STAT5A Regulates DNMT3 and Inactivates PTEN Tumor Suppressor Gene in CD34⁺/CD38⁻ AML Cells

Abstract 4087

Acute myelogenous leukemia (AML) is organized as a cellular hierarchy, initiated and maintained by a subset of self renewing leukemia stem cells (LSCs). We have recently reported that signal transducer and activator of transcription 5 (STAT5) is activated in CD34⁺/CD38⁻ AML cells which contain abundant leukemia stem cells (LSCs) and inhibition of STAT5 induced apoptosis and sensitized these cells to the growth inhibition mediated by conventional chemotherapeutic agents (Int J Cancer. 2011;128:2317-25). This study attempted to identify molecules which are regulated by STAT5 and support survival of CD34⁺/CD38⁻ AML cells. We lentivirally transduced a short hairpin (sh) RNA targeting STAT5 in CD34⁺/CD38⁻ AML cells and compared their gene expression profiles with those of control shRNA transduced CD34⁺/CD38⁻ AML cells by microarray analysis. Interestingly, DNA methyltransferase (DNMT) 3A, which catalyzes the transfer of a methyl group to DNA was differentially expressed between control and STAT5A down-regulated AML cells. In contrast, lentiviral transduction of STAT5A in CD34⁺/CD38⁻ AML cells increased levels of DNMT3A. Reporter gene assays found that exposure of leukemia cells to IL-10 increased levels of the phosphorylated forms of STAT5, leading to transcriptional activation of DNMT3A. On the other hand, dephosphorylation of STAT5A by AZ960, a selective JAK2 inhibitor decreased this transcriptional activity in leukemia cells. Of note, forced expression of STAT5A in leukemia cells caused hypermethylation on the promoter region of tumor suppressor gene PTEN and down-regulated its mRNA levels as measured by methylation-specific PCR and real-time RT-PCR, respectively. Taken together, STAT5A may positively regulate levels of DNMT3A, resulting in inactivation of tumor suppressor gene by epigenetic mechanisms in LSCs.