Phosphoinositide 3-kinase regulation of T cell receptor-mediated interleukin-2 gene expression in normal T cells

PTEN drives Th17 cell differentiation by preventing IL-2 production

Th17 cells mediate inflammation and autoimmunity. Although it was known that cytokine IL-2 inhibits Th17 cell differentiation, how it does so was elusive. Using IL-17–specific PTEN-deficient mice, Kim et al. show that phosphatase PTEN inhibits IL-2 production and thus promotes Th17 cell differentiation.

T helper 17 (Th17) cells are a CD4⁺ T cell subset that produces IL-17A to mediate inflammation and autoimmunity. IL-2 inhibits Th17 cell differentiation. However, the mechanism by which IL-2 is suppressed during Th17 cell differentiation remains unclear. Here, we show that phosphatase and tensin homologue (PTEN) is a key factor that regulates Th17 cell differentiation by suppressing IL-2 production. Th17-specific Pten deletion (Pten^fl/flIl17a^cre) impairs Th17 cell differentiation in vitro and ameliorated symptoms of experimental autoimmune encephalomyelitis (EAE), a model of Th17-mediated autoimmune disease. Mechanistically, Pten deficiency up-regulates IL-2 and phosphorylation of STAT5, but reduces STAT3 phosphorylation, thereby inhibiting Th17 cell differentiation. PTEN inhibitors block Th17 cell differentiation in vitro and in the EAE model. Thus, PTEN plays a key role in Th17 cell differentiation by blocking IL-2 expression.

Pten Regulates Retinal Amacrine Cell Number by Modulating Akt, Tgfβ, and Erk Signaling

All tissues are genetically programmed to acquire an optimal size that is defined by total cell number and individual cellular dimensions. The retina contains stereotyped proportions of one glial and six neuronal cell types that are generated in overlapping waves. How multipotent retinal progenitors know when to switch from making one cell type to the next so that appropriate numbers of each cell type are generated is poorly understood. Pten is a phosphatase that controls progenitor cell proliferation and differentiation in several lineages. Here, using a conditional loss-of-function strategy, we found that Pten regulates retinal cell division and is required to produce the full complement of rod photoreceptors and amacrine cells in mouse. We focused on amacrine cell number control, identifying three downstream Pten effector pathways. First, phosphoinositide 3-kinase/Akt signaling is hyperactivated in Pten conditional knock-out (cKO) retinas, and misexpression of constitutively active Akt (Akt-CA) in retinal explants phenocopies the reduction in amacrine cell production observed in Pten cKOs. Second, Akt-CA activates Tgfβ signaling in retinal explants, which is a negative feedback pathway for amacrine cell production. Accordingly, Tgfβ signaling is elevated in Pten cKO retinas, and epistatic analyses placed Pten downstream of TgfβRII in amacrine cell number control. Finally, Pten regulates Raf/Mek/Erk signaling levels to promote the differentiation of all amacrine cell subtypes, which are each reduced in number in Pten cKOs. Pten is thus a positive regulator of amacrine cell production, acting via multiple downstream pathways, highlighting its diverse actions as a mediator of cell number control.

SIGNIFICANCE STATEMENT

Despite the importance of size for optimal organ function, how individual cell types are generated in correct proportions is poorly understood. There are several ways to control cell number, including readouts of organ function (e.g., secreted hormones reach functional levels when enough cells are made) or counting of cell divisions or cell number. The latter applies to the retina, where cell number is regulated by negative feedback signals, which arrest differentiation of particular cell types at threshold levels. Herein, we show that Pten is a critical regulator of amacrine cell number in the retina, acting via multiple downstream pathways. Our studies provide molecular insights into how PTEN loss in humans may lead to uncontrolled cell division in several pathological conditions.

PI3K pathway inhibitors: potential prospects as adjuncts to vaccine immunotherapy for glioblastoma

Constitutive activation of the PI3K pathway has been implicated in glioblastoma (GBM) pathogenesis. Pharmacologic inhibition can both inhibit tumor survival and downregulate expression of programmed death ligand-1, a protein highly expressed on glioma cells that strongly contributes to cancer immunosuppression. In that manner, PI3K pathway inhibitors can help optimize GBM vaccine immunotherapy. In this review, we describe and assess the potential integration of various classes of PI3K pathway inhibitors into GBM immunotherapy. While early-generation inhibitors have a wide range of immunosuppressive effects that could negate their antitumor potency, further work should better characterize how contemporary inhibitors affect the immune response. This will help determine if these inhibitors are truly a therapeutic avenue with a strong future in GBM immunotherapy.

RAS/ERK signaling controls proneural genetic programs in cortical development and gliomagenesis

Neural cell fate specification is well understood in the embryonic cerebral cortex, where the proneural genes Neurog2 and Ascl1 are key cell fate determinants. What is less well understood is how cellular diversity is generated in brain tumors. Gliomas and glioneuronal tumors, which are often localized in the cerebrum, are both characterized by a neoplastic glial component, but glioneuronal tumors also have an intermixed neuronal component. A core abnormality in both tumor groups is overactive RAS/ERK signaling, a pro-proliferative signal whose contributions to cell differentiation in oncogenesis are largely unexplored. We found that RAS/ERK activation levels differ in two distinct human tumors associated with constitutively active BRAF. Pilocytic astrocytomas, which contain abnormal glial cells, have higher ERK activation levels than gangliogliomas, which contain abnormal neuronal and glial cells. Using in vivo gain of function and loss of function in the mouse embryonic neocortex, we found that RAS/ERK signals control a proneural genetic switch, inhibiting Neurog2 expression while inducing Ascl1, a competing lineage determinant. Furthermore, we found that RAS/ERK levels control Ascl1's fate specification properties in murine cortical progenitors--at higher RAS/ERK levels, Ascl1(+) progenitors are biased toward proliferative glial programs, initiating astrocytomas, while at moderate RAS/ERK levels, Ascl1 promotes GABAergic neuronal and less glial differentiation, generating glioneuronal tumors. Mechanistically, Ascl1 is phosphorylated by ERK, and ERK phosphoacceptor sites are necessary for Ascl1's GABAergic neuronal and gliogenic potential. RAS/ERK signaling thus acts as a rheostat to influence neural cell fate selection in both normal cortical development and gliomagenesis, controlling Neurog2-Ascl1 expression and Ascl1 function.

MiR-17-92 cluster: an apoptosis inducer or proliferation enhancer

Study of the non-coding RNA roles in the regulation of adaptive immune responses through T cells could be the basis of novel therapeutic applications. MicroRNAs (miRNAs) are a class of short non-coding RNAs that control the cell's functions and destination. To investigate the role of miRNAs in T cell activation, herein the expressions of miR-17-92 cluster and its paralogs were studied in naïve CD4(+)T cells that were activated by anti-CD2, -CD3, -CD28 microbeads and induced with or without IL-2. Proliferation and apoptosis rate of the cultured cells were determined by BrdU incorporation assay (ELISA) and propidium iodide staining, respectively. In continuation the expressions of eight miRNAs of the mentioned clusters were analyzed quantitatively. In addition their potential targets were predicted using multiple algorithms; as a confirmation, the transcription of PIK3R3 (a putative target of modulated miRNAs) was evaluated. Stimulation index (SI) of activated cells was decreased on day 6; whereas, the IL-2 induced cells showed increase in SI in the assay time. Evaluation of eight members of the aforementioned cluster showed upregulation of miR-92a-2* (~15 times) in IL-2 un-induced (activated) cells relative to the IL-2 induced cells. In silico investigations revealed that the suggested miRNAs targeted genes that were involved in cell proliferation, survival, and apoptosis. Transcriptional analysis of PIK3R3 illustrated decrease in activated cells relative to IL-2 induced cells. According to our findings, it seems that multiple members of miR-17-92 families in activated CD4(+)T cells inhibited negative regulators of IL-2 such as DUSP, PTPN, and SOCS families after IL-2 induction. According to our findings, it seems that multiple genes of cell proliferation-related families such as MAPK, E2F, AKT, STAT, and JAK as well as PIK3R3 are inhibited by miR-17-92 cluster in activated cells. As FASL is a putative target of over-expressed miRNAs in activated cell, antigen-induced cell death (AICD) might be occurred in FASL-independent manner. Altogether this study suggested that clonal expansion through IL-2 signaling pathway does not depend on the members of miR-17-92 family; while, it appears that AICD in activated CD4(+)T cells without IL-2 induction is affected by these miRNA clusters.

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

Knockdown of PIK3R1 by shRNA inhibits the activity of the splenic macrophages associated with hypersplenism due to portal hypertension

Phosphatidylinositol 3-kinase (PI3K) plays a central role in the metabolic actions of insulin. One 85 kDa regulatory subunit of PIK3 is encoded by phosphoinositide-3-kinase, the regulatory subunit 1 (PIK3R1). Our previous study has demonstrated that PIK3R1 was up-regulated significantly in the splenic macrophage (MΦ) of portal hypertensive spleen. In the present study, RNA interference specific to PIK3R1 was employed to investigate its inhibitive effects on the activity of MΦ associated with hypersplenism due to portal hypertension (HS-PHT). The expression of PIK3R1 in the spleen was detected by immunohistochemical staining. Plasmid vector pGenesil-1 expressing specific small hairpin RNA (shRNA) against PIK3R1 and the scrambled shRNA control was constructed. MΦ were isolated and purified by anchored cultivation from patients with HS-PHT (HS-PHT-MΦ) and traumatic rupture of the spleen (Con-MΦ). After transfection into MΦ, PIK3R1 expression at both the mRNA and the protein level was examined by real-time polymerase chain reaction and Western blot. The activities of MΦ were determined, and the expression and activity of NF-κB were also detected. Immunohistochemistry revealed expression and cellular distribution of PIK3R1 in the spleen. The PIK3R1-shRNA was successfully synthesized and cloned into the plasmid vector pGenesil-1, and specifically suppressed PIK3R1 expression at both the mRNA and the protein level. After transfection into HS-PHT-MΦ and Con-MΦ, PIK3R1 knockdown inhibited the viability of MΦ, reduced the phagocytic rate, the rate of antigen-presenting positive cells, the metabolic rate, and the secretion of IL-1β and TNF-α (all p<0.05), and decreased the expression and activity of NF-κB. Our data showed that the knocking down of PIK3R1 with shRNA produced by pGenesil-1 led to inhibition of viability and to decreased activity of MΦ associated with HS-PHT in vitro. Therefore, it is tempting to speculate that PIK3R1 might play a considerable role in the pathogenesis of HS-PHT, and inhibition of PIK3R1 expression might be a novel therapeutic strategy for HS-PHT.

Cyclin D1 induction by benzo[a]pyrene-7,8-diol-9,10-epoxide via the phosphatidylinositol 3-kinase/Akt/MAPK- and p70s6k-dependent pathway promotes cell transformation and tumorigenesis

Benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE), the major metabolite of B[a]P, has been well recognized as one ubiquitous carcinogen, but the molecular mechanism involved in its carcinogenic effect remains obscure. In the present study, we found that bronchial epithelial cells (Beas-2B) and hepatocytes treated with B[a]PDE presented a significant increase of cyclin D1 expression. Moreover, Akt, p70(s6k), and MAPKs including JNK, Erks, and p38 were notably activated in B[a]PDE-treated Beas-2B cells, whereas NF-kappaB, NFAT, and Egr-1 were not. Our results demonstrated that JNK and Erks were required in B[a]PDE-induced cyclin D1 expression because the inhibition of JNK or Erks by a selective chemical inhibitor or dominant negative mutant robustly impaired the cyclin D1 induction by B[a]PDE. Furthermore, we found that overexpression of the dominant negative mutant of p85 (regulatory subunit of phosphatidylinositol 3-kinase) or Akt dramatically suppressed B[a]PDE-induced JNK and Erk activation as well as cyclin D1 expression, suggesting that cyclin D1 induction by B[a]PDE is via the phosphatidylinositol 3-kinase/Akt/MAPK-dependent pathway. In addition, we clarified that p70(s6k) is also involved in B[a]PDE-induced cyclin D1 expression because rampamycin pretreatment dramatically reduced cyclin D1 induction by B[a]PDE. More importantly, we demonstrated that up-regulated cyclin D1 by B[a]PDE plays a critical role in oncogenic transformation and tumorigenesis of Beas-2B cells. These results not only broaden our knowledge of the molecular mechanism of B[a]PDE carcinogenicity but also lead to the further study of chemoprevention of B[a]PDE-associated human cancers.

PI3K/Akt/JNK/c-Jun signaling pathway is a mediator for arsenite-induced cyclin D1 expression and cell growth in human bronchial epithelial cells

Arsenite exposure is associated with an increased risk of human lung cancer. However, the molecular mechanisms underlying the arsenite-induced human lung carcinogenesis remain elusive. In this study, we demonstrated that arsenite upregulates cyclin D1 expression/activity to promote the growth of human bronchial epithelial Beas-2B cells. In this process, the JNKs (c-Jun N-terminal kinases)/c-Jun cascade is elicited. The inhibition of JNKs or c-Jun by chemical or genetic inhibitors blocks the cyclin D1 induction mediated by arsenite. Furthermore, using a loss of function mutant of p85 (Deltap85, a subunit of PI3K) or dominant-negative Akt (DN-Akt), we showed that PI3K and Akt act as the upstream regulators of JNKs and c-Jun in arsenite-mediated growth promotion. Overall, our data suggest a pathway of PI-3K/Akt/JNK/c-Jun/cylin D1 signaling in response to arsenite in human bronchial epithelial cells.

Kupffer cells mediate leptin-induced liver fibrosis

BACKGROUND & AIMS

Leptin has profibrogenic effects in liver, although the mechanisms of this process are unclear. We sought to elucidate the direct and indirect effects of leptin on hepatic stellate cells (HSCs).

METHODS

HSCs from Sprague-Dawley rats were exposed to leptin and expression of collagen-I, tissue inhibitor of matrix metalloproteinases-1 (TIMP1), transforming growth factor beta1 (TGF-beta1), and connective tissue growth factor (CTGF/CCN2) was assessed. The effects of medium from Kupffer cells (KCs) and sinusoidal endothelial cells (SECs) following leptin were evaluated in HSCs; alpha-smooth muscle actin (alphaSMA) production and KC signaling were analyzed.

RESULTS

HSCs were not activated by incubation with leptin. However, HSCs cultured with medium taken from KCs that were incubated with leptin had increased expression of collagen I, TIMP1, TGF-beta1, and CTGF/CCN2, as well as alphaSMA protein levels and proliferation. These effects were leptin receptor dependent because conditioned medium from KCs isolated from leptin receptor-deficient Zucker (fa/fa) rats did not activate HSCs. In KCs incubated with leptin, messenger RNA and protein expression of TGF-beta1 and CTGF/CCN2 increased. Leptin potentiated signal transducer and activator of transcription 3, AKT, and extracellular signal-related kinase 1/2 phosphorylation in KCs and increased AP-1 and nuclear factor-kappaB DNA binding. Finally, addition of anti-TGF-beta to KC-conditioned medium inhibited HSC expression of collagen I, TIMP1, and CTGF/CCN2, whereas signal transducer and activator of transcription 3 inhibitor attenuated TGF-beta1 production by KC.

CONCLUSIONS

Leptin mediates HSC activation and liver fibrosis through indirect effects on KC; these effects are partly mediated by TGF-beta1.

Kupffer cells mediate leptin-induced liver fibrosis

BACKGROUND & AIMS

Leptin has profibrogenic effects in liver, although the mechanisms of this process are unclear. We sought to elucidate the direct and indirect effects of leptin on hepatic stellate cells (HSCs).

METHODS

HSCs from Sprague-Dawley rats were exposed to leptin and expression of collagen-I, tissue inhibitor of matrix metalloproteinases-1 (TIMP1), transforming growth factor beta1 (TGF-beta1), and connective tissue growth factor (CTGF/CCN2) was assessed. The effects of medium from Kupffer cells (KCs) and sinusoidal endothelial cells (SECs) following leptin were evaluated in HSCs; alpha-smooth muscle actin (alphaSMA) production and KC signaling were analyzed.

RESULTS

HSCs were not activated by incubation with leptin. However, HSCs cultured with medium taken from KCs that were incubated with leptin had increased expression of collagen I, TIMP1, TGF-beta1, and CTGF/CCN2, as well as alphaSMA protein levels and proliferation. These effects were leptin receptor dependent because conditioned medium from KCs isolated from leptin receptor-deficient Zucker (fa/fa) rats did not activate HSCs. In KCs incubated with leptin, messenger RNA and protein expression of TGF-beta1 and CTGF/CCN2 increased. Leptin potentiated signal transducer and activator of transcription 3, AKT, and extracellular signal-related kinase 1/2 phosphorylation in KCs and increased AP-1 and nuclear factor-kappaB DNA binding. Finally, addition of anti-TGF-beta to KC-conditioned medium inhibited HSC expression of collagen I, TIMP1, and CTGF/CCN2, whereas signal transducer and activator of transcription 3 inhibitor attenuated TGF-beta1 production by KC.

CONCLUSIONS

Leptin mediates HSC activation and liver fibrosis through indirect effects on KC; these effects are partly mediated by TGF-beta1.

IL-4 protects the B-cell lymphoma cell line CH31 from anti-IgM-induced growth arrest and apoptosis: contribution of the PI-3 kinase/AKT pathway

Interleukin-4 (IL-4) promotes lymphocyte survival and protects primary lymphomas from apoptosis. Previous studies reported differential requirements for the signal transducer and activator of transcription 6 (STAT6) and IRS2/phosphatidylinositol 3 kinase (PI-3K) signaling pathways in mediating the IL-4-induced protection from Fas-mediated apoptosis. In this study, we characterized IL-4-activated signals that suppress anti-IgM-mediated apoptosis and growth arrest of CH31, a model B-cell lymphoma line. In CH31, anti-IgM treatment leads to the loss of mitochondrial membrane potential, phospho-Akt, phospho-CDK2, and c-myc protein. These losses are followed by massive induction of p27(Kip1) protein expression, cell cycle arrest, and apoptosis. Strikingly, IL-4 treatment prevented or reversed these changes. Furthermore, IL-4 suppressed the activation of caspases 9 and 3, and, in contrast to previous reports, induced the phosphorylation (deactivation) of BAD. IL-4 treatment also induced expression of BclxL, a STAT6-dependent gene. Pharmacologic inhibitors and dominant inhibitory forms of PI-3K and Akt abrogated the anti-apoptotic function of IL-4. These results suggest that the IL-4 receptor activates several signaling pathways, with the Akt pathway playing a major role in suppression of the apoptotic program activated by anti-IgM.

PI-3K/Akt signal pathway plays a crucial role in arsenite-induced cell proliferation of human keratinocytes through induction of cyclin D1

Exposure of arsenite can induce hyperproliferation of skin cells, which is believed to play important roles in arsenite-induced carcinogenesis by affecting both promotion and progression stages. However, the signal pathways and target genes activated by arsenite exposure responsible for the proliferation remain to be defined. In the present study, we found that: (1) exposure of human keratinocytic HaCat cells to arsenite caused an increase in cell proliferation, which was significantly inhibited by pretreatment of wortmannin, a specific chemical inhibitor of PI-3K/Akt signal pathway; (2) arsenite exposure was also able to activate PI-3K/Akt signal pathway, which thereby induced the elevation of cyclin D1 expression level in both HaCat cells and human primary keratinocytes based on that inhibition of PI-3K/Akt pathway by either pretreatment of wortmannin or the transfection of their dominant mutants, significantly inhibited cyclin D1 expression upon arsenite exposure; (3) PI-3K/Akt pathway is implicated in arsenite-induced proliferation of HaCat cells through the induction of cyclin D1 because either knockdown of cyclin D1 by its siRNA or inhibition of PI-3K/Akt signal pathway by their dominant mutants markedly impaired the proliferation of HaCat cells induced by arsenite exposure. Taken together, we provide the direct evidence that PI-3K/Akt pathway plays a role in the regulation of cell proliferation through the induction of cyclin D1 in human keratinocytes upon arsenite treatment. Given the importance of aberrant cell proliferation in cell transformation, we propose that the activation of PI-3K/Akt pathway and cyclin D1 induction may be the important mediators of human skin carcinogenic effect of arsenite.

Cutting Edge: Abortive Proliferation of CD46-Induced Tr1-Like Cells due to a Defective Akt/Survivin Signaling Pathway

T regulatory cell 1 (Tr1) are low proliferating peripherally induced suppressive T cells. Engaging CD3 and CD46 on human CD4+ T cells induces a Tr1-like phenotype. In this study, we report that human Tr1-like cells do not sustain proliferation over time. The weak proliferation of these cells results first from their inability to sustain expression of various cell cycle-associated proteins, to efficiently degrade the inhibitor of cell cycle progression p27/Kip1 and, as a consequence, in their accumulation in the G0-G1 phase. Also, the reduced proliferation of Tr1-like cells results from their increased sensitivity to death as they divide, through a mechanism that is neither Fas-mediated nor Bcl2/Bcl-xL related. Both properties, impaired cell cycle and death sensitivity, are explained by a specific defective activation of Akt that impairs the expression of Survivin. Thus, our results show that CD3/CD46-induced Tr1-like cells die through a process of abortive proliferation.

Phosphatidylinositol 3-kinase improves the efficiency of positive selection

We have generated transgenic mice expressing the amino-terminal fragment of the phosphatidylinositol 3-kinase (PI3K) catalytic subunit (p110ABD) in thymocytes. Expression of p110ABD results in constitutive activation of PI3K and in significant increases in the numbers of mature, single-positive thymocytes. We previously reported that the increase in mature cells was in part due to a defect in thymic emigration. In this study we identify another component to this phenotype. Expression of p110ABD results in an enhancement of positive selection, without alterations in thymocyte lifespan or negative selection. Since PI3K can affect activation of Btk, which in turn potentiates calcium fluxes, during B cell development, our results suggest that PI3K could play a role in the regulation of Itk kinases in T cells, and that both cell types share a common signaling network to modulate calcium responses downstream of their antigen receptor.

Involvement of phosphatidylinositol-3 kinase–Akt and nuclear factor kappa-B pathways in the effect of frutalin on human lymphocyte

The mechanisms involved in the mitogenic effect of lectins are not fully understood and are thought to involve a cascade of intracellular signals related to T cell receptor activation. This study shows that frutalin, the alpha-D-galactose-binding lectin from Artocarpus incisa seeds, is a potent mitogenic activator of human lymphocytes. This effect is inhibited by D-galactose and PI3K inhibitors, and is accompanied by an increase in IL-2 receptor expression and by a PI3K-dependent IL-2 gene expression and IL-2 protein synthesis. Frutalin also induces Akt-phosphorylation and activates NF-kappaB, inducing its translocation from the cytosol to the nucleus. Both effects are blocked in the presence of D-galactose or by PI3K inhibitors. In summary, frutalin, interacting with alpha-D-galactose, activates signaling pathways related to TCR, and thereby triggers PI3K/Akt and NF-kappaB pathway, which modulates T cell proliferation, IL-2 synthesis and IL-2R expression. Frutalin might be a useful tool to study intracellular mechanisms following T cell activation that link upstream signaling pathways to downstream events.

Essential roles of PI-3K/Akt/IKKbeta/NFkappaB pathway in cyclin D1 induction by arsenite in JB6 Cl41 cells

Skin is a major target of carcinogenic trivalent arsenic (arsenite, As3+). It has been thought that cell proliferation is one of the central events involved in the carcinogenic effect of arsenite. Cyclin D1, a nuclear protein playing a pivotal role in cell proliferation and cell cycle transition from G1 to S phases, has been reported to be induced in human fibroblast by arsenite via uncertain molecular mechanisms. In the present study, the potential roles of PI-3K/Akt/IKKbeta/NFkappaB signal pathway in cyclin D1 induction by arsenite were addressed in mouse epidermal Cl41 cells. We found that exposure of Cl41 cells to arsenite was able to induce cell proliferation, activate PI-3K-->Akt/p70(S6k) signal pathway and increase cyclin D1 expression at both transcription and protein levels. Pre-treatment of Cl41 cells with PI-3K inhibitor, wortmannin, significantly inhibited the phosphorylation of Akt and p70(S6k) and thereby dramatically impaired the cyclin D1 induction by arsenite, implicating the importance of the PI-3K signal pathway in the cyclin D1 induction by arsenite. Furthermore, inhibition of PI-3K/Akt by overexpression of Deltap85 or DN-Akt blocked arsenite-induced IKK phosphorylation, IkappaBalpha degradation and cyclin D1 expression, indicating that IKK/NFkappaB is the downstream transducer of arsenite-triggered PI-3K/Akt cascade. Moreover, inhibition of IKKbeta/NFkappaB signal pathway by overexpression of its dominant negative mutant, IKKbeta-KM, also significantly blocked arsenite-induced cyclin D1 expression. Overall, arsenite exposure triggered PI-3K/Akt/IKKbeta/NFkappaB signal cascade which in turn plays essential roles in inducing cyclin D1 expression.

A Role for Phosphatidylinositol 3-Kinase in TCR-Stimulated ERK Activation Leading to Paxillin Phosphorylation and CTL Degranulation

PI3K is an important regulator of a number of cellular processes. We examined the contribution of PI3K to mouse CTL signaling, leading to degranulation. We show that TCR-triggered, but not phorbol ester and calcium ionophore-induced, CTL degranulation is dependent on PI3K activity. Although PI3K activity is required for optimal LFA-1-mediated adhesion and cell spreading, this most likely does not account for its full contribution to degranulation. We demonstrate that PI3K is required for TCR-stimulated ERK activation in CTL, which we have shown previously to be required for CTL degranulation. We thus define a pathway through which PI3K most likely regulates degranulation and in which ERK appears to be a key signaling molecule. Furthermore, we identified the cytoskeletal adaptor paxillin as a target of ERK downstream of TCR stimulation. Consistent with a role in degranulation, we demonstrate that paxillin is localized to the microtubule organizing center in resting cells and upon target cell binding is recruited to the contact point with the target cell. These studies demonstrate that PI3K regulates ERK activity leading to CTL degranulation, and identify paxillin as a target of ERK downstream of the TCR. That paxillin is independently phosphorylated by both tyrosine kinase(s) and ERK downstream of the TCR and localized both at the microtubule organizing center and at the target cell contact point suggests an important role for paxillin in CTL-mediated killing.

Effect of Mycobacterium vaccae on cytokine responses in children with atopic dermatitis

The increasing prevalence of atopic diseases over the last few decades is thought to be due to reduced exposure to environmental microbes that normally down-regulate allergic responses (hygiene hypothesis). We have shown previously that administration of the environmental microbe Mycobacterium vaccae ameliorates atopic dermatitis in school-age children at 3 months post-treatment. The present study tested the hypothesis that M. vaccae suppresses Th2-type cytokine activity and increases transforming growth factor (TGF)-beta(1) immunomodulatory activity in these children. Interleukin (IL)-4, IL-5, TGF-beta(1) and interferon (IFN)-gamma activity were assessed in resting and stimulated peripheral blood mononuclear cells (PBMC) isolated from 12 of the children who received M. vaccae in our original clinical trial. A cDNA expression array was used to examine a broader range of cytokine pathway transcripts. There were no significant changes in either Th2-type or TGF-beta(1) activity. A 5- to 10-fold increase in Th1-type activity was found at 1 month post-M. vaccae administration (P < 0.05), but it had returned to baseline by 3 months. The results do not support the hypothesis that M. vaccae reduces Th2-type or increases TGF-beta(1) activity of PBMC isolated from children with atopic dermatitis. The transient surge in IFN-gamma at 1 month is unlikely to explain any improvement in eczema score at 3 months.

CD28 regulates the translation of Bcl-xL via the phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway

In concert with the TCR, CD28 promotes T cell survival by regulating the expression of the antiapoptotic protein Bcl-x(L). The mechanism by which CD28 mediates the induction of Bcl-x(L) remains unknown. We show that although signaling through the TCR is sufficient to stimulate transcription of Bcl-x(L) mRNA, CD28, by activating PI3K and mammalian target of rapamycin, provides a critical signal that regulates the translation of Bcl-x(L) transcripts. We observe that CD28 induced 4E-binding protein-1 phosphorylation, an inhibitor of the translational machinery, and that CD28 costimulation directly augmented the translation of a Bcl-x(L) 5'-untranslated region reporter construct. Lastly, costimulation by CD28 shifted the distribution of Bcl-x(L) mRNA transcripts from the pretranslation complex to the translationally active polyribosomes. These results demonstrate that CD28 relieves the translational inhibition of Bcl-x(L) in a PI3K/mammalian target of rapamycin-dependent manner.

PTEN permits acute increases in D3-phosphoinositide levels following TCR stimulation but inhibits distal signaling events by reducing the basal activity of Akt

Phosphoinositide 3-kinase (PI3K) is important in TCR signaling. PI3K generates phosphatidylinositol 3, 4, 5-trisphosphate (PI-3,4,5-P3), which regulates membrane localization and/or activity of multiple signaling proteins. PTEN (phosphatase and tensin homologue deleted on chromosome 10) opposes PI3K, reversing this reaction. Maintaining the balance between these two enzymes is important for normal T cell function. Here we use the PTEN-null Jurkat T cell line to address the role of PTEN in modulating proximal and distal TCR-signaling events. PTEN expression at levels that restored low basal Akt phosphorylation (an indicator of PI-3,4,5-P3 levels), but which were not themselves cytotoxic, had minimal effect on TCR-stimulated activation of phospholipase Cgamma1 and Ca2+ flux, but reduced the duration of extracellular signal-regulated kinase (Erk) activation. Distal signaling events, including nuclear factor of activated T cells (NFAT) activation, CD69 expression and IL-2 production, were all inhibited by PTEN expression. Notably, PTEN did not block TCR-stimulated PI-3,4,5-P3 accumulation. The effect of PTEN on distal TCR signaling events was strongly correlated with the loss of the constitutive Akt activation and glycogen synthase kinase-3 (GSK3) inhibition that is typical of Jurkat cells, and could be reversed by expression of activated Akt or pharmacologic inhibition of GSK3. These results suggest that PTEN acts in T cells primarily to control basal PI-3,4,5-P3 levels, rather than opposing PI3K acutely during TCR stimulation.

Enhanced T cell proliferation in mice lacking the p85beta subunit of phosphoinositide 3-kinase

Phosphoinositide 3-kinase activation is important for lymphocyte proliferation and survival. Disrupting the gene that encodes the major phosphoinositide 3-kinase regulatory isoform p85alpha impairs B cell development and proliferation. However, T cell functions are intact in the absence of p85alpha. In this study, we test the hypothesis that the related isoform p85beta is an essential regulatory subunit for T cell signaling. Unexpectedly, T cells lacking p85beta showed a marked increase in proliferation and decreased death when stimulated with anti-CD3 plus IL-2. Both CD4(+) and CD8(+) T cells completed more cell divisions. Transcriptional profiling revealed reduced levels of caspase-6 mRNA in p85beta-deficient T cells, which was paralleled by reduced caspase-6 enzyme activity. Increased T cell accumulation was also observed in vivo following infection of p85beta-deficient mice with mouse hepatitis virus. Together, these results suggest a unique role for p85beta in limiting T cell expansion.

Phosphoinositide 3-kinase: diverse roles in immune cell activation

Cells of the immune system carry out diverse functions that are controlled by surface receptors for antigen, costimulatory molecules, cytokines, chemokines, and other ligands. A shared feature of signal transduction downstream of most receptors on immune cells, as in nonhematopoietic cell types, is the activation of phosphoinositide 3-kinase (PI3K). The mechanism by which this common signaling event is elicited by distinct receptors and contributes to unique functional outcomes is an intriguing puzzle. Understanding how specificity is achieved in PI3K signaling is of particular significance because altered regulation of this pathway is observed in many disease states, including leukemia and lymphoma. Here we review recent advances in the understanding of PI3K signaling mechanisms in different immune cells and receptor systems. We emphasize the concept that PI3K and its products are components of complex networks of interacting proteins and second messengers, rather than simple links in linear signaling cascades.

Cutting Edge: Extracellular Signal-Regulated Kinases 1/2 Function as Integrators of TCR Signal Strength

Altered signaling through the TCR is currently showing promise for immunotherapy. However, the molecular mechanisms are not completely understood. Therefore, we investigated whether varying the strength of TCR engagement in various human T cells would yield different second messenger responses. The kinetics and duration of extracellular signal-regulated kinase (ERK) activation, central to multiple cellular responses, are distinctly dependent on the T cell activation state (naive vs effector), strength of TCR cross-linking, and input from the phosphatidylinositol-3 kinase pathway, which is regulated by cytokines and growth factors. Moreover, the duration of ERK activation affects c-Fos expression, a component of the AP-1 transcription complex. Thus, the character of ERK activation, transient or sustained, acts as a signal integrator to quantify the strength of TCR engagement and direct the cellular response.

EphA2 receptor tyrosine kinase regulates endothelial cell migration and vascular assembly through phosphoinositide 3-kinase-mediated Rac1 GTPase activation

Angiogenesis is critical for vascular remodeling during development and contributes to the pathogenesis of diseases such as cancer. Targeted disruption of several EphB class receptor tyrosine kinases results in vascular remodeling defects during embryogenesis. The role of EphA class receptors in vascular remodeling, however, is not well-characterized. We recently demonstrated that global inhibition of EphA receptors disrupts endothelial migration induced by ephrin, VEGF or tumor-derived signals, though the specific target remained undefined. Here, we report that EphA2 regulates endothelial cell assembly and migration through phosphoinositide (PI) 3-kinase-mediated activation of Rac1 GTPase in two model systems: primary bovine and murine pulmonary microvascular endothelial cells. EphA2-deficient endothelial cells fail to undergo vascular assembly and migration in response to ephrin-A1 in vitro. Ephrin-A1 stimulation induces PI3-kinase-dependent activation of Rac1 in wild-type endothelial cells, whereas EphA2-deficient cells fail to activate Rac1 upon stimulation. Expression of dominant negative PI3-kinase or Rac1 inhibits ephrin-A1-induced endothelial cell migration. Consistent with in vitro data, EphA2-deficient mice show a diminished angiogenic response to ephrin-A1 in vivo. Moreover, EphA2-deficient endothelial cells fail to assemble in vivo when transplanted into recipient mice. These data suggest that EphA2 is an essential regulator of post-natal angiogenesis.

CD152 (CTLA-4) determines the unequal resistance of Th1 and Th2 cells against activation-induced cell death by a mechanism requiring PI3 kinase function

Survival of antigen-experienced T cells is essential for the generation of adaptive immune responses. Here, we show that the genetic and antibody-mediated inactivation of CD152 (cytotoxic T lymphocyte antigen 4) in T helper (Th) effector cells reduced the frequency of nonapoptotic cells in a completely Fas/Fas ligand (FasL)–dependent manner. CD152 cross-linking together with stimulation of CD3 and CD28 on activated Th2 cells prevented activation-induced cell death (AICD) as a result of reduced Fas and FasL expression. Apoptosis protection conferred by CD152 correlated with the up-regulation of Bcl-2 and was mediated by phosphatidylinositol 3 kinase, which prevented FasL expression through the inhibitory phosphorylation of Forkhead transcription factor FKHRL1. We show that signals induced by CD152 act directly on activated T lymphocytes and, due to its differential surface expression on activated Th1 and Th2 cells, induce resistance to AICD mainly in Th2 cells.

PI3K/Akt and apoptosis: size matters

Recent research has examined Akt and Akt-related serine-threonine kinases in signaling cascades that regulate cell survival and are important in the pathogenesis of degenerative diseases and in cancer. We seek to recapitulate the research that has helped to define the current understanding of the role of the Akt pathway under normal and pathologic conditions, also in view of genetic models of Akt function. In particular, we will evaluate the mechanisms of Akt regulation and the role of Akt substrates in Akt-dependent biologic responses in the decisions of cell death and cell survival. Here, we hope to establish the mechanisms of apoptosis suppression by Akt kinase as a framework for a more general understanding of growth factor-dependent regulation of cell survival.

Apoptosis of infiltrating T cells in the central nervous system of mice infected with Theiler's murine encephalomyelitis virus

Theiler murine encephalomyelitis virus (TMEV), DA strain, induces in susceptible strain of mice a biphasic disease consisting of early acute disease followed by late chronic demyelinating disease. Both phases of the disease are associated with inflammatory infiltrates of the central nervous system (CNS). Late chronic demyelinating disease induced by TMEV serves as an excellent model to study human demyelinating disease, multiple sclerosis. During early acute disease, the virus is partially cleared from the CNS by CD3(+) T cells. These T cells express Fas, FasL, negligible levels of Bcl-2 proteins and undergo activation-induced cell death as determined by TUNEL assay leading to resolution of the inflammatory response. In contrast, during late chronic demyelinating disease, and despite dense perivascular and leptomeningeal infiltrates, only very few cells undergo apoptosis. Mononuclear cells infiltrating the CNS express Bcl-2. It appears that the lack of apoptosis of T cells during late chronic demyelinating disease leads to the accumulation of these cells in the CNS. These cells may play a role in the pathogenesis of the demyelinating disease.

Growth factor signaling induces metastasis genes in transformed cells: molecular connection between Akt kinase and osteopontin in breast cancer

Malignant tumors are characterized by excessive growth, immortalization, and metastatic spread, whereas benign tumors do not express gene products that mediate invasion. The molecular basis for this difference is incompletely understood. We have screened signal transduction molecules associated with the epidermal growth factor (EGF) receptor and have identified constitutive phosphorylation, indicative of activation, of Akt kinase in MT2994 breast cancer cells. In contrast, cells of the benign breast epithelial cell lines Comma-D and FSK-7 are immortalized through pathways that are independent of the EGF-phosphatidylinositol 3-kinase-Akt kinase cascade, but this is not associated with invasiveness. Transfection of constitutively active Akt kinase causes accelerated cell division and osteopontin expression. Conversely, dominant-negative Akt kinase slows cell cycle progression and suppresses osteopontin expression. The manipulation of osteopontin expression in this setting by transfection of the gene or its antisense does not affect the growth rate of the cells but alters cell motility and anchorage independence. Therefore, Akt kinase activates two distinct genetic programs: the program of growth and survival, which is independent of osteopontin expression, and the program of invasiveness and anchorage independence, which is mediated by osteopontin. These studies define Akt kinase as a molecular bridge between cell cycle progression and dissemination.

PI3K in lymphocyte development, differentiation and activation

Phosphoinositide 3-kinases (PI3Ks) regulate numerous biological processes, including cell growth, differentiation, survival, proliferation, migration and metabolism. In the immune system, impaired PI3K signalling leads to immunodeficiency, whereas unrestrained PI3K signalling contributes to autoimmunity and leukaemia. New insights into the role of PI3Ks in lymphocyte biology have been derived from gene-targeting studies, which have identified the PI3K subunits that are involved in B-cell and T-cell signalling. In particular, the catalytic subunit p110delta seems to be adapted to transmit antigen-receptor signalling in B and T cells. Additional recent work has provided new insights into the molecular interactions that lead to PI3K activation and the signalling pathways that are regulated by PI3K.

T cell anergy and costimulation

T lymphocytes play a key role in immunity by distinguishing self from nonself peptide antigens and regulating both the cellular and humoral arms of the immune system. Acquired, antigen-specific unresponsiveness is an important mechanism by which T cell responses to antigen are regulated in vivo. Clonal anergy is the term that describes T cell unresponsiveness at the cellular level. Anergic T cells do not proliferate or secrete interleukin (IL)-2 in response to appropriate antigenic stimulation. However, anergic T cells express the IL-2 receptor, and anergy can be broken by exogenous IL-2. Anergy can be induced by submitogenic exposure to peptide antigen in the absence of a costimulatory signal provided by soluble cytokines or by interactions between costimulatory receptors on T cells and counter-receptors on antigen-presenting cells. The molecular events that mediate the induction and maintenance of T cell anergy are the focus of this review. The molecular consequences of CD28-B7 interaction are discussed as a model for the costimulatory signal that leads to T cell activation rather than the induction of anergy.

Lipid phosphatases in the regulation of T cell activation: living up to their PTEN-tial

The initiating events associated with T activation in response to stimulation of the T cell antigen receptor (TCR) and costimulatory receptors, such as CD28, are intimately associated with the enzymatically catalyzed addition of phosphate not only to key tyrosine, threonine and serine residues in proteins but also to the D3 position of the myo-inositol ring of phosphatidylinositol (PtdIns). This latter event is catalyzed by the lipid kinase phosphoinositide 3-kinase (PI3K). The consequent production of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 serves both to recruit signaling proteins to the plasma membrane and to induce activating conformational changes in proteins that contain specialized domains for the binding of these phospholipids. The TCR signaling proteins that are subject to regulation by PI3K include Akt, phospholipase Cgamma1 (PLCgamma1), protein kinase C zeta (PKC-zeta), Itk, Tec and Vav, all of which play critical roles in T cell activation. As is the case for phosphorylation of protein substrates, the phosphorylation of PtdIns is under dynamic regulation, with the D3 phosphate being subject to hydrolysis by the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), thereby placing PTEN in direct opposition to PI3K. In this review we consider recent data concerning how PTEN may act in regulating the process of T cell activation.

Lymphocide: cytokines and the control of lymphoid homeostasis

In a human, about 10(11) excess peripheral lymphocytes die every day. This death process maintains a constant lymphocyte population size in the face of a continuous influx of new lymphocytes and the homeostatic proliferation of old ones. Death is triggered when a lymphocyte fails to acquire signals from survival factors, the availability of which, therefore, determines the size of the pool of lymphocytes. A lymphocyte acquires survival signals through receptors for cytokines, antigens, hormones and probably other extracellular factors. Here, we discuss current concepts of the intracellular signalling pathways for survival versus death that establish cytokine-regulated lymphocyte homeostasis.

YopH prevents monocyte chemoattractant protein 1 expression in macrophages and T-cell proliferation through inactivation of the phosphatidylinositol 3-kinase pathway

Phosphatidylinositol 3-kinase (PI 3-kinase) and its target protein kinase B (Akt) are involved in various processes including internalization, chemotaxis and proliferation. We analysed the activation of Akt in J774 macrophages infected with virulent (pYV+) or avirulent (pYV-) Yersinia enterocolitica. During the early stage of infection with pYV+ and pYV- bacteria, Akt and its targets, glycogen synthase kinase 3 (GSK-3) and forkhead transcription factor (FKHRL1), became phosphorylated. This phosphorylation induction was inhibited by wortmannin and thus dependent on PI 3-kinase. When infection was carried out with pYV+ bacteria but not with pYV- bacteria, Akt and its targets became dephosphorylated at later time points. Using single knock-out mutants in bacterial effector genes, we have determined that the tyrosine phosphatase YopH was responsible for the inactivation of the PI 3-kinase cascade. In macrophages, this inactivation correlated with the downregulation of mRNA coding for monocyte chemoattractant protein 1 (MCP-1), suggesting that YopH inhibits recruitment of macrophages to lymph nodes. We also analysed the effects of Y. enterocolitica infection on the proliferation of T lymphocytes. Consistent with the observation that YopH inactivated the Akt pathway, YopH inhibited PI 3-kinase-dependent secretion of interleukin 2 and proliferation. These data reveal a new effect of YopH in Yersinia pathogenesis.

The basis for TCR-mediated regulation of the IL-2 receptor alpha chain gene: role of widely separated regulatory elements

The interleukin-2 receptor alpha (IL-2Ralpha) chain is a component of high-affinity IL-2 receptors and thus is a key regulator of lymphocyte proliferation. Lineage-restricted and activation-dependent IL-2Ralpha transcription is controlled by four upstream positive regulatory regions (PRRs) and one downstream PRR. We now demonstrate that T-cell receptor (TCR) responsiveness requires both upstream sequences and an intronic region, PRRIV, previously identified as an IL-2 response element. Whereas IL-2 responsiveness requires Stat5 and HMG-I(Y) binding, TCR responsiveness of PRRIV requires two AP-1- and two NFAT-binding sites that bind Jun, Fos and NFAT family members in vitro and in vivo. Moreover, IL-2Ralpha induction is impaired in T lymphocytes from transgenic mice expressing a dominant-negative c-jun construct, or following treatment with cyclosporin A. Thus, our data indicate an important role for both AP-1 and NFAT proteins for TCR-induced IL-2Ralpha expression and establish that both upstream and intronic sequences mediate TCR responsiveness of the IL-2Ralpha gene. Moreover, our data reveal a previously unappreciated link between the TCR-mediated up-regulation of the IL-2 and IL-2Ralpha genes.

CD28 costimulation mediates down-regulation of p27kip1 and cell cycle progression by activation of the PI3K/PKB signaling pathway in primary human T cells

CD28 provides a costimulatory signal that cooperates with the TCR/CD3 complex to induce T cell activation, cytokine production, and clonal expansion. We have recently shown that CD28 directly regulates progression of T lymphocytes through the cell cycle. Although a number of signaling pathways have been linked to the TCR/CD3 and to CD28, it is not known how these two receptors cooperate to induce cell cycle progression. Here, using cell-permeable pharmacologic inhibitors of phosphatidylinositol 3-hydroxykinase (PI3K) and mitogen-activated protein kinase kinase (MEK1/2), we show that cell cycle progression of primary T lymphocytes requires simultaneous activation of PI3K- and MEK1/2-dependent pathways. Decreased abundance of cyclin-dependent kinase inhibitor p27(kip1), which requires simultaneous TCR/CD3 and CD28 ligation, was dependent upon both MEK and PI3K activity. Ligation of TCR/CD3, but not CD28 alone, resulted in activation of MEK targets extracellular signal-related kinase 1/2, whereas ligation of CD28 alone was sufficient for activation of PI3K target protein kinase B (PKB; c-Akt). CD28 ligation alone was also sufficient to mediate inactivating phosphorylation of PKB target glycogen synthase kinase-3 (GSK-3). Moreover, direct inactivation of GSK-3 by LiCl in the presence of anti-CD3, but not in the presence of anti-CD28, resulted in down-regulation of p27(kip1), hyperphosphorylation of retinoblastoma tumor suppressor gene product, and cellular proliferation. Thus, inactivation of the PI3K-PKB target GSK-3 could substitute for CD28 but not for CD3 signals. These results show that the PI3K-PKB pathway links CD28 to cell cycle progression and suggest that p27(kip1) integrates mitogenic MEK- and PI3K-dependent signals from TCR and CD28 in primary T lymphocytes.

HIV Nef increases T cell ERK MAP kinase activity

The human immunodeficiency regulatory protein Nef enhances viral replication and is central to viral pathogenesis. Although Nef has displayed a capacity to associate with a diverse assortment of cellular molecules and to increase T cell activity, the biochemical activity of Nef in T cells remains poorly defined. In this report we examine the bioactivity of Nef in primary CD4 T cells and, in particular, focus on the biochemical pathways known to be central to T cell activity. The extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathway was dramatically affected by Nef expression with increases in ERK, MEK, and Elk induction. The capacity of Nef to increase the MAP kinase pathway activity was dependent on T cell receptor stimulation. By increasing ERK MAP kinase activity, Nef is functionally associated with a kinase known to affect T cell activity, viral replication, and viral infectivity.

Phosphoinositide 3-kinase in immunological systems

Phosphoinositide 3-kinases (PI3Ks) are an evolutionarily conserved family of signal transducing enzymes. A great variety of stimuli activate PI3K, leading to the transient accumulation of its lipid products in cell membranes. These lipids serve as second messengers to regulate the location and activity of an array of downstream effector molecules. In cells of the mammalian immune system, PI3K is activated by receptors for antigen, cytokines, costimulatory molecules, immunoglobulins and chemoattractants. Signaling via PI3K regulates immune cell proliferation, survival, differentiation, chemotaxis, phagocytosis, degranulation, and respiratory burst. Here we review our current understanding of PI3K signaling in leukocytes.

Signaling pathways of D3-phosphoinositide-binding kinases in T cells and their regulation by PTEN

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the D3 position of the myo -inositol ring of inositol phospholipids, producing, amongst others, phosphatidylinositol-(3,4,5)-trisphosphate. This activity is opposed by the lipid phosphatase PTEN, which catalyzes the removal of this phosphate. Stimulation of PI3Ks is elicited by engagement of receptors for antigen, cytokines and chemokines, and by co-stimulatory molecules. Kinases and other enzymes containing pleckstrin homology domains are activated by binding to these phospholipids, affecting a variety of cellular processes that control lymphocyte function, including cell survival, proliferation, chemotaxis and cytoskeletal reorganization. This review highlights the signaling pathways of these kinases and other enzymes in T cells, their biological effects, and their regulation by PTEN.

Identification of AKT-regulated genes in inducible MERAkt cells

AKT/protein kinase B plays a critical role in the phosphoinositide 3-kinase (PI3-kinase) pathway regulating cell growth, differentiation, and oncogenic transformation. Akt1-regulated genes were identified by cDNA array hybridization analysis using an inducible AKT1 protein, MERAKT. Treatment of MERAkt cells with estrogen receptor ligands resulted in phosphorylative activation of MERAKT. Genes differentially expressed in MERAkt/NIH3T3 cells treated with tamoxifen, raloxifene, ICI-182780, and ZK955, were identified at 3 and 20 h. AKT activation resulted in the repression of c-myc, early growth response 1 (EGR1), transforming growth factor beta receptor III (TGF-betar III), and thrombospondin-1 (THBS1). Although c-myc induction is often associated with oncogenic transformation, the c-myc repression observed here is consistent with the anti-apoptotic function of AKT. Repression of THBS1 and EGR1 is consistent with the known pro-angiogenic functions of AKT. AKT-regulated genes were found to be largely distinct from platelet-derived growth factor-beta (PDGFbeta)-regulated genes; only T-cell death-associated gene 51 (TDAG51) was induced in both cases. In contrast to their repression by AKT, c-myc, THBS1, and EGR1 were induced by PDGFbeta, indicating negative interference between elements upstream and downstream of AKT1 in the PDGFbeta signal transduction pathway.

Activator protein 1 activation following hypoosmotic stress in HepG2 cells is actin cytoskeleton dependent

BACKGROUND

Following hypoosmotic stress-induced cell volume change, the actin cytoskeleton reorganizes itself. The role of this reorganization in the activation of the phosphatidylinositol 3-OH-kinase/protein kinase B/activator protein 1 (PI-3-K/PKB/AP-1) proliferative signaling cascade is unknown. Focal adhesion kinase (FAK) participates in the cytoskeleton-based activation of PI-3-K. We hypothesized that hypoosmotic stress-induced activation of PKB and AP-1 in HepG2 cells is dependent on an intact actin cytoskeleton and subsequent FAK phosphorylation.

METHODS

HepG2 cells were incubated for 1 h with or without 20 microM cytochalasin D, an actin disrupter, and were then exposed for up to 30 min to hypoosmotic medium (200 mOsm/L) to induce swelling. Tumor necrosis factor alpha (1.4 nM) and medium alone served as positive and negative controls, respectively. Western blots measured cytoplasmic phosphorylated or total FAK and PKB. EMSAs measured nuclear AP-1. All experiments were performed in triplicate.

RESULTS

Exposure to hypoosmotic stress resulted in activation of the following signaling messengers in a sequential fashion: (1) phosphorylation of FAK occurred by 2 min, (2) phosphorylation of PKB occurred by 10 min, (3) nuclear translocation of AP-1 occurred by 30 min. All three signaling events were abolished when these cells were pretreated with cytochalasin D.

CONCLUSION

Actin reorganization following hypoosmotic stress is essential for the FAK-mediated activation of the PI-3-K/PKB/AP-1 proliferative cascade. These data delineate a possible mechanism by which the cell swelling-induced cytoskeletal changes can initiate proliferative signal transduction in human liver cancer.

The effects of calpain inhibition upon IL-2 and CD25 expression in human peripheral blood mononuclear cells

Calcium is an important contributor to T cell activation; it is also the major factor in the activation of the calcium-activated neutral proteinase, calpain. For this reason, we wanted to investigate if calpain has a role in T cell activation and what aspects of this activation calpain affects. As measured by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), calpain inhibition decreased interleukin-2 (IL-2) and CD25 mRNA expression in a dose-dependent manner, at early time points following the initial activation, and over extended periods of time in activated human peripheral blood mononuclear cells (PBMCs). Using an enzyme-linked immuno-sorbent assay (ELISA) specific for human IL-2, we found that calpain inhibition decreased IL-2 secretion in a dose-dependent manner, shortly after activation, and continuously over time. Inhibiting calpain caused a dose-dependent inhibition of CD25 cell surface expression and also inhibited expression shortly after activation and for at least 48 h. This study showed that calpain has an integral role in the synthesis of the two important T cell activation factors, IL-2 and CD25.

Costimulation by extracellular matrix proteins determines the response to TCR ligation

Although ligation of the T-cell antigen receptor (TCR) is central to the responsiveness and antigen specificity of T-cells, it is insufficient to elicit a response. To determine whether the need for costimulation reflects inadequate strength of signal transduction through the TCR or an absolute block of signaling in the absence of a coligand, we studied T-cell activation under serum-free conditions eliminating costimulation by various extracellular matrix proteins which otherwise have an omnipresent and frequently overlooked effect. Engagement of the TCR leads to induction of Fas, but not to measurable IL-2 secretion or apoptosis. Those activation parameters are induced by costimulation through integrin alphaVbeta3. Furthermore, T-cell survival or elimination is determined by the type of ligand binding to this coreceptor with vitronectin, fibronectin, and fibrinogen efficiently inducing apoptosis and IL-2 production while osteopontin and entactin mediate IL-2 secretion comparably without causing programmed cell death. Consistent with the cytokine properties of these ligands, differential costimulation depends on their presentation in soluble rather than immobilized form. The determination of elimination versus survival of activated T-cells by coligation of beta3-integrins may have bearing on the fundamental postthymic mechanisms that shape the T-cell repertoire.

Cutting edge: distinct motifs within CD28 regulate T cell proliferation and induction of Bcl-XL

CD28 provides an important costimulatory signal in T cell activation that regulates multiple cellular processes including proliferation and survival. Several signal transduction pathways are activated by CD28; however, the precise biochemical mechanism by which CD28 regulates T cell function remains controversial. Retroviral gene transfer into primary T cells from TCR-transgenic, CD28-deficient mice was used to determine the specific sequences within CD28 that determine function. Discrete regions of the cytoplasmic domain of CD28 were identified that differentially regulate T cell proliferation and induction of the anti-apoptotic protein Bcl-X(L). Mutation of C-terminal proline residues abrogated the proliferative and cytokine regulatory features of CD28 costimulation while preserving Bcl-X(L) induction. Conversely, mutation of residues important in phosphatidylinositol 3-kinase activation partially inhibited proliferation but prevented induction of Bcl-X(L.) Thus the ability of CD28 to regulate proliferation and induction of Bcl-X(L) map to distinct motifs, suggesting independent signaling cascades modulate these biologic effects.

Activation of cGMP-dependent protein kinase Ibeta inhibits interleukin 2 release and proliferation of T cell receptor-stimulated human peripheral T cells

Several major functions of type I cGMP-dependent protein kinase (cGK I) have been established in smooth muscle cells, platelets, endothelial cells, and cardiac myocytes. Here we demonstrate that cGK Ibeta is endogenously expressed in freshly purified human peripheral blood T lymphocytes and inhibits their proliferation and interleukin 2 release. Incubation of human T cells with the NO donor, sodium nitroprusside, or the membrane-permeant cGMP analogs PET-cGMP and 8-pCPT-cGMP, activated cGK I and produced (i) a distinct pattern of phosphorylation of vasodilator-stimulated phosphoprotein, (ii) stimulation of the mitogen-activated protein kinases ERK1/2 and p38 kinase, and, upon anti-CD3 stimulation, (iii) inhibition of interleukin 2 release and (iv) inhibition of cell proliferation. cGK I was lost during in vitro culturing of primary T cells and was not detectable in transformed T cell lines. The proliferation of these cGK I-deficient cells was not inhibited by even high cGMP concentrations indicating that cGK I, but not cGMP-regulated phosphodiesterases or channels, cAMP-dependent protein kinase, or other potential cGMP mediators, was responsible for inhibition of T cell proliferation. Consistent with this, overexpression of cGK Ibeta, but not an inactive cGK Ibeta mutant, restored cGMP-dependent inhibition of cell proliferation of Jurkat cells. Thus, the NO/cGMP/cGK signaling system is a negative regulator of T cell activation and proliferation and of potential significance for counteracting inflammatory or lymphoproliferative processes.

Role of phosphatidylinositol 3-kinase in anti-IgM- and anti-IgD-induced apoptosis in B cell lymphomas

Cross-linking of surface Ig receptors with anti IgM (anti-mu heavy chain, anti-mu), but not anti-IgD (anti-delta heavy chain, anti-delta), Abs leads to growth arrest and apoptosis in several extensively characterized B cell lymphomas. By poorly understood mechanisms, both Igs transiently stimulate c-Myc protein expression. However, ultimately, only anti-mu causes a severe loss in c-Myc and a large induction of p27(Kip1) protein expression. Because phosphatidylinositol 3-kinase (PI3K) has been established as a major modulator of cellular growth and survival, we investigated its role in mediating anti-Ig-stimulated outcomes. Herein, we show that PI3K pathways regulate cell cycle progression and apoptosis in the ECH408 B cell lymphoma. Anti-mu and anti-delta driven c-Myc protein changes precisely follow their effects on the PI3K effector, p70(S6K). Upstream of p70(S6K), signaling through both Ig receptors depresses PI3K pathway phospholipids below control with time, which is followed by p27(Kip1) induction. Conversely, anti-delta, but not anti-mu stimulated PI3K-dependent phospholipid return to control levels by 4-8 h. Abrogation of the PI3K pathway with specific inhibitors mimics anti-mu action, potentiates anti-mu-induced cell death and, importantly, converts anti-delta to a death signal. Transfection with active PI3K kinase construct induces anti-mu resistance, whereas transfection with dominant negative PI3K augments anti-mu sensitivity. Our results show that prolonged disengagement of PI3K or down-regulation of its products by anti-mu (and not anti-delta) determines B cell fate.

Akt provides the CD28 costimulatory signal for up-regulation of IL-2 and IFN-gamma but not TH2 cytokines

A region of the interleukin-2 (IL-2) promoter known as the RE/AP element is activated in concert by signals that originate from the T cell antigen receptor and the CD28 coreceptor. We show here that the serine-threonine kinase Akt can provide a costimulatory signal for RE/AP activation that is indistinguishable from the signal provided by CD28. This includes the ability of Akt, like antibodies to CD28, to synergize with protein kinase C theta (PKC-theta) in the induction of RE/AP. Retrovirus-mediated expression of activated Akt in primary T cells from CD28-deficient mice is capable of selectively restoring production of IL-2 and interferon gamma, but not IL-4 or IL-5. Our results provide evidence that CD28 costimulation of different cytokines is mediated by discrete signaling pathways, one of which includes Akt.