Activation-dependent α5β1 integrin-mediated adhesion to fibronectin decreases proliferation of chronic myelogenous leukemia progenitors and K562 cells

Chronic myelogenous leukemia (CML) is a malignant disease of the hematopoietic stem cell characterized by abnormal circulation and proliferation of malignant progenitors. In contrast to their normal counterparts, CML progenitors adhere poorly to bone marrow stroma or fibronectin (FN). Aside from anchoring progenitors in the marrow microenvironment, β1 integrin-dependent adhesion of normal progenitors is also associated with inhibition of their proliferation. As the β1 integrin expression on CML progenitors is normal, we hypothesized that decreased integrin affinity may underlie the abnormal adhesive and proliferative characteristics of CML progenitors. We examined the effect of affinity modulation by the activating antibody 8A2 on the adhesion and proliferation of CML progenitors and the CML cell line, K562. 8A2 induced α5β1-dependent adhesion of Philadelphia chromosome-positive (Ph+) CD34+/HLA-DR+ cells and K562 cells to FN. Increased adhesion was 8A2- and FN concentration-dependent, time-dependent, and energy-dependent. Further, 8A2-induced adhesion to FN significantly inhibited the proliferation of malignant CML progenitors as well as K562 cells independent of cell differentiation, necrosis, or apoptosis. These studies demonstrate that affinity modulation of the α5β1 integrin on CML progenitors and K562 cells by 8A2 results in increased adhesion to FN with subsequent decreased proliferation, suggesting that decreased β1 integrin affinity contributes to the abnormal circulation and proliferation of malignant progenitors in CML.