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

Pleiotropy of PP2A Phosphatases in Cancer with a Focus on Glioblastoma IDH Wildtype

Serine/Threonine protein phosphatase 2A (PP2A) is a heterotrimeric (or occasionally, heterodimeric) phosphatase with pleiotropic functions and ubiquitous expression. Despite the fact that they all contribute to protein dephosphorylation, multiple PP2A complexes exist which differ considerably by their subcellular localization and their substrate specificity, suggesting diverse PP2A functions. PP2A complex formation is tightly regulated by means of gene expression regulation by transcription factors, microRNAs, and post-translational modifications. Furthermore, a constant competition between PP2A regulatory subunits is taking place dynamically and depending on the spatiotemporal circumstance; many of the integral subunits can outcompete the rest, subjecting them to proteolysis. PP2A modulation is especially important in the context of brain tumors due to its ability to modulate distinct glioma-promoting signal transduction pathways, such as PI3K/Akt, Wnt, Ras, NF-κb, etc. Furthermore, PP2A is also implicated in DNA repair and survival pathways that are activated upon treatment of glioma cells with chemo-radiation. Depending on the cancer cell type, preclinical studies have shown some promise in utilising PP2A activator or PP2A inhibitors to overcome therapy resistance. This review has a special focus on "glioblastoma, IDH wild-type" (GBM) tumors, for which the therapy options have limited efficacy, and tumor relapse is inevitable.

From the Biology of PP2A to the PADs for Therapy of Hematologic Malignancies

Over the past decades, an emerging role of phosphatases in the pathogenesis of hematologic malignancies and solid tumors has been established. The tumor-suppressor protein phosphatase 2A (PP2A) belongs to the serine-threonine phosphatases family and accounts for the majority of serine-threonine phosphatase activity in eukaryotic cells. Numerous studies have shown that inhibition of PP2A expression and/or function may contribute to leukemogenesis in several hematological malignancies. Likewise, overexpression or aberrant expression of physiologic PP2A inhibitory molecules (e.g., SET and its associated SETBP1 and CIP2A) may turn off PP2A function and participate to leukemic progression. The discovery of PP2A as tumor suppressor has prompted the evaluation of the safety and the efficacy of new compounds, which can restore PP2A activity in leukemic cells. Although further studies are needed to better understand how PP2A acts in the intricate phosphatases/kinases cancer network, the results reviewed herein strongly support the development on new PP2A-activating drugs and the immediate introduction of those available into clinical protocols for leukemia patients refractory or resistant to current available therapies.

Protein phosphatase 2A: a target for anticancer therapy

Protein phosphatase 2A (PP2A), one of the main serine-threonine phosphatases in mammalian cells, maintains cell homoeostasis by counteracting most of the kinase-driven intracellular signalling pathways. Unrestrained activation of oncogenic kinases together with inhibition of tumour suppressors is often required for development of cancer. PP2A has been shown to be genetically altered or functionally inactivated in many solid cancers and leukaemias, and is therefore a tumour suppressor. For example, the phosphatase activity of PP2A is suppressed in chronic myeloid leukaemia and other malignancies characterised by aberrant activity of oncogenic kinases. Preclinical studies show that pharmacological restoration of PP2A tumour-suppressor activity by PP2A-activating drugs (eg, FTY720) effectively antagonises cancer development and progression. Here, we discuss PP2A as a druggable tumour suppressor in view of the possible introduction of PP2A-activating drugs into anticancer therapeutic protocols.

Arsenic in cancer treatment: challenges for application of realgar nanoparticles (a minireview)

While intensive efforts have been made for the treatment of cancer, this disease is still the second leading cause of death in many countries. Metastatic breast cancer, late-stage colon cancer, malignant melanoma, multiple myeloma, and other forms of cancer are still essentially incurable in most cases. Recent advances in genomic technologies have permitted the simultaneous evaluation of DNA sequence-based alterations together with copy number gains and losses. The requirement for a multi-targeting approach is the common theme that emerges from these studies. Therefore, the combination of new targeted biological and cytotoxic agents is currently under investigation in multimodal treatment regimens. Similarly, a combinational principle is applied in traditional Chinese medicine, as formulas consist of several types of medicinal herbs or minerals, in which one represents the principal component, and the others serve as adjuvant ones that assist the effects, or facilitate the delivery, of the principal component. In Western medicine, approximately 60 different arsenic preparations have been developed and used in pharmacological history. In traditional Chinese medicines, different forms of mineral arsenicals (orpiment—As₂S₃, realgar—As₄S₄, and arsenolite—arsenic trioxide, As₂O₃) are used, and realgar alone is included in 22 oral remedies that are recognized by the Chinese Pharmacopeia Committee (2005). It is known that a significant portion of some forms of mineral arsenicals is poorly absorbed into the body, and would be unavailable to cause systemic damage. This review primary focuses on the application of arsenic sulfide (realgar) for treatment of various forms of cancer in vitro and in vivo.

Hematopoietic stem cell differentiation affects expression and function of MRP4 (ABCC4), a transport protein for signaling molecules and drugs

Several types of peripheral blood cells express ABC transporters. ABCC4 (MRP4) and ABCC5 (MRP5) localize to different cellular sites and fulfill lineage-specific functions such as mediator storage in platelets' dense granules. All mature blood cells originate from the same precursor and specific functionalities arise during differentiation. To characterize this process, expression, localization and function of MRP4 and MRP5 were assessed in differentiating human CD34+ progenitors and leukemia cell lines using real time polymerase chain reaction (PCR), immunofluorescence microscopy and cell viability assays. Median MRP4 mRNA copy numbers were significantly enhanced by megakaryocytic differentiation from 7.9 x 10(3) to 5.8 x 10(4) copies per nanograms of total RNA (p < 0.05) in CD34+ progenitors and in M-07e cells (MRP4 mRNA/18S rRNA ratios: 5.4 +/- 3.8 x 10(-4) vs. 2.7 +/- 0.9 x 10(-3) for native and differentiated cells, respectively, p < 0.05), and MRP4 protein was localized to granular structures and to the plasma membrane both in differentiated progenitors and bone marrow megakaryocytes. In contrast, expression of MRP4 decreased during maturation to leukocytes (MRP4 mRNA/18S rRNA ratios: 5.2 x 10(-3) for native vs. 3.5 x 10(-3) for CD34+ cells in the presence of G-CSF, p < 0.05) and was significantly reduced in mature monocytes and granulocytes compared with progenitors (MRP4 mRNA/18S rRNA ratios: 8.1 +/- 5.4 x 10(-5) and 2.8 +/- 1.6 x 10(-4) vs. 1.2 +/- 0.7 x 10(-3), respectively, p < 0.05). Expression of MRP5 was not significantly altered under all differentiation conditions. These results indicate that MRP4 expression is differentially regulated during hematopoiesis. The increase of MRP4 together with its specific localization during differentiation toward megakaryocytes supports the concept of platelet specific functions whereas decreased transporter expression in leukocyte differentiation may have implications for chemotherapy.

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.

The effect of steroid on myeloid leukemic cells: The potential of short-course high-dose methylprednisolone treatment in inducing differentiation, apoptosis and in stimulating myelopoiesis

Several in vitro studies have shown that dexamethasone (Dex) and prednisolone can induce differentiation of some mouse and human myeloid leukemic cells to macrophages and granulocytes. Based on in vitro experiments, we have shown that short-course (3-7 days) high-dose methylprednisolone (HDMP) (20-30 mg/kg/day) treatment can induce differentiation of myeloid leukemic cells in vivo in children with different subtypes of acute myeloblastic leukemia (AML) (AML-M1, -M2, -M3, -M4, -M7). We have also shown that induction of apoptosis of myeloid leukemic cells with or without differentiation is possible by short-course HDMP treatment. In addition, short-course HDMP treatment has been shown to be effective in accelerating leukocyte recovery, possibly stimulating normal CD34-positive hematopoietic progenitor cells. Addition of HDMP to mild cytotoxic chemotherapy (low-dose cytosine arabinoside (LD-Ara-c), weekly mitoxantrone and Ara-c or 6-thioguanine) increased the remission rate (87-89%) and improved the outcome of AML children. We believe that the results of our 17-year clinical experience will provide important benefits to AML patients.

Arsenic trioxide: an anti cancer missile with multiple warheads

The proven efficacy of ATO in the treatment of APL and the emerging importance of ATO in other diseases prompted extensive studies of the mechanisms of action of ATO in APL and in other types of cancers. In this review we will focus on downstream events in ATO-induced intrinsic and extrinsic apoptotic pathways with an emphasis on the role of pro-apoptotic and anti-apoptotic proteins and the role of p53 in ATO-induced apoptosis including its effect on cell cycle, its anti-mitotic effect and the role of apoptosis inducing factors (AIF) in ATO-induced apoptosis, chromatin condensation and nuclear fragmentation in myeloma cells as a model.

Effect of ionizing radiation on the pteridine metabolic pathway and evaluation of its cytotoxicity in exposed hospital staff

Investigations carried out to estimate the effect of long-term occupational exposure to low levels of external ionizing radiation indicated that exposed hospital staff showed an increase in chromosome aberrations. The purpose of this study was to evaluate whether genomic instability or an alteration in pteridine synthesis could be used as a marker of the potential hazard of ionizing radiation in hospital workers. Twenty gamma-radiation- and 33 X-ray-exposed technicians working in radiotherapy and radio-diagnostic units were included in this study, along with 22 healthy matched individuals. Plasma concentrations of nitrite plus nitrate (NO(x)) were measured to estimate reactive nitrogen species. Urinary neopterin, biopterin and creatinine concentrations were measured by high-performance liquid chromatography to determine metabolic activity along the pteridine pathway. Sister chromatid exchange was used as a measure of mutagenicity. Apoptosis was evaluated morphologically and also with a DNA-fragmentation test. The plasma NO(x) levels of both gamma-radiation- and X-ray-exposed technicians were significantly higher than those of the healthy controls (p<0.05). While the urinary biopterin concentrations were significantly higher in radiation-exposed groups compared with the healthy subjects (p<0.05), urinary neopterin concentrations remained unchanged. The apoptosis rates of gamma-radiation- and X-ray-exposed workers were significantly elevated in comparison with those in the control group (both p<0.05). Also, the increase in sister chromatid exchange frequency was significant in each of the radiation-exposed groups (exposed groups versus controls; p<0.05). These results indicate that long-term exposure to low-dose ionizing radiation, even below the permitted levels, could result in increased oxidative stress, which may lead to DNA damage and mutagenicity.

Phase II trial of arsenic trioxide in relapsed and refractory acute myeloid leukemia, secondary leukemia and/or newly diagnosed patients at least 65 years old

The prognosis for patients with relapsed/refractory AML, secondary leukemia and AML in older adults is extremely poor. An appealing alternative approach to intensive cytotoxic chemotherapy is to induce apoptosis with a novel agent. There is in vitro evidence that arsenic trioxide (ATO) has anti-proliferative and pro-apoptotic effects on myeloid leukemia cell lines. To evaluate efficacy and toxicities of ATO, we conducted a phase II trial including subjects with relapsed/refractory or secondary AML or age > or = 65 years with de novo disease. Eleven subjects were entered with a median age of 77 years (56-90) and a median total dose of ATO of 415.55 mg (91.5-793) with a daily dose of 0.25 mg/kg. Median survival following the first dose of ATO was 2.25 months (0.4-19). Myelosuppression was the major adverse effect, most likely due to disease progression rather than drug-related. All subjects had progressive disease. There was no direct treatment-related mortality. Based on this study, we do not recommend single agent ATO as a treatment option for AML.

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.

The potential effect of short-course high-dose steroid on the maturation and apoptosis of leukemic cells in a child with acute megakaryoblastic leukemia

High-dose methylprednisolone (HDMP) treatment has been shown to induce differentiation of myeloid leukemic cells in children with acute promyelocytic leukemia and other subtypes (FAB AML M1-M2-M4) of acute myeloblastic leukemia. In the present study, a child with acute megakaryoblastic leukemia (AMKL) was given HDMP (30 mg/kg/day) orally in a single dose for the first 4 days of induction therapy. A marked decrease in peripheral blood blast cells and an increase in platelet count associated with a striking change in bone marrow (BM) morphology was observed following a short-course of HDMP treatment alone. BM cells developed distinct morphology characterized by cytoplasmic blebbing and some appeared as platelet producing micromegakaryocytes. Flow cytometric analysis of the BM cells 4 days after HDMP treatment demonstrated a decrease in the percentage of cells co-expressing CD34 and CD117 antigens and a marked increase in CD42a antigen. These changes in BM morphology and immunophenotype may suggest maturation effect of HDMP on megakaryocytic leukemic cells. In addition ultrastructural analysis of BM cells cultured with methylprednisolone (10(-3) and 10(-6) M) for 24 and 48 h showed numerous apoptotic cells. This was coincident with a significant increase in the percentage of annexin positive cells. These results suggest that HDMP treatment may induce differentiation and apoptosis of leukemic cells in a child with AMKL and it could be a promising agent for remission induction of patients with AMKL.