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The B Cell Antigen Receptor Controls AP-1 and NFAT Activity through Ras-Mediated Activation of Ral¹

Department of Pathology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands

Signaling by the BCR involves activation of several members of the Ras superfamily of small GTPases, among which is Ras itself. Ras can control the activity of multiple effectors, including Raf, PI3K, and guanine nucleotide exchange factors for the small GTPase Ral. Ras, Raf, and PI3K have been implicated in a variety of processes underlying B cell development, differentiation, and function; however, the role of Ral in B lymphocytes remains to be established. In this study, we show that Ral is activated upon BCR stimulation in human tonsillar and mouse splenic B lymphocytes and in B cell lines. Using signaling molecule-deficient B cells, we demonstrate that this activation is mediated by Lyn and Syk, Btk, phospholipase C-2, and inositol-1,4,5-trisphosphate receptor-mediated Ca²⁺ release. In addition, although Ral can be activated by Ras-independent mechanisms, we demonstrate that BCR-controlled activation of Ral is dependent on Ras. By means of expression of the dominant-negative mutants RasN17 and RalN28, or of RalBPGAP, a Ral effector mutant which sequesters active Ral, we show that Ras and Ral mediate BCR-controlled transcription of c-fos. Furthermore, while not involved in NF-B activation, Ras and Ral mediate BCR-controlled activation of JUN/ATF2 and NFAT transcription factors. Taken together, our data show that Ral is activated upon BCR stimulation and mediates BCR-controlled activation of AP-1 and NFAT transcription factors. These findings suggest that Ral plays an important role in B cell development and function.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by a grant from the Dutch Cancer Society.

² Address correspondence and reprint requests to Dr. Marcel Spaargaren, Department of Pathology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. E-mail address: marcel.spaargaren{at}amc.uva.nl

³ Abbreviations used in this paper: GEF, guanine nucleotide exchange factor; PKB, protein kinase B; Btk, Bruton’s tyrosine kinase; RalBP, Ral-binding protein; PLC, phospholipase C; IP₃R, inositol-1,4,5-trisphosphate receptor; WT, wild type; RBD, Ral/Ras-binding domain; DAG, diacylglycerol; GAP, GTPase-activating protein.

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