EP1- and FP-mediated cross-desensitization of the alpha (α) and beta (β) isoforms of the human thromboxane A2 receptor

Abstract

1. Heterologous desensitization or intermolecular cross-talk plays a critical role in regulating intracellular signalling by diverse members of the G-protein-coupled receptor superfamily. We have previously established that the α and β isoforms of the human thromboxane A2 receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E2, an agonist of the EP1 subtype of the PGE2 receptor (EP) family. 2. Herein, we investigated the molecular basis of TPα and TPβ desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE2 and in response to the PGF2α receptor (FP) agonist PGF2α, and sought to identify the target site(s) of those desensitizations. 3. Our results demonstrated that TPα and TPβ receptors are subject to desensitization in response to both EP 1 and FP receptor activation and that these effects are mediated by direct protein kinase (PK)C phosphorylation of the individual TP isoforms within their unique carboxyl-terminal (C)-tail domains. 4. Moreover, deletion/site-directed mutagenesis and metabolic labelling studies identified Thr337, within TPα, and Thr399, within TPβ, as the specific target residues for PKC phosphorylation and EP1-and FP-mediated desensitization of TPα and TPβ signalling, respectively. 5. Hence, in conclusion, while the TPα and TPβ diverge within their C-tail domains, they have evolved to share a similar mechanism of PKC-induced phosphorylation and desensitization in response to EP1 and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.