GILZ, a new target for the transcription factor FoxO3, protects T lymphocytes from interleukin-2 withdrawal–induced apoptosis

Abstract

Interleukin-2 (IL-2) withdrawal is a physiologic process inducing cell death in activated T lymphocytes. Glucocorticoidinduced leucine zipper (GILZ) has recently been identified as a protein modulating T-cell receptor activation by repressing various signaling pathways. We report here that IL-2 deprivation leads to expression of GILZ in T lymphocytes. We then characterized the human gilz promoter and showed that FoxO3 (Forkhead box class O3) binding to the Forkhead responsive elements identified in the promoter is necessary for induction of gilz expression upon IL-2 withdrawal. To assess the functional consequences of this induction, we used 2 strategies, GILZ overexpression and GILZ silencing in murine IL-2-dependent CTLL-2 cells. GILZ overexpression protects CTLL-2 cells from IL-2 withdrawal-induced apoptosis, whereas cell death is accelerated in cells unable to express GILZ. Concomitantly, the expression of Bim is inhibited in GILZ-overexpressing cells and enhanced when GILZ expression is impaired. Furthermore, GILZ inhibits FoxO3 transcriptional activity that leads to inhibition of Bim expression but also to down-regulation of GILZ itself. Therefore, GILZ is a transiently expressed protein induced upon IL-2 withdrawal that protects T cells from the onset of apoptosis. ( IntroductionA complex network of cytokines controls the development, maturation, homeostasis, and responses of the immune system. Immune responses typically involve clonal expansion of activated T cells, which must be eliminated at the end of the response to preserve homeostasis. 1 Interleukin-2 (IL-2) is secreted by activated T cells and plays a role in their survival and proliferation. IL-2 is also involved in the regulation of homeostasis of the lymphoid tissue, as IL-2 induces apoptosis of activated T cells in an "active" way involving regulation of Fas and Fas ligand (FasL) expression and in a "passive" way due to deprivation of this survival factor. 2 Binding of IL-2 to its receptor triggers a signaling cascade that induces, among others, the phosphoinositide 3-kinase (PI3K) pathway involved in T-cell proliferation and survival. 3,4 One of the downstream effectors of PI3K signaling is protein kinase B (PKB)/Akt. PKB is a serine/threonine kinase promoting cell survival by targeting proapoptotic proteins. 5 PKB exerts one of its inhibitory effects by phosphorylating members of the Forkhead family of transcription factors (Forkhead box class O1 [FoxO1], FoxO3, and FoxO4), which contain 3 RXRXXS/T consensus phosphorylation sites for PKB. 6 Interruption of the PI3K/PKB pathway following IL-2 deprivation results in dephosphorylation of FoxO proteins leading to their translocation to the nucleus and transcription of target genes.Brunet et al 7 were the first to ascribe a role to FoxO proteins in programmed cell death, reporting that overexpression of a constitutive active form of FoxO3 induces apoptosis of Jurkat T cells in a Fas-dependent manner. Fas-independent mechanisms of apoptosis induced by FoxO proteins have also been observed in othe...

Decrease of Bcl-xL and augmentation of thymocyte apoptosis in GILZ overexpressing transgenic mice

“…This rise in apoptosis apparently contrasts with reports, indicating that GILZ inhibits apoptosis induced by anti-CD3 stimulation 13,20 or IL-2 withdrawal. 32 However, some molecules are known to play opposing roles in apoptosis. GCs promote thymic apoptosis but inhibit ovary apoptosis.…”

Les protéines de la famille TSC-22D

“…Cette maladie auto-immune inflammatoire chronique est caractérisée par une forte production de TNF- et d'IL-1-. L'analyse de prélèvements de synovies de patients atteints de PR et de donneurs sains montre que GILZ est exprimée en quantité plus de BIM (membre pro-apoptotique de la famille Bcl-2) et à l'apoptose des LT [16,25] (Figure 2). Inversement, TSC-22 favorise l'apoptose de ces cellules (Figure 2) : sa surexpression diminue l'expression de GILZ induite lors de la déprivation en IL-2, résultant en une augmentation de l'expression de BIM (A. Pépin et al, résultats non publiés).…”

“…Le promoteur murin de Gilz a été cloné par notre équipe. Il comporte des GRE (GC responsive element) et des FHRE (forkhead responsive element) qui coopèrent pour la transactivation de ce promoteur [25]. Le facteur FOXO3 (forkhead box class O3), activé lors de la déprivation en IL-2 de la lignée de lymphocytes T murins CTLL-2, participe à la transcription de Gilz [25].…”

“…Il comporte des GRE (GC responsive element) et des FHRE (forkhead responsive element) qui coopèrent pour la transactivation de ce promoteur [25]. Le facteur FOXO3 (forkhead box class O3), activé lors de la déprivation en IL-2 de la lignée de lymphocytes T murins CTLL-2, participe à la transcription de Gilz [25]. En présence d'IL-2, FOXO3 est phosphorylé et donc inactif, et l'expression de Gilz est réprimée [18].…”

“…En présence d'IL-2, FOXO3 est phosphorylé et donc inactif, et l'expression de Gilz est réprimée [18]. Le promoteur de Gilz comporte aussi des sites potentiels de fixation pour les facteurs de transcription STAT6 (signal transducer and activator of transcription 6), NFAT (nuclear factor of activated T cell), OCT-1 (octamer binding transcription factor-1) et c-Myc [25]. Enfin, un mécanisme de régulation post-transcriptionnelle de Gilz a été décrit dans lequel l'activation par le LPS (lipopolysaccharide) déstabilise l'ARNm de Gilz dans la région 3'UTR (untranslated region), diminuant ainsi son expression [22].…”