Qualitative and quantitative characterization of Fas (CD95) expression and its role in primary human acute leukemia cells

Soluble Fas and Fas ligand and prognosis in children with acute lymphoblastic leukemia

The soluble forms of Fas and its ligand (sFas and sFasL) correlate with disease progression in various malignancies. We compared serum levels of sFas and sFasL in children with acute lymphoblastic leukemia and healthy children to determine the prognostic significance of these molecules. Serum levels of sFas and sFasL were measured with an enzyme-linked immunosorbent assay in 48 patients with newly diagnosed childhood acute lymphoblastic leukemia and 38 healthy children. Cut-off values of sFas and sFasL levels were based on their levels in controls. Clinical and laboratory characteristics were recorded on admission. The mean serum concentration of sFas was 243 ± 40 pg/mL in patients and 238 ± 29 pg/mL in controls. Serum levels of sFasL were 4.33 ± 0.25 ng/mL in patients and 4.27 ± 0.11 ng/mL in controls. Neither difference was significant. Based on the cut-off value, 12.5% of the patients were positive for sFas, and 16.6% were positive for sFasL. Survival was significantly longer in sFasL-positive patients (394 ± 69.6 vs. 254 ± 24.3 days) and the duration of complete remission was also longer (380 ± 65.0 vs. 246 ± 26.0 days) than in sFasL-negative patients (P < 0.02), indicating the important role of this molecule in the response to therapy. Higher sFas levels were associated with hepatosplenomegaly (P < 0.047). In conclusion, sFasL positivity was associated with a favorable outcome in ALL patients.

Regulation of the expression of death receptors and their ligands by melatonin in haematological cancer cell lines and in leukaemia cells from patients

Incorporation of new therapeutic agents remains as a major challenge for treatment of patients with malignant haematological disorders. Melatonin is an indolamine without relevant side effects. It has been shown previously to exhibit synergism with several chemotherapeutic drugs in Ewing sarcoma cells by potentiating the extrinsic pathway of apoptosis. It also sensitizes human glioma cells against TRAIL by increasing DR5 expression. Here, we report the induction of cell death by melatonin in several human malignant haematological cell lines through the activation of the extrinsic pathway of apoptosis. Such activation was mediated by the increase in the expression of the death receptors Fas, DR4 and DR5 and their ligands Fas L and TRAIL, with a remarkable rise in the expression of Fas and Fas L. The cytotoxic effect and the increase in Fas and Fas L were dependent on Akt activation. Results were corroborated in blasts from bone marrow and peripheral blood of acute myeloid leukaemia patients, where melatonin induced cell death and increased both Fas and Fas L expressions. We conclude that melatonin may be considered as a potential antileukaemic agent and its therapeutic use, either alone or in combination with current chemotherapeutic drugs, should be taken into consideration for further research.

Small molecule XIAP inhibitors sensitize childhood acute leukemia cells for CD95-induced apoptosis

Escape of apoptosis may contribute to treatment failure in childhood acute lymphoblastic leukemia (ALL) calling for new approaches to overcome apoptosis resistance. Here, we provide for the first time evidence that small molecule inhibitors that target the anti-apoptotic protein X-linked inhibitor of apoptosis (XIAP) sensitize ALL cells for CD95-induced apoptosis. XIAP inhibitors at subtoxic concentrations, but not a structurally related control compound, act synergistically with agonistic anti-CD95 antibodies or MegaFasL, a hexameric form of CD95 ligand, to induce apoptosis in ALL cells. Further, XIAP inhibitors co-operate with MegaFasL to reduce clonogenic survival of ALL cells demonstrating their effect also on long-term survival. In contrast, XIAP inhibitors show little effect on MegaFasL-mediated apoptosis in normal peripheral blood lymphocytes (PBLs), pointing to some tumor selectivity. Molecular studies reveal that XIAP inhibitors enhance CD95-induced activation of caspases, loss of mitochondrial membrane potential and cytochrome c release in a caspase-dependent manner. Importantly, XIAP inhibitors sensitize primary leukemic blasts from children with ALL for MegaFasL-induced apoptosis. Thus, small molecule XIAP inhibitors present a promising novel approach to enhance CD95-induced apoptosis in childhood acute leukemia.

Identification, characterisation and regulation by CD40 activation of novel CD95 splice variants in CD95-apoptosis-resistant, human, B-cell non-Hodgkin's lymphoma

CD95 gene and splicing aberrations have been detected in B-cell non-Hodgkin lymphoma (B-NHL) where they are thought to contribute to CD95 apoptosis resistance. To further investigate this, we have performed extensive CD95 transcript sequencing and functional analysis in B-NHL with demonstrated resistance to CD95-induced apoptosis (B-NHLr). Strikingly, instead of showing CD95 mutations per se, B cells from B-NHLr co-expressed wild-type and multiple, normal (CD95nv) and novel alternatively spliced variant CD95 transcripts (CD95av). CD95av were predicted, by sequencing, to encode soluble, potentially apoptosis inhibitory proteins. However, their overexpression, by transfection, in Jurkat cells did not interfere with endogenous CD95 death signalling. Furthermore, CD95av-expressing B-NHLr did not show mutations in CD95 splice-regulatory elements and CD95av expression was 'reversible' by CD40 activation. This, in conjunction with treatment by the protein synthesis inhibitor, cycloheximide, could sensitise a subset of B-NHLr to CD95 apoptosis. In normal and lymphoma B cells, this correlated to increased CD95 membrane expression, enhanced DISC activity and engagement of the mitochondrial death pathway via Bid cleavage, although the latter occurred less efficiently in B-NHLr. Thus, immune modulation of CD95 transcription and alternative splicing combined with enhanced engagement of mitochondrial death signalling offer potential for restoration of CD95 apoptosis sensitivity in B-NHLr.

Up-regulation of c-FLIPS+R upon CD40 stimulation is associated with inhibition of CD95-induced apoptosis in primary precursor B-ALL

Previous studies on apoptosis defects in acute lymphoblastic leukemia (ALL) have focused on chemotherapy-induced, primarily mitochondrial death pathways. Yet, immunologic surveillance mechanisms including sensitization to apoptotic signals mediated via the death receptor CD95 might contribute to leukemic control. Here, we show that primary B-cell precursor ALL cells from children escape from receptor-dependent cell death in 2 ways: Resting ALL blasts are protected from receptor-mediated apoptosis due to the absence of CD95 surface expression. However, even though CD40 ligation results in up-regulation of CD95, ALL blasts, unlike normal B cells, remain resistant to apoptosis. We show that this apoptosis resistance involves the selective up-regulation of the short isoforms of the caspase-8 inhibitor c-FLIP acting directly at the CD95 receptor level. Treatment with cycloheximide during CD40 activation prevents up-regulation of those c-FLIP isoforms and sensitizes ALL cells toward CD95-mediated apoptosis. We therefore propose that induction of the short c-FLIP isoforms inhibits the onset of CD95-induced apoptosis in primary CD40-stimulated ALL cells despite high CD95 expression.

A Fas agonist induces high levels of apoptosis in haematological malignancies

We developed and tested a potent hexameric Fas agonist, termed MegaFasL, for its cytotoxic effects on a panel of human haematopoietic malignant cells and healthy human haematopoietic progenitor cells (CD34+CD38low). Results demonstrated that MegaFasL induced apoptosis in cell lines and primary cells representing multiple myeloma (MM), acute myeloid leukaemia (AML), acute lymphoblastic leukaemia (ALL) and Burkitt's lymphoma. Cells from a chronic myeloid leukaemia (CML) line and from patients with chronic lymphocytic leukaemia (CLL) were resistant. Furthermore, CD34+CD38low progenitor cells were also resistant to MegaFasL. The data indicate that MegaFasL could be a highly efficient therapeutic agent ex vivo or potentially in vivo.

Apoptosis induction in peripheral leukemia cells by remission induction treatment in vivo: selective depletion and apoptosis in a CD34+ subpopulation of leukemia cells

In vitro studies demonstrating the induction of programmed cell death by cytotoxic drugs used in anticancer chemotherapy suggested that antileukemic treatment eliminates leukemia cells by apoptosis. We therefore analyzed apoptosis induction and activation of apoptosis signaling molecules in patients receiving remission induction treatment for AML and ALL during the initial phase of leukemia cell reduction. A coexistence of distinct populations of CD34(+) and CD34(-) leukemia cells could be identified. During chemotherapy, CD34(+) leukemia cells were more rapidly depleted than CD34(-) cells. Furthermore, a significant increase in leukemia cell apoptosis ex vivo was detected in CD34(+) cells, while no such increase was observed in the CD34(-) subpopulation, suggesting that CD34(+) leukemia cells are the main targets for apoptosis induction through antileukemic treatment. No alterations in Bax and Bcl-2 expression were found during in vivo chemotherapy, and CD95 expression and sensitivity remained low, indicating the induction of apoptosis independent of the CD95 system or regulation of protein levels of Bax and Bcl-2. The data suggest that analysis of leukemia cell subpopulations is required for further identification of apoptosis signaling molecules relevant for response to treatment and assessment of drug efficacy in vivo and in vitro.