We report herein that expression of ␣2␤1 integrin increased human erythroleukemia K562 transfectant (KX2C2) cell movement after extravasation into liver parenchyma. In contrast, a previous study demonstrated that ␣2␤1 expression conferred a stationary phenotype to human rhabdomyosarcoma RD transfectant (RDX2C2) cells after extravasation into the liver. We therefore assessed the adhesive and migratory function of ␣2␤1 on KX2C2 and RDX2C2 cells using a ␣2␤1-specific stimulatory monoclonal antibody (mAb), JBS2, and a blocking mAb, BHA2.1. In comparison with RDX2C2 cells, KX2C2 were only weakly adherent to collagen and laminin. JBS2 stimulated ␣2␤1-mediated interaction of KX2C2 cells with both collagen and laminin resulting in increases in cell movement on both matrix proteins. In the presence of Mn 2ϩ , JBS2-stimulated adhesion on collagen beyond an optimal level for cell movement. In comparison, an increase in RDX2C2 cell movement on collagen required a reduction in its adhesive strength provided by the blocking mAb BHA2.1. Consistent with these in vitro findings, in vivo videomicroscopy revealed that ␣2␤1-mediated postextravasation cell movement of KX2C2 cells in the liver tissue could also be stimulated by JBS2. Thus, results demonstrate that ␣2␤1 expression can modulate postextravasation cell movement by conferring either a stationary or motile phenotype to different cell types. These findings may be related to the differing metastatic activities of different tumor cell types. INTRODUCTIONIt is well established that ␤1 integrins represent the major receptors for providing the functional linkage between extracellular matrix (ECM) proteins and cytoskeletal components (for review, Hynes, 1992). The expression of ␣2␤1 integrin as receptors for collagen and laminin has been associated with the morphogenesis of mammary epithelial cells (Berdichevsky et al., 1992; Keely et al., 1995a,b) and differentiation of the human erythroleukemia cell line K562 (Burger et al., 1992). In comparison, ␣2␤1 expression was down-regulated on keratinocytes undergoing terminal differentiation (Adams and Watt, 1990). In addition, ␣2␤1 has also been associated with the metastatic activities of tumor cells. Interestingly, there is both a direct correlation (Dedhar and Saulnier, 1990;Chan et al., 1991;Klein et al., 1991;Mortarini et al., 1991;Danen et al., 1993;Chen et al., 1994;Santala et al., 1994) and an inverse correlation (Pignatelli et al., 1990(Pignatelli et al., , 1991Zutter, et al., 1990Zutter, et al., , 1995 of ␣2␤1 expression with tumor metastasis. The exact mechanisms whereby ␣2␤1 enhances or, in some cases, reduces the metastatic activities of tumor cells are still unclear. One possibility is that ␣2␤1 may confer distinct cell functions among the tumor cells. The present study focuses on ␣2␤1 interaction with ECM proteins and its in vivo effect on cell movement.Studies in recent years have demonstrated three distinct modes of ligand binding for ␣2␤1: no ligandbinding activity, binding collagen but not laminin, and binding both ...