Lower Phosphorylation of p38 MAPK Blocks the Oxidative Stress-induced Senescence in Myeloid Leukemic CD34+CD38- Cells

Leukemia seems to depend on a small population of leukemia stem cells (LSCs) for its growth and metastasis. However, the precise surviving mechanisms of LSCs remain obscure. Cellular senescence is an important obstacle for production and surviving of tumor cells. In this study we investigated the activated state of a pathway, in which reactive oxygen species (ROS) induces cellular senescence through DNA damage and phophorylation of p38 MAPK (p38), in myeloid leukemic CD34(+)CD38(-) cells. Bone marrow samples were obtained from patients with acute myeloid leukemia (AML, n=11) and chronic myeloid leukemia (CML, n=9). CD34(+)CD38(-) cells were isolated from mononuclear cells from these bone marrow samples, and K562 and KG1a cells (two kinds of myeloid leukemia cell lines) by mini-magnetic activated cell sorting. Hematopoietic stem cells (HSCs) from human cord blood served as controls. Intracellular ROS level was detected by flow cytometry. DNA damage defined as the gamma H2AX level was measured by immunofluorescence staining. Real-time RT-PCR was used to detect the expression of p21, a senescence-associated gene. Western blotting and immunofluorescence staining were employed to determine the p38 expression and activation. The proliferation and apoptosis of CD34(+)CD38(-) cells were detected by MTT assay and flow cytometry. Our results showed that ROS and DNA damage were substantially accumulated and p38 was less phosphorated in myeloid leukemic CD34(+)CD38(-) cells as compared with HSCs and H2O2-induced senescent HSCs. Furthermore, over-phosphorylation of p38 by anisomycin, a selective activator of p38, induced both the senescence-like growth arrest and apoptosis of CD34(+)CD38(-) cells from K562 and KG1a cell lines. These findings suggested that, although excessive accumulation of oxidative DNA damage was present in LSCs, the relatively decreased phosphorylation of p38 might help leukemic cells escape senescence and apoptosis.