Vav3 and Rac2 Activation Are Required for B Progenitor Transformation Induced by p190-BCR-ABL.

Abstract 449

The Philadelphia chromosome-positive (Ph⁺) hematologic malignancies are characterized by the reciprocal translocation between the BCR serine/threonine kinase and the ABL tyrosine kinase genes. Fifty percent of adult B-cell Ph+ acute lymphoblastic leukemia (B-ALL) and most cases of pediatric Ph+ B-ALL express the p190 BCR-ABL isoform, which causes abnormal expansion of lymphoid progenitors and a poor prognosis despite the administration of tyrosine kinase inhibitors. A major signaling pathway activated by BCR-ABL is the Rac/Rho GTPase signaling pathway. We had previously demonstrated that p210-BCR-ABL activates Rac GTPases and the deficiency of Rac2, or even more, the deficiency of both Rac1 and Rac2, impairs myeloid leukemogenesis induced by p210 BCR-ABL in vitro and in vivo (Thomas EK et al., Cancer Cell 2007). The p190-BCR-ABL isoform also activates Rac GTPases and the deficiency of Rac2 induces resistance to develop proB-ALL (Sanchez-Aguilera et al., ASH 2008). Since p190-BCR-ABL is devoid of guanine nucleotide exchange factor (GEF) activity through the absence of the BCR Dbl-homology (DH) domain, it probably requires the recruitment of intermediate GEFs to activate Rac GTPases. The Vav family of GEF has been shown to bind the SH3/SH2 domains of ABL and become activated. Three isoforms of Vav (Vav1, Vav2 and Vav3) are expressed in hematopoietic cells. Several studies have indicated that Vav3 has a role distinct from Vav1 or Vav2. Here, we hypothesized that the hematopoietic specific Rho GTPase, Rac2, may mediate key signals in the transformation of B-cell progenitors upon Vav-mediated activation. We have found that both Vav1 and Vav3 are overexpressed and hyperphosphorylated in p190 BCR-ABL expressing human B-ALL cells as well as in BCR-ABL transduced murine proB cells. While the combined deficiency of Vav1 and Vav2 does not impair leukemogenesis in vivo, Vav3 deficiency reverses p190-BCR-ABL induced transformation phenotypes. Specifically, Vav3 deficiency in p190-BCR-ABL expressing proB-ALL cells associated with a significant reduction of Rho and Rac activation (75% and 56%), apoptosis induction (4-fold) and impaired ex-vivo expansion (33% reduction). In addition, p190-BCR-ABL-induced adhesion-to-fibronectin defect and hypermigratory response to CXCL12 were corrected by deletion of Vav3 (2.1-fold increase and 60% decrease, respectively), suggesting that Vav3 deficiency reverses the transformation-associated phenotype associated to BCR-ABL expression. Collectively, our work identifies a critical role of Rac2 and its activator Vav3 for p190 BCR-ABL induced B-cell progenitor leukemic transformation and may define a novel therapeutic target for this poor prognosis disease.