Azacitidine Blocks GATA-1-Mediated Repression of the PU.1 Gene in Human Leukemic Cells

Introduction: GATA-1 and PU.1 are two important hematopoietic transcription factors that mutually inhibit each other in progenitor cells to guide entrance into the erythroid or myeloid lineage, respectively. Expression of PU.1 is controlled by several transcription factors including PU.1 itself by binding to the distal URE enhancer (upstream regulatory element) whose deletion leads to acute myeloid leukemia (AML) (Rosenbauer F et al. 2004). Co-expression of PU.1 and GATA-1 in AML-erythroleukemia (EL) blasts prevents efficient differentiation regulated by these transcription factors. Inhibition of transcriptional activity of PU.1 protein by GATA-1 has been reported (Nerlov C et al. 2000), however it is not known whether GATA-1 can inhibit PU.1 gene in human early erythroblasts directly. We have recently found that MDS/AML erythroblasts display repressive histone modifications and DNA methylation status of PU.1 gene that respond to 5-azacitidine (AZA) leading to inhibited blast cell proliferation and stimulated myeloid differentiation (Curik N et al. 2012). We hypothesize that l eukemia blockade during early erythroid differentiation includes direct GATA-1-mediated inhibition of the PU.1 gene.

Results: We herein document the GATA-1 mediated repression of the PU.1 gene in human EL cell lines (OCI-M2 and K562) together with the recruitment of DNA methyl transferase I (DNMT1) to the URE known to guide most of the PU.1 gene transcription. Repression of the PU.1 gene involves both DNA methylation at the URE and methylation/deacetylation of the histone H3 lysine-K9 residue and methylation of H3K27 at additional DNA elements and the PU.1 promoter. Inhibition of GATA-1 by siRNA as well as the AZA treatment in AML-EL led to the significant DNA-demethylation of the URE thorough the mechanism of DNMT1 depletion leading to upregulation of the PU.1 expression.

Conclusions: Our data indicate that GATA-1 binds to the PU.1 gene at the URE and initiate events leading to the PU.1 gene repression in human ELs. The mechanism includes repressive epigenetic remodeling of the URE that is important for the PU.1 downregulation and leukemogenesis and that is also simultaneously sensitive to the DNA demethylation treatment with AZA. The GATA-1-mediated inhibition likely contributes to the PU.1 downregulation during progenitor cell differentiation that could be employed during leukemogenesis. Importantly, we also observed important differences between murine and human ELs and found that repression of the PU.1 gene in human ELs can become reverted by the epigenetic therapy with AZA. Our work also suggests that hypomethylating therapy using DNA methylation inhibitors in MDS/AML may become potentially effective in MDS/EL patients. We think that during early erythroid differentiation the GATA-1 binds and represses the PU.1 gene, however this is not fully completed in EL and therefore the erythroid as well as myeloid differentiation are blocked. Grants: GACR P305/12/1033, UNCE 204021, PRVOUK-P24/LF1/1.