[Molecular biology of apoptosis]

Apoptosis, a mechanism involving programmed cell death, is important for normal development and maintenance of tissue homeostasis of multicellular organisms. Apoptotic cells are defined by their fragmented nuclei with condensed chromatin, fragmented or condensed cytoplasm and formation of apoptotic bodies. The apoptotic signal transducing pathways activated by a variety of stimuli, including depletion of growth factors, heat shock, cytokines, DNA damaging reagents and crosslinking of Fas receptor, finally converge into the phylogenically conserved apoptotic main machinery, consisting of death-driving ICE-family proteases and anti-cell death protein Bcl-2. Recently, we noted that necrotic cell death induced by chemical hypoxia shares at least some part of the apoptotic main machinery. Using this system, we have shown that Bcl-2 prevents the loss of the mitochondrial membrane potential observed in both apoptotic and necrotic cell death. We also showed that the ICE protease cascade operates in apoptosis and that Bcl-2 functions upstream of the ICE prolease cascade. Here, we review the signal transducing pathway of the apoptotic main machinery.

Bcl-2 and Bcl-xL block apoptosis as well as necrosis: possible involvement of common mediators in apoptotic and necrotic signal transduction pathways

The proto-oncogene bcl-2 and a bcl-2-related gene bcl-x prevent apoptotic cell death induced by various treatments. Although a mechanism has been proposed that involves Bcl-2 activity on reactive oxygen species (ROS), we find that expression of Bcl-2 or Bcl-xL prevents cell death induced by withdrawal of oxygen (hypoxia) and that the cell death does not involve ROS, suggesting that Bcl-2 or Bcl-xL exerts an anti-cell death function by a mechanism other than through regulation of ROS activity. Using electron microscopy, and confocal and non-confocal fluorescence microscopy, we show that hypoxia induces both necrosis and apoptosis. Overexpression of Bcl-2 or Bcl-xL blocks hypoxia-induced apoptosis and, although to a lesser extent, necrosis. The anti-apoptotic proteins Bcl-2 and Bcl-xL effectively inhibit KCN-induced cell death which is characterized by necrotic features including apparently intact chromatin, remarkable mitochondrial swelling with loss of crista structure and loss of plasma membrane integrity. The necrotic cell death is also inhibited by inhibitors of ICE (interleukin-1 beta converting enzyme)(-like) proteases, the common mediators of apoptosis. These results indicate that Bcl-2/Bcl-xL and ICE(-like) proteases modulate both apoptotic and at least some forms of necrotic cell death, suggesting that both cell death pathways involve some common mediators.

Susceptibility of cerebellar granule neurons derived from Bcl-2-deficient and transgenic mice to cell death

Overproduced Bcl-2 oncoprotein has been shown to suppress cell death induced by a variety of stimuli in many cell types, including neuronal cells. Because bcl-2 is expressed in the nervous system where massive cell death is observed during development, endogenous Bcl-2 is likely to be involved in regulating neuronal cell death. Here we examined the possible role of endogenous Bcl-2 in the regulation of neuronal cell survival in the central nervous system using primary cultured cerebellar granule neurons from bcl-2-deficient, wild-type and NSE-bcl-2-transgenic mice. Cerebellar granule neurons from bcl-2-deficient mice were more susceptible than those from normal littermates to death induced by reducing the K+ concentration of the medium from high (25 mM) to low (5 mM), and neurons from bcl-2-transgenic mice were least susceptible. Similar results were obtained when cell death was induced by serum withdrawal under high K+ conditions or by the presence of etoposide, A23187 or nimodipine. Consistently, bcl-2 deficiency reduced the number of cerebellar granule neurons per mouse. These results indicate that Bcl-2 impedes neuronal cell death induced by various stimuli in a dose-dependent manner, and that endogenous levels of Bcl-2 are able to regulate neuronal cell survival in the central nervous system.

Direct interaction of the mitochondrial membrane protein carnitine palmitoyltransferase I with Bcl-2

To demonstrate interaction between the mitochondrial membrane proteins CPT I and Bcl-2 the yeast two-hybrid system was used. Full-length CPT I was required for binding to Bcl-2. Direct protein interaction was confirmed in a GST binding assay and in coimmunoprecipitations using two different kinds of anti-Bcl-2 antibodies.

Gene transfection of mouse primordial germ cells in vitro and analysis of their survival and growth control

We evaluated electroporation, liposome-mediated transfection, and the calcium phosphate (CaPO4) coprecipitation method for gene transfection of mouse primordial germ cells (PGCs) in culture as a prelude to the investigation of molecular mechanisms of the germ cell development. We found that electroporation severely damaged PGCs, and the efficiency of liposome-mediated transfection was very low. In contrast, using the CaPO4 coprecipitation method, 18% of PGCs transfected with plasmid pSV-LT expressed simian virus 40 large tumor antigen (SV 40 T-Ag) transiently. However, we did not detect any effects on the proliferation and survival of PGCs obtained from the embryonic gonads at 11.5 days postcoitum (d.p.c.) during 2 days of culture after the transfection. PGCs isolated from the 11.5-d.p.c. gonads change from spread- to round-shape and exhibit growth arrest during a few days of culture, and these rounded PGCs quickly disappear from the culture. We found that the transfection and expression of Bcl-XL or adenovirus type 2 E1B 19,000-molecular-weight protein (E1B 19K) significantly promoted the survival of PGCs and retarded the disappearance of rounded PGCs from the culture system. These results suggest that the Bcl-XL or E1B 19K can prevent the apoptosis of PGCs and inhibit the cell death of the rounded PGCs in culture.

Essential role of active nuclear transport in apoptosis

BACKGROUND

Apoptosis is defined by chromatin condensation, nuclear fragmentation and the formation of apoptotic bodies. Because apoptotic signals are transmitted through a common pathway that includes the target steps of death-driving ICE-family proteases and anti-cell death protein Bcl-2 in the cytoplasm, the signals must be transferred from the cytoplasm to the nucleus, at least to induce the apoptotic manifestation of the nucleus. Small signal molecules might diffuse across nuclear pores, but larger molecules are transported by active mechanisms requiring ATP and GTP hydrolysis. It is not known whether apoptotic signals are transmitted into the nucleus by the mechanisms of active nuclear transport.

RESULTS

To test the possibility that active nuclear transport is involved in apoptotic signal transmission, we have analysed the effects of molecules that inhibit active nuclear transport on apoptosis. Wheat germ agglutinin (WGA), excess amounts of p10 protein, Ran-GTPgammaS complex, and anti-PTAC58 antibody, which all inhibit active nuclear transport when exogenously microinjected, prevent Fas-induced apoptotic nuclear manifestation. WGA also prevents apoptotic nuclear change promoted by microinjected active CPP32beta/Yama protease (an ICE family member), which plays an essential role in most apoptosis.

CONCLUSIONS

The results presented here strongly suggest that active nuclear transport is essential for apoptotic signal transduction.

Involvement of ICE family proteases in apoptosis induced by reoxygenation of hypoxic hepatocytes

Cell death due to reoxygenation after hypoxia was characterized in primary cultured hepatocytes. Fluorescence and electron microscopic analyses of reoxygenated hepatocytes revealed morphological characteristics of apoptosis, including chromatin condensation, nuclear fragmentation, and formation of apoptotic bodies. Few necrotic hepatocytes, defined by loss of plasma membrane integrity, mitochondrial swelling, and formation of large vacuoles, were observed. Activation of interleukin-1 beta-converting enzyme (ICE)-like and CPP32/Yama-like proteases, which are known to drive apoptosis, was observed during reoxygenation, and addition of their respective inhibitors inhibited the induction of apoptosis, indicating the involvement of ICE family proteases in apoptosis by reoxygenation. Production of oxygen radicals was enhanced by reoxygenation of hypoxic cells, and reoxygenation-induced apoptosis was inhibited by oxygen radical scavengers, suggesting a role for reactive oxygen species as a triggering factor in cell death. Electrophoretic analysis revealed the presence of 50-kb DNA fragments but not oligonucleosomal DNA fragments in reoxygenation-induced apoptotic hepatocytes.

Functional conservation of mouse Notch receptor family members

All the known members of the mouse Notch receptor family were examined for their biochemical function by interaction with a DNA binding protein RBP-Jkappa. mNotch2, mNotch3 and int3 (= mNotch4) were shown to interact with RBP-Jkappa by the GST-fusion pull down assay and dominant negative competition with Epstein Barr virus nuclear antigen 2. Furthermore the intracellular region of int3 was shown to transactivate the Epstein Barr virus TP1 promoter. These results indicate that all mouse Notch family members have biochemical functions similar to mNotch1, which transduces proliferative signal by direct interaction with the DNA binding protein RBP-Jkappa.

Accelerated disappearance of melanocytes in bcl-2-deficient mice

Follicular melanocytes in bcl-2(-/-) mice have been reported to turn gray during the second hair cycle. Light microscopic analysis revealed that about half of bcl-2(-/-) mouse hair shafts had no detectable melanin granules after the second hair follicle cycle, but the remaining hair appeared to be pigmented normally. After depilation to induce new anagen hair, more than 97% of the hair shafts did not have visible melanin granules in bcl-2(-/-) mice, whereas 100% of the hair shafts in bcl-2(+/+) mice were pigmented. In bcl-2(+/+) mice, dopa-positive melanocytes appeared on day 4 after depilation, whereas bcl-2(-/-) mice developed few dopa-positive melanocytes after depilation, as assessed by light and electron microscopic observation. bcl-2(-/-) mouse hair in the second hair cycle contained about 60-70% less melanin than normal mouse hair, and newly generated bcl-2(-/-) mouse hair after depilation contained a level of melanin as low as that of albino mouse hair. These observations suggest that the expression of bcl-2 might be essential for melanocyte maintenance after the second hair cycle.

Localization of bcl-2, bax, and bcl-x mRNAs in the developing inner ear of the mouse

In situ hybridization was employed to study the expression of bcl-2 mRNA and its family members, bax and bcl-x mRNAs, in the developing inner ear. We found that in the cochlear structure, sensory epithelial cells, the spiral ganglion and stria vascularis expressed these mRNAs in postnatal period in a temporally similar manner, but in embryos, neither bax nor bcl-x mRNA were expressed in the sensory epithelium from embryonic day (E) 13 to 19. In contrast to these patterns, bcl-2 mRNA was expressed by E15 to E19, and the expression at E13 was below the lower limit of detection. Non-neuronal tissue (stria vascularis) also expressed these three transcripts during development. These results suggest that bcl-2 family members may be differentially involved in the differentiation of sensory epithelial cells, spiral ganglia, and stria vascularis. In particular, the differential expression patterns in the cochleovestibular neurons suggest that proliferating and differentiating neurons utilize distinct members of the bcl-2 family.

Bcl-2 blocks loss of mitochondrial membrane potential while ICE inhibitors act at a different step during inhibition of death induced by respiratory chain inhibitors

Bcl-2, Bcl-xL, CrmA and tetrapeptide ICE inhibitor reduce the extent of necrotic cell death induced by cyanide, which primarily damages mitochondria. Although none of them affects the drastic decrease in ATP levels induced by cyanide, Bcl-2 and Bcl-xL but not CrmA or ICE inhibitor inhibit the cyanide-induced decrease in mitochondrial membrane potential. A similar blocking effect is observed on necrotic cell death induced by other respiration inhibitors, rotenone and antimycin A, and on apoptotic cell death induced by etoposide or calcium ionophore. These results indicate that Bc1-2 and Bcl-xL protect mitochondria against the loss of function during both apoptosis and at least some forms of necrotic cell death. The ICE family proteases act at a different step other than the loss of mitochondrial membrane potential.

[Prevention of cell death by Bcl-2 family genes]

Apoptosis is a tightly regulated and evolutionally conserved suicide mechanism. Proteases encoded by ced-3 and ice gene family appear to play a key role in driving apoptosis. On the other hand, Bcl-2 and its relatives have the ability to inhibit apoptosis induced by a variety of stimuli, indicating that Bcl-2 family negatively regulates a key event in a common pathway of apoptosis in which different apoptotic signals finally converge, although the biochemical basis of the function remains unknown. Here, I overview Bcl-2 gene family, and discuss the molecular basis of anti-apoptotic activity of Bcl-2, focusing on our recent findings on Bcl-2 function in mitochondria.

Oxidative stress response in yeast: purification and characterization of glutathione reductase from Hansenula mrakii

Glutathione reductase was purified from a yeast. Hansenula mrakii IFO 0895, to approximately 3500-fold with 59% activity yield. The enzyme was homogeneous on polyacrylamide gel electrophoresis. The molecular weight of the enzyme was estimated to be 56 kDa by SDS-polyacrylamide gel electrophoresis, and 123 kDa by gel filtration using a calibrated Sephadex G-150 column. The Km values for glutathione disulfide and NADPH were 21.3 microM and 14.3 microM, respectively. The enzyme was most active at pH 7.5, 55 degrees C. The enzyme was stable up to 40 degrees C, and between pHs 4 and 10. The enzyme was inhibited by p-chloromercuribenzoate and metal ions such as Fe3+, Cd2+, Cu2+, and Zn2+.

[Inhibition of apoptosis by a baculovirus p35 gene]

The baculovirus gene p35 inhibits virus-induced apoptosis in insect cells. p35 can also inhibit developmentally programmed cell death in Caenorhabditis elegans and Drosophila, mammalian neuronal cell death induced by serum or NGF deprivation, and Fas- and tumor necrosis factor (TNF)-induced apoptosis in mammalian cells, indicating that p35 may interrupt an evolutionally conserved component of the death machinery. Recently it has been shown that p35 protein functions as an inhibitor of ICE/CED-3 cysteine protease family that seem to play an important role in an apoptotic pathway. This observation indicates that p35 may inhibit apoptosis by directly blocking the activities of these cysteine proteases in diverse animals.

Bcl-2 expression prevents activation of the ICE protease cascade

The Bcl-2 family and the ICE family of cysteine proteases play important roles in regulating cell death. We show here that induction of cell death by a Ca2+ ionophore or hypoxia results in increased levels and activity of active ICE(-like) proteases and the subsequent activation of CPP32/Yama(-like) proteases, and that inhibition of these protease activities reduces the extent of cell death. Overexpression of the anti-apoptotic proteins Bcl-2 or Bcl-xL inhibits the cell death and the activation of ICE(-like) and CPP32/Yama(-like) proteases, indicating that Bcl-2 and Bcl-xL act upstream of these proteases. We also show that specific inhibition of ICE(-like) proteases in vivo prevents activation of CPP32/Yama(-like) proteases, whereas inhibition of CPP32/Yama(-like) proteases does not prevent activation of ICE(-like) proteases, suggesting the existence of a protease cascade in vivo that requires ICE(-like) proteases for activation of CPP32/Yama(-like) proteases. Induction of necrotic cell death by KCN also induces activation of ICE(-like) proteases but not of CPP32/Yama(-like) proteases, and Bcl-2 and Bcl-xL inhibit the activation and the cell death, suggesting that the functional site of Bcl-2 and Bcl-xL is also upstream of ICE(-like) proteases in at least some forms of necrosis.

Retardation of chemical hypoxia-induced necrotic cell death by Bcl-2 and ICE inhibitors: possible involvement of common mediators in apoptotic and necrotic signal transductions

Inhibition of the respiratory chain reaction by cyanide, rotenone or antimycin A (chemical hypoxia) induces necrotic cell death characterized by apparently intact chromatin, remarkable mitochondrial swelling with loss of crista structure, and loss of plasma membrane integrity. The treatments induce no apoptotic cell death, as defined by fragmented nuclei with condensed chromatin, fragmented or condensed cytoplasm. The anti-apoptotic proteins Bcl-2 and Bcl-xL effectively retard the chemical hypoxia-induced necrotic cell death. The necrotic cell death is also retarded by inhibitors of ICE(-like) proteases, including interleukin-1beta converting enzyme (ICE), which are common mediators of apoptosis. These results indicate that Bcl-2/Bcl-xL and ICE(-like) proteases modulate apoptotic and at least some forms of necrotic cell death. Both cell death pathways appear to involve some common mediators; however necrotic or apoptotic cell death signals might be transduced through multiple pathways, because Bcl-2/ Bcl-xL or inhibitors of ICE(-like) proteases are relatively less potent in blocking necrotic cell death than in preventing apoptosis.

Cytokine-induced apoptotic cell death in a mouse pancreatic beta-cell line: inhibition by Bcl-2

Cytokines are thought to contribute to the induction of pancreatic beta-cell destruction in insulin-dependent diabetes mellitus. The molecular mechanisms that underlie beta-cell death were investigated by studying cytokine-induced cell death in beta-cell lines. A combination of three cytokines (interleukin-1 beta, tumour necrosis factor-alpha, and interferon-gamma) induced apoptotic cell death in the mouse pancreatic beta-cell line beta TC1, as judged from the appearance of cells with hypodiploid nuclei and oligonucleosomal DNA fragmentation. The same treatment also induced apoptosis in the mouse pancreatic alpha-cell line alpha TC1 and the NOD/Lt mouse beta-cell line NIT-1, although to a lesser extent than in beta TC1 cells. The abundance of endogenous Bcl-2 in beta TC1 cells was lower than that in the other two cell lines. Overexpression of human Bcl-2 in beta TC1 cells partially protected them from cytokine-induced cell death. These results suggest that apoptosis may be responsible, at least in part, for cytokine-induced beta-cell destruction and that Bcl-2 prevents apoptosis in pancreatic islet cells.

Induction of apoptosis as well as necrosis by hypoxia and predominant prevention of apoptosis by Bcl-2 and Bcl-XL

The molecular mechanism of cell death due to hypoxia has not been elucidated. Our recent observations that overexpression of the anti-apoptotic proto-oncogene bcl-2 and a bcl-2-related gene, bcl-x, prevents hypoxic cell death suggest that hypoxia induces apoptosis. Using electron microscopy and confocal and nonconfocal fluorescence microscopy, we show here that hypoxia does, in fact, induce both necrosis and apoptosis, and that the proportion of these two modes is highly dependent on the cell type. Overexpression of Bcl-2 or Bcl-Xl blocks hypoxia-induced apoptosis in a dose-dependent manner.

Involvement of CPP32/Yama(-like) proteases in Fas-mediated apoptosis

Fas (Apo-1/CD95) belongs to the tumor necrosis factor/nerve growth factor receptor family and transmits apoptotic signals by binding to its ligand. Interleukin-1beta-converting enzyme (ICE), which shows substantial homology to the product of the cell death gene, ced-3, of Caenorhabditis elegans, is reported to be involved in Fas-mediated apoptosis. Using two human carcinoma-derived cell lines with undetectable levels of ICE, we found that an agonistic antihuman Fas antibody induces the activation of CPP32/Yama(-like) proteases that are ICE(-like) protease family members, and that a tetrapeptide inhibitor of CPP32/Yama protease, DEVD-CHO, inhibits the Fas-mediated activation of the proteases, Fas-mediated apoptosis, and CPP32/Yama(-like) proteolytic activities in vitro. Fas-mediated apoptosis is inhibited by the CPP32/Yama inhibitor DEVD-CHO, but not by the ICE inhibitor YVAD-CHO, suggesting a dominant role for the CPP32/Yama(-like) proteases and not ICE itself in Fas-mediated apoptosis of the human carcinoma cell lines.

Nitric oxide induces upregulation of Fas and apoptosis in vascular smooth muscle

Interleukin-1 induced a time-dependent release of high levels of nitric oxide from rat vascular smooth muscle cells up to 96 hours. A time-dependent release of lactate dehydrogenase was also induced by Interleukin-1 from 72 to 96 hours after its stimulation. In situ nick end-labeling assay revealed that incubation for 48 hours with interleukin-1 induced a positive staining of fragmented nuclei. However, NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthase, inhibited both lactate dehydrogenase release and DNA fragmentation induced by interleukin-1. Furthermore, sodium nitroprusside, a nitric oxide donor, also induced lactate dehydrogenase release and DNA fragmentation. Fluorescent staining of DNA revealed patches of irregularly dispersed, brightly staining, and condensed chromatin in rat vascular smooth muscle cells treated with sodium nitroprusside. Flow cytometric analysis with monoclonal antibody against human Fas revealed that expression of Fas was upregulated by sodium nitroprusside in human vascular smooth muscle cells. Methylene blue, an inhibitor of soluble guanylate cyclase, did not affect sodium nitroprusside-induced upregulation of Fas. Furthermore, 8-bromo-guanosine 3':5'-cyclic monophosphate, an analogue of cGMP, did not upregulate Fas expression. These findings indicate that nitric oxide released from vascular smooth muscle cells may induce apoptosis in vascular smooth muscle cells themselves and also induced upregulation of Fas via a cGMP-independent mechanism. Thus, nitric oxide could trigger the remodeling of atherosclerotic plaques.

Molecular cloning of the breakpoint of t(11;22) (q23;q11) chromosome translocation in an adult acute myelomonocytic leukaemia

Southern blot analysis with a cDNA probe of MLL indicated that the breakpoint is in a BamHI 8.3 kb fragment which carries the exon 5-11 of MLL gene in DNA from an adult acute myelomonocytic leukaemia with a t(11;22) (q23;q11) translocation. The structural analysis of the rearranged MLL locus demonstrated that the breakpoint is localized between exon 8 and 9 of MLL locus. The normal counterpart fused to the MLL locus was proved to be derived from chromosome 22q11(AF-22) by somatic cell hybrids analysis and FISH. By FISH, AF-22 was localized to the region more centromeric to the BCR gene.

Involvement of Fas in regression of vaginal epithelia after ovariectomy and during an estrous cycle

Fas, also called APO-1, belongs to the tumor necrosis factor/nerve growth factor receptor family and transmits an apoptotic signal within the cell by binding to the Fas ligand. Fas has been implicated in the activation-induced suicide of T cells and cytotoxic T cell activity in the immune system. Non-immune cells such as those in liver, lung and ovary also express Fas, but its role in these cells remains unclear. Ovariectomy has been used to study homeostasis of female reproductive organs, which is regulated by sex hormones. Here we analyzed Fas function in the ovariectomy-induced regression of mouse vaginal epithelial cells. Fas expression was detected in vagina and was elevated after ovariectomy. Fas-deficient lpr and lpr(cg) mice did not exhibit ovariectomy-induced regression of vaginal epithelia, whereas uterine regression induced by ovariectomy was not affected in these mice. The vaginas of lpr and lpr(cg) mice were in a persistent estrous stage with cornification of vaginal epithelia, as judged from the cell types in the vaginal fluid. Thus, Fas appears to be involved directly in the regression of vaginal epithelia induced by ovariectomy and during the estrous cycle, suggesting that the physiological role of this receptor extends beyond that exerted on immune cells. This is the first evidence of a role for Fas inducing physiological apoptosis in non-immune cells.

Chromosomal assignment of the Bcl2-related genes, Bcl2l and Bax, in the mouse and rat

The Bcl2-related genes, Bax and Bcl2l (formerly called Bclx), are involved in the regulation of apoptotic cell death, and the chromosome location of these two genes was determined in the mouse and rat by fluorescence in situ hybridization. Bcl2l was localized to mouse chromosome 2H1 and rat chromosome 3q41.2, and Bax was localized to mouse chromosome 7B5 and rat chromosome 1q31.2. Both genes mapped in a region of conserved linkage homology between the two the species. Molecular linkage analysis using interspecific backcross mice revealed the murine Bcl2l locus at 0.7 cM terminal to D2Mit22, and the murine Bax locus at 0.8 cM terminal to D7Nds5.