Glycogen Synthase Kinase-3 Modulates Cbl-b and Constrains T Cell Activation.

The decision between T cell activation and tolerance is governed by the spatial and temporal integration of diverse molecular signals and events occurring downstream of TCR and costimulatory or coinhibitory receptor engagement. The PI3K-protein kinase B (PKB; also known as Akt) signaling pathway is a central axis in mediating proximal signaling events of TCR and CD28 engagement in T cells. Perturbation of the PI3K-PKB pathway, or the loss of negative regulators of T cell activation, such as the E3 ubiquitin ligase Cbl-b, have been reported to lead to increased susceptibility to autoimmunity. In this study, we further examined the molecular pathway linking PKB and Cbl-b in murine models. Our data show that the protein kinase GSK-3, one of the first targets identified for PKB, catalyzes two previously unreported phosphorylation events at Ser476 and Ser480 of Cbl-b. GSK-3 inactivation by PKB abrogates phosphorylation of Cbl-b at these two sites and results in reduced Cbl-b protein levels. We further show that constitutive activation of PKB in vivo results in a loss of tolerance that is mediated through the downregulation of Cbl-b. Altogether, these data indicate that the PI3K-PKB-GSK-3 pathway is a novel regulatory axis that is important for controlling the decision between T cell activation and tolerance via Cbl-b.

Regulation of retinoid X receptor responsive element-dependent transcription in T lymphocytes by Ser/Thr phosphatases: functional divergence of protein kinase C (PKC)theta; and PKC alpha in mediating calcineurin-induced transactivation.

T lymphocyte activation signals regulate the expression and transactivation function of retinoid X receptor (RXR) alpha through an interplay of complex signaling cascades that are not yet fully understood. We show that cellular Ser/Thr protein phosphatases (PPs) play an important role in mediating these processes. Inhibitors specific for PP1 and PP2A decreased basal expression of RXR alpha RNA and protein in T lymphocyte leukemia Jurkat cells and prevented activation-induced RXR alpha accumulation in these cells. In addition, these inhibitors attenuated the RXR responsive element (RXRE)-dependent transcriptional activation in transient transfection assays. Inhibitors of calcineurin (CN), by contrast, did not have any effect on the basal RXR alpha expression and even augmented activation-induced RXR alpha expression. Expression of a dominant-active (DA) mutant of CN together with a DA mutant of protein kinase C (PKC)theta;, a novel PKC isoform, significantly increased RXRE-dependent transcription. Expression of catalytically inactive PKC theta; or a dominant-negative mutant of PKC theta; failed to synergize with CN and did not increase RXRE-dependent transcription. Expression of a DA mutant of PKC alpha or treatment with PMA was found to attenuate PKC theta; and CN synergism. We conclude that PP1, PP2A, and CN regulate levels and transcriptional activation function of RXR alpha in T cells. In addition, CN synergizes with PKC theta; to induce RXRE-dependent activation, a cooperative function that is antagonized by the activation of the conventional PKC alpha isoform. Thus, PKC theta; and PKC alpha may function as positive and negative modulators, respectively, of CN-regulated RXRE-dependent transcription during T cell activation.