toll-like receptors in human dendritic cells

Introduction

Nonhypoxic stimuli can induce the expression of hypoxia-inducible factor 1α (HIF1α). Only recently has it been demonstrated that lipopolysaccharide (LPS) induces the expression of HIF1α in macrophages and that the induction of hypoxic genes in macrophages is also Toll-like receptor 4 (TLR4)-dependent. We hypothesized that HIF1α expression is induced in dendritic cells in a TLR-dependent manner, plays a crucial role in linking the innate with the adaptive immune system and may also influence mitochondrial respiration.

Methods

Human monocyte-derived immature dendritic cells (iDC) were stimulated with different TLR ligands (hyaluronic acid (HA), LPS or lipoteichoic acid) under normoxia. Furthermore, iDC were incubated under hypoxic conditions (1.5% oxygen) at the same time points with or without additional stimulation with LPS. HIF1α expression was examined by western blot at 2 hours, 4 hours, 6 hours, 8 hours, 12 hours and 24 hours after TLR stimulation. In parallel, the cells were analyzed for the expression of the costimulatory molecules and maturation markers CD80 and CD86 by flow cytometry (FACScan; B&D). Finally, iDC were incubated in the presence or absence of 1 mg/ml LPS, and mitochondrial respiration of digitonin-permeabilized iDC was determined using the Oxygraph 2 K (Oroboros Instruments, Innsbruck, Austria) and DatLab 4.2 software for data acquisition and analysis.

Results

All tested TLR ligands stimulated the expression of HIF1α in a time-dependent manner. Interestingly, TLR induced HIF1α expression levels in normoxia were even higher than in hypoxia. Hyaluronic acid, LPS and lipoteichoic acid led to dendritic cell maturation, as shown by CD80 and CD86 induction. LPS also increased complex II-dependent mitochondrial respiration of iDC (complex II respiratory control ratio: 1.5 ± 0.5 for controls vs 3.8 ± 1.2 for LPS, P < 0.05; n = 3).

Conclusion

The current data demonstrate that HIF1α expression in dendritic cells is induced under normoxic conditions via TLR2 and TLR4 agonists in a time-dependent manner. LPS also increases complex II-dependent mitochondrial respiration of dendritic cells.