The unrestrained growth of tumor cells is generally attributed to mutations in essential growth control genes, but tumor cells are also influenced by signals from the environment. In multiple myeloma (MM), the factors and signals coming from the bone marrow microenvironment are possibly even essential for the growth of the tumor cells. As targets for intervention, these signals may be equally important as mutated oncogenes. Given their oncogenic potential, WNT signals form a class of paracrine growth factors that could act to influence MM cell growth. In this paper, we report that MM cells have hallmarks of active WNT signaling, whereas the cells have not undergone detectable mutations in WNT signaling genes such as adenomatous polyposis coli and ␤-catenin (CTNNB1). We show that the malignant MM plasma cells overexpress ␤-catenin, including its N-terminally unphosphorylated form, suggesting active ␤-catenin͞T cell factor-mediated transcription. Further accumulation and nuclear localization of ␤-catenin, and͞or increased cell proliferation, was achieved by stimulation of WNT signaling with either Wnt3a, LiCl, or the constitutively active S33Y mutant of ␤-catenin. In contrast, by blocking WNT signaling by dominant-negative T cell factor, we can interfere with the growth of MM cells. We therefore suggest that MM cells are dependent on an active WNT signal, which may have important implications for the management of this incurable form of cancer.␤-catenin ͉ T cell factor ͉ lymphoma ͉ plasma cell

Integrin-mediated adhesion and B cell antigen receptor (BCR) signaling play a critical role in B cell development and function, including antigen-specific B cell differentiation. Here we show that the BCR controls integrin α4β1 (VLA-4)-mediated adhesion of B cells to vascular cell adhesion molecule-1 and fibronectin. Molecular dissection of the underlying signaling mechanism by a combined biochemical, pharmacological, and genetic approach demonstrates that this BCR-controlled integrin-mediated adhesion requires the (consecutive) activation of Lyn, Syk, phosphatidylinositol 3-kinase, Bruton's tyrosine kinase (Btk), phospholipase C (PLC)γ2, IP3R-mediated Ca2+ release, and PKC. In contrast, activation of mitogen-activated protein kinase kinase (MEK) or extracellular signal–regulated kinase (ERK) is not required, and simultaneous activation of MEK, ERK, and PKB is not sufficient either. Furthermore, Btk is also involved in the control of integrin-mediated adhesion of preB cells. The control of integrin α4β1-mediated B cell adhesion by the BCR involves cytoskeletal reorganization and integrin clustering. These results reveal a novel function for the BCR and Btk, i.e., regulation of integrin α4β1 activity, thereby providing new insights into the control of B cell development and differentiation, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulineamia (XLA).

The evolution of multiple myeloma (MM) depends on complex signals from the bone marrow (BM) microenvironment, supporting the proliferation and survival of malignant plasma cells. An interesting candidate signal is hepatocyte growth factor/ scatter factor (HGF), since its receptor Met is expressed on MM cells, while HGF is produced by BM stromal cells and by some MM cell lines, enabling para-or autocrine interaction. To explore this hypothesis, we studied the biological effects of HGF stimulation on MM cell lines and on primary MMs. We observed that Met is expressed by the majority of MM cell lines and by approximately half of the primary plasma cell neoplasms tested. Stimulation of MM cells with HGF led to the activation of the RAS/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/protein kinase B (PI3K/PKB) pathways, signaling routes that have been implicated in the regulation of cell proliferation and survival. Indeed, functional studies demonstrated that HGF has strong proliferative and antiapoptotic effects on both MM cell lines and primary MM cells. Furthermore, by applying specific signal-transduction inhibitors, we demonstrated that MEK is required for HGF-induced proliferation, whereas activation of PI3K is required for both HGF-induced proliferation and for rescue of MM cells from apoptosis. Taken together, our data indicate that HGF is a potent myeloma growth and survival factor and suggest that the HGF/Met pathway is a potential therapeutic target in MM.