Phosphotyrosine phosphatase activity prevents the detection of P210bcr/abl protein in mature cells in chronic myelogenous leukemia even by an immunoblotting technique

P210bcr/abl protein with tyrosine protein kinase has been implicated in the proliferation and differentiation of chronic myelogenous leukemia cells. Using an immunoblotting technique with antiphosphotyrosine (anti-P-Tyr) antibodies, we examined whether P210bcr/abl protein was expressed in chronic phase cells in patients with chronic myelogenous leukemia (CML). We could detect P210bcr/abl protein in blast cells regardless of myeloid or lymphoid lineage but not in chronic phase cells from patients. However, in a patient with both blast cells and chronic phase cells, we could identify the protein only after the enrichment of the blast crisis cells by Percoll gradient centrifugation. When K562 cells were mixed with mature granuloid cells, the P210bcr/abl in K562 cells detected by immunoblotting was decreased. Using phosphotyrosyl proteins in K562 cells as substrates, high phosphotyrosyl (P-Tyr) phosphatase activity was observed, not only in the lysate of chronic phase cells from CML patients but also in the lysate of neutrophils from normal subjects. These findings suggest the possibility that high P-Tyr phosphatase activity prevents the detection of P210bcr/abl in CML cells in the chronic phase. The activity may be characteristic of mature cells and may regulate cellular events through dephosphorylation of P210bcr/abl.     

A small molecule significantly inhibits the bcr/abl fusion gene at the mRNA level in human chronic myelogenous leukemia

Bcr/abl fusion gene is the marker gene in chronic myelogenous leukemia (CML) and becomes the target for CML therapy. Although Imatinib opened a new way to treat CML, the resistance to the drug caused by bcr/abl fusion protein mutation stimulated search for new molecules to inhibit bcr/abl expression. In our research, it was found that a novel 2-aminosteroid (H89465) possessed special mechanism in treating CML. H89465 inhibits the proliferation of both non-resistant and resistant CML cells such as K562, Meg-01 and clinical primary CML cells. It prolongs the survival time of NOD/SCID mice inoculated with K562 leukemia cells. The mechanism underlying the effects is concerned with down-regulation of bcr/abl mRNA expression followed by decreasing the BCR/ABL protein expression and tyrosine kinase activity in CML cells. Our results demonstrate that H89465 possesses the therapeutic potential in treating human CML.     

Regulation of biosynthesis and phosphorylation of P210bcr/abl protein during differentiation induction of K 562 cells

The changes of P210bcr/abl and other tyrosine phosphorylated proteins in K562 cells during growth and differentiation were studied by metabolic labeling with 32PO4 and immunoprecipitation with anti-phosphotyrosine sera. The anti-phosphotyrosine sera recognized P210bcr/abl and other phosphoproteins with mol wt of 150, 115, 100, 70 and 64 kD. The 2-day incubation of K562 cells with inducers for differentiation, hemin, sodium butyrate and TPA, decreased the level of P210bcr/abl protein and other phosphoproteins. Inhibitors of tyrosine protein kinase, amiloride and genistein, also reduced the phosphorylation of P210bcr/abl protein and other substrates, but did not induce differentiation of the cells. Although most of these additives inhibited the cell growth, cytotoxic agents such as adriamycin, vincristine and Ara-C did not affect the level of P210bcr/abl protein. The experiments using 35S-methionine labeled cells and the immunoprecipitation with anti-abl sera suggested that reduced biosynthesis but not dephosphorylation of P210bcr/abl protein mainly accounted for the reduction of P210bcr/abl protein reacting to anti-phosphotyrosine sera in differentiation-induced cells. These results indicate that the reduction of P210bcr/abl protein synthesis plays some roles in cellular differentiation of K562 cells.     

bcr-abl融合基因抑制K562细胞凋亡的研究

目的研究ｂｃｒａｂｌ融合基因在慢性粒细胞白血病细胞（ＣＭＬ）凋亡调控中的作用。方法用人工合成的与ｂｃｒａｂｌｍＲＮＡ（ｂ３ａ２型）融合点碱基序列互补的反义寡聚脱氧核苷酸（ＡＳＯＤＮ）作用于体外培养的ＣＭＬ细胞株Ｋ５６２细胞,观察Ｋ５６２细胞凋亡的情况。结果ｂｃｒａｂｌ融合基因表达受抑后,Ｋ５６２细胞凋亡显著增加。结论ｂｃｒａｂｌ融合基因可抑制ＣＭＬ细胞凋亡,这可能是ＣＭＬ发病的主要机制。     

Lineage non-specific down regulation of P210bcr/abl in the CML cell line, KU-812-F, during differentiation

CML cell line, KU-812-F, originally established from a patient with Philadelphia-chromosome-positive chronic myelocytic leukemia has maintained the ability to differentiate into both granuloid (basophilic) and erythroid lineages. The expression of P210bcr/abl in KU-812-F cells during differentiation was studied by immunoblotting and immunoprecipitation. Immunoblotting with anti-phosphotyrosine sera revealed the down-regulation of P210bcr/abl in both granuloid and erythroid lineages. Immunoprecipitation with anti-abl antibodies of 35S-methionine-labelled cells revealed a reduced rate of synthesis of P210bcr/abl protein. Cytotoxic agents that caused growth inhibition of the cells did not alter the expression of P210bcr/abl. These results indicate that the down regulation of P210bcr/abl protein is a lineage non-specific event accompanied by differentiation.     

Induction of Erythroid Differentiation by 5-Fluorouracil in K562 Leukemia Cells

K562 cell line, established from a patient in the blast crisis of chronic myeloid leukemia, expresses high levels of c-myc and bcr/abl gene products. Exposure of K562 cells to 5-fluorouracil (5-FU) resulted in a marked benzidine-positive erythroid differentiation with concomitant reduction in cell proliferation. No change in c-myc mRNA or protein levels occurred during 96 h of drug treatment. In contrast, a biphasic change of p210 ^bcr/abl and the abl -associated kinase activities was observed upon treatment with 5-FU. The change in p210 ^bcr/abl expression may be mediated at the translational level, since the steady-state level and the enzymatic activity of p210 ^bcr/abl are reduced, whereas bcr/abl mRNA levels are unaltered. The results are consistent with the existence of pleiotropic differentiation pathways in K562 cells.     

Berbamine: a novel inhibitor of bcr/abl fusion gene with potent anti-leukemia activity

Gleevec, which is an inhibitor of the bcr/abl tyrosine kinase, has been a remarkable success for the treatment of chronic myelogenous leukemia (CML). However, a significant proportion of patients chronically treated with Gleevec develop resistance. Here we describe the activity of a natural small molecular compound, berbamine from plant Berberis amurensis that can selectively induce cell death of both Gleevec-sensitive and -resistant Ph+ CML cells. The IC50 values of berbamine were 8.80 microg/ml in Gleevec-sensitive Ph+ CML cells, 11.34 microg/ml in Gleevec-resistant Ph+ CML cells, and 54.40 microg/ml in Ph- KG-1 cells, respectively. Similarly, berbamine was also found to display a selective anti-proliferative activity of primary leukemia cells from CML patients, and its IC50 values were 4.20-10.50 microg/ml in primary CML cells, and 185.20 microg/ml in normal bone marrow cells, respectively. More importantly, our studies demonstrate that berbamine down-regulates p210bcr/abl oncoprotein level, and induces apoptosis of bcr/abl+ cells through caspase-3-dependent pathway. These data suggest that berbamine might be a novel bcr/abl inhibitor with potent anti-leukemia activity.     

Enhanced growth suppression of Philadephia1 leukemia cells by targeting bcr3/abl2 and VEGF through antisense strategy.

An antisense strategy by targeting both bcr3/abl2 and VEGF was designed to suppress the growth of Philadephia1 leukemia cells in vitro and in vivo in mice. In vitro, although bcr3/abl2 or VEGF antisense oligodeoxyribonucleotides (AS-ODNs) alone was able to inhibit the proliferation of K562 cells, the combination of bcr3/abl2 and VEGF AS-ODNs produced an additive inhibitory effect on the growth of K562 cells and significantly enhanced the sensibility of K562 cells to apoptosis-inducing stimuli including STI571. In vivo, the nude mice xenografted with K562 cells received intratumoral injections of bcr3/abl2 and VEGF AS-ODNs showed a significant reduction in leukemia tumor size and microvessel density and an increase of apoptosis in the tumors when compared to the mice that received an individual agent. These results demonstrate that targeting both bcr3/abl2 and VEGF can result in an additive tumor-suppressive action and may represent an excellent strategy to augment the efficacy of chemotherapy in CML.     

Long-template DNA polymerase chain reaction for the detection of the bcr/abl translocation in patients with chronic myelogenous leukemia.

In most patients with chronic myelogenous leukemia (CML) primitive hematopoietic progenitors carry the acquired reciprocal bcr/abl gene rearrangement t(9;22)(q34.1; q11.21). However, not all of the progenitor cells express the bcr/abl hybrid mRNA or the p210 fusion protein. These cells, therefore, might escape detection by techniques that are based on expression of the fusion gene. To circumvent this problem, we established a new detection method for the rearrangement at the DNA level. Because breakpoints might occur in a very large genomic region (>200 kb), we developed a long-template DNA-PCR (LT-DNA-PCR). In 22 of 59 CML patients, fragments of up to 19 kb could be amplified. Furthermore, 6 of 7 leukapheresis products of three bcr/abl-positive patients which were collected after mobilization chemotherapy and had been shown to be negative for the bcr/abl rearrangement by FISH and by RT-PCR were clearly positive by LT-DNA-PCR. Using a specific pair of primers, it is possible to detect the presence of, and to characterize, the individual gene rearrangement. This approach could serve for diagnostic purposes as well as detection of minimal residual disease under cytotoxic therapy or after purging regimens, being independent of expression of the bcr/abl hybrid mRNA or the fusion protein.     

In vitro and in vivo evaluations of the tyrosine kinase inhibitor NSC 680410 against human leukemia and glioblastoma cell lines

PURPOSE: NSC 680410, the novel adamantyl ester of AG957, an inhibitor of the p210bcr/abl tyrosine kinase (CML, Ph(+)) and possibly other kinases, was tested for antitumor activity in ten human leukemia and human glioblastoma cell lines. METHODS: CEM/0, seven ara-C- and/or ASNase-resistant clones, Jurkat/0, the myelomonocytic line U937 and U87 MG glioblastoma cell lines were used for these studies. The drug-resistant leukemic clones lack p53, express bcl-2 and VEGF-R1, and thus are refractory to apoptosis. Since tyrosine kinases drive many proliferative pathways and these activities are increased in many leukemic cells, we hypothesized that NSC 680410 may induce cytotoxicity in drug-resistant leukemia clones, independently of p210bcr/abl expression. RESULTS: NSC 680410 exhibited significant antileukemic activity in CEM/0, Jurkat E6-1, and in the drug-resistant leukemic cell lines. The IC(50) values in nine leukemia lines ranged from 17 to 216 n M. Western blot analyses after NSC 680410 treatment demonstrated caspase-3 cleavage and ELISAs showed a fivefold upregulation of its activity in cellular extracts. In addition, U87 MG human glioblastoma cells, which express VEGF-R1, were treated with the Flt-1/Fc chimera, a specific inhibitor of VEGF, and showed 30-43% cell kill in the MTT assay. Furthermore, the combination of NSC 680410 plus Flt-1/Fc chimera demonstrated an eightfold synergism against U87 MG cells in vitro. To verify this observation in vivo, athymic mice were inoculated orthotopically into the caudate putamen with 10(6) U87 MG cells. On day 3, five mice per group were treated i.p. with either 8.3 mg/kg NSC 680410 daily for three doses per week for 4 weeks alone or in combination with one dose of Flt-1/Fc chimera 100 mg/kg subcutaneously. Treatment with NSC 680410 alone produced no weight changes and increased the median survival to 133%, whereas treatment with NSC680410 plus Flt-1/Fc chimera increased survival to 142% over control. Control animals had large intra- and extracranial tumors while the NSC 680410-treated mice had small, only intracranial tumors with necrotic centers. The combination treatment resulted in small residual tumors around the needle track, indicating significant inhibition of tumor growth. CONCLUSIONS: These studies demonstrate that the tyrosine kinase inhibitor NSC 680410 has significant antileukemic activity in p53-null, drug-resistant human leukemia cell lines, as well as significant antitumor activity in combination with Flt-1/Fc chimera against U87 MG glioblastoma brain tumors implanted in situ in athymic mice.     

Activation of protein kinase A (PKA) by 8-Cl-cAMP as a novel approach for antileukaemic therapy

Activation of PKA by cAMP agonists, such as 8-Cl-cAMP activation, selectively causes rapid apoptosis in v-abl transformed fibroblasts by inhibiting the Raf-1 kinase. Here we investigated whether 8-Cl-cAMP is useful for the treatment of chronic myelogenous leukaemia (CML), which is hallmarked by the expression of the p210(bcr/abl) oncogene. Autologous bone marrow transplantation is a feasible alternative for patients with no suitable donor, but hampered by the risk of relapse due to the persistence of leukaemia cells in the transplant. To study the effects of 8-Cl-cAMP on primary leukaemic cells, bone marrow cells (BMCs) from eight CML patients (one at diagnosis, three in chronic and four in accelerated phase) were treated. Ex vivo treatment of BMCs obtained in chronic phase of CML with 100 microM 8-Cl-cAMP for 24-48 h led to the selective purging of Philadelphia Chromosome (Ph1 chromosome) without toxic side effects on BMCs from healthy donors as measured by colony-forming unit (CFU) assays. BMCs from patients in accelerated phase showed selective, but incomplete elimination of Ph1 chromosome positive colony forming cells. The mechanism of 8-Cl-cAMP was investigated in FDCP-mix cells transformed by p210(bcr/abl), a cell culture model for CML. The results showed that 8-Cl-cAMP reduced DNA synthesis and viability independent of Raf inhibition as Raf inhibitors had no effect. MEK inhibitors interfered with DNA synthesis, but not with viability. In summary, our results indicate that 8-Cl-cAMP could be useful to purge malignant cells from the bone marrow of patients with CML and certain other forms of leukaemias.     

bcr／abl基因修饰树突细胞疫苗诱导CTLs杀伤K562细胞体外实验研究

背景与目的：T细胞介导的特异性免疫效应在慢性粒细胞白血病的治疗中发挥着重要作用,而以树突细胞（dendriticcell,DC）为中心的免疫治疗是目前肿瘤生物免疫学治疗的热点之一。本研究以复制缺陷型重组腺病毒为载体,介导慢性粒细胞白血病bcr／abl基因片段转导DC制备疫苗,观察bcr／abl基因修饰DC疫苗诱导产生的特异性细胞毒性T淋巴细胞（cytotoxic T lymphocytes,CTLs）在体外对白血病K562细胞的杀伤效应。方法：应用RT—PCR法扩增慢性粒细胞白血病bcr／abl基因片段．构建复制缺陷型重组腺病毒质粒。并包装产生重组腺病毒。体外诱导培养外周血单个核细胞来源DC,观察DC分别经重组腺病毒转染及多肽负载后,诱导产生特异性的CTLs在体外对白血病K562细胞的杀伤效应。结果：成功构建了携带bcr／abl基因片段的复制缺陷型重组腺病毒表达载体,包装产生的重组腺病毒滴度高达2．0×10^10pfu／mL。体外转染DC效率达到50％～60％,成功制备了bcr／abl特异性DC疫苗。体外实验中,在40：1和20：1效靶比下,bcr／abl基因修饰DC疫苗对K562细胞的杀伤效应分别为（47．6±4．7）％和（47．5±1．6）％,多肽负载DC为（25．8±4．4）％和（24．6±6．3）％,空白DC为（5．7±1．3）％和（4．5±1．6）％,基因修饰DC疫苗诱导的杀伤效应明显高于多肽负载及空白组（P〈0．05）。结论：以重组腺病毒为载体介导bcr／abl基因片段转导DC制备的基因修饰DC疫苗,具有显著的诱导CTLs杀伤白血病K562细胞的作用。     

反义ras基因增强足叶乙甙对白血病K562细胞的诱导凋亡作用

目的 研究ras p21蛋白在bcr-abl p210蛋白抗凋亡信号传导通路中的作用。方法 应用逆转录病毒载体pLXSN将反义N－ras1 cDNA转导到K562细胞系中后，通过足叶乙甙对细胞进行诱导凋亡。结果 转导反义N－ras1 cDNA的K562细胞内N－ras p21表达下降。反义N－ras转民细胞较对照细胞K562易发生凋亡，对药物的敏感性显著增高。结论 ras p21蛋白是bcr-abl p210蛋白介导抗凋亡信号的重要的下游蛋白，阻断该蛋白的功能有可能成为治疗人类慢性髓性白血病的1个选择点。     

两种慢性髓系白血病细胞株对干扰素-α的敏感性

背景和目的：临床研究表明干扰素-α(interferon-alpha,IFN-α)是造血系统恶性肿瘤和淋巴瘤的有效治疗剂之一。然而,IFN-α仅可使约70％～80％的慢性髓系白血病(chronic myeloid leukemia,CML)患者获得血液学缓解。不同的CML患者对IFN-α反应性不同的机制尚未阐明。本研究旨在比较两种CML细胞株KA-1／A3、K562对IFN-α的不同敏感性。方法：(1)采用MTT、半固体集落形成和台盼蓝染色液体培养细胞检测,不同浓度IFN-α(100、500、1000、5000和10000u／ml)对KT-1／A3及K562细胞生长的影响;(2)IFN-α(1000u／ml)诱导KT-1／A3、K562细胞48h后,采用流式细胞分析(flow cytometry,FCM)、荧光染色和DNA片段检测细胞凋亡;(3)加入IFN-α(1000u／m1)培养KT-1／A3、K562细胞48h后采用相对定量RT-PCR分析细胞bcr／abl基因表达水平。结果：(1)IFN-α呈剂量依赖性(从100u／ml到10000u／ml)抑制KT-1／A3细胞生长;(2)IFN-α(1000u／m1)诱导48h后使KT-1／A3细胞凋亡率从3．3％升到11．8％,并使KT-1／A3细胞中bcr／abl嵌合基因的表达水平降至对照组的66．7％;(3)IFN-α对K562细胞的生长、凋亡及bcr／abl嵌合基因表达无明显调节效应。结论：不同类型的CML细胞对IFN-α的反应性不同。     

K562多药耐药细胞系中肿瘤干细胞样细胞对伊马替尼耐药机制的初步研究

 目的 进一步阐明一些高表达P-糖蛋白（P-gp）的慢性粒细胞白血病细胞对伊马替尼耐药的机制。方法 经过对K562细胞系长期的足叶乙苷（VP16）诱导和克隆筛选,建立一株耐药细胞系K562／VP16;利用干细胞高效能将Hoechst 33342 荧光染料泵出细胞的特性,采用流式细胞术,从K562／VP16细胞系中分选出一小群细胞,即边缘细胞（SP）,称为K562／VP16 SP细胞,并初步探讨其抗伊马替尼的机制。结果 bcr／abl和abl 蛋白在K562细胞、K562／VP16 SP细胞及非K562／VP16 SP细胞（non-SP K562／VP16）中的表达水平差异无统计学意义;P-gp在K562细胞中不表达,在K562／VP16 SP及non-SP K562／VP16细胞中均高表达且表达水平一致;与non-SP K562／VP16细胞比较,K562／VP16 SP细胞对伊马替尼的耐药性更强,并且这种抗性几乎不能被多种多药耐药逆转剂逆转;另外,体内外实验显示,K562／VP16细胞的致瘤性几乎全部来源于K562／VP16 SP细胞。结论 bcr／abl基因的扩增、过度表达和多药耐药基因及其蛋白表达产物P-gp的高表达,可能不是白血病细胞产生对伊马替尼临床耐药的重要机制;白血病细胞对伊马替尼具有一定的抗性,可能与数量极少的白血病干细胞有直接的关系。因此,这类数量极少的干细胞样的肿瘤细胞应当成为有效治疗肿瘤的靶细胞。     

姜黄素衍生物FM17对K562细胞增殖影响及其与P210^kcr/abl激活的信号途径关系

目的探讨姜黄素衍生物FM17对体外培养的慢性粒细胞白血病（CML）急变期细胞K562细胞增生的影响,及其P210^kcr/abl激活的信号途径的关系。方法四甲基偶氮唑蓝（MTT）法检测姜黄素衍生物不同时间点和不同的浓度对K562细胞增殖的抑制作用。western blot的方法分析姜黄素衍生物FM17不同浓度、不同时间点对P210b刮ab0激活的信号的影响。结果2～8μg／mL浓度的FM17在12h、24h、36h及48h均可明显抑制K562细胞增殖,并在一定范围内呈时间和剂量依赖性;FM17可抑制P210^kcr/abl蛋白的含量,且可下调P210^kcr/abl激活的下游信号分子PKC和P-Erk。结论姜黄素衍生物FM17较姜黄素有更强的抗增殖能力,对P210^kcr/abl及其激活的信号途径也有较强的抑制作用。     

Cross-talk between extracellular S1P/S1P2 and P42/44 MAPK in bcr/abl positive chronic myeloid leukemia cells

Objective: BCR/ABL oncoprotein-expression is associated with uncontrolled cell growth. Sphingosine kinase 1 (SPK1) regulates the production of sphingosine 1-phosphate (S1P), a key lipid signal molecular in cell proliferation and survival. The objective of this study was to elucidate the roles of S1P and its receptors in bcr/abl positive chronic myeloid leukemia (CML) cells.Methods: The expressions of S1P receptors: S1P1, S1P2 and S1P3 in CML cells were detected by RT-PCR. SPK1 expression, activity and extracellular S1P were determined in ECV304 and HL-60 cells which were transfected with bcr/abl gene. To elucidate the relationship between the BCR/ABL, ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase), SPK/S1P and S1P/S1P2 signal pathways, bcr/abl positive CML cell line K562 was treated with STI571, PD98059, N,N-dimethyl sphingosine (DMS) and JTE-013.Results: Retrovirus-mediated overexpression of bcr/abl gene in ECV304 and HL-60 cells resulted in upregulation of the expression, activity of SPK1 and increase of the secretion of S1P, whereas treatment of STI571 and PD98059 decreased the BCR/ABL-induced S1P secretion. Treatment of DMS reduced S1P secretion and P42/44MAPK phosphorylation. S1P2-selective antagonist JTE-013 could also decrease P42/44MAPK phosphorylation. Conclusion: These results suggest that BCR/ABL up-regulates extracellular sphingosine 1-phosphate through sphingosine kinase 1 and there is cross-talk between SPK1/S1P/S1P2 and P42/44MAPK in bcr/abl positive CML cells.     

Expression of the chronic myelogenous leukemia-associated p210bcr/abl oncoprotein in a murine IL-3 dependent myeloid cell line.

We have studied the effect of a replication-defective murine retroviral vector expressing the chronic myelogenous leukemia associated oncoprotein p210bcr/abl in murine IL-3 dependent myeloid 32D C13(G) cells. This cell line can be induced to differentiate along either the granulocytic or monocytic lineages thus permitting an independent assessment of the effect of p210bcr/abl on growth and differentiation. Cells expressing p210bcr/abl displayed a complete non-autocrine abrogation of IL-3 dependence and an enhanced response to an activity in FBS which is not IGF-I or IGF-II. During the first few generations following infection with the bcr/abl vector, cells became larger with an increased fraction of cells in G2/M and monocyte/macrophage markers were expressed. Four cytoplasmic proteins phosphorylated in response to IL-3 in the parental cell line with apparent molecular weights of 98, 70, 62, and 52 Kd were amongst those constitutively phosphorylated in p210bcr/abl expressing cells. These results suggest that the functional substitution of IL-3 by p210bcr/abl is due to constitutive activation of proteins involved in IL-3 signal transduction. Alterations of cell differentiation, cell cycle and growth which cannot be attributed to IL-3 like effects indicate that p210bcr/abl has pleiotropic effects involving several other pathways of cellular regulation.     

A novel 53 kDa protein complexed with P210bcr-abl in human chronic myelogenous leukemia cells

Leukemic cells from patients with Philadelphia chromosome (Ph1)-positive chronic myelogenous leukemia (CML) contain a 210 kDa protein (P210bcr-abl) with a protein tyrosine kinase activity that is a product of fused bcr and abl genes. We have prepared two monoclonal anti-peptide antibodies, one from each gene product, and have affinity purified each. Incubation of anti-abl (c-abl 51-64) immunoprecipitates of K562 cells with [gamma-32P]ATP in protein kinase assays resulted in the labeling of P210bcr-abl and a 53 kDa (ph-P53) protein. Increasing concentrations of antibody detected similar ratios of P210bcr-abl: ph-P53, suggesting the presence of a complex between the proteins. Several different anti-abl and anti-bcr antibodies detected the ph-P53/P210 complex. Sodium dodecyl sulfate (SDS) treatment without 2-mercaptoethanol eluted P210bcr-abl and ph-P53 from the monoclonal antibody in the form of complexes which migrated on 6% SDS-polyacrylamide gels and had apparent molecular weights of 275,000 and more than 500,000. Both complexes yielded ph-P53 and P210bcr-abl upon treatment with SDS-mercaptoethanol. Studies involving glycerol gradient centrifugation also detected complexes of P210bcr-abl and ph-P53. Our results indicate that ph-P53 is not a degraded product of P210bcr-abl, does not share antigenic determinants with P210bcr-abl since it is not recognized by anti-abl and bcr antibodies in immunoblots, is not the phosphorylated heavy chain of immunoglobulin G, and is different from p53 (the nonviral T protein) complexed to the large T antigen of simian virus 40. Previous studies (Maxwell et al., 1987) have shown that ph-P53 has a different peptide map than P210bcr-abl. Therefore, we conclude that ph-P53 is a distinct cellular protein complexed to P210bcr-abl in K562 cells.