Expression of Apoptosis-Regulating Proteins in Chronic Lymphocytic Leukemia: Correlations With In Vitro and In Vivo Chemoresponses

Update on the biology of chronic lymphocytic leukemia

An understanding of the molecular biology of B-cell chronic lymphocytic leukemia (B-CLL) has led to the appreciation that several different B-cell diseases are represented under this name. Variability in the bcl-2 family of proteins, p53 mutation, or the presence of various chromosomal abnormalities corresponds to variability of the clinical course of disease and response to therapy. Differential expression of cell surface adhesion molecules by B-CLL cells have also been shown to influence clinical outcome, as have the expression of immune regulatory molecules (eg, CD80, CD40R, CD27 and CD79b). Recent work studying immunoglobulin-heavy chain gene rearrangement postulates at least two subsets of B-CLL originating from different stages of B-cell development and following different clinical courses. The knowledge that B-CLL is the final consequence of many different molecular perturbations may allow the development of chemotherapies, immunotherapies, and gene therapies that target the specific molecular defect in a given case of B-CLL.

Chemosensitivity of B cell chronic lymphocytic leukemia and correlated expression of proteins regulating apoptosis, cell cycle and DNA repair

B cell chronic lymphocytic leukemia (B-CLL) cannot be cured with conventional chemotherapy. This clinical enigma appears to be at least partially due to the fact that B-CLL cells are resistant to programmed cell death (apoptosis) and that they are arrested in G0/G1 phase of the cell cycle. The reasons for the dysregulation of these two key cellular events in B-CLL are unclear. The present study aimed at determining correlations between the expression levels of proteins regulating apoptosis, cell cycle and DNA repair in B-CLL cells and normal B cells. In addition, the differential sensitivity of B-CLL cells to drug-induced apoptosis was quantified. We show that in B-CLL cells levels of the death-suppressor Bcl-2 correlated positively with those of the pro-apoptotic protein Bax and of the cyclin-dependent kinase (cdk) inhibitor p27Kip1. In B-CLL cells levels of the anti-apoptotic Bcl-xL showed a positive correlation with levels of the 80 kDa regulatory component (Ku80) of the DNA-dependent protein kinase that is involved in DNA double-stranded break repair. These correlations were not detected in normal B cells. The sensitivity of leukemic cells to FLUD but not to ADM, CPM or to DEX was reduced in pre-treated patients. These data support the hypothesis that in B-CLL cells death-modulators and molecules modulating cell cycle and DNA repair are regulated in a coordinated manner. Leukemia (2000) 14, 40-46.

Acute myeloid leukemia possessing jumping translocation is related to highly elevated levels of EAT/mcl-1, a Bcl-2 related gene with anti-apoptotic functions

Jumping translocations (JTs) are unbalanced chromosomal translocations in which an identical chromosomal region is translocated to the telomeric region of different chromosomes. JTs are rare in hematological malignancies where they are second translocations and may be an indicator of poor prognosis. We report a case of acute myeloid leukemia with t(16;21) and a JT in which the long arm of chromosome 1 distal to q21 is translocated to the terminal region of chromosome 10. The leukemic cells exhibit high expression of EAT/mcl1, an anti-apoptotic Bcl-2 related gene. Since EAT/mcl1 is mapped to 1q21 near the breakpoint in the JTs, high level expression of EAT/mcl1 may be associated with the poor prognosis of leukemia with JTs.

Apoptosis regulating proteins as targets of therapy for haematological malignancies

Most chemotherapeutic agents used in the treatment of haematological malignancies cause cell death by inducing apoptosis through undefined means. The discovery of the proteins involved in apoptosis and the description of apoptotic pathways suggest new potential targets for therapeutic intervention. Both 'intrinsic' and 'extrinsic' pathways can be activated separately, but activation of caspases appears central to most apoptotic pathways. Novel approaches attempt to induce apoptosis by directly targeting a portion of an apoptotic pathway. Agents that trigger signalling of Fas or tumour necrosis factor- (TNF-) related apoptosis inducing ligand (TRAIL) receptor seek to induce the extrinsic pathway at the cell surface. The BCL-2 family of proteins seems central to the regulation of those apoptotic pathways that involve mitochondrial sequestration or the release of cytochrome c, with subsequent activation of Apaf-1, caspase-9 and caspase-3. The activity of this family may depend upon both the phosphorylation state of different members and the relative level of pro- and anti-apoptotic members. New agents such as the staurosporine analogue UCN-01 and bryostatin are thought to affect apoptosis induction by altering BCL-2 phosphorylation. Others, such as BCL-2 antisense and ATRA attempt to modulate the protein levels to promote apoptosis. Direct activation of caspase-3 is a probable target, but as yet no agent with this direct function is in trial. Clinical trials of several agents have been completed or are underway. It is likely that agents that target particular points in apoptosis pathways will have antileukaemia/lymphoma activity, however, the optimal utilisation may involve combination with other more conventional agents that also activate apoptosis.

Antisense-mediated suppression of Bcl-2 highlights its pivotal role in failed apoptosis in B-cell chronic lymphocytic leukaemia

Although advances have been made in the development of more effective treatment modalities, B-cell chronic lymphocytic leukaemia (B-CLL) remains incurable due to the development of drug resistance. Defective programmed cell death mechanisms rather than dysregulation of cell cycle appears to predominate in B-CLL and it is likely that a failure to initiate apoptosis contributes to chemoresistance. Most B-CLL cells contain high levels of the anti-apoptotic protein Bcl-2 and high Bcl-2/Bax ratios have been associated with in vitro resistance to cytotoxic agents. In this study we evaluated the cellular responses to a Bcl-2 antisense oligonucleotide in terms of Bcl-2 mRNA and protein expression and the induction of apoptosis. The antisense molecule induced a specific reduction in Bcl-2 mRNA and protein expression over the 48 h culture period and was associated with increased apoptosis. The study indicates that Bcl-2 protein is central to the mediation of resistance to apoptosis in B-CLL. Therefore Bcl-2 antisense oligonucleotides might be useful in the treatment of B-CLL.

Immune complexes inhibit apoptosis of chronic lymphocytic leukaemia B cells

In the present study we examined the effect of immune complexes (IC) on the survival of chronic lymphocytic leukaemia (B-CLL) B cells. Our results showed that either precipitating IC (pIC), Ab-coated erythrocytes (E-IgG) or heat-aggregated IgG (aIgG) significantly inhibited spontaneous apoptosis of B-CLL cells, as well as that induced by fludarabine, chlorambucil or dexamethasone. After depletion of T lymphocytes, monocytes and NK cells, incubation with IC was no longer able to delay B-CLL cells apoptosis, suggesting that prevention of apoptosis depends on IC interaction with accessory leucocytes. The release of IFNgamma by non-malignant cells upon activation with IC was responsible, to some extent, for IC effects as shown by the fact that neutralizing anti-IFNgamma MoAb partially prevented their ability to inhibit B-CLL cells apoptosis. The observation that treatment with IC resulted in increased expression of HLA-DR on B-CLL cells suggests that inhibition of apoptosis is associated with cellular activation.

Pleiotropic drug resistance in B-cell chronic lymphocytic leukaemia--the role of Bcl-2 family dysregulation

B-cell chronic lymphocytic leukaemia (B-CLL) is an incurable clonal disease which shows initial responsiveness to a number of chemotherapeutic drugs. However, most treated patients become resistant to treatment and this represents a major problem in the successful management of the condition. Experimental evidence points to the fact that most chemotherapeutic drugs ultimately exert their cell killing effect through the process of apoptosis. In this study we compared the apoptotic responses of B-CLL cells in vitro following exposure to several chemotherapeutic drugs. We found that there was a correlation between ID50 values for all the drugs under investigation; particularly between Chlorambucil and Fludarabine (P = 0.0002). In addition, we analysed the expression of Bcl-2 and Bax, two proteins pivotal to the regulation of apoptosis, both immediately ex vivo and in viable and apoptotic sub-populations following exposure to drug. Our data suggest that high Bcl-2/Bax ratios may be predictive of a drug resistant phenotype in B-CLL cells and that modulation of these proteins is essential for the induction of cell death. Furthermore, it seems likely that the superior potency that has been ascribed to Fludarabine is due to it being administered in a more optimised dose. A recently reported clinical trial of Fludarabine against high-dose Chlorambucil supports this view since it showed that both treatment modalities were comparable in terms of response rate and survival times.

CD47 ligation induces caspase-independent cell death in chronic lymphocytic leukemia

Thrombospondin forms a 'molecular bridge' between phagocytic and apoptotic cells through interaction with alphavbeta3/CD36. We report here that engagement of CD47, a newly described thrombospondin receptor, by immobilized monoclonal antibody against CD47 or by thrombospondin induced in all B-cell chronic lymphocytic leukemia clones the cytoplasmic features of apoptosis (cell shrinkage, decrease in mitochondrial transmembrane potential and phosphatidylserine externalization) without the nuclear features (chromatin condensation, appearance of single-stranded DNA, DNA fragmentation and cleavage of poly ADP-ribose polymerase). These cytoplasmic events of apoptosis were not prevented by the addition of caspase inhibitor z-VAD-fmk, or by the presence of survival factors (such as interleukin-4 and gamma interferon) or cell activation. Morphological studies confirmed the integrity of the nucleus and showed swelling of the mitochondria. This caspase-independent death pathway may be relevant to the development of alternate therapeutic strategies in chronic lymphocytic leukemia, which remains an incurable disease.

In Vitro Evaluation of Fludarabine in Combination With Cyclophosphamide and/or Mitoxantrone in B-Cell Chronic Lymphocytic Leukemia

B-chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of long-lived CD5+ B lymphocytes. We have analyzed the effect in vitro of the combination of fludarabine with cyclophosphamide and/or mitoxantrone on cells from 20 B-CLL patients. Mafosfamide, the active form of cyclophosphamide in vitro, increased the cytotoxicity of fludarabine in all of the patients studied and produced a significant synergistic effect (P < .01) after 48 hours of incubation. The addition of mitoxantrone to this combination increased the cytotoxic effect in cells from 8 patients, but in the remaining 12 patients no significant increase was observed. The effect of fludarabine and mafosfamide was dose-dependent. Mafosfamide and fludarabine had a synergistic effect in inducing apoptosis of B-CLL cells as determined by DNA staining with propidium iodide and analysis of phosphatidylserine exposure. Mafosfamide significantly increased the apoptosis induced by fludarabine on CD19+ cells (P = .007), but not on CD3+ cells (P= .314). Cell viability was correlated with a decrease in Mcl-1 levels and an increase in p53 levels. These results support that fludarabine in combination with cyclophosphamide and/or mitoxantrone can be highly effective in the treatment of B-CLL.

Bryostatin and CD40-ligand enhance apoptosis resistance and induce expression of cell survival genes in B-cell chronic lymphocytic leukaemia

Modulating signal transduction pathways represents a promising approach for altering the biological behaviour of haemopoietic malignancies. B-cell chronic lymphocytic leukaemia (B-CLL) cells were treated in vitro with CD40-ligand (CD40L) (CD154) or the protein kinase C modulator Bryostatin-1, exploring the effects on: (a) sensitivity to apoptosis induction by chemotherapeutic drugs (fludarabine, dexamethasone) or anti-Fas antibody; (b) expression of apoptosis-regulatory proteins (Bcl-2, Bcl-X, Mcl-1, Bax, Bak, BAG-1, Flip, XIAP); (c) expression of cell surface co-stimulatory antigens (CD80 [B7.1]; CD54 [ICAM-1]; CD70); and (d) expression of immune modulatory receptors (CD27, CD40, CD95 [Fas]). CD40L and Bryostatin decreased both spontaneous and drug-induced apoptosis in most B-CLL specimens tested. Apoptosis resistance was associated with CD40L- and Bryostatin-induced elevations in the anti-apoptotic Bcl-2 family protein Mcl-1. CD40L also induced striking increases in the levels of the anti-apoptotic protein Bcl-XL in B-CLLs. CD40L stimulated increases in the surface expression of CD40, CD54, CD69, CD70, CD80 and CD95, whereas Bryostatin induced expression of CD40, CD54, CD69 and CD95 but not the co-stimulatory molecules CD70 and CD80. Despite elevations in the expression of CD95 (Fas), anti-Fas antibodies failed to induce apoptosis of CD40L- and Bryostatin-treated B-CLL cells. This Fas-resistance was associated with increased expression of the Fas-antagonist Flip in CD40L-treated, and with elevations in the caspase inhibitor XIAP in Bryostatin-treated B-CLLs. The potential anti-apoptotic properties of CD40L and Bryostatin should be taken into consideration when employing these agents in clinical trials involving patients with B-CLL.

Chronic lymphocytic leukemia

Significant strides have been made in our understanding of the biology and treatment of B cell chronic lymphocytic leukemia. Recent studies have defined cytogenetic and molecular lesions that may be responsible for leukemogenesis or disease progression. Molecular analyses of immunoglobulin genes have delineated two or more subgroups of chronic lymphocytic leukemia that may differ in their clinical behavior. Research in the biochemistry of chronic lymphocytic leukemia has provided insight into the noted resistance of leukemia cells to cytotoxic drugs. Investigations into the immunology has revealed mechanisms whereby chronic lymphocytic leukemia cells can contribute to the immune deficiency that commonly develops in patients with this disease. Clinical studies have delineated factors that are helpful in predicting prognosis and have provided data on promising new therapies for patients with this disease, including stem cell transplantation, monoclonal antibodies, and gene therapy.

Activation of the cAMP signaling pathway increases apoptosis in human B-precursor cells and is associated with downregulation of Mcl-1 expression

During B- and T-cell ontogeny, extensive apoptosis occurs at distinct stages of development. Agents that increase intracellular levels of cAMP induce apoptosis in thymocytes and mature B cells, prompting us to investigate the role of cAMP signaling in human CD10+ B-precursor cells. We show for the first time that forskolin (which increases intracellular levels of cAMP) increases apoptosis in the CD10- cells in a dose-dependent manner (19%-94% with 0-1,000 microM forskolin after 48 hours incubation, IC50 = 150 microM). High levels of apoptosis were also obtained by exposing the cells to the cAMP analogue 8-chlorophenylthio-cAMP (8-CPT-cAMP). Specific involvement of cAMP-dependent protein kinase (PKA) was demonstrated by the ability of a cAMP antagonist, Rp-isomer of 8-bromo-adenosine- 3', 5'- monophosphorothioate (Rp-8-Br-cAMPS), to reverse the apoptosis increasing effect of the complementary cAMP agonist, Sp-8-Br-cAMPS. Furthermore, we investigated the expression of Bcl-2 family proteins. We found that treatment of the cells with forskolin or 8-CPT-cAMP for 48 hours resulted in a fourfold decline in the expression of Mcl-1 (n = 6, P = 0.002) compared to control cells. The expression of Bcl-2, Bcl-xL, or Bax was largely unaffected. Mature peripheral blood B cells showed a smaller increase in the percentage of apoptotic cells in response to 8-CPT-cAMP (1.3-fold, n = 6, P = 0.045) compared to B-precursor cells, and a smaller decrease in Mcl-1 levels (1.5-fold, n = 4, P = 0.014). Taken together, these findings show that cAMP is important in the regulation of apoptosis in B-progenitor and mature B cells and suggest that cAMP-increased apoptosis could be mediated, at least in part, by a decrease in Mcl-1 levels.

Apoptosis and growth inhibition in malignant lymphocytes after treatment with arsenic trioxide at clinically achievable concentrations

BACKGROUND

Arsenic trioxide (As2O3) can induce clinical remission in patients with acute promyelocytic leukemia via induction of differentiation and programmed cell death (apoptosis). We investigated the effects of As2O3 on a panel of malignant lymphocytes to determine whether growth-inhibitory and apoptotic effects of As2O3 can be observed in these cells at clinically achievable concentrations.

METHODS

Eight malignant lymphocytic cell lines and primary cultures of lymphocytic leukemia and lymphoma cells were treated with As2O3, with or without dithiothreitol (DTT) or buthionine sulfoximine (BSO) (an inhibitor of glutathione synthesis). Apoptosis was assessed by cell morphology, flow cytometry, annexin V protein level, and terminal deoxynucleotidyl transferase labeling of DNA fragments. Cellular proliferation was determined by 5-bromo-2'-deoxyuridine incorporation into DNA and flow cytometry and by use of a mitotic arrest assay. Mitochondrial transmembrane potential (delta psi(m)) was measured by means of rhodamine 123 staining and flow cytometry. Protein expression was assessed by western blot analysis or immunofluorescence.

RESULTS

Therapeutic concentrations of As2O3 (1-2 microM) had dual effects on malignant lymphocytes: 1) inhibition of growth through adenosine triphosphate (ATP) depletion and prolongation of cell cycle time and 2) induction of apoptosis. As2O3-induced apoptosis was preceded by delta psi(m) collapse. DTT antagonized and BSO enhanced As2O3-induced ATP depletion, delta psi(m) collapse, and apoptosis. Caspase-3 activation, usually resulting from delta psi(m) collapse, was not always associated with As2O3-induced apoptosis. As2O3 induced PML (promyelocytic leukemia) protein degradation but did not modulate expression of cell cycle-related proteins, including c-myc, retinoblastoma protein, cyclin-dependent kinase 4, cyclin D1, and p53, or expression of differentiation-related antigens.

CONCLUSIONS

Substantial growth inhibition and apoptosis without evidence of differentiation were induced in most malignant lymphocytic cells treated with 1-2 microM As2O3. As2O3 may prove useful in the treatment of malignant lymphoproliferative disorders.

Protease activation and glucocorticoid-induced apoptosis in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is at present an incurable disease. All of the drugs used in the treatment of CLL induce apoptosis in the cells, and in vitro responses to glucocorticoid or analogs correlate with in vivo sensitivity to these agents. Since CLL lymphocytes accumulate rather than proliferate, the idea that CLL is a disease involving defective apoptosis is particularly attractive. Recent studies have identified many of the central components of the apoptotic pathway that appear to be conserved from one cell type to another. Thus, investigation into the functionality of these molecules should reveal where the defect(s) in apoptosis may lie in CLL cells. Protease activation is a central event during apoptosis. and leads to many of the familiar characteristics of apoptosis. Here we will examine the role of apoptotic proteases in CLL and speculate on their contribution to disease emergence and drug resistance.

Chlorambucil resistance in B-cell chronic lymphocytic leukaemia is mediated through failed Bax induction and selection of high Bcl-2-expressing subclones

Our previous data have shown that high Bct-2/ Bax ratios in chronic lymphocytic leukaemia (B-CLL) correlate with in vitro apoptosis and clinical resistance. We have now monitored the in vitro viability of B-CLL cells in relation to Bcl-2 and Bax expression over a 48 h time course following exposure to chlorambucil. The results showed that Bax up-regulation was essential for chlorambucil-induced apoptosis in B-CLL cells and a 3-fold increase in expression within 4 h of exposure to drug was typically observed in sensitive cells; resistant cells failed to up-regulate Bax at all. In contrast, the constitutively high levels of Bcl-2 found in B-CLL cells were found to be down-regulated in apoptotic cells but the mean Bcl-2 expression in viable cells was increased, probably as a result of the loss of lower Bcl-2-expressing cells into the apoptotic compartment. Taken together, these data add further weight to the suggestion that Bcl-2/Bax ratios may be pivotal in determining the fate of B-CLL cells. Furthermore, the Bcl-2/Bax ratios found in apoptotic B lymphocytes were remarkably similar in the treated, untreated and normal control cells, which suggests that there is a universal Bcl-2/Bax ratio threshold for cell survival and cell death.

Apoptosis in B-chronic lymphocytic leukaemia

B-chronic lymphocytic leukaemia (B-CLL) is characterised by the progressive accumulation of monoclonal B cells, which may be the result of several factors leading to extended B-CLL cell lifespan, increased proliferative capacity and diminished cell death. Here we review the implications of several signals mediated by receptors, such as surface IgM, CD6 and CD40, for the B-CLL cell survival, together with data on gene modulation in relation to the apoptosis process in B-CLL cells. We also describe some features of the Fas/FasL system in B-CLL that hypothetically might contribute to the accumulation of leukaemic cells and the progression of the disease, by downregulating the apoptotic response or avoiding the autologous immune response.