BCR/ABL Expression of Myeloid Progenitors Increases β₁-Integrin Mediated Adhesion to Stromal Cells

The expression of the fusion protein BCR/ABL is a hallmark of chronic myeloid leukemia. BCR/ABL is a constitutively active tyrosine kinase influencing cell proliferation, apoptosis, and differentiation. To what extent and by which mechanism BCR/ABL affects the adhesion of leukemic cells to bone marrow stromal cells (BMSC) is controversial. To characterize adhesion of BCR/ABL-transformed 32D cells (32D-BCR/ABL) to the BMSC line M2-10B4, we used washing assays and single-cell force spectroscopy (SCFS). Compared to control 32D cells (32D-V), 32D-BCR/ABL developed threefold higher adhesion forces. This enhanced cell adhesion could be reduced to control levels after specifically inhibiting the activity of the tyrosine kinase BCR/ABL using imatinib mesylate (IM). SCFS showed that the adhesion forces of 32D-BCR/ABL were strongest to fibronectin and collagen type I, suggesting that β₁-integrin has a major role in mediating the adhesion of leukemic cells to BMSC. Indeed, the β₁-integrin blocking antibody Ha2/5 abrogated the attachment of 32D-V and 32D-BCR/ABL cells to BMSC. Although 32D-BCR/ABL cells show significantly increased β₁-integrin expression, no significant difference of β₁-integrin mRNA levels could be detected, indicating a post-transcriptional regulation of β₁-comprising integrin heterodimers by BCR/ABL. The data presented here argue that the interaction of β₁-integrin and extracellular matrix components is functionally important in leukemic cells expressing high-levels of BCR/ABL, and could provide a rationale for the development of optimized targeted therapies.