Organic anion transporting polypeptide 1B3 (OATP1B3) is a hepatocyte plasma membrane protein that transports various endogenous and xenobiotic compounds. Although it is exclusively expressed in the human liver under normal conditions, OATP1B3 can be also expressed in various human cancer tissues that have been associated with prognosis and clinical outcomes. However, despite the potential significance of the latter finding, no experimental evidence addressing the molecular entity of cancer-associated OATP1B3 has been provided to date. In this paper, we report the identification of a new OATP1B3 mRNA isoform expressed in human colon and lung cancer tissues, which we named cancer-type OATP1B3 (Ct-OATP1B3). Our results also make known a previously unidentified transcription start site and an alternative promoter region, localized at intron 2, from which Ct-OATP1B3 mRNA is generated. Isoform specific mRNA quantification showed that the Ct-OATP1B3 mRNA level was strikingly higher than that of Lt-OATP1B3 mRNA in human cancer tissues. In addition, the results showed that the translation occurred at three out of four open reading frames. To summarize, our results clearly demonstrate that the newly-identified Ct-OATP1B3 (but not Lt-OATP1B3) is the primary mRNA isoform, at least in the human cancerous samples we have examined. In line with the possibility that its translation products play important biological roles in cancer cells, we strongly believe that the existence of Ct-OATP1B3 should be taken into account during future studies of OATP1B3 associated with cancer prognosis and clinical outcomes.
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