Phosphoinositide 3-kinase/Akt involvement in arsenic trioxide resistance of human leukemia cells

An overview of arsenic trioxide-involved combined treatment algorithms for leukemia: basic concepts and clinical implications

Arsenic trioxide is a first-line treatment drug for acute promyelocytic leukemia, which is also effective for other kinds of leukemia. Its side effects, however, limit its clinical application, especially for patients with complex leukemia symptoms. Combination therapy can effectively alleviate these problems. This review summarizes the research progress on the combination of arsenic trioxide with anticancer drugs, vitamin and vitamin analogs, plant products, and other kinds of drugs in the treatment of leukemia. Additionally, the new progress in arsenic trioxide-induced cardiotoxicity was summarized. This review aims to provide new insights for the rational clinical application of arsenic trioxide.

Synergistic Effects of PI3K Inhibition on Arsenic Trioxide Cytotoxicity in Acute Promyelocytic Leukemia Cells: A New Portrait of Idelalisib as an Adjuvant Therapy

The advent of small-molecule inhibitors targeting the components of oncogenic signaling pathways has revolutionized cancer treatment, where the pharmacological approaches have gone from an era of non-specific chemotherapeutic drugs to the golden age of targeted therapies. In the present study, we evaluated the therapeutic value of an isoform-specific inhibitor of PI3K (Idelalisib) in potentiating the anti-leukemic effects of arsenic trioxide (ATO), an eminent drug used in the treatment of acute promyelocytic leukemia (APL). We found that the abrogation of the PI3K axis profoundly reinforced the anti-leukemic effects of the lower concentrations of ATO, as revealed by the superior reduction in the viability, cell number, and metabolic activity of APL-derived NB4 cells as compared to either agent alone. The cytotoxic effect of Idelalisib in combination with ATO was probably mediated through suppression of c-Myc that was coupled with the elevation in the intracellular level of reactive oxygen species and induction of caspase-3-dependent apoptosis. Notably, our results showed that the suppression of autophagy reinforced the ability of the drugs in eradicating the leukemic cells, suggesting that the compensatory activation of this system may probably overshadow the success of Idelalisib-plus-ATO in APL cells. All in all and given the significant efficacy of Idelalisib against NB4 cells, we proposed the application of this PI3K inhibitor as a foreseeable approach with a safe profile in the treatment of APL.

Apatinib inhibits cell proliferation and migration of osteosarcoma via activating LINC00261/miR-620/PTEN axis

Apatinib has been recently identified as a potential treatment option for osteosarcoma (OS). Nonetheless, the molecular mechanism of Apatinib in regulating OS progression remains unclear. To explore the downstream molecules that mediated the tumor-suppressive effect of Apatinib on OS. Expression levels of genes were detected by RT-qPCR and western blot assays. Functional assays including Transwell assay were applied to detect the proliferation, apoptosis and migration of OS cells. Molecular interactions were detected by luciferase reporter assay and RIP assay. Apatinib inhibited the proliferation and migration of OS cells. LINC00261 was down-regulated in OS cells but then up-regulated after the treatment by Apatinib. Silencing LINC00261 abrogated the suppressive effect of Apatinib on OS cell proliferation and migration. MicroRNA-620 (miR-620) could be sponged by LINC00261. Besides, miR-620 was up-regulated in OS cells and Apatinib treatment reduced miR-620 expression. Furthermore, LINC00261 acted as a competitive endogenous RNA (ceRNA) by sequestering miR-620 to up-regulate the expression of phosphatase and tensin homolog (PTEN). Moreover, Apatinib hindered in vitro cell proliferation and migration as well as the in vivo tumorigenesis of OS through LINC00261/miR-620/PTEN axis. Apatinib-enhanced LINC00261 restrained OS via miR-620/PTEN axis, indicating LINC00261 might promote the efficacy of Apatinib on OS.

MicroRNA-524 promotes cell proliferation by down-regulating PTEN expression in osteosarcoma

Background

Increasing numbers of studies have examined the correlation between specific miRNAs and tumours to enable their diagnosis and treatment. However, there are few reports regarding the concrete role and mechanism of miRNA in osteosarcoma.

Methods

The expression of miR-524 in osteosarcoma tissues and cell lines was examined by qRT-PCR. The cell proliferation was examined using CCK-8 in vitro. A series of bioinformatics and molecular biology techniques were adopted to investigate the regulatory relationship between miR-524 and target genes in osteosarcoma.

Results

The results showed that the miRNA with the most significant differential expression in osteosarcoma was miR-524, which was significantly up-regulated in both osteosarcoma tissues and cell lines. MiR-524 knockdown inhibited proliferation and promoted apoptosis of osteosarcoma cells, while overexpression of miR-524 induced their proliferation. Bioinformatics analysis and luciferase assay confirmed that PTEN was a direct target gene of miR-524 and that miR-524 induced proliferation of osteosarcoma cells through activation of the PI3K/AKT pathway via inhibition of PTEN.

Conclusions

MiR-524 induces the proliferation of osteosarcoma cells through activation of the PI3K/AKT pathway via inhibition of the target gene PTEN, which provides a theoretical basis for selecting a new therapeutic target for osteosarcoma.

Electronic supplementary material

The online version of this article (10.1186/s12935-018-0612-1) contains supplementary material, which is available to authorized users.

Arsenic trioxide-mediated suppression of miR-182-5p is associated with potent anti-oxidant effects through up-regulation of SESN2

Arsenic trioxide (ATO) is a traditional Chinese medicine that can induce oxidative stress for treatment of cancer cells. However, ATO may generate anti-oxidative responses to compromise the cytotoxic effect, but the underlying mechanisms remain unclear. Here we found that ATO could inhibit miR-182-5p expression in patient-derived primary S1 glioblastoma (GBM) cells accompanied by up-regulation of Sestrin-2 (SESN2) mRNA, a known anti-oxidant molecule. This phenomenon was also detected in a U87MG glioma cell line, human lung adenocarcinoma H1299 cell line and A549 cell line. Pretreatment with a free radical scavenger N-acetylcysteine (NAC) reduced the oxidative stress induced by ATO. Concomitantly, ATO mediated suppression of miR-182-5p and enhancement of SESN2 expression were also compromised. The MTT assay further showed that ATO induced cytotoxicity was enhanced by transfection of miR-182-5p mimics. Overexpression of miR-182-5p mimics significantly suppressed the expression of SENS2 and a firefly luciferase reporter gene fused to 3’- untranslated region (UTR) of SESN2 mRNA. Use of ribonucleoprotein immunoprecipitation (RNP-IP), ATO mediated suppression of miR-182-5p led to the stabilization of SESN2 mRNA as a result of Argonaute-2 (AGO2) dependent gene silencing. Furthermore, high expression of miR-182-5p and low expression of SESN2 mRNA tend to be associated with longer survival of glioma or lung cancer patients using public available gene expression datasets and online tools for prediction of clinical outcomes. Taken together, current data suggest that the miR-182-5p/SENS2 pathway is involved in ATO induced anti-oxidant responses, which may be important for the design of novel strategy for cancer treatment.

Multifactorial Modes of Action of Arsenic Trioxide in Cancer Cells as Analyzed by Classical and Network Pharmacology

Arsenic trioxide is a traditional remedy in Chinese Medicine since ages. Nowadays, it is clinically used to treat acute promyelocytic leukemia (APL) by targeting PML/RARA. However, the drug's activity is broader and the mechanisms of action in other tumor types remain unclear. In this study, we investigated molecular modes of action by classical and network pharmacological approaches. CEM/ADR5000 resistance leukemic cells were similar sensitive to As2O3 as their wild-type counterpart CCRF-CEM (resistance ratio: 1.88). Drug-resistant U87.MG ΔEGFR glioblastoma cells harboring mutated epidermal growth factor receptor were even more sensitive (collateral sensitive) than wild-type U87.MG cells (resistance ratio: 0.33). HCT-116 colon carcinoma p53-/- knockout cells were 7.16-fold resistant toward As2O3 compared to wild-type cells. Forty genes determining cellular responsiveness to As2O3 were identified by microarray and COMPARE analyses in 58 cell lines of the NCI panel. Hierarchical cluster analysis-based heat mapping revealed significant differences between As2O3 sensitive cell lines and resistant cell lines with p-value: 1.86 × 10-5. The genes were subjected to Galaxy Cistrome gene promoter transcription factor analysis to predict the binding of transcription factors. We have exemplarily chosen NF-kB and AP-1, and indeed As2O3 dose-dependently inhibited the promoter activity of these two transcription factors in reporter cell lines. Furthermore, the genes identified here and those published in the literature were assembled and subjected to Ingenuity Pathway Analysis for comprehensive network pharmacological approaches that included all known factors of resistance of tumor cells to As2O3. In addition to pathways related to the anticancer effects of As2O3, several neurological pathways were identified. As arsenic is well-known to exert neurotoxicity, these pathways might account for neurological side effects. In conclusion, the activity of As2O3 is not restricted to acute promyelocytic leukemia. In addition to PML/RARA, numerous other genes belonging to diverse functional classes may also contribute to its cytotoxicity. Network pharmacology is suited to unravel the multifactorial modes of action of As2O3.

Deficiency of SUMO-specific protease 1 induces arsenic trioxide-mediated apoptosis by regulating XBP1 activity in human acute promyelocytic leukemia

Small ubiquitin-like modifier (SUMO)/sentrin-specific protease 1 (SENP1), a member of the SENP family, is highly expressed in several neoplastic tissues. However, the effect of SENP1 in acute promyelocytic leukemia (APL) has not been elucidated. In the present study, it was observed that SENP1 deficiency had no effect on the spontaneous apoptosis or differentiation of NB4 cells. Arsenic trioxide (As₂O₃) could induce the upregulation of endoplasmic reticulum (ER) stress, resulting in the apoptosis of NB4 cells. Additionally, knockdown of SENP1 significantly increased As₂O₃-induced apoptosis in NB4 cells transfected with small interfering RNA targeting SENP1. SENP1 deficiency also increased the accumulation of SUMOylated X-box binding protein 1 (XBP1), which was accompanied by the downregulation of the messenger RNA expression and transcriptional activity of the XBP1 target genes endoplasmic reticulum-localized DnaJ 4 and Sec61a, which were involved in ER stress and closely linked to the apoptosis of NB4 cells. Taken together, these results revealed that the specific de-SUMOylation activity of SENP1 for XBP1 was involved in the ER stress-mediated apoptosis caused by As₂O₃ treatment in NB4 cells, thus providing insight into potential therapeutic targets for APL treatment via manipulating XBP1 signaling during ER stress by targeting SENP1.

Arsenic trioxide synergistically potentiates the cytotoxic effect of fludarabine in chronic lymphocytic leukemia cells by further inactivating the Akt and ERK signaling pathways

CLL remains an incurable disease, making it crucial to continue searching for new therapies efficient in all CLL cases. We have studied the effect of combining arsenic trioxide (ATO) with fludarabine, a frontline drug in CLL. We have found a synergistic interaction between 1 μM ATO and 5 μM fludarabine that significantly enhanced the cytotoxic effect of the individual drugs. Importantly, ATO sensitized fludarabine-resistant cells to the action of this drug. The mechanism behind this effect included the downregulation of phospho-Akt, phospho-ERK, and the Mcl-1/Bim and Bcl-2/Bax ratios. The combination of ATO and fludarabine partially overcame the survival effect induced by co-culturing CLL cells with stromal cells. Therefore, low concentrations of ATO combined with fludarabine may be an efficient therapeutic strategy in CLL patients.

Inactivation of Akt by arsenic trioxide induces cell death via mitochondrial-mediated apoptotic signaling in SGC-7901 human gastric cancer cells

Arsenic trioxide (As2O3) has been recognized as a potential chemotherapeutic agent, yet the details concerning its mechanism of action in solid cancers remain undetermined. The present study assessed the role of Akt in the cell death induced by As2O3. The MTT assay showed that As2O3 suppressed the proliferation of SGC-7901 cells in a dose- and time-dependent manner. Characteristic apoptotic changes were observed in the As2O3‑treated cells by Hoechst 33342 staining, and FACS analysis showed that As2O3 caused dose-dependent apoptotic cell death. As2O3 activated caspase-3 and -9, and PARP cleavage in a dose-dependent manner. Compromised mitochondrial membrane potential and an increased protein level of Bax indicated involvement of mitochondia. As2O3 decreased the levels of p-Akt (Ser473), p-Akt (Thr308) and p-GSK-3β (Ser9), suggesting that As2O3 inactivated Akt kinase. In addition, LY294002 (a PI3 kinase inhibitor) augmented the apoptosis induced by As2O3. These results demonstrated that inhibition of PI3K/Akt signaling was involved in As2O3-induced apoptosis of gastric cancer SGC-7901 cells.

Glaucocalyxin A, a negative Akt regulator, specifically induces apoptosis in human brain glioblastoma U87MG cells

Akt is becoming an attractive target in the development of anti-tumor agents. In the present study, we aimed to discover novel negative Akt regulators against malignant glioma. An Akt regulator screening platform performed in an Akt-GFP overexpression cell line was developed, and natural product library was screened and evaluated using this platform. In addition, the cytotoxic effect of the regulator was detected by MTT assay. Cell apoptosis was assayed by Hoechst 33342 staining and flow cytometry analysis. Afterwards, the apoptotic signaling pathway was investigated by western blot analysis. Glaucocalyxin A, isolated from Rabdosia japonica, was identified as a potent negative regulator of Akt. In human-derived malignant glioma U87MG cells, glaucocalyxin A inhibited Akt phosphorylation, suppressed proliferation, and promoted apoptosis in a dose-dependent manner, but not in normal glial cells. Furthermore, glaucocalyxin A activated caspase-3, decreased BAD phosphorylation, and reduced the expression of X-linked inhibitor of apoptosis protein. Taken together, these results indicated that glaucocalyxin A may become a promising candidate in the treatment of malignant glioma.

Chemical genomic screening identifies LY294002 as a modulator of glucocorticoid resistance in MLL-rearranged infant ALL

Successful treatment results for MLL-rearranged Acute Lymphoblastic Leukemia (ALL) in infants remain difficult to achieve. Significantly contributing to therapy failure is poor response to glucocorticoids (GCs), like prednisone. Thus, overcoming resistance to these drugs may be a crucial step towards improving prognosis. We defined a gene signature that accurately discriminates between prednisolone-resistant and prednisolone-sensitive MLL-rearranged infant ALL patient samples. In the current study, we applied Connectivity Map analysis to perform an in silico screening for agents capable of reversing the prednisolone-resistance profile and induce sensitivity. These analyses revealed that LY294002, a PI3K inhibitor, would potentially fulfill this task. Subsequent validation experiments demonstrated that indeed LY294002, and other known PI3K inhibitors, markedly sensitized otherwise resistant MLL-rearranged ALL cells to prednisolone in vitro. Using quantitative RT-PCR analyses, we validated the modulating effects of the PI3K inhibitors on the expression of the genes present in our prednisolone-resistance profile. Interestingly, prednisolone-sensitizing actions may be mediated by inhibition of FCGR1B. Moreover, only high-level expression of FCGR1B showed to be predictive for a poor prognosis and shRNA-mediated knock-down of FCGR1B led to in vitro prednisolone sensitization. Thus, implementing FDA-approved PI3K inhibitors in current treatments may potentially improve the GC response and prognosis in patients with MLL-rearranged ALL.

Two hits are better than one: targeting both phosphatidylinositol 3-kinase and mammalian target of rapamycin as a therapeutic strategy for acute leukemia treatment

Phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) are two key components of the PI3K/Akt/mTOR signaling pathway. This signal transduction cascade regulates a wide range of physiological cell processes, that include differentiation, proliferation, apoptosis, autophagy, metabolism, motility, and exocytosis. However, constitutively active PI3K/Akt/mTOR signaling characterizes many types of tumors where it negatively influences response to therapeutic treatments. Hence, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors may improve cancer patient outcome. The PI3K/Akt/mTOR signaling cascade is overactive in acute leukemias, where it correlates with enhanced drug-resistance and poor prognosis. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds targeting the catalytic site of both kinases. In preclinical models, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against acute leukemia cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin. At variance with rapamycin, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenic protein translation. Therefore, they strongly reduced cell proliferation and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors may represent a promising option for future targeted therapies of acute leukemia patients.

Triptolide modulates the sensitivity of K562/A02 cells to adriamycin by regulating miR-21 expression

CONTEXT

Multidrug-resistance is a serious obstacle encountered in leukemia treatment. Recent studies have shown microRNA-21 (miR-21) is overexpressed in several types of cancer and contributes to tumor resistance to chemotherapy. In our previous studies, we found triptolide (TPL) could enhance adriamycin-induced cytotoxicity and apoptosis in K562/A02 cells.

OBJECTIVE

In the present study, we investigated the mechanism of TPL on the sensitivity of K562/A02 cells to adriamycin.

MATERIALS AND METHODS

Cell viability was assessed by methyl thiazolyl tetrazolium (MTT) assay. Expression of mature miR-21 was determined by SYBER green PCR. The miR-21 mimics and inhibitors were chemically synthesized and transfected into K562 cells or K562/A02 cells. PTEN protein levels was determined by western blots. PTEN promoter activity was measured by luciferase assays.

RESULTS

TPL (5 nmol/L) increased the sensitivity of K562/A02 to adriamycin. When adriamycin was combined with 5 nmol/L TPL, the mean apoptotic population of K562/A02 cells was increased from 4.3 to 18.5%, respectively. K562/A02 cells showed a significant reduction in miR-21 and phosphatase and tensin homolog deleted on chromosome ten (PTEN) expressions after TPL treatment. K562/A02 cells that were transfected with the miR-21 inhibitor had a significantly higher PTEN protein level than the control. K562 cells that were pre-treated with PTEN siRNA had increased survival rate compared to the control group.

DISCUSSION AND CONCLUSION

Our findings indicated that triptolide modulates the sensitivity of K562/A02 cells to adriamycin by regulating miR-21 expression. Triptolide inhibited miR-21 expression and enhanced PTEN levels in K562/A02 cells.

Inhibition of the heat shock response by PI103 enhances the cytotoxicity of arsenic trioxide

Heat shock factor 1 (HSF1) is a key regulator of the cytoprotective and anti-apoptotic heat shock response and can be activated by arsenite. Inhibition of HSF1 activation may therefore enhance the cytotoxicity of arsenic trioxide (ATO). We show that ATO induced HSF1 phosphorylation at serine 326 (S326) and induced HSF1-dependent expression of heat shock proteins (HSPs) 27 and 70 in cultured cells. HSF1 significantly reduced cell sensitivity to ATO by reducing apoptosis. Disruption of HSF1 function not only reduced ATO induction of HSP27 and 70 but also enhanced ATO cytotoxicity by elevating apoptosis. These results reveal that HSF1 activation and the resulting induction of HSPs may protect cells from ATO cytotoxicity. The diminished expression of HSPs and hypersensitivity to ATO in cells stably depleted of HSF1 was rescued by ectopic expression of wild-type HSF1 but not an S326A substitution mutant, indicating that phosphorylation at S326 was critical for the protective effect of HSF1. Simultaneous treatment of cells with ATO and PI103, an inhibitor of members of the phosphatidylinositol 3-kinase (PI3K) family, suppressed not only ATO-induced expression of an HSP70 promoter-reporter construct and endogenous HSP70 but also phosphorylation of HSF1 S326. PI103 considerably reduced HSF1 transactivation in ATO-treated cells but had only a limited effect on HSF1 nuclear translocation and DNA binding. Furthermore, PI103 enhanced ATO cytotoxicity in an HSF1-dependent manner. Thus, inhibition of S326 phosphorylation by PI103 blocks the transactivation of HSF1 and may consequently suppress ATO induction of the heat shock response and sensitize cells to ATO.

Class I phosphoinositide 3-kinases in normal and pathologic hematopoietic cells

Class I phosphoinositide 3-kinases which produce the D3-phosphoinositide second messenger phosphatidylinositol 3,4,5-trisphosphate in response to membrane receptors activation play a critical role in cell proliferation, survival, metabolism, and motility. These lipid kinases and the phosphatases regulating the level of D3-phosphoinositides have been an intense area of research these last two decades. The class I phosphoinositide 3-kinases signaling is found aberrantly activated in numerous human cancers, including in malignant hemopathies, and are important therapeutic targets for cancer therapy. Haematopoiesis is an ongoing process which generates the distinct blood cell types from a common hematopoietic stem cell through the action of a variety of cytokines. In the human adult hematopoiesis occurs primarily in the bone marrow, and defects in hematopoiesis result in diseases, such as anemia, thrombocytopenia, myeloproliferative syndromes, or leukemia. Here we give a brief overview of the role of class I phosphoinositide 3-kinases in hematopoietic stem cells, in hematopoietic lineage development and in leukemia, particularly in acute myeloid leukemia and summarize the potential therapeutic implications.

Synergism between arsenite and proteasome inhibitor MG132 over cell death in myeloid leukaemic cells U937 and the induction of low levels of intracellular superoxide anion

Increased oxygen species production has often been cited as a mechanism determining synergism on cell death and growth inhibition effects of arsenic-combined drugs. However the net effect of drug combination may not be easily anticipated solely from available knowledge of drug-induced death mechanisms. We evaluated the combined effect of sodium arsenite with the proteasome inhibitor MG132, and the anti-leukaemic agent CAPE, on growth-inhibition and cell death effect in acute myeloid leukaemic cells U937 and Burkitt's lymphoma-derived Raji cells, by the Chou-Talalay method. In addition we explored the association of cytotoxic effect of drugs with changes in intracellular superoxide anion (O₂⁻) levels. Our results showed that combined arsenite+MG132 produced low levels of O₂⁻ at 6h and 24h after exposure and were synergic on cell death induction in U937 cells over the whole dose range, although the combination was antagonistic on growth inhibition effect. Exposure to a constant non-cytotoxic dose of 80μM hydrogen peroxide together with arsenite+MG132 changed synergism on cell death to antagonism at all effect levels while increasing O₂⁻ levels. Arsenite+hydrogen peroxide also resulted in antagonism with increased O₂⁻ levels in U937 cells. In Raji cells, arsenite+MG132 also produced low levels of O₂⁻ at 6h and 24h but resulted in antagonism on cell death and growth inhibition. By contrast, the combination arsenite+CAPE showed high levels of O₂⁻ production at 6h and 24 h post exposure but resulted in antagonism over cell death and growth inhibition effects in U937 and Raji cells. We conclude that synergism between arsenite and MG132 in U937 cells is negatively associated to O₂⁻ levels at early time points after exposure.

Suppression of TG-interacting factor sensitizes arsenic trioxide-induced apoptosis in human hepatocellular carcinoma cells

HCC (hepatocellular carcinoma) is among the most common and lethal cancers worldwide with a poor prognosis mainly due to a high recurrence rate and chemotherapy resistance. ATO (arsenic trioxide) is a multi-target drug that has been effectively used as an anticancer drug in acute promyelocytic leukaemia. However, a Phase II trial involving patients with HCC indicates that the use of arsenic as a single agent is not effective against HCC. TGIF (TG-interacting factor) is a transcriptional co-repressor that interferes with TGF-β (transforming growth factor-β) signalling which plays a growth-inhibitory role in HCC. In the present study, we demonstrated that ATO induced hepatocellular apoptosis via TGF-β/Smad signalling and led to downstream induction of p21(WAF1/CIP1) (p21). However, ATO could also induce TGIF expression via a post-transcriptional regulation mechanism to antagonize this effect. Using a biotin-labelled RNA probe pull-down assay and in vivo RNA immunoprecipitation analysis, we identified that HuR (human antigen R) bound to the TGIF mRNA 3'-UTR (3'-untranslated region) and prevented it from degradation. ATO treatment increased the interaction between HuR and TGIF mRNA, and reduction of HuR expression inhibited ATO-induced TGIF expression. Moreover, the EGFR (epidermal growth factor receptor)/PI3K (phosphoinositide 3-kinase)/Akt pathway was shown to mediate the post-transcriptional regulation of TGIF in response to ATO. Finally, we also demonstrated that the down-regulation of TGIF could sensitize ATO-induced HepG2 cell apoptosis. Collectively, we propose that the EGFR/PI3K/Akt pathway may regulate the post-transcriptional regulation of TGIF expression to antagonize ATO-induced apoptosis in HCC. Blockage of the PI3K/Akt pathway or TGIF expression combined with ATO treatment may be a promising strategy for HCC therapy.

Involvement of miR-21 in resistance to daunorubicin by regulating PTEN expression in the leukaemia K562 cell line

Recent studies have shown microRNA-21 (miR-21) is overexpressed in several types of cancer and contributes to tumor resistance to chemotherapy. In this study, we investigated whether miR-21 mediated resistance of the leukaemia cell line K562 to the chemotherapeutic agent daunorubicin (DNR). miR-21 expression was upregulated in the DNR resistant cell line K562/DNR compared to its parental line K562. Stable transfection of miR-21 induced drug resistance in K562, while suppression of miR-21 in K562/DNR led to enhanced DNR cytotoxicity. Additional experiments indicate that the mechanism of miR-21 drug resistance involves the PI3K/Akt pathway and changes following PTEN protein expression. This study provides a novel mechanism for understanding leukaemia drug resistance.

Arsenic trioxide cooperates with all trans retinoic acid to enhance mitogen-activated protein kinase activation and differentiation in PML-RARalpha negative human myeloblastic leukemia cells

Arsenic trioxide (ATO) synergistically promotes all trans retinoic acid (ATRA)-induced differentiation of PML-RARalpha negative HL-60 myeloblastic leukemia cells. In PML-RARalpha positive myeloid leukemia cells, ATO is known to cause degradation of PML-RARalpha with subsequent induced myeloid differentiation. We found that ATO by itself does not cause differentiation of the PML-RARalpha negative HL-60 cells, but enhances ATRA's capability to cause differentiation. ATO augmented ATRA-induced RAF/MEK/ERK axis signaling, expression of CD11b and p47(PHOX), and inducible oxidative metabolism. ATO enhanced ATRA-induced population growth retardation without evidence of apoptosis or enhanced G1/G0 growth arrest. Compared to ATRA-treated cells, the ATRA plus ATO-treated cells progressed more slowly through the cell cycle as detected by a slower rate of accumulation in G2/M following nocodazole treatment. Hoechst/PI staining showed that low-dose ATO did not induce apoptosis. In summary, our results indicate that ATO in conjunction with ATRA is of potential chemotherapeutic use in PML-RARalpha negative leukemias.

E3 ubiquitin ligase Cbl-b potentiates the apoptotic action of arsenic trioxide by inhibiting the PI3K/Akt pathway

Arsenic trioxide (ATO) is a strong inducer of apoptosis in malignant hematological cells. Inducible phosphatidyl inositol 3 kinase (PI3K)-Akt activation promotes resistance to ATO. In the present study, we evaluated whether E3 ubiquitin ligase Cbl-b, a negative regulator of PI3K activation, is involved in the action of ATO. The effect of ATO on cell viability was measured by the Trypan blue exclusion assay or by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Apoptosis was determined by flow cytometry and protein expression was assayed by Western blotting. ATO decreased the viability of HL60 cells and induced cellular apoptosis, which was accompanied by transient activation of Akt. The PI3K/Akt inhibitor, LY294002, significantly increased ATO-induced apoptosis (P < 0.05). In addition, ATO up-regulated the expression of Cbl-b proteins. Furthermore, ATO inhibited cell viability with an IC50 of 18.54 μM at 24 h in rat basophilic leukemia-2H3 cells. ATO induced cellular apoptosis with transient activation of Akt and Cbl-b was also up-regulated. Rat basophilic leukemia-2H3 cells transfected with a dominant negative (DN) Cbl-b mutation showed overexpression of Cbl-b (DN) and enhanced Akt activation. Compared with cells transfected with vector, ATO-induced apoptosis was decreased and G2/M phase cells were increased at the same concentration (P < 0.05). The PI3K/Akt inhibitor, LY294002, re-sensitized Cbl-b (DN) overexpressing cells to ATO and reversed G2/M arrest (P < 0.05). Taken together, these results suggest that Cbl-b potentiates the apoptotic action of ATO by inhibition of the PI3K/Akt pathway.

The impact of PTEN regulation by CK2 on PI3K-dependent signaling and leukemia cell survival

Gene alterations affecting elements of PI3K signaling pathway do not appear to be sufficient to explain the extremely high frequency of PI3K signaling hyperactivation in leukemia. It has been known for long that PTEN phosphorylation at the C-terminal tail, in particular by CK2, contributes to the stabilization and simultaneous inhibition of this critical tumor suppressor. However, direct evidence of the involvement of this mechanism in cancer has been gathered only recently. It is now known that CK2-mediated posttranslational, non-deleting, inactivation of PTEN occurs in T-ALL, CLL and probably other leukemias and solid tumors. To explore this knowledge for therapeutic purposes remains one of the challenges ahead.

Synergistic antiproliferative effect of arsenic trioxide combined with bortezomib in HL60 cell line and primary blasts from patients affected by myeloproliferative disorders

Both arsenic trioxide (ATO) and bortezomib show separate antileukemic activity. With the purpose of evaluating whether the combination of ATO and bortezomib would be an option for patients with acute leukemia, we incubated HL60 leukemic cells with ATO alone and in combination with bortezomib. ATO and bortezomib cooperated to induce cell death and to inhibit proliferation and apoptosis in a synergistic way. The combined treatment resulted in a stronger activation of caspase 8 and 9, moderate activation of caspase 3, and increased expression of Fas and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-DR5 receptors. When bortezomib was added, some proapoptotic genes (CARD9, TRAIL) were upregulated, and some antiapoptotic genes (BCL2, BCL3, FLICE) were downregulated. When coincubated, approximately 80% of cells showed altered mitochondrial membrane permeability. Moreover, ATO alone and in combination with bortezomib abrogated DNA-binding activity of nuclear factor kappa beta (NF-kappaB). Gene expression assays showed that more deregulated genes were related to proliferation of leukocytes, tumorigenesis, control of cell cycle, hypoxia and oxidative stress, cytokines, PI3K-AKT, ERK-MAPK, EGF pathways, and ubiquitination. Finally, in three cases of acute myeloid leukemia, the addition of bortezomib to ATO significantly increased cytotoxicity. We conclude that the combination of bortezomib and ATO may be efficacious in the treatment of myeloid disorders.

Signal transduction pathways and transcription factors triggered by arsenic trioxide in leukemia cells

Arsenic trioxide (As(2)O(3)) is widely used to treat acute promyelocytic leukemia (APL). Several lines of evidence have indicated that As(2)O(3) affects signal transduction and transactivation of transcription factors, resulting in the stimulation of apoptosis in leukemia cells, because some transcription factors are reported to associate with the redox condition of the cells, and arsenicals cause oxidative stress. Thus, the disturbance and activation of the cellular signaling pathway and transcription factors due to reactive oxygen species (ROS) generation during arsenic exposure may explain the ability of As(2)O(3) to induce a complete remission in relapsed APL patients. In this report, we review recent findings on ROS generation and alterations in signal transduction and in transactivation of transcription factors during As(2)O(3) exposure in leukemia cells.

Arsenic trioxide induces apoptosis and G2/M phase arrest by inducing Cbl to inhibit PI3K/Akt signaling and thereby regulate p53 activation

Arsenic trioxide (ATO) strongly induces apoptosis in acute promyelocytic leukemia (APL), but it induces cell cycle arrest in most solid tumors. In this study, we investigated the mechanism of ATO action on APL-derived NB4 cells and gastric cancer cell lines. ATO decreased the viability of both cell lines, but gastric cancer cells were much less susceptible. ATO-induced G2/M phase arrest and p53 degradation in gastric cancer MGC803 cells. In contrast, ATO-induced apoptosis in NB4 cells without degradation of p53. Both processes were accompanied by transient activation of Akt. The PI3K/Akt inhibitor LY294002 significantly increased the amount of p53 protein and ATO-induced apoptosis in both cell lines and decreased G2/M phase arrest of MGC803 cells. In addition, ATO up-regulated the expression of Cbl proteins in both cell lines. Inhibition of Cbl with the proteasome inhibitor Ps341 decreased apoptosis in NB4 cells and increased the G2/M phase arrest of MGC803 cells, and it also prolonged the activation of PI3K/Akt by ATO. Consistent results with those in MGC803 cells were showed in gastric cancer cell BGC823 and SGC7901 after ATO treatment. These results demonstrate that inhibition of PI3K/Akt signaling by Cbl is involved in both ATO-induced apoptosis of NB4 cells and ATO-induced G2/M phase arrest of gastric cancer cells. Cbl achieved these effects probably via its regulating PI3K/Akt pathway, and thereby modulated p53 activation.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.

Role of mTOR in anticancer drug resistance: perspectives for improved drug treatment

The mammalian target of rapamycin (mTOR) pathway plays a central role in regulating protein synthesis, ribosomal protein translation, and cap-dependent translation. Deregulations in mTOR signaling are frequently associated with tumorigenesis, angiogenesis, tumor growth and metastasis. This review highlights the role of the mTOR in anticancer drug resistance. We discuss the network of signaling pathways in which the mTOR kinase is involved, including the structure and activation of the mTOR complex and the pathways upstream and downstream of mTOR as well as other molecular interactions of mTOR. Major upstream signaling components in control of mTOR activity are PI3K/PTEN/AKT and Ras/Raf/MEK/ERK pathways. We discuss the central role of mTOR in mediating the translation of mRNAs of proteins related to cell cycle progression, those involved in cell survival such as c-myc, hypoxia inducible factor 1* (HIF-1*) and vascular endothelial growth factor (VEGF), cyclin A, cyclin dependent kinases (cdk1/2), cdk inhibitors (p21(Cip1) and p27(Kip1)), retinoblastoma (Rb) protein, and RNA polymerases I and III. We then discuss the potential therapeutic opportunities for using mTOR inhibitors rapamycin, CCI-779, RAD001, and AP-23573 in cancer therapy as single agents or in combinations to reverse drug resistance.

Arsenic trioxide sensitizes promonocytic leukemia cells to TNFα-induced apoptosis via p38-MAPK-regulated activation of both receptor-mediated and mitochondrial pathways

Treatment with the anti-leukemic drug arsenic trioxide (As(2)O(3), 1-4 microM) sensitizes U937 promonocytes and other human myeloid leukemia cell lines (HL60, NB4) to apoptosis induction by TNFalpha. As(2)O(3) plus TNFalpha increases TNF receptor type 1 (TNF-R1) expression, decreases c-FLIP(L) expression, and causes caspase-8 and Bid activation, and apoptosis is reduced by anti-TNF-R1 neutralizing antibody and caspase-8 inhibitor. The treatment also causes Bax translocation to mitochondria, cytochrome c and Omi/HtrA2 release from mitochondria, XIAP down-regulation, and caspase-9 and caspase-3 activation. Bcl-2 over-expression inhibits cytochrome c release and apoptosis, and also prevents c-FLIP(L) down-regulation and caspase-8 activation, but not TNF-R1 over-expression. As(2)O(3) does not affect Akt phosphorylation/activation or intracellular GSH content, nor prevents the TNFalpha-provoked stimulation of p65-NF-kappaB translocation to the nucleus and the increase in NF-kappaB binding activity. Treatments with TNFalpha alone or with As(2)O(3) plus TNFalpha cause TNF-R1-mediated p38-MAPK phosphorylation/activation. P38-MAPK-specific inhibitors attenuate the As(2)O(3) plus TNFalpha-provoked activation of caspase-8/Bid, Bax translocation, cytochrome c release, and apoptosis induction. In conclusion, the sensitization by As(2)O(3) to TNFalpha-induced apoptosis in promonocytic leukemia cells is an Akt/NF-kappaB-independent, p38-MAPK-regulated process, which involves the interplay of both the receptor-mediated and mitochondrial executioner pathways.

Specific inhibition of basal mitogen-activated protein kinases and phosphatidylinositol 3 kinase activities in leukemia cells: a possible therapeutic role for the kinase inhibitors

OBJECTIVE

The roles of phosphatidylinositol 3 (PI3K) and mitogen-activated protein kinases (MAPK) have been widely studied in terms of the differentiation process induced by several drugs (phorbol ester, vitamin D-3, retinoic acid, etc.), but their exact functions in leukemic cells' phenotype and their potential therapeutic role remain incompletely clarified.

MATERIALS AND METHODS

In order to investigate this query, leukemia cells were cultured in presence of kinase inhibitors (KIs). Proliferation, apoptosis, and differentiation were analyzed at the cellular and molecular levels, using flow cytometry and reverse transcriptase quantitative polymerase chain reaction.

RESULTS

SB203580, a P38 MAPK inhibitor, had no effect on cell proliferation, whereas LY294002, a PI3K inhibitor, and PD098059, a selective inhibitor of mitogen-activated extracellular regulated kinase (MEK) phosphorylation, arrested cells in G(0)/G(1). However, LY294002 and PD098059 acted using different mechanisms: LY294002 decreased the expression of phosphorylated S6RP, whereas PD098059 increased P21/waf1 antigen expression. SP600125, an inhibitor of N-terminal c-jun kinases, arrested cells in G(2) and induced an endoreplicative process. SP600125 increased p21 at both the mRNA and protein levels. G(2) blockage is dependent on the PI3K pathway and the endoreplicative process is dependent on the PI3K and extracellular regulated kinase (ERK) pathways and mRNA synthesis. On the other hand, PD098059 potentiated the apoptotic process induced by either SP600125 or LY294002. Modulation of the expression of CD11, CD15, CD18, and CD54 was cell-dependent.

CONCLUSION

Our results suggest that KIs modulate proliferation of leukemia cells and that the MEK/ERK inhibitor, PD098059, in combination with either SP600125 or LY294002, could have clinical value.

Potentiation of arsenic trioxide cytotoxicity by Parthenolide and buthionine sulfoximine in murine and human leukemic cells

PURPOSE

To possibly increase the in vitro cytotoxic activity of arsenic trioxide (ATO) by combining it with Parthenolide (PRT), a known NF-kappaB inhibitor and buthionine sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase.

METHODS

Several cell lines representing various hematological malignancies were treated in vitro with the study drugs alone or in combinations. Flow cytometry was used to assess cell death rates and reative oxygen species production. Glutathione and ATP levels were determinded using a photometric and a luminometric assay, respectively. Cell death was characterised by fluorescence microscopy and DNA fragmentation analysis.

RESULTS

PRT increased cytotoxicity of ATO in seven out of eight cell lines. Addition of buthionine sulfoximine (BSO) further potentiated cytotoxicity of the combined treatment. When combined with PRT and BSO, clinically achievable concentrations of ATO (2.5 microM) induced cytotoxicity rates of 80-98% after 24 h. Importantly, lymphocytes from healthy donors were largely unaffected by these treatment modalities, also after growth stimulation in cell culture. N-acetylcysteine inhibited the cytotoxic effects of the triple combination. Treatment of leukemic cells with ATO, PRT and BSO rapidly depleted cells from glutathione, induced oxidative stress and decreased intracellular ATP levels. Cell death showed characteristics of necrosis presumably as a result of ATP loss.

CONCLUSION

Based on the observed selectivity towards malignant cells this combination may offer a therapeutic option applicable to different kinds of leukemia.

The Akt/Mammalian Target of Rapamycin Signal Transduction Pathway Is Activated in High-Risk Myelodysplastic Syndromes and Influences Cell Survival and Proliferation

The Akt/mammalian target of rapamycin (mTOR) signaling pathway is important for both cell growth and survival. In particular, an impaired regulation of the Akt/mTOR axis has been strongly implicated in mechanisms related to neoplastic transformation, through enhancement of cell proliferation and survival. Myelodysplastic syndromes (MDS) are a group of heterogeneous hematopoietic stem cell disorders characterized by ineffective hematopoiesis and by a high risk of evolution into acute myelogenous leukemia (AML). The pathogenesis of the MDS evolution into AML is still unclear, although some recent studies indicate that aberrant activation of survival signaling pathways could be involved. In this investigation, done by means of immunofluorescent staining, we report an activation of the Akt/mTOR pathway in high-risk MDS patients. Interestingly, not only mTOR was activated but also its downstream targets, 4E-binding protein 1 and p70 ribosomal S6 kinase. Treatment with the selective mTOR inhibitor, rapamycin, significantly increased apoptotic cell death of CD33(+) (but not CD33(-)) cells from high-risk MDS patients. Rapamycin was ineffective in cells from healthy donors or low-risk MDS. Moreover, incubation of high-risk MDS patient CD34(+) cells with rapamycin decreased the in vitro clonogenic capability of these cells. In contrast, the phosphoinositide 3-kinase inhibitor, LY294002, did not significantly affect the clonogenic activity of high-risk MDS cells. Taken together, our results indicate that the Akt/mTOR pathway is critical for cell survival and proliferation in high-risk MDS patients. Therefore, this signaling network could become an interesting therapeutic target for treating more advanced MDS cases.

The insulin-like growth factor-I receptor kinase inhibitor NVP-AEW541 induces apoptosis in acute myeloid leukemia cells exhibiting autocrine insulin-like growth factor-I secretion

Insulin-like growth factor-I (IGF-I) and its receptor (IGF-IR) have been implicated in the pathophysiology of many human cancers, including those of hematopoietic lineage. We investigated the therapeutic potential of the novel IGF-IR tyrosine kinase activity inhibitor, NVP-AEW541, on human acute myeloid leukemia (AML) cells. NVP-AEW541 was tested on a HL60 cell subclone, which is dependent on autocrine secretion of IGF-I for survival and drug resistance, as well as primary drug resistant leukemia cells. NVP-AEW541 treatment (24 h) induced dephosphorylation of IGF-IR. NVP-AEW541 also caused Akt dephosphorylation and changes in the expression of key regulatory proteins of the cell cycle. At longer incubation times (48 h), NVP-AEW541-induced apoptotic cell death, as demonstrated by caspase-3 cleavage. Apoptosis was accompanied by decreased expression of anti-apoptotic proteins. NVP-AEW541 enhanced sensitivity of HL60 cells to either cytarabine or etoposide. Moreover, NVP-AEW541 reduced the clonogenic capacity of AML CD34(+) cells cultured in the presence of IGF-I. Chemoresistant AML blasts displayed enhanced IGF-I secretion, and were sensitized to etoposide-induced apoptosis by NVP-AEW541. Our findings indicate that NVP-AEW541 might be a promising therapeutic agent for the treatment of those AML cases characterized by IGF-I autocrine secretion.

Arsenic trioxide induces not only apoptosis but also autophagic cell death in leukemia cell lines via up-regulation of Beclin-1

Although recent data shows that arsenic trioxide (As2O3) is capable of inducing cell death via cell cycle arrest and apoptosis both in acute promyelocytic leukemia (APL) and in non-APL cells, the mechanisms of As2O3-mediated cell death are not fully understood. In this study, we investigated the in vitro effects of As2O3 on cell growth inhibition and cell death in human T-lymphocytic leukemia and myelodysplastic syndrome (MDS) cell lines. As2O3 significantly inhibited the proliferation of Molt-4 and Mutz-1 cells in dose- and time-dependent manner. Autophagic cell death (programmed cell death type II) and apoptosis (programmed cell death type I) were activated together in leukemia cell lines after exposed to As2O3. Numerous large cytoplasmic inclusions and vacuoles were observed in As2O3-treated cells using electron microscope. Furthermore, 3-methyladenine (an autophagy inhibitor) significantly reduced autophagic cell death and sequentially induced apoptosis. Finally, leukemia cells treated with 4 microM As2O3 showed a considerable up-regulation of Beclin-1 (a Bcl-2-interacting protein) expression, which was independent of transcription of mRNA and required protein synthesis. In addition, Molt-4 cells treated with As2O3 exhibited the down-regulation of Bax protein expression, suggesting that Bax may be involved in accumulating of Beclin-1 and triggering autophagic cell death in As2O3-treated leukemia cells. These results may lead to a better understanding of the mechanism of action of As2O3, and provide a suggestion that As2O3 may be of therapeutic value for the treatment of patients with human T-lymphocytic leukemia and myelodysplastic syndrome.

Phosphoinositide 3-kinase/Akt signaling pathway and its therapeutical implications for human acute myeloid leukemia

The phosphoinositide 3-kinase (PI3K)/Akt signaling pathway is crucial to many aspects of cell growth, survival and apoptosis, and its constitutive activation has been implicated in the both the pathogenesis and the progression of a wide variety of neoplasias. Hence, this pathway is an attractive target for the development of novel anticancer strategies. Recent studies showed that PI3K/Akt signaling is frequently activated in acute myeloid leukemia (AML) patient blasts and strongly contributes to proliferation, survival and drug resistance of these cells. Upregulation of the PI3K/Akt network in AML may be due to several reasons, including FLT3, Ras or c-Kit mutations. Small molecules designed to selectively target key components of this signal transduction cascade induce apoptosis and/or markedly increase conventional drug sensitivity of AML blasts in vitro. Thus, inhibitory molecules are currently being developed for clinical use either as single agents or in combination with conventional therapies. However, the PI3K/Akt pathway is important for many physiological cellular functions and, in particular, for insulin signaling, so that its blockade in vivo might cause severe systemic side effects. In this review, we summarize the existing knowledge about PI3K/Akt signaling in AML cells and we examine the rationale for targeting this fundamental signal transduction network by means of selective pharmacological inhibitors.

Targeting Hsp90 by 17-AAG in leukemia cells: mechanisms for synergistic and antagonistic drug combinations with arsenic trioxide and Ara-C

17-Allylamino-17-demethoxygeldanamycin (17-AAG) is a new anticancer agent currently in clinical trials. The ability of 17-AAG to abrogate the function of heat-shock protein Hsp90 and modulate cellular sensitivity to anticancer agents has prompted recent research to use this compound in drug combination therapy. Here we report that 17-AAG has striking opposite effects on the activity of arsenic trioxide (ATO) and ara-C. Combination of 17-AAG with ATO exhibited a synergistic effect in leukemia cells, whereas coincubation of 17-AAG and ara-C showed antagonistic activity. Mechanistic studies revealed that ATO exerted cytotoxic action by reactive oxygen species generation, and activated Akt survival pathway. 17-AAG abrogated Akt activation and enhanced the activity of ATO. In contrast, treatment of leukemia cells with 17-AAG caused a G1 arrest, a decrease in DNA synthesis and reduced ara-C incorporation into DNA, leading to antagonism. The ability of 17-AAG to enhance the antileukemia activity of ATO was further demonstrated in primary leukemia cells isolated from patients with acute myeloid leukemia and chronic lymphocytic leukemia, including cells from refractory patients. Our data suggest that combination of 17-AAG and ATO may be an effective therapeutic regimen. Caution should be exercised in using 17-AAG together with ara-C, as their combination effects are schedule dependent.

Low dose leflunomide activates PI3K/Akt signalling in erythroleukemia cells and reduces apoptosis induced by anticancer agents

Rheumatoid arthritis (RA) is characterized by persistent joint synovial tissue inflammation. Leflunomide is an immunomodulatory agent that has been approved for treatment of active RA. In the past few years, uses other than RA treatment have appeared. Leflunomide has been reported to show antitumor potential through inhibition of cancer cell proliferation. We thus tested the antiproliferative potential of leflunomide on HEL and K562 erythroleukemia cells. The findings summarized in this report demonstrate for the first time that low dose leflunomide prolonged survival and reduced apoptosis induced by several anticancer agents in erythroleukemia cells. We showed that in treated cells, leflunomide reduced the signalling pathways involved in promoting apoptosis by reducing p38 MAPK and JNK basal activity. On the other hand, leflunomide transiently activated the ERK signalling pathway and induced a sustained activation of Akt. We also showed that leflunomide reduced caspase-3 activity and DNA fragmentation induced by anticancer agents. By using an inhibitory strategy, we showed that inhibition of Akt activation but not ERK abolished the protective effect of leflunomide. Thus our findings suggested that leflunomide reduced apoptosis induced by anticancer agents through PI3K/Akt signalling activation.

Activation of the mitogen- and stress-activated kinase 1 by arsenic trioxide

Arsenic trioxide (As2O3) is a potent inducer of apoptosis of leukemic cells in vitro and in vivo, but the precise mechanisms by which it mediates such effects are not well defined. We provide evidence that As2O3 induces activation of the mitogen- and stress-activated kinase 1 (MSK1) and downstream phosphorylation of its substrate, histone H3, in leukemia cell lines. Such activation requires upstream engagement of p38 MAPK, as demonstrated by experiments using pharmacological inhibitors of p38 or p38alpha knock-out cells. Arsenic-induced apoptosis was enhanced in cells in which MSK1 expression was decreased using small interfering RNA and in Msk1 knock-out mouse embryonic fibroblasts, suggesting that this kinase is activated in a negative feedback regulatory manner to regulate As2O3 responses. Consistent with this, pharmacological inhibition of MSK1 enhanced the suppressive effects of As2O3 on the growth of primary leukemic progenitors from chronic myelogenous leukemia patients. Altogether, these findings indicate an important role for MSK1 downstream of p38 in the regulation of As2O3 responses.

The role of the nuclear Akt activation and Akt inhibitors in all-trans-retinoic acid-differentiated HL-60 cells

The pharmacological inhibitors of phosphoinositide 3-kinase (PI3K)/Akt pathway have been proposed in the treatment of leukemia based on their antiproliferative effects. However, several studies demonstrated the activation of PI3K in the nuclei of all-trans-retinoic acid (ATRA) - differentiated HL-60 cells, raising the possibility that PI3K/Akt-inhibitors may block antitumor properties of retinoids. The aim of the present study was to investigate the possible activation of nuclear Akt in ATRA-treated cells and to test the effects of Akt-inhibitors on ATRA-mediated differentiation. The Akt-activity was found to be increased in the nuclei and lysates of ATRA-differentiated HL-60 and NB4 cells. The down-modulation of the expression of Akt protein in HL-60 cells using siRNA reduces the CD11b expression in ATRA-treated cells. The treatment of both cell lines with the commercially available Akt inhibitors inhibited the growth of both control and ATRA-treated cells. Akt-inhibitors had no inhibitory effects on ATRA-mediated growth arrest and the expression of CD11b in HL-60 cells, but increased the percentage of control cells expressing CD11b. In contrast, the presence of Akt inhibitors reduced the expression of CD11b in ATRA-treated NB4 cells.

The Akt/PKB pathway: molecular target for cancer drug discovery

The serine/threonine kinase Akt/PKB pathway presents an exciting new target for molecular therapeutics, as it functions as a cardinal nodal point for transducing extracellular (growth factor and insulin) and intracellular (receptor tyrosine kinases, Ras and Src) oncogenic signals. In addition, alterations of the Akt pathway have been detected in a number of human malignancies. Ectopic expression of Akt, especially constitutively activated Akt, is sufficient to induce oncogenic transformation of cells and tumor formation in transgenic mice as well as chemoresistance. Akt has a wide range of downstream targets that regulate tumor-associated cell processes such as cell growth, cell cycle progression, survival, migration, epithelial-mesenchymal transition and angiogenesis. Blockage of Akt signaling results in apoptosis and growth inhibition of tumor cells with elevated Akt. The observed dependence of certain tumors on Akt signaling for survival and growth has wide implications for cancer therapy, offering the potential for preferential tumor cell killing. In the last several years, through combinatorial chemistry, high-throughput and virtual screening, and traditional medicinal chemistry, a number of inhibitors of the Akt pathway have been identified. This review focuses on ongoing translational efforts to therapeutically target the Akt pathway.

Phosphoinositide 3-kinase/Akt inhibition increases arsenic trioxide-induced apoptosis of acute promyelocytic and T-cell leukaemias

Recent studies suggest that the prosurvival signal transduction pathway involving phosphoinositide 3-kinase (PI3K)/Akt can confer an aggressive, apoptosis-resistant phenotype to acute leukaemia cells. We have investigated the effect of modulating this signalling pathway on the sensitivity of leukaemic cell lines (NB-4, CEM, Jurkat, MOLT-4) and acute promyelocytic primary blasts to apoptosis induced by 1 micromol/l As2O3. Whereas parental NB-4 cells did not display any phosphorylated (active) Akt, CEM, Jurkat and MOLT-4 cells exhibited high levels of Akt activation. Consistently, treatment of NB-4 cells with pharmacological inhibitors of the PI3K/Akt pathway (LY294002, wortmannin) did not increase sensitivity of these cells to arsenic trioxide (As2O3), whereas siRNA knock-down of Akt enhanced As2O3-induced apoptosis of CEM, Jurkat and MOLT-4 cells. Overexpression of a constitutively active Akt cDNA rendered NB-4 cells less susceptible to As2O3. Upon prolonged exposure to As2O3, we isolated a NB-4 cell clone that was resistant to As2O3 and displayed high levels of active Akt. LY294002 treatment of acute promyelocytic primary blasts with elevated Akt phosphorylation levels resulted in an increased sensitivity to As2O3. These results may provide a rationale for the development of combined or sequential treatment with PI3K/Akt inhibitors to improve the efficacy of As2O3 on acute leukaemias and also to overcome As2O3 resistance.

N/A

N/A

Over-expression of Flt3 induces NF-kappaB pathway and increases the expression of IL-6

Activating mutations or over-expression of the Flt3 is prevalent in acute myeloblastic leukemia (AML), associated with activation of Ras/MAP kinase and other signaling pathways. In this study, we addressed the role of Flt3 in the activation of nuclear factor-kappa B (NF-kappaB), which is a target molecule of these kinase pathways. In BaF3 cells stably expressing Flt3, a NF-kappaB-responsive reporter was upregulated and its target gene, IL-6, was increased by the involvement of Flt3-ERK/MAPK-NF-kappaB pathway. Furthermore, we found a modest positive correlation (r=0.35, p=0.096) between Flt3 and IL-6 mRNA expression in 24 AML specimens. These results suggest a role of Flt3 over-expression in NF-kappaB pathway.