The Allosterically Modulated Free Fatty Acid Receptor 2 is Transactivated by an Increase in the Cytosolic Concentration of Calcium Ions

Abstract

Allosterically modulated free fatty acid receptor 2 (FFA2R/GPR43) can be activated without the involvement of any orthosteric FFA2R agonist, by signals generated for example by P2Y_2_R, the G protein coupled receptor for ATP. An FFA2R specific positive allosteric modulator (PAM; Cmp58) was used to disclose the molecular mechanism by which signals generated by ATP/P2Y_2_R transactivates FFA2R. The P2Y_2_R induced signal that transactivates the allosterically modulated FFA2R was generated downstream of the Gα_q_ containing G protein that couple to P2Y_2_R. A receptor induced rise in the cytosolic concentration of ionized calcium ([Ca^2+^]_i_) was hypothesized to be the receptor transactivation signal. The Gα_q_ dependent transient rise in [Ca^2+^]_i_ induced by the ATP activated P2Y_2_Rs was not affected by Cmp58. The hypothesis gained, however, support from the finding that the modulator transferred FFA2R to a Ca^2+^ sensitive state. The rise in [Ca^2+^]_i_ induced by the Ca^2+^ specific ionophore ionomycin, activated the allosterically modulated FFA2R. The response induced by ionomycin was rapidly terminated and the FFA2Rs could then no longer be activated by the orthosteric FFA2R agonist propionate or be transactivated by the signal generated by the activated ATP receptor. The desensitized/non-responding state of FFA2R was, however, revoked by an earlier described cross-sensitizing/activating allosteric FFA2R modulator. The receptor transactivation of the allosterically modulated FFA2Rs, represent a unique regulatory receptor cross-talk mechanism by which the activity of a G protein coupled receptor is controlled by a signaling system operating from the cytosolic side of the plasma membrane. A P2Y_2_ receptor signal generated downstream of a Gαq containing G protein transactivates the allosterically modulated FFA2 receptor