Regulation of the human prostanoid TPα and TPβ receptor isoforms mediated through activation of the EP₁ and IP receptors

Abstract

1 The intermolecular cross-regulation mediated by the prostanoid IP-receptor (IP)/EP 1 receptor (EP 1 ) agonists PGI 2 and 17 phenyl trinor PGE 2 on TP receptor (TP) signalling within platelets was compared to that which occurs to the individual TPa and TPb receptors over-expressed in human embryonic kidney (HEK) 293 cells. Ligand mediated TP receptor activation was monitored by analysing mobilization of intracellular calcium ([Ca 2+ ] i ) following stimulation with the selective thromboxane (TX) A 2 mimetic U46619. 2 Consistent with previous studies, in platelets, PGI 2 acting through endogenous IP receptors completely inhibited U46619-mediated TP receptor signalling in a protein kinase (PK) A-dependent, PKC-independent manner. 3 In HEK 293 cells, PGI 2 , acting through endogenous AH6809 sensitive EP 1 rather than IP receptors, and the selective EP 1 receptor agonist 17 phenyl trinor PGE 2 antagonized U46619-mediated signalling by both TPa and TPb receptors in a PKC-dependent, PKA-independent manner. 4 The maximum response induced by either ligand was signi®cantly (P50.005) greater for the TPa receptor than the TPb receptor, pointing to possible physiologic dierences between the TP isoforms, although the potency of each ligand was similar for both TP receptors. TP D328, a truncated variant of TP receptor lacking the C-tail sequences unique to TPa or TPb receptors, was not sensitive to EP 1 receptor-mediated regulation by PGI 2 or 17 phenyl trinor PGE 2 . 6 In conclusion, these data con®rm that TPa and TPb receptors are subject to cross regulation by EP 1 receptor signalling in HEK 293 cells mediated by PKC at sites unique to the individual TP receptors and that TPa receptor responses are signi®cantly more reduced by EP 1 receptor regulation than those of the TPb receptor.

α1‐Adrenoceptor activation of PKC‐ε causes heterologous desensitization of thromboxane receptors in the aorta of spontaneously hypertensive rats

“…However, the present results with the selective PKC-ε inhibitor ε-V1-2 (Lee et al, 1999) make the latter interpretation unlikely but suggest that PKC activation contributes to the TP receptor desensitization in the SHR aorta. Transfection studies on cell cultures also suggest that PKC mediates modifications of TP receptors causing their desensitization (Spurney and Coffman, 1997;Yukawa et al, 1997;Walsh and Kinsella, 2000;Yan et al, 2002;Kelley-Hickie and Kinsella, 2004). The present findings indicate that the kinase indeed contributes to TP receptor desensitization in aortae isolated from live animals and that such heterologous desensitization results from crosstalk with α1-adrenoceptors.…”

“…The present results thus support the interpretation that variations in PKC isozymes underlie the desensitization of TP receptors observed in the SHR aorta. A similar conclusion was reached in TP receptor desensitization studies on cell cultures (Spurney and Coffman, 1997;Yukawa et al, 1997;Spurney, 1998;Walsh and Kinsella, 2000;Kelley-Hickie and Kinsella, 2004).…”

“…This expression increase may result in a protective response in the vasculature of hypertensive animals. Crosstalk between TP, IP, EP, DP and FP receptors in transfected cells has been studied by others (Walsh and Kinsella, 2000;Foley et al, 2001;Kelley-Hickie and Kinsella, 2004). The present study reports a crosstalk between non-prostanoid and prostanoid receptors that occurs ex vivo and is modulated by disease.…”

Thromboxane A2 Signalling in Humans: a ‘Tail’ of Two Receptors

“…Both TPα and TPβ have recently been established to undergo agonist-induced homologous desensitization and phosphorylation in transfected HEK 293 cells [96]. In addition, recent studies indicate that TPβ, but not TPα, may undergo agonist-induced internalization [97,98] and we have established that the TP isoforms are subject to differential prostanoid EP 1 receptor-induced desensitization mediated at PKC sites unique to the individual TPs [63].…”

Prostacyclin release and receptor activation: differential control of human pulmonary venous and arterial tone

“…Similarly, the relaxations induced by ACh via endogenous release of PGI 2 were also increased in human pulmonary veins treated with the EP 1 antagonists (AH6809, SC19220). Other studies have indicated the ability of PGI 2 to activate EP 1 -receptor in human cells (Walsh & Kinsella, 2000) or to contract guinea-pig smooth muscle via the EP 1 -receptor (Dong et al, 1986;Lawrence et al, 1992). This dual control of the venous tone via EP 1 -and/or IP-receptor in the human lung seems also present in the human hand vein (Arner & Hogesta¨tt, 1991).…”