We report that chlamydiae, which are obligate intracellular bacterial pathogens, possess a novel antiapoptotic mechanism. Chlamydia-infected host cells are profoundly resistant to apoptosis induced by a wide spectrum of proapoptotic stimuli including the kinase inhibitor staurosporine, the DNA-damaging agent etoposide, and several immunological apoptosis-inducing molecules such as tumor necrosis factor-α, Fas antibody, and granzyme B/perforin. The antiapoptotic activity was dependent on chlamydial but not host protein synthesis. These observations suggest that chlamydia may encode factors that interrupt many different host cell apoptotic pathways. We found that activation of the downstream caspase 3 and cleavage of poly (ADP-ribose) polymerase were inhibited in chlamydia-infected cells. Mitochondrial cytochrome c release into the cytosol induced by proapoptotic factors was also prevented by chlamydial infection. These observations suggest that chlamydial proteins may interrupt diverse apoptotic pathways by blocking mitochondrial cytochrome c release, a central step proposed to convert the upstream private pathways into an effector apoptotic pathway for amplification of downstream caspases. Thus, we have identified a chlamydial antiapoptosis mechanism(s) that will help define chlamydial pathogenesis and may also provide information about the central mechanisms regulating host cell apoptosis.

During embryonic development, a large number of cells die naturally to shape the new organism. Members of the caspase family of proteases are essential intracellular death effectors. Herein, we generated caspase-2-deficient mice to evaluate the requirement for this enzyme in various paradigms of apoptosis. Excess numbers of germ cells were endowed in ovaries of mutant mice and the oocytes were found to be resistant to cell death following exposure to chemotherapeutic drugs. Apoptosis mediated by granzyme B and perforin was defective in caspase-2-deficient B lymphoblasts. In contrast, cell death of motor neurons during development was accelerated in caspase-2-deficient mice. In addition, caspase-2-deficient sympathetic neurons underwent apoptosis more effectively than wild-type neurons when deprived of NGF. Thus, caspase-2 acts both as a positive and negative cell death effector, depending upon cell lineage and stage of development.

Many apoptotic signaling pathways are directed to mitochondria, where they initiate the release of apoptogenic proteins and open the proposed mitochondrial permeability transition (PT) pore that ultimately results in the activation of the caspase proteases responsible for cell disassembly. BNIP3 (formerly NIP3) is a member of the Bcl-2 family that is expressed in mitochondria and induces apoptosis without a functional BH3 domain. We report that endogenous BNIP3 is loosely associated with mitochondrial membrane in normal tissue but fully integrates into the mitochondrial outer membrane with the N terminus in the cytoplasm and the C terminus in the membrane during induction of cell death. Surprisingly, BNIP3-mediated cell death is independent of Apaf-1, caspase activation, cytochrome c release, and nuclear translocation of apoptosis-inducing factor. However, cells transfected with BNIP3 exhibit early plasma membrane permeability, mitochondrial damage, extensive cytoplasmic vacuolation, and mitochondrial autophagy, yielding a morphotype that is typical of necrosis. These changes were accompanied by rapid and profound mitochondrial dysfunction characterized by opening of the mitochondrial PT pore, proton electrochemical gradient (⌬m) suppression, and increased reactive oxygen species production. The PT pore inhibitors cyclosporin A and bongkrekic acid blocked mitochondrial dysregulation and cell death. We propose that BNIP3 is a gene that mediates a necrosis-like cell death through PT pore opening and mitochondrial dysfunction.

The baculovirus antiapoptotic protein p35 inhibited the proteolytic activity of human interleukin-1 beta converting enzyme (ICE) and three of its homologs in enzymatic assays. Coexpression of p35 prevented the autoproteolytic activation of ICE from its precursor form and blocked ICE-induced apoptosis. Inhibition of enzymatic activity correlated with the cleavage of p35 and the formation of a stable ICE-p35 complex. The ability of p35 to block apoptosis in different pathways and in distantly related organisms suggests a central and conserved role for ICE-like proteases in the induction of apoptosis.

No abstract

Activation of the serine-threonine kinase p34cdc2 at an inappropriate time during the cell cycle leads to cell death that resembles apoptosis. Premature activation of p34cdc2 was shown to be required for apoptosis induced by a lymphocyte granule protease. The kinase was rapidly activated and tyrosine dephosphorylated at the initiation of apoptosis. DNA fragmentation and nuclear collapse could be prevented by blocking p34cdc2 activity with excess peptide substrate, or by inactivating p34cdc2 in a temperature-sensitive mutant. Premature p34cdc2 activation may be a general mechanism by which cells induced to undergo apoptosis initiate the disruption of the nucleus.

Nip3 (nineteen kD interacting protein-3) is an E1B 19K and Bcl-2 binding protein of unknown function. Nip3 is detected as both a 60- and 30-kD protein in vivo and in vitro and exhibits strong homologous interaction in a yeast two-hybrid system indicating that it can homodimerize. Nip3 is expressed in mitochondria and a mutant (Nip3163) lacking the putative transmembrane domain and COOH terminus does not dimerize or localize to mitochondria. Transient transfection of epitope-tagged Nip3 in Rat-1 fibroblasts and MCF-7 breast carcinoma induces apoptosis within 12 h while cells transfected with the Nip3163 mutant have a normal phenotype, suggesting that mitochondrial localization is necessary for induction of cell death. Nip3 overexpression increases the sensitivity to apoptosis induced by granzyme B and topoisomerase I and II inhibitors. After transfection, both Nip3 and Nip3163 protein levels decrease steadily over 48 h indicating that the protein is rapidly degraded and this occurs in the absence of cell death. Bcl-2 overexpression initially delays the onset of apoptosis induced by Nip3 but the resistance is completely overcome in longer periods of incubation. Nip3 protein levels are much higher and persist longer in Bcl-2 expressing cells. In conclusion, Nip3 is an apoptosis-inducing dimeric mitochondrial protein that can overcome Bcl-2 suppression.

Transforming growth factor-beta (TGF-beta) can regulate cell growth and differentiation as well as production of extracellular matrix proteins. Elevated production of TGF-beta has been associated with human and rodent chronic inflammatory and fibrotic diseases. Using immunohistochemical staining, we have examined lung sections of patients with advanced idiopathic pulmonary fibrosis (IPF), a disease characterized by chronic inflammation and fibrosis and demonstrated a marked and consistent increase in TGF-beta production in epithelial cells and macrophages when compared to patients with nonspecific inflammation and those with no inflammation or fibrosis. In patients with advanced IPF, intracellular staining with anti-LC (1-30) TGF-beta antibody was seen prominently in bronchiolar epithelial cells. In addition, epithelial cells of honeycomb cysts and hyperplastic type II pneumocytes stained intensely. Anti-CC (1-30) TGF-beta antibody, which reacts with extracellular TGF-beta, was localized in the lamina propria of bronchioles and in subepithelial regions of honeycomb cysts in areas of dense fibroconnective tissue deposition. The close association of subepithelial TGF-beta to the intracellular form in advanced IPF suggests that TGF-beta was produced and secreted primarily by epithelial cells. Because of the well-known effects of TGF-beta on extracellular matrix formation and on epithelial cell differentiation, the increased production of TGF-beta in advanced IPF may be pathogenic to the pulmonary fibrotic and regenerative responses seen in this disease.