Proteinase-3, a serine protease which mediates doxorubicin-induced apoptosis in the HL-60 leukemia cell line, is downregulated in its doxorubicin-resistant variant

The Epigenetic Reader Methyl-CpG-Binding Protein 2 (MeCP2) Is an Emerging Oncogene in Cancer Biology

Epigenetic mechanisms are gene regulatory processes that control gene expression and cellular identity. Epigenetic factors include the "writers", "readers", and "erasers" of epigenetic modifications such as DNA methylation. Accordingly, the nuclear protein Methyl-CpG-Binding Protein 2 (MeCP2) is a reader of DNA methylation with key roles in cellular identity and function. Research studies have linked altered DNA methylation, deregulation of MeCP2 levels, or MECP2 gene mutations to different types of human disease. Due to the high expression level of MeCP2 in the brain, many studies have focused on its role in neurological and neurodevelopmental disorders. However, it is becoming increasingly apparent that MeCP2 also participates in the tumorigenesis of different types of human cancer, with potential oncogenic properties. It is well documented that aberrant epigenetic regulation such as altered DNA methylation may lead to cancer and the process of tumorigenesis. However, direct involvement of MeCP2 with that of human cancer was not fully investigated until lately. In recent years, a multitude of research studies from independent groups have explored the molecular mechanisms involving MeCP2 in a vast array of human cancers that focus on the oncogenic characteristics of MeCP2. Here, we provide an overview of the proposed role of MeCP2 as an emerging oncogene in different types of human cancer.

Differential Sensitivity of the Protein Translation Initiation Machinery and mTOR Signaling to MECP2 Gain- and Loss-of-Function Involves MeCP2 Isoform-Specific Homeostasis in the Brain

Eukaryotic gene expression is controlled at multiple levels, including gene transcription and protein translation initiation. One molecule with key roles in both regulatory mechanisms is methyl CpG binding protein 2 (MeCP2). MECP2 gain- and loss-of-function mutations lead to Rett Syndrome and MECP2 Duplication Syndrome, respectively. To study MECP2 gain-of-function, we generated stably transduced human brain cells using lentiviral vectors for both MECP2E1 and MECP2E2 isoforms. Stable overexpression was confirmed by Western blot and immunofluorescence. We assessed the impact of MeCP2E1-E2 gain-of-function on the MeCP2 homeostasis regulatory network (MECP2E1/E2-BDNF/BDNF-miR-132), mTOR-AKT signaling, ribosome biogenesis, markers of chromatin structure, and protein translation initiation. We observed that combined co-transduction of MeCP2 isoforms led to protein degradation of MeCP2E1. Proteosome inhibition by MG132 treatment recovered MeCP2E1 protein within an hour, suggesting its induced degradation through the proteosome pathway. No significant change was detected for translation initiation factors as a result of MeCP2E1, MeCP2E2, or combined overexpression of both isoforms. In contrast, analysis of human Rett Syndrome brains tissues compared with controls indicated impaired protein translation initiation, suggesting that such mechanisms may have differential sensitivity to MECP2 gain- and loss-of-function. Collectively, our results provide further insight towards the dose-dependent functional role of MeCP2 isoforms in the human brain.

Simvastatin Induces Apoptosis in Medulloblastoma Brain Tumor Cells via Mevalonate Cascade Prenylation Substrates

Medulloblastoma is a common pediatric brain tumor and one of the main types of solid cancers in children below the age of 10. Recently, cholesterol-lowering “statin” drugs have been highlighted for their possible anti-cancer effects. Clinically, statins are reported to have promising potential for consideration as an adjuvant therapy in different types of cancers. However, the anti-cancer effects of statins in medulloblastoma brain tumor cells are not currently well-defined. Here, we investigated the cell death mechanisms by which simvastatin mediates its effects on different human medulloblastoma cell lines. Simvastatin is a lipophilic drug that inhibits HMG-CoA reductase and has pleotropic effects. Inhibition of HMG-CoA reductase prevents the formation of essential downstream intermediates in the mevalonate cascade, such as farnesyl pyrophosphate (FPP) and gernaylgerany parophosphate (GGPP). These intermediates are involved in the activation pathway of small Rho GTPase proteins in different cell types. We observed that simvastatin significantly induces dose-dependent apoptosis in three different medulloblastoma brain tumor cell lines (Daoy, D283, and D341 cells). Our investigation shows that simvastatin-induced cell death is regulated via prenylation intermediates of the cholesterol metabolism pathway. Our results indicate that the induction of different caspases (caspase 3, 7, 8, and 9) depends on the nature of the medulloblastoma cell line. Western blot analysis shows that simvastatin leads to changes in the expression of regulator proteins involved in apoptosis, such as Bax, Bcl-2, and Bcl-xl. Taken together, our data suggests the potential application of a novel non-classical adjuvant therapy for medulloblastoma, through the regulation of protein prenylation intermediates that occurs via inhibition of the mevalonate pathway.

Expression of myeloperoxidase in acute myeloid leukemia blasts mirrors the distinct DNA methylation pattern involving the downregulation of DNA methyltransferase DNMT3B

Myeloperoxidase (MPO) has been associated with both a myeloid lineage commitment and favorable prognosis in patients with acute myeloid leukemia (AML). DNA methyltransferase inhibitors (decitabine and zeburaline) induced MPO gene promoter demethylation and MPO gene transcription in AML cells with low MPO activity. Therefore, MPO gene transcription was directly and indirectly regulated by DNA methylation. A DNA methylation microarray subsequently revealed a distinct methylation pattern in 33 genes, including DNA methyltransferase 3 beta (DNMT3B), in CD34-positive cells obtained from AML patients with a high percentage of MPO-positive blasts. Based on the inverse relationship between the methylation status of DNMT3B and MPO, we found an inverse relationship between DNMT3B and MPO transcription levels in CD34-positive AML cells (P=0.0283). In addition, a distinct methylation pattern was observed in five genes related to myeloid differentiation or therapeutic sensitivity in CD34-positive cells from AML patients with a high percentage of MPO-positive blasts. Taken together, the results of the present study indicate that MPO may serve as an informative marker for identifying a distinct and crucial DNA methylation profile in CD34-positive AML cells.

Taiwanin A inhibits MCF-7 cancer cell activity through induction of oxidative stress, upregulation of DNA damage checkpoint kinases, and activation of p53 and FasL/Fas signaling pathways

This study investigates the anti-MCF-7 breast cancer cell effects and the underlying pharmacological activity and mechanism of taiwanin A, a major lignan isolated from Taiwania cryptomerioides. Our results show that taiwanin A time-dependently induced reactive oxygen species level and DNA damage in MCF-7 cells, which were likely activated kinases ataxia telangiectasia mutated (ATM) and checkpoint kinase (Chk). Taiwanin A could also up-regulate p53, phosphorylated p53, p21(Cip1), and p27(Kip1) and down-regulate the G(2)/M checkpoint cyclin-dependent kinase1 (Cdk1)-cyclin A/B, leading to induction of G(2)/M cell-cycle arrest in MCF-7 cells. Blockade of p53 gene expression by siRNA further demonstrated that the cell-cycle arrest induced by taiwanin A was p53-dependent. The FasL/Fas-mediated apoptotic signaling cascade was involved in taiwanin A-induced apoptosis via activation of caspases-10 and -7 (but not caspase-8), and proteolytic cleavage of poly(ADP-ribose) polymerase (PARP). In contrast, mitochondria-initiated apoptotic pathway was not involved. This is the first report to delineate novel mechanism of the action of taiwanin A against MCF-7 cells, suggesting this lignan may have value for development as an anti-breast cancer agent.

Apoptosis induces expression of sphingosine kinase 1 to release sphingosine-1-phosphate as a "come-and-get-me" signal

Sphingosine-1-phosphate (S1P) is a bioactive lipid that regulates myriad important cellular processes, including growth, survival, cytoskeleton rearrangements, motility, and immunity. Here we report that treatment of Jurkat and U937 leukemia cells with the pan-sphingosine kinase (SphK) inhibitor N,N-dimethylsphingosine to block S1P formation surprisingly caused a large increase in expression of SphK1 concomitant with induction of apoptosis. Another SphK inhibitor, D,L-threo-dihydrosphingosine, also induced apoptosis and produced dramatic increases in SphK1 expression. However, up-regulation of SphK1 was not a specific effect of its inhibition but rather was a consequence of apoptotic stress. The chemotherapeutic drug doxorubicin, a potent inducer of apoptosis in these cells, also stimulated SphK1 expression and activity and promoted S1P secretion. The caspase inhibitor ZVAD reduced not only doxorubicin-induced lethality but also the increased expression of SphK1 and secretion of S1P. Apoptotic cells secrete chemotactic factors to attract phagocytic cells, and we found that S1P potently stimulated chemotaxis of monocytic THP-1 and U937 cells and primary monocytes and macrophages. Collectively, our data suggest that apoptotic cells may up-regulate SphK1 to produce and secrete S1P that serves as a "come-and-get-me" signal for scavenger cells to engulf them in order to prevent necrosis.

Overcoming MDR-associated chemoresistance in HL-60 acute myeloid leukemia cells by targeting sphingosine kinase-1

We examined the involvement of sphingosine kinase-1, a critical regulator of the sphingolipid balance, in susceptibility to antineoplastic agents of either sensitive or multidrug-resistant acute myeloid leukemia cells. Contrary to parental HL-60 cells, doxorubicin and etoposide failed to trigger apoptosis in chemoresistant HL-60/Doxo and HL-60NP16 cells overexpressing MRP1 and MDR1, respectively. Chemosensitive HL-60 cells displayed sphingosine kinase-1 inhibition coupled with ceramide generation. In contrast, chemoresistant HL-60/ Doxo and HL-60/VP16 had sustained sphingosine kinase-1 activity and did not produce ceramide during treatment. Enforced expression of sphingosine kinase-1 in chemosensitive HL-60 cells resulted in marked inhibition of apoptosis that was mediated by blockade of mitochondrial cytochrome c efflux hence suggesting a control of apoptosis at the pre-mitochondrial level. Incubation with cell-permeable ceramide of chemoresistant cells led to a sphingosine kinase-1 inhibition and apoptosis both prevented by sphingosine kinase-1 over-expression. Furthermore, F-12509a, a new sphingosine kinase inhibitor, led to ceramide accumulation, decrease in sphingosine 1-phosphate content and caused apoptosis equally in chemosensitive and chemoresistant cell lines that is inhibited by adding sphingosine 1-phosphate or overexpressing sphingosine kinase-1. F-12509a induced classical apoptosis hallmarks namely nuclear fragmentation, caspase-3 cleavage as well as downregulation of antiapoptotic XIAP, and release of cytochrome c and SMAC/Diablo.

P-glycoprotein enhances TRAIL-triggered apoptosis in multidrug resistant cancer cells by interacting with the death receptor DR5

The death-inducing cytokine TRAIL is a promising agent for anticancer therapy since it preferentially kills cancer versus normal cells; however, some cancer cells are TRAIL-resistant. We initially explored whether overexpression of the MDR1 gene product P-glycoprotein (P-gp), which causes multidrug resistance (MDR) in cancer cells, also contributes to TRAIL-resistance. Surprisingly, our results revealed that P-gp-overexpression enhances TRAIL-induced apoptosis not only in neoplastic cells transfected with the MDR1 gene but also in MDR variants selected with cytotoxic anticancer agents. Mechanistic analysis of TRAIL-induced apoptosis in the MDR1-transfected MCF-7 breast cancer cell line BC-19 revealed that TRAIL-triggered significantly more apoptosis in these cells compared with parental MCF-7 cells by binding to the TRAIL receptor DR5. DR5 but not DR4 engagement by TRAIL attenuated cellular ATP levels by robustly stimulating P-gp ATPase activity, and thus triggered P-gp-dependent apoptosis by depletion of the cellular ATP pool. In addition to hyperactive P-gp-mediated ATP hydrolysis, TRAIL-induced, P-gp-potentiated apoptosis was associated with activation of caspases-6, -7, -8, and -9; Bid cleavage; and mitochondrial depolarization. P-gp interacted with the TRAIL receptors DR4, DR5, and DcR1 in plasma membranes and enhanced TRAIL binding to DR5. Interestingly, the decreased level of the decoy TRAIL receptor, DcR1, in BC-19 cells further sensitized these cells to TRAIL. Therefore, both extrinsic and intrinsic apoptosis pathways are involved in this process. These findings for the first time reveal that TRAIL treatment preferentially causes apoptosis in P-gp-overexpressing MDR cells, and suggests significant clinical implications for the use of TRAIL in treating neoplasms that have failed chemotherapy.

Antigen-specific cellular immunotherapy of leukemia

Advances in cellular and molecular immunology have led to the characterization of leukemia-specific T-cell antigens and to the development of strategies for effective augmentation of T-cell immunity in leukemia patients. While several leukemia-related antigens have been identified, this review focuses on the Wilms' tumor 1 (WT1) antigen and the proteinase 3 (Pr3) antigen that are overexpressed in leukemic cells and are already being used in the clinical setting. Moreover, WT1 is also overexpressed in a vast number of nonhematological solid tumors, thereby expanding its use as a promising target for cancer vaccines. Examples of spontaneous immune responses against WT1 and Pr3 in leukemia patients are presented and the potential of WT1 and Pr3 for adoptive T-cell immunotherapy of leukemia is discussed. We also elaborate on the use of professional antigen-presenting cells loaded with mRNA encoding WT1 exploiting the advantage of broad HLA coverage for therapeutic vaccination purposes. Finally, the summarized data underscore the potential of WT1 for the manipulation of T-cell immunity in leukemia and in cancer in general, that will likely pave the way for the development of more effective and generic cancer vaccines.

Proteinase-3 induces procaspase-3 activation in the absence of apoptosis: potential role of this compartmentalized activation of membrane-associated procaspase-3 in neutrophils

In the present study, we provide evidence that procaspase-3 is a novel target of proteinase 3 (PR3) but not of human neutrophil elastase (HNE). Human mast cell clone 1 (HMC1) and rat basophilic leukemia (RBL) mast cell lines were transfected with PR3 or the inactive mutated PR3 (PR3S203A) or HNE cDNA. In both RBL/PR3 and HMC1/PR3, a constitutive activity of caspase-3 was measured with DEVD substrate, due to the direct processing of procaspase-3 by PR3. No caspase-3 activation was observed in cells transfected with the inactive PR3 mutant or HNE. Despite the high caspase-3 activity in RBL/PR3, no apoptosis was detected as demonstrated by an absence of 1) phosphatidylserine externalization, 2) mitochondria cytochrome c release, 3) upstream caspase-8 or caspase-9 activation, or 4) DNA fragmentation. In vitro, purified PR3 cleaved procaspase-3 into an active 22-kDa fragment. In neutrophils, the 22-kDa caspase-3 activation fragment was present only in resting neutrophils but was absent after apoptosis. The 22 kDa fragment was specific of myeloid cells because it was absent from resting lymphocytes. This 22-kDa fragment was not present when neutrophils were treated with pefabloc, an inhibitor of serine proteinase. Like in HMC1/PR3, the 22-kDa caspase-3 fragment was restricted to the plasma membrane compartment. Double immunofluorescence labeling after streptolysin-O permeabilization further showed that PR3 and procaspase-3 could colocalize in an extragranular compartment. In conclusion, our results strongly suggest that compartmentalized PR3-induced caspase-3 activation might play specific functions in neutrophil survival.

Variability in apoptotic response to poliovirus infection

In several cell types, poliovirus activates the apoptotic program, implementation of which is suppressed by viral antiapoptotic functions. In such cells, productive infection leads to a necrotic cytopathic effect (CPE), while abortive reproduction, associated with inadequate viral antiapoptotic functions, results in apoptosis. Here, we describe two other types of cell response to poliovirus infection. Murine L20B cells expressing human poliovirus receptor responded to the infection by both CPE and apoptosis concurrently. Interruption of productive infection decreased rather than increased the proportion of apoptotic cells. Productive infection was accompanied by the early efflux of cytochrome c from the mitochondria in a proportion of cells and by activation of DEVD-specific caspases. Inactivation of caspase-9 resulted in a marked, but incomplete, prevention of the apoptotic response of these cells to viral infection. Thus, the poliovirus-triggered apoptotic program in L20B cells was not completely suppressed by the viral antiapoptotic functions. In contrast, human rhabdomyosarcoma RD cells did not develop appreciable apoptosis during productive or abortive infection, exhibiting inefficient efflux of cytochrome c from mitochondria and no marked activation of DEVD-specific caspases. The cells were also refractory to several nonviral apoptosis inducers. Nevertheless, typical caspase-dependent signs of apoptosis in a proportion of RD cells were observed after cessation of viral reproduction. Such "late" apoptosis was also observed in productively infected HeLa cells. In addition, a tiny proportion of all studied cells were TUNEL positive even in the presence of a caspase inhibitor. Degradation of DNA in such cells appeared to be a postmortem phenomenon. Biological relevance of variable host responses to viral infection is discussed.

Taxol Induces Caspase-10-dependent Apoptosis

Taxol (paclitaxel) is known to inhibit cell growth and trigger significant apoptosis in various cancer cells. Although taxol induces apoptosis of cancer cells, its exact mechanism of action is not yet known. In this study we investigated death receptors, FAS-associated death domain protein (FADD), the activation of caspases-10 and -8 as well as the downstream caspases, and reactive oxygen species (ROS) in taxol-induced apoptosis in the CCRF-HSB-2 human lymphoblastic leukemia cell line. Pretreating the cells with neutralizing antibodies to Fas, tumor necrosis factor (TNF)-alpha receptor 1, or TNF-related apoptosis-inducing ligand receptors (DR4 and DR5) did not affect taxol-induced apoptosis, but transfection of the cells with a dominant negative FADD plasmid resulted in inhibition of taxol-induced apoptosis, revealing that taxol induces apoptosis independently of these death receptors but dependently on FADD. Furthermore, the drug induced activation of caspases-10, -8, -6, and -3, cleaved Bcl-2, Bid, poly(ADP-ribose) polymerase, and lamin B, and down-regulated cellular levels of FLICE-like inhibitory protein (FLIP) and X-chromosome-linked inhibitor of apoptosis protein (XIAP). However, despite the release of cytochrome c from the mitochondria in taxol-treated cells, caspase-9 was not activated. Inhibitors of caspases-8, -6, or -3 partially inhibited taxol-induced apoptosis, whereas the caspase-10 inhibitor totally abrogated this process. Taxol-induced apoptosis was also associated with decreased mitochondrial membrane potential (Deltapsim) and a significant increase in ROS generation. However, increased ROS production was not directly involved in taxol-triggered apoptosis. Therefore, these results demonstrate for the first time that taxol induces FADD-dependent apoptosis primarily through activation of caspase-10 but independently of death receptors.

N-terminal deletion augments the cell-death-inducing activity of BAX in adenoviral gene delivery to nonsmall cell lung cancers

Therapeutic modalities that overcome the antiapoptotic function of Bcl-2 that is often overexpressed in cancer cells are expected to be a novel strategy for cancer treatment. We previously reported that the leukemic cell death induced by an N-terminally truncated Bax (deltaN Bax: corresponding to amino acid 112-192 of full-length Bax) was not blocked by Bcl-2 or Bcl-x(L) owing to the lack of the BH3 domain needed to interact with the antiapoptotic Bcl-2 family molecules. In this study, we used the Cre-loxP system that allowed us to propagate adenoviruses expressing deltaN Bax, and investigated the effects of the deltaN Bax gene transfer into A549 and NCI-H1299 nonsmall cell lung cancer cell lines. deltaN Bax showed more cell-death-inducing activity in both cells than did the full-length Bax in vitro. It was found that the deltaN Bax-induced cell death was not inhibited by the pan-caspase inhibitor z-VAD-fmk, suggesting that deltaN Bax induces cell death through a caspase-independent mechanism. Intratumoral injection of adenoviruses expressing deltaN Bax into A549 tumors in Balb/c nude mice showed a significantly stronger suppression of tumor growth (74%) than full-length Bax (25%) compared to the control. Our results suggest that deltaN Bax may provide a better alternative than currently used cytotoxic genes in cancer gene therapy trials.

Ham-Wasserman Lecture

Conventional treatment of acute leukemia involves the use of cytotoxic agents (chemotherapy), but other strategies have been explored. All-trans retinoic acid (ATRA) and arsenic have clearly been effective in the treatment of acute promyelocytic leukemia (APL), which creates the possibility that other types of acute leukemia can be conquered by selectively inducing differentiation and/or apoptosis. A great number of investigations have been performed to elucidate the mechanisms and search for effective agents in the treatment of other types of acute leukemia by these new strategies. Progress at the molecular level has been achieved in explaining the mechanisms of action of ATRA and arsenic compounds, and several new agents have emerged, although their clinical effectiveness remains to be confirmed. Mechanism-/gene-based targeted therapy and a combination of different strategies will improve the treatment of acute leukemia.

Ham-Wasserman Lecture

Conventional treatment of acute leukemia involves the use of cytotoxic agents (chemotherapy), but other strategies have been explored. All-trans retinoic acid (ATRA) and arsenic have clearly been effective in the treatment of acute promyelocytic leukemia (APL), which creates the possibility that other types of acute leukemia can be conquered by selectively inducing differentiation and/or apoptosis. A great number of investigations have been performed to elucidate the mechanisms and search for effective agents in the treatment of other types of acute leukemia by these new strategies. Progress at the molecular level has been achieved in explaining the mechanisms of action of ATRA and arsenic compounds, and several new agents have emerged, although their clinical effectiveness remains to be confirmed. Mechanism-/gene-based targeted therapy and a combination of different strategies will improve the treatment of acute leukemia.