Human CYP2E1 is one of the pharmacologically and toxicologically important cytochrome P450 isoforms. Earlier studies have reported that the CYP2E1 expression is extensively regulated by post-transcriptional and post-translational mechanisms, but the molecular basis remains unclear. In the present study, we examined the possibility that microRNA may be involved in the post-transcriptional regulation of human CYP2E1. In silico analysis identified a potential recognition element of miR-378 (MRE378) in the 3'-untranslated region (UTR) of human CYP2E1 mRNA. Luciferase assays using HEK293 cells revealed that the reporter activity of the plasmid containing the MRE378 was decreased by co-transfection of precursor miR-378, indicating that miR-378 functionally recognized the MRE378. We established two HEK293 cell lines stably expressing human CYP2E1 including or excluding 3'-UTR. When the precursor miR-378 was transfected into the cells expressing human CYP2E1 including 3'-UTR, the CYP2E1 protein level and chlorzoxazone 6-hydroxylase activity were significantly decreased, but were not in the cells expressing CYP2E1 excluding 3'-UTR. In both cell lines, the CYP2E1 mRNA levels were decreased by overexpression of miR-378, but miR-378 did not affect the stability of CYP2E1 mRNA. In a panel of 25 human livers, no positive correlation was observed between the CYP2E1 protein and CYP2E1 mRNA levels, supporting the post-transcriptional regulation. Interestingly, the miR-378 levels were inversely correlated with the CYP2E1 protein levels and the translational efficiency of CYP2E1. In conclusion, we found that human CYP2E1 expression is regulated by miR-378, mainly via translational repression. This study could provide new insight into the unsolved mechanism of the post-transcriptional regulation of CYP2E1.
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