Two thromboxane A2 receptor isoforms in human platelets. Opposite coupling to adenylyl cyclase with different sensitivity to Arg60 to Leu mutation.

Abstract

Thromboxane A 2 (TXA 2 ) receptor is a key molecule in hemostasis as its abnormality leads to bleeding disorders. Two isoforms of the human TXA 2 receptor have been cloned; one from placenta and the other from endothelium, here referred to as TXR ␣ and TXR ␤ , respectively. These isoforms differ only in their carboxyl-terminal tails. We report that both isoforms are present in human platelets. The two isoforms expressed in cultured cells show similar ligand binding characteristics and phospholipase C (PLC) activation but oppositely regulate adenylyl cyclase activity; TXR ␣ activates adenylyl cyclase, while TXR ␤ inhibits it. The Arg 60 to Leu mutant of TXR ␣ , which has been shown to impair PLC activation (Hirata, T., A. Kakizuka, F. Ushikubi, I. Fuse, M. Okuma, and S. Narumiya. 1994. J. Clin. Invest. 94: 1662-1667), also impairs adenylyl cyclase stimulation, whereas that of TXR ␤ retains its activity to inhibit adenylyl cyclase. These findings suggest that the pathway linked to adenylyl cyclase inhibition might be involved in some of the TXA 2 -induced platelet responses such as shape change and phospholipase A 2 activation which remain unaffected in the patients with this mutation. ( J. Clin. Invest. 1996. 97:949-956.)

Exploratory aspirin resistance trial in healthy Japanese volunteers (J-ART) using platelet aggregation as a measure of thrombogenicity

“…However, a-type activates adenyl cyclase, but b-type inhibits the activity. 22 This SNP may influence the splicing. In this study, aspirin administration markedly inhibited platelet TXA 2 production in all subjects.…”

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

“…TPα and TPβ are encoded by the single TP gene, on chromosome 19p13.3 [31], and arise by a novel differential splicing mechanism within Exon 3 whereby nucleotides 984 -1642 of the TPα mRNA behave as an Intron (Intron 2b) within the TPβ mRNA [20,31,32; Figure 1]. Reverse transcriptase polymerase chain reaction (RT PCR) experiments indicated that HUVECs express only TPβ [32] whereas human platelets were reported to express both TPα and TPβ isoforms [33]. The physiologic significance for the existence of 2 receptors for TXA 2 in humans, but not in other species thus far investigated [21][22][23][24][25], is currently unknown but is an area of extensive research interest within my laboratory.…”

“…From structure/ function relationship studies carried out with other prototypical GPCRs, including the β 2 adrenergic receptor [26,42,43], it is widely accepted that while the C-tail domains of GPCRs do not appreciably influence ligand binding, they can indeed play an essential role in determining the specificity and / or efficiency of receptor: heterotrimeric G protein coupling and effector regulation, and may also play an essential role in GPCR desensitization following ligand activation [26,42,43] [33] demonstrated that the TP isoforms over-expressed in…”

“…Chinese hamster ovary cells oppositely regulate adenylyl cyclase activity with TPα activating adenylyl cyclase, through Gα s and TPβ inhibiting it, through Gα i [33], suggesting a possible role for the C-tail of TP in determining G protein specificity. Moreover, TPα, but not TPβ, mediates agonist activation of G h , the novel high molecular weight G protein [56][57][58][59], leading to PLC activation and PI turnover [60].…”

Mechanisms Responsible for Serotonin Vascular Reactivity Sex Differences in the Internal Mammary Artery

“…Serotonin and thromboxane A 2 function through their cell surface receptors (serotonin 2A+2B and thromboxane A 2 receptors), coupled to both the G q/11 and G 12/13 families of G‐protein–coupled receptors, respectively. In turn, these receptors activate sarcoplasmic reticulum calcium release, activate voltage‐gated L‐type calcium entry, and increase calcium sensitivity via Rho A–mediated activation of Rho kinases and the protein kinase C–dependent activation of C‐kinase–activated protein phosphatase‐1 inhibitor for serotonin but not thromboxane A 2 signalling 13, 14, 15, 16. Therefore, vascular response to serotonin and thromboxane A 2 involves a balance of direct contractile effects, mainly attributable to activation of their respective cell surface receptors on smooth muscle, balanced with endothelial activation and the release of endothelial‐dependent vasodilator substances, including NO and prostanoids 17, 18, 19, 20…”