Protein phosphatase 2A plays a critical role in interleukin-2-induced beta 2-integrin dependent homotypic adhesion in human CD4+ T cell lines

Protein phosphatase 1 is involved in IL-2-induced IL-5 and IL-13 expression in human T cells

IL-2 plays an important role in immunological and other biological functions. This cytokine directly induces the production of several cytokines, such as IL-5 and IL-13. The mechanisms of IL-2-mediated cytokine synthesis are mostly unclear; however, the involvement of IL-2 receptor (IL-2R)β has been suggested. In this study, the signaling molecule downstream of IL-2Rβ was investigated, employing a proteomic approach. Full-length IL-2Rβ and its mutant in which the intracellular component was truncated were introduced in an IL-2Rα- and IL-2Rγ-stably transfected T cell hybridoma, S1. The differential phosphorylation profiles of protein tyrosine residues in these cells upon IL-2 stimulation were examined by two-dimensional gel electrophoresis. The candidate phosphoproteins of interest were re-covered, in-gel digested and mass spectrometry fingerprinted. Among proteins specifically phosphorylated in full-length IL-2Rβ-expressing cells in response to IL-2 stimulation, protein phosphatase (PP)1β and FK506-binding protein 4 were identified. Particularly, PP1β augmented IL-5 and IL-13 expression stimulated by IL-2 but not by anti-CD3 antibody in human peripheral CD4+ T cells upon ectopic expression. IL-2-induced cytokine expression was suppressed by overexpression of PP1 regulatory subunit 2. A PP1 inhibitor, tautomycin, but not a PP2A inhibitor, okadaic acid, also inhibited the IL-2R-mediated responses. It was conclusively shown that PP1 is crucially involved in IL-2-mediated IL-5 and IL-13 synthesis in human T cells.

Programmed cell death-10 enhances proliferation and protects malignant T cells from apoptosis

The programmed cell death-10 (PDCD10; also known as cerebral cavernous malformation-3 or CCM3) gene encodes an evolutionarily conserved protein associated with cell apoptosis. Mutations in PDCD10 result in cerebral cavernous malformations, an important cause of cerebral hemorrhage. PDCD10 is associated with serine/threonine kinases and phosphatases and modulates the extracellular signal-regulated kinase pathway suggesting a role in the regulation of cellular growth. Here we provide evidence of a constitutive expression of PDCD10 in malignant T cells and cell lines from peripheral blood of cutaneous T-cell lymphoma (Sezary syndrome) patients. PDCD10 is associated with protein phosphatase-2A, a regulator of mitogenesis and apoptosis in malignant T cells. Inhibition of oncogenic signal pathways [Jak3, Notch1, and nuclear factor-κB (NF-κB)] partly inhibits the constitutive PDCD10 expression, whereas an activator of Jak3 and NF-κB, interleukin-2 (IL-2), enhances PDCD10 expression. Functional data show that PDCD10 depletion by small interfering RNA induces apoptosis and decreases proliferation of the sensitive cells. To our knowledge, these data provide the first functional link between PDCD10 and cancer.

The uptake kinetics and immunotoxic effects of microcystin-LR in human and chicken peripheral blood lymphocytes in vitro

Microcystin-LR is a cyanobacterial heptapeptide that presents acute and chronic hazards to animal and human health. We investigated the influence of this toxin on human and chicken immune system modulation in vitro. Peripheral blood lymphocytes were treated with microcystin-LR at environmentally relevant doses of 1, 10 and 25 microg/ml for 12, 24, 48, 72 h (for proliferation assay cells were treated for 72 h). T-cell and B-cell proliferation as well as apoptosis and necrosis were determined in human and chicken samples. IL-2 and IL-6 production by human lymphocytes also was measured. In addition, uptake kinetics of microcystin-LR into human and chicken peripheral blood lymphocytes were calculated by Liquid Chromatography (LS) /Mass Spectrometry (MS) analysis. At the highest dose microcystin-LR decreased T-cell proliferation and all doses of microcystin-LR inhibited B-cell proliferation. The frequency of apoptotic and necrotic cells increased in a dose and time-dependent manner. Human lymphocytes responded to stimulation with microcystin-LR by increased production of IL-6 and decreased production of IL-2. Human lymphocytes were able to uptake from 0.014 to 1.663 microg/ml and chicken lymphocytes from 0.035 to 1.733 microg/ml of the microcystin-LR added to the cultures, depending on the treatment time and dose. In conclusion, microcystin-LR acted as an immunomodulator in cytokine production and down-regulated lymphocyte functions by induction of apoptosis and necrosis. However, further studies dealing with the influence of microcystin-LR on expression cytokine genes and transcription factors are necessary to confirm these hypotheses.

Sites of protein kinase A activation of the human ClC-2 Cl(-) channel

Human ClC-2 Cl(-) (hClC-2) channels are activated by protein kinase A (PKA) and low extracellular pH(o). Both of these effects are prevented by the PKA inhibitor, myristoylated PKI. The aims of the present study were to identify the PKA phosphorylation site(s) important for PKA activation of hClC-2 at neutral and low pH(o) and to examine the relationship between PKA and low pH(o) activation. Recombinant hClC-2 with point mutations of consensus phosphorylation sites was prepared and stably expressed in HEK-293 cells. The responses to forskolin plus isobutylmethylxanthine at neutral and acidic pH(o) were studied by whole cell patch clamp in the presence and absence of phosphatase inhibitors. The double phosphorylation site (RRAT655(A) plus RGET691(A)) mutant hClC-2 lost PKA activation and low pH(o) activation. Either RRAT or RGET was sufficient for PKA activation of hClC-2 at pH(o) 7.4, as long as phosphatase inhibitors (cyclosporin A or endothal) were present. At pH(o) 6 only RGET was needed for PKA activation of hClC-2. Low pH(o) activation of hClC-2 Cl(-) channel activity was PKA-dependent, retained in RGET(A) mutant hClC-2, but lost in RRAT(A) mutant hClC-2. RRAT655(D) mutant hClC-2 was constitutively active and was further activated by PKA at pH(o) 7.4 and 6.0, consistent with the above findings. These results show that activation of hClC-2 is differentially regulated by PKA at two sites, RRAT655 and RGET691. Either RRAT655 or RGET691 was sufficient for activation at pH(o) 7.4. RGET, but not RRAT, was sufficient for activation at pH(o) 6.0. However, in the RGET691(D) mutant, there was PKA activation at pH(o) 6.0.

Differences in phosphatase modulation of α4 β1 and α5 β1 integrin-mediated adhesion and migration of B16F1 cells

It is well established that a biphasic relationship exists between the adhesive strength of β1 integrins and their ability to mediate cell movement. Thus, cell movement increases progressively with adhesive strength, but beyond a certain point of optimal interaction, cell movement is reduced with further increases in adhesive function. The interplay between the various kinase and phosphatase activities provides the balance in β1 integrin-mediated cell adhesion and migration. In the present study, the significance of protein tyrosine phosphatases (PTP) and ser/thr protein phosphatases (PP) in α4β1 and α5β1 integrin-mediated mouse melanoma B16F1 cell anchorage and migration on fibronectin was characterized using phosphatase inhibitors. At low fibronectin concentration, α5β1 functioned as the predominant receptor for cell movement; a role for α4β1 in B16F1 cell migration increased progressively with fibronectin concentration. Treatment of B16F1 cells with PTP inhibitors, sodium orthovanadate (Na3VO4) and phenylarsine oxide (PAO), or PP-1/2A inhibitor, okadaic acid (OA), abolished cell movement. Inhibition of cell movement by PAO and OA was associated by a reduction in the adhesive strength of α4β1 and α5β1. In contrast, treatment of B16F1 cells with Na3VO4 resulted in selective stimulation of the adhesive function of α5β1, but not α4β1. Therefore, our results demonstrate that (i) both PTP and PP-1/2A have roles in cell movement, (ii) modulation of cell movement by PTP and PP-1/2A may involve either a stimulation or reduction of β1 integrin adhesive strength, and (iii) distinct phosphatase-mediated signaling pathways for differential regulation of the various β1 integrins exist. Key words: phosphatases, integrins, cell movement, cell adhesion.

Interleukin-2 induces beta2-integrin-dependent signal transduction involving the focal adhesion kinase-related protein B (fakB)

beta2 integrin molecules are involved in a multitude of cellular events, including adhesion, migration, and cellular activation. Here, we studied the influence of beta2 integrins on interleukin-2 (IL-2)-mediated signal transduction in human CD4(+) T cell lines obtained from healthy donors and a leukocyte adhesion deficiency (LAD) patient. We show that IL-2 induces tyrosine phosphorylation of a 125-kDa protein and homotypic adhesion in beta2 integrin (CD18)-positive but not in beta2-integrin-negative T cells. EDTA, an inhibitor of integrin adhesion, blocks IL-2-induced tyrosine phosphorylation of the 125-kDa protein but not other proteins in beta2-integrin-positive T cells. Likewise, a beta2 integrin (CD18) antibody selectively inhibits induction of the 125-kDa phosphotyrosine protein, whereas cytokine-mediated tyrosine phosphorylation of other proteins is largely unaffected. Immunoprecipitation experiments indicate that the IL-2-induced 125-kDa phosphotyrosine protein is the focal adhesion kinase-related protein B (fakB). Thus, IL-2 induces strong tyrosine phosphorylation of fakB in beta2-integrin-positive but not in beta2-integrin-negative T cells, and CD18 mAb selectively blocks IL-2-induced fakB-tyrosine phosphorylation in beta2-integrin-positive T cells. In parallel experiments, IL-2 does not induce or augment tyrosine phosphorylation of p125(FAK). In conclusion, our data indicate that IL-2 induces beta2-integrin-dependent signal transduction events involving the tyrosine kinase substrate fakB.