Activation of beta 1 integrins on CML progenitors reveals cooperation between beta 1 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 alpha 4 beta 1 and alpha 5 beta 1 integrins and the CD44 receptor. Aside from anchoring progenitors in the marrow microenvironment, beta 1 integrin-dependent adhesion of normal CFC is associated with inhibition of their proliferatian. 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 alpha 4, alpha 5, and beta 1 integrins and CD44 are present on CML and normal CD34(+) cells, We have previously demonstrated that beta 1-dependent adhesion to and subsequent proliferation inhibition by FN can be restored when CML Ph+ CFC are incubated with the beta 1 integrin activating antibody, 8A2, and demonstrated a role for the alpha 5 beta 1 integrin in this phenomenon, Since the integrin alpha 4 beta 1 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 beta 1 integrins and CD44. We demonstrate that 8A2 induces adhesion of CML Ph+ CFC not only to intact FN, but also to alpha 4 beta 1, alpha 5 beta 1, 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 alpha 5, alpha 4, or beta 1 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 beta 1 integrins on CML Ph+ CFC not only results in upregulation of beta 1 integrin-dependent adhesion and adhesion-mediated inhibition of proliferation, but also in the restoration of cooperation between beta 1 integrins and CD44, These studies suggest that decreased beta 1 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.