Effects of combined traffic noise on the synaptic ultrastructure and expressions of p-CaMKII and NMDAR1 in the hippocampus of young SD rats.

In order to explore the effects of combined traffic noise (CTN) on learning and memory function, young Sprague-Dawley (SD) rats were exposed to CTN from highway and high-speed railway for 52 days, whose day-night equivalent continuous A-weighted sound pressure level (Ldn) was 70 dB(A) (corresponding sound pressure level was 80 dB). The synaptic ultrastructure and the expressions of phosphorylated calcium/calmodulin-dependent protein kinase II (p-CaMKII) and N-methyl-D-aspartate receptor 1 (NMDAR1 or NR1) in the hippocampus were tested by transmission electron microscopy (TEM) and Western blot, respectively. Results showed that there was no significant difference in the synaptic ultrastructure and the expressions of p-CaMKII and NR1 in the hippocampus of young rats between the experimental group and control group. Compared with single high-speed railway noise (HSRN) with Ldn of 70 dB(A), CTN had less influences on learning and memory function, which was closely related to smaller intermittency of CTN and less anxiety caused by CTN. In comparison with white noise with a sound pressure level of 80 dB, CTN had less impacts on learning and memory function, which was mainly associated with CTN's smaller R-weighted sound pressure level based on rats' auditory sensitivity.

A comparative study on influences of static electric field and power frequency electric field on cognition in mice.

Recently, electromagnetic fields around ultra-high voltage transmission lines have received considerable attentions for their potential biological effects. This study aimed to investigate the effects of static electric field (SEF) and power frequency electric field (PFEF) on cognition. Mice were exposed to SEF and PFEF with the same strength (35 kV/m) for 49 days, respectively. Behaviors in Morris water maze test and amino acid neurotransmitter levels in hippocampus were examined during exposure. Results indicated that the exposure of 35 kV/m SEF would not cause significant influences on learning and memory ability in mice, while the exposure of 35 kV/m PFEF would cause significant positive effects on learning and memory ability in mice on day 33. This difference in effects from SEF and PFEF on cognition was possibly induced by the difference in the degree of molecular polarization and ion migration in organisms under exposure of two kinds of electric fields with different frequency.

Sensitization of multidrug-resistant cancer cells to Hsp90 inhibitors by NSAIDs-induced apoptotic and autophagic cell death.

NSAIDs (non-steroidal anti-inflammatory drugs) have potential use as anticancer agents, either alone or in combination with other cancer therapies. We found that NSAIDs including celecoxib (CCB) and ibuprofen (IBU) significantly potentiated the cytotoxicity of Hsp90 inhibitors in human multidrug-resistant (MDR) cells expressing high levels of mutant p53 (mutp53) protein and P-glycoprotein (P-gp), and reversed Hsp90 inhibitor resistance caused by activation of heat shock factor 1 (HSF1) and by up-regulation of heat shock proteins (Hsps) and P-gp. Inhibition of Akt/mTOR and STAT3 pathways by CCB induced autophagy, which promoted the degradation of mutp53, one of Hsp90 client proteins, and subsequently down-regulated HSF1/Hsps and P-gp. Inhibition of autophagy prevented mutp53 degradation and CCB-induced apoptosis, and inhibition of caspase-3-mediated apoptotic pathway by Z-DEVD-FMK did not completely block CCB-induced cell death in MDR cells, suggesting that autophagic and apoptotic cell death may contribute to CCB-induced cytotoxicity in MDR cells. Furthermore, CCB and IBU suppressed Hsp90 inhibitor-induced HSF1/Hsp70/P-gp activity and mutp53 expression in MDR cells. Our results suggest that NSAIDs can be used as potential Hsp90 inhibitor chemosensitizers and reverse resistance of MDR cells to Hsp90 inhibitors via induction of apoptosis and autophagy. These results might enable the use of lower, less toxic doses of Hsp90 inhibitors and facilitate the design of practically applicable, novel combination therapy for the treatment of MDR cancer.

Potential Role of CD133 Expression in the Susceptibility of Human Liver Cancer Stem-Like Cells to TRAIL.

Hepatocellular carcinoma (HCC) is one of the most common malignancies, with a poor prognosis and high recurrence rate. In the present study, we identified CD133, one of the markers of cancer stem cells, as a novel molecular target of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In four human HCC cell lines established from primary HCC tumors, we found that CD133-high human liver cancer stem-like cells (CD133hi) derived from the SNU-475 cell line were highly susceptible to TRAIL compared to other HCC cell lines with a small population of CD133. CD133hi SNU-475 cells showed upregulation of TRAIL receptor DR5 and stemness-related genes such as c-Myc and ABC transporters compared to their CD133-low (CD133lo) cells. Hypersensitivity of CD133hi cells to TRAIL was associated with c-Myc-mediated upregulation of DR5 and downregulation of c-FLIPL in the cells. Knockdown of CD133 expression in CD133hi cells resulted in the downregulation of c-Myc, and depletion of c-Myc caused a decrease in the cell surface expression of DR5 and an increase in the expression of c-FLIPL and, consequently, attenuated TRAIL-induced cytotoxicity and apoptosis of CD133hi cells. These results suggest that TRAIL may provide a new strategy for CD133hi CSCs of HCC-targeted therapies and, potentially, for therapies of other CD133-expressing types of cancer.

Sensitization of multidrug-resistant human cancer cells to Hsp90 inhibitors by down-regulation of SIRT1.

The effectiveness of Hsp90 inhibitors as anticancer agents was limited in multidrug-resistant (MDR) human cancer cells due to induction of heat shock proteins (Hsps) such as Hsp70/Hsp27 and P-glycoprotein (P-gp)-mediated efflux. In the present study, we showed that resistance to Hsp90 inhibitors of MDR human cancer cells could be overcome with SIRT1 inhibition. SIRT1 knock-down or SIRT1 inhibitors (amurensin G and EX527) effectively suppressed the resistance to Hsp90 inhibitors (17-AAG and AUY922) in several MDR variants of human lymphoblastic leukemia and human breast cancer cell lines. SIRT1 inhibition down-regulated the expression of heat shock factor 1 (HSF1) and subsequently Hsps and facilitated Hsp90 multichaperone complex disruption via hyperacetylation of Hsp90/Hsp70. These findings were followed by acceleration of ubiquitin ligase CHIP-mediated mutant p53 (mut p53) degradation and subsequent down-regulation of P-gp in 17-AAG-treated MDR cancer cells expressing P-gp and mut p53 after inhibition of SIRT1. Therefore, combined treatment with Hsp90 inhibitor and SIRT1 inhibitor could be a more effective therapeutic approach for Hsp90 inhibitor-resistant MDR cells via down-regulation of HSF1/Hsps, mut p53 and P-gp.

Sensitization of chemo-resistant human chronic myeloid leukemia stem-like cells to Hsp90 inhibitor by SIRT1 inhibition.

Development of effective therapeutic strategies to eliminate cancer stem-like cells (CSCs), which play a major role in drug resistance and disease recurrence, is critical to improve cancer treatment outcomes. The current investigation was undertaken to examine the effectiveness of the combination treatment of Hsp90 inhibitor and SIRT1 inhibitor in inhibiting the growth of chemo-resistant stem-like cells isolated from human chronic myeloid leukemia K562 cells. Inhibition of SIRT1 by use of SIRT1 siRNA or SIRT1 inhibitors (amurensin G and EX527) effectively potentiated sensitivity of Hsp90 inhibitors (17-AAG and AUY922) in CD44(high) K562 stem-like cells expressing high levels of CSC-related molecules including Oct4, CD34, ß-catenin, c-Myc, mutant p53 (mut p53), BCRP and P-glycoprotein (P-gp) as well as CD44. SIRT1 depletion caused significant down-regulation of heat shock factor 1 (HSF1)/heat shock proteins (Hsps) as well as these CSC-related molecules, which led to the sensitization of CD44(high) K562 cells to Hsp90 inhibitor by SIRT1 inhibitor. Moreover, 17-AAG-mediated activation of HSF1/Hsps and P-gp-mediated efflux, major causes of Hsp90 inhibitor resistance, was suppressed by SIRT1 inhibitor in K562-CD44(high) cells. Our data suggest that combined treatment with Hsp90 inhibitor and SIRT1 inhibitor could be an effective therapeutic approach to target CSCs that are resistant to current therapies.

Ku70 acetylation and modulation of c-Myc/ATF4/CHOP signaling axis by SIRT1 inhibition lead to sensitization of HepG2 cells to TRAIL through induction of DR5 and down-regulation of c-FLIP.

In this study, we investigated the role of c-Myc/ATF4/CHOP signaling pathway in sensitization of human hepatoma HepG2 cells to TRAIL. Knockdown of SIRT1 or treatment with SIRT1 inhibitor caused the up-regulation of DR5 and down-regulation of c-FLIP through modulation of c-Myc/ATF4/CHOP pathway, and subsequently enhanced the cytotoxic and apoptotic effects of TRAIL on HepG2 cells. Interestingly, SIRT1 interacted directly with c-FLIP(L) and Ku70, and treatment with SIRT1 inhibitor enhanced acetylation of Ku70 and subsequently decreased its binding to c-FLIP. And this was followed by degradation of c-FLIP. Moreover, Ku70(-/-) MEF and Ku70-knockdown HepG2 cells showed the increased levels of c-Myc, ATF4, CHOP, and DR5 and decreased level of c-FLIP. These results were followed by increased sensitivity of Ku70(-/-) MEF cells and Ku70-knockdown HepG2 cells to TRAIL compared with their control cells. These findings reveal for the first time that SIRT1 inhibition increases Ku70 acetylation, and the acetylated Ku70 with a decreased function mediates the induction of DR5 and the down-regulation of c-FLIP by up-regulating c-Myc/ATF4/CHOP pathway, and consequently promotes the TRAIL-induced apoptosis of HepG2 cells. This study provides important mechanistic insight of the synergism exhibited by SIRT1 inhibition and TRAIL.

Amurensin G, a novel SIRT1 inhibitor, sensitizes TRAIL-resistant human leukemic K562 cells to TRAIL-induced apoptosis.

Many types of cancer cells remain resistant towards TRAIL-induced cytotoxicity by the blockade of apoptotic signaling cascades. Thus, sensitizers are needed to enhance the effect of TRAIL-based cancer therapies. Although synergistic tumor cell death has been reported when various HDAC inhibitors were administered with TRAIL in a variety of human cancers, the effect of inhibitors of Class III HDAC such as SIRT1 have not been reported. We reported here for the first time that inhibition of SIRT1 augmented the cytotoxic and apoptotic effects of TRAIL on human leukemic K562 cells. Knockdown of SIRT1 or treatment with amurensin G, a potent new SIRT1 inhibitor, up-regulated the levels of DR5 and c-Myc and down-regulated the level of c-FLIP(L/S). Furthermore, knockdown of SIRT1 or treatment with amurensin G augmented the molecular responses to TRAIL, including activation of caspase-8, -9 and -3, PARP cleavage, up-regulation of Bax, and down-regulation of Bcl-2. Amurensin G-enhanced TRAIL-induced apoptosis was abrogated by caspase inhibitor Z-VAD-FMK. These findings suggest that the suppression of SIRT1 with siRNA or amurensin G sensitize the TRAIL-resistant K562 cell to TRAIL-induced apoptosis, possibly by the up-regulation of c-Myc and DR5 surface expression and the down-regulations of c-FLIP and Mcl-1. In addition, amurensin G, a potent new SIRT1 inhibitor, would be used as a sensitizer of TRAIL in TRAIL-resistant leukemic cells.

Involvement of SIRT1 in hypoxic down-regulation of c-Myc and ß-catenin and hypoxic preconditioning effect of polyphenols.

SIRT1 has been found to function as a Class III deacetylase that affects the acetylation status of histones and other important cellular nonhistone proteins involved in various cellular pathways including stress responses and apoptosis. In this study, we investigated the role of SIRT1 signaling in the hypoxic down-regulations of c-Myc and ß-catenin and hypoxic preconditioning effect of the red wine polyphenols such as piceatannol, myricetin, quercetin and resveratrol. We found that the expression of SIRT1 was significantly increased in hypoxia-exposed or hypoxic preconditioned HepG2 cells, which was closely associated with the up-regulation of HIF-1α and down-regulation of c-Myc and ß-catenin expression via deacetylation of these proteins. In addition, blockade of SIRT1 activation using siRNA or amurensin G, a new potent SIRT1 inhibitor, abolished hypoxia-induced HIF-1α expression but increased c-Myc and ß-catenin expression. SIRT1 was also found to stabilize HIF-1α protein and destabilize c-Myc, ß-catenin and PHD2 under hypoxia. We also found that myricetin, quercetin, piceatannol and resveratrol up-regulated HIF-1α and down-regulated c-Myc, PHD2 and ß-catenin expressions via SIRT1 activation, in a manner that mimics hypoxic preconditioning. This study provides new insights of the molecular mechanisms of hypoxic preconditioning and suggests that polyphenolic SIRT1 activators could be used to mimic hypoxic/ischemic preconditioning.

Suppression of multidrug resistance by treatment with TRAIL in human ovarian and breast cancer cells with high level of c-Myc.

In this study, we investigated the role of c-Myc in overcoming multidrug resistance (MDR) in human ovarian and breast cancer cells by TRAIL. We showed that P-gp expressing MDR variants (Hey A8-MDR and MCF7-MDR cells) with high level of c-Myc were highly susceptible to TRAIL treatment when compared to their drug-sensitive parental human ovarian cancer Hey A8 and breast MCF-7 cells, respectively. Up-regulation of DR5 TRAIL receptor and down-regulation of c-FLIP and the promotion of caspase-dependent cell death, which contribute to TRAIL sensitization of MDR cells, were regulated by the over-expressed c-Myc in the MDR cells. After targeted inhibition of c-Myc with specific siRNA, these responses to TRAIL disappeared and TRAIL-induced apoptosis was also suppressed in MCF7-MDR cells. Treatment with TRAIL significantly reduced P-glycoprotein (P-gp)-mediated efflux of rhodamine123 in both Hey A8-MDR and MCF7-MDR cells. Furthermore, TRAIL significantly potentiated the cytotoxicity of vinblastine, vincristine, doxorubicin and VP-16 that are P-gp substrate anticancer drugs in both MDR cells, which resulted in the reversal effect of TRAIL on the MDR phenotype. The present study shows for the first time that elevated c-Myc expression in the MDR cells plays a critical role in overcoming MDR by TRAIL that can act as a specific sensitizer for P-gp substrate anticancer drug.

High susceptibility of metastatic cells derived from human prostate and colon cancer cells to TRAIL and sensitization of TRAIL-insensitive primary cells to TRAIL by 4,5-dimethoxy-2-nitrobenzaldehyde.

BACKGROUND: Tumor recurrence and metastasis develop as a result of tumors' acquisition of anti-apoptotic mechanisms and therefore, it is necessary to develop novel effective therapeutics against metastatic cancers. In this study, we showed the differential TRAIL responsiveness of human prostate adenocarcinoma PC3 and human colon carcinoma KM12 cells and their respective highly metastatic PC3-MM2 and KM12L4A sublines and investigated the mechanism underlying high susceptibility of human metastatic cancer cells to TRAIL. RESULTS: PC3-MM2 and KM12L4A cells with high level of c-Myc and DNA-PKcs were more susceptible to TRAIL than their poorly metastatic primary PC3 and KM12 cells, which was associated with down-regulation of c-FLIPL/S and Mcl-1 and up-regulation of the TRAIL receptor DR5 but not DR4 in both metastatic cells. Moreover, high susceptibility of these metastatic cells to TRAIL was resulted from TRAIL-induced potent activation of caspase-8, -9, and -3 in comparison with their primary cells, which led to cleavage and down-regulation of DNA-PKcs. Knockdown of c-Myc gene in TRAIL-treated PC3-MM2 cells prevented the increase of DR5 cell surface expression, caspase activation and DNA-PKcs cleavage and attenuated the apoptotic effects of TRAIL. Moreover, the suppression of DNA-PKcs level with siRNA in the cells induced the up-regulation of DR5 and active caspase-8, -9, and -3. We also found that 4,5-dimethoxy-2-nitrobenzaldehyde (DMNB), a specific inhibitor of DNA-PK, potentiated TRAIL-induced cytotoxicity and apoptosis in relatively TRAIL-insensitive PC3 and KM12 cells and therefore functioned as a TRAIL sensitizer. CONCLUSION: This study showed the positive relationship between c-Myc expression in highly metastatic human prostate and colon cancer cells and susceptibility to TRAIL-induced apoptosis and therefore indicated that TRAIL might be used as an effective therapeutic modality for advanced metastatic cancers overexpressing c-Myc and combination of TRAIL therapy with agent that inhibits the DNA-PKcs/Akt signaling pathway might be clinically useful for the treatment of relatively TRAIL-insensitive human cancers.

TRAIL sensitize MDR cells to MDR-related drugs by down-regulation of P-glycoprotein through inhibition of DNA-PKcs/Akt/GSK-3beta pathway and activation of caspases.

BACKGROUND: The development of new modulator possessing high efficacy, low toxicity and high selectivity is a pivotal approach to overcome P-glycoprotein (P-gp) mediated multidrug resistance (MDR) in cancer treatment. In this study, we suggest a new molecular mechanism that TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) down-regulates P-glycoprotein (P-gp) through inhibition of DNA-PKcs/Akt/GSK-3beta pathway and activation of caspases and thereby sensitize MDR cells to MDR-related drugs. RESULTS: MDR variants, CEM/VLB10-2, CEM/VLB55-8 and CEM/VLB100 cells, with gradually increased levels of P-gp derived from human lymphoblastic leukemia CEM cells, were gradually more susceptible to TRAIL-induced apoptosis and cytotoxicity than parental CEM cells. The P-gp level of MDR variants was positively correlated with the levels of DNA-PKcs, pAkt, pGSK-3beta and c-Myc as well as DR5 and negatively correlated with the level of c-FLIPs. Hypersensitivity of CEM/VLB100 cells to TRAIL was accompanied by the activation of mitochondrial apoptotic pathway as well as the activation of initiator caspases. In addition, TRAIL-induced down-regulation of DNA-PKcs/Akt/GSK-3beta pathway and c-FLIP and up-regulation of cell surface expression of death receptors were associated with the increased susceptibility to TRAIL of MDR cells. Moreover, TRAIL inhibited P-gp efflux function via caspase-3-dependent degradation of P-gp as well as DNA-PKcs and subsequently sensitized MDR cells to MDR-related drugs such as vinblastine and doxorubicin. We also found that suppression of DNA-PKcs by siRNA enhanced the susceptibility of MDR cells to vincristine as well as TRAIL via down-regulation of c-FLIP and P-gp expression and up-regulation of DR5. CONCLUSION: This study showed for the first time that the MDR variant of CEM cells was hypersensitive to TRAIL due to up-regulation of DR5 and concomitant down-regulation of c-FLIP, and degradation of P-gp and DNA-PKcs by activation of caspase-3 might be important determinants of TRAIL-induced sensitization of MDR cells to MDR-related drugs. Therefore, combination of TRAIL and chemotherapeutic drugs may be a good strategy for treatment of cancer with multidrug resistance.

Trichostatin A sensitizes human ovarian cancer cells to TRAIL-induced apoptosis by down-regulation of c-FLIPL via inhibition of EGFR pathway.

TRAIL-resistant cancer cells can be sensitized to TRAIL by combination therapy. In this study, we investigated the effect of trichostatin A (TSA), a histone deacetylase inhibitor, to overcome the TRAIL resistance in human ovarian cancer cells. Co-treatment of human ovarian cancer cells with TSA and TRAIL synergistically inhibited cell proliferation and induced apoptosis. The combined treatment of ovarian cancer SKOV3 cells with TSA and TRAIL significantly activated caspase-8 and truncated Bid, resulting in the cytosolic accumulation of cytochrome c as well as the activation of caspase-9 and -3. Moreover, we found that down-regulation of c-FLIP(L) might contribute to TSA-mediated sensitization to TRAIL-induced apoptosis in SKOV3 cells, and this result was supported by showing that down- or up-regulation of c-FLIP(L) with transfection of siRNA or plasmid sensitized or made SKOV3 cells resistant to TRAIL-induced apoptosis, respectively. TSA or co-treatment with TSA alone and TRAIL also resulted in down-regulation of EGFR1/2 and dephosphorylation of its downstream targets, AKT and ERK. Treatment of SKOV3 cells with PKI-166 (EGFR1/2 inhibitor), LY294002 (AKT inhibitor), and PD98059 (ERK inhibitor) decreased c-FLIP(L) expression and co-treatment with TRAIL further reduced the level of c-FLIP(L,) respectively, as did TSA. Collectively, our data suggest that TSA-mediated sensitization of ovarian cancer cells to TRAIL is closely correlated with down-regulation of c-FLIP(L) via inhibition of EGFR pathway, involving caspase-dependent mitochondrial apoptosis, and combination of TSA and TRAIL may be an effective strategy for treating TRAIL-resistant human ovarian cancer cells.

Sensitization of human K562 leukemic cells to TRAIL-induced apoptosis by inhibiting the DNA-PKcs/Akt-mediated cell survival pathway.

Despite the fact that many cancer cells are sensitive to TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, human K562 leukemic cells showed resistance to TRAIL-induced apoptosis. Interestingly, K562/R3 cells, a stable TRAIL-sensitive variant isolated from K562 cells, showed down-regulation of DNA-PK/Akt pathway and a high responsiveness to TRAIL-mediated growth inhibition and apoptosis. We revealed that siRNA-mediated suppression of DNA-PKcs led to decreased phosphorylation of Akt and Bad, a target molecule of Akt, and increased expression of DR4/DR5. Also, we found that suppression of DNA-PKcs using siRNA down-regulated c-FLIP and sensitized K562 cells to TRAIL-induced apoptosis through activation of caspase-8, -9 and -3. In addition, we revealed that treatment with DMNB, a specific inhibitor of DNA-PK, resulted in an increase of DR4/DR5 mRNA levels and their surface expression and a decrease of c-FLIP mRNA levels in K562 cells. DMNB potentiated TRAIL-induced cytotoxicity and apoptosis through inhibition of DNA-PK/Akt pathway and activation of caspase-8, -9 and -3 in K562 cells. This study is the first to show that a protective role of DNA-PK/Akt pathway against TRAIL-induced apoptosis and thus TRAIL in combination with agents that inhibit DNA-PK/Akt pathway would have clinical applicability in treating TRAIL-insensitive human leukemic cells. This model may provide a novel framework for overcoming TRAIL resistance of other cancer cells with agents that inhibit DNA-PK/Akt pathway.

Cotreatment with apicidin overcomes TRAIL resistance via inhibition of Bcr-Abl signaling pathway in K562 leukemia cells.

TNF-related apoptosis-inducing ligand (TRAIL) is a pro-apoptotic cytokine that is capable of inducing apoptosis in a wide variety of cancer cells but not in normal cells. Although many cancer cells are sensitive to TRAIL-induced apoptosis, chronic myeloid leukemia (CML) develops resistance to TRAIL. In this study, we investigated whether apicidin, a novel histone deacetylase inhibitor, could overcome the TRAIL resistance in CML-derived K562 cells. Compared to treatment with apicidin or TRAIL alone, cotreatment with apicidin and TRAIL-induced apoptosis synergistically in K562 cells. This combination led to activation of caspase-8 and Bcl-2 interacting domain (Bid), resulting in the cytosolic accumulation of cytochrome c from mitochondria as well as an activation of caspase-3. Treatment with apicidin resulted in down-regulation of Bcr-Abl and inhibition of its downstream target, PI3K/AKT-NF-kappaB pathway. In addition, apicidin decreased the level of NF-kappaB-dependent Bcl-x(L), leading to caspase activation and Bid cleavage. These results suggest that apicidin may sensitize K562 cells to TRAIL-induced apoptosis through caspase-dependent mitochondrial pathway by regulating expression of Bcr-Abl and its related anti-apoptotic proteins. Therefore, the present study suggests that combination of apicidin and TRAIL may be an effective strategy for treating TRAIL-resistant Bcr-Abl expressing CML cells.

Sensitization of imatinib-resistant CML cells to TRAIL-induced apoptosis is mediated through down-regulation of Bcr-Abl as well as c-FLIP.

Resistance to imatinib is commonly associated with reactivation of Bcr-Abl signalling. However, Bcr-Abl-independent signalling pathways may be activated and contributed to imatinib resistance in some CML (chronic myelogenous leukaemia) patients. We had isolated three imatinib-resistant K562/R1, R2 and R3 variants with gradual loss of Bcr-Abl from K562 cells to develop effective therapeutic strategies for imatinib-resistant CML. Interestingly, we found that these cells became highly sensitive to TRAIL (tumour necrosis factor-related apoptosis-inducing factor) in comparison with K562 cells showing high resistance to TRAIL. Treatment of K562/R3 cells with TRAIL resulted in activation of TRAIL receptor pathway by including caspase 8 activation, Bid cleavage, cytochrome c release and caspase 3 activation. These results were accompanied by down-regulation of c-FLIP {cellular FLICE [FADD (Fas-associated death domain)-like interleukin 1beta-converting enzyme]-inhibitory protein} in imatinib-resistant K562 variants compared with K562 cells. Overexpression of c-FLIP in K562/R3 cells acquired TRAIL resistance and conversely, c-FLIP-silenced K562 cells became sensitive to TRAIL. Moreover, Bcr-Abl-silenced K562 cells showed down-regulation of c-FLIP and the subsequent overcome of TRAIL resistance. Taken together, our results demonstrated for the first time that the loss of Bcr-Abl in imatinib-resistant cells led to the down-regulation of c-FLIP and subsequent increase of TRAIL sensitivity, suggesting that TRAIL could be an effective strategy for the treatment of imatinib-resistant CML with loss of Bcr-Abl.