Up-regulation of serine/threonine protein phosphatase type 2A regulatory subunits during methylprednisolone-induced differentiation of leukaemic HL-60 cells

Evaluation of protein tyrosine phosphatase activity in patients with acute leukemia

Protein tyrosine phosphatases regulate physiological processes including growth, differentiation, metabolism and the cell cycle. Together with tyrosine kinases, they control the phosphorylation state of tyrosine residues of signaling proteins. An increased level of protein phosphorylation results in abnormal proliferation and many cancer types show a mutation or deletion of a protein tyrosine phosphatase gene. In this study we evaluated the protein tyrosine phosphatase activity in acute leukemia patients. Tyrosine phosphatase activity in bone marrow mononuclear cells of acute leukemia patients was measured using a tyrosine phosphatase assay system kit and compared with a control group. We found that tyrosine phosphatase activity in acute leukemia patients was high compared to the controls. According to subgroups of acute leukemia, tyrosine phosphatase activity in the AML-M2 subgroup was high compared to the controls. The effect of increased level of protein tyrosine phosphatase activity on leukemogenesis needs further evaluation. Studies in a large group of patients are needed to emphasize the importance of tyrosine phosphatase activity in acute leukemia patients.

The interaction between taxoids and serine/threonine protein phosphatase activities during taxan-induced apoptosis of HL 60 leukemic cells

Paclitaxel and docetaxel (taxoids) are chemotherapy agents whose mode of action is through an effect on cellular microtubules. Several studies have investigated their potential in the treatment of myeloid malignancies. The aim of our study was to investigate the potential role of the serine/threonine protein phosphatase system in docetaxel/paclitaxel induced cytotoxicity on HL 60 cells. The IC50 dose of paclitaxel and docetaxel were found as 20 and 5 nM respectively using trypan blue dye exclusion and XTT assays. Treating HL 60 cells with docetaxel and paclitaxel resulted in dose and time dependent cytotoxicity. Docetaxel induced the decrease in the activity of protein phosphatase 1 (PP1) and increase in the activity of PP2 subgroups, while paclitaxel induced the increase in the activity of PP1 and decrease in the activity of PP2 subgroups. Potential use of specific protein phosphatase inhibitors or activators in combination with taxoids will open new windows in the treatment of myeloid leukemias.

Induction of human promyelocytic leukemia HL-60 cell differentiation into monocytes by arsenic sulphide: Involvement of serine/threonine protein phosphatases

The clinical efficacy of arsenic sulfide (As(4)S(4)), also known as realgar, in the treatment of leukemia in China is prompting people to explore the underlying mechanism. We examined the realgar-induced differentiation in human promyelocytic leukemia cell line HL-60. Cells exhibited proliferation inhibition when treated with 0.10-1.5 microM of realgar, and underwent monocytic differentiation as indicated by morphological changes, NBT reduction assay, and cytofluorometric analyses of the cell surface antigens, CD11b and CD14. Accompanying the differentiation, the activity of serine/threonine protein phosphatase type 1 (PP1) and type 2A (PP2A) were enhanced, whereas the activity of PP2B remained virtually the same compared to the control. When cells were treated with realgar in the presence of an inhibitor of PP1 and 2A or an inhibitor of PP2B, the differentiation of the cells was partially suppressed as revealed by NBT reduction assay and the expression of CD14. Our data demonstrate that realgar induces monocytic differentiation in HL-60 cells and that some serine/threonine protein phosphatases may be involved in the process.

Protein phosphatase 2A is a negative regulator of IL-2 production in patients with systemic lupus erythematosus

Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.

Involvement of protein phosphatase 2A in interferon-alpha-2b-induced apoptosis in K562 human chronic myelogenous leukaemia cells

Interferon-alpha (IFN-alpha)-2b is known to have antiproliferative effects on hematological malignant cells, especially chronic myelogenous leukaemia (CML). However, it can induce cytogenetical remissions in a very small percentage of the patients. Also during interferon therapy, resistance can emerge in the CML clones. K562 is an in vitro model cell line transformed from a Ph positive CML patient. It can be induced to differentiate to granulocytic and/or monocytic lineages with certain molecules. IFN-alpha-2b generally exerts its effects on CML cells by Janus family kinases (Jak/Stat) pathway, mostly through tyrosine kinase system. However, there is almost no data on the relevance of serine/threonine (Ser/Thr) protein phosphatase (PP) system in the interferon induced signal transduction pathways. In this study, we investigated serine/threonine protein phosphatases in the IFN-alpha-2b induced K562 cytotoxicity. Trypan blue dye exclusion test and MTT assay were utilised for determining cytotoxicity. IC(50) of IFN-alpha-2b on K562 cells was found to be 600IU/ml. However, no differentiation was determined by analysis of cell surface antigen expressions. Serine/threonine protein phosphatase inhibitors calyculin A (Cal A) and okadaic acid (OKA) augmented the IFN-alpha-2b induced cytotoxicity. Apoptosis assay by the mono-oligonucleosome detection and acridine orange/propidium iodide dye revealed marked apoptosis underlying cytotoxicity. Phosphatase enzyme assay revealed a gradual increase in protein phosphatase 2A (PP2A) activity during interferon induced cytotoxicity. Conversely, immunoblots showed no change in the expression of PP2A catalytic and regulatory subunits. In conclusion, PP2A plays a role in IFN-alpha-2b induced apoptosis of K562 cells and should be investigated as a new window furthermore.

Protein phosphatase 2A enhances activation of human immunodeficiency virus type 1 by phorbol myristate acetate

The viral replication rate in patients infected with human immunodeficiency virus type 1 (HIV-1) is controlled in part by regulation of the transcription of viral genes. The rate of transcription is determined by a complex interplay between cellular and viral proteins and the promoter elements found in the long terminal repeats. Protein phosphatase 2A (PP2A) is a phosphoprotein that plays important roles in the regulation of signal transduction and cell growth. In this report, we demonstrate that overexpression of the catalytic subunit of protein phosphatase 2A (PP2Ac) increases the basal activity of the HIV-1 promoter and, especially, enhances the promoter's response to the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol-13-acetate (PMA). Additionally, ectopic PP2Ac enhances activation of HIV-1 provirus by PMA. Okadaic acid, a potent inhibitor of PP2A, markedly reduces both HIV-1 enhancer and proviral activation. Fostriecin, a PP2A inhibitor which has been used as an antineoplastic agent in clinical trials, is also able to inhibit PMA-stimulated HIV-1 proviral activation. These observations demonstrate a role for the important cellular phosphatase PP2A in HIV-1 transcription and replication and also suggest that PKC can potentiate the activity of PP2A. PP2A is a potential target for therapeutic intervention in patients infected with HIV-1.

Arsenic trioxide and methylprednisolone use different signal transduction pathways in leukemic differentiation

Certain cell lines like HL 60 and K 562 are utilised as leukemic cell models for leukemogenesis research, which differentiate along the granulocytic and/or monocytic pathway when treated with certain inducer molecules. High dose methylprednisolone treatment has been shown to induce in vivo and in vitro differentiation of myeloid leukemia cells to mature granulocytes in patients with acute promyelocytic leukemia (APL) and other subtypes of acute myeloid leukemia (AML). Arsenic trioxide (As(2)O(3)) has been confirmed to have remission induction effects on APL. However, there are conflicting results on the effects with other AML subtypes. Also, it has been well established that the reversible phosphorylation of proteins is a major regulatory mechanism in the signal transduction pathways that control cell growth and differentiation. Serine/threonine protein phosphatases (PP) are major components of phosphorylation. In this study, we investigated the effect of As(2)O(3) on HL 60 and K 562 myeloid leukemic differentiation and compared the signalling cascades of the two inducers with respect to serine/threonine PP 1 and 2A. We utilised PP1 and PP2A inhibitors okadaic acid and calyculin A. In contrast to methylprednisolone, there was no effect of phosphatase inhibitors on As(2)O(3)-induced leukemic differentiation. Incomplete leukemic differentiation occurred with lower As(2)O(3) concentration as 10(-6)M. Unlike As(2)O(3), methylprednisolone induced complete granulocytic and/or monocytic differentiation of HL 60 and K 562 cells via upregulation of PP2A regulatory subunits. Therefore, As(2)O(3) and methylprednisolone are promising agents that have the potential to be used together in myeloid leukemic differentiation therapy.