Activation of β1 integrins on CML progenitors reveals cooperation between β1 integrins and CD44 in the regulation of adhesion and proliferation

Adhesion of normal colony-forming cells (CFC) to bone marrow (BM) stroma and the extracellular matrix (ECM) component fibronectin (FN) depends at least in part on the α4β1 and α5β1 integrins and the CD44 receptor. Aside from anchoring progenitors in the marrow microenvironment, β1 integrin-dependent adhesion of normal CFC is associated with inhibition of their proliferation. In contrast to normal CFC, chronic myelogenous leukemia (CML) Ph+ CFC adhere significantly less to either stroma or FN. CML Ph+ CFC proliferation is also not inhibited by coculture with stroma or FN. However, equal numbers of α4, α5, and β1 integrins and CD44 are present on CML and normal CD34⁺ cells. We have previously demonstrated that β1-dependent adhesion to and subsequent proliferation inhibition by FN can be restored when CML Ph+ CFC are incubated with the β1 integrin activating antibody, 8A2, and demonstrated a role for the α5β1 integrin in this phenomenon. Since the integrin α4β1 and the proteoglycan form of CD44 may cooperate in establishing normal CFC adhesion to FN, we examined if treatment of CML Ph+ CFC with 8A2 also restores the cooperativity between β1 integrins and CD44. We demonstrate that 8A2 induces adhesion of CML Ph+ CFC not only to intact FN, but also to α4β1, α5β1, and proteoglycan binding fragments of FN. 8A2-induced adhesion to these fragments and peptides also results in a significant inhibition of the proliferation of CML Ph+ CFC. Addition of antibodies to either the α5, α4, or β1 integrins, antibodies against the CD44 receptor, or removal of chondroitin sulfate glycosaminoglycans from the surface of CML CD34⁺ HLA-DR⁺ cells significantly reduced the 8A2-induced adhesion to and adhesion-mediated inhibition of proliferation by FN. These studies demonstrate that activation of β1 integrins on CML Ph+ CFC not only results in upregulation of β1 integrin-dependent adhesion and adhesion-mediated inhibition of proliferation, but also in the restoration of cooperation between β1 integrins and CD44. These studies suggest that decreased β1 integrin avidity may also affect the function of the proteoglycan adhesion receptor CD44, both of which may contribute to the abnormal circulation and expansion of malignant progenitors in CML.