Tin mesoporphyrin (SnMP), a competitive heme oxygenase (HO) inhibitor, also 
induces HO-1 mRNA and protein expression by a mechanism that is not fully 
understood. We examined whether the induction by SnMP is mediated by a 
de-repression of Bach1, a transcription factor that suppresses the HO-1 gene. 
Incubation of NIH3T3-HO-1-luc cells with SnMP attenuated HO activity with a 
concomitant increase in HO-1 mRNA and protein and a decrease in Bach1 and HO-2 
proteins, which was not due to transcriptional down-regulation, but accelerated 
protein decay. Similarly, HO-1 protein degradation was increased by SnMP, 
despite of an elevation in HO-1 transcription. Transfection of Bach1 shRNA in 
Hepa cells raised basal HO-1 expression significantly, and SnMP treatment 
further increased HO-1 mRNA. In conclusion, SnMP induces HO-1 expression not 
only by de-repressing the HO-1 promoter by binding Bach1, but also by 
accelerating Bach1 degradation.