The
transcription factor Bach1 heterodimerizes with small
Maf proteins to repress Maf recognition
element (MARE)-dependent gene expression. The repressor activity of Bach1 is inhibited by the direct binding of
heme. To investigate the involvement of Bach1 in the
heme-dependent regulation of the expression of the
beta-globin gene, mouse
erythroleukemia (MEL) cells were cultured with
succinylacetone (SA), a specific inhibitor of
heme biosynthesis, and the level of
beta-globin mRNA was examined. A marked decrease of
beta-globin mRNA in SA-treated cells was observed, and this decrease was reversed by the addition of
hemin. An
iron chelator,
desferrioxamine, also lowered the level of
beta-globin mRNA. The
heme-dependent expression of
beta-globin is a transcriptional event since the expression of the human
beta-globin gene promoter-reporter gene containing the microlocus control region (microLCR) was inhibited when human
erythroleukemia K562 cells and MEL cells were cultured with SA.
Hemin treatment restored the decrease in promoter activity caused by SA. The control of the microLCR-
beta-globin promoter reporter gene by
heme was dependent on
DNase I-hypersensitive site 2 (HS2), which contains MARE. The MARE binding activity of Bach1 in K562 and MEL cells increased upon SA treatment, and the increase was diminished by the treatment with
hemin. Transient expression of Bach1 suppressed the microLCR activity, and this repressor activity was cancelled by treatment with
hemin. The expression of a mutated Bach1 lacking
heme-binding sites led to a loss in the
heme responsiveness of the microLCR. Furthermore,
chromatin immunoprecipitation experiments revealed that Bach1 bound to the MARE of HS2 increased by the treatment of MEL cells with SA, and this was cancelled by
hemin. We propose that
heme positively regulates the
beta-globin gene expression by blocking the interaction of Bach1 with the MARE in the LCR.