Regulation of drug sensitivity by ribosomal protein S3a

When bcl-2 is immunoprecipitated from (32)P-labeled cell extracts of all-trans retinoic acid (ATRA)-treated acute myeloblastic leukemia (AML) blasts, a phosphorylated protein of approximately 30 kd is coprecipitated. This protein has been identified as ribosomal protein S3a. The biologic effects of S3a include favoring apoptosis and enhancing the malignant phenotype. We sought to determine whether S3a, like bcl-2, influenced the response of cells to chemotherapeutic drugs and ATRA. Cell lines were studied in which S3a was genetically increased or disrupted; increased S3a was regularly associated with increased plating efficiency and increased sensitivity to either cytosine arabinoside (ara-C) or doxorubicin (DNR). S3a did not affect the sensitivity of cells to paclitaxel. Pulse exposures to either (3)HTdR or ara-C showed a greater percentage of clonogenic cells in the S phase of the cell cycle in cells with increased S3a than in controls. Cells with increased S3a responded to ATRA by increased ara-C or DNR sensitivity, whereas cells with reduced S3a protein were either protected by ATRA or not affected. We studied cryopreserved blast cells from patients with AML or chronic myelomonocytic leukemia (CMML). S3a protein levels were heterogeneous in these populations. In 32 cryopreserved blast populations, S3a levels were significantly correlated with both bcl-2 and with cell growth in culture. As in cell lines, high S3a in cryopreserved blasts was associated with ATRA-induced sensitization to ara-C. No significant association was seen between S3a levels and response to treatment.

Antileukemic action of buthionine sulfoximine: evidence for an intrinsic death mechanism based on oxidative stress

The glutathione-depleting agent buthionine sulfoximine (BSO) was found to be toxic to some AML blast populations. This toxicity was manifested as the appearance of high levels of reactive oxygen generation in GSH-depleted cells, and later by the loss of mitochondrial membrane potential and an increase in intracellular calcium. Striking heterogeneity in BSO sensitivity was observed in a series of four human AML cell lines, and in fresh leukemic blasts obtained from eight AML patients. In some cases, toxicity was seen at BSO concentrations as low as 1 microM; approximately 100-fold less than the plasma levels achieved in patients treated with BSO as a drug resistance reversing agent. Based on these results we propose that some AML blast populations are unusually dependent on GSH-based antioxidant mechanisms, due to high intrinsic rates of reactive oxygen generation. The mitochondrial respiratory chain is the most likely source of this reactive oxygen. Because toxicity is seen at clinically achievable concentrations of BSO, this agent might have antileukemic activity in patients.

Phosphorylation of BCL-2 after exposure of human leukemic cells to retinoic acid

Serine phosphorylation of bcl-2 has been reported after treatment of cells with protein kinase C, okadaic acid, taxol, and other chemotherapeutic agents that attack microtubules. We report here that bcl-2 is phosphorylated on serine in acute myeloblastic leukemia (AML) blasts exposed to all trans retinoic acid (ATRA). Two-dimension gels (isoelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE]) disclosed a novel acidic isoform of bcl-2 in ATRA-treated blast cells from a continuous line and from two AML patients; when the cell lysates were digested with lambda-phosphatase, bcl-2 reverted to the control position, indicating that it was phosphorylated. Metabolic labeling experiments using 32Pi showed that, while control bcl-2 was labeled, incorporation was greatly increased when cells were treated with ATRA. A comparison of bcl-2 from blasts treated with ATRA or taxol showed that bcl-2 was phosphorylated on serine in cells treated with either agent; however, both qualitative and quantitative differences were seen. Qualitatively, the phosphorylated isoform from taxol-treated cells was slightly larger than the native isoform and could be distinguished on 10% to 20% SDS-polyacrylamide gradient gels, while the phosphorylated bcl-2 after ATRA ran as a single band on gradient gels at the same position as control bcl-2. Quantitatively, all bcl-2 from ATRA-treated cells was in the phosphorylated isoform, while after taxol, both phosphorylated and native bcl-2 was present; incorporation of 32Pi into bcl-2 was stimulated to greater extent in ATRA-treated compared with taxol-treated cells. We used immunoprecipitation experiments to ask if bcl-2 phosphorylated after ATRA or taxol had altered capacity to dimerize with bax. No change in dimerization was demonstrated. We conclude that: bcl-2 is phosphorylated on serine after treatment of AML blasts with ATRA; bcl-2 phosphorylation after ATRA is different from that seen after taxol; bcl-2 phosphorylated after either agent retains capacity to dimerize with bax. The ATRA or taxol-induced phosphorylation of bcl-2 can also be seen in blast cells obtained from AML patients.

Toward a leukemia treatment strategy based on the probability of stem cell death: an essay in honor of Dr. Emil J Freireich

Dr. Emil J Freireich is a pioneer in the rational treatment of cancer in general and leukemia in particular. This essay in his honor suggests that the cell kill concept of chemotherapy of acute myeloblastic leukemia be extended to include two additional ideas. The first concept is that leukemic blasts, like normal hemopoietic cells, are organized in hierarchies, headed by stem cells. In both normal and leukemic hemopoiesis, killing stem cells will destroy the system; furthermore, both normal and leukemic cells respond to regulators. It follows that acute myelogenous leukemia should be considered as a dependent neoplasm. The second concept is that cell/drug interaction should be considered as two phases. The first, or proximal phase, consists of the events that lead up to injury; the second, or distal phase, comprises the responses of the cell that contribute to either progression to apoptosis or recovery. Distal responses are described briefly. Regulated drug sensitivity is presented as an example of how distal responses might be used to improve treatment.

Relationships between the mitochondrial permeability transition and oxidative stress during ara-C toxicity

The mitochondrial permeability transition and oxidative stress seem to be critical alterations in cellular physiology that take place during programmed cell death. Failure to undergo apoptosis is associated with drug resistance in acute myeloid leukemia and other cancers. Therefore, it is important to establish causal relationships between the physiological changes that take place in apoptosis, because these are potential targets for novel treatment strategies to overcome this form of drug resistance. We describe the use of multilaser flow cytometry methods to make correlated measurements of mitochondrial membrane potential (MMP), the generation of reactive oxygen intermediates, the cellular content of reduced glutathione (GSH), intracellular calcium, and exposure of phosphatidylserine on the cell surface. Using these combined methods, we have mapped a "death sequence" that occurs after treatment of leukemic blasts with clinically relevant concentrations of 1-beta-D-arabinofuranosylcytosine (ara-C). Dual labeling of MMP and cellular glutathione content showed that loss of MMP, indicative of the permeability transition, took place in cells that were depleted of glutathione. The loss of MMP coincided with phosphatidylserine exposure and preceded a state of high reactive oxygen generation. Finally, there was an increase in intracellular calcium. These results demonstrate that the mitochondrial permeability transition takes place during ara-C toxicity but suggest that this occurs downstream of the loss of GSH. Thus, oxidative stress after ara-C-induced toxicity seems to be a biphasic phenomenon, with the permeability transition occurring after a depletion of GSH and preceding a state of high reactive oxygen generation.

Regulation of the synthesis of bcl-2 protein by growth factors

The sensitivity of AML blast stem cells can be measured in cell culture, using a clonogenic assay to determine survival after each of a graded series of drug concentrations. For cytosine arabinoside, the dose-response curve is a simple negative exponential that can be described by a D10 value, a measure of slope. This D10 value can be affected by regulatory molecules added to the cultures. All-trans retinoic acid (ATRA) usually sensitizes cells, while hydrocortisone (HC) is protective. Growth factor responsive cells are more Ara-C sensitive in G-CSF than in GM-CSF or IL-3. The proto-oncogene bcl-2 may be part of the mechanism by which drug sensitivity is regulated. Previous work has shown that ATRA decreases bcl-2 RNA expression and the half-life of the protein; in contrast, the protein from cells treated with HC is more stable than controls. Growth factors were not shown to change either expression of bcl-2 RNA or the stability of its protein. In this paper, we describe experiments where OCI/AML-1 cells were grown in G-CSF and then transferred to medium containing both G-CSF and the GM-CSF-IL-3 fusion protein pIXY. Steady-state levels of bcl-2 protein were measured by Western blot and synthesis by incorporation of 35S methionine into protein. We observed that both measures doubled within 12-24 h after transfer from G-CSF in G-CSF with pIXY, but promptly returned to the previous state when pIXY was withdrawn. We conclude that growth factors regulate that activity of bcl-2 post-transcriptionally by altering the rate of synthesis of the protein.

A role for paclitaxel in the combination chemotherapy of acute myeloblastic leukaemia: preclinical cell culture studies

Paclitaxel dose responses in culture have been investigated alone and in association with cytosine arabinoside (ARA-C) and all-trans retinoic acid (ATRA), with the objective of identifying a role for paclitaxel in the treatment of acute myeloblastic leukaemia (AML). Initial studies were done to determine if paclitaxel dose responses of AML blast cell precursors were altered by regulatory compounds known to modify the dose responses of ARA-C. In contrast to ARA-C, paclitaxel dose responses were independent of cell culture method, the growth factors G-CSF and GM-CSF, and the ligands all-trans retinoic acid (ATRA) and hydrocortisone. Most blast cell populations were sensitive to paclitaxel; compared with normal marrow progenitors the dose responses were markedly heterogenous with some more, and others less, sensitive. Remission marrow progenitor paclitaxel responses resembled those of AML blasts in heterogeneity. The cell culture model tested the effect of pacliataxel and ATRA on the ARA-C dose responses of OCI/ AML-5; paclitaxel exposure was either before or after ARA-C to test for an effect of schedule; ATRA was added to the MEC cultures after paclitaxel and ARA-C. Repeat experiments were done to test three dose levels each of paclitaxel and ATRA. When paclitaxel was given after ARA-C, synergism was found for all but one of the dose combinations tested; only three examples of synergy were seen when paclitaxel preceded ARA-C. The studies justify trials combining ARA-C, paclitaxel and ATRA using a schedule suggested by the cell culture findings.

Generation of reactive oxygen intermediates after treatment of blasts of acute myeloblastic leukemia with cytosine arabinoside: role of bcl-2

Cytosine arabinoside is usually considered to be lethal by incorporation into DNA followed by chain termination. Recently, we have reported that the radical scavenger N-acetyl-cysteine (NAC) protects cultured clonogenic AML blast cells from the lethal affects of Ara-C if given before the drug. This observation provides indirect evidence that toxic reactive oxygen intermediates (ROI) are generated in AML blast cells following Ara-C-induced damage to DNA. In the present paper we present evidence in support of this hypothesis. Using flow cytometry and multiple fluorescent probes for live cell function, we have mapped a sequence of discrete stages that occur during Ara-C cytotoxicity. An early event was the increased generation of ROI. Initially this oxidative stress was countered by an increase in the cellular content of reduced glutathione (GSH), but cells then underwent an abrupt transition to a state characterized by low GSH and very high ROI generation indicative of collapse of cellular redox balance. Next, the capacity to maintain low intracellular ionized calcium was lost, probably due to lipid peroxidation at membrane sites of calcium regulation. Finally, surface membrane integrity was lost. Concurrent measurements of clonogenic cell survival insured the relevance of these flow cytometry measurements to the stem cell population. We used OCI/AML-2 cells transfected with bcl-2 to look for the place in this sequence where bcl-2 protein protects cells against apoptosis; bcl-2 transfectants showed an increase in ROI generation similar to controls, but were able to maintain GSH levels in the face of this oxidative stress. We conclude that oxidative stress plays a major role in Ara-C toxicity, and that bcl-2 protein protects cells by maintaining cellular redox balance in a reducing state. These studies complement previous work showing how regulators of AML growth affect the sensitivity of blast cells to Ara-C by changing the concentration or stability of bcl-2 protein.

Post-transcriptional regulation of bcl-2 in acute myeloblastic leukemia: significance for response to chemotherapy

The blast stem cells of acute myeloblastic leukemia become more sensitive in culture to the chemotherapeutic agents cytosine arabinoside (Ara-C) and daunorubicin (DNR) when exposed to all-trans retinoic acid (ATRA) after drug. We have proposed that down regulation of bcl-2 by ATRA is part of the mechanism of sensitization. The hypothesis is based on reduced expression of bcl-2 mRNA, as seen in Northern blots, after ATRA. Nuclear run on experiments, however, failed to account completely for the effect at the transcriptional level. Accordingly, we looked for post-transcriptional effects of ATRA on bcl-2, using metabolic labelling of the protein to measure stability. We found that the half-life of bcl-2 protein is markedly shortened after treatment with ATRA. Hydrocortisone (HC) protects cells against the toxic effects of Ara-C or DNR when given before drug. HC does not alter bcl-2 expression at the level of mRNA; however, metabolic labelling shows that newly synthesized bcl-2 protein is stabilized in blast cells treated with HC. Response to Ara-C by growth factor responsive blast cells is influenced by the factor in the cultures; cells are more sensitive in cultures with G-CSF and less sensitive when GM-CSF is present. We compared two blast cell lines, OCI/AML-5, primarily responsive to GM-CSF, and OCI/AML-10, primarily responsive to G-CSF. Growth factor did not influence the stability of bcl-2 protein in either line. In contrast, Western blots showed that the amount of bcl-2 protein was greater in cultures with GM-CSF or GM-CSF in combination with G-CSF than in cultures with G-CSF or no added factor. This pattern was seen regardless of the mitogenic response to G-CSF or GM-CSF. We interpret our findings as indicating that bcl-2 protein is transcriptionally activated; that the stability of the protein is decreased after ATRA and increased after HC; that the amount of bcl-2 protein is greater in cultures with GM-CSF than in cultures with G-CSF, regardless of which factor gives the greater mitogenic response. We propose that these post-transcriptional modifications of transcriptionally activated bcl-2 account, in part, for the regulation of drug sensitivity by ATRA, HC and growth factors.

Direct evidence for the participation of bcl-2 in the regulation by retinoic acid of the Ara-C sensitivity of leukemic stem cells

All-trans retinoic acid (ATRA) increases the sensitivity of AML blast cells to cytosine arabinoside (Ara-C) or daunorubicin (DNR) when ATRA is given after drug. We have proposed that down-regulation of bcl-2 is part of the mechanism by which ATRA regulates drug sensitivity. To test this hypothesis cDNA encoding bcl-2 was transfected into cells of the continuous lines OCI/AML-2 and OCI/AML-5. Four transfectant lines were isolated; three contained transfected bcl-2 in the sense orientation (AML5-BCL2sa, AML5-BCL2sb and 2-bcl2) and one with anti-sense bcl-2(AML5-bcl2as). The presence of the transfected gene was demonstrated by Northern blot; translation of the sense transfected genes into protein was demonstrated by Western blotting. Lines with sense-oriented transfected bcl-2 were significantly less sensitive to Ara-C or H2O2 than the parental lines; the cells with anti-sense transfected genes were more sensitive than their parent but the difference did not reach statistical significance. The effect of ATRA on bcl-2 expression was compared in sense-transfected cells and their parents; by Northern blotting it was shown that the endogenous but not the transfected genes were down-regulated after ATRA exposure. The capacity of cells with transfected genes to respond to ATRA was tested by obtaining Ara-C survival curves for ATRA-treated cells. Compared to controls not exposed to ATRA, the transfected cells showed little or statistically insignificant changes in Ara-C sensitivity after ATRA treatment. We conclude that data from the transfectants provides evidence that expression of bcl-2 is a determinant of sensitivity to Ara-C and H2O2; and that the effect of ATRA on sensitivity requires the presence of bcl-2 genes in association with regulatory elements.

Recombinant human retinoblastoma protein inhibits cancer cell growth

Aberrant expression of the tumor suppressor gene RB1 is associated with a variety of solid tumors and hematopoietic neoplasms. Certain cancer cell lines in which the protein encoded by RB1 (p110RB) is absent have been reported to show decreased growth rate, clonogenicity, or tumorigenicity following insertion of a transcriptionally active RB1 gene. We asked whether these RB-deficient cells could be growth inhibited by direct exposure to purified p110RB. We report a decrease in uptake of tritiated thymidine by 5637 bladder carcinoma cells (RB-negative) when purified recombinant p110RB is added to culture media. Internalization of the protein by cells and translocation to the nucleus are demonstrated by immunohistochemistry, FACS, and detection of radiolabeled protein in subcellular fractions. Next, we chose a well-described leukemia cell culture model to investigate the potential effect of recombinant p110RB in clinical disease. We observed dose-related decreases in cell number of colony formation in vitro in 8 of 20 acute myelogenous leukemia samples, 7 of which did show endogenous p110RB detectable by immunohistochemistry. Histological appearance following exposure to p110RB shows cytoplasmic vacuolization and nuclear lobulation of degenerating cells. We conclude that purified p110RB added to culture media is internalized by cells, translocated to the nucleus, and exerts a growth-inhibitory effect on certain cancer cell types.

Mechanism of cytosine arabinoside toxicity to the blast cells of acute myeloblastic leukemia: involvement of free radicals

Retinoic acid and hydrocortisone (HC) have been shown to regulate the drug sensitivity of the blast cells of acute myeloblastic leukemia (AML). We asked if the proto-oncogene bcl-2 played a role in this regulation. As target cells we used the continuous lines, OCI/AML-1, OCI/AML-2 or OCI/AML-5; expression of bcl-2 can be detected by Northern analysis of RNA from OCI/AML-2 or OCI/AML-5 cells; bcl-2 expression can be found in OCI/AML-1 cells only by using RT-PCR. Exposure of OCI/AML-2 or OCI/AML-5 cells to retinoic acid (all-trans retinoic acid, ATRA) led to a down-regulation of bcl-2 expression that was first seen after 2 h of exposure and was complete after a day. The down-regulation could be prevented by exposing the cells to ara-C either before or after ATRA; decrease in bcl-2 protein was moderate and only obvious after 36 h of ATRA treatment. Nuclear run-on experiments provided evidence that bcl-2 down-regulation was occurring at transcriptional and post-translational levels. Since bcl-2 is considered to have anti-oxidant activity, we tested the sensitivity of the three cell lines to H2O2; we found that OCI/AML-1, the line with very low bcl-2 expression, was a 100-fold more H2O2-sensitive than OCI/AML-2 or OCI/AML-5, where bcl-2 expression can be detected readily. We then asked if H2O2 sensitivity could be regulated. We found that exposure of cells to HC before H2O2 was protective while ATRA after peroxide treatment increased killing; this is the same pattern of regulation observed when AML blasts are exposed to HC before, or ATRA after ara-C. Finally, we asked whether N-acetylcysteine (NAC), a known radical scavenger would protect cells against ara-C killing. Significant protection was observed when NAC was given before drug, but not if given after drug. NAC protection against ara-C killing was seen for OCI/AML-1 and 2 cells, but not for OCI/AML-5 cells. We interpret the results as follows: ara-C kills cells in two ways: first, directly, by incorporation into DNA and chain termination; second, indirectly, by inducing the production of toxic radicals. Bcl-2 reduces the oxidant activity of such radicals, and is protective. ATRA regulates ara-C toxicity by its action on bcl-2. Left unexplained are the action of HC, which does not affect bcl-2 expression and the mechanism by which ara-C prevents down-regulation of bcl-2 by ATRA.

Sensitivities of AML blast stem cells to idarubicin and daunorubicin: a comparison with normal hematopoietic progenitors

The objective of this study was to determine whether or not cell culture studies could contribute to the correct choice between idarubicin (IDA) and daunorubicin (DNR) for combination with ara-C in remission induction therapy of AML. Two growth factor-sensitive AML cell lines and the peripheral blood blast cells from 10 patients with AML were studied in culture for sensitivity to IDA and DNR under four culture conditions; cells were grown either in methylcellulose or suspension culture in the presence of G-CSF or GM-CSF. Normal bone marrow cells were cultured in methylcellulose in the presence of IDA or DNR under conditions suitable for the detection of BFU-E and CFU-C. Dose-response curves for AML blast cells were characterized by an initial shoulder and then an exponential decrease in survival. Marked patient-to-patient variation was observed for both portions of the survival curves. IDA was significantly more toxic to blast cells than DNR, especially for more sensitive cell populations. Consistent differences in drug sensitivity in the four culture conditions were not observed. BFU-E and CFU-C dose-response curves of normal marrow progenitors resembled those of AML blast cells but in contrast fell within a narrow range. The culture studies support the clinical finding in AML of modest superiority of IDA over DNR. The heterogeneity in sensitivity of AML blasts in culture suggests an opportunity to individualize treatment. Preclinical studies may help in developing such a therapeutic approach.

Fluorescence-labeling of nicks in DNA from leukemic blast cells as a measure of damage following cytosine arabinoside. Application to the study of regulated drug sensitivity

Damage to DNA can be assessed using a technique for labeling nicks in DNA by incubating paraformaldehyde-fixed cells in a mixture of biotin-labeled dUTP, dATP with dNTP and DNA polymerase I. The addition of labeled nucleotides can then be identified by fluorescence by their reaction with streptavidin. We have used this method to examine damage to the DNA of OCI/AML-2 cells caused by cytosine arabinoside (ara-C) and the effects of hydrocortisone and retinoic acid on this damage (regulated drug sensitivity). Concurrent measurements of clonogenic cells were used to allow a comparison of damage as shown by labeled nicks in DNA with loss of colony-forming capacity. Both methods gave comparable ara-C dose-response curves, for cells incubated with the drug for 24 h. Both methods showed that exposure of OCI/AML-2 cells to hydrocortisone before ara-C greatly reduced the toxicity of the drug; and that retinoic acid given after ara-C increased both its lethal effects on colony formation and the extent of DNA damage as assessed by labeled nicks. Clonogenic assays required for colony formation are not readily adapted to the study of development and repair of damage. The labeled nick assay is suitable for such kinetic studies. OCI/AML-2 cells were exposed in suspension to either hydrocortisone before ara-C or retinoic acid after ara-C. At 24 h intervals thereafter, cells were harvested, assayed by both methods, and recultured after dilution to the original cell concentration. In cultures exposed only to ara-C (controls), the number of cells with labeled nicks increased during the first 24 h and cells with damaged DNA could be detected for 48-72 h, depending on the ara-C dose in spite of the dilution at each passage. OCI/AML-2 cells exposed to hydrocortisone before drug showed fewer nick-labeled cells than controls at the first observation and damaged cells rapidly disappeared from the population with increasing time. For cells treated with retinoic acid after ara-C, the nick-labeled cell population was greater than controls and remained greater throughout subsequent observations. We propose that in the control cultures, sublethal damage either became lethal with time and was seen as increased numbers of cells with damaged DNA, or alternatively, sublethal damage was repaired. From this point of view we consider that hydrocortisone promotes repair of sublethal damage while retinoic acid inhibits repair.

Regulation by retinoic acid and hydrocortisone of the anthracycline sensitivity of blast cells of acute myeloblastic leukemia

The experiments reported here continue the study of regulated drug sensitivity by extending the observations to anthracyclines. Previous work has shown that hydrocortisone (HC) protects AML blast stem cells from the lethal effects of cytosine arabinoside (ara-C) while retinoic acid (ATRA) increases ara-C sensitivity; further mechanisms of regulation of ara-C sensitivity might include increase or decrease in repair of sublethal damage. Anthracycline dose-response curves are characterized by an initial shoulder, followed by exponential decrease in survival with increasing dose. The shoulder portion of such curves may indicate the accumulation of sublethal damage. We used two assays to look for evidence of regulation of anthracycline sensitivity by HC or ATRA; the clonogenic assay for blast stem cells detects drug effects on this crucial population, but only after several days on incubation, during which time repair might occur. Measurements of nicks in DNA show damage in the bulk population of cells, but these can be detected very soon after exposure to drug. Both methods showed the HC protected cells in two continuous cell lines (OCI/AML-2 and OCI/AML-5) while ATRA made the cells more sensitive. Blast cells freshly-obtained from six AML patients were also tested. Both assays showed HC protection and ATRA sensitization in three populations. The clonogenic assay detected both effects in cells from a fourth patient; the nicked DNA assay confirmed both effects in a fifth patient, where the results of the clonogenic assay did not reach statistical significance. Neither ATRA nor HC influenced the sensitivity of blasts from a sixth patient; but these cells were highly resistant to drug. Kinetic studies showed that damage persisted longer after treatment with anthracyclines than with ara-C. OCI/AML-2 cells treated with HC before drug accumulated fewer cells with nicked DNA after daunorubicin (DNR). Cells exposed to ATRA after DNR showed increased toxicity in kinetic experiments. We conclude that sensitivity to anthracyclines may be regulated by ligands for steroid receptors. Furthermore, since growth factors do not regulate anthracyclines' sensitivity, different mechanisms may be operative for the action of ligands for cell surface receptors. Finally, we suggest that retinoic acid might be considered for inclusion in standard anthracycline/ara-C regimens for the treatment of AML.

Stem cell renewal and determination during clonal expansion in normal and leukaemic haemopoiesis

Normal haemopoiesis is a cellular hierarchy headed by pluripotent stem cells capable of both self renewal and, after determination, the generation of differentiating lineages that end in terminal functional cells. The role of stem cells is crucial because only these have the capacity to generate clonal populations during development or after injury. During clonal expansion the cells are affected by many sets of receptors and ligands. These belong to at least two classes: one consists of growth factors that bind cell surface receptors and initiate signalling events; the other class contains receptors which act as ligand-dependent transcription factors such as the intracellular steroid superfamily. In spite of this elaborate regulatory apparatus, control during clonal expansion is lax, perhaps stochastic, as evident from the great heterogeneity disclosed by examining the cellular compositions of haemopoietic clones. It may be that the large number of signals impinging on binary possible outcomes (for example self-renewal or determination) serve to set probabilities rather than to determine outcomes. In leukaemia, many of the features of normal haemopoiesis are retained. The disease begins as transformations in normal stem cells; after additional leukaemogenic events clonal expansion yields malignant populations which are clonal in each affected individual. These dominant clonal populations retain the hierarchical organization found in the normal, the major difference is that post-deterministic divisions in leukaemia yield descendants that retain primitive (blast) morphology although proliferative capacity is lost. In acute myeloblastic leukaemia (AML) cell culture methods are available that permit the measurement of clonogenic blast stem cells. These methods have shown that regulatory mechanisms active in normal haemopoiesis are retained in AML, including lax regulation during clonal expansion. The biological features of blast stems cells displayed by the culture technique reflect in part, events in vivo, as associations have been found between results in cell culture and clinical outcome. Thus, study of leukaemic populations provides a challenge for basic science and an opportunity for successful application in control of disease.

Influence of schedule on regulated sensitivity of AML blasts to cytosine arabinoside

Regulatory molecules that affect the growth culture of blast cells from acute myeloblastic leukemia (AML) may also alter drug sensitivity, a phenomenon that may be called regulated drug sensitivity. Previous studies have shown: (i) blast cells exposed to retinoic acid before cytosine arabinoside (Ara-C) usually show increased sensitivity, but after some retinoic acid exposure times, sensitivity may be decreased; (ii) factor-sensitive or responsive blasts cultured with granulocyte colony-stimulating factor (G-CSF) are regularly more Ara-C-sensitive than when cultured with granulocyte-macrophage CSF (GM-CSF). This paper is concerned with the effects of schedule on drug sensitivity as regulated by either retinoic acid or the myelopoietic growth factors, G-CSF and GM-CSF. We measured the effects of retinoic acid on the sensitivity of blasts cells from the two continuous AML lines to Ara-C or arabinofuranosyl 5-azacytosine (Ara-AC). Cells from seven patients with AML were tested for Ara-C sensitivity in conjunction with retinoic acid. The cells were treated with retinoic acid before or after administration of the drug. Both increases and decreases in Ara-C sensitivity were seen for both schedules. Consistent increases in Ara-C sensitivity were obtained when retinoic acid was included in the methylcellulose cultures used to determine clonogenic cell recovery at each drug dose. In studies of growth factors, a single factor-dependent cell line (OCI/AML-5) was used to compare the effects of G-CSF and GM-CSF on Ara-C sensitivity. An experimental design was used that permitted factors to present in culture for 24 h before Ara-C, during the next 24 h period with the drug, for a subsequent day in suspension without drug, and during the 5-7 days required for colony formation in methylcellulose cultures. G-CSF and GM-CSF were most effective in increasing or decreasing Ara-C, respectively, when the factor under test was included in the methylcellulose cultures. Thus, like retinoic acid, growth factors influenced drug sensitivity when they were present after the drug had been removed. These data, therefore, are compatible with the hypothesis that repair mechanism may contribute to regulated drug sensitivity.

A cell culture model for the treatment of acute myeloblastic leukemia with fludarabine and cytosine arabinoside

The purpose of this paper was to ascertain whether results obtained in cell cultures of AML clonogenic blast cells would provide a useful model for a clinical regimen that combines fludarabine (F-ara-AMP) and cytosine arabinoside (ara-C). In the cultures the nucleoside F-ara-A was used. Blast cells from the continuous lines OCI/AML-2 and OCI/AML-3 were grown, either in methylcellulose to quantify clonogenic cells, or in suspension to measure self-renewal as reflected in changes in numbers of clonogenic cells. F-ara-A, like ara-C, was found to be more toxic to blast stem cells in suspension than in the clonogenic assay, indicating that F-ara-A might, in addition to general cytotoxicity, have some specific inhibitory effects on self-renewing stem cells. F-ara-A was less cytotoxic than ara-C; but, when F-ara-A was given before ara-C, synergism was seen at some F-ara-A doses, as manifested by increased ara-C cytotoxicity. In contrast, when ara-C was given before F-ara-A, protection was observed. Control experiments make it unlikely that this effect is related to changes in the cell cycle following ara-C exposure. We conclude that the cellular studies reported here confirm previous pharmacological data indicating that F-ara-A before ara-C increases the effectiveness of ara-C by increasing the accumulation of ara-CTP. However the present experiments show that the synergism between F-ara-A and ara-C is dependent on both dose and schedule.

Relationship between c-Kit expression and proliferation in acute myeloblastic leukemia cell lines

We have proposed that the transmembrane receptor encoded by the c-Kit protooncogene and its ligand play an important role in regulating the proliferation of blasts cells in acute myeloblastic leukemia (AML). To test this hypothesis, immunobeads were used to separate blasts from three Kit-expression positive cell lines into strongly Kit-protein positive and weakly Kit-protein positive fractions. The strongly positive fraction had greater proliferative potential than the weakly positive fraction as assessed both by colony-formation in methylcellulose and growth of clonogenic cells in suspension. The reproducibility of the percentage of each blast population found in the strongly and weakly positive fractions provided evidence that Kit-protein expression is regulated. Kinetic experiments provided evidence for reversible transitions between strong and weak Kit protein expression. Thus regulated expression of the Kit receptor may be a mechanism for controlling blast cell growth in culture.

High-dose cytosine arabinoside remission induction for acute myelogenous leukemia: comparison of two regimens of remission maintenance

We report a single institution sequential trial of two maintenance treatment regimens for patients with acute myelogenous leukemia (AML). A total of 175 consecutive patients with AML received initial remission induction therapy with high-dose cytosine arabinoside (ara-C) and glucocorticoids. For the initial 63 patients (group A), the control population, planned maintenance treatment was with conventional-dose ara-C given over 4 days for up to 18 months. The subsequent 107 patients (group B) had planned maintenance therapy of up to 6 courses of daunorubicin, ara-C and prednisone and daily cis-retinoic acid for up to two years. The presenting features of group A and B patients were similar as were the response to remission induction, 60 and 52%, respectively. Severe neurological toxicity was encountered once after high-dose ara-C; no drug-related deaths occurred during maintenance treatment. Median duration of remission for group B patients was 9.9 months compared with 5.5 for group A (p = 0.0685). Median survival duration for the two groups was similar, 9.1 months for group A and 10.4 for group B. Survival of patients in group B who attained a complete remission was significantly better than that of patients in group A (p = 0.0439). The studies confirm our initial experience with remission induction using single agent high-dose ara-C and suggest a positive role for maintenance therapy in AML.

Comparison of the effects of all-trans and cis-retinoic acid on the blast stem cells of acute myeloblastic leukemia in culture

Recent work has shown that acute promyelocytic leukemia (APL) cells have a characteristic translocation involving the retinoic acid receptor on chromosome 17 and the myl protein on chromosome 15. Patients with APL respond to the administration of all-trans-retinoic acid. A cell line with t15;17 (NB4) has recently been reported; this line responds to all-trans-retinoic acid with differentiation. There is also a recent report showing that all-trans-retinoic acid is more active than cis-retinoic acid in inducing differentiation in freshly obtained APL cells. All-trans-retinoic and cis-retinoic acid are compared for their effects on growth in culture of freshly obtained AML cells, cell lines without t15;17, and NB4 cells. While all of these AML populations responded to both forms of retinoic acid, NB4 cells only were much more sensitive to all-trans-retinoic acid compared to cis-retinoic acid. The difference was seen when the NB4 cells were exposed in suspension and not when colony-formation in methylcellulose was used as an end point. Both forms of retinoic acid increased the sensitivity of blast cells to cytosine arabinoside; for NB4 cells, the sensitization was much greater when all-trans-retinoic acid was used rather than cis-retinoic acid. We conclude that the increased effects of all-trans-retinoic acid are specific for APL cells, and that a major effect of retinoic acid is on blast stem cell self-renewal.

Interactions between retinoic acid and colony-stimulating factors affecting the blast cells of acute myeloblastic leukemia

The responses to retinoic acid (RA) of acute myeloblastic leukemia (AML) blasts and normal hemopoietic progenitors was examined under defined growth factor conditions. For the leukemic cells marked patient to patient variation was seen; blast colony formation by cells from some patients was stimulated by RA without growth factors or in the presence of recombinant granulocyte colony-simulating factor (rG-CSF), recombinant granulocyte-macrophage-CSF rGM-CSF and recombinant interleukin-3 (rIL-3); for other populations inhibition was observed under the same conditions. Some blast cells were stimulated by RA in the presence of rGM-CSF and rIL-3 and inhibited when cultured with RA and rG-CSF. Supernatants prepared from blasts cultured with RA and growth factors did not show activities that were not readily explained by the carry-over of growth factors; this result did not provide evidence that RA and growth factors interact to produce factors. Titrations of RA showed that activity was first observed at concentrations of 10(-9) M and was maximum at concentrations of 10(-7) M. Different effects of RA in combination with rG-CSF compared with rGM-CSF or IL-3 were not seen when the cells were tested in suspension culture rather than in methylcellulose, a finding that may be interpreted to mean that the interaction between RA and factors affects terminally-dividing blast cells. Three normal bone marrow samples were cultured with RA and growth factors. Colony formation was stimulated by RA in the presence of rGM-CSF or rIL-3 but inhibited by RA with rG-CSF. Thus a differential effect of RA in combination with growth factors occurs in normal hemopoietic cells and persists in some AML populations.

Granulocyte-macrophage colony-stimulating factor and interleukin-3 protect leukemic blast cells from ara-C toxicity

The blast cells of acute myeloblastic leukemia (AML) usually require growth factors for optimum proliferation in cell culture. Growth factors also affect the sensitivity of AML blast cells to cytosine arabinoside (ara-C). Others have reported that factor-treated cells are more ara-C sensitive than blasts in culture without factors. These authors have reported previously that AML blasts grown with rG-CSF, with or without GM-CSF, are more sensitive than cells in GM-CSF alone. This paper reports experiments which show that changes in the ara-C sensitivities of blast cells in different growth factors are not explained by changes in the percentage of cells in the DNA synthesis (S) phase of the cycle. Blasts freshly obtained from five AML patients were cultured in either rG-CSF, rGM-CSF, or rIL-3; they were then exposed to 20 min pulses of either high specific activity tritiated thymidine (3HTdR) or a high concentration of ara-C. Regardless of the factor present, the pulse of 3HTdR decreased the number of clonogenic cells by about 50%, the result expected for actively proliferating cells with an S phase occupying about half the cycle time. The same result was found for four of the five blast cell populations grown in G-CSF and pulsed with ara-C; in contrast, clonogenic cells grown in GM-CSF or IL-3 from these four populations were not killed by ara-C. The blasts from the fifth patient were ara-C resistant under all conditions. It was concluded that exposure to GM-CSF or IL-3 decreased ara-C sensitivity in blasts that were actively making DNA. The observation was explored in more detail using a cell line (OCI/AML-1a) that is both ara-C sensitive and growth factor dependent. These studies showed that about 15 h of growth in factor are required for a change in ara-C sensitivity.

OCI/AML-4 an acute myeloblastic leukemia cell line: regulation and response to cytosine arabinoside

This paper describes the properties of a continuous cell line derived from the blast cells of a patient with acute myeloblastic leukemia (AML), secondary to the treatment of Hodgkin's disease. The line grows slowly without stimulation but responds to interleukin-3 (IL-3), GM-CSF and mast cell growth factor (MGF), a ligand for the receptor encoded by the c-kit oncogene. When OCI/AML-4 cells are exposed to MGF with IL-3 or GM-CSF, additive or synergistic effects are seen. Combinations of MGF and G-CSF, IL-6 or CSF-1 give less growth than MGF alone. OCI/AML-4 cells are sensitive to retinoic acid; a dose related decrease in clonogenic cells is observed when OCI/AML-4 cells are exposed to retinoic acid in suspension culture. OCI/AML-4 cells are sensitive to cytosine arabinoside (ara-C), but the ara-C dose-response curve can be changed by altering the regulatory milieu in suspension culture. The cells are more ara-C sensitive in MGF or G-CSF than in IL-3 or GM-CSF. Following a 24 h exposure to retinoic acid, the ara-C sensitivity increases; in contrast, after a similar exposure to hydrocortisone, the cells become less ara-C sensitive. These changes in ara-C sensitivity occur in cells that are actively making DNA, as indicated by the reduction in colony formation after exposure to tritiated thymidine. Since OCI/AML-4 cells respond to many of the regulators that affect the growth of freshly obtained AML blast cells, it is proposed that this cell line may be useful for the study of regulation on AML in general and the interaction between different regulators in particular.

Hydrocortisone in culture protects the blast cells in acute myeloblastic leukemia from the lethal effects of cytosine arabinoside

The blast cells in acute myeloblastic leukemia (AML) respond to many of the same regulatory mechanisms that control normal hemopoiesis. These include the growth factors that bind to membrane receptors and steroid hormones or vitamins that have intracellular receptors. We report the effects in culture of the steroid glucocorticoid hydrocortisone on freshly explanted AML blasts from patients and on two continuous AML cell lines. Only small changes in clonogenic cell numbers in suspension cultures were seen in the presence of hydrocortisone. The most striking effect of the hormone was on the sensitivity of blasts cells to cytosine arabinoside (ara-C). In contrast to the response of AML blast cells to retinoic acid, a ligand for intracellular steroid receptors that sensitizes some blast populations to ara-C, hydrocortisone reduced the toxic effects of the drug. The protective action of hydrocortisone was not mediated through the cell cycle since exposure of blasts to hydrocortisone did not affect the percentage of cells in DNA synthesis as measured with the tritiated thymidine (3HTdR) "suicide" technique. The hydrocortisone effect could be demonstrated using a pulse (20 min) exposure protocol. Blasts pulsed with increasing specific activities of 3HTdR showed the usual response pattern with an initial loss in plating efficiency to about 50% of control, followed by a plateau, regardless of whether the cells had been exposed to hydrocortisone. Control blasts exposed to increasing ara-C concentrations gave very similar dose-response curves; in striking contrast, blast cells cultured in hydrocortisone, then pulsed with ara-C did not lose colony-forming ability even though the same population was sensitive to 3HTdR. The hydrocortisone effect was dose and time related; protection from ara-C increased from 10(-8) to 10(-5) M and was seen after 4 hr exposure but required 8 hr to reach a maximum. We conclude that hydrocortisone can protect blasts from the lethal effects of ara-C even while the cells are in active DNA synthesis.

Mast cell growth factor, a ligand for the receptor encoded by c-kit, affects the growth in culture of the blast cells of acute myeloblastic leukemia

The c-kit proto-oncogene encodes a transmembrane receptor with a tyrosine kinase internal domain. C-kit has been mapped to the W locus in the mouse, and the gene encoding the ligand has been shown to be the product of the murine SI locus. Previous genetic studies have shown that the murine W and SI loci play important roles in the normal function of hemopoietic stem cells. As these stem cells have been identified as the origins of abnormal clones in acute myeloblastic leukemia (AML), a study was begun of c-kit in AML. By Northern blot analysis, it was shown that all of 21 blast populations from AML patients were kit expression positive, but some AML cell lines did not transcribe detectable c-kit mRNA. This study is now extended to the responses of freshly obtained AML cells and cell lines to the ligand, mast-cell growth factor (MGF). In culture, fresh cells usually responded to added ligand with increases in both self-renewal and terminal divisions. The most obvious effects were seen when MGF was combined with either IL-3 or G-CSF. The response of cell lines to MGF mirrored their expression of c-kit; expression positive lines responded in culture with patterns similar to those seen for fresh cells. C-kit expression negative cells did not respond to MGF. RNA prepared from the cells giving rise to one such line, OCI/AML-5, was available for study. mRNA for c-kit could not be detected in this RNA sample by Northern blot analysis or the polymerase chain reaction. Thus the heterogeneity found in AML blast populations extends to the involvement of c-kit and its ligand in growth regulation, although blast populations without this regulatory apparatus appear to be rare.

Cytosine arabinoside (ara-C) and cis-dichlorodiammineplatinum II (cisplatin) alone and in combination: effects on acute myeloblastic leukemia blast cells in culture and in vivo

Cytosine arabinoside (ara-C) and cis-dichlorodiammineplatinum II (cisplatin) are lethal to mammalian cells by very different mechanisms; however, they share interactions with the biology of blast cells in acute myelogeneous leukemia (AML). Both agents are more toxic to AML blasts in suspension than when a clonogenic assay in methyl cellulose is used; both agents are more toxic in suspension in the presence of rG-CSF than with rGM-CSF. Accordingly, preclinical tests were undertaken of cisplatin and ara-C in combination. At the same time, a phase I/II clinical trial of the combination was conducted, using AML patients refractory to treatment or in relapse. In the laboratory, blasts from eight AML patients were tested against each agent singly and in combination. The observed survival values for the mixture were compared with those predicted by assuming either an additive effect or a more general effect that allows synergism or antagonism. Blasts from two patients were tested with this design in the presence of rG-CSF or rGM-CSF. In most instances the toxic effects of ara-C and cisplatin were additive. Evidence of synergism was seen in blasts from three patients.

Effects of rGM-CSF and rG-CSF on the cisplatin sensitivity of the blast cells of acute myeloblastic leukemia

Recombinant growth factors have been shown to alter the sensitivity of acute myeloblastic leukemia (AML) blast cells to cytosine arabinoside (ara-C) in culture. The mechanism is controversial and suggestions for it include changes in ara-C metabolism, changes in cell cycle parameters, and changes in the balance between self-renewal and determination in blast stem cells. We addressed this issue by measuring the cisplatin sensitivity of freshly obtained AML blasts in rG-CSF, rGM-CSF, or the two together. For comparison, simultaneous measurements of ara-C sensitivity were made. We found that exposure to different factors in suspension altered the cisplatin sensitivity of AML blasts in the same direction as the change observed in ara-C sensitivity. Similar changes in cisplatin sensitivity were seen when cells were briefly exposed to the drug, washed, and then grown in suspension in the presence of different growth factors. Control experiments showed that the conditions in suspension, not in the clonogenic assay in methylcellulose, were responsible for the changes in cisplatin sensitivity. The capacity of high specific activity to inactivate clonogenicity was tested at several times under growth conditions which altered the sensitivity of cells to cisplatin. Whereas changes in survival after 3HTdR and cisplatin both were seen with time, growth conditions that altered cisplatin sensitivity were not associated with changes in 3HTdR toxicity. The data do not support explanations of the effects of growth conditions on drug toxicity which depend either on drug metabolism or cell cycle effects. Instead, the findings are consistent with a model that postulates an association between drug sensitivity and the balance between self-renewal and differentiation in the blast population.

The effects of leukemia inhibitory factor (LIF) on the blast stem cells of acute myeloblastic leukemia

Leukemia inhibitory factor (LIF) was tested using three established acute myelocytic leukemia (AML) cell lines. In growth assays in two of the three lines, we found that the addition of LIF increased the doubling time of the clonogenic population but not the total population as assessed by nucleated cell counts. Similarly, tritiated thymidine uptake into total AML populations was not affected by LIF, but the percentage of clonogenic cells killed by exposure to high specific activity 3HTdR was reduced in LIF-treated cultures compared to controls. We interpret these results to indicate that LIF prolongs the cell cycle of stem cells in some AML lines, possibly by increasing the time spent in the G2-M-G1 parts of the cycle. Consistent with this interpretation, we observed a decrease in ara-C sensitivity in LiF treated cultures. Variable results were obtained when freshly obtained AML blasts were exposed to LIF.

Interaction between retinoic acid and cytosine arabinoside affecting the blast cells of acute myeloblastic leukemia

Retinoic acid (RA) is a potent morphogen that has been shown to increase differentiation in some leukemic cell populations. RA has been used in treatment of some patients with acute myeloblastic leukemia (AML) and myelodysplastic syndromes. In previous experiments we had observed that RA may decrease the self-renewal of blast cells in established cell lines, and in our clinic RA has been tested as maintenance treatment in association with chemotherapeutic drugs. Accordingly, we asked if exposure of AML blast cells to RA affected their subsequent response to ara-C. We found that brief exposure to RA regularly increased the ara-C sensitivity of cells from two established AML cell lines. A similar, though less marked, effect was seen when the blast cells from one patient were tested directly; in a second instance, highly ara-C resistant blasts did not become sensitive when exposed to RA. Experiments using high specific activity tritiated thymidine did not disclose any changes in the proportion of AML cells in the DNA synthesis phase of the cycle at times when their responses to ara-C were changing. We interpret our findings as support for continuing efforts to integrate RA in the management of AML patients and suggest that the mechanism of ara-C sensitization may not depend on changes in the cell cycle.

The expression of the proto-oncogene C-kit in the blast cells of acute myeloblastic leukemia

The oncogene kit has been shown genetically to map in the W locus of the mouse. This locus is known to have an important role in the regulation of normal hemopoietic stem cell growth. The blast cells of acute myeloblastic leukemia may be considered to arise in predeterministic stem cells. Accordingly, we sought evidence that kit was involved in the regulation of AML blast growth, using a cDNA probe to the external domain of c-kit. With this probe the gene was found to be in germline configuration in blast cells from AML, ALL, and continuous myeloblastic cell lines. However, expression could be detected by Northern analysis or RNA dot blots only in fresh AML blast cells. Fresh cells from ALL patients, normal bone marrow, PHA-stimulated lymphocytes, and four myeloblastic continuous cell lines were expression negative by the same techniques.

Growth factors influence the sensitivity of leukemic stem cells to cytosine arabinoside in culture

We have proposed that the blasts in acute myeloblastic leukemia (AML) are renewal populations maintained by a small subpopulation of stem cells. The balance between self-renewal and differentiation in blast stem cells may be an important attribute contributing to treatment outcome. Cytosine arabinoside (ara-C) is included in most chemotherapeutic regimens for the treatment of AML. When ara-C survival curves are constructed, the drug appears to be more toxic when an assay is used that detects principally self-renewing divisions, compared with a procedure that depends on terminal divisions. AML blasts usually respond in culture to myelopoietic growth factors; their response often includes a change in self-renewal, differentiation, or both. These features of the model for AML blasts led to the prediction that growth factors would alter ara-C survival curves in a way that depended on the effects of the culture conditions on self-renewal and differentiation. Four AML blast populations were chosen to test this prediction on the basis of our ability to manipulate them by adding or withholding one or more growth factors. Highly significant changes were seen in the ara-C survival curves, depending on the growth factors present in the cultures as was predicted by the observed effects of the factors on renewal and differentiation.

Expression of a retinoic acid receptor gene in myeloid leukemia cells

Retinoic acid (RA) has been shown to increase differentiation in some leukemia cell lines (HL-60 and KG-1) but not others (K562). Similarly, RA has been reported to have variable effects on fresh blast cells. Recently, molecular clones have been obtained for the nuclear receptor for retinoic acid. The experiments described in this paper were designed to compare expression of the receptor to biological activity in myeloblastic leukemia cells. In four continuous AML cell lines, a positive correlation was found between retinoic acid receptor (RAR) expression by Northern analysis or RNA dot blot and the ability of RA to inhibit colony formation. Kinetic studies of the most sensitive cell line showed that inhibition of colony formation was associated with reduced blast cell self-renewal and differentiation-like events. RAR was detected in freshly obtained blast cells from 23 AML patients. Patient-to-patient variation was observed; however, a correlation was not found between RAR expression and response of the freshly obtained blast cells to RA.

Heterogeneity in acute myeloblastic leukemia

Morphological-identified blast populations are the hallmark of the malignant clones that dominate hemopoiesis in acute myeloblastic leukemia (AML). Marked heterogenity is characteristic of AML blasts. Patient-to-patient variation is seen in their biological properties but is particularly evident in the response to treatment. Intraclonal variation is generated during clonal expansion, particularly as blast stem cells either undergo self-renewal or enter into a series of terminal divisions. These two alternative activities can be monitored in cell culture using a clonogenic assay and a suspension assay. The balance between renewal and differentiation can be altered by exposing blast populations to various growth factors in culture. Further, certain drugs, particularly ara-C, appear to be more toxic for self-renewing divisions than cell-cycle events generally. We suggest that both drugs and growth factors should be assessed for their effects on self-renewal as part of preclinical testing.

The effects of combinations of the recombinant growth factors GM-CSF, G-CSF, IL-3, and CSF-1 on leukemic blast cells in suspension culture

The blast cells of acute myeloblastic leukemia may be considered as a renewal population maintained by stem cells that are capable of both self-renewal and differentiation. Blast stem cells grow in culture usually when stimulated by growth factors normally active on myelopoietic cells. Two culture methods permit an evaluation of the balance between self-renewal and differentiation; previous studies have shown that this balance can be affected by recombinant growth factors. These include interleukin 3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF), active on early cells in normal myelopoiesis, and G-CSF and CSF-1, restricted in normal hemopoiesis to the granulopoietic and macrophage/monocytic lineages, respectively. In this paper we report the results of evaluating the effects on these recombinant growth factors alone or in mixtures of two at optimal concentrations. The results were obtained either using titrations of colony formation in methylcellulose or growth in suspension. Star diagrams, a technique from exploratory data analysis, were used to provide quantitative and graphic displays of the results of the recombinant factors on the balance between blast self-renewal and differentiation. Blasts from 4 acute myeloblastic leukemia patients and one patient with the blast crisis of chronic myeloblastic leukemia were examined in detail. The great patient-to-patient variation usually observed was seen in both plating efficiency in methylcellulose and growth pattern in suspension. In spite of this variation, a common pattern of response to growth factors emerged. When the early acting factors, IL-3 and GM-CSF, were combined, the effect was quantitatively and qualitatively similar to the largest stimulation seen with either of the factors alone. In contrast, late-acting factors, G-CSF and CSF-1, influenced each other's effects when present together and each affected the activities of GM-CSF and IL-3. Notably, CSF-1, which often led to the accumulation of adherent, terminal cells in suspension, usually maintained or increased this differentiation-like activity in combination. G-CSF also favored differentiation in combination, although the effect was usually to increase the number of colonies in methylcellulose, most of which consist of blast cells incapable of further divisions. The results are discussed as they relate to the postulated structure of the blast population and the normal targets of the recombinant growth factors.

The effects of recombinant CSF-1 on the blast cells of acute myeloblastic leukemia in suspension culture

Recombinant hemopoietic colony-stimulating factors (CSFs), including GM-CSF, G-CSF and IL-3, have been shown to be effective stimulators of both self-renewal and terminal differentiation of blast stem cells in acute myeloblastic leukemia (AML). We have examined the activity of a fourth growth factor, recombinant CSF-1 (or M-CSF), on the growth of leukemic blasts in culture. CSF-1 was found to be active on some, but not all, blast populations. In sensitive cells, CSF-1 often stimulated the production of adherent blast cells incapable of division. This observation leads us to suggest that CSF-1 may be useful in the treatment of selected cases of AML.

Binding of iodinated recombinant human GM-CSF to the blast cells of acute myeloblastic leukemia

Granulocyte/macrophage-colony-stimulating factor (GM-CSF) is an effective growth factor for the blasts of acute myeloblastic leukemia (AML). Radioiodinated Chinese hamster ovary (CHO)-cell derived GM-CSF was prepared using Bolton-Hunter reagent to label free amino groups on the protein. Normal human neutrophils and the blast cells from AML patients were examined for binding. We found that there were fewer receptors of higher affinity on blast cells compared with neutrophils. After brief culture in suspension, receptor number increased and affinity decreased. Experiments provided evidence that GM-CSF from Escherichia coli had a higher affinity for neutrophils (kd = 20 pM) than the CHO-cell derived protein (kd = 500 pM-1 nM). This difference was reflected in the increased effectiveness of the E. coli protein over the CHO protein to stimulate colony formation in both normal bone marrow cells and AML blasts.

Expression of the CSF-1 gene in the blast cells of acute myeloblastic leukemia: association with reduced growth capacity

Myelopoietic growth factors are known to influence the growth in culture of malignant blast cells from human Acute Myeloblastic Leukemia (AML). We have used cDNA clones for the factor CSF-1 and its receptor fms to study DNA and RNA from the blasts of 25 AML patients. The CSF-1 gene was always in the germline configuration. CSF-1 mRNA was found in about half the blast populations. The cells were also studied for their growth properties in culture. A highly significant association was found between CSF-1 expression and poor growth in suspension culture. Most blast populations expressed fms; the number of fms expression negative samples was to small to permit the detection of any association between fms expression and growth or any interaction between the effects of the expression of the growth factor and its receptor. We propose that CSF-1 may be an important part of the mechanism determining the balance between self-renewal and determination in AML blast clones.

Stem cell renewal and differentiation in acute myeloblastic leukaemia

The defining properties of stem cells are capacities for self-renewal and, after determination, a limited number of terminal divisions. The blast cells of acute myeloblastic leukaemia (AML) are maintained by stem cells with these two properties. Since renewal and differentiation can be assessed separately in cultures of AML blasts, these cancer cells provide a useful model for examining stem regulation; such studies have practical importance for future developments in the treatment of AML. This paper considers three aspects of blast cell biology. First, evidence is presented that self-renewal and differentiation are regulated by specific genes; further, the DNA encoding these genes has structural features that affect the chemosensitivity of self-renewal. This sensitivity varies from patient-to-patient and is an important attribute contributing to variation in treatment efficacy. Second, the effects of myelopoietic growth factors on blast stem cells are presented and discussed, as these bear on the regulation of the balance between renewal and differentiation. Finally, models of leukaemic haemopoiesis are considered in light of the experimental findings. The suggestion is advanced that leukaemia can be explained better by abnormalities of gene expression than by blocked differentiation.

Structure and expression of genes of GM-CSF and G-CSF in blast cells from patients with acute myeloblastic leukemia

The hematopoietic growth factors granulocyte/macrophage colony-stimulating factor (GM-CSF) and G-CSF, available as recombinant products, stimulate the growth in culture of blasts from patients with acute myeloblastic leukemia (AML). We used cDNA probes for each gene to study the genomic organization in blast cells of 22 patients and expression in the blast cells of 18 patients. Alteration in the structure of G-CSF (two instances) and GM-CSF (two instances) was found. In two patients in whom it was possible to study DNA from bone marrow obtained at remission, the new bands detected in the leukemic cells were not found. Fifteen of 18 patients showed no RNA expression of either growth factor. Both patients with GM-CSF abnormalities as seen by Southern analysis expressed an abnormally large GM-CSF message but no G-CSF messages. One patient with an abnormal Southern pattern with G-CSF expressed normal-sized G-CSF and GM-CSF messages. The biologic significance of these findings remains to be determined. Nonetheless, the abnormal Southern patterns may prove to be useful clonal markers in the study of AML.

Response to 5-azacytidine of leukemic blast cells in suspension: a biological parameter associated with response to chemotherapy

Sensitivities to drugs acting on cells in culture can be measured as dose-response curves, provided a quantitative assay is available for a relevant cell function. We have used two such assays in the study of the blast cells of acute myeloblastic leukemia. Colony formation in culture with methylcellulose detects principally terminal divisions, while growth of clonogenic cells in suspension reflects self-renewal. In a previous study different cytosine arabinoside and 5-azacytidine dose-response curves were obtained with the two assays. For the former the slope of the dose-response curve measured in suspension was steeper than that obtained using the clonogenic assay. For the latter, 5-azacytidine, the relationship between sensitivity in suspension and in methylcellulose was reversed. Further, for cytosine arabinoside, sensitivity in suspension but not in methylcellulose was associated with successful remission-induction. In this article we report an association between 5-azacytidine sensitivity in suspension and successful remission induction, for patients treated only with high-dose cytosine arabinoside. There was no correlation between the 5-azacytidine dose-response curve in methyl-cellulose and clinical outcome. A model is presented that may explain these findings, based on the hypothesis that there are genetic mechanisms responsible for blast cell renewal.

The effects of three recombinant growth factors, IL-3, GM-CSF, and G-CSF, on the blast cells of acute myeloblastic leukemia maintained in short-term suspension culture

The blast stem cells of acute myeloblastic leukemia (AML) respond in cell culture to growth factors by both self-renewal and terminal divisions. Both of these functions have been shown to be stimulated by the recombinant growth factors granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF). In this paper, recombinant gibbon interleukin-3 (IL-3), homologous to human IL-3, was tested on blast cells and compared with the effects of GM-CSF, G-CSF, and medium conditioned by the bladder cell line 5637 (5637-CM). We found that IL-3 was an effective stimulator of blast renewal and terminal divisions. However, great patient-to-patient variation was found. A graphic method of presenting complex comparisons between growth factors is also included.

Synergism between recombinant growth factors, GM-CSF and G-CSF, acting on the blast cells of acute myeloblastic leukemia

The genes for the hemopoietic growth factors, GM colony-stimulating factor (CSF) and G-CSF have been cloned, and recombinant material is available for both. We tested these recombinant factors for their effects on the blast cells of acute myeloblastic leukemia (AML). Culture methods are available that support both colony formation by AML blasts and the growth of blast stem cells in suspension. Recombinant GM-CSF is active in both culture systems, although to a varying degree. We found that recombinant G-CSF was also effective; however, the two recombinant factors showed striking synergism for the stimulation of blast growth of cells from five of eight AML patients. In these cases, the combination was equivalent to the stimulating activity of supernatants from the continuous cell line 5637. This conditioned medium (HTB9-CM) is considered the standard for blast growth. Blasts from one of the patients grew without added factor. In another instance, recombinant GM-CSF alone was almost as effective as HTB9-CM. In the third case, both recombinant factors were active, but synergism was not observed and their combined effect was not equivalent to that of HTB9-CM. Both GM-CSF and G-CSF were active on normal bone marrow granulopoietic progenitors, but synergism was not observed. We conclude that the marked heterogeneity observed when AML blasts are examined by other criteria is also observed when their response to growth factors is evaluated.

High-dose cytosine arabinoside in the treatment of acute myelogenous leukemia: contributions to outcome of clinical and laboratory attributes

High-dose cytosine arabinoside (HDAra-C) has been used for remission induction, and in conventional doses for maintenance in a trial of single-agent therapy in 43 previously untreated patients with acute myelogenous leukemia (AML). Rationale for the trial was provided by the observed decrease in leukemic blast cell self-renewal in culture following exposure to Ara-C. Compared with a previous trial of 57 patients treated with multidrug therapy, single-drug Ara-C was associated with a significantly improved complete remission rate (P = .010), although the survival time was not increased. All patients with low self-renewal responded to HDAra-C in contrast to the previous trial where some patients with this phenotype failed remission induction. The clinical observations are consistent with the view that the antileukemic effect of Ara-C has some specificity for cellular events required for self-renewal of blast cells. Exposure in vivo to Ara-C was associated with an increase in blast stem cell renewal at relapse, indicating that maintenance with other drugs should be tested. The study demonstrates the importance of biological attributes in design and analysis of clinical trials.

Sensitivity to 5-azacytidine of blast progenitors in acute myeloblastic leukemia

In a previous study, we showed that the blast stem cells of acute myeloblastic leukemia (AML) were more sensitive to cytosine arabinoside (ara-C) when growing in suspension culture than during colony formation in methylcellulose. We suggested that the difference might be explained by considering the cellular mechanisms responsible for growth in suspension and colony formation. In the former, the clonogenic cells increase in number (self-renewal); in the latter, most of the divisions are terminal. The increased sensitivity to ara-C in suspension might then be attributed to its ability to inhibit self-renewal to a greater degree than cell division generally. A test of this hypothesis would be to compare the survival curves in suspension and in methylcellulose using a drug that spared or stimulated self-renewal. Such an agent is 5-azacytidine (5-aza) and has the additional advantage that its analogue, 6-azacytidine (6-aza) has no effect on self renewal. The data supported the hypothesis, since clonogenic AML blasts were much less sensitive to 5-aza in suspension than in methylcellulose. The effect of 6-aza, while qualitatively similar, was much less marked. Controls showed that the difference in survival curves could not be explained on a kinetic basis or by the secretion of growth factors by 5-aza-treated cells. We suggest that a comparison of the effects of drugs in suspension and in methylcellulose may be useful in preclinical screening of putative anti-AML compounds.