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Cancerous inhibitor of protein phosphatase 2A (CIP2A) is a predictive biomarker of disease progression in many malignancies, including imatinib-treated chronic myeloid leukemia (CML). Although high CIP2A levels correlate with disease progression in CML, the underlying molecular mechanisms remain elusive. In a screen of diagnostic chronic phase samples from patients with high and low CIP2A protein levels, high CIP2A levels correlate with an antiapoptotic phenotype, characterized by downregulation of proapoptotic BCL-2 family members, including BIM, PUMA and HRK, and upregulation of the antiapoptotic protein BCL-XL. These results suggest that the poor prognosis of patients with high CIP2A levels is due to an antiapoptotic phenotype. Disrupting this antiapoptotic phenotype by inhibition of BCL-XLvia RNA interference or A-1331852, a novel, potent and BCL-XL-selective inhibitor, resulted in extensive apoptosis either alone or in combination with imatinib, dasatinib or nilotinib, both in cell lines and in primary CD34+cells from patients with high levels of CIP2A. These results demonstrate that BCL-XLis the major antiapoptotic survival protein and may be a novel therapeutic target in CML.

Apaf-1, by binding to and activating caspase-9, plays a critical role in apoptosis. Oligomerization of Apaf-1, in the presence of dATP and cytochrome c, is required for the activation of caspase-9 and produces a caspase activating apoptosome complex. Reconstitution studies with recombinant proteins have indicated that the size of this complex is very large in the order of approximately 1.4 MDa. We now demonstrate that dATP activation of cell lysates results in the formation of two large Apaf-1-containing apoptosome complexes with M(r) values of approximately 1.4 MDa and approximately 700 kDa. Kinetic analysis demonstrates that in vitro the approximately 700-kDa complex is produced more rapidly than the approximately 1.4 MDa complex and exhibits a much greater ability to activate effector caspases. Significantly, in human tumor monocytic cells undergoing apoptosis after treatment with either etoposide or N-tosyl-l-phenylalanyl chloromethyl ketone (TPCK), the approximately 700-kDa Apaf-1 containing apoptosome complex was predominately formed. This complex processed effector caspases. Thus, the approximately 700-kDa complex appears to be the correctly formed and biologically active apoptosome complex, which is assembled during apoptosis.

Release of cytochrome c is important in many forms of apoptosis. Recent studies of CD95 (Fas/APO-1)-induced apoptosis have implicated caspase-8 cleavage of Bid, a BH3 domain-containing proapoptotic member of the Bcl-2 family, in this release. We now demonstrate that both receptor-induced (CD95 and tumor necrosis factor) and chemical-induced apoptosis result in a similar time-dependent activation of caspases-3, -7, -8, and -9 in Jurkat T cells and human leukemic U937 cells. In receptor-mediated apoptosis, the caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone (Z-VAD. FMK), inhibits apoptosis prior to commitment to cell death by inhibiting the upstream activator caspase-8, cleavage of Bid, release of mitochondrial cytochrome c, processing of effector caspases, loss of mitochondrial membrane potential, and externalization of phosphatidylserine. However, Z-VAD.FMK inhibits chemical-induced apoptosis at a stage after commitment to cell death by inhibiting the initiator caspase-9 and the resultant postmitochondrial activation of effector caspases. Cleavage of Bid but not release of cytochrome c is blocked by Z-VAD.FMK demonstrating that in chemical-induced apoptosis cytochrome c release is caspase-independent and is not mediated by activation of Bid. We propose that caspases form an integral part of the cell death-inducing mechanism in receptor-mediated apoptosis, whereas in chemical-induced apoptosis they act solely as executioners of apoptosis.

In mammals, apoptotic protease-activating factor 1 (Apaf-1), cytochrome c, and dATP activate caspase-9, which initiates the postmitochondrial-mediated caspase cascade by proteolytic cleavage/activation of effector caspases to form active approximately 60-kDa heterotetramers. We now demonstrate that activation of caspases either in apoptotic cells or following dATP activation of cell lysates results in the formation of two large but different sized protein complexes, the "aposome" and the "microaposome". Surprisingly, most of the DEVDase activity in the lysate was present in the aposome and microaposome complexes with only small amounts of active caspase-3 present as its free approximately 60-kDa heterotetramer. The larger aposome complex (M(r) = approximately 700,000) contained Apaf-1 and processed caspase-9, -3, and -7. The smaller microaposome complex (M(r) = approximately 200,000-300,000) contained active caspase-3 and -7 but little if any Apaf-1 or active caspase-9. Lysates isolated from control THP.1 cells, prior to caspase activation, showed striking differences in the distribution of key apoptotic proteins. Apaf-1 and procaspase-7 may be functionally complexed as they eluted as an approximately 200-300-kDa complex, which did not have caspase cleavage (DEVDase) activity. Procaspase-3 and -9 were present as separate and smaller 60-90-kDa (dimer) complexes. During caspase activation, Apaf-1, caspase-9, and the effector caspases redistributed and formed the aposome. This resulted in the processing of the effector caspases, which were then released, possibly bound to other proteins, to form the microaposome complex.

Caspases plays a key role in the execution phase of apoptosis. "Initiator" caspases, such as caspase-8, activate "effector" caspases, such as caspase-3 and -7, which subsequently cleave cellular substrates thereby precipitating the dramatic morphological changes of apoptosis. Following treatment of mice with an agonistic anti-Fas antibody to induce massive hepatocyte apoptosis, we now demonstrate a distinct subcellular localization of the effector caspases-3 and -7. Active caspase-3 is confined primarily to the cytosol, whereas active caspase-7 is associated almost exclusively with the mitochondrial and microsomal fractions. These data suggest that caspases-3 and -7 exert their primary functions in different cellular compartments and offer a possible explanation of the presence of caspase homologs with overlapping substrate specificities. Translocation and activation of caspase-7 to the endoplasmic reticulum correlates with the proteolytic cleavage of the endoplasmic reticular-specific substrate, sterol regulatory element-binding protein 1. Liver damage, induction of apoptosis, activation and translocation of caspase-7, and proteolysis of sterol regulatory element-binding protein 1 are all blocked by the caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone (Z-VAD. fmk). Our data demonstrate for the first time the differential subcellular compartmentalization of specific effector caspases following the induction of apoptosis in vivo.

A human receptor for the cytotoxic ligand TRAIL (TRAIL receptor-1, designated DR4) was identified recently as a member of the tumor necrosis factor receptor family. In this report we describe the identification of two additional human TRAIL receptors, TRAIL receptor-2 and TRAIL receptor-3, that belong to the tumor necrosis factor receptor family. Interestingly, TRAIL receptor-2 but not TRAIL receptor-3 contains a cytoplasmic "death domain" necessary for induction of apoptosis and is hence designated death receptor-5 (DR5). Like DR4, DR5 engages the apoptotic pathway independent of the adaptor molecule FADD/MORT1. Because of its lack of a death domain, TRAIL receptor-3 is not capable of inducing apoptosis. However, by competing for TRAIL, it is capable of inhibiting TRAIL-induced apoptosis. Thus, TRAIL receptor-3 may function as an antagonistic decoy receptor to attenuate the cytotoxic effect of TRAIL in most tissues that are TRAIL+, DR4+, and DR5+.

Apoptosis is a programmed form of cell death characterized by biochemical and morphological changes affecting the nucleus, cytoplasm, and plasma membrane. These changes in various cellular compartments are widely regarded as mechanistically linked events in a single "program" in which activation of caspases and proteolysis of intracellular substrates represent a final common pathway leading to cell death. To date there has been very limited exploration of the linkage of this program to the plasma membrane changes, which bring about swift recognition, uptake, and safe degradation of apoptotic cells by phagocytes. Using the mitochondrial inhibitors antimycin A and oligomycin in human monocytic THP.1 cells triggered into apoptosis, we report the uncoupling of plasma membrane changes from other features of apoptosis. These inhibitors blocked increased plasma membrane permeability, externalization of phosphatidylserine, and recognition by two classes of phagocytes but not activation of caspase-3, cleavage of poly(ADP-ribose) polymerase and DNA fragmentation. Externalization of phosphatidylserine in apoptotic human leukemic U937 cells was also dissociated from caspase activation. Thus changes governing safe clearance of apoptotic cells may be regulated by an independent pathway to those bringing about caspase activation. This finding could have important consequences for attempts to manipulate cell death for therapeutic gain in vivo.

Inhibitors of macromolecular synthesis and topoisomerases induce apoptosis in the human leukaemic cell line, U937. In this study, U937 cells were treated with the RNA synthesis inhibitor, actinomycin D (1 microM), the protein synthesis inhibitors, emetine (1 microM) and cycloheximide (100 microM), the topoisomerase II inhibitor, teniposide (5 microM), or the topoisomerase I inhibitor, camptothecin (1 microM). Apoptotic cell death was assessed both by flow cytometry and agarose gel electrophoresis, and was correlated to the appearance of large (20 to > or = 580 kilobase pairs) DNA fragments, as assessed by field inversion gel electrophoresis. In all cases, the appearance of DNA fragments of 20-50 kilobase pairs accompanied the appearance of an apoptotic population and of internucleosomal cleavage. However, teniposide additionally induced a marked increase in fragmentation to > or = 580 kilobase pairs. The cotreatment of cells with zinc (1 mM) inhibited the formation of all large DNA fragments, internucleosomal cleavage and the appearance of an apoptotic population. We conclude that the generation of large DNA fragments is characteristic of apoptosis induced by various stimuli in U937, as has been found previously in rat thymocytes. However, unlike what occurs in rat thymocytes, zinc treatment does not dissociate the formation of large fragments from conventional markers of apoptosis.