Regulation of thromboxane receptor trafficking through the prostacyclin receptor in vascular smooth muscle cells - Role of receptor heterodimerization

Background-Prostacyclin ( PGI(2)) and thromboxane ( TxA(2)) effect disparate outcomes for atherogenesis and the response to vascular injury; PGI(2), a vasodilator and inhibitor of platelet aggregation, limits the deleterious actions of TxA(2), a vasoconstrictor and platelet activator. Dimerization of their G protein-coupled receptors, IP and TP, evokes a modified cellular response through which IP/TP counter-balance may be effected. We examined the consequence of IP/TP interaction for the regulatory pathways of both receptors. Methods and Results-TP alpha overexpressed in HEK293 cells or expressed endogenously in aortic smooth muscle cells ( ASMCs) was internalized after selective activation of either TP or IP. Homologous trafficking of TP was unaltered by coexpression of IP. Heterologous sequestration of TP alpha required the physical presence of activated IP, in transfected and native cells, but was independent of IP signaling to adenylyl cyclase. Reciprocal heterologous regulation of IP, via activated TP, was evident in both HEK293 cells and ASMCs. Homologous TP internalization led to receptor retention and degradation. In contrast, when internalization was IP-induced, TP alpha was recycled to the cell surface in coexpressing HEK293 cells, but not in ASMCs, in accord with the postendocytotic pathway of IP. Conclusions-IP/TP alpha interaction permits reciprocal regulation of receptor endocytosis via the trafficking pathway determined by the activated dimeric partner.