Akt/Protein kinase B inhibits cell death by preventing the release of cytochrome c from mitochondria.
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Growth factors signaling through the phosphoinositide 3-kinase/Akt pathway promote cell survival. The mechanism by which the serine/threonine kinase Akt prevents cell death remains unclear. We have previously shown that Akt inhibits the activity of DEVD-targeted caspases without changing the steady-state levels of Bcl-2 and Bcl-x(L). Here we show that Akt inhibits apoptosis and the processing of procaspases to their active forms by delaying mitochondrial changes in a caspase-independent manner. Akt activation is sufficient to inhibit the release of cytochrome c from mitochondria and the alterations in the inner mitochondrial membrane potential. However, Akt cannot inhibit apoptosis induced by microinjection of cytochrome c. We also demonstrated that Akt inhibits apoptosis and cytochrome c release induced by several proapoptotic Bcl-2 family members. Taken together, our results show that Akt promotes cell survival by intervening in the apoptosis cascade before cytochrome c release and caspase activation via a mechanism that is distinct from Bad phosphorylation. (+info)
Safety and antitumor activity of recombinant soluble Apo2 ligand.
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TNF and Fas ligand induce apoptosis in tumor cells; however, their severe toxicity toward normal tissues hampers their application to cancer therapy. Apo2 ligand (Apo2L, or TRAIL) is a related molecule that triggers tumor cell apoptosis. Apo2L mRNA is expressed in many tissues, suggesting that the ligand may be nontoxic to normal cells. To investigate Apo2L's therapeutic potential, we generated in bacteria a potently active soluble version of the native human protein. Several normal cell types were resistant in vitro to apoptosis induction by Apo2L. Repeated intravenous injections of Apo2L in nonhuman primates did not cause detectable toxicity to tissues and organs examined. Apo2L exerted cytostatic or cytotoxic effects in vitro on 32 of 39 cell lines from colon, lung, breast, kidney, brain, and skin cancer. Treatment of athymic mice with Apo2L shortly after tumor xenograft injection markedly reduced tumor incidence. Apo2L treatment of mice bearing solid tumors induced tumor cell apoptosis, suppressed tumor progression, and improved survival. Apo2L cooperated synergistically with the chemotherapeutic drugs 5-fluorouracil or CPT-11, causing substantial tumor regression or complete tumor ablation. Thus, Apo2L may have potent anticancer activity without significant toxicity toward normal tissues. (+info)
Herpes simplex virus thymidine kinase/ganciclovir-induced apoptosis involves ligand-independent death receptor aggregation and activation of caspases.
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Suicide gene therapy systems such as the herpes simplex thymidine kinase/ganciclovir system (TK/GCV) may kill cancer cells by apoptosis through as yet undefined mechanisms. Here we show that TK/GCV treatment induces p53 accumulation and increases cell surface expression of CD95 and tumor necrosis factor receptor, which is likely to involve p53-mediated translocation of CD95 to the cell surface. TK/GCV-induced apoptosis involves CD95-L-independent CD95 aggregation leading to the formation of a Fas-associated death domain protein (FADD) and caspase-8-containing, death-inducing signaling complex. Dominant negative FADD, the caspase-8 inhibitor zIETD-fmk [Z-Ile-Glu(OMe)-Thr-Asp(OMe)-fluoromethylketone], and zVAD-fmk (Z-Val-Ala-Asp-fluoromethylketone) partially abrogate TK/GCV-induced apoptosis. In addition to apoptosis induction, TK/GCV treatment strongly sensitizes for CD95-L-, TNF-, and TNF-related, apoptosis-inducing, ligand (TRAIL)-induced cell death in constitutively resistant cells. These findings may be used to increase the efficacy of TK/GCV and other suicide gene therapy systems for the treatment of cancer. (+info)
Inherited human Caspase 10 mutations underlie defective lymphocyte and dendritic cell apoptosis in autoimmune lymphoproliferative syndrome type II.
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Caspases are cysteine proteases that mediate programmed cell death in phylogenetically diverse multicellular organisms. We report here two kindreds with autoimmune lymphoproliferative syndrome (ALPS) type II, characterized by abnormal lymphocyte and dendritic cell homeostasis and immune regulatory defects, that harbor independent missense mutations in Caspase 10. These encode amino acid substitutions that decrease caspase activity and interfere with death receptor-induced apoptosis, particularly that stimulated by Fas ligand and TRAIL. These results provide evidence that inherited nonlethal caspase abnormalities cause pleiotropic apoptosis defects underlying autoimmunity in ALPS type II. (+info)
Activated human T cells release bioactive Fas ligand and APO2 ligand in microvesicles.
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Activation-induced cell death is a process by which overactivated T cells are eliminated, thus preventing potential autoimmune attacks. Two known mediators of activation-induced cell death are Fas(CD95) ligand (FasL) and APO2 ligand (APO2L)/TNF-related apoptosis-inducing ligand (TRAIL). We show here that upon mitogenic stimulation, bioactive FasL and APO2L are released from the T cell leukemia Jurkat and from normal human T cell blasts as intact, nonproteolyzed proteins associated with a particulate, ultracentrifugable fraction. We have characterized this fraction as microvesicles of 100-200 nm in diameter. These microvesicles are released from Jurkat and T cell blasts shortly (+info)
Expression of the death gene Bik/Nbk promotes sensitivity to drug-induced apoptosis in corticosteroid-resistant T-cell lymphoma and prevents tumor growth in severe combined immunodeficient mice.
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Members of the Bcl-2 gene family have been implicated in the regulation of cell death induced by cytostatic drugs. In some malignancies such as B-cell lymphoma, there is evidence that high expression of Bcl-2 is an independent negative prognostic marker and the overexpression of Bcl-2 has been shown to confer resistance to cytotoxic drugs by preventing drug-induced apoptosis. This function of Bcl-2 can be antagonized by apoptosis-promoting members of the Bcl-2 family. We previously showed that overexpression of Bax restores the chemosensitivity of Bax-deficient breast cancer cell lines. Therefore, we investigated whether the death-promoting Bcl-2 homologue Bik/Nbk can enhance cytostatic drug-induced apoptosis. As a model, we used the T-cell leukemia H9 (CD3(+) and CD4(+)CD8(-)), which is resistant to corticosteroid-induced cell death and does not express endogenous Bik/Nbk. Sensitivity for drug-induced apoptosis was increased 10- to 39-fold in cells transfected with the full-length coding sequence of Bik/Nbk. In addition, apoptosis induced via CD95/Fas or heat shock was increased to a similar extent. These data show that Bik/Nbk, which, unlike Bax, carries only a BH3 but no BH1 or BH2 domain may be a target to enhance chemosensitivity. The complete suppression of tumor growth in a severe combined immunodeficient mouse xenotransplant model suggests that, in analogy to Bax, Bik/Nbk may function as a tumor suppressor gene. (+info)
Animal cell-death suppressors Bcl-x(L) and Ced-9 inhibit cell death in tobacco plants.
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In plants, events similar to programmed cell death have been reported [1] [2], although little is known of their mechanisms at the molecular level. To investigate the mechanism(s) involved, we overexpressed bcl-x(L), which encodes a mammalian suppressor of programmed cell death, in tobacco plants, under the control of a strong promoter [3]. In plants expressing Bcl-x(L), cell death induced by UV-B irradiation, paraquat treatment or the hypersensitive reaction (HR) to tobacco mosaic virus (TMV) infection was suppressed. The extent of suppression of cell death depended on the amount of Bcl-x(L) protein expressed. Similar enhanced resistance to cell death was found in transgenic tobacco plants overexpressing the ced-9 gene, a Caenorhabditis elegans homolog of bcl-x(L) [4], indicating that Bcl-x(L) and Ced-9 can function to inhibit cell death in plants. (+info)
The antiapoptotic decoy receptor TRID/TRAIL-R3 is a p53-regulated DNA damage-inducible gene that is overexpressed in primary tumors of the gastrointestinal tract.
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Both DR4 and DR5 have recently been identified as membrane death receptors that are activated by their ligand TRAIL to engage the intracellular apoptotic machinery. TRID (also named as TRAIL-R3) is an antagonist decoy receptor and lacks the cytoplasmic death domain. TRID protects from TRAIL-induced apoptosis by competing with DR4 and DR5 for binding to TRAIL. TRID has been shown to be overexpressed in normal human tissues but not in malignantly transformed cell lines. DR5 is a p53-regulated gene and we have recently reported that DR5 expression is induced in response to genotoxic stress in both a p53-dependent and independent manner (Sheikh et al., 1998). In the current study, we demonstrate that TRID gene expression is also induced by the genotoxic agents ionizing radiation and methyl methanesulfonate (MMS) in predominantly p53 wild-type cells, whereas UV-irradiation does not induce TRID gene expression. Consistent with these results, exogenous wild-type p53 also upregulates the expression of endogenous TRID in p53-null cells. Thus, TRID appears to be a p53 target gene that is regulated by genotoxic stress in a p53-dependent manner. Using primary gastrointestinal tract (GIT) tumors and their matching normal tissue, we also demonstrate for the first time that TRID expression is enhanced in primary tumors of the GIT. It is, therefore, possible that TRID overexpressing GIT tumors may gain a selective growth advantage by escaping from TRAIL-induced apoptosis. (+info)