Evaluation of Apaf-1 and procaspases-2, -3, -7, -8, and -9 as potential prognostic markers in acute leukemia

Caspase-10 involvement in cytotoxic drug-induced apoptosis of tumor cells

Anticancer drugs can induce tumor cell death by caspase-dependent apoptosis. The observation that procaspase-10 expression decreased in leukemic cells from acute myeloblastic leukemia patients at first relapse led us to explore the role of caspase-10 in cytotoxic drug-induced apoptosis. We show that caspase-10 is activated in etoposide-treated cells in a dose- and time-dependent manner. A caspase-10 peptide inhibitor, a caspase-10 dominant-negative mutant or a small interfering RNA (siRNA)-mediated downregulation of the enzyme negatively interfere with drug-induced cell death and caspase-2, -3, -8 and -9 activation. The extrinsic pathway to apoptosis is not involved in drug-induced caspase-10 activation that occurs downstream of Bax redistribution to mitochondria and cytochrome c release from this organelle. siRNA-mediated downregulation of Apaf-1 prevents etoposide-mediated activation of caspase-10. In a cell-free assay, cytochrome c and dATP treatment of cell extracts after immunodepletion of either caspase-3 or caspase-9 indicates that caspase-10 is activated downstream of caspase-9. Then, caspase-10 is involved in a feedback amplification loop that amplifies caspase-9 and -3 activities. Altogether, these data indicate an active role for caspase-10 in cytotoxic drug-induced tumor cell death, downstream of the mitochondria.

Methylation silencing of the Apaf-1 gene in acute leukemia

Apaf-1 is important for tumor suppression and drug resistance because it plays a central role in DNA damage-induced apoptosis. Inactivation of the Apaf-1 gene is implicated in disease progression and chemoresistance of some malignancies. In this study, we attempted to clarify the role of Apaf-1 in leukemogenesis. Apaf-1 mRNA levels were below the detection limit or very low in 5 of 20 human leukemia cell lines (25%) and 5 of 12 primary acute myeloblastic leukemia cells (42%). There were no gross structural abnormalities in the Apaf-1 gene in these samples. Expression of factors regulating Apaf-1 transcription, such as E2F-1, p53, and Sp-1, did not differ between Apaf-1-positive and Apaf-1-negative cells. Methylation of CpG in the region between +87 and +128 of the Apaf-1 gene was almost exclusively observed in Apaf-1-defective cell lines. Treatment of these cells with 5-aza-2'-deoxycytidine, a specific inhibitor of DNA methylation, restored the expression of Apaf-1. Furthermore, we showed that the region between +87 and +128 could act as a repressor element by recruiting corepressors such as methylated DNA-binding domain 2 and histone deacetylase 1 upon methylation. Overexpression of Dnmt1, a mammalian maintenance DNA methyltransferase, was associated with Apaf-1 gene methylation. DNAs from Dnmt1-overexpressing cells were more resistant to digestion with methylation-sensitive enzyme HpaII than those from cells with low Dnmt1 expression, suggesting that Dnmt1 mediates aberrant methylation of multiple genes. In conclusion, methylation silencing is a mechanism of the inactivation of Apaf-1 in acute leukemia, and Dnmt1 overexpression may underlie hypermethylation of the Apaf-1 gene.

High expression levels of x-linked inhibitor of apoptosis protein and survivin correlate with poor overall survival in childhood de novo acute myeloid leukemia

PURPOSE

Apoptosis-related proteins are important molecules for predicting chemotherapy response and prognosis in adult acute myeloid leukemia (AML). However, data on the expression and prognostic impact of these molecules in childhood AML are rare.

EXPERIMENTAL DESIGN

Using flow cytometry and Western blot analysis, we, therefore, investigated 45 leukemic cell samples from children with de novo AML enrolled and treated within the German AML-BFM93 study for the expression of apoptosis-regulating proteins [CD95, Bcl-2, Bax, Bcl-xL, procaspase-3, X-linked inhibitor of apoptosis protein (XIAP), cellular inhibitor of apoptosis protein-1 (cIAP-1), survivin].

RESULTS

XIAP (P < 0.002) but no other apoptosis regulators showed maturation-dependent expression differences as determined by French-American-British (FAB) morphology with the highest expression levels observed within the immature M0/1 subtypes. XIAP (P < 0.01) and Bcl-xL (P < 0.01) expression was lower in patients with favorable rather than intermediate/poor cytogenetics. After a mean follow-up of 34 months, a shorter overall survival was associated with high expression levels of XIAP [30 (n = 10) versus 41 months (n = 34); P < 0.05] and survivin [27 (n = 10) versus 41 months (n = 34); P < 0.05].

CONCLUSIONS

We conclude that apoptosis-related molecules are associated with maturation stage, cytogenetic risk groups, and therapy outcome in childhood de novo AML. The observed association of XIAP with immature FAB types, intermediate/poor cytogenetics, and poor overall survival should be confirmed within prospective pediatric AML trials.

Latent sensitivity to Fas-mediated apoptosis after CD40 ligation may explain activity of CD154 gene therapy in chronic lymphocytic leukemia

Patients with chronic lymphocytic leukemia (CLL) treated with adenovirus (Ad)-CD154 (CD40L) gene therapy experience reductions in leukemia cell counts and lymph node size associated with induction of the death receptor Fas (CD95). CD4 T cell lines can induce apoptosis of CD40-activated CLL cells via a CD95 ligand (CD95-L)-dependent mechanism. To examine whether CD95-L was sufficient to induce cytolysis of CD40-activated CLL cells, we used Chinese hamster ovary cells transfected with CD95-L as cytotoxic effector cells. CD40-activated CLL cells were initially resistant to CD95-mediated apoptosis despite high-level expression of CD95. However, after 72 h, CLL cells from seven of seven patients became increasingly sensitive to CD95-mediated apoptosis. This sensitivity correlated with a progressive decline in Flice-inhibitory protein (FLIP), which was induced within 24 h of CD40 ligation. Down-regulation of FLIP with an antisense oligonucleotide or a pharmacologic agent, however, was not sufficient to render CLL cells sensitive to CD95-mediated apoptosis in the 24-72 h after CD40 activation. Although the levels of pro-Caspase-8 appeared sufficient, inadequate levels of Fas-associated death domain protein (FADD) and DAP3 may preclude assembly of the death-inducing signaling complex. Seventy-two hours after CD40 ligation, sensitivity to CD95 and a progressive increase in FADD and DAP3 were associated with the acquired ability of FADD and FLIP to coimmunoprecipitate with the death-inducing signaling complex after CD95 ligation. Collectively, these studies reveal that CD40 ligation on CLL B cells induces a programmed series of events in which the cells initially are protected and then sensitized to CD95-mediated apoptosis through shifts in the balance of the anti- and proapoptotic proteins FLIP and FADD.

Bax translocation is crucial for the sensitivity of leukaemic cells to etoposide-induced apoptosis

Bax translocation from cytosol to mitochondria is believed to be a crucial step for triggering cytochrome c release from mitochondria. However, it is unclear whether Bax translocation is associated with Bax induction by DNA damaging agents. The induction of Bax in response to DNA damaging agents has been considered to be linked with p53. In this study, we used the p53 negative human chronic myeloid leukaemia K562 cell line. Bax up-regulation occurred at the whole cell level after DNA damage induced by etoposide. However, after incubation with etoposide, Bax failed to translocate to mitochondria and as a result, the apoptotic process was blocked. A Bax stable transfectant, the K/Bax cell line, expressed more Bax protein in the cytosol, mitochondria and nuclei. This Bax overexpression induced cytochrome c release, a reduction of cytochrome c oxidase activity and mitochondrial membrane potential (Delta(Psi)m). However, Bax-induced apoptosis was blocked downstream of mitochondria in K562 cells. The increased levels of mitochondrial Bax sensitized cells to etoposide-induced activation of caspases-2, -3 and -9 and apoptosis. However, after transient transfection with the Apaf-1 gene, K/Bax cells were sensitized to etoposide-induced caspase activation and apoptosis to a larger extent compared with Bax or Apaf-1 transfection alone. We therefore conclude that two mechanisms contribute to the resistance of K562 cells to etoposide-induced apoptosis; firstly failure of Bax targeting to mitochondria and, secondly, deficiency of Apaf-1. Uncoupling of Bax translocation from Bax induction can occur in response to etoposide-induced DNA damage.