Now showing 1 - 2 of 2
  • Publication
    Investigation of the role of the carboxyl terminal tails of the alpha and beta isoforms of the human thromboxane A2 receptor (TP) in mediating receptor : effector coupling
    We have investigated the functional coupling of alpha and beta isoforms of the human thromboxane A2 receptor (TP) to Galpha16 and Galpha12 members of the Gq and G12 families of heterotrimeric G proteins in human embryonic kidney (HEK) 293 cell lines HEK.alpha10 or HEK.beta3, stably over-expressing TPalpha and TPbeta, respectively. Moreover, using HEK.TP-328 cells which over-expresses a variant of TP truncated at the point of divergence of TPalpha and TPbeta we investigated the requirement of the C-tail per se in mediating G protein coupling and effector activation. Both TPalpha and TPbeta couple similarly to Galpha16 to affect increases in IP3 and mobilization of intracellular calcium ([Ca2+]i) in response to the TP agonist U46619. Whilst both TP isoforms mediated [Ca2+]i mobilization in cells co-transfected with Galpha12, neither receptor generated corresponding increases in IP3 indicating that the Galpha12 mediated increases in [Ca2+]i do not involve PLC activation. Verapamil, an inhibitor of voltage dependent Ca2+ channels reduced [Ca2+]i mobilization in TPalpha and TPbeta cells co-transfected with Galpha12 to approximately 40% of that mobilized in its absence whereas 3,4,5-trimethyloxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), an antagonist of intracellular Ca2+ release, had no effect on [Ca2+]i mobilization by either receptor isoform co-transfected with Galpha12. Despite the lack of differential coupling specificity by TPalpha and TPbeta, TP-328 signaled more efficiently in the absence of a co-transfected G protein compared to the wild type receptors but, on the other hand, displayed an impaired ability to couple to co-transfected Galpha11, Galpha12 or Galpha16 subunits. In studies investigating the role of the C-tail in influencing coupling to the effector adenylyl cyclase, similar to TPalpha but not TPbeta, TP-328 coupled to GalphaS, leading to increased cAMP, rather than to Galphai. Whereas TP-328 signaled more efficiently in the absence of co-transfected G protein compared to the wild type TPalpha co-transfection of Galphas did not augment cAMP generation by TP-328. Hence, from these studies involving the wild type TPalpha, TPbeta and TP-328, we conclude that the C-tail sequences of TP are not a major determinant of G protein coupling specificity to Galpha11 and Galpha16 members of the Gq family or to Galpha12; it may play a role in determining GS versus Gi coupling and may act as a determinant of coupling efficiency.
      302Scopus© Citations 45
  • Publication
    Prostaglandin D2 receptor-mediated desensitization of the alpha isoform of the human thromboxane A2 receptor
    Thromboxane (TX) A2 and prostaglandin (PG) D2 mediate opposing actions in platelets and in vascular and non-vascular smooth muscle. Here, we investigated the effects of stimulation of the PGD2 receptor (DP) on signaling by the TXA2 receptor (TP) expressed in human platelets and in human embryonic kidney (HEK) 293 cells over-expressing the individual TPalpha and TPbeta isoforms. In platelets, the selective DP agonist BW245C abolished TP-mediated mobilization of intracellular calcium ([Ca2+]i) and inhibited platelet aggregation in response to the TXA2 mimetic U46619. DP-mediated desensitization of TP signaling in platelets was prevented by pre-treatment with the cAMP-dependent PKA inhibitor, H-89, but was unaffected by the PKC inhibitor GF 109203X. In HEK 293 cells signaling by TPalpha, but not TPbeta, was subject to DP mediated desensitization in a PKA dependent, PKC independent manner. U46619-induced signaling by TP-328, a truncated variant of TP containing only those residues common to TPalpha and TPbeta, was insensitive to prior DP stimulation indicating that the carboxyl terminal tail of TPalpha contains the target site(s) for DP-mediated desensitization. Mutation of Ser329 to Ala329 within a consensus PKA site in TPalpha rendered the mutant TPalphaS329A insensitive to BW245C-mediated desensitization. Whole cell phosphorylation assays established that TPalpha, but not TPbeta or TPalphaS329A, was subject to DP-mediated phosphorylation and that TPalpha phosphorylation was blocked by the PKA inhibitor H-89. These data establish that TPalpha, but not TPbeta, is subject to DP mediated cross desensitization, which occurs through direct PKA mediated phosphorylation of TPalpha at Ser329.
    Scopus© Citations 30  340