Interplay between hnRNP Al and a cis-acting element in the 3′ UTR of CYP2A5 mRNA is central for high expression of the gene

Our previous evidence suggests that heterogeneous nuclear ribonucleoprotein (hnRNP) At plays a part in the regulation of the Cyp2a5 gene by interacting with the 3' untranslated region (UTR) of the CYP2A5 mRNA. However, the exact role of this interaction is not clear. The aim of the present work was to gain further insight into the regulation process of Cyp2a5. For this purpose the 3' UTR of CYP2A5 was fused to the coding region of luciferase mRNA. Luciferase recombinants containing either the full length 3' UTR, or the 3' UTR lacking a previously described 71 nucleotide (nt) region (the hnRNP At primary binding site), were transiently expressed in cells expressing or lacking hnRNP A1. The expression of the luciferase recombinants was examined both at mRNA and enzyme activity levels. The results disclosed that the presence of hnRNP At was required for the high expression of the recombinant carrying the full length 3' UTR of CYP2A5. Deletion of the hnRNP At primary binding site dramatically modified the expression pattern: the mRNA levels and luciferase activities of the deletion mutant were independent from hnRNP At. These results conclusively demonstrate that the 71 nt region in the 3' UTR of CYP2A5 mRNA can confer hnRNP A1-dependent regulation to a gene. In addition, comparison of RNA levels and luciferase activities suggested that regions flanking the hnRNP At binding site could regulate translation of the CYP2A5 mRNA. These results are consistent with a model in which the binding of hnRNP At to the 71 nt putative hairpin-loop region in the CYP2A5 mRNA 3' UTR upregulates mRNA levels possibly by protecting the mRNA from degradation. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.