Physiological significance of thromboxane A2 receptor dimerization

The thromboxane A(2) receptor (TP), one of the G protein-coupled receptors (GPCRs), consists of two splicing variants, TP alpha and TP beta, which differ in their C-terminal regions. In the present study, we investigated whether TP alpha and TP beta formed homo- or hetero-dimers and whether the dimerization changed the function of TP. The immunofluorescent analysis using human embryonic kidney (HEK) 293 cells expressing either FLAG-tagged TP alpha or TP beta showed that TP alpha is mainly distributed on plasma membranes and TP beta existed on plasma membranes and within the cells. Co-immunoprecipitation analysis using HEK293 cells expressing both TP alpha and TP beta showed that TP alpha and TP beta formed homo- and hetero-dimers. U46619, a TP agonist, caused phosphoinositide hydrolysis and elevation of [Ca2+], in a concentration-dependent manner in Chinese hamster ovary (CHO) cells expressing TP alpha or TP beta. The responses were observed to a greater extent in the cells expressing TP alpha than TP beta. In the cells expressing both TP alpha and Tp beta, U46619-induced responses were observed to a lesser extent than in the cells expressing TPa alone. Furthermore, [H-3]SQ29548 binding showed that the level of the cell surface expression of TP was the following order: the cells expressing TP alpha > TP alpha and TP beta > TP beta. These results indicate that TP alpha and TP beta formed homo- and lietero-dirners, and TP-mediated signaling may be regulated by the hetero-dirner.