Stromal Cell Line Enhances Survival of Myeloid Leukemia Cells by Regulating the Expression of bcl-2 Family Genes and Caspase-3.

Approximately 20–30% acute myeloid leukemia (AML) patients are not cured by traditional chemotherapy. The possible explanation is that residual leukemia cells are harbored in association with stromal cells in the bone marrow, which causes late relapse of leukemia. This study was designed to investigate interactions between marrow stromal microenvironment and leukemia cells. AML cells were cultured in different conditions with or without serum, Ara-c, the monolayers or the conditioned medium of a stromal cell line HESS-5. Apoptosis frequencies, cell viability, cell cycle distribution, DNA fragment, and bcl-2/bcl-xl gene as well as caspase-3 expression were determined by Typan blue exclusion, MTT assay, flow cytometry analysis, PCR and Western blot respectively. Direct contact of HL-60 cells with HESS-5 significantly inhibited serum-deprivation- or Ara-c-induced apoptosis of HL-60 cells, and resulted in a significant increase of short-term proliferation and viability. Soluble factors had negligible effect. The interaction of HL-60 with fibronectin did not confer a significant resistance to serum-deprivation and Ara-c agent. HESS-5 cells were also able to maintain cell cycle progression of HL-60 during chemotherapy exposure. HESS-5 cells-mediated apoptosis inhibition was associated with increased bcl-2/bcl-xl expression and down-regulation of caspase-3 in primary myeloid leukemia cells. These results indicate that stromal cells enhance the survival of myeloid leukemia cells by regulating bcl-2 family genes as well as caspase-3 protein and suggest that specific interactions between stromal cells and leukemia cells may enhance the resistance of leukemia cells to chemotherapy.