Bone marrow stromal cells protect acute myeloid leukemia cells from anti-CD44 therapy partly through regulating PI3K/Akt-p27(Kip1) axis

The anti-CD44 monoclonal antibody (mAb) A3D8 induces differentiation or apoptosis in vitro in various subtypes of acute myeloid leukemia (AML) via p27(Kip1) upregulation. Bone marrow (BM) stromal cells play a vital role in the development of chemoresistance in AML cells attached to the stroma. To investigate the effect of BM stroma adhesion induced AML resistance to A3D8, we developed a co-culture system composed of an AML-derived cell line (NB4) cultured with either a human BM stroma cell line (HS-5) or mesenchymal stem cells (MSCs). We found that NB4 cells adhered to HS-5 cells or MSCs developed resistance against the anti-proliferative effects of A3D8, and this action is caused by the activation of PI3K/Akt signaling following p27(Kip1) down-regulation and cytoplasmic re-localization. The stromal co-culture-induced resistance can be partially abolished by inhibiting the PI3K/Akt signaling pathway. Such findings were confirmed in two additional AML-derived cell lines as well as in primary AML cells. Our results suggest that BM stroma can induce A3D8 resistance in part via the PI3K/Akt-p27(Kip1) axis, and blocking PI3K/Akt pathway maybe necessary for anti-CD44 treatment on AML in BM microenvironment.