Rap1-GTP Augments Activation of Smad and p38 Mediated Signaling Downstream of TGF-β Receptor In T Lymphocytes

Abstract 956

Rap1A, a member of the Ras superfamily, was discovered as a gene product that reverted K-Ras-induced transformation. Although it was initially thought that Rap1A (thereafter referred to as Rap1) opposes Ras-induced transformation by competing for common downstream effector(s), it has now become apparent that Ras and Rap1 proteins operate in different signaling networks and mediate distinct functions. In lymphocytes Rap1 is activated by BCR and TCR mediated signals and is involved in inside-out activation of integrins. To understand the role of Rap1 in T cell responses we generated transgenic (Tg) mice that express the active Rap1 mutant Rap1E63 in T cells. Rap1E63-Tg and littermate control mice had no statistically significant difference in absolute thymocyte numbers and differentiation profiles of double positive and single positive thymocytes. In contrast to thymocyte numbers, Rap1E63-Tg mice had reduced T cell numbers in peripheral lymphoid organs. In peripheral lymph nodes and spleen, the total T cell numbers were decreased to approximately one third to one forth of those in littermate controls. Strikingly, Rap1E63-Tg mice had a 4-fold increase in the CD4+CD103+ T cell fraction in spleens and lymph nodes compared to their littermate controls. CD103 defines a subset of peripherally generated Treg with potent suppressive function. CD4+CD103+ Treg cells in RapE63-Tg mice expressed CD45RBlow, CD44high, CD54high, CD62Llow and LFA-1high, findings consistent with an effector memory phenotype. TGF-β is the strongest stimulus for induction of CD103 expression. To examine whether Rap1 affected TGF-β-mediated signaling in T cells, we used stable Jurkat T cell lines expressing Rap1E63, Jurkat T cell lines, in which endogenous Rap1 was depleted by shRNA (Rap1-KD), and primary T cells from Rap1E63-Tg mice and Rap1-KO mice. TGF-β signaling involves the interaction of two membrane-bound receptors, TGF-βRI and II. TGF-β binds the exracellular domain of TGF-βRII, which then recruits TGF-βRI resulting in transphosphorylation. TGF-βRI activation propagates downstream signaling via the Smad family proteins Smad2 and Smad3 and also via p38 MAPK, in a Smad-independent manner. Incubation of Rap1E63 Jurkat T cells with TGF-β resulted in enhanced and sustained phosphorylation of Smad2 and Smad3, which was observed with very low concentrations of TGF-β that were incapable of inducing detectable phosphorylation of Smad2 or Smad3 in control cells. In contrast, diminished level and duration of Smad2 and Smad3 phosphorylation was observed in Rap1-KD Jurkat cells. Activation of p38 downstream of TGF-βRI displayed a dramatically augmented and sustained induction, whereas the opposite effect was observed in Rap1-KD cells. Similar patterns of responses to those observed in Rap1E63 Jurkat T cells and in Rap1-KD cells were observed in primary mouse T cells isolated from Rap1E63-Tg mice and Rap1 deficient mice, respectively. Assessment of TGF-β binding on TGF-β surface receptors by using biotinylated TGF-β revealed that Rap1E63 expressing cells displayed significantly enhanced TGF-β binding compared to their relevant controls, whereas reduced TGF-β binding was observed in Rap1 deficient cells. These results indicate that active Rap1 modulates TGF-β receptor binding and enhances TGF-β mediated signaling, gene transcription, and functional outcome. Our data reveal a novel and unexpected mechanism by which Rap1 regulates T cell responses that may have important implications on TGF-β-mediated T cell homeostasis and maintenance of immune quiescence.