Activation of c-Abl tyrosine kinase requires caspase activation and is not involved in JNK/SAPK activation during apoptosis of human monocytic leukemia U937 cells

STI571 reduces TRAIL-induced apoptosis in colon cancer cells: c-Abl activation by the death receptor leads to stress kinase-dependent cell death

BACKGROUND

In an effort to achieve better cancer therapies, we elucidated the combination cancer therapy of STI571 (an inhibitor of Bcr-Abl and clinically used for chronic myelogenous leukemia) and TNF-related apoptosis-inducing ligand (TRAIL, a developing antitumor agent) in leukemia, colon, and prostate cancer cells.

METHODS

Colon cancer (HCT116, SW480), prostate cancer (PC3, LNCaP) and leukemia (K562) cells were treated with STI571 and TRAIL. Cell viability was determined by MTT assay and sub-G1 appearance. Protein expression and kinase phosphorylation were determined by Western blotting. c-Abl and p73 activities were inhibited by target-specific small interfering (si)RNA. In vitro kinase assay of c-Abl was conducted using CRK as a substrate.

RESULTS

We found that STI571 exerts opposite effects on the antitumor activity of TRAIL. It enhanced cytotoxicity in TRAIL-treated K562 leukemia cells and reduced TRAIL-induced apoptosis in HCT116 and SW480 colon cancer cells, while having no effect on PC3 and LNCaP cells. In colon and prostate cancer cells, TRAIL caused c-Abl cleavage to the active form via a caspase pathway. Interestingly, JNK and p38 MAPK inhibitors effectively blocked TRAIL-induced toxicity in the colon, but not in prostate cancer cells. Next, we found that STI571 could attenuate TRAIL-induced c-Abl, JNK and p38 activation in HCT116 cells. In addition, siRNA targeting knockdown of c-Abl and p73 also reduced TRAIL-induced cytotoxicity, rendering HCT116 cells less responsive to stress kinase activation, and masking the cytoprotective effect of STI571.

CONCLUSIONS

All together we demonstrate a novel mediator role of p73 in activating the stress kinases p38 and JNK in the classical apoptotic pathway of TRAIL. TRAIL via caspase-dependent action can sequentially activate c-Abl, p73, and stress kinases, which contribute to apoptosis in colon cancer cells. Through the inhibition of c-Abl-mediated apoptotic p73 signaling, STI571 reduces the antitumor activity of TRAIL in colon cancer cells. Our results raise additional concerns when developing combination cancer therapy with TRAIL and STI571 in the future.

c-Abl in neurodegenerative disease

The c-Abl tyrosine kinase participates in a variety of cellular functions, including regulation of the actin cytoskeleton, regulation of the cell cycle, and the apoptotic/cell cycle arrest response to stress, and the Abl family of kinases has been shown to play a crucial role in development of the central nervous system. Recent studies have shown c-Abl activation in human Alzheimer's and Parkinson's diseases and c-Abl activation in mouse models and neuronal culture in response to amyloid beta fibrils and oxidative stress. Overexpression of active c-Abl in adult mouse neurons results in neurodegeneration and neuroinflammation. Based on this evidence, a potential role for c-Abl in the pathogenesis of neurodegenerative disease is discussed, and we attempt to place activation of c-Abl in context with other known contributors to neurodegenerative pathology.

NF-kappaB activity is constitutively elevated in c-Abl null fibroblasts

The c-abl proto-oncogene encodes a nonreceptor tyrosine kinase involved in many cellular processes, including signaling from growth factor and antigen receptors, remodeling the cytoskeleton, and responding to DNA damage and oxidative stress. Many downstream pathways are affected by c-Abl. Elevated c-Abl kinase activity can inhibit NF-kappaB activity by stabilizing the inhibitory protein IkappaB alpha, raising the possibility that c-Abl-deficient cells might have increased NF-kappaB activity. We examined the levels of NF-kappaB activity in primary mouse embryonic fibroblasts (MEFs) derived from wild-type and c-Abl knockout mice and found that the knockout MEFs indeed exhibited elevated NF-kappaB activity in response to stimulation as well as constitutively elevated NF-kappaB activity. Thus, endogenous c-Abl is a negative regulator of basal and inducible NF-kappaB activity. Examination of various points of NF-kappaB regulation revealed that unstimulated c-Abl knockout MEFs do not exhibit an increase in IkappaB alpha degradation, p65/RelA nuclear translocation, or DNA binding of NF-kappaB subunits. They do, however, show reduced levels of the histone deacetylase HDAC1, a negative regulator of basal NF-kappaB activity. Unstimulated c-Abl knockout MEFs are less responsive to induction of NF-kappaB activity by trichostatin A, an HDAC inhibitor, suggesting that c-Abl might play a role in the HDAC-mediated repression of basal NF-kappaB activity.

Consequences of Shb and c-Abl interactions for cell death in response to various stress stimuli

The adaptor protein Shb has previously been shown to regulate apoptosis in response to cytokines and inhibitors of angiogenesis although the mechanisms governing these effects have remained obscure. We currently demonstrate interactions between Shb and c-Abl and that Shb regulates c-Abl kinase activity. The data suggest that c-Abl binds to tyrosine phosphorylated Shb via a concerted effort involving both the c-Abl SH3 and SH2 domains. The biological significance of the Shb/c-Abl interaction was presently tested in overexpression experiments and was found to promote hydrogen peroxide-induced cell death. We also show by Shb knockdown experiments that Shb regulates c-Abl activity and modulates cell death in response to the genotoxic agent cisplatin and the endoplasmic reticulum stress-inducer tunicamycin. The findings are in agreement with the notion of Shb playing a pivotal role in modulating c-Abl pro-apoptotic signaling in response to various stress stimuli.

c-Abl acetylation by histone acetyltransferases regulates its nuclear-cytoplasmic localization

c-Abl function is strictly dependent on its subcellular localization. Using an in vitro approach, we identify c-Abl as a new substrate for p300, CBP (CREB-binding protein) and PCAF (p300/CBP-associated factor) histone acetyltransferases. Remarkably, acetylation markedly alters its subcellular localization. Point mutagenesis indicated that Lys 730, located in the second nuclear localization signal, is the main target of p300 activity. It has previously been reported that c-Abl accumulates in the cytoplasm during myogenic differentiation. Here, we show that c-Abl protein is acetylated at early stages of myogenic differentiation. Indeed, acetylation on Lys 730 drives c-Abl accumulation in the cytoplasm and promotes differentiation. Thus, Lys 730 acetylation is a novel post-translational modification of c-Abl and a novel mechanism for modulating its subcellular localization that contributes to myogenic differentiation.

Inhibition of ABL tyrosine kinase potentiates radiation-induced terminal growth arrest in anaplastic thyroid cancer cells

Gleevec, a selective tyrosine kinase inhibitor, retarded the growth of anaplastic thyroid cancer cell lines in vitro and in vivo through selective inhibition of ABL tyrosine kinase activity. In the present study, we investigated the ability of Gleevec to modulate the in vitro and in vivo radiation response of anaplastic thyroid cancer cells. Cell growth assays, colony formation assays and xenograft models were used to quantify the radiosensitizing effect of Gleevec in cells of the anaplastic thyroid cancer cell lines ARO and FRO. FACS, Western blotting and histochemical techniques were employed to study the mechanisms of radiation response after exposure to Gleevec. Gleevec (7.0 microM) increased the anti-proliferative effect of radiation on the growth ARO and FRO cells in vitro. Clonogenic analysis demonstrated that Gleevec reduced cell survival after irradiation. Gleevec combined with radiation produced an increase in tumor growth inhibition compared to treatment with either modality alone in mice bearing anaplastic thyroid cancer xenografts. The drug suppressed radiation-induced ABL activation and promoted CDKN1A (p21(cip1)) accumulation in irradiated anaplastic thyroid cancer cells. Gleevec had an additional effect on radiation-induced apoptosis in cells of both cell lines and potentiated the induction of terminal growth arrest accompanied by the expression of senescence-associated beta-galactosidase. The antitumor effect of Gleevec is potentiated in adjunctive therapy with radiation not only due to inhibition of proliferative cell growth with transient cell cycle arrest and apoptosis, but also due to the terminal growth arrest associated with senescence, suggesting that tumor cell senescence is a mechanism for tumor targeting therapy in combination with ionizing radiation.

Nucleo-cytoplasmic communication in apoptotic response to genotoxic and inflammatory stress

Genotoxic agents or inflammatory cytokines activate cellular stress responses and trigger programmed cell death. We have identified a signal transduction module, including three nuclear proteins that participate in the regulation of cell death induced by chemotherapeutic agents and tumor necrosis factor (TNF). In this nuclear signaling module, retinoblastoma protein (Rb) functions as an inhibitor of apoptotic signal transduction. Inactivation of Rb by phosphorylation or caspase-dependent cleavage/degradation is required for cell death to occur. Rb inhibits the Abl tyrosine kinase. Thus, Rb inactivation is a pre-requisite for Abl activation by DNA damage or TNF. Activation of nuclear Abl and its downstream effector p73 induces mitochondriadependent cell death. The involvement of these nuclear signal transducers in TNF induced apoptosis, which does not require new gene expression, indicates that nuclear events other than transcription can contribute to extrinsic apoptotic signal transduction.

Apollon ubiquitinates SMAC and caspase-9, and has an essential cytoprotection function

Apollon (also known as BRUCE or BIRC6) is a large protein containing baculoviral-IAP-repeat (BIR) and ubiquitin-conjugating enzyme (UBC) domains at the amino- and carboxy termini, respectively. Apollon inhibits apoptosis, but its molecular and physiological function remains unclear. Here we report that Apollon binds to, ubiquitinates and facilitates proteasomal degradation of SMAC and caspase-9, which both contain IAP-binding motifs. Targeted disruption of Apollon in mice caused embryonic and neonatal lethality. Notably, SMAC induced apoptosis in Apollon-deficient cells, but not in Apollon-expressing cells. Furthermore, the IAP-binding motif of SMAC was required to induce apoptosis in Apollon-deficient cells. These results suggest that Apollon has an essential function in preventing SMAC-induced apoptosis.

Tumor necrosis factor alpha-induced apoptosis requires p73 and c-ABL activation downstream of RB degradation

The retinoblastoma protein (RB) suppresses cell proliferation and apoptosis. We have previously shown that RB degradation is required for tumor necrosis factor alpha (TNF-alpha) to induce apoptosis. We show here the identification of two apoptotic effectors, i.e., c-ABL tyrosine kinase and p73, which are activated by TNF-alpha following RB degradation. In cells expressing a degradation-resistant RB protein (RB-MI), TNF-alpha does not activate c-ABL. RB-MI also inhibits TNF-alpha-mediated activation of p73. Genetic deletion and pharmacological inhibition of c-ABL or p73 diminish the apoptotic response to TNF-alpha in human cell lines and mouse fibroblasts. Thymocytes isolated from Rb(MI/MI), Abl(-/-), or p73(-/-) mice are resistant to TNF-alpha-induced apoptosis compared to their wild-type counterparts. This is in contrast to p53(-/-) thymocytes, which exhibit a wild-type level of apoptosis in response to TNF-alpha. Thus, c-ABL and p73 contribute to apoptosis induced by TNF-alpha, in addition to their role in promoting DNA damage-associated cell death.

Requirement of caspase-mediated cleavage of c-Abl during stress-induced apoptosis

c-Abl protein tyrosine kinase plays an important role in cell cycle control and apoptosis. Furthermore, induction of apoptosis correlates with the activation of c-Abl. Here, we demonstrate the cleavage of c-Abl by caspases during apoptosis. Caspases separate c-Abl into functional domains including a Src-kinase, a fragment containing nuclear import sequences, a fragment with an actin-binding domain and nuclear export sequence. Caspase cleavage increases the kinase activity of c-Abl as demonstrated by in vitro kinase assays as well as by auto- and substrate phosphorylation. Cells in which c-Abl expression was knocked down by RNA interference resisted cisplatin- but not TNFalpha-induced apoptosis. A similar selective resistance against cisplatin-induced apoptosis was observed when cleavage resistant c-Abl was overexpressed in treated cells. Our data suggest the selective requirement of c-Abl cleavage by caspases for stress-induced, but not for TNFalpha-induced apoptosis.

Caspase-mediated cell death in hematological malignancies: theoretical considerations, methods of assessment, and clinical implications

Apoptosis, the caspase-mediated cell death, plays an important role in the etiology, pathogenesis and therapy of a variety of diseases. Abnormalities of apoptosis regulation, resulting in either its inhibition or enhancement, play a key role in the development of various malignant hematological disorders. Several routine and new therapeutic strategies in Oncohematology are based on apoptosis modulation. Cytotoxic effects of most antineoplastic drugs are based on induction of apoptosis. The accurate estimate of incidence of apoptosis, therefore, is of importance in Oncohematology. In this review we provide an overview of the methods designed to measure the incidence of apoptosis, including the recently developed assays that are based on detection of caspases activation. We also review recent findings on the role of caspase-mediated cell death in hematological malignancies and discuss their clinical implications, including new therapeutical strategies that evolve from these findings.

Caspase-dependent cleavage of c-Abl contributes to apoptosis

The nonreceptor tyrosine kinase c-Abl may contribute to the regulation of apoptosis. c-Abl activity is induced in the nucleus upon DNA damage, and its activation is required for execution of the apoptotic program. Recently, activation of nuclear c-Abl during death receptor-induced apoptosis has been reported; however, the mechanism remains largely obscure. Here we show that c-Abl is cleaved by caspases during tumor necrosis factor- and Fas receptor-induced apoptosis. Cleavage at the very C-terminal region of c-Abl occurs mainly in the cytoplasmic compartment and generates a 120-kDa fragment that lacks the nuclear export signal and the actin-binding region but retains the intact kinase domain, the three nuclear localization signals, and the DNA-binding domain. Upon caspase cleavage, the 120-kDa fragment accumulates in the nucleus. Transient-transfection experiments show that cleavage of c-Abl may affect the efficiency of Fas-induced cell death. These data reveal a novel mechanism by which caspases can recruit c-Abl to the nuclear compartment and to the mammalian apoptotic program.

Inhibition of c-Abl with STI571 attenuates stress-activated protein kinase activation and apoptosis in the cellular response to 1-beta-D-arabinofuranosylcytosine

The response of myeloid leukemia cells to treatment with 1-beta-D-arabinofuranosylcytosine (ara-C) includes activation of the c-Abl protein tyrosine kinase and the stress-activated protein kinase (SAPK). The present studies demonstrate that treatment of human U-937 leukemia cells with ara-C is associated with translocation of SAPK to mitochondria. STI571 (imatinib mesylate), an inhibitor of c-Abl, blocked both activation and mitochondrial targeting of SAPK in the ara-C response. In concert with these effects of STI571, similar findings were obtained in c-Abl-deficient cells. The results further show that STI571 inhibits ara-C-induced loss of mitochondrial transmembrane potential, caspase-3 activation, and apoptosis. These findings demonstrate that STI571 down-regulates c-Abl-mediated signals that target the mitochondria in the apoptotic response to ara-C.

Succession of events in desquamation of superficial urothelial cells as a response to stress induced by prolonged constant illumination

The effect of moderate stress induced by prolonged illumination was analysed on urothelial cells of female mouse urinary bladders at ultrastructural and cytochemical levels. This study demonstrates that the urothelium responds to moderate stress with desquamation which involves two subsequent steps. The first step includes a local detachment of tight junctions and consequently the loss of the permeability barrier leading to expanded intercellular spaces among urothelial cells. During the second step, the disjunction of desmosomes accompanied by exocytosis of lysosomal enzymes (NADPase) in the intercellular space results in exfoliation of superficial cells. It is evident that moderate stress elicits an enhanced desquamation of only superficial cells by a subsequent dysfunction of first tight junctions and after that adherens-type junctions. A rapid restoration of the new tight junctions prevents a long-term malfunction of the blood-urine barrier.

Repression of cyclin B1 expression after treatment with adriamycin, but not cisplatin in human lung cancer A549 cells

Most anticancer drugs cause DNA strand breaks and finally induce cell cycle arrest or cell death. To identify genes involved in these effects, we examined gene expression profiles in human lung cancer A549 cells before and after adriamycin treatment, using a cDNA array technique. In this manner, we identified several up- or down-regulated genes in cells undergoing G2 arrest following adriamycin treatment; among them, cyclin B1 expression was dramatically reduced. The reduction in cyclin B1 expression and G2 arrest were also seen after treatment with etoposide and irinotecan. Previous reports have shown that overexpression of p53 represses cyclin B1 transcription. However, cisplatin neither reduced cyclin B1 expression nor induced G2 phase arrest, while it induced a comparable amount of p53 protein. These results suggest that a reduction in cyclin B1 expression plays a role in the mechanism of action of certain anticancer drugs, including adriamycin, which induce G2 arrest in cancer cells.

The BRCT regions of tumor suppressor BRCA1 and of XRCC1 show DNA end binding activity with a multimerizing feature

The BRCT regions are two repeating structures in BRCA1 at the carboxyl-terminus and are ubiquitous in some proteins involved in cell cycle checkpoint and in DNA repair. Here, using electron microscopy, we show direct evidence that the BRCT regions of BRCA1 bound double-strand breaks of DNA. The BRCT regions could multimerize thus forming large protein particles. Smeared patterns of DNA fragments were consistently shown in the gel retardation assay. A single BRCT was sufficient for DNA binding. The smeared patterns were also observed in BRCTs of TopBP1, suggesting that multimerization may be an important feature of BRCTs. The recombinant second BRCT of XRCC1 (X-ray repair cross-complementing group 1), whose folding was determined by X-ray crystallography, also showed similar DNA end binding images. It is possible that some BRCTs are fundamental structures that detect DNA damages.

Regulation of cell death by the Abl tyrosine kinase

The c-abl proto-oncogene encodes a protein tyrosine kinase that is distributed in the nucleus and the cytoplasm of proliferating cells. In the nucleus, c-Abl activity is negatively regulated by the retinoblastoma protein (RB) and positively regulated by DNA damage signals. Activation of the c-Abl kinase by DNA damage requires the function of ATM, which regulates cell cycle checkpoint, DNA repair and apoptosis in response to DNA damage. Cells lacking c-Abl can activate cell cycle checkpoints and DNA repair, but show defects in apoptosis. The apoptosis defect of c-Abl deficient cells is correlated with a defect in the induction and activation of p73, which is a functional homologue of the p53 tumor suppressor protein and has pro-apoptotic activity. The inhibition of c-Abl by RB is consistent with RB's ability to block apoptosis; while the activation of c-Abl by ATM is consistent with ATM's ability to activate cell death. The oncogenic Bcr-Abl tyrosine kinase is a potent inhibitor of apoptosis, and it is retained exclusively in the cytoplasm of transformed cells. Interestingly, when Bcr-Abl is trapped inside of the nucleus through a combined disruption of its cytoplasmic retention and its nuclear export, this oncogenic Abl kinase induces apoptosis. Taken together, the current results support a role for the nuclear c-Abl tyrosine kinase in the regulation of apoptosis. Whether the cytoplasmic c-Abl kinase can actively inhibit apoptosis remains to be determined; however, a deliberate retention of c-Abl in the cytoplasm could potentially contribute to the attenuation of apoptosis response.

Glyoxalase I is involved in resistance of human leukemia cells to antitumor agent-induced apoptosis

Abnormality in the machinery of apoptosis is associated with a resistant phenotype of the tumor cell to chemotherapy. To determine the molecular basis of resistance to antitumor agent-induced apoptosis, we performed a complementary DNA (cDNA) subtractive hybridization with messenger RNA (mRNA) from human monocytic leukemia U937 and its variant UK711, which is resistant to apoptosis induced by antitumor agents. We found that glyoxalase I (GLO1), an enzyme that detoxifies methylglyoxal, is selectively overexpressed in the apoptosis-resistant UK711 cells. The GLO1 enzyme activity was significantly elevated in UK711 and UK110 cells, another drug-resistant mutant, as well as in K562/ADM, adriamycin-resistant leukemia cells, compared with their parental cells. When overexpressed in human Jurkat cells, GLO1 inhibited etoposide- and adriamycin-induced caspase activation and apoptosis, indicating the involvement of GLO1 in apoptosis suppression caused by these drugs. Moreover, cotreatment withS-p-bromobenzylglutathione cyclopentyl diester (BBGC), a cell-permeable inhibitor of GLO1, enhanced etoposide-induced apoptosis in resistant UK711 cells but not in parental U937 cells. Taken together, these results indicate that GLO1 is a resistant factor to antitumor agent-induced apoptosis in human leukemia cells and that the GLO1 inhibitor could be a drug resistance-reversing agent.

Glyoxalase I is involved in resistance of human leukemia cells to antitumor agent-induced apoptosis

Abnormality in the machinery of apoptosis is associated with a resistant phenotype of the tumor cell to chemotherapy. To determine the molecular basis of resistance to antitumor agent-induced apoptosis, we performed a complementary DNA (cDNA) subtractive hybridization with messenger RNA (mRNA) from human monocytic leukemia U937 and its variant UK711, which is resistant to apoptosis induced by antitumor agents. We found that glyoxalase I (GLO1), an enzyme that detoxifies methylglyoxal, is selectively overexpressed in the apoptosis-resistant UK711 cells. The GLO1 enzyme activity was significantly elevated in UK711 and UK110 cells, another drug-resistant mutant, as well as in K562/ADM, adriamycin-resistant leukemia cells, compared with their parental cells. When overexpressed in human Jurkat cells, GLO1 inhibited etoposide- and adriamycin-induced caspase activation and apoptosis, indicating the involvement of GLO1 in apoptosis suppression caused by these drugs. Moreover, cotreatment withS-p-bromobenzylglutathione cyclopentyl diester (BBGC), a cell-permeable inhibitor of GLO1, enhanced etoposide-induced apoptosis in resistant UK711 cells but not in parental U937 cells. Taken together, these results indicate that GLO1 is a resistant factor to antitumor agent-induced apoptosis in human leukemia cells and that the GLO1 inhibitor could be a drug resistance-reversing agent.

c-Abl: activation and nuclear targets

The c-Abl tyrosine kinase and its transforming variants have been implicated in tumorigenesis and in many important cellular processes. c-Abl is localized in the nucleus and the cytoplasm, where it plays distinct roles. The effects of c-Abl are mediated by multiple protein-protein and protein-DNA interactions and its tyrosine kinase domain. At the biochemical level, the mechanism of c-Abl kinase activation and the identification of its target proteins and cellular machineries have in part been solved. However, the phenotypic outcomes of these molecular events remained in large elusive. c-Abl has been shown to regulate the cell cycle and to induce under certain conditions cell growth arrest and apoptosis. In this respect the interaction of c-Abl with p53 and p73 has attracted particular attention. Recent findings have implicated c-Abl in an ionizing irradiation signaling pathway that elicits apoptosis. In this pathway p73 is an important immediate downstream effector. Here I review the current knowledge about these nuclear processes in which c-Abl is engaged and discuss some of their possible implications on cell physiology. Cell Death and Differentiation (2000) 7, 10 - 16.

Pathophysiological mechanisms of TNF during intoxication with natural or man-made toxins

Intoxication with different natural toxins or man-made toxicants has been associated with the induction of tumor necrosis factor alpha (TNF). These include endotoxin, superantigens, Pseudomonas aeruginosa exotoxin A, bacterial DNA, T cell stimulatory agents such as agonistic anti-CD3 mAbs or concanavalin A, alpha-amanitin, paracetamol, ethanol, carbon tetrachloride, dioxin, and dimethylnitrosamine. In this paper we compile and discuss the current knowledge on the pathophysiological role of TNF during intoxication with all mentioned toxins and toxicants. A possible role of gut-derived endotoxin in several TNF-dependent toxic events has been considered. The development of pharmaceuticals that selectively interfere with the detrimental pathways induced by TNF during intoxication with bacteria, viruses, drugs, or other chemicals requires detailed knowledge of the signaling pathways originating from the two TNF receptors (TNFR1 and TNFR2). Major characteristics of these signaling pathways are described and put together.