Expression and tissue distribution of the mRNAs encoding the human thromboxane A2 receptor (TP) α and β isoforms

Abstract

The human thromboxane A2 receptor (TP), a G protein-coupled receptor, exists as two isoforms, TPalpha and TPbeta, which arise by alternative mRNA splicing and differ exclusively in their carboxyl terminal cytoplasmic regions. In this study, a reverse transcriptase-polymerase chain reaction (RT-PCR)-based strategy was developed to examine the expression of the TPs in tissues of physiologic relevance to TXA2. Although most of the 17 different cell/tissue types examined expressed both TP isoforms, the liver hepatoblastoma HepG2 cell line was found to exclusively express TPalpha mRNA. In most cell types, TPalpha mRNA predominated over TPbeta mRNA. Moreover, although the levels of TPalpha mRNA expression were similar in most of the cell/tissue types examined, extensive differences in the levels of TPbeta mRNA were observed. Consequently, the relative expression of TPalpha: TPbeta mRNA varied considerably due to extensive differences in TPbeta mRNA expression. Most strikingly, primary HUVECs were found to express: (i) low levels of TPbeta and (ii) approximately 6-fold greater levels of TPalpha than TPbeta. These data were confirmed in the spontaneously transformed HUVEC derived ECV304 cell line. Expression of TP mRNAs in the various tissue/cells correlated with protein expression, as assessed by radioligand binding using the selective TP antagonist [3H]SQ29,548.

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

“…Thus, the relative expression of TPα / TPβ varied considerably due to extensive differences in the level of TPβ expression. Most strikingly, contrary to previous reports that HUVECs expressed only TPβ mRNA sequences [32], our studies established that primary HUVECs expressed only low levels of TPβ but rather expressed 6-fold greater levels of TPα than TPβ mRNA [34].…”

“…Expression of TP mRNAs in the various cell / tissue types examined correlated with protein expression, as assessed by radioligand binding using the selective TP antagonist [ 3 H]SQ29,548 [34]. Additional immunofluorescence studies, employing TP isoform specific antibodies directed to peptide sequences within the unique C-tail domains of TPα and TPβ, also confirmed the expression and cellular localisation of the TP receptors [38,39].…”

“…Whereas Northern blot analyses had previously confirmed the existence of TP mRNA in human MEG-01 cells, placenta and lung [20], in human erythroleukemic (HEL) cells [35][36][37], these studies did not discriminate between the expression of the TPα and TPβ isoforms. Thus, through a series of studies involving RT PCR, Southern blot and phosphorimage analyses and radioligand binding studies, we found that both TPα and TPβ were expressed in the 17 tissues and cell types studied, with rare exceptions [34]. Whereas the levels of TPα expression predominated and were similar in most of the cell /tissue types examined, extensive differences in the levels of TPβ expression were observed [34].…”

“…Thus, through a series of studies involving RT PCR, Southern blot and phosphorimage analyses and radioligand binding studies, we found that both TPα and TPβ were expressed in the 17 tissues and cell types studied, with rare exceptions [34]. Whereas the levels of TPα expression predominated and were similar in most of the cell /tissue types examined, extensive differences in the levels of TPβ expression were observed [34]. Thus, the relative expression of TPα / TPβ varied considerably due to extensive differences in the level of TPβ expression.…”

“…Thus, as an essential prerequisite into studies investigating the potential differential roles of the TPα and TPβ receptors, we initially investigated the expression and tissue distribution of the TP isoforms in cells and tissues of relevance to TXA 2 biology [34]. Whereas Northern blot analyses had previously confirmed the existence of TP mRNA in human MEG-01 cells, placenta and lung [20], in human erythroleukemic (HEL) cells [35][36][37], these studies did not discriminate between the expression of the TPα and TPβ isoforms.…”