Sensitisation of HL60 human leukaemic cells to cytotoxic drug-induced apoptosis by inhibition of PI3-kinase survival signals

Inhibition of PI3K Signaling Intensified the Antileukemic Effects of Pioglitazone: New Insight into the Application of PPARγ Stimulators in Acute Lymphoblastic Leukemia

Over the past two decades, molecular targeted therapy has revolutionized the landscape of cancer treatment due to lower side effects as well as higher anticancer effects. Peroxisome proliferator-activated receptor gamma (PPARγ) is a member of the nuclear hormone receptor which plays a crucial role in cell proliferation and death and the efficacy of PPARγ ligands either as monotherapy or in combination with traditional chemotherapy drugs has been proved by recent studies. In this study, we aimed to investigate the effects of pioglitazone, a well-known PPARγ stimulator, in ALL-derived NALM6 cells by using trypan blue assay, MTT assay, and flow cytometry analysis. Moreover, to investigate the molecular mechanism action of pioglitazone in these cells, we assessed the possible alterations in the expression of some target genes which regulate cell proliferation, apoptosis, and autophagy system. Our result demonstrated that pioglitazone induced a remarkable antileukemic effect on NALM6 cells through a PTEN-mediated manner. Based on the fact that PI3K hyperactivation is one of the main properties of ALL cells, the effects of PI3K inhibition using CAL-101 on pioglitazone-induced cytotoxicity were evaluated by combinatorial experiments. Moreover, the result of cell cycle assay and qRT-PCR demonstrated that pioglitazone-CAL-101 induced antileukemic effect mainly through induction of p21 and p27-mediated G1 arrest. Additionally, our result showed that inhibition of proteasome and autophagy system, two main cellular processes, increased the antileukemic effects of the agents. Taken together, we suggest a novel therapeutic application for PPARγ stimulators as a single agent or in combination with PI3K inhibitors that should be clinically evaluated in ALL patients.

DeSUMOylation of chromatin-bound proteins limits the rapid transcriptional reprogramming induced by daunorubicin in acute myeloid leukemias

Genotoxicants have been used for decades as front-line therapies against cancer on the basis of their DNA-damaging actions. However, some of their non-DNA-damaging effects are also instrumental for killing dividing cells. We report here that the anthracycline Daunorubicin (DNR), one of the main drugs used to treat Acute Myeloid Leukemia (AML), induces rapid (3 h) and broad transcriptional changes in AML cells. The regulated genes are particularly enriched in genes controlling cell proliferation and death, as well as inflammation and immunity. These transcriptional changes are preceded by DNR-dependent deSUMOylation of chromatin proteins, in particular at active promoters and enhancers. Surprisingly, inhibition of SUMOylation with ML-792 (SUMO E1 inhibitor), dampens DNR-induced transcriptional reprogramming. Quantitative proteomics shows that the proteins deSUMOylated in response to DNR are mostly transcription factors, transcriptional co-regulators and chromatin organizers. Among them, the CCCTC-binding factor CTCF is highly enriched at SUMO-binding sites found in cis-regulatory regions. This is notably the case at the promoter of the DNR-induced NFKB2 gene. DNR leads to a reconfiguration of chromatin loops engaging CTCF- and SUMO-bound NFKB2 promoter with a distal cis-regulatory region and inhibition of SUMOylation with ML-792 prevents these changes.

Targeting HDAC3, a new partner protein of AKT in the reversal of chemoresistance in acute myeloid leukemia via DNA damage response

Resistance to cytotoxic chemotherapy drugs remains as the major cause of treatment failure in acute myeloid leukemia. Histone deacetylases (HDAC) are important regulators to maintain chromatin structure and control DNA damage; nevertheless, how each HDAC regulates genome stability remains unclear, especially under genome stress conditions. Here, we identified a mechanism by which HDAC3 regulates DNA damage repair and mediates resistance to chemotherapy drugs. In addition to inducing DNA damage, chemotherapy drugs trigger upregulation of HDAC3 expression in leukemia cells. Using genetic and pharmacological approaches, we show that HDAC3 contributes to chemotherapy resistance by regulating the activation of AKT, a well-documented factor in drug resistance development. HDAC3 binds to AKT and deacetylates it at the site Lys20, thereby promoting the phosphorylation of AKT. Chemotherapy drug exposure enhances the interaction between HDAC3 and AKT, resulting in decrease in AKT acetylation and increase in AKT phosphorylation. Whereas HDAC3 depletion or inhibition abrogates these responses and meanwhile sensitizes leukemia cells to chemotoxicity-induced apoptosis. Importantly, in vivo HDAC3 suppression reduces leukemia progression and sensitizes MLL-AF9⁺ leukemia to chemotherapy. Our findings suggest that combination therapy with HDAC3 inhibitor and genotoxic agents may constitute a successful strategy for overcoming chemotherapy resistance.

MK-2206 induces apoptosis of AML cells and enhances the cytotoxicity of cytarabine

Genetic alterations in the PI3K/AKT cascade have been linked to various human cancers including acute myeloid leukemia (AML) and have emerged to be promising targets for treatment. In this study, we explored the molecular mechanism and clinical implication of a specific allosteric AKT inhibitor, MK-2206, in the treatment of AML. Four leukemia cell lines, MV-4-11, MOLM-13, OCI/AML3, and U937, were used. Apoptosis and cell cycle distribution were determined by flow cytometry analysis. Expression of anti-apoptotic protein family and glycogen synthase kinase 3β (GSK3β) signaling was determined by western blotting. Drug combination effects of MK-2206 with cytarabine were evaluated by cell proliferation assay, and the combination index values were calculated by CompuSyn software. MK-2206 had no effect on normal peripheral blood mononuclear cells, but induced G1-phase arrest and apoptosis in leukemia cells. Among anti-apoptotic Bcl-2 family members, only myeloid cell leukemia-1 (Mcl-1) was significantly suppressed. Mcl-1 suppression by MK-2206 was closely associated with decreased GSK3β phosphorylation at Ser9, an event leads to GSK3β activation. Furthermore, the effect of MK-2206 on Mcl-1 downregulation was abolished by GSK3β inhibitor, lithium chloride and proteasome inhibitor, MG-132, suggesting that MK-2206 acted through a GSK3β-mediated, proteasome-dependent protein degradation. In addition, co-administration of MK-2206 with cytarabine could enhance the cytotoxic efficacy of cytarabine in leukemia cell lines. In conclusion, we have demonstrated that MK-2206 is an active agent in AML and its efficacy as in combination with cytarabine is implicated.

Reversal of boswellic acid analog BA145 induced caspase dependent apoptosis by PI3K inhibitor LY294002 and MEK inhibitor PD98059

PI3K/Akt and ERK pathways are important for growth and proliferation of many types of cancers. Therefore, PI3K inhibitor LY294002 (LY) and MEK1/2 inhibitor PD98059 (PD) are used to sensitize many types of cancer cell lines to chemotherapeutic agents, where AKT and ERK pathways are over activated. However, in this study, we show for the first time that PD could protect the leukemia cells independent of ERK pathway inhibition, besides, we also report a detailed mechanism for antiapoptotic effect of LY in HL-60 cells against the cytotoxicity induced by a boswellic acid analog BA145. Apoptosis induced by BA145 is accompanied by downregulation of PI3K/Akt and ERK pathways in human myelogenous leukemia HL-60 cells, having activating N-Ras mutation. Both LY and PD protected the cells against mitochondrial stress caused by BA145, and reduced the release of cytochrome c and consequent activation of caspase-9. LY and PD also diminished the activation of caspase-8 without affecting the death receptors. Besides, LY and PD also reversed the caspase dependent DNA damage induced by BA145. Further studies revealed that LY and PD significantly reversed the inhibitory effect of BA145 on cell cycle regulatory proteins by upregulating hyperphosphorylated retinoblastoma, pRB (S795) and downregulating p21 and cyclin E. More importantly, all these events were reversed by caspase inhibition by Z-VAD-fmk, suggesting that both LY and PD act at the level of caspases to diminish the apoptosis induced by BA145. These results indicate that inhibitors of PI3K/Akt and ERK pathways can play dual role and act against chemotherapeutic agents.

Context-dependent antagonism between Akt inhibitors and topoisomerase poisons

Signaling through the phosphatidylinositol-3 kinase (PI3K)/Akt pathway, which is aberrantly activated in >50% of carcinomas, inhibits apoptosis and contributes to drug resistance. Accordingly, several Akt inhibitors are currently undergoing preclinical or early clinical testing. To examine the effect of Akt inhibition on the activity of multiple widely used classes of antineoplastic agents, human cancer cell lines were treated with the Akt inhibitor A-443654 [(2S)-1-(1H-indol-3-yl)-3-[5-(3-methyl-2H-indazol-5-yl)pyridin-3-yl]oxypropan-2-amine; ATP-competitive] or MK-2206 (8-[4-(1-aminocyclobutyl)phenyl]-9-phenyl-2H-[1,2,4]triazolo[3,4-f][1,6]naphthyridin-3-one;dihydrochloride; allosteric inhibitor) or with small interfering RNA (siRNA) targeting phosphoinositide-dependent kinase 1 (PDK1) along with cisplatin, melphalan, camptothecin, or etoposide and assayed for colony formation. Surprisingly different results were observed when Akt inhibitors were combined with different drugs. Synergistic effects were observed in multiple cell lines independent of PI3K pathway status when A-443654 or MK-2206 was combined with the DNA cross-linking agents cisplatin or melphalan. In contrast, effects of the Akt inhibitors in combination with camptothecin or etoposide were more complicated. In HCT116 and DLD1 cells, which harbor activating PI3KCA mutations, A-443654 over a broad concentration range enhanced the effects of camptothecin or etoposide. In contrast, in cell lines lacking activating PI3KCA mutations, partial inhibition of Akt signaling synergized with camptothecin or etoposide, but higher A-443654 or MK-2206 concentrations (>80% inhibition of Akt signaling) or PDK1 siRNA antagonized the topoisomerase poisons by diminishing DNA synthesis, a process that contributes to effective DNA damage and killing by these agents. These results indicate that the effects of combining inhibitors of the PI3K/Akt pathway with certain classes of chemotherapeutic agents might be more complicated than previously recognized.

Rapamycin interacts synergistically with idarubicin to induce T-leukemia cell apoptosis in vitro and in a mesenchymal stem cell simulated drug-resistant microenvironment via Akt/mammalian target of rapamycin and extracellular signal-related kinase signaling pathways

T-cell acute lymphoblastic leukemias (T-ALLs) are clonal lymphoid malignancies with a poor prognosis, and still a lack of effective treatment. Here we examined the interactions between the mammalian target of rapamycin (mTOR) inhibitor rapamycin and idarubicin (IDA) in a series of human T-ALL cell lines Molt-4, Jurkat, CCRF-CEM and CEM/C1. Co-exposure of cells to rapamycin and IDA synergistically induced T-ALL cell growth inhibition and apoptosis mediated by caspase activation via the intrinsic mitochondrial pathway and extrinsic pathway. Combined treatment with rapamycin and IDA down-regulated Bcl-2 and Mcl-1, and inhibited the activation of phosphoinositide 3-kinase (PI3K)/mTOR and extracellular signal-related kinase (ERK). They also played synergistic pro-apoptotic roles in the drug-resistant microenvironment simulated by mesenchymal stem cells (MSCs) as a feeder layer. In addition, MSCs protected T-ALL cells from IDA cytotoxicity by up-regulating ERK phosphorylation, while rapamycin efficiently reversed this protective effect. Taken together, we confirm the synergistic antitumor effects of rapamycin and IDA, and provide an insight into the potential future clinical applications of combined rapamycin-IDA regimens for treating T-cell malignancies.

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.

Perifosine--a new option in treatment of acute myeloid leukemia?

INTRODUCTION

Perifosine is a novel targeted oral Akt inhibitor. In preclinical leukemia models, perifosine has an independent cytotoxic potential but also synergizes well with other rationally selected targeted agents. The evidence from clinical trials supporting the use of perifosine in the therapy of leukemias is limited. The optimal dose and schedule have yet to be defined. However, given its favorable toxicity profile and mechanism of action, the therapeutic potential of perifosine should be evaluated in well-designed clinical trials.

AREAS COVERED

The role of the phosphatidylinositol-3 kinase (PI3K)/Akt zpathway in normal cells, cancer and leukemias is discussed. The mechanism of action of perifosine and the basic information on the development and chemical properties are summarized. The evidence from in vivo and in vitro studies is presented. The efficacy and side effect profile are summarized.

EXPERT OPINION

The safety and tolerability profile of perifosine are satisfactory. The evidence from clinical trials in patients with leukemias is very limited. The preclinical data are encouraging. Perifosine has the potential to play a role in the treatment of leukemias in the future. Its role needs to be confirmed in clinical trials.

A review of molecular mechanisms of the anti-leukemic effects of phenolic compounds in honey

Hematologic malignancies constitute about 9% of all new cases of cancers as reported via the GLOBOCAN series by International Agency for Research on Cancer (IARC) in 2008. So far, the conventional therapeutic and surgical approaches to cancer therapy have not been able to curtail the rising incidence of cancers, including hematological malignancies, worldwide. The last decade has witnessed great research interest in biological activities of phenolic compounds that include anticancer, anti-oxidation and anti-inflammation, among other things. A large number of anticancer agents combat cancer through cell cycle arrest, induction of apoptosis and differentiation, as well as through inhibition of cell growth and proliferation, or a combination of two or more of these mechanisms. Various phenolic compounds from different sources have been reported to be promising anticancer agents by acting through one of these mechanisms. Honey, which has a long history of human consumption both for medicinal and nutritional uses, contains a variety of phenolic compounds such as flavonoids, phenolic acids, coumarins and tannins. This paper presents a review on the molecular mechanisms of the anti-leukemic activity of various phenolic compounds on cell cycle, cell growth and proliferation and apoptosis, and it advocates that more studies should be conducted to determine the potential role of honey in both chemoprevention and chemotherapy in leukemia.

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.

Testosterone exerts antiapoptotic effects against H2O2 in C2C12 skeletal muscle cells through the apoptotic intrinsic pathway

Experimental data indicate that apoptosis is activated in the aged skeletal muscle, contributing to sarcopenia. We have previously demonstrated that testosterone protects against hydrogen peroxide (H(2)O(2))-induced apoptosis in C2C12 muscle cells. Here we identified molecular events involved in the antiapoptotic effect of testosterone. At short times of exposure to H(2)O(2) cells exhibit a defense response but at longer treatment times cells undergo apoptosis. Incubation with testosterone prior to H(2)O(2) induces BAD inactivation, inhibition of poly(ADP-ribose) polymerase cleavage, and a decrease in BAX levels, and impedes the loss of mitochondrial membrane potential, suggesting that the hormone participates in the regulation of the apoptotic intrinsic pathway. Simultaneous treatment with testosterone, H(2)O(2), and the androgen receptor (AR) antagonist, flutamide, reduces the effects of the hormone, pointing to a possible participation of the AR in the antiapoptotic effect. The data presented allow us to begin to elucidate the mechanism by which the hormone prevents apoptosis in skeletal muscle.

Induction of a 55 kDa acetylcholinesterase protein during apoptosis and its negative regulation by the Akt pathway

Acetylcholinesterase (AChE) is emerging as an important contributor to apoptosis in various cell types. However, overexpression of AChE does not initiate apoptosis, and cells which express AChE at basal levels grow normally, suggesting that AChE may function differently between normal and apoptotic conditions. In this study, we determined that an AChE-derived protein (∼55 kDa) positively correlated with cellular apoptotic levels. The 55 kDa AChE protein was not a result of a novel splice variant of the AChE primary transcript. Instead, it was determined to be a cleaved fragment of the full-length 68 kDa AChE protein that could not be inhibited by cycloheximide (CHX) but could be suppressed by caspase inhibitors in apoptotic PC-12 cells. Furthermore, activation of the Akt cascade abolished the 55 kDa protein, and both AChE protein forms (68 and 55 kDa) accumulated in the nucleus during apoptosis. In a mouse model for ischemia/reperfusion (I/R)-induced acute renal failure, the 55 kDa AChE protein was detected in the impaired organs but not in the normal ones, and its levels correlated with the genotype of the mice. In summary, a 55 kDa AChE protein resulting from the cleavage of 68 kDa AChE is induced during apoptosis, and it is negatively regulated by the Akt pathway. This study suggests that an alternative form of AChE may play a role in apoptosis.

Activity of any class IA PI3K isoform can sustain cell proliferation and survival

Small molecule inhibitors of PI3K for oncology mainly target the class I PI3Ks, comprising the p110alpha, beta, gamma, and delta isoforms, of which only p110alpha is mutated in cancer. To assess the roles of class I PI3K isoforms in cell proliferation and survival, we generated immortalized mouse leukocyte and fibroblast models in which class I PI3Ks were inactivated by genetic and pharmacological approaches. In IL3-dependent hemopoietic progenitor cells (which express all four class I PI3K isoforms), genetic inactivation of either p110alpha or p110delta did not affect cell proliferation or survival or sensitize to p110beta or p110gamma inactivation. Upon compound inactivation of p110alpha and p110delta, which removed >90% of p85-associated PI3K activity, remarkably, cells continued to proliferate effectively, with p110beta assuming an essential role in signaling and cell survival. Furthermore, under these conditions of diminished class I PI3K activity, input from the ERK pathway became important for cell survival. Similar observations were made in mouse embryonic fibroblasts (which mainly express p110alpha and p110beta) in which p110alpha or p110beta could sustain cell proliferation as a single isoform. Taken together, these data demonstrate that a small fraction of total class I PI3K activity is sufficient to sustain cell survival and proliferation. Persistent inhibition of selected PI3K isoforms can allow the remaining isoform(s) to couple to upstream signaling pathways in which they are not normally engaged. Such functional redundancy of class IA PI3K isoforms upon sustained PI3K inhibition has implications for the development and use of PI3K inhibitors in cancer.

Rapamycin, the mTOR kinase inhibitor, sensitizes acute myeloid leukemia cells, HL-60 cells, to the cytotoxic effect of arabinozide cytarabine

The mammalian target of rapamycin (mTOR) kinase is a key regulator of cell growth and proliferation. Overexpression of the mTOR signaling pathway has been described in several tumor cells, including the majority of acute myeloid leukemia (AML) cases. The anti-tumor efficacy of mTOR inhibitors was shown in several preclinical and clinical studies. In AML, however, the potential antineoplastic effect of mTOR inhibitors has received little attention thus far. In this in-vitro study of the human AML cell line, HL-60, we aimed to assess the antileukemic activity of rapamycin (RAPA), an mTOR inhibitor, alone and in combination with cytarabine (Ara-C). The study showed that RAPA in concentrations of 1-10 nmol/l arrested the cell cycle progression of Hl-60 cells in the G1 phase, without evident cytotoxic effect. This effect was associated with significant inhibition of cyclin E expression. At concentrations higher than 10 nmol/l, RAPA exerted a significant proapoptotic effect, with the collapse of mitochondrial potential and caspase-3 activation. The most prominent proapoptotic effect was observed for a combination of 1 nmol/l of RAPA and 50 nmol/l of Ara-C, especially when Ara-C was added at a 24-h interval after RAPA. In conclusion, these data indicate that RAPA might be effective in the treatment of acute leukemia patients, especially in combination with Ara-C, the drug routinely used in AML treatment. On the basis of these results, attempts to combine classical induction chemotherapy with an inhibitor of the mTOR kinase in AML treatment could be warranted.

Novel postremission strategies in adults with acute myeloid leukemia

PURPOSE OF REVIEW

Given the high rates of relapse in acute myeloid leukemia (AML), there is tremendous opportunity for the development of new therapeutic strategies in the postremission state. Unfortunately, the currently available modalities for postremission therapy, namely chemotherapy, have proven largely ineffective in changing the natural history of AML. The challenges to overcome therapeutic failure in the minimal residual disease status may relate to an incomplete understanding of the mechanisms and cell populations that are directly related to disease relapse as well as suboptimal ability to identify patients at highest risk for relapse.

RECENT FINDINGS

Being a heterogeneous disease, relapsed AML is unlikely to emanate from one predominant mechanism; instead, there are likely multiple biologic factors at play that allow for clinical relapse to occur. These factors likely include multidrug resistance proteins, aberrant signal transduction pathways, survival of leukemia stem cells, microenvironmental interactions, and immune tolerance. Many novel strategies are in development that target these mechanisms, ranging from chemotherapeutic modalities, to signal transduction inhibitors, to upregulation of antileukemic immune responses.

SUMMARY

Understanding the underlying mechanisms of leukemic cell survival and resistance has spurred the development of novel therapeutic approaches to overcome these mechanisms in the hope of eradicating minimal residual disease and improving survival in AML.

Enhancing the in vitro cytotoxic activity of Delta9-tetrahydrocannabinol in leukemic cells through a combinatorial approach

Delta(9)-Tetrahydrocannabinol (THC) is the active metabolite of cannabis, which has demonstrable cytotoxic activity in vitro. In support of our previously published data, we have investigated the interactions between THC and anti-leukemia therapies and studied the role of the signalling pathways in mediating these effects. Results showed clear synergistic interactions between THC and the cytotoxic agents in leukemic cells. Additionally, exposure of cells to sub lethal levels of THC (1 microM) sensitised cells to these cytotoxic agents, by reducing IC(50) values by approximately 50%. Sensitisation appeared to be dependent upon the ability of THC to down regulate phosphorylated ERK, as cells dominantly expressive of MEK were not sensitised to the cytotoxic drugs by equi-molar amounts of THC. Overall, these results demonstrate for the first time that a combination approach with THC and established cytotoxic agents may enhance cell death in vitro. Additionally the MAPK/ERK pathway appears responsible in part for these effects.

Enhanced anti-cancer effect of a phosphatidylinositol-3 kinase inhibitor and doxorubicin on human breast epithelial cell lines with different p53 and oestrogen receptor status

New efforts are being focused on signalling pathways as targets for cancer therapy. This particular study was designed to investigate whether blockade of the phosphatidylinositol 3OH-kinase (PI3K) pathway (a survival/anti-apoptosis pathway, overexpressed in various tumours) could sensitise human breast cancer cells to the effect of chemotherapeutics. Doxorubicin (Dox) and LY294002 (LY, a PI3K inhibitor) were used individually or in combination on MDA-MB-231 (p53 mutant, ER-), T47D (p53 mutant, ER+), and MCF-7 (p53 wildtype, ER+) human breast cancer cell lines, and on 184A1, a nonmalignant human breast epithelial cell line (p53 wildtype, ER-). Each drug showed time- and dose-dependent growth inhibition of cell proliferation on all 4 cell lines. The combination of Dox+LY resulted in enhanced cell growth inhibition in MDA-MB-231 and T47D cells, and additive inhibition in MCF-7 and 184A1 cells. Cell cycle analysis showed that Dox+LY enhanced the arrest of MDA-MB-231 and T47D cells in G2 with the appearance of a sub-G1 peak indicating apoptosis/necrosis, a notion supported by enhanced depolarisation of mitochondrial membrane potential in these cell types. The combination also caused a greater additive increase in Cyclin B1. Thus, the synergistic effect of the combination on cell proliferation in some, but not all, breast cancer cells may be through enhanced induction of both G2 arrest and apoptosis, in which p53 may play a role. Substantially lower doses of doxorubicin could be used with low doses of inhibitors of the PI3K pathway, without compromising the anti-cancer effect, but also lowering detrimental side-effects of doxorubicin. This study supports the notion that survival signalling pathways offer special targets for chemotherapy in cancer.

Targeting Akt to increase the sensitivity of neuroblastoma to chemotherapy: lessons learned from the brain-derived neurotrophic factor/TrkB signal transduction pathway

Neuroblastoma (NB) is a neural crest precursor cell-derived extracranial solid tumor in children. Patients with a poor prognosis are often resistant to chemotherapy and have tumors that express the neuronal growth/survival factor brain-derived neurotrophic factor and its tyrosine kinase receptor, TrkB. In this article, the authors discuss a growth/survival factor-stimulated mechanism leading to chemoresistance in NB that is mediated by the PI3K/Akt signaling pathway. Besides brain-derived neurotrophic factor/TrkB, other growth/survival factors and their receptors also activate the PI3K/Akt pathway and have the potential to mediate chemoresistance in NB. These findings raise the possibility of a new therapeutic approach in NB that would target Akt, the common downstream mediator of multiple growth/survival factor signaling pathways, to enhance the efficacy of chemotherapeutics. Several classes of Akt inhibitors, including phosphatidylinositol ether lipid analogs, alkylphospholipid analogs, allosteric Akt kinase inhibitors, HSP90 inhibitor and HIV protease inhibitors are discussed.

Apoptotic pathways in tumor progression and therapy

Apoptosis is a cell suicide program that plays a critical role in development and tissue homeostasis. The ability of cancer cells to evade this programmed cell death (PCD) is a major characteristic that enables their uncontrolled growth. The efficiency of chemotherapy in killing such cells depends on the successful induction of apoptosis, since defects in apoptosis signaling are a major cause of drug resistance. Over the past decades, much progress has been made in our understanding of apoptotic signaling pathways and their dysregulation in cancer progression and therapy. These advances have provided new molecular targets for proapoptotic cancer therapies that have recently been used in drug development. While most of those therapies are still at the preclinical stage, some of them have shown much promise in the clinic. Here, we review our current knowledge of apoptosis regulation in cancer progression and therapy, as well as the new molecular targeted molecules that are being developed to reinstate cancer cell death.

Mapping genes that contribute to daunorubicin-induced cytotoxicity

Daunorubicin is an anthracycline antibiotic agent used in the treatment of hematopoietic malignancies. Toxicities associated with this agent include myelosuppression and cardiotoxicity; however, the genes or genetic determinants that contribute to these toxicities are unknown. We present an unbiased genome-wide approach that incorporates heritability, whole-genome linkage analysis, and linkage-directed association to uncover genetic variants contributing to the sensitivity to daunorubicin-induced cytotoxicity. Cell growth inhibition in 324 Centre d' Etude du Polymorphisme Humain lymphoblastoid cell lines (24 pedigrees) was evaluated following treatment with daunorubicin for 72 h. Heritability analysis showed a significant genetic component contributing to the cytotoxic phenotypes (h2 = 0.18-0.63 at 0.0125, 0.025, 0.05, 0.1, 0.2, and 1.0 mumol/L daunorubicin and at the IC50, the dose required to inhibit 50% cell growth). Whole-genome linkage scans at all drug concentrations and IC50 uncovered 11 regions with moderate peak LOD scores (> 1.5), including 4q28.2 to 4q32.3 with a maximum LOD score of 3.18. The quantitative transmission disequilibrium tests were done using 31,312 high-frequency single-nucleotide polymorphisms (SNP) located in the 1 LOD confidence interval of these 11 regions. Thirty genes were identified as significantly associated with daunorubicin-induced cytotoxicity (P < or = 2.0 x 10(-4), false discovery rate < or = 0.1). Pathway and functional gene ontology analysis showed that these genes were overrepresented in the phosphatidylinositol signaling system, axon guidance pathway, and GPI-anchored proteins family. Our findings suggest that a proportion of susceptibility to daunorubicin-induced cytotoxicity may be controlled by genetic determinants and that analysis using linkage-directed association studies with dense SNP markers can be used to identify the genetic variants contributing to cytotoxicity.

Targeting receptor tyrosine kinase signaling in acute myeloid leukemia

Acute myeloid leukemia (AML) is a quickly progressing, heterogeneous clonal disorder of hematopoietic progenitor cells. Significant progress in understanding the pathogenesis of AML has been achieved in the past few years. Two major types of genetic events are thought to give rise to leukemic transformation: alterations in the activity of transcription factors controlling hematopoietic differentiation and activation of components of receptor tyrosine kinase (RTK) signaling pathways. This has led to the development of promising new therapeutic strategies for the disease. In this article, we will discuss recent developments in the field of molecularly targeted therapies for AML, which involve RTKs such as FMS-like tyrosine kinase 3 (Flt3), c-Kit and signal transduction via the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways. Initial results imply that targeting RTKs is a very promising approach for AML and that other receptors, such as the insulin-like growth factor receptor (IGF-IR), could also represent new targets in the future.

NFV, an HIV-1 protease inhibitor, induces growth arrest, reduced Akt signalling, apoptosis and docetaxel sensitisation in NSCLC cell lines

HIV-1 protease inhibitor (PI), nelfinavir (NFV) induced growth arrest and apoptosis of NCI-H460 and -H520, A549, EBC-1 and ABC-1 non-small-cell lung cancer (NSCLC) cells in association with upregulation of p21^waf1, p27 ^kip1 and p53, and downregulation of Bcl-2 and matrix metalloproteinase (MMP)-2 proteins. We found that NFV blocked Akt signalling in these cells as measured by Akt kinase assay with glycogen synthase kinase-3α/β (GSK-3α/β) as a substrate. To explore the role of Akt signalling in NFV-mediated growth inhibition of NSCLC cells, we blocked this signal pathway by transfection of Akt small interfering RNA (siRNA) in these cells; transient transfection of Akt siRNA in NCI-H460 cells decreased the level of Bcl-2 protein and slowed their proliferation compared to the nonspecific siRNA-transfected cells. Conversely, forced-expression of Akt partially reversed NFV-mediated growth inhibition of these cells, suggesting that Akt may be a molecular target of NFV in NSCLC cells. Also, we found that inhibition of Akt signalling by NFV enhanced the ability of docetaxel to inhibit the growth of NCI-H460 and -H520 cells, as measured by MTT assay. Importantly, NFV slowed the proliferation and induced apoptosis of NCI-H460 cells present as tumour xenografts in nude mice without adverse systemic effects. Taken together, this family of compounds might be useful for the treatment of individuals with NSCLC.

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.

Generation of survival signal by differential interaction of p38MAPKalpha and p38MAPKbeta with caveolin-1 and caveolin-3 in the adapted heart

Sphingomyelin breakdown product ceramide has recently been found to induce an adaptive response and reduce myocardial ischemia/reperfusion injury. Since activation of MAP kinases plays an essential role in myocardial adaptation to ischemic stress and since ceramide is involved in lipid raft formation where MAP kinases can be translocated in response to stress, we reasoned that preconditioning may potentiate the translocation of MAP kinases into the lipid raft. To test the hypothesis, rats were divided into five groups: (i) control, (ii) ischemia/reperfusion (I/R), (iii) I/R+C-2 ceramide, (iv) adapted and (v) adapted+desipramine, an inhibitor of ceramide formation. Isolated hearts were preperfused for 15 min with Krebs Henseleit bicarbonate (KHB) buffer in the absence or presence of 10 microM desipramine followed by adaptation induced by four cyclic episodes of 5 min ischemia and 10 min reperfusion. For myocardial adaptation to ischemia with ceramide, the hearts were perfused with 1 microM C-2 ceramide. All hearts were then subjected to 30 min ischemia and 2 h of reperfusion. As expected, both ischemic adaptation and ceramide adaptation made the heart resistant to I/R injury as evidenced by improved ventricular performance and reduced myocardial infarct size and cardiomyocyte apoptosis, which were significantly blocked with desipramine indicating the involvement of ceramide in ischemic adaptation. Ceramide also participated in the formation of lipid raft, and desipramine disrupted the raft formation. In the adapted hearts, there was an increased association of the proapoptotic p38MAPKalpha with caveolin-1 while there was a reduced association of anti-apoptotic p38MAPKbeta with caveolin-3 indicating reduced amount of p38MAPKalpha and increased amount of p38MAPKbeta were available to the adapted hearts thereby generating a survival signal. Desipramine decreased the association of P38MAPKalpha and C-2 ceramide increased the association of P38MAPKalpha with the lipid raft. The survival signal was further confirmed by increased phosphorylation of AKT and enhanced induction of expression of Bcl-2 during adaptation and its reversal with desipramine. The results indicated a unique ceramide signaling the ischemic and PC hearts involving lipid rafts, which generated a survival signal by differentially associating the p38MAPKalpha and p38MAPKbeta with the caveolin-1 and caveoli-3, respectively.

Rapamycin induces a caspase-independent cell death in human monocytes

The immunosuppressive activity of rapamycin (RAPA) and its efficacy as an anti-rejection agent in organ transplantation have been ascribed principally to its anti-proliferative effects on T cells, while the activity on monocytes is partially unknown. In vitro, RAPA reduced monocyte survival by inducing a caspase-independent cell death. RAPA-induced monocyte cell death (RAPA-CD) was impeded by activation of granulocyte macrophage-colony stimulating factor family receptors or toll-like receptor 4, and by exposure to inflammatory cytokines. In vivo, in patients who received RAPA monotherapy as part of pre-conditioning for islet transplantation, RAPA affected survival of myeloid lineage cells. In the peripheral blood, CD33(+) and CD14(+) cells decreased, whereas lymphocytes appeared unaffected. In the bone marrow, myeloid precursors such as CD15(+) and CD15(+)/CD16(+) were selectively and significantly decreased, but no major cytotoxic effects were observed. The RAPA-CD suggests a dependence of monocytes on mammalian target of RAPA pathways for nutrient usage, and this feature implies that RAPA could be selectively useful as a treatment to reduce monocytes or myeloid cells in conditions where these cells negatively affect patient, suggesting a potential anti-inflammatory action of this drug.

Selective inhibition of Ras, phosphoinositide 3 kinase, and Akt isoforms increases the radiosensitivity of human carcinoma cell lines

Ras activation promotes the survival of tumor cells after DNA damage. To reverse this survival advantage, Ras signaling has been targeted for inhibition. Other contributors to Ras-mediated DNA damage survival have been identified using pharmacologic inhibition of signaling, but this approach is limited by the specificity of the inhibitors used and their toxicity. To better define components of Ras signaling that could be inhibited in a clinical setting, RNA interference was used to selectively block expression of specific isoforms of Ras, phosphoinositide 3 (PI3) kinase, and Akt. Inhibition of oncogenic Ras expression decreased both phospho-Akt and phospho-p42/44 mitogen-activated protein (MAP) kinase levels and reduced clonogenic survival. Because pharmacologic inhibition of PI3 kinases and Akt radiosensitized cell lines with active Ras signaling, whereas inhibition of the MAP/extracellular signal-regulated kinase (ERK) kinase/ERK pathway did not, we examined the contribution of PI3 kinases and Akts to radiation survival. Selective inhibition the PI3 kinase P110alpha + p85beta isoforms reduced Akt phosphorylation and radiation survival. Similarly, inhibition of Akt-1 reduced tumor cell radiation survival. Inhibition of Akt-2 or Akt-3 had less effect. Retroviral transduction and overexpression of mouse Akt-1 was shown to rescue cells from inhibition of endogenous human Akt-1 expression. This study shows that Ras signaling to the PI3 kinase-Akt pathway is an important contributor to survival, whether Ras activation results from mutation of ras or overexpression of epidermal growth factor receptor. This study further shows that selective inhibition of the PI3 kinase P110alpha + p85beta isoforms or Akt-1 could be a viable approach to sensitizing many tumor cells to cytotoxic therapies.

LY294002 and LY303511 sensitize tumor cells to drug-induced apoptosis via intracellular hydrogen peroxide production independent of the phosphoinositide 3-kinase-Akt pathway

The phosphoinositide 3-kinase (PI3K)-Akt pathway is constitutively active in many tumors, and inhibitors of this prosurvival network, such as LY294002, have been shown to sensitize tumor cells to death stimuli. Here, we report a novel, PI3K-independent mechanism of LY-mediated sensitization of LNCaP prostate carcinoma cells to drug-induced apoptosis. Preincubation of tumor cells to LY294002 or its inactive analogue LY303511 resulted in a significant increase in intracellular hydrogen peroxide (H2O2) production and enhanced sensitivity to non-apoptotic concentrations of the chemotherapeutic agent vincristine. The critical role of intracellular H2O2 in LY-induced death sensitization is corroborated by transient transfection of cells with a vector containing human catalase gene. Indeed, overexpression of catalase significantly blocked the amplifying effect of LY pretreatment on caspase-2 and caspase-3 activation and cell death triggered by vincristine. Furthermore, the inability of wortmannin, another inhibitor of PI3K, to induce an increase in H2O2 production at doses that effectively blocked Akt phosphorylation provides strong evidence to unlink inhibition of PI3K from intracellular H2O2 production. These data strongly support death-sensitizing effect of LY compounds independent of the PI3K pathway and underscore the critical role of H2O2 in creating a permissive intracellular milieu for efficient drug-induced execution of tumor cells.

Pharmacologic inhibitors of PI3K/Akt potentiate the apoptotic action of the antileukemic drug arsenic trioxide via glutathione depletion and increased peroxide accumulation in myeloid leukemia cells

Treatment for 14 to 24 hours with low concentrations of arsenic trioxide (As2O3, 1-4 μM) caused apoptosis in U-937 promonocytes and other human myeloid leukemia cell lines (HL-60, NB4). This effect was potentiated by cotreatment with the phosphatidylinositol 3-kinase (PI3K) inhibitors LY294002 and wortmannin, and the Akt inhibitor Akti5. However, the inhibitors did not increase the toxicity of the mitochondria-targeting drug lonidamine, and the DNA-specific drugs camptothecin and cisplatin, when used under similar experimental conditions as As2O3. The potentiation of As2O3-provoked apoptosis involved the increased disruption of mitochondrial transmembrane potential, increased caspase-3 activation and cytochrome c release from mitochondria, increased Bax and Bid activation, and attenuation of 27-kDa heat shock protein (HSP27) expression; the potentiation was prevented by Bcl-2 overexpression. The PI3K/Akt inhibitors decreased the intracellular glutathione content, and caused intracellular oxidation, as measured by peroxide accumulation. Cotreatment with subcytotoxic concentrations of hydrogen peroxide increased apoptosis induction by As2O3. On the other hand, the treatments did not significantly affect glutathione S-transferase π expression and activity. These results, which indicate that glutathione is a target of PI3K/Akt in myeloid leukemia cells, may partially explain the selective increase of As2O3 toxicity by PI3K/Akt inhibitors, and may provide a rationale to improve the efficacy of these inhibitors as therapeutic agents.

PI3-kinase/Akt is constitutively active in primary acute myeloid leukaemia cells and regulates survival and chemoresistance via NF-kB, MAPkinase and p53 pathways

The phosphoinositide 3-kinase (PI3-kinase) signalling pathway plays a key role in the regulation of cell survival and proliferation. We show that the PI3-kinase/Akt pathway is constitutively active in primary acute myeloid leukaemia (AML) cells and that blockade by the selective inhibitor LY294002 reduces survival of the total blast population (mean 52%). The ERK/MAPK module is also constitutively active and treatment with the MAPKK inhibitor U0126 reduces cell survival by 22%. In 10 of 18 samples, PI3-kinase contributes to MAPK activation as incubation with LY294002 leads to a marked reduction in its phosphorylation. PI3-kinase inhibition reduces survival of the CD34+38- AML progenitor subset by 44%, whereas MAPKK inhibition has little effect. Reporter assays in primary AML cells show that blocking PI3-kinase leads to a marked reduction of constitutive NF-kappaB activity and promotes p53-mediated transcription. This is associated with a synergistic interaction between LY294002 and Ara-C. An inducible activated form of Akt protects normal myeloid cells from Ara-C and etoposide-mediated apoptosis. These results show that blocking PI3-kinase has direct antileukaemic effects and potentiates the response to conventional cytotoxics via a number of targets including NF-kappaB, p53 and MAPK. Inhibitors of PI3-kinase and Akt may be useful in the treatment of AML.

PI-3-kinase/NF-kappaB mediated response of Jurkat T leukemic cells to two different chemotherapeutic drugs, etoposide and TRAIL

Jurkat T leukemic cells respond to Etoposide, antineoplastic agent which targets the DNA unwinding enzyme, Topoisomerase II, and TNF-Related-Apoptosis-Inducing-Ligand (TRAIL), 34 kDa transmembrane protein, which displays minimal or no toxicity on normal cells and tissues, not only disclosing the occurrence of apoptosis but also a kind of resistance. A similar rate of viability upon the exposure to these two drugs up to 24 h has been evidenced, followed by the occurrence of a rescue process against TRAIL, not performed against Etoposide, along with an higher number of dead cells upon Etoposide exposure, in comparison with TRAIL treatment. These preliminary results let us to speculate on the possible involvement of PI-3-kinase in TRAIL resistance disclosed by surviving cells (20%), may be phosphorylating Akt-1 and, in parallel, IkappaB alpha on both serine and tyrosine residues. On the other hand, in Etoposide Jurkat exposed cells Ser 32-36 phosphorylation of IkappaB alpha is not sufficient to overbalance the apoptotic fate of the cells, since Bax increase, IAP decrease, and caspase-3 activation determine the persistence of the apoptotic state along with the occurrence of cell death by necrosis. Thus, the existence of a balance between apoptotic and rescue response in 20% of cells surviving to TRAIL suggests the possibility of pushing it in favor of cell death in order to improve the yield of pharmacological strategies.

Novel 2′-substituted, 3′-deoxy-phosphatidyl-myo-inositol analogues reduce drug resistance in human leukaemia cell lines with an activated phosphoinositide 3-kinase/Akt pathway

Activation of the phosphoinositide 3-kinase (PI3-K)/Akt signalling pathway has been linked with resistance to chemotherapeutic drugs, and its down-regulation, by means of pharmacological inhibitors of PI3-K, considerably lowers resistance to various types of therapy in cell lines derived from solid tumours. Recently, a new class of Akt inhibitors, referred to as phosphatidylinositol ether lipids (PIAs), have been synthesized. We tested whether two new PIAs could lower the sensitivity threshold to chemotherapeutic drugs of human leukaemia cell lines with an activated PI3-K/Akt network. We used HL60AR (for apoptosis resistant), K562 and U937 cells. The two pharmacological inhibitors, used at 5 micromol/l, down-regulated Akt kinase activity and phosphorylation. Neither of the two chemicals affected the activity of other signalling proteins in the Akt pathway, such as phosphoinositide-dependent protein kinase-1 or PTEN. When employed at 5 micromol/l, the Akt inhibitors markedly reduced the resistance of the leukaemic cell lines to etoposide or cytarabine. Remarkably, a 5 micromol/l concentration of the inhibitors did not negatively affect the survival rate of human cord blood CD34(+) cells. Overall, our results indicate that new selective Akt pharmacological inhibitors might be used in the future for overcoming Akt-mediated resistance to therapeutic treatments of acute leukaemia cells.

Long-term androgen-ablation causes increased resistance to PI3K/Akt pathway inhibition in prostate cancer cells

BACKGROUND

In advanced stages of prostate cancer, the phosphatidylinositol-3' kinase (PI3K)/Akt signaling cascade, one of the major survival pathways in the cell, is frequently constitutively activated due to mutation or loss of the tumor suppressor protein phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Using cell culture models representing different tumor stages, we explored the effect of inhibition of this survival pathway on the induction of apoptosis.

METHODS

Inhibition of the survival kinase Akt and induction of apoptosis was analyzed in androgen-insensitive DU145 and PC-3 cells, in androgen-responsive LNCaP, and in androgen-independent long-term androgen-ablated LNCaP-abl cells representing therapy-resistant prostate cancer cells. Activated Akt was determined by immunoblotting using a phospho-Akt specific antibody. Induction of apoptosis was analyzed employing annexing V and propidium iodide staining and flow cytometry and measurement of cleavage of the caspases substrate poly-ADP-ribose polymerase (PARP).

RESULTS

IGF-1, EGF, and heregulin but not PDGF or activators of protein kinase A induced phosphorylation of Akt in DU145 cells and activation was completely blocked by the PI3K inhibitor LY294002. In the hormone-responsive prostate cancer cell line LNCaP that has a constitutively switched-on Akt kinase, LY294002 caused a dose- and time-dependent Akt inhibition, which was absent in long-term androgen-ablated LNCaP sublines. In agreement with the resistance to inhibition of the PI3K/Akt pathway, long-term androgen-ablated LNCaP sublines remained relatively resistant to induction of cell death by LY294002 or the cytotoxic drug etoposide. Inhibition of the PI3K/Akt pathway restored the sensitivity of long-term androgen-ablated cells to induction of apoptosis by a cytotoxic drug almost completely.

CONCLUSION

These results suggest that long-term androgen ablation therapy for prostate cancer reinforces the PI3K/Akt pathway and impedes its inhibition thus contributing to increased resistance of tumor cells to induction of apoptosis. With regard to treatment of therapy-refractory prostate cancer, these findings suggest effectiveness of a combination of cytotoxic treatment and inhibition of the PI3K-Akt survival pathway in tumor cells after failure of androgen-ablation therapy.

Constitutive NF-kappaB DNA-binding activity in AML is frequently mediated by a Ras/PI3-K/PKB-dependent pathway

In the present study, we aimed to elucidate the mechanism responsible for constitutive NF-kappaB DNA-binding activity in AML cells. Intervening in aberrant signaling pathway provides a rational approach for in vivo targeting of AML cells. Constitutive NF-kappaB DNA-binding activity was observed in 16 of 22 (73%) investigated AML cases and was, in general, associated with resistance to spontaneous apoptosis. Indeed, inhibition of NF-kappaB activity by the NF-kappaB inhibitor SN-50 peptide resulted in enhanced chemotherapy-induced apoptosis. In the majority of cases, constitutive NF-kappaB activity was mediated by a Ras/PI3 kinase (PI3-K)/protein kinase B (PKB)-mediated pathway. The PI3-K inhibitor Ly294002 and the Ras inhibitor L-744832 both inhibited PKB phosphorylation and NF-kappaB DNA-binding activity. The constitutive activation of Ras GTP-ase was caused by mutations in the gene encoding for N-Ras in 29% of the cases. The constitutive NF-kappaB activity could so far not be ascribed to the autocrine production of growth factors or to mutations in the Flt3 receptor, since anti-GM-CSF, -IL-1, -IL6, -TNFalpha or the tyrosine kinase inhibitor AG1296 did not affect the NF-kappaB DNA-binding activity. The present study demonstrates that Ras activation is an important pathway for triggering the NF-kappaB pathway in AML cells.

Farnesyltransferase inhibitors in hematologic malignancies: new horizons in therapy

Farnesyltransferase inhibitors (FTIs) are small-molecule inhibitors that selectively inhibit farnesylation of a number of intracellular substrate proteins such as Ras. Preclinical work has revealed their ability to effectively inhibit tumor growth across a wide range of malignant phenotypes. Many hematologic malignancies appear to be reasonable disease targets, in that they express relevant biologic targets, such as Ras, mitogen-activated protein kinase (MAPK), AKT, and others that may depend on farnesyl protein transferase (FTase) activity to promote proliferation and survival. A host of phase 1 trials have been recently launched to assess the applicability of FTIs in hematologic malignancies, many of which demonstrate effective enzyme target inhibition, low toxicity, and some clinical responses. As a result, phase 2 trials have been initiated in a variety of hematologic malignancies and disease settings to further validate clinical activity and to identify downstream signal transduction targets that may be modified by these agents. It is anticipated that these studies will serve to define the optimal roles of FTIs in patients with hematologic malignancies and provide insight into effective methods by which to combine FTIs with other agents.

Inhibition of phosphatidylinositol 3-kinase dephosphorylates BAD and promotes apoptosis in myeloid leukemias

The phosphatidylinositol 3-kinase (PI3K)/AKT protein kinase pathway is involved in cell growth, proliferation, and apoptosis. The functional activation of PI3K/AKT provides survival signals and blockade of this pathway may facilitate cell death. Downstream targets of PI3K-AKT include the proapoptotic protein BAD, caspase-9, NF-kappaB, and Forkhead. We have previously reported that BAD is constitutively phosphorylated in primary acute myeloid leukemia (AML) cells, a post-transcriptional modification, which inactivates its proapoptotic function. In this study, we tested the hypothesis that the inhibition of PI3K by LY294002 results in the dephosphorylation of AKT and BAD, and thus promote leukemia cell apoptosis. We investigated the effects of LY294002 in megakaryocytic leukemia-derived MO7E cells, primary AML and normal bone marrow progenitor cells. In MO7E cells, LY294002 reduced AKT kinase activity, induced dephosphorylation of AKT and BAD, and increased apoptosis. Concomitant inhibition of mitogen-activated protein kinase signaling or combination with all-trans retinoic acid further enhanced apoptosis of leukemic cells. In primary AML samples, clonogenic cell growth was significantly reduced. Normal hematopoietic progenitors were less affected, suggesting preferential targeting of leukemia cells. In conclusion, the data suggest that the inhibition of the PI3K/AKT signaling pathway restores apoptosis in AML and may be explored as a novel target for molecular therapeutics in AML.

Vanillins--a novel family of DNA-PK inhibitors

Non-homologous DNA end-joining (NHEJ) is a major pathway of double strand break (DSB) repair in human cells. Here we show that vanillin (3-methoxy-4-hydroxybenzaldehyde)--a naturally occurring food component and an acknowledged antimutagen, anticlastogen and anticarcinogen--is an inhibitor of NHEJ. Vanillin blocked DNA end-joining by human cell extracts by directly inhibiting the activity of DNA-PK, a crucial NHEJ component. Inhibition was selective and vanillin had no detectable effect on other steps of the NHEJ process, on an unrelated protein kinase or on DNA mismatch repair by cell extracts. Subtoxic concentrations of vanillin did not affect the ATM/ATR-dependent phosphorylation of Chk2 or the S-phase checkpoint response after ionising radiation. They significantly potentiated the cytotoxicity of cisplatin, but did not affect sensitivity to UVC. A limited screen of structurally related compounds identified two substituted vanillin derivatives that were 100- and 50-fold more potent than vanillin as DNA-PK inhibitors. These compounds also sensitised cells to cisplatin. The inhibition of NHEJ is consistent with the antimutagenic and other biological properties of vanillin, possibly altering the balance between DSB repair by NHEJ and homologous recombination.

A new selective AKT pharmacological inhibitor reduces resistance to chemotherapeutic drugs, TRAIL, all-trans-retinoic acid, and ionizing radiation of human leukemia cells

It is now well established that the reduced capacity of tumor cells of undergoing cell death through apoptosis plays a key role both in the pathogenesis of cancer and in therapeutic treatment failure. Indeed, tumor cells frequently display multiple alterations in signal transduction pathways leading to either cell survival or apoptosis. In mammals, the pathway based on phosphoinositide 3-kinase (PI3K)/Akt conveys survival signals of extreme importance and its downregulation, by means of pharmacological inhibitors of PI3K, considerably lowers resistance to various types of therapy in solid tumors. We recently described an HL60 leukemia cell clone (HL60AR cells) with a constitutively active PI3K/Akt pathway. These cells were resistant to multiple chemotherapeutic drugs, all-trans-retinoic acid (ATRA), and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Treatment with two pharmacological inhibitors of PI3K, wortmannin and Ly294002, restored sensitivity of HL60AR cells to the aforementioned treatments. However, these inhibitors have some drawbacks that may severely limit or impede their clinical use. Here, we have tested whether or not a new selective Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2(R)-2-O-methyl-3-O-octadecylcarbonate (Akt inhibitor), was as effective as Ly294002 in lowering the sensitivity threshold of HL60 cells to chemotherapeutic drugs, TRAIL, ATRA, and ionizing radiation. Our findings demonstrate that, at a concentration which does not affect PI3K activity, the Akt inhibitor markedly reduced resistance of HL60AR cells to etoposide, cytarabine, TRAIL, ATRA, and ionizing radiation. This effect was likely achieved through downregulation of expression of antiapoptotic proteins such as c-IAP1, c-IAP2, cFLIP(L), and of Bad phosphorylation on Ser 136. The Akt inhibitor did not influence PTEN activity. At variance with Ly294002, the Akt inhibitor did not negatively affect phosphorylation of protein kinase C-zeta and it was less effective in downregulating p70S6 kinase (p70S6K) activity. The Akt inhibitor increased sensitivity to apoptotic inducers of K562 and U937, but not of MOLT-4, leukemia cells. Overall, our results indicate that selective Akt pharmacological inhibitors might be used in the future for enhancing the sensitivity of leukemia cells to therapeutic treatments that induce apoptosis or for overcoming resistance to these treatments.

Requirement for the PI3K/Akt pathway in MEK1-mediated growth and prevention of apoptosis: identification of an Achilles heel in leukemia

The Raf/MEK/ERK kinase cascade plays a critical role in transducing growth signals from activated cell surface receptors. Using DeltaMEK1:ER, a conditionally active form of MEK1 which responds to either beta-estradiol or the estrogen receptor antagonist 4 hydroxy-tamoxifen (4HT), we previously documented the ability of this dual specificity protein kinase to abrogate the cytokine-dependency of human (TF-1) and murine (FDC-P1 and FL5.12) hematopoietic cells lines. Here we demonstrate the ability of DeltaMEK1:ER to activate the phosphatidylinositol 3-kinase (PI3K)/Akt/p70 ribosomal S6 kinase (p70(S6K)) pathway and the importance of this pathway in MEK1-mediated prevention of apoptosis. MEK1-responsive cells can be maintained long term in the presence of beta-estradiol, 4HT or IL-3. Removal of hormone led to the rapid cessation of cell proliferation and the induction of apoptosis in a manner similar to cytokine deprivation of the parental cells. Stimulation of DeltaMEK1:ER by 4HT resulted in ERK, PI3K, Akt and p70(S6K) activation. Treatment with PI3K, Akt and p70(S6K) inhibitors prevented MEK-responsive growth. Furthermore, the apoptotic effects of PI3K/Akt/p70(S6K) inhibitors could be enhanced by cotreatment with MEK inhibitors. Use of a PI3K inhibitor and a constitutively active form of Akt, [DeltaAkt(Myr(+))], indicated that activation of PI3K was necessary for MEK1-responsive growth and survival as activation of Akt alone was unable to compensate for the loss of PI3K activity. Cells transduced by MEK or MEK+Akt displayed different sensitivities to signal transduction inhibitors, which targeted these pathways. These results indicate a requirement for the activation of the PI3K pathway during MEK-mediated transformation of certain hematopoietic cells. These experiments provide important clues as to why the identification of mutant signaling pathways may be the Achilles heel of leukemic cell growth. Leukemia treatment targeting multiple signal transduction pathways may be more efficacious than therapy aimed at inhibiting a single pathway.

Bcl-2 and apoptosis in chronic lymphocytic leukemia

The Bcl-2 family of pro- and antiapoptotic proteins are key regulators of the apoptosis cascade and the mitochondrial-mediated pathway of caspase activation. Of this family, Bcl-2 was the first identified and remains the best characterized. Aberrant expression of Bcl-2 is common in chronic lymphocytic leukemia (CLL) and is associated with poor response to chemotherapy and decreased overall survival. Bcl-2 is an attractive target for novel therapeutic agents. Antisense oligonucleotides directed against Bcl-2 are effective in vitro and are being evaluated in clinical trials in CLL. Small molecule Bcl-2 inhibitors are in preclinical development and should be ready for clinical evaluation in the next few years. Strategies that induce apoptosis and bypass Bcl-2 may also be therapeutically useful in CLL. Thus, over the next decade, one can envision incorporating measurements of apoptotic proteins such as Bcl-2 into the risk assessment and treatment algorithms for individual patients. In addition, we anticipate that in the next decade, rationally designed therapies targeting specific molecular defects in the malignant CLL lymphocytes will be introduced into the clinic.

Albumin activates the AKT signaling pathway and protects B-chronic lymphocytic leukemia cells from chlorambucil- and radiation-induced apoptosis

Activation of the phosphatidylinositol 3- kinase/AKT pathway antagonizes apoptosis in diverse cellular systems. We previously showed that human plasma activated AKT and potently blocked the ability of chlorambucil or gamma radiation to induce apoptosis of B-chronic lymphocytic leukemia (CLL) cells. Here we report experiments that identify albumin as the major component of plasma that blocks CLL cell killing by chlorambucil or radiation. Intact plasma depleted of albumin by chromatography on Cibacron blue-Sepharose or plasma from a subject with analbuminemia failed either to activate AKT or to protect CLL cells from chlorambucil-induced apoptosis. Both functions were restored by re-addition of albumin. The protective action of albumin as well as AKT activation was compromised by the binding of lipids. Fluorescence-activated cell sorter (FACScan) analysis demonstrated the uptake of fluoresceinated albumin by CLL cells. Accumulation of albumin in intracellular vesicles was also shown by confocal microscopy. Indirect inhibition of AKT activation by the phosphatidylinositol 3-kinase inhibitor LY294002 reversed the blockade of chlorambucil-induced killing by plasma albumin. The data suggest that activation of AKT consequent to binding of albumin by CLL cells blocks chlorambucil- and radiation-induced apoptosis. Strategies designed to block albumin-induced antiapoptotic signaling may, therefore, be of value in enhancing cytotoxic drug action on CLL cells.

Differential activation of apoptosis regulatory pathways during monocytic vs granulocytic differentiation: a requirement for Bcl-X(L)and XIAP in the prolonged survival of monocytic cells

Neutrophils and monocytes/macrophages are derived from common progenitors, but exhibit markedly different lifespans. Differentiated neutrophils are short-lived and die rapidly by apoptosis, while monocytic cells are longer-lived. In this report we used the HL-60 cell line as a model system to identify differences in apoptotic pathways which might account for the differing lifespans of granulocytic vs monocytic cells. We observed that induction of granulocytic differentiation by retinoic acid led to robust activation of the executioner protease caspase-3, and early onset of apoptosis. By contrast, caspase-3 was not appreciably activated during phorbol 12-myristate 13-acetate (PMA)-induced monocytic differentiation, and apoptosis was delayed in these cells. Since the activation of caspase-3 is inhibited by members of the inhibitor of apoptosis (IAP) and Bcl-2 protein families, we investigated the expression of anti-apoptotic members of these families. Induction of monocytic differentiation led to marked upregulation of the IAP protein XIAP, as well as the Bcl-2 family member Bcl-X(L). During granulocytic differentiation the levels of XIAP progressively declined, while Bcl-X(L) levels remained unchanged. A different IAP protein, survivin, was downregulated during differentiation along either lineage, as was expression of Bcl-2. The upregulation of Bcl-X(L) during monocytic differentiation coincided with phosphorylation/activation of STAT3, a known activator of bcl-X gene transcription. Moreover, Bcl-X(L) upregulation was dependent on MEK/ERK signaling. Upregulation of XIAP proceeded in a MEK/ERK-independent fashion. Treatment with antisense Bcl-X(L) or XIAP oligonucleotides resulted in significant loss of viability in cells differentiating along the monocytic lineage. Together, these findings indicate that the levels of XIAP and Bcl-X(L) are regulated by distinct pathways during monocytic differentiation, and that upregulation of these proteins contributes to the increased longevity of cells in the monocytic lineage.

Growth inhibition of human malignant glioma cells induced by the PI3-K-specific inhibitor

OBJECT

The phosphatase and tensin homolog deleted from chromosome 10 (PTEN) functions as a tumor suppressor by negatively regulating the growth/survival signals of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway. The PI3-K/Akt pathway in PTEN-deficient tumors may be one of the key targets for anticancer therapy. The authors examined the effects of the PI3-K inhibitor 2-(4-morpholinyl)-8-phenylchromone (LY294002) on human malignant glioma cells, and compared these effects on PTEN-deficient cells with those on PTEN-wild-type (PTEN-wt) cells.

METHODS

Using human malignant glioma cell lines, including the PTEN-deficient cells A172 and U87MG and the PTEN-wt cells LN18 and LN229, the effects of LY294002 on cell growth, apoptosis, and chemotherapeutic agent-induced cytotoxicity were evaluated. The LY294002 inhibited the growth of U87MG cells associated with reduced phosphatidylinositol 3,4,5,-trisphosphate and phosphorylated Akt, and also induced growth inhibition in three other cell lines. Although LY294002 caused apoptosis in all four cell lines, apoptosis seemed to contribute to only a small portion of growth inhibition induced by LY294002. There was no link between the status of PTEN and the median inhibitory concentration values for LY294002 or between the gene status and the extent of LY294002-induced apoptosis. The LY294002 significantly augmented the cytotoxicity induced by etoposide in PTEN-deficient cells, but not in PTEN-wt cells. Enhancement of 1,3-bis(2-chloroethyl)-1-nitrosourea- and cisplatin-induced cytotoxicity by LY294002 was not linked to the status of PTEN. No marked difference in the amounts of phosphorylated Akt was found between PTEN-deficient and PTEN-wt cells.

CONCLUSIONS

The findings show that PI3-K is a possible target for therapy in patients with gliomas, and PI3-K inhibitors in combination with chemotherapeutic agents could be potent therapeutic modalities for patients with malignant gliomas.

Activation of Bak, Bax, and BH3-only proteins in the apoptotic response to doxorubicin

The anthracyclin doxorubicin (DXR) is a major antitumor agent known to cause cellular damage via a number of mechanisms including free radical formation and inhibition of topoisomerase II. It is not clear, however, how the subsequent lesions may lead to the apoptotic death of the cell. We have here examined the effects of DXR on activation of pro-apoptotic members of the Bcl-2 family, all of which are connected to the mitochondrial events of apoptosis. In two human cell lines (lymphoma and myeloma), clinically relevant concentrations of DXR were found to induce apoptosis, first observed after 24 h of treatment. Apoptosis correlated with modulation of Bak and Bax to their active conformations. bax- as well as bak-deficient mouse embryo fibroblasts were resistant to DXR compared with wild-type mouse embryo fibroblasts further supporting a role for these proteins as main DXR-induced apoptosis regulators. Furthermore, using immunocytochemistry as well as chemical blocking of putative apical pathways we could demonstrate that Bak is activated prior to Bax. In the human cell lines, DXR was furthermore found to induce high protein levels of Bik, another BH3-only protein. DXR-induced apoptosis was completely blocked in Bcl-2-overexpressing U266 cells. Interestingly, in Bcl-2-transfected cells Bak activation was also blocked, while Bax was still partially active in agreement with differential regulation of these two proteins. Furthermore, co-incubation of the phosphatidylinositol 3-kinase (PI3K)-inhibitor LY294002 potentiated the apoptotic response to DXR. This enhanced apoptosis was preceded by enhanced Bak and Bax activation, and both responses as well as apoptosis were blocked in transfectants overexpressing Bcl-2. In summary, several pieces of evidence suggest that DXR induces apoptosis through a sequential and differential activation of Bak and Bax.

Induction of apoptosis by stomach cancer-associated protein-tyrosine phosphatase-1

Stomach cancer-associated protein-tyrosine phosphatase-1 (SAP-1), a transmembrane-type protein-tyrosine phosphatase, is thought to inhibit integrin signaling by mediating the dephosphorylation of focal adhesion-associated proteins. Adenovirus-mediated overexpression of wild-type SAP-1, but not that of a catalytically inactive mutant of this enzyme, has now been shown to induce apoptosis in NIH 3T3 fibroblasts. This effect of SAP-1 was dependent on cellular caspase activities and was preceded by inactivation of two serine-threonine protein kinases, Akt and integrin-linked kinase (ILK), both of which function downstream of phosphoinositide (PI) 3-kinase to promote cell survival. Coexpression of constitutively active forms of PI 3-kinase or Akt (which fully restored Akt and ILK activities) resulted in partial inhibition of SAP-1-induced cell death. Furthermore, expression of a dominant negative mutant of PI 3-kinase did not induce cell death as efficiently as did SAP-1, although this mutant inhibited Akt and ILK activities more effectively than did SAP-1. Overexpression of SAP-1 had no substantial effect on Ras activity. These results suggest that SAP-1 induces apoptotic cell death by at least two distinct mechanisms: inhibition of cell survival signaling mediated by PI 3-kinase, Akt, and ILK and activation of a caspase-dependent proapoptotic pathway.

Doxorubicin resistant neuroblastoma cells secrete factors that activate AKT and attenuate cytotoxicity in drug-sensitive cells

The present study aims to verify the hypothesis that tumor cells with acquired resistance to chemotherapy may secrete survival factors and thus, participate in the protection of drug-sensitive cells against drug toxicity. A human neuroblastoma cell line, SK-N-SH, and its doxorubicin resistant derivative, RDOX6, have been used. Conditioned medium from RDOX6 cells attenuated the cytotoxic response of SK-N-SH cells to doxorubicin. This protective effect was associated with activation of the Akt survival pathway and inhibition of caspase-3. These data indicate that secretion, by drug-resistant cells, of factors that activate anti-apoptotic pathways in drug-sensitive cells, may constitute a novel mechanism of drug resistance.