The small GTPase Ral mediates SDF-1–induced migration of B cells and multiple myeloma cells

Abstract

IntroductionDepending on their stage of development or differentiation, B cells express specific chemokine receptors enabling them to respond to chemokines that control homing to the bone marrow (BM) and secondary lymphoid organs. [1][2][3] Here they are provided with signals crucial for progression through the consecutive stages of B-cell development and differentiation. The chemokine stromal cellderived factor-1 (SDF-1, also known as CXCL12) plays an important role in lymphocyte trafficking. SDF-1, originally identified as a growth-stimulatory factor for pre-B cells, 4 is constitutively expressed by BM stromal cells. [5][6][7] Its cognate receptor, the 7-transmembrane G-protein-coupled receptor CXCR4, is broadly expressed by cells of the immune system and mediates SDF-1-induced migration of hematopoietic progenitors and lymphocytes. [1][2][3]8,9 In the B-cell lineage, CXCR4 is prominently expressed by pre-B cells and controls localization in cellular niches in the BM obligatory for B-cell development. 3,8 Furthermore, CXCR4 expression by mature B cells is required for proper germinal center organization, which is essential for antigen-specific B-cell differentiation. 10 In addition, SDF-1 controls plasma cell migration, 6 and plasma cells require CXCR4 expression to localize in the BM compartment. 11,12 Multiple myeloma (MM), an incurable hematologic malignancy with a median survival of 3 to 4 years, is characterized by the expansion of malignant plasma cells in the BM. These tumor cells also express CXCR4, 13 and SDF-1 controls their ␣4␤1-mediated adhesion to VCAM-1, fibronectin, and endothelial cells, as well as their transendothelial migration and homing/retention in the BM tumor microenvironment. [13][14][15][16][17][18][19] MM cells are critically dependent on the BM microenvironment, where cytokines produced by BM stromal cells provide them with proliferation and survival signals required for their expansion. 16 Thus, by controlling homing and retention of MM cells in the BM, SDF-1 plays an important role in the pathogenesis of MM.The important role of chemokine-induced migration in B-cell development and differentiation, as well as in the pathogenesis of MM, prompted us to explore the underlying signaling pathways. Recently, we have established an important role for Bruton tyrosine kinase (Btk) and phospholipase C-␥2 (PLC␥2) in chemokineinduced integrin-mediated migration and homing of B cells. 20 Importantly, however, Btk is not expressed in plasma cells and MM cells. [21][22][23] Several members of the Ras superfamily of small GTPases have also been implicated in chemokine-controlled B-cell migration. [24][25][26][27] This includes members of the Rho family 24,25 and members of the Ras family, such as Ras and Rap1. 26,27 Ral, another member of the Ras family of small GTPases, has been shown to mediate a wide variety of cellular responses, including regulation of cytoskeletal rearrangements and cell motility. 28 Methods MaterialsMonoclonal antibodies used were mouse anti-RalA (IgG2a), anti-Rap1 (IgG1)...

Dissection of RAS downstream pathways in melanomagenesis: a role for Ral in transformation

“…Activated RalGEFs can transform human and rodent cells (Urano et al , 1996; Hamad et al , 2002; Rangarajan et al , 2004; Lim et al , 2005), but activating mutations have yet to be detected in human melanoma (Omholt and Hansson, 2007). Loss of Ral function in genetically engineered mice and in cultured cells through RNAi-mediated knockdown results in reduction in the malignant phenotype (Chien and White, 2003; Oxford et al , 2005; Chien et al , 2006; Lim et al , 2006; Rosse et al , 2006; de Gorter et al , 2008; C Counter, personal communication). Moreover, the RalGEF-Ral pathway has been shown to promote oncogenic transformation in several KRas- and HRas-driven cancer cell lines and tumor tissues (Lim et al , 2005, 2006; Smith et al , 2007).…”

Beyond Desensitization: Physiological Relevance of Arrestin-Dependent Signaling

“…Of these, the involvement of G proteinindependent signaling is most convincing for the AT 1A receptor, because cells will follow a gradient of the arrestin pathway-selective AT 1A receptor agonist Sar 1 -Ile 4 -Ile 8 , a response that is lost when arrestins are down-regulated. In addition to regulating cortical actin assembly through arrestin-dependent activation of ERK1/2 and a cofilin-LIM kinase-chronophin complex (DeFea, 2007), recent evidence suggests that other arrestin-regulated effectors, including the Src family kinase, Lyn (Cheung et al, 2009), PP2A (Basu et al, 2007), and the small GTPase, Ral (de Gorter et al, 2008;Li et al, 2009) are involved in chemokineinduced chemotaxis in vitro.…”

The RAS-RAL axis in cancer: evidence for mutation-specific selectivity in non-small cell lung cancer

“…RALA is also promigratory, assisting in the metastasis of prostate cancer to bone and aiding in the invasion of squamous cell carcinoma of the skin and renal cancers [28,68,70,71] . RALB, however, is important for the invasion and migration of bladder cancer, oral squamous cell carcinoma, B cells and multiple myeloma cells [53,72,73] . RALBP1 has been identified as the main RAL effector in the majority of these tumors.…”