# The free fatty acid 2 receptor regulates C5a induced neutrophil superoxide production and its sensitivity to inhibition by the antagonist avacopan

## TL;DR

This study shows that the C5a receptor activates neutrophil superoxide production through direct and indirect pathways involving free fatty acid receptors.

## Contribution

The paper reveals a new regulatory mechanism where the C5a receptor transactivates free fatty acid receptors to modulate NADPH oxidase activity.

## Key findings

- The C5a receptor activates NADPH oxidase through two distinct signaling pathways.
- The free fatty acid receptor is higher in the neutrophil receptor hierarchy than the C5a receptor.
- Avacopan's inhibitory effect is reduced by an allosteric modulator of the free fatty acid receptor.

## Abstract

Receptor transactivation is a novel signaling mechanism used by several different neutrophil G protein-coupled receptors (GPCRs) to activate an allosterically modulated receptor that recognizes short chain free fatty acids. The C5a receptor recognizes the complement-derived chemoattractant C5a, a potent activator of the neutrophil superoxide anion generating NADPH oxidase. The aim of the study was to investigate the ability for the C5a receptor to generate receptor transactivating signals.

A rise in the cytosolic concentration of free calcium ions and activation of the NADPH oxidase were used as functional read-out systems.

An allosteric modulator (Cmp58) for the fatty acid receptor increased the activating potency but not the efficacy of C5a. The allosteric modulator also reduced the inhibitory effect of the C5a receptor antagonist avacopan, suggesting that the NADPH oxidase is activated by two different signaling pathways downstream of the C5a receptor. While the allosteric modulator affected the NADPH oxidase activity, the C5a-induced rise in the intracellular concentration of free calcium ions was unaffected. Activated C5a receptors thus generate signals that both directly activate the NADPH oxidase and transactivate free fatty acid receptors, which subsequently generate signals that elicit NADPH oxidase activity. In addition, the free fatty acid receptor was ranked higher than the receptor for C5a, in the neutrophil receptor hierarchy.

The results are in line with an earlier described receptor transactivation model, in which the free fatty acid receptor is activated by signal signals generated by other neutrophil GPCRs to which we now add the C5a receptor. The dual receptor trans-regulatory effects, whereby the receptor for C5a activates the free fatty acid receptor and by which this receptor attenuates the C5a response, represent new regulatory mechanisms of importance for the NADPH oxidase activity in neutrophils.