Transcriptional start site in the mouse Cyp1a1 (cytochrome P1450) gene

Two transcriptional initiation sites have been described by different laboratories for the mouse Cyp1a1 (cytochrome P1450) gene: start site TI, at the beginning of the nontranslated 87-bp first exon, and start site TII, 635 bp downstream from site TI within the 2,380-bp first intron. Site TI was characterized in normal C57BL/6 mouse liver, whereas site TII was described in a high activity variant line (HAV cells) derived from the mouse hepatoma Hepa-1c1c7 wild type (wt) established cell line. It is conceivable that a tumor-specific promoter might be used in the Cyp1a1 gene in malignant tissue, as compared with an alternative transcriptional start site in normal tissue. To test this hypothesis, we constructed fusion plasmids containing the two putative transcription initiation sites and performed transcriptional mapping on the Cyp1a1 transcripts from control and tetrachlorobenzo-p-dioxin (TCDD)-treated wt and HAV cells; Cyp1a1 mRNA sequencing was also carried out in the TCDD-treated wt and HAV cell lines. We conclude that the Cyp1a1 transcription initiation site in both of these hepatoma-derived cell lines is the same (start site TI) as that used in normal mouse liver.