The responses of secondary endings of cat soleus muscle spindles to succinyl choline

In this report we examine biochemical and genetic alterations in DNA topoisomerase II (topoisomerase II) in K562 cells selected for resistance in the presence of etoposide (VP-16). Previously, we have demonstrated that the 30-fold VP-16-resistant K/VP.5 cell line exhibits decreased stability of drug-induced topoisomerase II/DNA covalent complexes, requires greater ATP concentrations to stimulate VP-16-induced topoisomerase II/DNA complex formation, and contains reduced mRNA and protein levels of the M(r) 170,000 isoform of topoisomerase II, compared with parental K562 cells. K/VP.5 cells grown in the absence of VP-16 for 2 years maintained resistance to VP-16, decreased levels of topoisomerase II, and attenuated ATP stimulation of VP-16-induced topoisomerase II/DNA binding, compared with K562 cells. Sequencing of cDNA coding for two consensus ATP binding sites and the active site tyrosine in the K/VP.5 topoisomerase II gene indicated that no mutations were present in these domains. In addition, single-strand conformational polymorphism analysis of restriction fragments encompassing the entire topoisomerase II cDNA revealed no evidence of mutations in the gene for this enzyme in K/VP.5 cells. Nuclear extracts from K562 (but not K/VP.5) cells contained a heat-labile factor that potentiated VP-16-induced topoisomerase II/DNA covalent complex formation in isolated nuclei from K/VP.5 cells. Immunoprecipitated topoisomerase II from K/VP.5 cells was 2.5-fold less phosphorylated, compared with enzyme from K562 cells. Collectively, our data suggest that acquired VP-16 resistance is mediated, at least in part, by altered levels or activity of a kinase that regulates topoisomerase II phosphorylation and hence drug-induced topoisomerase II/DNA covalent complex formation and stability.     

Identification of ligand binding sites on integrin alpha4beta1 through chemical cross-linking

We have used chemical cross-linking to identify sequences in integrin alpha4beta1 that are involved in its interactions with ligands. A recently described leucine-aspartic acid-valine (LDV)-based small molecule inhibitor of alpha4beta1 (BIO-1494), that contained a single reactive amino group for targeting the cross-linking, was used for these studies. The specificity of the interaction was defined by (i) the ability to block the interaction with a competitive inhibitor lacking the reactive group, (ii) the absolute requirement of divalent cations for cross-linking, and (iii) the lack of cross-linking to the functionally related integrin alpha4beta7. With ANB-NOS as the cross-linker, only the beta1 chain was labeled with BIO-1494, while with the more flexible cross-linker DSS both the alpha4 and beta1 chains were modified. Similar results were obtained when cross-linking was performed on K562 cells expressing alpha4beta1 but not on K562 cells expressing alpha2beta1. The site of cross-linking on the beta1 chain was localized by CNBr peptide mapping within residues 130-146, a region that contains the putative metal binding site DXSXS and for which analogous data had been generated with RGD binding to integrin alphaIIbbeta3. The striking similarity between the data we generated for an LDV ligand and published data for the RGD family supports the notion of a common ligand binding pocket formed by both integrin chains. The cross-linking strategy developed here should serve as a useful tool for studying alpha4beta1 function.     

D2 dopamine receptor antisense increases the activity and mRNA of tyrosine hydroxylase and aromatic L-amino acid decarboxylase in mouse brain

OBJECTIVE: Cytolytic activity of TNF was analysed at L929 and K562 tumor cell lines. METHODS: TNF-mediated cytotoxicity was studied within 10(-6)-10(-17) M concentration range after 18 h of incubation with target cells. RESULTS: TNF caused reliable cytotoxicity values in both cell lines, while L929 cells were more sensitive to cytolytic action of the protein than K562 cells. Three cytotoxicity maxima were detected at each cell line: at concentrations of 10(-6) M, 10(-17) M and 10(-15) M in K562 cells and at concentrations of 10(-7) M, 10(-11) M, 10(-14), 10(-16) M in L929 cells. CONCLUSIONS: DNA fragmentation analysis demonstrated that all cytolytic processes induced by TNF in L929 cells are associated with apoptotic mechanism of cell death, while cytolytic process induced in K562 cells differed in DNA fragmentation patterns: cytolytic processes induced by 10(-6) M of TNF was of apoptotic type, while the other processes were not associated with internucleosomal DNA cleavage.     

Single- and multiple-dose pharmacokinetic comparison of a sustained-release tablet and conventional tablets of naproxen in healthy volunteers

We investigated the cytotoxicity and cellular pharmacology of idarubicin (IDA), idarubicinol (IDAol) and daunorubicin (DNR) in K562/VP-H2 cells, which show topoisomerase II-related multidrug resistance but do not overexpress P-glycoprotein. K562/VP-H2 cells were less resistant to IDA and IDAol than to DNR. There was no significant difference in the accumulation of each drug between K562 and K562/VP-H2 cells. The cleavage of DNA induced by each drug was decreased in K562/VP-H2 cells, however, the decrease in cleavage in K562/VP-H2 cells was less with IDA and IDAol than with DNR. These results suggest that IDA and IDAol have more cytotoxic potency than DNR in topoisomerase II-related multidrug-resistant leukemia cells.     

Enforced expression of Bcl-XS induces differentiation and sensitizes chronic myelogenous leukemia-blast crisis K562 cells to 1-beta-D-arabinofuranosylcytosine-mediated differentiation and apoptosis

Human chronic myelogenous leukemia-blast crisis K562 cells have been demonstrated to be relatively resistant to antileukemic drug-induced apoptosis. This has been attributed to the activity of p210bcr-abl tyrosine kinase present in the K562 cells, which is known to suppress drug-induced apoptosis. Recently, K562 cells have been shown to express the antiapoptosis Bcl-xL but not Bcl-2 proteins. To investigate the contribution of Bcl-xL toward resistance to drug-induced apoptosis, we created K562/Bcl-xS and K562/neo cells by electroporating the expression plasmids pSFFVneo-Bcl-xS and pSFFVneo, containing the bcl-xS and neomycin resistance genes, respectively, into K562 cells. K562/Bcl-xS but not K562/neo cells expressed the bcl-xS mRNA and p19Bcl-xS protein. In contrast, both cell types expressed equivalent levels of Bcl-xL, Bax, Bcl-2, Myc, retinoblastoma, p21cbor-abl, and p145abl proteins. A significant increase in the hemoglobin levels was observed in the K562/Bcl-xS compared with the K562/neo cells (P < 0.05). In addition, K562/Bcl-xS cells were significantly more sensitive than K562/neo cells to undergoing erythroid differentiation induced by low-dose 1-beta-D-arabinofuranosylcytosine (ara-C) and hexamethyl bisacetamide (P < 0.05), but not by all-trans-retinoic acid. Low-dose ara-C- or hexamethyl bisacetamide-induced differentiation was not associated with apoptosis of K562/Bcl-xS or K562/neo cells. Low-dose ara-C-induced erythroid differentiation was accompanied by conversion of the retinoblastoma protein to predominantly its underphosphorylated isoform as well as by down-regulation of Myc levels in K562/Bcl-xS and K562/neo cells. Importantly, exposure to high-dose ara-C (HIDAC; 100 microM ara-C for 4 h) caused internucleosomal DNA fragmentation and the morphological features of apoptosis in K562/Bcl-xS cells. These effects were modestly enhanced by cotreatment with HIDAC plus herbimycin A. In contrast, K562/neo cells were completely resistant to HIDAC- and herbimycin A-induced apoptosis. These results indicate that the expression of Bcl-xS induces erythroid differentiation and partially sensitizes chronic myelogenous leukemia-blast crisis-derived K562 cells to ara-C-induced differentiation and apoptosis.     

The K562 chronic myeloid leukemia cell line undergoes apoptosis in response to interferon-alpha

BACKGROUND AND OBJECTIVE: The K562 cell line, derived from a chronic myeloid leukemia (CML) patient and expressing B3A2 bcr-abl hybrid gene, is known to be particularly resistant to apoptotic death. IFN-alpha treatment of CML patients impairs malignant cell clone, apparently protecting from progression to terminal blast crisis. The mechanisms underlying this kind of cell deletion are analyzed here by multiple technical approaches. DESIGN AND METHODS: K562 cells, variably treated with IFN-alpha, were examined by agarose gel DNA electrophoresis, light and electron microscopy. The presence of bcr-abl rearrangement was revealed by RT-PCR. RESULTS: At 4 day treatment both DNA ladder and apoptotic nuclear changes were identified, consistently in the presence of bcr-abl expression. INTERPRETATION AND CONCLUSIONS: Even cells expressing bcr-abl, such as K562, can be triggered to apoptosis. Therefore, this genetic condition, commonly preventing PCD, does not prevent IFN-alpha-mediated apoptosis. PCD seems thus to be the mechanism underlying IFN-alpha-treated K562 cell deletion and it could be the basis of malignant clone reduction in IFN-alpha treated CML patients.     

Phosphorylation of GATA-1 increases its DNA-binding affinity and is correlated with induction of human K562 erythroleukaemia cells

We have investigated by electrophoretic mobility shift assay (EMSA) the level of GATA-1 DNA-binding activity in nuclear extracts prepared from the human erythroleukaemic cell line, K562, after erythroid induction by hemin, sodium butyrate (NaB) or Trichostatin A or treatment with N -acetylcysteine (NAC). Relative to extract from untreated cells, GATA-1 binding activity increased markedly in all cases. However, immunoblot analysis revealed unchanged levels of GATA-1 protein after induction. Incubation of induced but not uninduced K562 extracts with phosphatase prior to EMSA weakened the binding activity, suggesting that the increase in GATA-1 binding following induction of K562 cells was a consequence of phosphorylation. When the mouse erythroleukaemic cell line MEL was induced with dimethylsulphoxide (DMSO), NaB or NAC, GATA-1 binding activity fell with DMSO, rose significantly with NaB and remained at about the same level in NAC-induced cells. In this case immunoblotting revealed that GATA-1 protein levels were in accord with the EMSA data. The DNA-binding activities of induced and uninduced MEL cell nuclear extracts were decreased by incubation with phosphatase, showing that phosphoryl-ation and DNA binding of GATA-1 are already optimalin these cells. The DNA-binding activity of affinity-purified GATA-1 from MEL cells was also reduced by phosphatase treatment, showing that phosphorylation/dephosphorylation is directly affecting the factor. Furthermore, when a comparison was made by EMSA of nuclear extracts prepared from K562 and MEL cells untreated or incubated with okadaic acid, a phosphatase inhibitor, GATA-1 binding was seen to increase with K562 cells, whereas with MEL cells there was no change in GATA-1 binding. Overall the results suggest that the level of GATA-1 phosphorylation increases after the induction of K562, but not MEL cells, where GATA-1 is already highly phosphorylated. Furthermore, phosphorylation increases the binding affinity of GATA-1 for a canonical binding site.     

7-Hydroxystaurosporine (UCN-01) induces apoptosis in human colon carcinoma and leukemia cells independently of p53

7-hydroxystaurosporine (UCN-01) is a more selective protein kinase C inhibitor than staurosporine. UCN-01 exhibits antitumor activity in experimental tumor models and is presently in clinical trials. Our study reveals that human myeloblastic leukemia HL60 and K562 and colon carcinoma HT29 cells undergo internucleosomal DNA fragmentation and morphological changes characteristic of apoptosis after UCN-01 treatment. These three cell lines lack functional p53, and K562 and HT29 cells are usually resistant to apoptosis. DNA fragmentation in HT29 and K562 cells occurred after 1 day of treatment while it took less than 4 h in HL60 cells. Cycloheximide prevented UCN-01-induced DNA fragmentation in HT-29 cells, but not in HL60 and K562 cells, suggesting that macromolecular synthesis is selectively required for apoptotic DNA fragmentation in HT29 cells. UCN-01-induced DNA fragmentation was preceded by activation of cyclin B1/cdc2 kinase. Further studies in HL60 cells showed that UCN-01-induced apoptosis was associated with degradation of CPP32, PARP, and lamin B and that the inhibitor of caspases (ICE/CED-3 cysteine proteases), Z-VAD-FMK, and the serine protease inhibitor, DCI, protected HL60 cells from UCN-01-induced DNA fragmentation. However, only DCI and TPCK, but not Z-VAD-FMK, inhibited DNA fragmentation in the HL60 cell-free system, suggesting that serine protease(s) may play a role in the execution phase of apoptosis in HL60 cells treated with UCN-01. Z-VAD-FMK and DCI also inhibited apoptosis in HT29 cells. These data demonstrate that the protein kinase C inhibitor and antitumor agent, UCN-01 is a potent apoptosis inducer in cell lines that are usually resistant to apoptosis and lack p53 and that caspases and probably serine proteases are activated during UCN-01-induced apoptosis.     

K562细胞硫氧还蛋白还原酶活力测定及其抑制剂体外抗白血病的初步研究

目的 探讨慢性粒细胞白血病(CML)细胞株K562中硫氧还蛋白还原酶(TrxR)的活力及其新型抑制剂乙烷硒啉(BBSKE)体外抗白血病作用.方法 应用胰岛素还原法检测K562细胞株及健康人骨髓单个核细胞中TrxR的活力.运用CCK-8法测定BBSKE对K562细胞的增殖抑制率.应用激光共聚焦显微镜、琼脂糖凝胶电泳以及Annexin V-FITC/PI双标记流式细胞术观察BBSKE的抗白血病作用.结果 K562细胞中TrxR的活性明显高于健康人骨髓单个核细胞,10 μmol/L BBSKE与K562细胞作用24 h,激光共聚焦显微镜可见典型的细胞凋亡表现,琼脂糖凝胶电泳后可见典型的DNA"梯"条带出现,流式细胞术检测凋亡率为(10.28±2.74)%;10 μmol/LBBSKE对CML患者原代细胞有诱导凋亡的作用,凋亡率为(5.70±0.48)%.结论 慢性粒细胞白血病细胞株K562中TrxR活力高于健康人骨髓单个核细胞,BBSKE有抑制TrxR活力、抑制K562细胞增殖和诱导凋亡的作用,是治疗CML潜在的有效药物.     

Expression-dependent perturbation of nucleosomal phases at HS2 of the human beta-LCR: possible correlation with periodic bent DNA

DNA bend sites appear periodically at average intervals of 680 bp, corresponding to a length of four nucleosomes, in the human epsilon-, beta- and Ggamma-Agamma-psibeta-globin gene regions. We found that the HS2 region flanked by two DNA bend sites accommodated five nucleosomes and they were regularly phased throughout the region with the exception of that located in the middle, which corresponded to the precise location of HS2 and included the binding site for NF-E2. There appeared to be several phases in this region in the reconstituted chromatin and in erythroid K562 cells where the globin genes are expressed, whereas only one phase was adopted in non-erythroid HeLa cells. Meanwhile, almost unique phases were adopted at the flanking bend sites in vitro as well as in vivo. Sequences of 30 bp containing the bend centers cloned into the vector alone showed identical nucleosomal phases to those observed with the in vitro and in vivo experiments and removing the bend sites caused disruption of the phases at the bend sites as well as those in their direct vicinity. Finally, the nucleosome in this HS2 region had an inhibitory effect on NF-E2 binding, although remodeling occurred with the nuclear extract from K562 cells in the presence of ATP. This suggests that HS2 is placed at a region of weak nucleosome phasing activity along with factor binding sites.     

Interrelationship between ectoparasites and wild rodents from Tijucas do Sul, state of Paraná, Brazil

NK cells kill sensitive targets via exocytosis of cytoplasmic granules containing membrane damaging perforins and DNA damaging granzymes. Therefore, the target cell can either die by necrosis or by apoptosis. A third, non-secretory, mechanism of killing mediated by Fas-FasL interaction can be used by activated NK cells to kill Fas+ targets. Here, we have studied the modulation exerted by two NK active cytokines, IL-2 and IL-12, on the apoptosis-inducing activity of NK cells in NK-sensitive Fas- K562 and NK-insensitive Fas+ Raji cells. Our results show that apoptosis is preferentially induced at low target:effector ratios. Resting NK cells virtually do not induce apoptosis in target cells while IL-2 stimulation endows NK cells with the capacity of inducing apoptosis and IL-12 further enhances this effect against both targets. Finally, we demonstrate that cytokine-stimulated NK cells use both granzyme and Fas-FasL pathways of apoptosis induction.     

Prospective study of mutant frequencies at the hprt and T-cell receptor gene loci in pediatric cancer patients during chemotherapy

With the use of three different hematopoietic cell lineages, the downregulation of telomerase activity was found to be a general response to the induction of differentiation. The decrease in telomerase activity occurred as early as 24 h when HL-60 and K562 cells were cultured in the presence of 1alpha, 25 dihydroxyvitamin D3 (VD3), all-trans-retinoic acid (ATRA) and hemin, and completely disappeared after 3 days. On the other hand, MEG-01 cells showed a marked inhibition of telomerase activity after 6 days of culture with 12-0-tetradecanoylphorbal 13-acetate (TPA). The analysis of telomeric DNA in the HL-60 cells and K562 cells demonstrated no detectable loss of telomeric DNA with cellular differentiation, with a loss of telomerase activity. The repression of telomerase is a common molecular event during leukemic cell differentiation.     

Distribution of herpes simplex virus DNA in the brains of human long-term survivors of encephalitis

The influence of morphine on proliferation of human tumor K562 and lymphoid cells was studied and compared with that on the mitogen-induced proliferation of human peripheral blood mononuclear cells (PBMC). Morphine was shown to act as a suppressor of both cellular DNA synthesis (50% and more as compared to control) and the cellular population growth of mitogen-induced PBMC, B-lymphoma Namalva cells and EBV-transformed lymphocytes. Morphine activated proliferation of myeloid K562 and T-lymphoma Yurkat cells 1.5-fold. It is supposed that the opposite effects of morphine on proliferation of cell lines of immune origin reveal the difference in modulation of diverse immune cell types by morphine.     

A cytotoxic NK-cell line (NK-92) for ex vivo purging of leukemia from blood

Myeloablative chemo-/radiotherapy supported by transplantation of autologous bone marrow or blood progenitor cells for acute leukemia, lymphoma, or myeloma continues to be associated with a high relapse rate because of the infusion of malignant stem cells and the lack of an in vivo graft-vs.-leukemia (GVL) effect. Although various methods of purging are established for marrow, purging procedures for blood progenitor cell preparations have not been widely used primarily because of the technical challenges to process a higher number of cells. As a broadly applicable method for immunological purging, we tested whether highly cytotoxic cells from a natural killer (NK) cell line characterized previously (NK-92) could be used for immunological purging of blood preparations. The NK-92 cell line, which was established from a patient with non-Hodgkin's lymphoma, can lyse in vitro a broad range of leukemia, lymphoma, and myeloma cell lines even at very low effector:target (E:T) ratios; this lysis is superior to cytotoxicity obtained from normal peripheral blood mononuclear cells (PBMCs) stimulated for 4 days with interleukin (IL)-2. In an attempt to quantitate the purging achievable with NK-92 cells, normal PBMCs were spiked with 10% K562 cells that had been transfected with the neo(r) marker gene (K562-neo(r). Various numbers of NK-92 cells were then added to the cell mixtures, which were incubated for 4 or 48 hours at 37 degrees C with or without IL-2 (500 U/mL). In order to prevent their proliferation, NK-92 cells were irradiated with 1000 cGy (cesium source). This radiation dose was determined to suppress proliferation of NK-92 cells, but at the same time maintain full cytotoxic activity. After co-culture, the cells were plated in methylcellulose containing 0.8 mg/mL G418. The number of surviving K562-neo(r) colonies was counted under the microscope 7 days later and the results were considered a quantitative readout for the purging efficacy of NK-92 cells. No neomycin-resistant K562 colonies could be detected up to a ratio of NK-92:K562-neo(r) cells of 5:1 (effective NK-92:PBMC ratio of 0.5:1). The presence or absence of IL-2 during the culture period did not affect the results. At this ratio of NK-92:PBMC, the growth of normal clonogenic hematopoietic progenitor cells was not compromised as determined by a standard methylcellulose assay. Considering that K562 is a rapidly proliferating cell line and that the input number of K562 cells (10%) tested here was high, the data suggest that the cytotoxic NK-92 clone (after irradiation to prevent proliferation) could be used effectively for immunological ex vivo purging without compromising hematopoietic cell function.     

Study of Dengue virus infection in SCID mice engrafted with human K562 cells

Here we report that severe combined immunodeficient (SCID) mice engrafted with human K562 cells (K562-SCID mice) can be used as an animal model to study dengue virus (DEN) infection. After intratumor injection into K562 cell masses of PL046, a Taiwanese DEN-2 human isolate, the K562-SCID mice showed neurological signs of paralysis and died at approximately 2 weeks postinfection. In addition to being detected in the tumor masses, high virus titers were detected in the peripheral blood and the brain tissues, indicating that DEN had replicated in the infected K562-SCID mice. In contrast, the SCID mice were resistant to DEN infection and the mock-infected K562-SCID mice survived for over 3 months. These data illustrate that DEN infection contributed directly to the deaths of the infected K562-SCID mice. Other serotypes of DEN were also used to infect the K562-SCID mice, and the mortality rates of the infected mice varied with different challenge strains, suggesting the existence of diverse degrees of virulence among DENs. To determine whether a neutralizing antibody against DEN in vitro was also protective in vivo, the K562-SCID mice were challenged with DEN-2 and received antibody administration at the same time or 1 day earlier. Our results revealed that the antibody-treated mice exhibited a reduction in mortality and a delay of paralysis onset after DEN infection. In contrast to K562-SCID, the persistently DEN-infected K562 cells generated in vitro invariably failed to be implanted in the mice. It seems that in the early stage of implantation, a gamma interferon activated, nitric oxide-mediated anti-DEN effect might play a role in the innate immunity against DEN-infected cells. The system described herein offers an opportunity to explore DEN replication in vivo and to test various antiviral protocols in infected hosts.     

Managing and understanding variances in clinical path methodology: a case study

We have analysed the cellular metabolism of a novel thymidylate synthase (TS) inhibitor, ZD1694, in MOLT-3 and K562 human leukaemia cell lines sensitive to or made resistant to ZD1694 by continuous exposure of the cells to ZD1694 with stepwise escalation of the drug concentration. The initial cellular uptake of [3H]ZD1694 was greater in K562 cells than in MOLT-3 cells and the drug accumulated approximately 3-fold more in the former cells following incubation with 0.1 microM ZD1694 at 37 degrees C for 24 h. TS and dihydrofolate reductase activities were not significantly different between the two cell lines. After a 30-min incubation with the drug at 37 degrees C, 85% of the total drug (2.3 pmol/mg protein) in K562 cells was found as tri- to pentaglutamates, whereas MOLT-3 cells accumulated less drug in this time (0.83 pmol/mg protein) and polyglutamates of chain length greater than triglutamate were not found to a significant extent. When the incubation time was extended to 24 h, the polyglutamate profile in K562 cells was progressively shifted towards those of long glutamate chain length and 59% of the total cellular drug (204 pmol/mg protein) was identified as the penta form. In contrast, even distribution between tri- and pentaglutamate was observed in MOLT-3 cells. Total cellular polyglutamates were approximately 3-fold higher in K562 cells than in MOLT-3 cells, and this may explain the 2.5-fold difference in the sensitivity to ZD1694 between the two cell lines. Continuous exposure of MOLT-3 and K562 cells to ZD1694 up to 1 microM or 0.1 microM resulted in 1600- and 4200-fold resistant sublines, respectively (MOLT-3/ZD1694.C and K562/ZD1694.C). The resistant MOLT-3 cells showed a markedly lower cellular accumulation and poor retention of [3H]ZD1694 with no significant change of initial drug uptake by 10 min and with a little increase of TS activity. HPLC analysis demonstrated that more than 90% of the 3H co-eluted with the monoglutamate (parent drug) in the resistant MOLT-3 cells, indicating extremely diminished polyglutamation in the cells. On the other hand, cellular uptake of [3H]ZD1694 was extensively impaired in K562/ZD1694.C cells and cellular accumulation of the drug was only 2.5% of that in the parent cells following 24 h incubation with the drug. Neither an increase of TS or dihydrofolate reductase activity nor a change in the polyglutamate formation profile was observed in the resistant K562 cells. These results indicate that the cellular ability to produce the polyglutamate metabolites of ZD1694 must influence the sensitivity of the tumour cells to this drug, and development of mechanisms involved in the ZD1694 resistance may relate to the intrinsic biochemical properties of the cells.     

Protein kinase Ciota activity is necessary for Bcr-Abl-mediated resistance to drug-induced apoptosis

K562 chronic myelogenous leukemia cells are highly resistant to chemotherapeutic drugs, such as taxol, that induce cell death by apoptosis. This resistance is mediated by the chimeric tyrosine kinase oncogene Bcr-Abl. However, little is known about the mechanism by which Bcr-Abl protects K562 cells from apoptosis. We recently demonstrated that expression of PKCiota is necessary for the resistance of K562 cells to taxol-induced apoptosis (Murray, N. R., and Fields, A. P. (1997) J. Biol. Chem. 272, 27521-27524). We now demonstrate that treatment of K562 cells with taxol leads to sustained activation of PKCiota. In contrast, Bcr-Abl-negative HL60 myeloid leukemia cells, which are sensitive to taxol-induced apoptosis, do not exhibit sustained PKCiota activation in response to taxol. Treatment of K562 cells with tyrphostin AG957, a selective Bcr-Abl inhibitor, blocks taxol-induced PKCiota activation and sensitizes these cells to taxol-induced apoptosis, indicating that PKCiota is a relevant downstream target of Bcr-Abl-mediated resistance. Furthermore, expression of constitutively active PKCiota by adenovirus-mediated gene transfer rescues AG957-treated K562 cells from taxol-induced apoptosis. Taken together, these results demonstrate that both Bcr-Abl and PKCiota activity are necessary for apoptotic resistance in K562 cells. Furthermore, they identify PKCiota as a critical downstream target of Bcr-Abl that is sufficient to mediate the anti-apoptotic effects of Bcr-Abl.