The let-7 microRNA (miRNA) plays important roles in human liver development and 
diseases such as hepatocellular carcinoma, liver fibrosis and hepatitis wherein 
oxidative stress accelerates the progression of these diseases. To date, the 
role of the let-7 miRNA family in modulation of heme oxygenase 1 (HMOX1), a key 
cytoprotective enzyme, remains unknown. Our aims were to determine whether let-7 
miRNA directly regulates Bach1, a transcriptional repressor of the HMOX1 gene, 
and whether indirect up-regulation of HMOX1 by let-7 miRNA attenuates oxidant 
injury in human hepatocytes. The effects of let-7 miRNA on Bach1 and HMOX1 gene 
expression in Huh-7 and HepG2 cells were determined by real-time qRT-PCR, 
Western blot, and luciferase reporter assays. Dual luciferase reporter assays 
revealed that let-7b, let-7c, or miR-98 significantly decreased Bach1 
3'-untranslated region (3'-UTR)-dependent luciferase activity but not mutant 
Bach1 3'-UTR-dependent luciferase activity, whereas mutant let-7 miRNA 
containing base complementarity with mutant Bach1 3'-UTR restored its effect on 
mutant reporter activity. let-7b, let-7c, or miR-98 down-regulated Bach1 protein 
levels by 50-70%, and subsequently up-regulated HMOX1 gene expression by 3-4 
fold, compared with non-specific controls. Furthermore, Huh-7 cells transfected 
with let-7b, let-7c or miR-98 mimic showed increased resistance against oxidant 
injury induced by tert-butyl-hydroperoxide (tBuOOH), whereas the protection was 
abrogated by over-expression of Bach1. In conclusion, let-7 miRNA directly acts 
on the 3'-UTR of Bach1 and negatively regulates expression of this protein, and 
thereby up-regulates HMOX1 gene expression. Over-expression of the let-7 miRNA 
family members may represent a novel approach to protecting human hepatocytes 
from oxidant injury.