The 3′-untranslated region of the mouse cholesterol 7α-hydroxylase mRNA contains elements responsive to post-transcriptional regulation by bile acids

Abstract

To investigate the importance of the 3ʹ-untranslated region (UTR) of the mouse cholesterol 7α-hydroxylase (cyp7) mRNA in post-transcriptional regulation of expression of the cyp7 gene, chimaeric genes encoding mRNA containing the structural sequence of chloramphenicol acetyltransferase (CAT) linked to either the 3ʹ-UTR of the mouse cyp7 mRNA or the SV40 early gene mRNA were constructed. The human cytomegalovirus (CMV) promoter was used to drive the expression of all the chimaeric genes. Thus the transgenes had identical sequences in the promoter, the regions encoding the 5ʹ-UTR and translated sequence but differed in the region encoding the 3ʹ-UTR of their respective mRNA species. The transgene containing the entire cyp7 3ʹ-UTR (designated CMV.CAT.CYP7) gave rise to CAT activity in transfected hepatoma cells that was one-quarter of that obtained in cells transfected with the transgene containing the SV40 3ʹ-UTR (designated CMV.CAT.SV40). The 3ʹ-UTR of the cyp7 mRNA contains sequences resembling AU-rich elements (AREs). Deleting eight of nine putative AREs from the CYP7 3ʹ-UTR sequence increased the CAT activity to a level greater than that observed for CMV.CAT.SV40, whereas deletion of the intron region had no effect. These results show that the AREs of the 3ʹ-UTR of the cyp7 mRNA decrease transgene expression. Bile acids are known to repress the expression of the cyp7 gene. To test whether the 3ʹ-UTR of the cyp7 mRNA has a role in this process, the expression of the chimaeric genes was evaluated in hepatoma cells competent for bile acid uptake. Conjugated bile acids, but not unconjugated bile acids, further decreased the expression of the CMV.CAT.CYP7 transgene. The same bile acids had no effect on the expression of the CMV.CAT.SV40 transgene. Deletion of the intron from the cyp7 sequence did not alter the CAT activity compared with the parental plasmid, and also did not alter the sensitivity of the transgene to the conjugated bile acids. Deletion of the AREs from the cyp7 3ʹ-UTR, which increased the expression of the transgene, did not abolish the sensitivity of the transgene to repression by conjugated bile acids. Thus the 3ʹ-UTR of the mouse cyp7 mRNA also contains elements that facilitate the further repression of transgene expression in the presence of conjugated bile acids. The results indicate that the 3ʹ-UTR of the mouse cyp7 mRNA contains information specifying regulation at the post-transcriptional level.