Activation of the interleukin 2 receptor: a possible role for tyrosine phosphatases

Engagement of interleukin-2 (IL-2) mediates the heterodimeridation of the common beta chain (beta(c)) and common gamma chain (gamma(c)) of the IL-2 receptor (IL-2R). This is sufficient and necessary for receptor activation and signal transduction. It is generally held that the IL-2R is activated by the trans-activity of the protein tyrosine kinases (PTKs) Jak1 and Jak3 associated with beta(c) and gamma(c) respectively. Transduction of proliferative signals requires Jak3 activity. A Jak3 independent signalling pathway involving p56(lck), generating anti-apoptotic signals, can be observed and requires the PROX domain of gamma(c). p56(lck) can be activated by dephosphorylation of an inhibitory carboxyl terminal phosphorylated tyrosine residue (Y505). We propose that this is mediated by a PROX domain associated protein tyrosine phosphatase (PTP). Activation of p56(lck) alone is insufficient for transduction of proliferative signals and thus works in concert with Jak3 mediated receptor activation. This indicates that both gamma(c) domains are vital for signal transduction.     

A membrane-proximal region of the interleukin-2 receptor gamma c chain sufficient for Jak kinase activation and induction of proliferation in T cells.

The interleukin-2 (IL-2) receptor (IL-2R) consists of three distinct subunits (alpha, beta, and gamma c) and regulates proliferation of T lymphocytes. Intracellular signalling results from ligand-mediated heterodimerization of the cytoplasmic domains of the beta and gamma c chains. To identify the residues of gamma c critical to this process, mutations were introduced into the cytoplasmic domain, and the effects on signalling were analyzed in the IL-2-dependent T-cell line CTLL2 and T-helper clone D10, using chimeric IL-2R chains that bind and are activated by granulocyte-macrophage colony-stimulating factor. Whereas previous studies of fibroblasts and transformed T cells have suggested that signalling by gamma c requires both membrane-proximal and C-terminal subdomains, our results for IL-2-dependent T cells demonstrate that the membrane-proximal 52 amino acids are sufficient to mediate a normal proliferative response, including induction of the proto-oncogenes c-myc and c-fos. Although gamma c is phosphorylated on tyrosine upon receptor activation and could potentially interact with downstream molecules containing SH2 domains, cytoplasmic tyrosine residues were dispensable for mitogenic signalling. However, deletion of a membrane-proximal region conserved among other cytokine receptors (cytoplasmic residues 5 to 37) or an adjacent region unique to gamma c (residues 40 to 52) abrogated functional interaction of the receptor chain with the tyrosine kinase Jak3. This correlated with a loss of all signalling events analyzed, including phosphorylation of the IL-2R beta-associated kinase Jak1, expression of c-myc and c-fos, and induction of the proliferative response. Thus, it appears in T cells that Jak3 is a critical mediator of mitogenic signaling by the gamma c chain.     

Possible mechanism for the alpha subunit of the interleukin-2 receptor (CD25) to influence interleukin-2 receptor signal transduction

The receptors for interleukin 2 (IL-2) and interleukin 15 (IL-15) in T cells share the IL-2R beta subunit (CD122) and gamma(C) subunit but have private alpha subunits. Despite utilizing the same receptor chains known to be necessary and sufficient to transduce IL-2 signals the two cytokines manifest different cellular effects. It is commonly held that the alpha subunit of the IL-2R (CD25) is involved solely in the generation of a high affinity receptor complex. This is questioned by the development of autoimmune diseases in instances where the expression of CD25 is absent. The timely expression of CD25 in the thymus has been linked with clonal deletion. Evidence from peripheral T cells indicates that survival signals arising from the intermediate affinity IL-2R (lacking CD25) do not require the activation of Janus kinase 3 (Jak3) but do require the presence of the membrane proximal region of the gamma(C) chain. This particular signalling pathway is not observed in the high affinity receptor complex where Jak3 is activated. Recent data point to CD25 having a surface distribution consistent with it being localized within membrane microdomains. Here we suggest that in the absence of CD25 expression, IL-2R activation occurs within the soluble membrane fraction. This membrane environment and the absence of CD25 promotes Jak3 independent signal transduction and induction of antiapoptotic mechanisms. T cell antigen receptor (TCR) signalling leads to the induction of CD25 expression, which localizes to membrane microdomains. There is a dynamic pre-association of CD25 and CD122 leading to the loose association of the heterodimer with membrane microdomains. High affinity IL-2R signalling in the context of CD25 and the microdomain environment is characterized by Jak3 activation. The relative levels of high to intermediate affinity receptor signalling determines whether a cell proliferates or undergoes activation induced cell death dependent upon cell status.     

Proliferation Response to Interleukin-2 and Jak/Stat Activation of T Cells Immortalized by Human T-Cell Lymphotropic Virus Type 1 Is Independent of Open Reading Frame I Expression

Human T-cell lymphotropic virus type 1 (HTLV-1), a complex retrovirus, encodes a hydrophobic 12-kD protein from pX open reading frame (ORF) I that localizes to cellular endomembranes and contains four minimal SH3 binding motifs (PXXP). We have demonstrated the importance of ORF I expression in the establishment of infection and hypothesize that p12(I) has a role in T-cell activation. In this study, we tested interleukin-2 (IL-2) receptor expression, IL-2-mediated proliferation, and Jak/Stat activation in T-cell lines immortalized with either wild-type or ORF I mutant clones of HTLV-1. All cell lines exhibited typical patterns of T-cell markers and maintained mutation fidelity. No significant differences between cell lines were observed in IL-2 receptor chain (alpha, beta, or gamma(c)) expression, in IL-2-mediated proliferation, or in IL-2-induced phosphorylated forms of Stat3, Stat5, Jak1, or Jak3. The expression of ORF I is more likely to play a role in early HTLV-1 infection, such as in the activation of quiescent T cells in vivo.     

The role of the common cytokine receptor gamma-chain in regulating IL-2-dependent, activation-induced CD8+ T cell death.

IL-2-dependent, activation-induced T cell death (AICD) plays an important role in peripheral tolerance. Using CD8+ TCR-transgenic lymphocytes (2C), we investigated the mechanisms by which IL-2 prepares CD8+ T cells for AICD. We found that both Fas and TNFR death pathways mediate the AICD of 2C cells. Neutralizing IL-2, IL-2R alpha, or IL-2R beta inhibited AICD. In contrast, blocking the common cytokine receptor gamma-chain (gamma c) prevented Bcl-2 induction and augmented AICD. IL-2 up-regulated Fas ligand (FasL) and down-regulated gamma c expression on activated 2C cells in vitro and in vivo. Adult IL-2 gene-knockout mice displayed exaggerated gamma c expression on their CD8+, but not on their CD4+, T cells. IL-4, IL-7, and IL-15, which do not promote AICD, did not influence FasL or gamma c expression. These data provide evidence that IL-2 prepares CD8+ T lymphocytes for AICD by at least two mechanisms: 1) by up-regulating a pro-apoptotic molecule, FasL, and 2) by down-regulating a survival molecule, gamma c.     

Absence of an essential role for thymic stromal lymphopoietin receptor in murine B-cell development

The murine cytokine thymic stromal lymphopoietin (TSLP) supports the development of B220+ IgM+ immature B cells and induces thymocyte proliferation in vitro. Human TSLP, by contrast, activates CD11c+ dendritic cells, but not B or T cells. Recent studies have demonstrated that the receptor for TSLP consists of a heterodimer of the interleukin 7 (IL-7) alpha chain and a novel protein that resembles the hematopoietic cytokine receptor common gamma chain. We examined signal transduction by the gamma-like chains using chimeric receptor proteins. The cytoplasmic domain of the human, but not of the murine, gamma-like chain, activates Jak2 and Stat5 and supports the proliferation of hematopoietic cell lines. In order to assess the role of the murine gamma-like chain in vivo, we generated gamma-like chain-deficient mice. Receptor-deficient mice are unresponsive to TSLP but exhibit no obvious phenotypic defects. In particular, hematopoietic cell development appeared normal. B-cell development, including the IgM+ compartment, was unaffected by loss of the TSLP pathway, as were T lymphopoiesis and lymphocyte proliferation in vitro. Cytokine receptors that utilize the common gamma chain signal through the lymphocyte-specific kinase Jak3. Mice deficient in Jak3 exhibit a SCID phenotype but harbor a residual B220+ splenic lymphocyte population. We demonstrate here that this residual lymphocyte population is lost in mice lacking both the gamma-like chain and Jak3.     

Cutting edge: an alternative pathway of CD4+ T cell differentiation is induced following activation in the absence of gamma-chain-dependent cytokine signals

This report addresses the role of gamma-chain cytokine signals in regulating CD4(+) T cell differentiation following activation. Using murine CD4(+) T cells lacking the Jak3 tyrosine kinase, we show that activation of these cells in the absence of gamma-chain-dependent cytokine signals induces an alternative pathway of T cell differentiation. Specifically, activated Jak3(-/-) CD4(+) T cells produce IL-10, TGF-beta, and IFN-gamma, but not IL-2 or IL-4, and are unable to proliferate in vitro. In addition, Jak3(-/-) CD4(+) T cells express high levels of programmed death-1 and lymphocyte activation gene-3 and modestly suppress the proliferation of wild-type CD4(+) T cells in coculture assays. Together, these features demonstrate a striking similarity between Jak3(-/-) CD4(+) T cells and the regulatory T cells that have been shown to suppress immune responses in vitro and in vivo. We conclude that Jak3 is a critical component of signaling pathways that regulate T cell differentiation into effector vs regulatory lineages.     

Activated lck tyrosine protein kinase stimulates antigen-independent interleukin-2 production in T cells.

p56lck, a member of the src family of cytoplasmic tyrosine kinases, is expressed predominantly in T cells where it associates with the T-cell surface molecules CD4 and CD8. Mutants of CD4 and CD8 that have lost the ability to associate with p56lck no longer enhance antigen-induced T-cell activation. This suggests that p56lck plays an important role during T-cell activation. In an effort to understand the function of p56lck in T cells, a constitutively activated lck gene (F505lck) was introduced into T-helper hybridoma cell lines by retroviral infection. In four T-cell lines we examined, the activated lck protein stimulated interleukin-2 (IL-2) production, a hallmark of T-cell activation, in the absence of antigenic stimulation. In addition, a marked increase in antigen-independent IL-2 production was apparent when T cells infected with a temperature-sensitive F505lck were shifted to the permissive temperature. Only one cell line expressing F505lck exhibited increased sensitivity to antigenic stimulation. The SH3 domain of p56lck was dispensable for the induction of antigen-independent IL-2 production. In contrast, deletion of the majority of the SH2 domain of p56F505lck reduced its ability to induce spontaneous IL-2 production markedly. Activated p60c-src also induced antigen-independent IL-2 production, whereas two other tyrosine kinases, v-abl and the platelet-derived growth factor receptor, did not. Tyrosine phosphorylation of a 70-kDa cellular protein was observed after cross-linking of CD4 in T cells expressing F505lck but not in cells expressing F527src.     

Cell type-specific roles of Jak3 in IL-2-induced proliferative signal transduction

Binding of interleukin-2 (IL-2) to its specific receptor induces activation of two members of Jak family protein tyrosine kinases, Jak1 and Jak3. An IL-2 receptor (IL-2R)-reconstituted NIH 3T3 fibroblast cell line proliferates in response to IL-2 only when hematopoietic lineage-specific Jak3 is ectopically expressed. However, the mechanism of Jak3-dependent proliferation in the fibroblast cell line is not known. Here, I showed that Jak3 expression is dispensable for IL-2-induced activation of Jak1 and Stat proteins and expression of nuclear proto-oncogenes in the IL-2R-reconstituted fibroblast cell line. Jak3 expression markedly enhanced these IL-2-induced signaling events. In contrast, Jak3 expression was essential for induction of cyclin genes involved in the G1-S transition. These data suggest a critical role of Jak3 in IL-2 signaling in the fibroblast cell line and may provide further insight into the cell type-specific mechanism of cytokine signaling.     

Cytokine-independent Jak3 activation upon T cell receptor (TCR) stimulation through direct association of Jak3 and the TCR complex

Jak3 is responsible for growth signals by various cytokines such as interleukin (IL)-2, IL-4, and IL-7 through association with the common gamma chain (gammac) in lymphocytes. We found that T cells from Jak3-deficient mice exhibit impairment of not only cytokine signaling but also early activation signals and that Jak3 is phosphorylated upon T cell receptor (TCR) stimulation. TCR-mediated phosphorylation of Jak3 is independent of IL-2 receptor/gammac but is dependent on Lck and ZAP-70. Jak3 was found to be assembled with the TCR complex, particularly through direct association with CD3zeta via its JH4 region, which is a different region from that for gammac association. These results suggest that Jak3 plays a role not only in cell growth but also in T cell activation and represents cross-talk of a signaling molecule between TCR and growth signals.     

The anti-apoptotic effect of Notch-1 requires p56lck-dependent, Akt/PKB-mediated signaling in T cells

The Notch family of transmembrane receptors have been implicated in a variety of cellular decisions in different cell types. Here we investigate the mechanism underlying Notch-1-mediated anti-apoptotic function in T cells using model cell lines as the experimental system. Ectopic expression of the intracellular domain of Notch-1/activated Notch (AcN1) increases expression of anti-apoptotic proteins of the inhibitors of apoptosis (IAP) family, the Bcl-2 family, and the FLICE-like inhibitor protein (FLIP) and inhibits death triggered by multiple stimuli that activate intrinsic or extrinsic pathways of apoptosis in human and murine T cell lines. Numb inhibited the AcN1-dependent induction of anti-apoptotic proteins and anti-apoptotic function. Using pharmacological inhibitors and dominant-negative approaches, we describe a functional role for phosphatidylinositol 3-kinase (PI3K)-dependent activation of the serine-threonine kinase Akt/PKB in the regulation of AcN1-mediated anti-apoptotic function and the expression of FLIP and IAP family proteins. Using a cell line deficient for the T cell-specific, Src family protein, the tyrosine kinase p56(lck) and by reconstitution approaches we demonstrate that p56(lck) is required for the Notch-1-mediated activation of Akt/PKB function. Furthermore, the Src tyrosine kinase inhibitor, PP2, abrogated ectopically expressed AcN1-mediated anti-apoptotic function and phosphorylation of p56(lck). We present evidence that endogenous Notch-1 associates with p56(lck) and PI3K but that Akt/PKB does not co-immunoprecipitate with the Notch1.p56(lck).PI3K complex. Finally, we demonstrate that the Notch1.p56(lck).PI3K complex is present in primary T cells that have been activated in vitro and sustained in culture with the cytokine interleukin-2.     

SOCS-3 is tyrosine phosphorylated in response to interleukin-2 and suppresses STAT5 phosphorylation and lymphocyte proliferation.

Members of the recently discovered SOCS/CIS/SSI family have been proposed as regulators of cytokine signaling, and while targets and mechanisms have been suggested for some family members, the precise role of these proteins remains to be defined. To date no SOCS proteins have been specifically implicated in interleukin-2 (IL-2) signaling in T cells. Here we report SOCS-3 expression in response to IL-2 in both T-cell lines and human peripheral blood lymphocytes. SOCS-3 protein was detectable as early as 30 min following IL-2 stimulation, while CIS was seen only at low levels after 2 h. Unlike CIS, SOCS-3 was rapidly tyrosine phosphorylated in response to IL-2. Tyrosine phosphorylation of SOCS-3 was observed upon coexpression with Jak1 and Jak2 but only weakly with Jak3. In these experiments, SOCS-3 associated with Jak1 and inhibited Jak1 phosphorylation, and this inhibition was markedly enhanced by the presence of IL-2 receptor beta chain (IL-2Rbeta). Moreover, following IL-2 stimulation of T cells, SOCS-3 was able to interact with the IL-2 receptor complex, and in particular tyrosine phosphorylated Jak1 and IL-2Rbeta. Additionally, in lymphocytes expressing SOCS-3 but not CIS, IL-2-induced tyrosine phosphorylation of STAT5b was markedly reduced, while there was only a weak effect on IL-3-mediated STAT5b tyrosine phosphorylation. Finally, proliferation induced by both IL-2- and IL-3 was significantly inhibited in the presence of SOCS-3. The findings suggest that when SOCS-3 is rapidly induced by IL-2 in T cells, it acts to inhibit IL-2 responses in a classical negative feedback loop.     

Interleukin-21 Is a Critical Regulator of CD4 and CD8 T Cell Survival during Priming under Interleukin-2 Deprivation Conditions

Optimal T cell activation and expansion require binding of the common gamma-chain (γc) cytokine Interleukin-2 (IL-2) to its cognate receptor that in turn engages a γc/Janus tyrosine kinase (Jak)3 signaling pathway. Because of its restricted expression by antigen-activated T cells and its obligatory role in promoting their survival and proliferation, IL-2 has been considered as a selective therapeutic target for preventing T cell mediated diseases. However, in order to further explore IL-2 targeted therapy, it is critical to precisely understand its role during early events of T cell activation. In this study, we delineate the role of IL-2 and other γc cytokines in promoting the survival of CD4 and CD8 T cells during early phases of priming. Under IL-2 inhibitory conditions (by neutralizing anti-IL-2 mAbs), the survival of activated CD8⁺ T cells was reduced, whereas CD4⁺ T cells remained much more resistant. These results correlated with reduced Bcl-2 expression, and mitochondrial membrane potential in CD8⁺ T cells in comparison to CD4⁺ T cells. However, using transwell co-culture assays we have found that CD4⁺ T cells could rescue the survival of CD8⁺ T cells even under IL-2 deprived conditions via secretion of soluble factors. A cytokine screen performed on CD8⁺ T cells cultured alone revealed that IL-21, another γc cytokine, was capable of rescuing their survival under IL-2 deprivation. Indeed, blocking the IL-21 signaling pathway along with IL-2 neutralization resulted in significantly reduced survival of both CD4⁺ and CD8⁺ T cells. Taken together, we have shown that under IL-2 deprivation conditions, IL-21 may act as the major survival factor promoting T cell immune responses. Thus, investigation of IL-2 targeted therapies may need to be revisited to consider blockade of the IL-21 signaling pathways as an adjunct to provide more effective control of T cell immune responses.     

Activation of ZAP-70 kinase activity by phosphorylation of tyrosine 493 is required for lymphocyte antigen receptor function.

ZAP-70 is a protein tyrosine kinase (PTK) required for T-cell development and T-cell antigen receptor (TCR) function. ZAP-70 is associated with the phosphorylated antigen receptor and undergoes tyrosine phosphorylation following receptor activation. We demonstrate here that tyrosine phosphorylation of ZAP-70 results in an increase in its catalytic activity and that this activation is mediated by the phosphorylation of tyrosine residue 493 by the src family of PTKs. The activity of baculoviral expressed ZAP-70 was up-regulated 10-fold when ZAP-70 was co-infected and phosphorylated by the src family PTK, lck. Mutation of Y493 alone abrogated the ability of ZAP-70 to be activated by lck. Moreover, we demonstrate that phosphorylation of Y493 and activation of ZAP-70 is required for antigen receptor-mediated induction of interleukin-2 (IL-2) secretion in lymphocytes.     

Jak3-independent trafficking of the common gamma chain receptor subunit: chaperone function of Jaks revisited

Janus kinases (Jaks) play an essential role in cytokine signaling and have been reported to regulate plasma membrane expression of their cognate receptors. In this study, we examined whether Jak3 and the common gamma chain (gamma(c)) reciprocally regulate their plasma membrane expression. In contrast to interleukin-2Ralpha, gamma(c) localized poorly to the plasma membrane and accumulated in endosomal-lysosomal compartments. However, gamma(c) was expressed at comparable levels on the surface of cells lacking Jak3, and plasma membrane turnover of gamma(c) was independent of Jak3. Nonetheless, overexpression of Jak3 enhanced accumulation of gamma(c) at the plasma membrane. Without gamma(c), Jak3 localized in the cytosol, whereas in the presence of the receptor, it colocalized with gamma(c) in endosomes and at the plasma membrane. Although the Jak FERM domain is necessary and sufficient for receptor binding, the requirement for full-length Jak3 in gamma(c) membrane trafficking was remarkably stringent; using truncation and deletion mutants, we showed that the entire Jak3 molecule was required, although kinase activity was not. Thus, unlike other cytokine receptors, gamma(c) does not require Jak3 for receptor membrane expression. However, full-length Jak3 is required for normal trafficking of this cytokine receptor/Jak pair, a finding that has important structural and clinical implications.     

Receptor expression is essential for proliferation induced by dimerized Jak kinases

Two members of Jak kinases, Jak1 and Jak3, are associated with the cytoplasmic domains of the interleukin-2 (IL-2) receptor (IL-2R) β chain (IL-2Rβ) and the common cytokine receptor γ chain (γc), respectively, and accumulating evidence indicates their functional importance in IL-2 signaling. Here, I showed that coumermycin-induced chemical heterodimerization of Jak1 and Jak3 but not homodimerization of Jak1 or Jak3 induces cell proliferation of an IL-2R-reconstituted cell line. In this regard, expression of IL-2Rβ was essential for cell proliferation by chemical heterodimerization of Jak1 and Jak3, indicating that dimerized Jak1 and Jak3 induce heterodimerization of IL-2Rβ and γc, which may activate receptor-bound signaling molecules. Previous reports using chemical dimerization suggest that dimerization of Jak kinases is sufficient to induce cell proliferation. The present study indicates that re-evaluation of this conclusion is necessary and that interpretation of functional analysis of signaling molecules using chemical dimerizers needs more careful assessment.     

Identification of a novel receptor/signal transduction pathway for IL-15/T in mast cells.

Interleukin-15/T(IL-15) is a growth factor that utilizes IL-2 receptor (IL-2R) components in addition to its private binding protein IL-15R(alpha) in T-cells. Here, we report that IL-15 induces mast cell proliferation in the absence of IL-2R alpha and beta. Using transfectants of these cells with a cytoplasmic-truncated mutant of gamma(c), we demonstrated that IL-15 signaling in mast cells does not involve gamma(c). Cross-linking of mast cells with [(125)I]IL-15 revealed a 60-65 kDa IL-15 binding protein that is distinct from known components of T-cell IL-15 receptors. Mast cell IL-15 receptors recruit JAK-2 and STAT-5, instead of JAK1/3 and STAT3/5 that are activated in T-cells. Thus IL-15 is a mast cell growth factor that utilizes a novel receptor and distinct signaling pathway.     

Fes mediates the IL-4 activation of insulin receptor substrate-2 and cellular proliferation

Although Jak kinases are essential for initiating cytokine signaling, the role of other nonreceptor tyrosine kinases in this process remains unclear. We have examined the role of Fes in IL-4 signaling. Examination of Jak1-deficient cell lines demonstrates that Jak1 is required for the activation of Fes by IL-4. Experiments studying signaling molecules activated by IL-4 receptor suggest that IL-4 signaling can be subdivided into Fes-dependent and Fes-independent pathways. Overexpression of kinase-inactive Fes blocks the IL-4 activation of insulin receptor substrate-2, but not STAT6. Fes appears to be a downstream kinase from Jak1/Jak3 in this process. Further examination of downstream signaling demonstrates that kinase-inactive Fes inhibits the recruitment of phosphoinositide 3-kinase to the activated IL-4 receptor complex and decreases the activation of p70(S6k) kinase in response to IL-4. This inhibition correlates with a decrease in IL-4-induced proliferation. In contrast, mutant Fes does not inhibit the activation of Akt by IL-4. These data demonstrate that signaling pathways activated by IL-4 require different tyrosine kinases. This differential requirement predicts that specific kinase inhibitors may permit the disruption of specific IL-4-induced functions.     

Overexpression of interleukin-2 receptor alpha in a human squamous cell carcinoma of the head and neck cell line is associated with increased proliferation,drug resistance,and transforming ability

It has been previously demonstrated that human carcinomas express interleukin-2 receptor (IL-2R) alpha, beta, and gamma chains. The beta and gamma chains of IL-2R have intermediate binding affinity for IL-2 and are responsible for the intracellular signaling cascades after IL-2 stimulation. IL-2Ralpha lacks the cytoplasmic domain, but is essential for increasing the IL-2-binding affinity of other receptors. Overexpression of IL-2Ralpha in tumor cells is associated with tumor progression and a poor patient prognosis. To define molecular mechanisms responsible for the effects associated with IL-2Ralpha expression, ex vivo experiments were performed with the squamous cell carcinoma head-and-neck cancer line, PCI-13, which was genetically engineered to overexpress the IL-2Ralpha chain. While IL-2Ralpha-overexpressing PCI-13 cells were capable of forming colonies in soft agar, PCI-13 cells transfected with the control vector or those expressing IL-2Rgamma did not. Consistently, IL-2Ralpha-expressing tumor cells proliferated more rapidly than the control or IL-2Rgamma+ cells, associated with increased levels of cyclins A and D1 and cyclin-dependent kinase (cdk(s)) 2 and 4 proteins. In addition, IL-2Ralpha-expressing cells were significantly more resistant to apoptosis induction by a tripeptidyl proteasome inhibitor (ALLN) and two chemotherapeutic drugs (VP-16 and taxol) than the control or IL-2Rgamma+ cells. Accompanying the drug resistance, high levels of anti-apoptotic Bcl-X(L) and Bcl-2 proteins were found in the mitochondria-containing fraction of IL-2Ralpha-expressing tumor cells. Treatment of IL-2Ralpha-expressing cells with a specific Janus kinase 3 (Jak3) inhibitor decreased expression of cyclin A, cyclin D1, Bcl-X(L), and Bcl-2 proteins. Finally, high levels of ubiquitinated proteins were detected in the proliferating IL-2Ralpha-expressing cells. Our data suggest that increased proliferation rates and decreased drug sensitivity of IL-2Ralpha-expressing tumor cells are responsible for the enhanced tumor aggressiveness and poor clinical prognosis of patients whose tumors express IL-2Ralpha.