EP 1 ‐ and FP‐mediated cross‐desensitization of the alpha ( α ) and beta ( β ) isoforms of the human thromboxane A 2 receptor

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 A 2 receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E 2 , an agonist of the EP 1 subtype of the PGE 2 receptor (EP) family. Herein, we investigated the molecular basis of TP α and TP β desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE 2 and in response to the PGF 2 α receptor (FP) agonist PGF 2 α , and sought to identify the target site(s) of those desensitizations. Our results demonstrated that TP α and TP β 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. Moreover, deletion/site‐directed mutagenesis and metabolic labelling studies identified Thr 337 , within TP α , and Thr 399 , within TP β , as the specific target residues for PKC phosphorylation and EP 1 ‐ and FP‐mediated desensitization of TP α and TP β signalling, respectively. Hence, in conclusion, while the TP α and TP β diverge within their C‐tail domains, they have evolved to share a similar mechanism of PKC‐induced phosphorylation and desensitization in response to EP 1 and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.British Journal of Pharmacology (2004) 142 , 203–221. doi: 10.1038/sj.bjp.0705695