Role of p21 RAS in p210 bcr-abl transformation of murine myeloid cells

The p21 RAS product has been implicated as part of the downstream signaling of certain nonreceptor tyrosine kinase oncogenes and several growth factor receptor-ligand interactions. We have reported that the chronic myelogenous leukemia oncogene p210 bcr-abl transforms a growth- factor-dependent myeloid cell line NFS/N1.H7 to interleukin-3 (IL-3) independence. In these p210 bcr-abl-transformed cells (H7 bcr-abl.A54) and in two other murine myeloid cell lines transformed to IL-3 independence by p210 bcr-abl, endogenous p21 RAS is activated as determined by an elevated ratio of associated guanosine triphosphate (GTP)/guanosine diphosphate (GDP), assayed by thin-layer chromatography of the nucleotides eluted from p21 RAS after immunoprecipitation with the Y13-259 antibody. Treatment of p210 bcr-abl-transformed cells with a specific tyrosine kinase inhibitor herbimycin A resulted in diminished tyrosine phosphorylation of p210 bcr-abl and associated proteins, without major reduction in expression of the p210 bcr-abl protein itself. Inhibition of p210 bcr-abl-dependent tyrosine phosphorylation resulted in a reduction of active p21RAS-GTP complexes in the transformed cells, in diminished expression of the nuclear early response genes c-jun and c-fos, and in lower cellular proliferation rate. To further implicate p21 RAS in these functional events downstream of p210 bcr-abl tyrosine phosphorylation, we targeted G- protein function directly by limiting the availability of GTP with the inosine monophosphate dehydrogenase inhibitor, tiazofurin (TR). In p210 bcr-abl-transformed cells treated for 4 hours with TR, in which the levels of GTP were reduced by 50%, but GDP, guanosine monophosphate, and adenosine triphosphate (ATP) were unaffected, p210 bcr-abl tyrosine phosphorylation was at control levels. However, expression of c-fos and c-jun nuclear proto-oncogenes were strongly inhibited and p21 RAS activity was downregulated. These findings show that p210 bcr-abl transduces proliferative signals, in part, through downstream activation of p21 RAS. Furthermore, p21 RAS activity is linked to pathways that regulate c-jun and c-fos expression.     

All-trans retinoic acid induces p62DOK1 and p56DOK2 expression which enhances induced differentiation and G0 arrest of HL-60 leukemia cells

p62(DOK1) (DOK1) and p56(DOK2) (DOK2) are sequence homologs that act as docking proteins downstream of receptor or nonreceptor tyrosine kinases. Originally identified in chronic myelogenous leukemia cells as a highly phosphorylated substrate for the chimeric p210(bcr-abl) protein, DOK1 was suspected to play a role in leukemogenesis. However, p62(DOK1-/-) fibroblast knockout cells were found to have enhanced MAPK signaling and proliferation due to growth factors, suggesting negative regulatory capabilities for DOK1. The role of DOK1 and DOK2 in leukemogeneis thus is enigmatic. The data in this report show that both the DOK1 and the DOK2 adaptor proteins are constitutively expressed in the myelomonoblastic leukemia cell line, HL-60, and that expression of both proteins is induced by the chemotherapeutic differentiation causing agents, all-trans retinoic acid (atRA) and 1,25-dihydroxyvitamin D3 (VD3). Ectopic expression of either protein enhances atRA- or VD3-induced growth arrest, differentiation, and G(0)/G(1) cell cycle arrest and results in increased ERK1/2 phosphorylation. DOK1 and DOK2 are similarly effective in these capabilities. The data provide evidence that DOK1 and DOK2 proteins have a similar role in regulating cell proliferation and differentiation and are positive regulators of the MAPK signaling pathway in this context.     

Crkl is constitutively tyrosine phosphorylated in platelets from chronic myelogenous leukemia patients and inducibly phosphorylated in normal platelets stimulated by thrombopoietin

Platelet functions such as aggregation and clot retraction are often abnormal in chronic mylogenous leukemia (CML) patients. However, the molecular mechanisms of these altered functions are unknown. As expression of the p210bcr-abl oncogene product, a constitutively active tyrosine kinase, is known to have an essential role in the pathogenesis of CML and tyrosine phosphorylation is intimately involved in various aspects of platelet activation, we examined the pattern of protein tyrosine phosphorylation in platelets from 15 CML patients by immunoblotting with a monoclonal antiphosphotyrosine antibody (4G10). Before and after stimulation with thrombin, the only consistent difference between normal and CML platelets was the presence of a tyrosine phosphorylated protein with a relative molecular weight of 39 kD. This tyrosine phosphorylated protein was identified as crid, an SH2, SH3 containing adapter protein. Thus, as previously demonstrated for neutrophils from CML patients, tyrosine phosphorylation of p39crkl persists in mature platelets. No tyrosine phosphorylation of crid was detected following stimulation with thrombin in normal platelets. However, crkl became incorporated into the Triton X-100 insoluble residue following thrombin stimulation in a manner dependent on platelet aggregation. Further, we found that crkl is an endogenous substrate for calpain, a protease that may be involved in postaggregation signaling processes. This suggests that crkl may be involved in the reorganization of the cytoskeleton during normal platelet aggregation and its tyrosine phosphorylation in CML platelets may contribute to the abnormal platelet function in CML patients. Finally, we found that thrombopoietin induces tyrosine phosphorylation of crk1 in normal platelets and FDCP cells genetically engineered to express human c-Mpl. This suggests that crk1 can be phosphorylated by a kinase other than p210bcr-abl and that crk1 may have a role in signaling by thrombopoietin.     

Protein tyrosine phosphatase PTP1B suppresses p210 bcr-abl-induced transformation of Rat-1 fibroblasts and promotes differentiation of K562 cells

The bcr-abl chimeric oncoprotein exhibits deregulated protein tyrosine kinase activity and is implicated in the pathogenesis of Philadelphia chromosome (Ph)-positive human leukemias, such as chronic myelogenous leukemia (CML). Recently we have shown that the levels of the protein tyrosine phosphatase PTP1B are enhanced in p210 bcr-abl-expressing cell lines. Furthermore, PTP1B recognizes p210 bcr-abl as a substrate, disrupts the formation of a p210 bcr-abl/Grb2 complex, and inhibits signaling events initiated by this oncoprotein PTK. In this report, we have examined whether PTP1B effects transformation induced by p210 bcr-abl. We demonstrate that expression of either wild-type PTP1B or the substrate-trapping mutant form of the enzyme (PTP1B-D181A) in p210 bcr-abl-transformed Rat-1 fibroblasts diminished the ability of these cells to form colonies in soft agar, to grow in reduced serum, and to form tumors in nude mice. In contrast, TCPTP, the closest relative of PTP1B, did not effect p210 bcr-abl-induced transformation. Furthermore, neither PTP1B nor TCPTP inhibited transformation induced by v-Abl. In addition, overexpression of PTP1B or treatment with CGP57148, a small molecule inhibitor of p210 bcr-abl, induced erythroid differentiation of K562 cells, a CML cell line derived from a patient in blast crisis. These data suggest that PTP1B is a selective, endogenous inhibitor of p210 bcr-abl and is likely to be important in the pathogenesis of CML.     

Rearrangement in the breakpoint cluster region and the clinical course in Philadelphia-negative chronic myelogenous leukemia

We have followed one patient with Philadelphia (Ph)-negative chronic myelogenous leukemia and identified an additional four patients from the literature who showed the rearrangement in the breakpoint cluster region (bcr) on chromosome 22 characteristic of Ph-positive chronic myelogenous leukemia. The clinical course of these five patients was similar to that of Ph-positive patients, with easily controlled leukocyte counts, a prolonged benign phase, and prolonged survival. Furthermore, we have shown, for the first time, that bcr rearrangement in Ph-negative chronic myelogenous leukemia can result in expression of the aberrant 210-kilodalton bcr-abl fusion protein, which has been strongly implicated in Ph-positive leukemogenesis. Research data pertaining to possible cytogenetic mechanisms leading to production of p210bcr-abl in the absence of the Ph chromosome are reviewed. Molecular analysis provides an important tool for classifying and predicting prognosis of some patients with Ph-negative chronic myelogenous leukemia.     

Expression of c-abl in Philadelphia-positive acute myelogenous leukemia

The identical cytogenetic marker, t(9;22)(q34;q11) (Philadelphia [Ph] translocation), is found in approximately 90%, 20%, and 2% of adult patients with chronic myelogenous leukemia (CML), acute lymphoblastic leukemia (ALL), and acute myelogenous leukemia (AML), respectively. In CML, the molecular events resulting from the Ph translocation include a break within the bcr locus on chromosome 22, transfer of the c-abl protooncogene from chromosome 9 to 22, and formation of an aberrant 210- kD bcr-abl fusion protein (p210bcr-abl). Recently, the absence of bcr rearrangement and expression of a distinct aberrant 190-kd abl protein (p190c-abl) has been described in Ph-positive ALL, with the suggestion that the two abl variants may be pathogenetically associated with myeloid v lymphoid leukemogenesis. Here we report that the genomic configuration and translation product of Ph-positive AML can be similar to that of Ph-positive ALL: the break at 22q11 may occur outside the 5.8 kb bcr region and result in expression of a 190-kD abl protein lacking these bcr sequences. Phosphokinase enzymatic activity, a fundamental property of p210bcr-abl, was also associated with AML- derived p190c-abl. Our current observations indicate that p190c-abl can be found in cells of lymphoid or myeloid lineage and is therefore unlikely to play a specific role in the development of lymphoid leukemias. Formation of p190c-abl instead of p210bcr-abl appears to be a characteristic of the acute rather than the chronic Ph-positive leukemic state.     

The role of protein tyrosine phosphatase 1B in Ras signaling

Protein tyrosine phosphatase (PTP) 1B has been implicated as a negative regulator of multiple signaling pathways downstream of receptor tyrosine kinases. Inhibition of this enzyme was initially thought to potentially lead to increased oncogenic signaling and tumorigenesis. Surprisingly, we show that platelet-derived growth factor-stimulated extracellular-regulated kinase signaling in PTP1B-deficient cells is not significantly hyperactivated. Moreover, these cells exhibit decreased Ras activity and reduced proliferation by way of previously uncharacterized pathways. On immortalization, PTP1B-deficient fibroblasts display increased expression of Ras GTPase-activating protein (p120RasGAP). Furthermore, we demonstrate that p62Dok (downstream of tyrosine kinase) is a putative substrate of PTP1B and that tyrosine phosphorylation of p62Dok is indeed increased in PTP1B-deficient cells. Consistent with the decreased Ras activity in cells lacking PTP1B, introduction of constitutively activated Ras restored extracellular-regulated kinase signaling and their proliferative potential to those of WT cells. These results indicate that loss of PTP1B can lead to decreased Ras signaling, despite enhanced signaling of other pathways. This finding may in part explain the absence of increased tumor incidence in PTP1B-deficient mice. Thus, PTP1B can positively regulate Ras activity by acting on pathways distal to those of receptor tyrosine kinases.     

Differences in tyrosine phosphorylated proteins between cells expressing P210bcr/abl and P190bcr/abl.

We analysed differences between the populations of tyrosine phosphorylated proteins in two cell lines, K-562 and MR-87, which express two different bcr-abl fusion gene products, using both immunoprecipitation and Western blotting with an anti-phosphotyrosine antibody. K-562 cells preferentially expressed P210bcr/abl (P210), while MR-87 expressed P190bcr/abl (P190). Tyrosine phosphorylated proteins with a molecular mass of 150 kDa (p150) and 115 kDa (p110) were found in both K-562 and MR-87. A 36 kDa protein (p36) was tyrosine phosphorylated in vivo only in K-562 cells, while proteins with a molecular mass of 140 kDa (p140) and 62 kDa (p62) were found only in MR-87 cells. Moreover, several proteins in the detergent-insoluble cell fraction were differently tyrosine phosphorylated in vitro in K-562 and MR-87 lysates. These results suggest that P210 and P190 may have different substrates, and thus, different signal transduction pathways for cell proliferation, although the differential association of such cellular proteins with the two bcr/abl products remains to be clarified.     

Effect of herbimycin A, an antagonist of tyrosine kinase, on bcr/abl oncoprotein-associated cell proliferations: abrogative effect on the transformation of murine hematopoietic cells by transfection of a retroviral vector expressing oncoprotein P210bcr/abl and preferential inhibition on Ph1-positive leukemia cell growth.

Herbimycin A, a benzoquinoid ansamycin antibiotic, was demonstrated to decrease intracellular phosphorylation by protein tyrosine kinase (PTK). In Philadelphia chromosome (Ph1)-positive leukemias such as chronic myelogenous leukemia (CML) and Ph1-positive acute lymphoblastic leukemia (ALL), both of which express bcr-abl fused gene products (P210bcr-abl or P190bcr-abl protein kinase) with augmented tyrosine kinase activities, herbimycin A markedly inhibited the in vitro growth of the Ph1-positive ALL cells and the leukemic cells derived from CML blast crisis. However, the same dose of herbimycin A did not inhibit in vitro growth of a broad spectrum of Ph1-negative human leukemia cells, and several other protein kinase antagonists also displayed no preferential inhibition. Furthermore, we demonstrated that herbimycin A has an antagonizing effect on the growth of transformed cells by a transfection of retroviral amphotrophic vector expressing P210bcr/abl into a murine interleukin (IL)-3-dependent myeloid FDC-P2 cell line. This inhibition was abrogated by the addition of sulfhydryl compounds, similar to the reaction previously described for Rous sarcoma virus transformation. The inhibitory effect of herbimycin A on the growth of Ph1-positive cells was associated with decreased bcr/abl tyrosine kinase activity, but no decrease of bcr-abl mRNA and protein, suggesting that the inactivation of bcr-abl tyrosine kinase activity by herbimycin A may be induced by its binding to the bcr-abl protein portion that is rich with sulfhydryl groups. The present study indicates that herbimycin A is a beneficial agent for the investigation of the role of the bcr-abl gene in Ph1-positive leukemias and further suggests that the development of agents inhibiting the bcr-abl gene product may offer a new therapeutic potential for Ph1-positive leukemias.     

Ras GTPase-activating protein: a substrate and a potential binding protein of the protein-tyrosine kinase p56lck.

Ras GTPase-activating protein (GAP) is a cytoplasmic factor that regulates the GTPase activity of p21ras. Phosphorylation of GAP on tyrosine has recently been reported by several groups and may be an important step in linking signaling pathways involving p21ras and protein-tyrosine kinases. p56lck, a src-like protein-tyrosine kinase, seems to play a crucial role in T-cell development and T-cell activation. However, the molecular mechanisms of T-cell signaling involving p56lck and the substrates of p56lck have not yet been identified. To test whether GAP is a substrate of p56lck, in vitro kinase reactions were performed with purified, recombinant GAP and p56lck. We found that GAP became specifically phosphorylated on tyrosine within one tryptic peptide. Furthermore, coimmunoprecipitation studies provided evidence that the tyrosine-phosphorylated form of GAP is bound to p56lck. These results suggest that in T cells the function of GAP might be regulated through its phosphorylation on tyrosine and binding to the protein-tyrosine kinase p56lck.     

Dok-3 plays a nonredundant role in negative regulation of B-cell activation

p62(dok) and Dok-3 are members of the Dok family of adaptors found in B cells, with the former cloned as a substrate of the p210(bcr/abl) oncoprotein in Ph + chronic myelogenous leukemia. A role for p62(dok) in FcgammaRIIB-mediated negative regulation of B-cell proliferation had been established previously. Here, we generated Dok-3(-/-) mice to assess the function of Dok-3 in B cells. Mice lacking Dok-3 have normal B-cell development but possess higher level of IgM antibodies in their sera. In comparison to wild-type mice, Dok-3(-/-) mice mounted significantly enhanced humoral immune responses to T cell-independent type I and II antigens. Dok-3-deficient B cells hyperproliferated, exhibited elevated level of calcium signaling as well as enhanced activation of NF-kappaB, JNK, and p38MAPK in response to B-cell receptor (BCR) engagement. In the absence of Dok-3, the localization of the inhibitory phosphatase SHIP-1 to the plasma membrane is intact while its phosphorylation is compromised, suggesting that Dok-3 could function to facilitate or sustain the activation of SHIP-1. The phenotype and responses of Dok-3(-/-) mice and B cells could be differentiated from those of the Dok-1(-/-) counterparts. Hence, we propose that Dok-3 plays a distinct and nonredundant role in the negative regulation of BCR signaling.     

Alpha-interferon reduces in vivo phosphorylation of P210bcr/abl protein during hemin-induced erythroid differentiation of K-562 cells

alpha-Interferon (IFN-alpha) is important in the management of chronic myelogenous leukemia (CML). The P210bcr/abl fusion protein, with enhanced tyrosine kinase activity, is implicated in the pathogenesis and progression of the disease. To elucidate the inhibitory mechanism of IFN-alpha on CML cell proliferation, we studied the effect of IFN-alpha on P210bcr/abl in K-562 cells. The phosphorylated level of P210bcr/abl was not altered by treatment with IFN-alpha alone despite its inhibiting cell proliferation. However, when K-562 cells were treated with either a low (5 x 10(2) U/ml) or high (10(4) U/ml) concentration of IFN-alpha in the presence of hemin, P210bcr/abl protein activity decreased through reduction of in vivo phosphorylation, but not through inhibition of de novo protein synthesis. Furthermore, hemoglobin content was increased by IFN-alpha at both low and high concentrations in tandem with hemin-induced erythroid differentiation and the change in P210bcr/abl. These results demonstrate that IFN-alpha synergises hemin-mediated erythroid differentiation as it reduces the in vivo tyrosine phosphorylation of P210bcr/abl in K-562 cells.     

Constitutive Activation of Extracellular Signal-Regulated Kinase and p38 Mitogen-Activated Protein Kinase in B-Cell Lymphoproliferative Disorders

Signaling molecules such as p21(ras) (Ras), mitogen-activated protein kinase (MAPK), and Akt kinase play pivotal roles in the proliferation and survival of lymphoid cells in response to many kinds of stimulation. It is not fully understood, however, how these molecules participate in the growth of malignant lymphoid cells. We determined whether Ras, MAPKs such as extracellular signal-regulated kinase (ERK), c-Jun amino-terminal kinase (JNK), and p38 MAPK, and Akt kinase are activated in B-cell tumors, including acute lymphoblastic leukemia, chronic lymphocytic leukemia, Burkitt-like lymphoma, diffuse large B-cell lymphoma, and plasma cell leukemia. We found that Lyn protein tyrosine kinase was constitutively phosphorylated on tyrosine, and that ERK and p38 MAPK were constitutively active in all cases of the B-cell tumor. In contrast, activation of Ras and Akt kinase was found in limited cases, and JNK kinase activity was not observed in any case. These results suggest that ERK and p38 play roles in the oncogenesis of B-cell tumors.     

Cell lines and peripheral blood leukocytes derived from individuals with chronic myelogenous leukemia display virtually identical proteins phosphorylated on tyrosine residues.

An aberrant p210BCR-ABL protein that possesses constitutive protein-tyrosine kinase activity is presumed to be involved in the development of the neoplastic phenotype in chronic myelogenous leukemia (CML). Using a highly specific antibody against phosphotyrosine, we have isolated the tyrosine-phosphorylated p210BCR-ABL and several other proteins containing phosphotyrosine from a variety of CML cell lines. p210BCR-ABL isolated by the monoclonal anti-phosphotyrosine antibody possessed protein-tyrosine kinase activity in vitro comparable to that of the p210BCR-ABL isolated by antibody to a specific peptide sequence in the ABL protein-tyrosine kinase. Other prominent proteins containing phosphorylated tyrosine residues were observed at 185, 150, 120, 105, 63, 56, 36, and 32 kDa, and less prominent proteins were observed at 195, 155, 94, 53, 40, and less than 29 kDa. Staphylococcal V8 peptide mapping indicated that proteins of similar molecular weights were highly homologous to each other across cell lines, despite the diverse hematopoietic lineages of these cells and the genetic heterogeneity of the patients from whom the CML cell lines were derived. Phosphopeptide mapping also revealed that these proteins were distinct from each other as well as from p210BCR-ABL. Because virtually identical phosphotyrosine-containing proteins were found in peripheral blood leukocytes taken directly from CML patients, these proteins are not an artifact of long-term tissue culture but appear to be an integral part of the CML phenotype.     

Multi-unit ribozyme-mediated cleavage of bcr-abl mRNA in myeloid leukemias

Chronic myelogenous leukemia is characterized by the Philadelphia chromosome, which at the molecular level results from the fusion of the bcr gene on chromosome 22 and the abl gene on chromosome 9. The bcr-abl fusion gene encodes a novel tyrosine kinase with transforming activity. In this study, we have synthesized a multi-unti ribozyme that targets bcr-abl mRNA. In vitro ribozyme cleavage reactions show increased cleavage efficiency of this multi-unit ribozyme compared with single or double ribozymes. The multiunit ribozyme was then transfected into murine myeloblasts transformed with the bcr-abl gene (32D cells). Ribozyme transfection was accomplished either by liposomes or using follic acid-polylysine as a carrier. Multi-unit ribozyme transfection reduced the level of bcr-abl mRNA 3 logs when transfected via folate receptor-mediated uptake into transformed 32D cells. These results suggest that a multi-unit ribozyme could be an effective therapeutic agent for the treatment of Philadelphia chromosome-positive chronic myelogenous leukemia.     

Ribozyme-mediated inhibition of bcr-abl gene expression in a Philadelphia chromosome-positive cell line

The bcr-abl fusion gene is the molecular counterpart of the Philadelphia chromosome (Ph1) and is directly involved in the pathogenesis of Ph1+ leukemia. Inhibition of bcr-abl gene expression may have profound effects on the cell biology of Ph1+ cells, as recent experiments with antisense oligonucleotides have shown. In this study we have designed and synthesized a unique ribozyme that is directed against bcr-abl mRNA. The ribozyme cleaved bcr-abl mRNA in a cell-free in vitro system. A DNA-RNA hybrid ribozyme was then incorporated into a liposome vector and transfected into EM-2 cells, a cell line derived from a patient with blast crisis of chronic myelogenous leukemia. The ribozyme decreased levels of detectable bcr-abl mRNA in these cells, inhibited expression of the bcr-abl gene product, p210bcr-abl, and inhibited cell growth. This anti-bcr-abl ribozyme may be a useful tool to study the cell biology of Ph1+ leukemia and may ultimately have therapeutic potential in treating patients with Ph1 leukemias.     

Tyrphostin-induced inhibition of p210bcr-abl tyrosine kinase activity induces K562 to differentiate

We report on the potency of two Tyrphostin tyrosine kinase blockers, AG 1112 and AG 568, to inhibit p210bcr-abl tyrosine kinase activity in K562 cells, concomitant with the induction of erythroid differentiation. AG 568 and especially AG 1112 represent a specific group of nontoxic protein tyrosine kinase blockers among more than 1,400 tested. These compounds possess therapeutic potential for purging Philadelphia chromosome-positive cells in preparation for autologous bone marrow transplantation in chronic myelogenous leukemia.     

T-cell immunity to the joining region of p210BCR-ABL protein.

The hallmark of chronic myelogenous leukemia is the translocation of the human c-abl protooncogene (ABL) from chromosome 9 to the specific breakpoint cluster region (bcr) of the BCR gene on chromosome 22. The t(9;22)(q34;q11) translocation results in the formation of a BCR-ABL fusion gene that encodes a 210-kDa chimeric protein with abnormal tyrosine kinase activity. The ABL and BCR genes are expressed by normal cells and thus the encoded proteins are presumably nonimmunogenic. However, the joining-region segment of the p210BCR-ABL chimeric protein is composed of unique sequences of ABL amino acids joined to BCR amino acids that are expressed only by malignant cells. The current study demonstrates that the joining region of BCR-ABL protein is immunogenic to murine T cells. Immunization of mice with synthetic peptides corresponding to the joining region elicited peptide-specific, CD4+, class II major histocompatibility complex-restricted T cells. The BCR-ABL peptide-specific T cells recognized only the combined sequence of BCR-ABL amino acids and not BCR or ABL amino acid sequences alone. Importantly, the BCR-ABL peptide-specific T cells could recognize and proliferate in response to p210BCR-ABL protein. The response of peptide-specific T cells to protein demonstrated that p210BCR-ABL can be processed by antigen-presenting cells so that the joining segment is bound to class II major histocompatibility complex molecules in a configuration similar to that of the immunizing peptide and in a concentration high enough to stimulate the antigen-specific T-cell receptor. Thus, BCR-ABL protein represents a potential tumor-specific antigen related to the transforming event and shared by many individuals with chronic myelogenous leukemia.     

Two bcr/abl fusion gene products, P210bcr/abl and P190bcr/abl, are equally sensitive to the protein tyrosine phosphatase of mature granulocytes.

Two bcr/abl fusion gene products with tyrosine kinase activity have been found in two phenotypes of Philadelphia chromosome (Ph1)-positive leukemia. P210bcr/abl (P210) is associated with Ph1-positive chronic myelogenous leukemia (CML), while P190bcr/abl is associated with Ph1-positive acute leukemia. We compared the susceptibility of 32Pi-labeled P210 from K-562 cells and P190 from MR-87 cells to protein tyrosine phosphatase (PTPase). PTPase, present in the lysate of mature granulocytes from CML patients as well as in the lysate of these cells from normal subjects, effectively dephosphorylated the CML-associated P210 and the acute leukemia associated P190. This PTPase activity was specifically inhibited by ZnCl2; it was not present in lymphocyte lysates, and was not inhibited by neutralization with anti-CD45 antibody. Since P210 and P190 were equally sensitive to the PTPase, the difference in leukemic phenotypes associated with the expression of these two tyrosine kinases cannot be explained by the differential dephosphorylation of P210 and P190.