p53 expression in K562 cells is associated with caspase-mediated cleavage of c-ABL and BCR-ABL protein kinases

CX‑5461 potentiates imatinib‑induced apoptosis in K562 cells by stimulating KIF1B expression

The selective RNA polymerase I inhibitor CX-5461 has been shown to be effective in treating some types of leukemic disorders. Emerging evidence suggests that combined treatments with CX-5461 and other chemotherapeutic agents may achieve enhanced effectiveness as compared with monotherapies. Currently, pharmacodynamic properties of the combination of CX-5461 with tyrosine kinase inhibitors remain to be explored. The present study tested whether CX-5461 could potentiate the effect of imatinib in the human chronic myeloid leukemia cell line K562, which is p53-deficient. It was demonstrated that CX-5461 at 100 nM, which was non-cytotoxic in K562 cells, potentiated the pro-apoptotic effect of imatinib. Mechanistically, the present study identified that the upregulated expression of kinesin family member 1B (KIF1B) gene might be involved in mediating the pro-apoptotic effect of imatinib/CX-5461 combination. Under the present experimental settings, however, neither CX-5461 nor imatinib alone exhibited a significant effect on KIF1B expression. Moreover, using other leukemic cell lines, it was demonstrated that regulation of KIF1B expression by imatinib/CX-5461 was not a ubiquitous phenomenon in leukemic cells and should be studied in a cell type-specific manner. In conclusion, the results suggested that the synergistic interaction between CX-5461 and imatinib may be of potential clinical value for the treatment of tyrosine kinase inhibitor-resistant chronic myeloid leukemia.

siRNA-mediated downregulation of BATF3 diminished proliferation and induced apoptosis through downregulating c-Myc expression in chronic myelogenous leukemia cells

OBJECTIVE

Despite considerable improvement in therapeutic approaches to chronic myeloid leukemia (CML) treatment, this malignancy is considered incurable due to resistance. However, investigating the molecular mechanism of CML may give rise to the development of extremely efficient targeted therapies that improve the prognosis of patients. Basic leucine zipper transcription factor ATF-like3 (BATF3), as transcription factor, is considered a key regulator of cellular activities and its function has been evaluated in tumor development and growth in several cancer types. This study aimed to evaluate the potential of the cellular impact of siRNA-mediated downregulation of BATF3 on CML cancer cells through cell proliferation, induction of apoptosis, and cell cycle distribution.

MATERIALS AND METHODS

The transfection of BATF3 siRNA to K562 CML cells was performed by electroporation device. To measure cellular viability and apoptosis, MTT assay and Annexin V/PI staining were carried out, respectively. Also, cell cycle assay and flow cytometry instrument were applied to assess cell cycle distribution of K562 cells. For more validation, mRNA expression of correlated genes was relatively evaluated by quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

The data indicated that siRNA-mediated BATF3 inactivating severely promoted the cell apoptosis. Also, the targeted therapy led to high expression of Caspase-3 gene and Bax/Bcl-2 ratio. Silenced BATF3 also induced cell cycle arrest in phase sub-G1 compared to control. Finally, a noticeable decrement was obtained in c-Myc gene expression through suppression of BATF3 in CML cells.

CONCLUSION

The findings of this research illustrated the suppression of BATF3 as an effective targeted therapy strategy for CML.

Isoalantolactone mediates the degradation of BCR-ABL protein in imatinib-resistant CML cells by down-regulating survivin

Isoalantolactone (Iso) is a bioactive lactone isolated from the root of Inula helenium L, which has been reported to have many pharmacological effects. To investigate the role and mechanism of isoalantolactone in chronic myeloid leukemia (CML), we first investigated isoalantolactone's anti-proliferative effects on imatinib-sensitive and imatinib-resistant CML cells by CCK8. Flow cytometry was used to detect isoalantolactone-induced cell apoptosis. Survivin was overexpressed in KBM5 and KBM5T315I cells using the lentivirus vector pSIN-3×flag-PURO. In KBM5 and KBM5T315I cells, shRNA was used to knockdown survivin. Cellular Thermal Shift Assay (CETSA) was used to detect the interaction between isoalantolactone and survivin. The ubiquitin of survivin induced by isoalantolactone was detected through immunoprecipitation. Quantitative polymerase-chain reaction (Q-PCR) and western blotting were used to detect the levels of mRNA and protein. Isoalantolactone inhibits the proliferation and promotes apoptosis of imatinib-resistant CML cells. Although isoalantolactone inhibits the proteins of BCR-ABL and survivin, it cannot inhibit survivin and BCR-ABL mRNA levels. Simultaneously, it was shown that isoalantolactone can degrade survivin protein by increasing ubiquitination. It was demonstrated that isoalantolactone-induced survivin mediated downregulation of BCR-ABL protein. It was also revealed that isoalantolactone triggered BCR-ABL protein degradation via caspase-3. Altogether, isoalantolactone inhibits survivin through the ubiquitin proteasome pathway, and mediates BCR-ABL downregulation in a caspase-3 dependent manner. These data suggest that isoalantolactone is a natural compound, which can be used as a potential drug to treat TKI-resistant CML.

Brefeldin A Induces Apoptosis, Inhibits BCR-ABL Activation, and Triggers BCR-ABL Degradation in Chronic Myeloid Leukemia K562 Cells

BACKGROUND

Chronic myeloid leukemia (CML) is a myeloproliferative disease caused by BCR-ABL oncoprotein. Tyrosine kinase inhibitors have been developed to inhibit the activity of BCR-ABL; however, drug resistance and side effect occur in clinic application. Therefore, it is urgent to find novel drugs for CML treatment. Under the guidance of cytotoxic activity, crude extracts of 55 fungal strains from the medicinal mangrove Acanthus ilicifolius were evaluated, and one potent cytotoxic natural compound, brefeldin A (BFA), was discovered from Penicillium sp. (HS-N-29).

OBJECTIVE

This study was aimed to determine the cytotoxic activity of BFA and the effect on the activation and expression of BCR-ABL in K562 cells.

METHOD

We evaluated cytotoxic activity by MTT assay and soft agar clone assay and apoptosis and cell cycle distribution by Muse cell analyzer. The protein level of BCR-ABL and signaling molecules were detected by western blotting, and the mRNA level of BCR-ABL was determined by RT-PCR.

RESULTS

BFA inhibited cell proliferation, induced G2/M cell cycle arrest, and stimulated cell apoptosis in K562 cells. Importantly, for the first time, we revealed that BFA inhibited the activation of BCR-ABL and consequently inhibited the activation of its downstream signaling molecules in K562 cells. Moreover, we found that BFA degraded BCR-ABL without affecting its transcription in K562 cells, and BFA-induced BCR-ABL degradation was related to caspase activation while not to autophagy or ubiquitinated proteasome degradation pathway.

CONCLUSION

Our present results indicate that BFA acts as a dual functional inhibitor and degrader of BCR-ABL, and BFA is a potential compound for chemotherapeutics to overcome CML.

A dominant-negative effect drives selection of TP53 missense mutations in myeloid malignancies

TP53, which encodes the tumor suppressor p53, is the most frequently mutated gene in human cancer. The selective pressures shaping its mutational spectrum, dominated by missense mutations, are enigmatic, and neomorphic gain-of-function (GOF) activities have been implicated. We used CRISPR-Cas9 to generate isogenic human leukemia cell lines of the most common TP53 missense mutations. Functional, DNA-binding, and transcriptional analyses revealed loss of function but no GOF effects. Comprehensive mutational scanning of p53 single-amino acid variants demonstrated that missense variants in the DNA-binding domain exert a dominant-negative effect (DNE). In mice, the DNE of p53 missense variants confers a selective advantage to hematopoietic cells on DNA damage. Analysis of clinical outcomes in patients with acute myeloid leukemia showed no evidence of GOF for TP53 missense mutations. Thus, a DNE is the primary unit of selection for TP53 missense mutations in myeloid malignancies.

Regulatory Molecules and Corresponding Processes of BCR-ABL Protein Degradation

The BCR-ABL fusion protein with strong tyrosine kinase activity is one of the molecular biological bases of leukemia. Imatinib (Gleevec), a specific targeted drug for the treatment of chronic myeloid leukemia (CML), was developed for inhibiting the kinase activity of the BCR-ABL fusion protein. Despite the positive clinical efficacy of imatinib, the proportion of imatinib resistance has gradually increased. The main reason for the resistance is a decrease in sensitivity to imatinib caused by mutation or amplification of the BCR-ABL gene. In response to this phenomenon, the new generation of tyrosine kinase inhibitors (TKIs) targeting the BCR-ABL fusion protein was developed to solve the problem. However this strategy only selectively inhibits the tyrosine kinase activity of the BCR-ABL protein without eliminating the BCR-ABL protein, it does not fundamentally cure the BCR-ABL-positive leukemia patients. With the accumulation of the knowledge of cellular molecular biology, it has become possible to specifically eliminate certain proteins by cellular proteases in a specific way. Therefore, the therapeutic strategy to induce the degradation of the BCR-ABL fusion protein is superior to the strategy of inhibiting its activity. The protein degradation strategy is also a solution to the TKI resistance caused by different BCR-ABL gene point mutations. In order to provide possible exploration directions and clues for eliminating the BCR-ABL fusion protein in tumor cells, we summarize the significant molecules involved in the degradation pathway of the BCR-ABL protein, as well as the reported potent compounds that can target the BCR-ABL protein for degradation.

Synthesis, characterization, and anticancer evaluation of some new N1-(anthraquinon-2-yl) amidrazone derivatives

A new series of novel N1-anthraquinon-2-yl amidrazones incorporating N-piperazines and related congeners were synthesized via reaction of the hydrazonoyl chloride derived from 2-qaminoanthraquinone with the appropriate piperazine (secondary amine). Structures of the new compounds were confirmed by a panel of spectroscopic methods including IR, NMR, and MS and by elemental analysis. The antitumor activity of the newly prepared compounds was evaluated in vitro against MCF-7 breast cancer, K562 chronic myelogenous leukemia, and dermal fibroblasts cell lines by means of a cell viability assay using the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Results revealed that compounds 13a and 13d exhibit the highest inhibitory activity against K562 and MCF-7 cell lines. These two compounds could be considered as promising as potential anticancer drugs.

Different BCR/Abl protein suppression patterns as a converging trait of chronic myeloid leukemia cell adaptation to energy restriction

BCR/Abl protein drives the onset and progression of Chronic Myeloid Leukemia (CML). We previously showed that BCR/Abl protein is suppressed in low oxygen, where viable cells retain stem cell potential. This study addressed the regulation of BCR/Abl protein expression under oxygen or glucose shortage, characteristic of the in vivo environment where cells resistant to tyrosine kinase inhibitors (TKi) persist. We investigated, at transcriptional, translational and post-translational level, the mechanisms involved in BCR/Abl suppression in K562 and KCL22 CML cells. BCR/abl mRNA steady-state analysis and ChIP-qPCR on BCR promoter revealed that BCR/abl transcriptional activity is reduced in K562 cells under oxygen shortage. The SUnSET assay showed an overall reduction of protein synthesis under oxygen/glucose shortage in both cell lines. However, only low oxygen decreased polysome-associated BCR/abl mRNA significantly in KCL22 cells, suggesting a decreased BCR/Abl translation. The proteasome inhibitor MG132 or the pan-caspase inhibitor z-VAD-fmk extended BCR/Abl expression under oxygen/glucose shortage in K562 cells. Glucose shortage induced autophagy-dependent BCR/Abl protein degradation in KCL22 cells. Overall, our results showed that energy restriction induces different cell-specific BCR/Abl protein suppression patterns, which represent a converging route to TKi-resistance of CML cells. Thus, the interference with BCR/Abl expression in environment-adapted CML cells may become a useful implement to current therapy.

Regulation of Endoribonuclease Activity of Alpha-Type Proteasome Subunits in Proerythroleukemia K562 Upon Hemin-Induced Differentiation

The proteasome is the main intracellular proteolytic machine involved in the regulation of numerous cellular processes, including gene expression. In addition to their proteolytic activity, proteasomes also exhibit ATPase/helicase (the 19S particle) and RNAse (the 20S particle) activities, which are regulated by post-translational modifications. In this report we uncovered that several 20S particle subunits: α1 (PSMA6), α2 (PSMA2), α4 (PSMA7), α5 (PSMA5), α6 (PSMA1) and α7 (PSMA3) possess RNAse activity against the p53 mRNA in vitro. Furthermore, we found that the RNAse activity of PSMA1 and PSMA3 was regulated upon hemin-induced differentiation of K562 proerythroleukemia cells. The decrease in RNAse activity of PSMA1 and PSMA3 was paralleled by changes in their status of phosphorylation and ubiquitylation. Collectively, our data support the notion that proteasomal RNAse activity may be functionally important and provide insights into the potential mechanism of p53 repression in erythroleukemia cells by RNAse activity of the 20S α-type subunits.

Anti-rheumatic agent auranofin induced apoptosis in chronic myeloid leukemia cells resistant to imatinib through both Bcr/Abl-dependent and -independent mechanisms

Resistance to Imatinib mesylate (IM) is an emerging problem for patients with chronic myelogenous leukemia (CML). T315I mutation in the Bcr-Abl is the predominant mechanism of the acquired resistance to IM and second generation tyrosine kinase inhibitors (TKI). Therefore it is urgent to search for new measures to overcome TKI-resistance. Auranofin (AF), clinically used to treat rheumatic arthritis, was recently approved by US Food and Drug Administration for Phase II clinical trial to treat cancer. In contrast to the reports that AF induces apoptosis by increasing intracellular reactive oxygen species (ROS) levels via inhibiting thioredoxin reductase, our recent study revealed that AF-induced apoptosis depends on inhibition of proteasomal deubiquitinases (UCHL5 and USP14). Here we report that (i) AF induces apoptosis in both Bcr-Abl wild-type cells and Bcr-Abl-T315I mutation cells and inhibits the growth of IM-resistant Bcr-Abl-T315I xenografts in vivo; (ii) AF inhibits Bcr-Abl through both downregulation of Bcr-Abl gene expression and Bcr-Abl cleavage mediated by proteasome inhibition-induced caspase activation; (iii) proteasome inhibition but not ROS is required for AF-induced caspase activation and apoptosis. These findings support that AF overcomes IM resistance through both Bcr/Abl-dependent and -independent mechanisms, providing great clinical significance for cancer treatment.

Gambogic acid induces apoptosis in imatinib-resistant chronic myeloid leukemia cells via inducing proteasome inhibition and caspase-dependent Bcr-Abl downregulation

PURPOSE

Chronic myelogenous leukemia (CML) is characterized by the constitutive activation of Bcr-Abl tyrosine kinase. Bcr-Abl-T315I is the predominant mutation that causes resistance to imatinib, cytotoxic drugs, and the second-generation tyrosine kinase inhibitors. The emergence of imatinib resistance in patients with CML leads to searching for novel approaches to the treatment of CML. Gambogic acid, a small molecule derived from Chinese herb gamboges, has been approved for phase II clinical trial for cancer therapy by the Chinese Food and Drug Administration (FDA). In this study, we investigated the effect of gambogic acid on cell survival or apoptosis in CML cells bearing Bcr-Abl-T315I or wild-type Bcr-Abl.

EXPERIMENTAL DESIGN

CML cell lines (KBM5, KBM5-T315I, and K562), primary cells from patients with CML with clinical resistance to imatinib, and normal monocytes from healthy volunteers were treated with gambogic acid, imatinib, or their combination, followed by measuring the effects on cell growth, apoptosis, and signal pathways. The in vivo antitumor activity of gambogic acid and its combination with imatinib was also assessed with nude xenografts.

RESULTS

Gambogic acid induced apoptosis and cell proliferation inhibition in CML cells and inhibited the growth of imatinib-resistant Bcr-Abl-T315I xenografts in nude mice. Our data suggest that GA-induced proteasome inhibition is required for caspase activation in both imatinib-resistant and -sensitive CML cells, and caspase activation is required for gambogic acid-induced Bcr-Abl downregulation and apoptotic cell death.

CONCLUSIONS

These findings suggest an alternative strategy to overcome imatinib resistance by enhancing Bcr-Abl downregulation with the medicinal compound gambogic acid, which may have great clinical significance in imatinib-resistant cancer therapy.

Effector caspases and leukemia

Caspases, a family of aspartate-specific cysteine proteases, play a major role in apoptosis and a variety of physiological and pathological processes. Fourteen mammalian caspases have been identified and can be divided into two groups: inflammatory caspases and apoptotic caspases. Based on the structure and function, the apoptotic caspases are further grouped into initiator/apical caspases (caspase-2, -8, -9, and -10) and effector/executioner caspases (caspase-3, -6, and -7). In this paper, we discuss what we have learned about the role of individual effector caspase in mediating both apoptotic and nonapoptotic events, with special emphasis on leukemia-specific oncoproteins in relation to effector caspases.

Synergy between proteasome inhibitors and imatinib mesylate in chronic myeloid leukemia

Background

Resistance developed by leukemic cells, unsatisfactory efficacy on patients with chronic myeloid leukemia (CML) at accelerated and blastic phases, and potential cardiotoxity, have been limitations for imatinib mesylate (IM) in treating CML. Whether low dose IM in combination with agents of distinct but related mechanisms could be one of the strategies to overcome these concerns warrants careful investigation.

Methods and Findings

We tested the therapeutic efficacies as well as adverse effects of low dose IM in combination with proteasome inhibitor, Bortezomib (BOR) or proteasome inhibitor I (PSI), in two CML murine models, and investigated possible mechanisms of action on CML cells. Our results demonstrated that low dose IM in combination with BOR exerted satisfactory efficacy in prolongation of life span and inhibition of tumor growth in mice, and did not cause cardiotoxicity or body weight loss. Consistently, BOR and PSI enhanced IM-induced inhibition of long-term clonogenic activity and short-term cell growth of CML stem/progenitor cells, and potentiated IM-caused inhibition of proliferation and induction of apoptosis of BCR-ABL+ cells. IM/BOR and IM/PSI inhibited Bcl-2, increased cytoplasmic cytochrome C, and activated caspases. While exerting suppressive effects on BCR-ABL, E2F1, and β-catenin, IM/BOR and IM/PSI inhibited proteasomal degradation of protein phosphatase 2A (PP2A), leading to a re-activation of this important negative regulator of BCR-ABL. In addition, both combination therapties inhibited Bruton's tyrosine kinase via suppression of NFκB.

Conclusion

These data suggest that combined use of tyrosine kinase inhibitor and proteasome inhibitor might be helpful for optimizing CML treatment.

Effective killing of Gleevec-resistant CML cells with T315I mutation by a natural compound PEITC through redox-mediated mechanism

Mutation of Bcr-Abl is an important mechanism by which chronic myelogenous leukemia (CML) cells become resistant to Gleevec. The T315I mutation is clinically significant since CML cells harboring this mutation are insensitive to Gleevec and other Bcr-Abl-targeted drugs. Identification of new agents capable of effectively killing CML cells with T315I mutation would have important therapeutic implications in Gleevec-resistant CML. Here, we showed that beta-phenylethyl isothiocyanate (PEITC), a natural compound found in vegetables, is effective in killing CML cells expressing T315I BCR-ABL. Treatment of leukemia cell lines harboring wild-type or mutant Bcr-Abl with 10 microM PEITC resulted in an elevated ROS stress and a redox-mediated degradation of the BCR-ABL protein, leading to massive death of the leukemia cells. Antioxidant NAC attenuated the PEITC-induced oxidative stress in CML cells and prevented the degradation of BCR-ABL, caspase-3 activation and cell death. We further showed that the ROS-induced degradation of BCR-ABL was mediated partially by caspase-3 and the proteasome pathway. The ability of PEITC to effectively kill T315I-positive CML cells was further confirmed using primary leukemia cells isolated from CML patients. Our results suggest that PEITC is a promising compound capable of killing Gleevec-resistant CML cells through a ROS-mediated mechanism and warrants further investigations.

ERK1/2 inactivation and p38 MAPK-dependent caspase activation during guanosine 5'-triphosphate-mediated terminal erythroid differentiation of K562 cells

Since differentiation therapy is one of the promising strategies for treatment of leukemia, universal efforts have been focused on finding new differentiating agents. In that respect, it was recently shown that guanosine 5'-triphosphate (GTP) induced the differentiation of K562 cells, suggesting its possible efficiency in treatment of chronic myelogenous leukemia (CML). However, further investigations are required to verify this possibility. Here, the effects of GTP on activation of mitogen-activated protein kinases (MAPKs) and caspases in K562 cells were examined. Exposure of K562 cells to 100muM GTP markedly inhibited growth (4-70%) and increased percent glycophorin A positive cells after 1-6 days. GTP-induced terminal erythroid differentiation of K562 cells was accompanied with activation of three key caspases, i.e., caspase-3, -6 and -9. More detailed studies revealed that mitochondrial pathway is activated along with down-regulation of Bcl-xL and releasing of cytochrome c into cytosol. Among MAPKs, ERK1/2and p38 were modulated after GTP treatment. Western blot analyses showed that sustained phosphorylation of p38 MAPK was accompanied by a decrease in ERK1/2 activation. These modulatory effects of GTP were observed at early exposure times before the onset of differentiation (3h), and followed for 24-96h. Interestingly, inhibition of p38 MAPK pathway by SB202190 impeded GTP-mediated caspases activation and differentiation in K562 cells, suggesting that p38 MAPK may act upstream of caspases in our system. These results point to a pivotal role for p38 MAPK pathway during GTP-mediated erythroid differentiation of K562 cells and will hopefully have important impact on pharmaceutical evaluation of GTP for CML treatment in differentiation therapy approaches.

Detection in primary chronic myeloid leukaemia cells of p210BCR-ABL1 in complexes with adaptor proteins CBL, CRKL, and GRB2

Chronic myeloid leukemia (CML) arises as a consequence of the expression of a chimeric fusion protein, p210BCR-ABL1, which is localized to the cytoplasm and has constitutive protein tyrosine kinase activity. Extensive publications report that p210BCR-ABL1 complexed with multiple cytoplasmic proteins can modulate several cell signaling pathways. However, while altered signaling states can be demonstrated in primary CML material, most of the reported analytical work on complexed proteins has been done in cell lines expressing p210BCR-ABL1. This has been necessary because primary hemopoietic cell lysates contain a degradative activity which rapidly and permanently destroys p210BCR-ABL1, precluding accurate p210BCR-ABL1 quantification by Western blotting or investigation of coimmunoprecipitating proteins in primary cells. This degradative activity has proven intractable to inhibition by conventional protease inhibitors. We show here that the degradative activity in primary cells is associated with cell lysosomes and is most likely to be an acid-dependent hydrolase. By lysing primary hemopoietic cells at high pH, we have demonstrated substantial inhibition of the p210BCR-ABL1-degradative activity and now report, to the best of our knowledge, the first published demonstration by coimmunoprecipitation of the association between p210BCR-ABL1 and cytoplasmic effector proteins in primary CML material.

TRAIL-induced cleavage and inactivation of SPAK sensitizes cells to apoptosis

Ste20-related proline-alanine-rich kinase (SPAK) has been linked to various cellular processes, including proliferation, differentiation, and ion transport regulation. Recently, we showed that SPAK mediates signaling by the TNF receptor, RELT. The presence of a caspase cleavage site in SPAK prompted us to study its involvement in apoptotic signaling induced by another TNF member, TRAIL. We show that TRAIL stimulated caspase 3-like proteases that cleaved SPAK at two distinct sites. Cleavage had little effect on the activity of SPAK but removed its substrate-binding domain. In addition, TRAIL reduced the activity of SPAK in HeLa cells in a caspase-independent manner. Thus, TRAIL inhibited SPAK by two mechanisms: activation of caspases, which removed its substrate-binding domain, and caspase-independent down-regulation of SPAK activity. Furthermore, reducing the amount of SPAK by siRNA increased the sensitivity of HeLa cells to TRAIL-induced apoptosis. Thus, TRAIL down-regulation of SPAK is an important event that enhances its apoptotic effects.

Tyrosine kinase inhibitor STI571 (Imatinib) cooperates with wild-type p53 on K562 cell line to enhance its proapoptotic effects

In order to ascertain whether p53 has a role in chronic myeloid leukemia hematopoietic progenitor response to the innovative tyrosine kinase inhibitor STI571 (Imatinib), we overexpressed a wild type (wt) p53 construct in the K562 cell line, generated from a human blast crisis and lacking endogenous p53. Wt p53 overexpression was associated with a significant reduction of bcr-abl expression levels resulting, at least in part, from post-transcriptional events affecting the stability of p210 bcr-abl fusion protein. Moreover, we demonstrated that p53 overexpression enhances the commitment to the apoptotic death fate of K562 following its in vitro exposure to 1 microM STI571. Multiple mechanisms are involved in p53 impact on K562 survival: Most importantly, we found that a greater reduction of bcr-abl transcription by STI571 was associated with the overexpression of wt p53. Further studies are required to elucidate the mechanisms involved in the transcriptional repression of bcr-abl by STI571 and p53 and in their synergic effects on the clonal hematopoiesis of chronic myeloid leukemia.

Synergistic apoptosis induction by proteasome and histone deacetylase inhibitors is dependent on protein synthesis

Proteasome inhibitors are able to efficiently induce apoptosis in many tumor cells while leaving quiescent, untransformed cells largely unharmed. Here we investigated the further enhancement of proteasome inhibitor-mediated apoptosis induction in Bcr-Abl positive K562 CML cells by simultaneous treatment with different histone deacetylase inhibitors (HDIs). Combining proteasome and HDIs resulted in rapid hyperacetylation of histone H3 and accumulation of polyubiquitinated proteins and the synergistic induction of apoptosis. Apoptosis induction was associated with caspase 8, 3 and 9 activation, Bid processing, destruction of the mitochondrial membrane potential, cleavage of PARP and lamin B and extensive DNA fragmentation. The pan-caspase inhibitor Z-VAD-FMK and the caspase-8 inhibitor Z-IETD-FMK could inhibit K562 cell apoptosis. Apoptosis was also delayed by overexpression of Bcl-xL, as well as by crmA, a known inhibitor of caspases 1 and 8. Caspase 8 activity could still be detected in the presence of ectopic Bcl-xL, but not in crmA transfected cells. The most striking anti-apoptotic effect though was obtained by the translational inhibitor cycloheximide, which abolished caspase 8 processing, blocked Bid cleavage and maintained the mitochondrial transmembrane potential. Apoptosis by the combination treatment occurred independently from CD95/Fas receptor stimulation. These results demonstrated that transcriptional activation by HDIs combined with proteasome inhibitor mediated posttranslational stabilization of protein(s) results in significantly enhanced CML apoptosis which was striktly dependent on uninterrupted protein synthesis.

ZnPcS2P2-based photodynamic therapy induces mitochondria-dependent apoptosis in K562 cells

Mitochondria play a key role in the regulation of apoptosis induced by numerous antitumor chemotherapeutic and other toxic agents. Photodynamic therapy (PDT) exerts significant cellular killing efficacy through either an apoptotic or necrotic cell death pathway. This study investigated the mechanism underlying the killing effects of a novel amphipathic photosensitizer [di-sulfonated di-phthalimidomethyl phthalocyanine zinc (ZnPcS2P2)]-mediated photodynamic therapy (ZnPcS2P2-PDT) on K562 cells. Apoptosis was evident in the post-PDT cells through the TdT-mediated dUTP nick end labeling (TUNEL) method and DNA fragmentation assay. After ZnPcS2P2-PDT, K562 cells underwent mitochondria-dependent apoptosis as evidenced by the release of cytochrome c from mitochondria into cytosol, accompanied by mitochondrial membrane potential (deltapsim) reduction, indicating the opening of the mitochondrial permeability transition pore (PTP). The activities of protease from the caspase family and caspase-3 were also significantly elevated. Furthermore, ZnPcS2P2-PDT down-regulated the expression of chimaeric Bcr-Abl oncoprotein, which is the molecular hallmark of chronic myelogenous leukemia (CML).

Role of p53 and ATM in photodynamic therapy-induced apoptosis

BACKGROUND AND OBJECTIVES

Photodynamic therapy (PDT) induces cell death through a laser light-activated photosensitizer and is a treatment option for tumors resistant to radio- and chemo-therapy.

STUDY DESIGN/MATERIALS AND METHODS

We investigated whether m-THPC-PDT induces cell death by necrosis and/or apoptosis, and whether these responses are modulated by p53 and/or ATM, two cancer-associated genes. Sensitivity of atm(+/+)p53(+/+), atm(+/+)p53(-/-), and atm(-/-)p53(-/-) mouse embryonic fibroblasts to m-THPC-PDT performed at a wavelength of 652 nm was determined by the MTT assay, trypan blue-exclusion, and the TUNEL and caspase3-cleavage apoptosis assays. c-Abl protein level was determined by immunoblotting.

RESULTS

m-THPC-PDT rapidly induced cell death in a substantial fraction of cells by p53- and Ataxia telangiectasia mutated (ATM)-independent non-apoptotic processes. However, in the subset of apoptotic cells, apoptosis was reduced by loss of p53 and was even more reduced by the additional loss of ATM. Apoptosis correlated inversely with c-Abl level.

CONCLUSIONS

p53 and ATM are not required for necrosis, but may be required for PDT-mediated apoptosis.

p53-induced inhibition of protein synthesis is independent of apoptosis

Activation of a temperature-sensitive form of p53 in murine erythroleukaemia cells results in a rapid impairment of protein synthesis that precedes inhibition of cell proliferation and loss of cell viability by several hours. The inhibition of translation is associated with specific cleavages of polypeptide chain initiation factors eIF4GI and eIF4B, a phenomenon previously observed in cells induced to undergo apoptosis in response to other stimuli. Although caspase activity is enhanced in the cells in which p53 is activated, both the effects on translation and the cleavages of the initiation factors are completely resistant to inhibition of caspase activity. Moreover, exposure of the cells to a combination of the caspase inhibitor z-VAD.FMK and the survival factor erythropoietin prevents p53-induced cell death but does not reverse the inhibition of protein synthesis. We conclude that the p53-regulated cleavages of eIF4GI and eIF4B, as well as the overall inhibition of protein synthesis, are caspase-independent events that can be dissociated from the induction of apoptosis per se.

Inducible p27(Kip1) expression inhibits proliferation of K562 cells and protects against apoptosis induction by proteasome inhibitors

Overexpression of the cyclin-dependent kinase inhibitor p27(Kip1) has been demonstrated to induce cell cycle arrest and apoptosis in various cancer cell lines, but has also been associated with the opposite effect of enhanced survival of tumor cells and increased resistance towards chemotherapeutic treatment. To address the question of how p27(Kip1) expression is related to apoptosis induction, we studied doxycycline-regulated p27(Kip1) expression in K562 erythroleukemia cells. p27(Kip1) expression effectively retards proliferation, but it is not sufficient to induce apoptosis in K562 cells. p27(Kip1)-expressing K562 cells, however, become resistant to apoptosis induction by the proteasome inhibitors PSI, MG132 and epoxomicin, in contrast to wild-type K562 cells that are efficiently killed. Cell cycle arrest in the S phase by aphidicolin, which is not associated with an accumulation of p27(Kip1) protein, did not protect K562 cells against the cytotoxic effect of the proteasome inhibitor PSI. The expression levels of p27(Kip1) thus constitute an important parameter, which decides on the overall sensitivity of cells against the cytotoxic effect of proteasome inhibitors.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Species-specific differences in the usage of several caspase substrates

The activation of caspases cleaving a plethora of specific substrates is pivotal for initiation as well as execution of apoptosis. The recognition motif for caspases is a tetrapeptide sequence containing an essential aspartic acid residue at the fourth position (often DXXD). Here, we report that the caspase cleavage sites of most identified substrates show a high degree of conservation between different species. However, we have identified differences in the cleavage sites of five substrates between murine and human proteins leading to either select processing in only one species or to different cleavage patterns. Finally, we provide evidence that murine c-Abl but not its human homolog serves as efficient substrate during apoptosis.