Sequential emergence of distinct resistance phenotypes in murine erythroleukemia cells under adriamycin selection: decreased anthracycline uptake precedes increased P-glycoprotein expression.

First-step Adriamycin (doxorubicin)-resistant mutants of the murine erythroleukemia cell line PC4 were cloned from Adriamycin-containing (10 ng/ml) methylcellulose at a frequency of 3 x 10(-4). They demonstrated 1.6- to 2.4-fold stable resistance to Adriamycin. Most were cross-resistant to etoposide, but not to vincristine, and were without enhanced expression of mdr genes, which code for P-glycoproteins. Two different murine erythroleukemia cell lines, PC4 and C7D, were passaged in suspension culture into stepwise increasing amounts of Adriamycin. No high-level resistant mutants were isolated de novo; cells initially displayed low-level resistance to Adriamycin and etoposide. Two stepwise doublings of the drug concentration were needed before PC4 cells acquired vincristine resistance, but there was no detectable overexpression of mdr or a change in anthracycline uptake. In a subsequent doubling of Adriamycin concentration, the cells showed a further increase in resistance to all three drugs and now a decreased anthracycline accumulation. However, there was still no detectable increase in mdr expression as judged by Northern analysis of poly(A)+ enriched RNA and Western blot analysis of membrane proteins. Only after a fourth doubling of Adriamycin concentration did the cells demonstrate enhanced expression of mdr and P-glycoprotein. Equivalent mutants of C7D were selected, but generally at lower Adriamycin concentrations. Verapamil partially lowered resistance, but failed to restore parental susceptibility in any mutant; it caused an increased uptake in those mutants showing decreased anthracycline accumulation, including those that did not overexpress mdr. This study demonstrated different resistance phenotypes among mutants appearing spontaneously under stepwise drug selection; mutants with vincristine resistance and decreased anthracycline uptake preceded those associated with over-expression of P-glycoprotein.

Overexpression of the multidrug resistance-associated protein (MRP) gene in vincristine but not doxorubicin-selected multidrug-resistant murine erythroleukemia cells.

Multidrug-resistant sublines of the murine erythroleukemia cell line PC4 were sequentially selected in increasing vincristine concentrations (5-160 ng/ml). The low- and intermediate-level resistant cell lines, selected in < or = 40 ng/ml of vincristine, demonstrated resistance to Vinca alkaloids and to an epipodophyllotoxin but little or none to an anthracycline. The expression of murine mdr genes, as analyzed by Northern blotting, revealed a baseline expression of murine mdr2 in parental cells that was unchanged in the drug-resistant cell lines. Overexpression of mdr3 was observed only in the highest-level resistant cell line, PC-V160, whereas mdr1 mRNA was not detected in any of the cell lines. The polymerase chain reaction, using mdr3-specific primers, excluded the possibility that low levels of P-glycoprotein expression contributed to the resistance phenotype in the low and intermediate-level resistant cell lines. Northern blot analysis using a human complementary DNA probe for the multidrug resistance-associated protein (MRP) demonstrated overexpression of murine mrp in each of the vincristine-selected sublines. Genomic amplification of the mrp gene was coincident with mrp overexpression. The expression of mrp was also examined in two series of previously characterized doxorubicin-selected cell lines derived from parental PC4 and C7D murine erythroleukemia cells. In contrast to the vincristine-selected cell lines, overexpression of mrp was not detected. These studies demonstrate that, in murine erythroleukemia cells selected for vincristine resistance, overexpression of murine mrp occurred prior to that for murine mdr. In contrast to human MRP, selection for vincristine, but not doxorubicin resistance, resulted in the overexpression of murine mrp.

Modulation by LY335979 of P-glycoprotein function in multidrug-resistant cell lines and human natural killer cells.

Resistance to chemotherapy by some human tumors may be due to overexpression of membrane-associated transport proteins. The best characterized of these is the multidrug resistance (MDR) transporter, P-glycoprotein (Pgp). The aim of this study was to measure the inhibitory effects of a potent new MDR modulator, (2R)-anti-5-(3-[4-(10,11-difluoromethanodibenzo-suber-5-yl) piperazin-1-yl]-2-hydroxypropoxy)quinoline trihydrochloride (LY335979), in the drug-resistant cell line HL60/VCR and in normal, human CD56(+) lymphocytes. We used flow cytometric methods to detect the accumulation of rhodamine 123 and daunorubicin, fluorescent MDR substrates, in these cells. Our results indicate that LY335979 was 500-1500 times more potent than cyclosporin A or verapamil in restoring Pgp substrate accumulation in the MDR cell line HL60/VCR. Moreover, LY335979 could effectively block Pgp function on isolated CD56(+) lymphocytes (IC(50) = 1.2 nM) or CD56(+) lymphocytes in whole blood (IC(50) = 174 nM). We conclude that LY335979 is among the most potent Pgp inhibitors described and that it maintains significant potency in whole-human blood. These latter findings are important for establishing the dosing regimens of LY335979 for future clinical studies.

Role of P-glycoprotein in dolastatin 10 resistance.

Dolastatin 10, a cytotoxic pentapeptide isolated from the mollusk Dolabella auricularia, exhibits potent antitumor activity. The present studies demonstrated that sublines of murine PC4 and human U-937 leukemia cells expressing a multidrug resistance (MDR) phenotype are cross-resistant to this agent. We also demonstrated that such resistance was reversed by verapamil. While these findings suggested the involvement of the P-glycoprotein (P-gp) in dolastatin 10 resistance, we performed similar studies in a CHO cell line transfected with the human mdr1 cDNA. Expression of P-gp in the transfected cells was associated with resistance to dolastatin 10 by a verapamil-sensitive mechanism. The demonstration that photoaffinity labeling of P-gp was decreased in the presence of dolastatin 10 further supports the interaction of this cytotoxic peptide with P-gp. Taken together, these findings suggest that resistance to dolastatin 10 is conferred, at least in part, by P-gp and that this cytotoxic peptide is a novel member of the MDR phenotype.

Increased efflux of vincristine, but not of daunorubicin, associated with the murine multidrug resistance protein (MRP).

The multidrug resistance protein (MRP) is a membrane protein that mediates altered transport of cytotoxic drugs. Although MRP overexpression has been described in doxorubicin-selected human tumor cell lines, the murine PC-V10 and PC-V40 cell lines are members of the only reported series of vincristine-selected cell lines that overexpress mrp. Western blotting, using an antiserum developed against human MRP, demonstrated high-level expression of murine MRP primarily in the plasma membranes in each of the vincristine-selected cell lines. Only PC-V160, selected for high level resistance, demonstrated concomitant overexpression of the P-glycoprotein. As compared with parental cells, each of the drug-selected cell lines demonstrated an energy-dependent, decreased net accumulation of vincristine without any changes in the initial rates of vincristine influx. However, there was an enhanced rate of vincristine loss, 2.3-fold from the PC-V40 cell line and 3.9-fold from the PC-V160 cell line. Selective plasma membrane permeabilization with digitonin equalized vincristine accumulation among the parental, the PC-V40, and the PC-V160 cell lines. No intracellular pH differences were detected among the cell lines. Despite high-level MRP expression, daunorubicin accumulation and the rate of daunorubicin loss in the PC-V40 cells were the same as that observed in parental PC4 cells. Fluorescence microscopy demonstrated no difference in the pattern of subcellular daunorubicin accumulation between parental and PC-V40 cells. These studies demonstrate that murine MRP, overexpressed and found predominantly in the plasma membrane of vincristine-selected PC-V40 cells, is associated with an energy-dependent increased efflux of vincristine, but not with efflux or altered distribution of daunorubicin.

Differential changes in genome structure and expression of the mdr gene family in multidrug-resistant murine erythroleukemia cell lines.

The genome structure and expression of mdr genes were examined in multidrug-resistant sublines of two different murine (DBA/2J) Friend erythroleukemia cell lines, PC4 and C7D, derived by stepwise exposure to increasing concentrations of adriamycin beginning with 5 ng/ml. The PC4 cell lines selected in higher drug concentrations (80-1280 ng/ml) demonstrated amplification of all three mdr genes with preferential amplification of mdr3. Overexpression of the mdr2 and mdr3 genes accompanied their genomic amplification; however, expression of mdr1 was not seen despite amplification. In the C7D cell lines selected with higher drug concentrations (40-160 ng/ ml), amplification and overexpression of mdr1 and mdr2 without mdr3 was observed. Increased expression of mdr1 occurred prior to gene amplification. The distribution of mdr-specific genes in micrococcal nuclease-generated chromatin fractions differing in transcriptionally active sequences and proteins was different between the parent and drug-resistant sublines. An enrichment (two- to threefold) of mdr3 genes in the H1-depleted mononucleosome fraction enriched for actively transcribed genes (e.g., globin) was detected by Southern analysis of chromatin fractions in PC4-80 cells (selected in 80 ng/ml of adriamycin and overexpressing mdr3), compared to the parental cells. mdr3 enrichment was also detected using a new PCR-based method, which examined mdr3 genes and repetitive sequences. Of note, the H1-depleted chromatin fraction from PC4-20 showed enrichment of the mdr3 gene, although mdr3 expression was not detected in the cell line. These studies showed a different pattern of gene amplification and overexpression in genetically related erythroleukemia cell lines selected for resistance to the same chemotherapeutic agent. A change in chromatin organization of mdr genes preceded overexpression and amplification of the mdr3 gene.

A phase I-II trial of continuous-infusion cisplatin, continuous-infusion 5-fluorouracil, and VP-16 in colorectal carcinoma.

Twenty-nine evaluable patients with colorectal adenocarcinoma were treated in a phase I-II trial of combination chemotherapy with a 72-h continuous infusion of cisplatin (CDDP) and 5-fluorouracil (5-FU) with an infusion of VP-16 given at 24 and 48 h after the start of therapy. There were five (17 +/- 14%) partial responses lasting 2-6 months (median, 3). Three of these responses occurred among the 10 previously untreated patients. The toxicity of this regimen was pronounced. Four of eight patients with severe neutropenia required hospitalization for infections, two of which were life-threatening; one of six patients with severe thrombocytopenia had a life-threatening hemorrhagic complication; and four patients experienced severe, dose-limiting fatigue. These complications occurred principally with CDDP and VP-16 at doses above 27.5 mg/m2/day and 110 mg/m2/dose, respectively. Mucositis occurred in six patients and limited the dose of 5-FU to 1,300 mg/m2/day. Although the response rate appeared to be high in previously untreated patients, the minimal palliative benefit of treatment and the brief duration of the responses do not compensate for the toxicity observed.

A phase II trial of continuous infusion cisplatin and 5-fluorouracil with oral calcium leucovorin in colorectal carcinoma.

Twenty previously untreated patients with advanced colorectal adenocarcinoma were entered on a Phase II trial of 3-day continuous infusion cisplatin (25 mg/m2/day) and 5-fluorouracil (800 mg/M2/day) with oral calcium leucovorin (30 mg/dose) every 6 hours. There were four partial responses (20%) and two complete responses (10%) for a total response rate of 30% (95% confidence limits +/- 20%). Patients received a median of 4.5 cycles of therapy (range 2-9 cycles). Three patients experienced neutropenia; one had a life-threatening infection. One developed neuropathy at 375 mg/M2 cumulative dose. Four patients developed mucositis. Treatment was stopped for one patient with stable disease after 5 cycles because of anorexia and nausea and vomiting; treatment was stopped for four patients because of excessive fatigue. The median duration of responses was 4 months (range 3-6 months). Although this regimen is active, the response rate, cumulative nature of the toxicity, and the requirement for hospitalization led us to conclude that this regimen does not warrant Phase III testing but might be a basis for further Phase II therapeutic trials.

Interferon protects normal human granulocyte/macrophage colony-forming cells from Ara-C cytotoxicity.

Interferons (IFN) have clinical efficacy in certain hematologic malignancies. Combining IFN with conventional cytotoxic agents has been proposed as a means of improving therapy for diseases such as chronic myelogenous leukemia (CML). In this study, we examined the effect of recombinant interferons alone and in combination with Ara-C on normal and leukemic human hematopoietic progenitor cells (CFU-GM) in vitro. Mononuclear cells from normal bone marrow, peripheral blood of patients with CML, or the acute nonlymphocytic leukemia cell line HL-60 were incubated with alpha-, beta-, or gamma-IFN (0-1,000 units/ml) followed by the addition of Ara-C. The survival of normal CFU-GM was significantly increased if cells were treated with IFN 1 h before 3 h of Ara-C exposure. Similar IFN pretreatment of CML and HL-60 progenitors failed to protect leukemic CFU-GM from Ara-C-induced toxicity. This selective protection of normal CFU-GM may have clinical application.

Effect of glucose transport inhibitors on vincristine efflux in multidrug-resistant murine erythroleukaemia cells overexpressing the multidrug resistance-associated protein (MRP) and two glucose transport proteins, GLUT1 and GLUT3.

The relationship between mammalian facilitative glucose transport proteins (GLUT) and multidrug resistance was examined in two vincristine (VCR)-selected murine erythroleukaemia (MEL) PC4 cell lines. GLUT proteins, GLUT1 and GLUT3, were constitutively coexpressed in the parental cell line and also in the VCR-selected cell lines. Increased expression of the GLUT1 isoform was noted both in the PC-V40 (a non-P-glycoprotein, mrp-overexpressing subline) and in the more resistant PC-V160 (overexpressing mrp and mdr3) cell lines. Overexpression of GLUT3 was detected only in the PC-V160 subline. An increased rate of facilitative glucose transport (Vmax) and level of plasma membrane GLUT protein expression paralleled increased VCR resistance, active VCR efflux and decreased VCR steady-state accumulation in these cell lines. Glucose transport inhibitors (GTIs), cytochalasin B (CB) and phloretin blocked the active efflux and decreased steady-state accumulation of VCR in the PC-V40 subline. GTIs did not significantly affect VCR accumulation in the parental or PC-V160 cells. A comparison of protein sequences among GLUT1, GLUT3 and MRP revealed a putative cytochalasin B binding site in MRP, which displayed 44% sequence similarity/12% identity with that previously identified in GLUT1 and GLUT3; these regions also exhibited a similar hydropathy plot pattern. The findings suggested that CB bound to MRP and directly or indirectly lowered VCR efflux and/or CB bound to one or both GLUT proteins, which acted to lower the VCR efflux mediated by MRP. This is the first report of a non-neuronal murine cell line that expressed GLUT3.

Expression of the multidrug resistance associated protein and P-glycoprotein in doxorubicin-selected human myeloid leukemia cells.

Drug-resistant sublines of the human U-937 myeloid leukemia cell line were selected in doxorubicin concentrations of 10, 40, and 200 ng/mL (designated U-A10, U-A40, and U-A200, respectively). Northern blot analysis showed overexpression of the multidrug resistance-associated protein (MRP) gene, but not MDR1, in U-A10 cells as compared with parental U-937 cells. Prolonged passage of U-A10 cells in 10 ng/mL of doxorubicin had little effect on MRP RNA levels, but increased MDR1 expression. The U-A40 and U-A200 cells, derived by selection of U-A10 cells, showed high levels of both MRP and MDR1 expression. None of the drug-resistant cell lines showed MRP or MDR1 gene amplification as judged by Southern blot analysis. U-A10 cells exhibited minimal decreased net accumulation of anthracycline, whereas U-A40 and U-A200 cells showed more significantly decreased drug accumulation as compared with U-937 cells. Subcellular anthracycline accumulation in U-937 cells as determined by fluorescence microscopy showed daunorubicin fluorescence predominately in the nucleus. However, the drug-resistant cell lines showed minimal nuclear drug accumulation with marked redistribution of drug into a vesicular compartment. Treatment with sodium azide/2-deoxyglucose, 2,4-dinitrophenol, or monensin, but not verapamil, abolished the vesicular accumulation. These studies in doxorubicin-selected U-937 cells indicate that induction of MRP overexpression occurs before that for the MDR1 gene. In addition, the drug-resistant cells possess an energy-dependent redistribution of anthracyclines into a nonnuclear vesicular compartment.

Differential protection of normal and malignant human myeloid progenitors (CFU-GM) from Ara-C toxicity using cycloheximide.

Cycloheximide, a reversible protein synthesis inhibitor, is thought to block DNA replication in normal cells by preventing synthesis of a labile protein. In animal systems, cycloheximide protects normal cells from cytotoxic S-phase specific agents, such as cytosine arabinoside (Ara-C). Malignant cells appear not to be susceptible to cycloheximide-induced cycle arrest and, subsequently, may not be protected from Ara-C cytotoxicity. The effect of cycloheximide on granulocyte/macrophage progenitors (CFU-GM) after in vitro Ara-C exposure was examined using normal human bone marrow, malignant progenitors from patients with chronic myelogenous leukemia (CML), and clonogenic cells from the human acute nonlymphocytic leukemia cell lines HL-60 and KG-1. Mononuclear or clonogenic cells were incubated for one hour with cycloheximide, followed by the addition, for three or 17 hours, of Ara-C before being plated in a methylcellulose culture system. CFU-GM survival was significantly increase if normal cells were treated with cycloheximide before Ara-C exposure. Similar cycloheximide pretreatment of CML progenitors and clonogenic HL-60 and KG-1 cells failed to protect CFU-GM from Ara-C-induced cytotoxicity.

Energy-dependent accumulation of daunorubicin into subcellular compartments of human leukemia cells and cytoplasts.

Anthracycline accumulation was evaluated by flow cytometry or radiolabeled drug assays in cells and cytoplasts (enucleated cells) prepared from parental and multidrug-resistant human K562 leukemia cells. Treatment with energy inhibitors, such as dinitrophenol (DNP) or sodium azide/deoxyglucose, led to a marked decrease in daunorubicin accumulation in parental cells and cytoplasts. Another ionophore, monensin, also caused a significant decrease in daunorubicin accumulation; however, ATPase inhibitors ouabain, vanadate, and N-ethylamaleimide had little or no effect. The lysosomatropic agents chloroquine and methylamine caused a moderate decrease in anthracycline accumulation. Fluorescence microscopy showed that the DNP-sensitive daunorubicin uptake occurred in a nonnuclear subcellular compartment. Studies using increasing daunorubicin concentrations demonstrated fluorescence quenching that occurred in the nonnuclear, DNP-sensitive compartment. The effect of inhibitors on the accumulation of rhodamine 123 and acridine orange strongly implicated lysosomes as the principal compartment of this inhibitable daunorubicin accumulation. Cytoplasts from P-glycoprotein containing multidrug-resistant K562 cells demonstrated a verapamil-reversible, decreased daunorubicin accumulation that was observed in resistant whole cells. Verapamil pretreatment of cytoplasts from resistant cells revealed the subcellular DNP-sensitive uptake present in parental cytoplasts. These studies demonstrate that cytoplasts are an effective means to study drug transport in mammalian cells without nuclear drug binding. Parental K562 cells and cytoplasts exhibit an energy-dependent accumulation of daunorubicin into cytoplasmic organelles that is also present in resistant cells and cytoplasts when P-glycoprotein mediated efflux is inhibited.

Treatment of acute myeloid leukemia in the elderly with low-dose cytarabine, hydroxyurea, and calcitriol.

Twenty-nine patients aged 62-82 years with acute myeloid leukemia (AML) were treated with a 21-day course of continuous infusion cytarabine, oral hydroxyurea, and 1,25-dihydroxyvitamin D3 (calcitriol). Ten patients had an antecedent myelodysplastic syndrome. Calcitriol was continued as the only postremission therapy. Thirteen patients (45%) obtained a complete remission, and 10 patients (34%) had a partial response for an overall 79% response rate. There were three early deaths. The median remission duration was 9.8 months. Overall median survival was 12 months for all patients and 14 months for responding patients. All responding patients had marked bone marrow hypoplasia. Twenty patients received part or all of their chemotherapy as outpatients. This regimen has acceptable toxicity and can result in prolonged remissions in elderly, high-risk patients with AML. The favorable results may be related to the synergistic effect of hydroxyurea, cytarabine, and calcitriol.

Expression of c-fos precedes MDR3 in vincristine and adriamycin selected multidrug resistant murine erythroleukemia cells.

Expression of murine P-glycoprotein (P-gp), encoded by mdrl or mdri3, confers a multidrug resistance phenotype. Higher expression of c-fos and c-jun has also been demonstrated in multidrug resistant human and murine cells. We detected increased expression of c-fos early in the derivation of two series of murine erythroleukemia sublines selected for resistance to vincristine or adriamycin which eventually overexpress mdr3. We speculate that early expression of c-fos prepares cells for overexpression of other genes, such as mdr3, that contribute to the multidrug resistance phenotype.

Vesicular anthracycline accumulation in doxorubicin-selected U-937 cells: participation of lysosomes.

The U-A10 cell line, a doxorubicin-selected variant of human U-937 myeloid leukemia cells, exhibits a redistribution of anthracyclines into a expanded vesicular compartment. The acidic nature of this compartment was confirmed by vital staining with a pH sensitive dye, LysoSensor yellow/blue DND-160. Identification of the vesicular compartment was performed by immunofluorescence analysis. Staining for the LAMP-1 and LAMP-2 antigens showed that the vesicles are enlarged lysosomes that are eccentrically placed near the nucleus of U-A10 cells. By contrast, the expression of the multidrug resistance-associated protein and the P-glycoprotein was observed predominately on the plasma membrane of the drug-resistant cells. The accumulation of daunorubicin into cellular compartments was quantified using radiolabeled drug. Exposing cells to 3[H]-daunorubicin and then isolating intact nuclei showed that nuclei from U-A10 cells accumulated twofold to threefold less anthracycline than nuclei from U-937 cells. However, when nuclei were isolated first and then exposed to 3[H]-daunorubicin, little difference in net nuclear drug accumulation was detected. Cytoplasts prepared from U-A10 and U-937 cells were exposed to 3[H]-daunorubicin to measure cytoplasmic drug accumulation. At external daunorubicin concentrations of 100 ng/mL or higher, cytoplasts from U-A10 cells accumulated significantly more daunorubicin than cytoplasts from U-937 cells. Moreover, studies with the lysosomotropic agent chloroquine showed that U-A10 cells accumulated twofold more chloroquine and showed twofold enhanced sensitivity to this agent as compared with parental U-937 cells. Fluorescence microscopy showed that chloroquine affects vesicular anthracycline sequestration in U-A10 cells with an associated increase in daunorubicin nuclear fluorescence. Although chloroquine did not alter anthracycline cytotoxicity in parental cells, it restored daunorubicin and doxorubicin sensitivity to U-A10 cells. Taken together, these studies demonstrate that U-A10 cells exhibit a redistribution of the lysosomal compartment. The trapping of drug into an expanded acidic vesicular compartment results in decreased nuclear drug accumulation and decreased cytotoxicity. Lysosomotropic agents, such as chloroquine, warrant further study as modulators of this acquired drug-resistance phenotype.

Defective translocation of protein kinase C in multidrug-resistant HL-60 cells confers a reversible loss of phorbol ester-induced monocytic differentiation.

Previous studies have demonstrated that human HL-60 myeloid leukemia cells differentiate in response to phorbol esters. This event is associated with induction of the c-jun early response gene and appearance of a monocytic phenotype. The present studies have examined the effects of vincristine-selected, multidrug resistance on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced HL-60 cell differentiation. The results demonstrate that multidrug-resistant HL-60 cells, designated HL-60/vinc, fail to respond to TPA with an increase in c-jun transcripts or other phenotypic characteristics of monocytic differentiation. By contrast, treatment of HL-60/vinc cells with okadaic acid, an inhibitor of serine/threonine protein phosphatases, induces c-jun transcription, growth arrest, and expression of the c-fms gene. Studies were also performed with an HL-60/vinc revertant (HL-60/vinc/R) line that has regained partial sensitivity to vincristine. The finding that HL-60/vinc/R cells respond to TPA with induction of a monocytic phenotype, but not c-jun expression, suggests that c-jun induction is not obligatory for monocytic differentiation. Other studies further demonstrate that the jun-B and fra-1 genes are induced by TPA in both HL-60/vinc and HL-60/vinc/R cells, whereas c-fos expression is attenuated in the HL-60/vinc line. Since TPA activates protein kinase C (PKC), we examined translocation of PKC from the cytosol to the membrane fraction. Although HL-60 and HL-60/vinc/R cells demonstrated translocation of PKC activity, this subcellular redistribution was undetectable in HL-60/vinc cells. Activity of the mitogen-activated protein kinase family with associated phosphorylation of c-Jun Y-peptide was markedly diminished in TPA-treated HL-60/vinc cells, but not in response to okadaic acid. Taken together, these findings suggest that vincristine resistance confers insensitivity to TPA-induced differentiation and can include defects in PKC-mediated signaling events and induction of jun/fos early response gene expression.

A syndrome of lymphoblastic lymphoma, eosinophilia, and myeloid hyperplasia/malignancy associated with t(8;13)(p11;q11): description of a distinctive clinicopathologic entity.

We report two patients with a distinctive biphenotypic hematologic disorder characterized by lymphoblastic lymphoma (LBL), eosinophilia, and myeloid malignancy and/or hyperplasia associated with a t(8;13)(p11;q11) chromosomal translocation in both bone marrow and lymph node specimens. Both patients presented with lymphadenopathy pathologically classified as LBL with a T-cell immunophenotype, myeloid hyperplasia of the bone marrow, and peripheral blood eosinophilia. The first patient achieved clinical complete remission after receiving several regimens of chemotherapy and remains disease-free 16 months after undergoing allogeneic bone marrow transplantation. The second patient developed progressive lymphadenopathy despite several courses of chemotherapy directed against non-Hodgkin's lymphoma. Eight months after his initial presentation, he developed acute myelogenous leukemia that was refractory to therapy. Comparison of these patients with four similar cases recently reported in the literature suggests that this constellation of findings constitutes a distinctive clinicopathologic syndrome. Molecular analysis of the t(8;13) translocation breakpoint may identify genes located in this region and provide insight into the pathogenesis of this interesting biphenotypic hematologic malignancy.

Overexpression of Bcl-XL by cytotoxic drug exposure confers resistance to ionizing radiation-induced internucleosomal DNA fragmentation.

Acquired resistance to diverse chemotherapeutic agents has been associated with overexpression of the P-glycoprotein. We have selected human U-937 cells for clones resistant to the cytotoxic agents doxorubicin (U-A20) and vincristine (U-V20). The results demonstrate that P-glycoprotein-positive U-A20 and U-V20 cells exhibit resistance to inducers of internucleosomal DNA fragmentation. Although parental U-937 cells responded to ionizing radiation with the DNA laddering characteristic of physiological cell death, the drug-resistant lines were insensitive to this effect. The U-A20 and U-V20 clones were also resistant to endonucleolytic DNA cleavage associated with exposure to tumor necrosis factor or ceramide. Previous work has demonstrated that physiological cell death is inhibited by overexpression of the Bcl-2 protein. However, analysis of Bcl-2 revealed similar levels in the parental and drug-resistant cells. In contrast, we show that U-A20 and U-V20 cells overexpress the Bcl-2-related protein, Bcl-xL. Moreover, studies with a U-937 cell line transfected with a Bcl-XL expression vector confirm resistance to ionizing radiation-induced DNA fragmentation and cell killing. These findings suggest that, unlike Bcl-2, Bcl-XL may be constitutively overexpressed as a result of selection for cytotoxic drug resistance and that Bcl-XL participates in an acquired form of multimodality resistance to chemotherapeutic agents and radiation.