Expression and tissue distribution of the mRNAs encoding the human thromboxane A2 receptor (TP) alpha and beta isoforms.

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.