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The BCR/ABL oncogene causes chronic myelogenous leukemia (CML) in humans and a CML-like disease, as well as lymphoid leukemia, in mice. p210 BCR/ABL is an activated tyrosine kinase that phosphorylates itself and several cellular signaling proteins. The autophosphorylation site tyrosine 177 binds the adaptor Grb2 and helps determine the lineage and severity of BCR/ABL disease: Tyr177 mutation (BCR/ABL-Y177F) dramatically impairs myeloid leukemogenesis, while diminishing lymphoid leukemogenesis. The critical signal(s) from Tyr177 has remained unclear. We report that Tyr177 recruits the scaffolding adaptor Gab2 via a Grb2/Gab2 complex. Compared to BCR/ABL-expressing Ba/F3 cells, BCR/ABL-Y177F cells exhibit markedly reduced Gab2 tyrosine phosphorylation and association of phosphatidylinositol-3 kinase (PI3K) and Shp2 with Gab2 and BCR/ABL, and decreased PI3K/Akt and Ras/Erk activation, cell proliferation, and spontaneous migration. Remarkably, bone marrow myeloid progenitors from Gab2 (-/-) mice are resistant to transformation by BCR/ABL, whereas lymphoid transformation is diminished as a consequence of markedly increased apoptosis. BCR/ABL-evoked PI3K/Akt and Ras/Erk activation also are impaired in Gab2 (-/-) primary myeloid and lymphoid cells. Our results identify Gab2 and its associated proteins as key determinants of the lineage and severity of BCR/ABL transformation.

In multiple myeloma (MM), migration is necessary for the homing of tumor cells to bone marrow (BM), for expansion within the BM microenvironment, and for egress into the peripheral blood. In the present study we characterize the role of vascular endothelial growth factor (VEGF) and beta(1) integrin (CD29) in MM cell migration. We show that protein kinase C (PKC) alpha is translocated to the plasma membrane and activated by adhesion of MM cells to fibronectin and VEGF. We identify beta(1) integrin modulating VEGF-triggered MM cell migration on fibronectin. We show that transient enhancement of MM cell adhesion to fibronectin triggered by VEGF is dependent on the activity of both PKC and beta(1) integrin. Moreover, we demonstrate that PKC alpha is constitutively associated with beta(1) integrin. These data are consistent with PKC alpha-dependent exocytosis of activated beta(1) integrin to the plasma membrane, where its increased surface expression mediates binding to fibronectin; conversely, catalytically active PKC alpha-driven internalization of beta(1) integrin results in MM cell de-adhesion. We show that the regulatory subunit of phosphatidylinositol (PI) 3-kinase (p85) is constitutively associated with FMS-like tyrosine kinase-1 (Flt-1). VEGF stimulates activation of PI 3-kinase, and both MM cell adhesion and migration are PI 3-kinase-dependent. Moreover, both VEGF-induced PI 3-kinase activation and beta(1) integrin-mediated binding to fibronectin are required for the recruitment and activation of PKC alpha. Time-lapse phase contrast video microscopy (TLVM) studies confirm the importance of these signaling components in VEGF-triggered MM cell migration on fibronectin.

CBL and the related CBL-B protein are two members of a family of RING finger type ubiquitin E3 ligases that are believed to function as negative regulators of signal transduction in hematopoietic and immune cells. In mice, expression of v-Cbl causes lymphomas, and targeted disruption of either the CBL gene or the CBL-B gene can result in a lymphoproliferative disorder or hypersensitivity of lymphocytes. CBL is one of the most prominent targets of the BCR/ABL tyrosine kinase oncogene. We compared the role of CBL and CBL-B in signal transduction of BCR/ABL using pairs of cell lines before and after expression of BCR/ABL. In contrast to CBL, BCR/ABL was found to rapidly downregulate the expression of CBL-B protein. The decrease in CBL-B protein induced by BCR/ABL was associated with downregulation of CBL-B mRNA. Downregulation and tyrosine phosphorylation of CBL-B required BCR/ABL kinase activity. However, despite their known similarities in structure and function, we found CBL and CBL-B proteins to be involved in distinct signaling complexes. CBL was predominantly in a complex with phosphatidylinositol 3'-kinase and CRKL, while CBL-B was not associated with any significant phosphatidylinositol 3'-kinase activity. A major CBL-B associated protein was identified as mono-ubiquitinated Vav, a nucleotide exchange factor for Rac1. These results demonstrate that BCR/ABL signals differentially through CBL and CBL-B, with downregulation of the CBL-B protein potentially contributing to BCR/ABL-mediated transformation.