LTB4 is converted into a potent human neutrophil NADPH oxidase activator via a receptor transactivation mechanism in which the BLT1 receptor activates the free fatty acid receptor 2

Abstract

The endogenous neutrophil chemoattractant leukotriene B₄ (LTB₄) is a biased signalling agonist that potently increases the intracellular concentration of free calcium ions ([Ca²⁺]_i), but alone is a weak activator of the neutrophil superoxide anion (O₂^-)-generating NADPH oxidase. However, in this study we show that an allosteric modulator of the free fatty acid 2 receptor (FFA2R) converts LTB₄ into a potent NADPH oxidase activating agonist. While an allosteric modulation of FFA2R was required for LTB₄ receptor 1 (BLT₁R)-mediated activation of the NADPH oxidase, the LTB₄-induced increase in [Ca²⁺]_i was not affected by the modulator. Thus, the biased BLT₁R signalling pattern was altered in the presence of the allosteric FFA2R modulator, being biased with a preference towards the signals that activate the NADPH oxidase relative to an increase in [Ca²⁺]_i. Both BLT₁R and FFA2R belong to the family of G protein-coupled receptors (GPCRs), and our results show that a communication between the activated BLT₁R and the allosterically modulated FFA2Rs generates signals that induce NADPH oxidase activity. This is consistent with a previously described receptor transactivation (crosstalk) model whereby the function of one neutrophil GPCR can be regulated by receptor downstream signals generated by another GPCR. Furthermore, the finding that an allosteric FFA2R modulator sensitises not only the response induced by orthosteric FFA2R agonists but also the response induced by LTB₄, violates the receptor restriction properties that normally define the selectivity of allosteric GPCR modulators.