Gene expression and transcriptional regulation of thrombopoietin

Thrombopoietin (TPO) is predominantly expressed in the liver among various tissues that express TPO transcripts. To investigate the transcriptional regulation of the human TPO gene in the liver, we determined the major transcription initiation site by means of 5'-RACE and Northern blotting. From these analyses, we concluded that TPO gene transcription started at various points, and the transcription initiation sites of the human TPO gene were localized downstream, close to a point we determined by S1 nuclease mapping. The human TPO promoter region contains consensus sequences of GATA, Evi-1, and Ets binding sites. We used the hepatocellular carcinoma cell line, HepG2, that expresses TPO mRNA to analyze its promoter activity by transfecting various reporter plasmids containing a sequentially 5'-deleted human TPO promoter. Although GATA binding factors increased the promoter activity, their effect was independent of the GATA binding consensus sequence. On the other hand, Evi-1 did not affect transcription. Moreover, we defined the core promoter region, in which an Ets binding consensus sequence was located. The deletion or mutation of the Ets binding site resulted in a loss of the promoter activity. These results suggested that TPO is regulated by the Ets family of transcription factors.