Regulation of the Src family kinase Lck by Hsp90 and ubiquitination

Regulation of the Src-related tyrosine kinase Lck is crucial to the outcome of T-cell receptor (TCR) stimulation. It was previously shown that the stability of the constitutively active mutant LckY505F is controlled by Hsp90 (M. J. Bijlmakers and M. Marsh, Mol. Biol. Cell. 11:1585-1595, 2000). Here we establish that following TCR stimulation, endogenous activated Lck in T cells is also degraded in the presence of the Hsp90 inhibitor geldanamycin. Using Lck constructs expressed in COS-7 cells, we show that the presence of activating Lck mutations results not only in the enhanced dependence on Hsp90 but also in enhanced ubiquitination of Lck. Although both processes were induced by mutations Y505F and W97A that release the SH2 and SH3 inhibitory intramolecular interactions, respectively, neither process required Lck kinase activity or activation-dependent phosphorylation at serines 42 and 59 or tyrosine 394. By binding to the ATP-binding site, the Src family inhibitor PP2 reduced ubiquitination and overcame the need for Hsp90 monitoring of active Lck. We conclude that the levels of active Lck are influenced by two opposing processes, targeting for degradation by ubiquitination and rescue from degradation by Hsp90 monitoring. Based on the PP2 result, we propose that activation-induced conformational changes of the Lck kinase domain instigate both regulatory processes.     

{beta}1 Integrin and IL-3R coordinately regulate STAT5 activation and anchorage-dependent proliferation

We previously demonstrated that integrin-dependent adhesion activates STAT5A, a well known target of IL-3-mediated signaling. Here, we show that in endothelial cells the active beta1 integrin constitutively associates with the unphosphorylated IL-3 receptor (IL-3R) beta common subunit. This association is not sufficient for activating downstream signals. Indeed, only upon fibronectin adhesion is Janus Kinase 2 (JAK2) recruited to the beta1 integrin-IL-3R complex and triggers IL-3R beta common phosphorylation, leading to the formation of docking sites for activated STAT5A. These events are IL-3 independent but require the integrity of the IL-3R beta common. IL-3 treatment increases JAK2 activation and STAT5A and STAT5B tyrosine and serine phosphorylation and leads to cell cycle progression in adherent cells. Expression of an inactive STAT5A inhibits cell cycle progression upon IL-3 treatment, identifying integrin-dependent STAT5A activation as a priming event for IL-3-mediated S phase entry. Consistently, overexpression of a constitutive active STAT5A leads to anchorage-independent cell cycle progression. Therefore, these data provide strong evidence that integrin-dependent STAT5A activation controls IL-3-mediated proliferation.     

Multilevel dysregulation of STAT3 activation in anaplastic lymphoma kinase-positive T/null-cell lymphoma.

Accumulating evidence indicates that expression of anaplastic lymphoma kinase (ALK), typically due to t(2;5) translocation, defines a distinct type of T/null-cell lymphoma (TCL). The resulting nucleophosmin (NPM) /ALK chimeric kinase is constitutively active and oncogenic. Downstream effector molecules triggered by NPM/ALK remain, however, largely unidentified. Here we report that NPM/ALK induces continuous activation of STAT3. STAT3 displayed tyrosine phosphorylation and DNA binding in all (four of four) ALK+ TCL cell lines tested. The activation of STAT3 was selective because none of the other known STATs was consistently tyrosine phosphorylated in these cell lines. In addition, malignant cells in tissue sections from all (10 of 10) ALK+ TCL patients expressed tyrosine-phosphorylated STAT3. Transfection of BaF3 cells with NPM/ALK resulted in tyrosine phosphorylation of STAT3. Furthermore, STAT3 was constitutively associated with NPM/ALK in the ALK+ TCL cell lines. Additional studies into the mechanisms of STAT3 activation revealed that the ALK+ TCL cells expressed a positive regulator of STAT3 activation, protein phosphatase 2A (PP2A), which was constitutively associated with STAT3. Treatment with the PP2A inhibitor calyculin A abrogated tyrosine phosphorylation of STAT3. Finally, ALK+ T cells failed to express a negative regulator of activated STAT3, protein inhibitor of activated STAT3. These data indicate that NPM/ALK activates STAT3 and that PP2A and lack of protein inhibitor of activated STAT3 may be important in maintaining STAT3 in the activated state in the ALK+ TCL cells. These results also suggest that activated STAT3, which is known to display oncogenic properties, as well as its regulatory molecules may represent attractive targets for novel therapies in ALK+ TCL.     

Expression of human tyrosine kinase, Lck, in yeast Saccharomyces cerevisiae: growth suppression and strategy for inhibitor screening

We report the successful expression and detection of a phosphorylated form of human T cell tyrosine kinase, Lck, in Saccharomyes cerevisiae, which leads to growth suppression of the yeast cells. Expression of an inactive Lck mutant resulted in no phosphorylation and no growth suppression, indicating that cell growth inhibition by Lck is due to the activity of the kinase, consistent with the observed tyrosine-phosphorylation of the Lck and yeast host cell proteins. The addition of a known inhibitor of Lck to the cell culture resulted in recovery of cell growth expressing the active Lck, suggesting that the growth inhibition by lck gene expression can be used to screen inhibitors for the gene product. We have extended such approach to Tob, another potential therapeutic target.     

Downregulation of Lck-mediated signal transduction by tip of herpesvirus saimiri.

A protein, called tip, of herpesvirus saimiri associates with Lck in transformed T cells. To investigate the effects of complex formation on cellular signal transduction, we constructed human Jurkat-T-cell lines expressing tip. The expression of tip in Jurkat-T cells dramatically suppressed cellular tyrosine phosphorylation and surface expression of lymphocyte antigens. The expression of tip also blocked the induction of tyrosine phosphorylation by anti-CD3 stimulation. The expression of tip in fibroblast cells suppressed the transforming activity of oncogenic F505 Lck. Binding assays showed that the SH3 domain of Lck is sufficient to form a stable complex with tip in vitro. These results demonstrate that tip acts at an early stage of the T-cell signal transduction cascade by associating with Lck and downregulating Lck-mediated activation. Inhibition of Lck-mediated signal transduction by tip in T cells appears to be analogous to the inhibition of Lyn/Syk-mediated signal transduction in B cells by LMP2A of the B-cell-tropic Epstein-Barr virus.     

The common gamma chain (gamma c) is a required signaling component of the IL-21 receptor and supports IL-21-induced cell proliferation via JAK3

The common cytokine receptor gamma chain (gamma c), an essential component of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15, is critical for the development and function of lymphocytes. Recently, a novel lymphokine (IL-21) and its receptor (IL-21R alpha) were described which profoundly affect the growth and activation state of B, T, and NK cells in concert with other lymphokines or stimuli [Parrish-Novak, J., et al. (2000) Nature 408, 57-63]. In this report, we show that gamma c is also a required signaling component of the IL-21 receptor (IL-21R) using the gamma c-deficient X-linked severe combined immunodeficiency (XSCID) lymphoblastoid cell line JT, and JT cells reconstituted with gamma c (JT/gamma c). Moreover, we demonstrate a functional requirement for both gamma c and the gamma c-associated Janus family tyrosine kinase 3 (JAK3) in IL-21-induced proliferation of pro-B-lymphoid cells engineered to express human IL-21R alpha (BaF3/IL-21R alpha). Retroviral-mediated transduction of wild-type gamma c into XSCID JT cells restored function to the IL-21R, as shown by IL-21-induced tyrosine phosphorylation of JAK1 and JAK3, and downstream activation of STAT5, in JT/gamma c cells as well as BaF3/IL-21R alpha and primary splenic B cells. In contrast, IL-21 failed to activate the JAK-STAT pathway in nonreconstituted JT cells. Monoclonal antibodies specific for the gamma c chain effectively inhibited IL-21-induced growth of BaF3/IL-21R alpha cells, supporting a functional role for this molecule in the IL-21R complex. In addition, the specific JAK3 tyrosine kinase inhibitor WHI-P131 significantly reduced IL-21-induced proliferation of BaF3/IL-21R alpha cells. Taken together, these results definitively demonstrate that IL-21-mediated signaling requires the gamma c chain, and indicate that JAK3 is an essential transducer of gamma c-dependent survival and/or mitogenic signals induced by this cytokine.     

Mutation of the Lck-Binding Motif of Tip Enhances Lymphoid Cell Activation by Herpesvirus Saimiri

The proline-rich SH3-binding (SH3B) motif of the tyrosine kinase-interacting protein (Tip) of herpesvirus saimiri (HVS) is required for binding to the cellular Src family kinase Lck. We constructed a mutant form of HVS in which prolines in the SH3B motif of Tip were altered to alanines. This mutant form of Tip was incapable of binding to Lck. The mutant virus, HVS/Tip mSH3B, retained its ability to immortalize common marmoset lymphocytes in culture. In fact, common marmoset lymphocytes immortalized by the HVS/Tip mSH3B mutant displayed increased expression of HLA-DR lymphocyte activation marker, an altered pattern of tyrosine phosphorylation, increased expression of the tyrosine kinase Lyn, and a shift in electrophoretic mobility of Lck compared to cells immortalized by wild-type HVS. Experimental infection of common marmosets resulted in fulminant lymphoma with both HVS/Tip mSH3B and wild-type HVS. However, HVS/Tip mSH3B produced greater infiltration of affected organs by proliferating lymphoid cells compared to wild-type HVS. These results demonstrate that Tip binding to Lck is not necessary for transformation and that abrogation of Tip binding to Lck alters the characteristics of transformed cells and the severity of the pathologic lesions.     

Activation of JAK2-V617F by Components of Heterodimeric Cytokine Receptors

The JAK2-V617F mutation is an important etiologic factor for the development of myeloproliferative neoplasms. The mechanism by which this mutated tyrosine kinase initiates deregulated signals in cells is not completely understood. It is believed that JAK2-V617F requires interactions with homodimeric cytokine receptors to elicit its transforming signal. In this study, we demonstrate that components of heterodimeric cytokine receptors can also activate JAK2-V617F. Expression of IL27Ra, a heterodimeric receptor component, enhanced the activation of JAK2-V617F and subsequent downstream signaling to activation of STAT5 and ERK. In addition, expression of components of the interleukin-3 receptor, IL3Ra and the common β chain, activated JAK2-V617F as well as STAT5 and ERK. Importantly, expression of IL27Ra functionally replaced the requirement of a homodimeric cytokine receptor to promote the activation and transforming activity of JAK2-V617F in BaF3 cells. Tyrosine phosphorylation of IL27Ra was not required to induce activation of JAK2-V617F or STAT5, or to enhance the transforming activity of JAK2-V617F. Expression of IL3Ra or the common β chain in BaF3 cells also enhanced the ability of JAK2-V617F to transform these hematopoietic cells. However, the heterodimeric receptor component IL12RB1 did not enhance the activation or transforming signals of JAK2-V617F in BaF3 cells. IL27Ra also activated the K539L and R683G JAK2 mutants. Together our data demonstrate that in addition to homodimeric receptors, some heterodimeric receptor components can support the activation and transforming signals of JAK2-V617F and other JAK2 mutants. Therefore, heterodimeric receptors may play unappreciated roles in JAK2 activation in the development of hematopoietic diseases including myeloproliferative neoplasms.     

Distinct roles for STAT1, STAT3, and STAT5 in differentiation gene induction and apoptosis inhibition by interleukin-9.

Interleukin-9 (IL-9) activates three distinct STAT proteins: STAT1, STAT3, and STAT5. This process depends on one tyrosine of the IL-9 receptor, which is necessary for proliferation, gene induction, and inhibition of apoptosis induced by glucocorticoids. By introduction of point mutations in amino acids surrounding this tyrosine, we obtained receptors that activated either STAT5 alone or both STAT1 and STAT3, thus providing us with the possibility to study the respective roles of these factors in the biological activities of IL-9. Both mutant receptors were able to prevent apoptosis, but only the mutant that activated STAT1 and STAT3 was able to support induction of granzyme A and L-selectin. In line with these results, constitutively activated STAT5 blocked glucocorticoid-induced apoptosis. In Ba/F3 cells, significant proliferation and pim-1 induction were observed with both STAT-restricted mutants, though proliferation was lower than with the wild-type receptor. These results suggest that survival and cell growth are redundantly controlled by multiple STAT factors, whereas differentiation gene induction is more specifically correlated with individual STAT activation by IL-9.     

STAT3 is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production

IL-27, a member of the IL-6/IL-12 family, activates both STAT1 and STAT3 through its receptor, which consists of WSX-1 and gp130 subunits, resulting in augmentation of Th1 differentiation and suppression of proinflammatory cytokine production. In the present study, we investigated the role of STAT3 in the IL-27-mediated immune functions. IL-27 induced phosphorylation of STAT1, -2, -3 and -5 in wild-type naive CD4+ T cells, but failed to induce that of STAT3 and STAT5 in STAT3-deficient cohorts. IL-27 induced not only proinflammatory responses including up-regulation of ICAM-1, T-box expressed in T cells, and IL-12Rbeta2 and Th1 differentiation, but also anti-inflammatory responses including suppression of proinflammatory cytokine production such as IL-2, IL-4, and IL-13 even in STAT3-deficient naive CD4+ T cells. In contrast, IL-27 augmented c-Myc and Pim-1 expression and induced cell proliferation in wild-type naive CD4+ T cells but not in STAT3-deficient cohorts. Moreover, IL-27 failed to activate STAT3, augment c-Myc and Pim-1 expression, and induce cell proliferation in pro-B BaF/3 transfectants expressing mutant gp130, in which the putative STAT3-binding four Tyr residues in the YXXQ motif of the cytoplasmic region was replaced by Phe. These results suggest that STAT3 is activated through gp130 by IL-27 and is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production. Thus, IL-27 may be a cytokine, which activates both STAT1 and STAT3 through distinct receptor subunits, WSX-1 and gp130, respectively, to mediate its individual immune functions.