Nuclear localization of procaspase-9 and processing by a caspase-3-like activity in mammary epithelial cells

Identification and functional characterization of Caspase-9 in goldfish (Carassius auratus L.) in response to Aeromonas hydrophila infection

Apoptosis plays a pivotal role in the immune response to combat pathogen infections. In mammals, caspase-9, abbreviated as Casp9, plays an irreplaceable role in the initiation phase of the apoptotic cascade. To investigate the role of Casp9 in teleosts, we conducted a functional characterization of Casp9 in goldfish (Carassius auratus L.). The open reading frame of GfCasp9 spans 1296 base pairs (bp), encoding a protein composed of 431 amino acids. GfCasp9 was ubiquitously expressed in various tissues, with the spleen and brain showing the highest levels of expression. Subcellular localization analysis revealed that GfCasp9 is distributed in both the cytoplasm and nucleus. Overexpressing of GfCasp9 in HEK293 cells elicits a robust apoptotic response. Additionally, infection with Aeromonas hydrophila significantly increases the mRNA and protein expression of GfCasp9. These findings underscore the critical importance of GfCasp9 in immune responses and apoptosis against bacterial infections.

Apoptosis regulation by subcellular relocation of caspases

The cleavage of nuclear proteins by caspases promotes nuclear breakdown and, therefore, plays a key role in apoptosis execution. However, the detailed molecular mechanisms of these events remain unclear. To get more insights into the mechanisms of nuclear events during apoptosis we set up a rapid fractionation protocol for the separation of the cytoplasmic and nuclear fractions of cells undergoing cisplatin-induced apoptosis. Importantly, nuclear accumulation of effector caspase-3 as well as initiator caspase-2, -8 and -9 was observed using the developed protocol and immunofluorescence microscopy. The detection of caspases and their cleavage products in the nucleus occurred within the same time interval after cisplatin treatment and took place shortly before nuclear fragmentation. The entry of initiator caspases to the nucleus was independent of caspase-3. Given that all three initiator caspases had catalytic activity in the nuclei, our findings indicate that initiator caspases might participate in the proteolysis of nuclear components during apoptosis, promoting its disintegration and apoptotic cell death.

Apoptosis-associated protein expression in human salivary gland morphogenesis

OBJECTIVE

Salivary gland (SG) development is based on branching morphogenesis, in which programmed cell death has been proposed to play a role in cell signalling and organ shaping. In the mouse salivary gland apoptosis has been suggested to play a key role in lumen formation, removing the central cells of the epithelial stalks. Here we analyse the expression of several anti- and pro-regulators of apoptosis during human SG development in a range of developmental stages.

DESIGN

Foetal SGs obtained from the University of São Paulo were analysed by immunohistochemistry to assess the expression of apoptosis-associated proteins: caspases (caspase-6, -7, -9 and cleaved caspase-3), Bcl-2 family members (Bax, Bak, Bad, Bid, Bcl-2, Bcl-x and Bcl-xL), Survivin (BIRC5), Cytochrome C and Apaf-1.

RESULTS

Nuclear expression of Bax and Bak was identified in presumptive luminal areas at initial stages, while Bcl-xL showed the most relevant anti-apoptotic activity. Caspase-6, -7 and -9 were expressed during all stages, while interestingly cleaved caspase-3 showed no prominent expression, indicating that caspase-7 is the main effector. Apoptosome complex components Apaf-1 and Cytochrome C, as well as survivin were all positive in developing glands.

CONCLUSIONS

The particular expression pattern of several apoptotic regulators in human SG development suggests the existence of a fundamental role for apoptosis during duct formation. The absence of Bad and Bid expressions indicates that the instrinsic pathway is more active then the extrinsic during human gland formation. The subcellular localisation of intrinsic-apoptosis proteins correlated with apoptotic activity, but also suggested additional non-apoptotic functions.

Grb7 and Hax1 may colocalize partially to mitochondria in EGF-treated SKBR3 cells and their interaction can affect Caspase3 cleavage of Hax1

Growth factor receptor bound protein 7 (Grb7) is a signal-transducing adaptor protein that mediates specific protein-protein interactions in multiple signaling pathways. Grb7, with Grb10 and Grb14, is members of the Grb7 protein family. The topology of the Grb7 family members contains several protein-binding domains that facilitate the formation of protein complexes, and high signal transduction efficiency. Grb7 has been found overexpressed in several types of cancers and cancer cell lines and is presumed involved in cancer progression through promotion of cell proliferation and migration via interactions with the erythroblastosis oncogene B 2 (human epidermal growth factor receptor 2) receptor, focal adhesion kinase, Ras-GTPases, and other signaling partners. We previously reported Grb7 binds to Hax1 (HS1 associated protein X1) isoform 1, an anti-apoptotic protein also involved in cell proliferation and calcium homeostasis. In this study, we confirm that the in vitro Grb7/Hax1 interaction is exclusive to these two proteins and their interaction does not depend on Grb7 dimerization state. In addition, we report Grb7 and Hax1 isoform 1 may colocalize partially to mitochondria in epidermal growth factor-treated SKBR3 cells and growth conditions can affect this colocalization. Moreover, Grb7 can affect Caspase3 cleavage of Hax1 isoform 1 in vitro, and Grb7 expression may slow Caspase3 cleavage of Hax1 isoform 1 in apoptotic HeLa cells. Finally, Grb7 is shown to increase cell viability in apoptotic HeLa cells in a time-dependent manner. Taken together, these discoveries provide clues for the role of a Grb7/Hax1 protein interaction in apoptosis pathways involving Hax1. Copyright © 2016 John Wiley & Sons, Ltd.

Preapoptotic cell stress response of primary hepatocytes

Primary hepatocytes are an important in vitro model for studying metabolism in man. Caspase-9 and Bcl-2-associated X protein (Bax) are regulators of the apoptotic pathway. Here we report on the translocation of procaspase-9 and Bax from cytoplasm to nuclei as well as on dispersion of mitochondria; these processes occur after isolation of primary hepatocytes. The observed changes appear similar to those at the beginning of apoptosis; however, the isolated hepatocytes are not apoptotic for the following reasons: (1) cells have a normal morphology and function; (2) the mitochondria are energized; (3) there is no apoptosis unless it is induced by, e.g., staurosporine or nodularin. Staurosporine does not trigger apoptosis through activation of caspase-9, as its activity is detected later than that of caspase-3. We propose that the translocation of procaspase-9 and Bax into the nuclei reduces the ability to trigger apoptosis through the intrinsic apoptotic pathway. The shifts of procaspase-9 and Bax are reversible in the absence of the apoptotic trigger; the spontaneous reversion was confirmed experimentally for procaspase-9, whereas Bax shifted from the nuclei to the cytosol and mitochondria after the initiation of apoptosis. To distinguish this process from apoptosis, we call it preapoptotic cell stress response. It shares some features with apoptosis; however, it is reversible and apoptosis has to be induced in addition to this process.

CONCLUSION

Knowledge on preapoptotic cell stress response is important for assessing the quality of the cells used in cell therapies, in regenerative medicine, and of those used for modeling metabolic processes.

Caspase-2(L), caspase-9, and caspase-3 during in vitro maturation and fragmentation of the mouse oocyte

Several studies have shown that apoptotic pathways control fragmentation of unfertilized ovulated oocyte, induced by doxorubicin. But very few have investigated the basis of this process, from prophase I to later stages. Our results revealed the presence of caspase-2(L), caspase-9, and caspase-3 in their zymogen and cleaved forms in the oocyte before meiosis resumption. Caspase-2(L) and caspase-9 were detected in the nucleus of GV-oocytes in a distribution related to chromatin configuration. The inhibition of caspase activity by Z-VAD-fmk accelerated the transition from metaphase I to metaphase II, and caspase-9 and caspase-3 were detected along the meiotic spindle. Surprisingly, Western blot analysis revealed that the three cleaved caspases were present in similar amounts in healthy and fragmented oocytes and caspase inhibition did not prevent doxorubicin-induced apoptosis. Our results suggest that, if cleaved, caspases may be dispensable for final oocyte death and they could be involved in regulating the maturation process.

DYRK1A phosphorylates caspase 9 at an inhibitory site and is potently inhibited in human cells by harmine

DYRK1A is a member of the dual-specificity tyrosine-phosphorylation-regulated protein kinase family and is implicated in Down's syndrome. Here, we identify the cysteine aspartyl protease caspase 9, a critical component of the intrinsic apoptotic pathway, as a substrate of DYRK1A. Depletion of DYRK1A from human cells by short interfering RNA inhibits the basal phosphorylation of caspase 9 at an inhibitory site, Thr125. DYRK1A-dependent phosphorylation of Thr125 is also blocked by harmine, confirming the use of this beta-carboline alkaloid as a potent inhibitor of DYRK1A in cells. We show that harmine not only inhibits the protein-serine/threonine kinase activity of mature DYRK1A, but also its autophosphorylation on tyrosine during translation, indicating that harmine prevents formation of the active enzyme. When co-expressed in cells, DYRK1A interacts with caspase 9, strongly induces Thr125 phosphorylation and inhibits caspase 9 auto-processing. Phosphorylation of caspase 9 by DYRK1A involves co-localization to the nucleus. These results indicate that DYRK1A sets a threshold for the activation of caspase 9 through basal inhibitory phosphorylation of this protease. Regulation of apoptosis through inhibitory phosphorylation of caspase 9 may play a role in the function of DYRK1A during development and in pathogenesis.

C-terminal cleavage of the amyloid-beta protein precursor at Asp664: a switch associated with Alzheimer's disease

In addition to the proteolytic cleavages that give rise to amyloid-beta (Abeta), the amyloid-beta protein precursor (AbetaPP) is cleaved at Asp664 intracytoplasmically. This cleavage releases a cytotoxic peptide, APP-C31, removes AbetaPP-interaction motifs required for signaling and internalization, and is required for the generation of AD-like deficits in a mouse model of the disease. Although we and others had previously shown that Asp664 cleavage of AbetaPP is increased in AD brains, the distribution of the Asp664-cleaved forms of AbetaPP in non-diseased and AD brains at different ages had not been determined. Confirming previous reports, we found that Asp664-cleaved forms of AbetaPP were increased in neuronal cytoplasm and nuclei in early-stage AD brains but were absent in age-matched, non-diseased control brains and in late-stage AD brains. Remarkably, however, Asp664-cleaved AbetaPP was prominent in neuronal somata and in processes in entorhinal cortex and hippocampus of non-diseased human brains at ages <45 years. Our observations suggest that Asp664 cleavage of AbetaPP may be part of the normal proteolytic processing of AbetaPP in young (<45 years) human brain and that this cleavage is down-regulated with normal aging, but is aberrantly increased and altered in location in early AD.

Lack of pathology in a triple transgenic mouse model of Alzheimer's disease after overexpression of the anti-apoptotic protein Bcl-2

Alzheimer's disease (AD) is characterized by the accumulation of plaques containing beta-amyloid (Abeta) and neurofibrillary tangles (NFTs) consisting of modified tau. Although Abeta deposition is thought to precede the formation of NFTs in AD, the molecular steps connecting these two pathologies is not known. Previous studies have suggested that caspase activation plays an important role in promoting the pathology associated with AD. To further understand the contribution of caspases in disease progression, a triple transgenic Alzheimer's mouse model overexpressing the anti-apoptotic protein Bcl-2 was generated. Here we show that overexpression of Bcl-2 limited caspase-9 activation and reduced the caspase cleavage of tau. Moreover, overexpression of Bcl-2 attenuated the processing of APP (amyloid precursor protein) and tau and reduced the number of NFTs and extracellular deposits of Abeta associated with these animals. In addition, overexpression of Bcl-2 in 3xTg-AD mice improved place recognition memory. These findings suggest that the activation of apoptotic pathways may be an early event in AD and contributes to the pathological processes that promote the disease mechanisms underlying AD.

Cell proliferation, apoptosis and mitochondrial damage in rat B50 neuronal cells after cisplatin treatment

OBJECTIVES

Cisplatin (cisPt) is used as a chemotherapeutic agent for the treatment of a variety of human tumours; more recently, it has been demonstrated that tumour cell exposure to cisPt ultimately results in apoptosis, but the mechanism by which nuclear cisPt/DNA generates the cytoplasmic cascade of events involved has not been clarified. We have investigated the effects of cisPt on proliferation in the neuronal cell line B50, with particular attention being given to understand whether mitochondria are a target of cisPt and their involvement in the apoptotic process.

MATERIALS AND METHODS

Rat neuronal B50 cells were used to investigate the mechanisms of cisPt-induced cytotoxicity; this line has been used as a model system for neurotoxicity in vivo.

RESULTS

Changes in proliferation, induction of apoptosis, activation of caspase-3 and DNA fragmentation were observed in the cells, as well as morphological and biochemical alterations of mithocondria. Activation of caspase-9 confirmed that mitochondria are a target of cisPt.

CONCLUSION

CisPt exerts cytotoxic effects in the neuronal B50 cell line via a caspase-dependent pathway with mitochondria being central relay stations.

Procaspase-9 is attached to the mitochondrial outer membrane in the early stages of apoptosis

Procaspase-9 is the zymogen form of one of the apoptosis initiators, caspase-9. Its cellular location may differ depending on the cell type; it is found throughout the cytosol, although some of it may be associated with the mitochondria. Procaspase-9 relocates from the cytosol to the mitochondria shortly after the triggering of apoptosis in rat hepatocytes. We investigated whether the mitochondrial protein import machineries import procaspase-9. The combined results of protein import analyses, mitochondrial fractionation and protease treatments of intact and swollen mitochondria imply that procaspase-9 attaches to the outer surface of the mitochondrial outer membrane.

Nuclear expression of cIAP-1, an apoptosis inhibiting protein, predicts lymph node metastasis and poor patient prognosis in head and neck squamous cell carcinomas

cIAP-1, an apoptosis inhibiting protein, has been suggested to play important roles in the development of cervical and esophageal squamous cell carcinomas (SCCs). In order to clarify the subcellular localization of cIAP-1 and to investigate its clinicopathological significance in head and neck SCCs (HNSCCs), we examined cIAP-1 expression in four oral SCC cell lines by immunocytochemistry and Western blot. Expressions of nuclear and cytoplasmic cIAP-1, caspase-3, and Smac/DIABLO were also examined immunohistochemically in 57 cases of the HNSCCs. cIAP-1 expression was detected in HSC-2, HSC-3, and HSC-4 cells by immunohistochemistry and Western blot. In HSC-2 and HSC-4 cells, cIAP-1 was detected in both the nuclear and cytoplasmic fractions. Nuclear cIAP-1 expression was positive in 17 (30%) of HNSCCs, was correlated with lymph node metastasis (P=0.020) and advanced disease stage (P=0.032), and tended to be correlated with poor patient prognosis (P=0.059). Cytoplasmic cIAP-1 expression showed similar but weaker clinicopathological correlations. Nuclear cIAP-1 expression was inversely correlated with caspase-3 expression, but was correlated with Smac/DIABLO expression. Nuclear cIAP-1 expression appears to be a useful marker for predicting poor patient prognosis in HNSCCs, and may play roles in HNSCCs through the signaling pathway mediated by Smac/DIABLO and caspase-3.

Caspase-9 can antagonize p53-induced apoptosis by generating a p76(Rb) truncated form of Rb

The tumor suppressor Rb (retinoblastoma protein) is known to regulate p53-dependent apoptosis, but the mechanisms involved are unclear. In a rat fibroblast model, we previously observed that caspase inhibition potentiates p53-dependent apoptosis and prevents the Rb cleavage associated with p53 activation. These results suggested that a caspase(s) can antagonize p53-mediated apoptosis via the production of a protective Rb truncated form. Here, we identify caspase-9 as the caspase that interferes, upstream of the mitochondrion, with p53-induced apoptosis in both immortalized and primary fibroblasts. This caspase can be detected as a p38 processed form in living cells, in the absence of apoptosome formation and apoptotic signal. We also provide evidence that the involvement of caspase-9 in a pre-mitochondrial protective pathway results from the previously undescribed cleavage of Rb, at a LExD site, into a p76(Rb) form, which antagonizes p53-induced apoptosis. These results establish that a truncated form of Rb can display an antiapoptotic activity, rather than just being a by-product of Rb degradation.

Constitutive nuclear localization of activated caspase 3 in subpopulations of the astroglial family of cells

In a study examining for apoptosis in spontaneously hypertensive rats (SHR), we observed the constitutive presence of activated caspase 3 in Bergmann glia. We then examined stroke-prone SHR as well as the normotensive strains Wistar, Wistar Kyoto, and Sprague-Dawley. In all these strains, we found that Bergmann glia expressed activated caspase 3 in nuclei. Furthermore, subpopulations of astrocytes in the granular layer of the cerebellar cortex, in the hippocampus, and spinal cord gray matter, particularly in the dorsal part of the dorsal horns, expressed nuclear activated caspase 3. This distribution corresponds to the distribution of astrocytes that express the glutamate transporter GLAST. We conclude that Bergmann glia and a subpopulation of astrocytes throughout the CNS express activated caspase 3 in nuclei to fulfill a yet-to-be defined non-apoptotic function. GLIA.

Immunohistochemical distribution of proteins belonging to the receptor-mediated and the mitochondrial apoptotic pathways in human placenta during gestation

The balance between cell death and cell proliferation and its regulation are essential features of many physiological processes and are particularly important in fetal morphogenesis and adult tissue homeostasis. Apoptosis is a type of cell suicide that is activated in two main ways: through a receptor-mediated pathway or through a mitochondrial pathway. We have investigated the immunohistochemical distribution of proteins belonging to these two pathways in human placenta during gestation by comparing their expression levels between the first and third trimester of gestation. In the first trimester, the receptor-mediated pathway prevails over the mitochondrial pathway with a moderate/intense expression of its three components, viz., Fas ligand (FasL), Fas, and caspase-8, and weak positivity of anti-apoptotic FLIP, these proteins being mainly localized in the cytotrophoblast compartment. In the third trimester of gestation, there is an increased expression of mitochondrial pathway proteins, viz., Apaf-1 and caspase-9. We have also investigated the expression level of caspase-3, the primary effector caspase of both pathways, and have observed that it is moderately expressed during gestation, being mainly localized in the cytotrophoblast during the first trimester and in both placental compartments during the third trimester of gestation. Thus, both pathways actively function in human placenta to execute cell death. By means of immunoelectron microscopy, we have further shown that, in human placenta, the two proteins of the mitochondrial pathway together with caspase-3 are localized both in the cytoplasm and in the nucleus. In particular, Apaf-1 and caspase-9 are distributed near to the nuclear envelope suggesting an important role for these two proteins in disrupting the nuclear-cytoplasmic barrier.

Nucling recruits Apaf-1/pro-caspase-9 complex for the induction of stress-induced apoptosis

Nucling is a novel protein isolated from murine embryonal carcinoma cells with an up-regulated expression during cardiac muscle differentiation. We show here that Nucling was up-regulated by proapoptotic stimuli and important for the induction of apoptosis after cytotoxic stress. We further demonstrated that overexpressed Nucling was able to induce apoptosis. In Nucling-deficient cells, the expression levels of Apaf-1 and cytochrome c, which are the major components of an apoptosis-promoting complex named apoptosome, were both down-regulated under cellular stress. A deficiency of Nucling also conferred resistance to apoptotic stress on the cell. After UV irradiation, Nucling was shown to reside in an Apaf-1/pro-caspase-9 complex, suggesting that Nucling might be a key molecule for the formation and maintenance of this complex. Nucling induced translocation of Apaf-1 to the nucleus, thereby distributing the Nucling/Apaf-1/pro-caspase-9 complex to the nuclear fraction. These findings suggest that Nucling recruits and transports the apoptosome complex during stress-induced apoptosis.

Nuclear localization of the hypoxia-regulated pro-apoptotic protein BNIP3 after global brain ischemia in the rat hippocampus

The 19 kD interacting protein 3, Nip3/BNIP3, is a pro-apoptotic member of the Bcl-2 family induced during hypoxia via the hypoxia-inducible factor (HIF) 1. BNIP3 has been linked to both apoptotic and necrotic cell death involving mitochondrial permeability transition. Since apoptotic and necrotic mechanisms may occur in brain ischemia, immunohistochemical changes of BNIP3 were studied at 1, 2, 3 and 7 days after transient global brain ischemia (12.5 min) in ventilated normothermic rats. In control brains, BNIP3-like immunoreactivity was moderately strong in neuronal processes or cytoplasm and absent in the nucleus. In the ischemia-vulnerable CA1 neurons, BNIP3-positive granules were seen in the nucleus at 1 and 2 days, and these neurons were damaged at 3 and 7 days. The resistant CA3 neurons showed nuclear BNIP3 labeling by 1 day and then returned to the normal state. BNIP3-positive granules did not overlap with the nucleolus. Constitutively expressed BNIP3 may participate in apoptotic and necrotic processes after brain ischemia. Nuclear location of BNIP3 after brain ischemia indicates a novel role for the regulation of cell survival in neurons or a general disturbance of the nuclear envelope.

ROS-dependent caspase-9 activation in hypoxic cell death

Mitochondria are known to play a fundamental role in apoptosis by releasing apoptogenic molecules such as cytochrome c into the cytoplasm, thereby sequentially activating initiator caspase-9. However, the mechanisms of cytochrome c release or caspase-9 activation in response to hypoxia are unclear. In this report, we show that caspase-9 is activated by reactive oxygen species (ROS) without involvement of cytochrome c release in hypoxic injury. In addition, activated caspase-9 induces permeability transition (PT)-independent cytochrome c release, suggesting that caspase-9 may disrupt mitochondrial diffusion limit of cytochrome c and serve to amplify further release of cytochrome c.

Apoptosis triggered redistribution of caspase-9 from cytoplasm to mitochondria

Caspase-9 is an apoptosis initiator protease activated as a response to the mitochondrial damage in the cytoplasmic complex apoptosome. By fluorescence labelling of proteins, confocal microscopy and subcellular fractionations we demonstrate that caspase-9 is in the cytoplasm of non-apoptotic pituitary cells. The activation of apoptosis with rotenone triggers the redistribution of caspase-9 to mitochondria. Experiments using the general caspase inhibitor z-VAD.fmk and the specific caspase-9 inhibitor z-LEHD.fmk show that the caspase-9 redistribution is a regulated process and requires the activity of a caspase other than the caspase-9. We propose that this spatial regulation is required to control the activity of caspase-9.

Caspase-12 processing and fragment translocation into nuclei of tunicamycin-treated cells

Excess endoplasmic reticulum (ER) stress induces processing of caspase-12, which is located in the ER, and cell death. However, little is known about the relationship between caspase-12 processing and cell death. We prepared antisera against putative caspase-12 cleavage sites (anti-m12D318 and anti-m12D341) and showed that overexpression of caspase-12 induced autoprocessing at D(318) but did not induce cell death. Mutation analysis confirmed that D(318) was a unique autoprocessing site. In contrast, tunicamycin, one of the ER stress stimuli, induced caspase-12 processing at the N-terminal region and the C-terminal region (both at D(318) and D(341)) and cell death. Anti-m12D318 and anti-m12D341 immunoreactivities were located in the ER of the tunicamycin-treated cells, and some immunoreactivities were located around and in the nuclei of the apoptotic cells. Thus, processing at the N-terminal region may be necessary for the translocation of processed caspase-12 into nuclei and cell death induced by ER stress. Some of the caspase-12 processed at the N-terminal and C-terminal regions may directly participate in the apoptotic events in nuclei.

Expression and transcriptional regulation of caspase-14 in simple and complex epithelia

Caspase-14 is a recent addition to the caspase family of aspartate proteases involved in apoptotic processes. Human caspase-14 appears to be only weakly processed during apoptosis, and it does not cleave classical caspase substrates. Post partum, caspase-14 is prominently expressed by human keratinocytes and reportedly participates in terminal differentiation of complex epithelia. Here we provide evidence challenging the view that caspase-14 expression or processing is linked exclusively to terminal keratinocyte differentiation. We demonstrate that caspase-14 expression extended to multiple cell lines derived from simple epithelia of the breast, prostate, and stomach. In keratinocytes and breast epithelial cells, caspase-14 expression was upregulated in high-density cultures and during forced suspension culture. These effects were primarily due to transcriptional activation as indicated by reporter gene assays using a 2 kb caspase-14 promoter fragment. Importantly, caspase-14 was not cleaved during forced suspension culture of either cell type although this treatment induced caspase-dependent apoptosis (anoikis). Forced expression of caspase-14 in immortalized human keratinocytes had no effect on cell death in forced suspension nor was the transfected caspase-14 processed in this setting. In contrast to postconfluent and forced suspension culture, terminal differentiation of keratinocytes induced in vitro by Ca2+ treatment was not associated with increased caspase-14 expression or promoter activity. Our results indicate that (1) caspase-14 is expressed not only in complex but also simple epithelia; (2) cells derived from complex and simple epithelia upregulate caspase-14 expression in conditions of high cell density or lack of matrix interaction and; (3) in both cell types this phenomenon is due to transcriptional regulation.

The role of mitochondria, cytochrome c and caspase-9 in embryonic lens fibre cell denucleation

During the differentiation of secondary lens fibre cells from the lens epithelium, the fibre cells lose all of their cytoplasmic organelles as well as their nuclei. The fibre cells, containing crystallins, which confer optical clarity, then persist in the adult lens. The process of denucleation of these cells has been likened to an apoptotic event which is not followed by the plasma membrane changes that are characteristic of apoptosis. We have examined the expression and subcellular translocation of molecules of the apoptotic cascade in differentiating lens epithelial cells in culture. In this culture system, the epithelial cells differentiate into lentoids composed of lens fibre cells. We find that caspase-9, which is expressed and activated before embryonic day 12 in intact lenses, is localized in the cytosol outside mitochondria in non-differentiating cultured cells. In lentoid cells, caspase-9 migrates into mitochondria after the latter undergo a membrane permeability transition that is characteristic of apoptotic cells. At the same time, caspase-9 co-localizes with cytochrome c in the cytosol. The cytochrome c is apparently released from the mitochondria in lentoid cells after the mitochondrial membrane permeability transition and during the period of nuclear shrinkage. Also during this time, the mitochondria aggregate around the degenerating nuclei. Cytochrome c disappears rapidly, while mitochondrial breakdown occurs approximately coincident with the disappearance of the nuclei, but mitochondrial remnants persist together with cytochrome c oxidase, which is a mitochondrial marker protein. Apaf-1, another cytosolic protein of the apoptotic cascade, also migrates to the permeabilized mitochondria and also co-localizes with caspase-9 and cytochrome c in the cytosol or mitochondria of denucleating cells, thus providing evidence for the formation of an 'apoptosome' in these cells, as in apoptotic cells. At no time did we observe the translocation of molecules between cytoplasmic compartments and the nucleus in differentiating lentoid cells. We suggest that the uncoupling of nuclear and membrane apoptotic events in these cells may be due to the early permeability changes in the mitochondria, resulting in the loss of mitochondrial signalling molecules, or to the failure of molecules to migrate to the nucleus in these cells, thus failing to activate nuclear-plasma membrane signalling pathways.

Phosphorylation by protein kinase CK2: a signaling switch for the caspase-inhibiting protein ARC

Caspases play a central role in apoptosis, but their activity is under the control of caspase-inhibiting proteins. A characteristic of caspase-inhibiting proteins is direct caspase binding. It is yet unknown how the localization of caspase-inhibiting proteins is regulated and whether there are upstream signals controlling their function. Here we report that the function of ARC is regulated by protein kinase CK2. ARC at threonine 149 is phosphorylated by CK2. This phosphorylation targets ARC to mitochondria. ARC is able to bind to caspase-8 only when it is localized to mitochondria but not to the cytoplasm. Our results reveal a molecular mechanism by which a caspase-inhibiting protein requires phosphorylation in order to prevent apoptosis.

Nuclear Apaf-1 and cytochrome c redistribution following stress-induced apoptosis

Apoptotic protease activating factor-1 (Apaf-1) and cytochrome c are cofactors critical for inducing caspase-9 activation following stress-induced apoptosis. One consequence of caspase-9 activation is nuclear-cytoplasmic barrier disassembly, which is required for nuclear caspase-3 translocation. In the nucleus, caspase-3 triggers proteolysis of the caspase-activated DNA nuclease (CAD) inhibitor, causing CAD induction and subsequent DNA degradation. Here we demonstrate that apoptotic cells show perinuclear cytochrome c aggregation, which may be critical for nuclear redistribution of cytochrome c and Apaf-1. We thus indicate that the nuclear redistribution of these cofactors concurs with the previously reported caspase-9-induced nuclear disassembly, and may represent an early apoptotic hallmark.

Mitochondria-dependent caspase-9 activation is necessary for antigen receptor-mediated effector caspase activation and apoptosis in WEHI 231 lymphoma cells

Engagement of the B cell Ag receptor (BCR) on immature B cells leads to growth arrest followed by apoptosis. Concomitant signaling through CD40 sustains proliferation and rescues the cells from apoptosis. Previously, we have shown that cross-linking CD40 on B cells stimulates the expression of A1, an antiapoptotic member of the Bcl-2 family, and that transduction of the murine B lymphoma line WEHI 231, a model for immature B cells, with A1 protected the cells against BCR-induced apoptosis. Here we demonstrate that A1 strongly interferes with activation of caspase-7, the major effector caspase activated after BCR cross-linking on WEHI 231 lymphoma cells. The pathway leading to activation of the effector caspase cascade including caspase-7 is unclear. Using retrovirally transduced WEHI 231 cell populations, we show that a catalytically inactive mutant of caspase-7 is cleaved almost as efficiently as the wild-type form, arguing against autocatalysis as the sole activating process. In contrast, overexpression of catalytically inactive caspase-9 strongly interferes with caspase-7 processing, poly(ADP-ribose) polymerase cleavage, and DNA laddering, suggesting a role for caspase-9 and hence for the mitochondrial pathway. The importance of the mitochondrial/caspase-9 pathway for BCR-triggered apoptosis is highlighted by our finding that both A1 and the mutant caspase-9 attenuate BCR-induced apoptosis. Thus, our data suggest that the BCR-mediated apoptotic signal in immature B cells spreads via a mitochondrial/caspase-9 pathway.

Caspase activation in retinas of diabetic and galactosemic mice and diabetic patients

Apoptosis of retinal capillary cells begins early in diabetes and likely contributes to the capillary obliteration that is an important feature of diabetic retinopathy. Caspases are proteolytic enzymes that are closely involved in the induction and execution phases of apoptosis, but their role in the development of diabetic retinopathy has not been studied previously. Our study focused on the measurement of activities of multiple caspases in retinas of mice at different durations of diabetes. Several caspases (including caspases-1, -2, -6, -8, and -9) were activated as early as 2 months of diabetes. The caspases activity pattern changed with increasing duration of disease, suggesting a slowly developing caspases cascade. Activities of executioner caspases (e.g., cas-6 and -3) became elevated after longer duration of diabetes, and the induction of cas-3 activity was associated with the duration of diabetes at which capillary cells begin to show evidence of undergoing apoptosis. Retinas from patients with type 2 diabetes likewise showed a significant increase in activities of cas-1, -3, -4, and -6. For comparison, retinal caspases were also measured in experimental galactosemia, another model that develops a diabetic-like retinopathy. The pattern of caspases activation differed between diabetes and galactosemia, but cas-1 activity became elevated soon after elevation of blood hexose concentration in both. Caspases offer new therapeutic targets to test the role of apoptosis in the development of diabetic retinopathy.

Use of fluorescently labeled caspase inhibitors as affinity labels to detect activated caspases

Activation of caspases is the key event of apoptosis and different approaches were developed to assay it. To detect their activation in situ, we applied fluorochrome labeled inhibitors of caspases (FLICA) as affinity labels of active centers of these enzymes. The FLICA ligands are fluorescein or sulforhodamine conjugated peptide-fluoromethyl ketones that covalently bind to enzymatic centers of caspases with 1:1 stoichiometry. The specificity of FLICA towards individual caspases is provided by the peptide sequence of amino acids. Exposure of live cells to FLICA results in uptake of these ligands and their binding to activated caspases; unbound FLICA is removed by cell rinse. Cells labeled with FLICA can be examined by fluorescence microscopy or subjected to quantitative analysis by cytometry. Intracellular binding sites of FLICA are consistent with known localization of caspases. Covalent binding of FLICA allowed us to identify the labeled proteins by immunoblotting: the proteins that bound individual FLICAs had molecular weight between 17 and 22 kDa, which corresponds to large subunits of the caspases. Detection of caspases activation by FLICA can be combined with other markers of apoptosis or cell cycle for multiparametric analysis. Because FLICA are caspase inhibitors they arrest the process of apoptosis preventing cell disintegration. The stathmo-apoptotic method was developed, therefore, that allows one to assay cumulative apoptotic index over long period of time and estimate the rate of cell entry into apoptosis for large cell populations. FLICA offers a rapid and convenient assay of caspases activation and can also be used to accurately estimate the incidence of apoptosis.

Differential subcellular localization of caspase family proteins in the adult rat brain

The distribution of the caspase family (caspase-2, -3, -6, -7, -8, -9, -10) was assessed using immunochemical detection of subcellular fractions of 8-week-old rat brain tissues. The present study demonstrated that the relative protein level of caspase-2, -3, -6, -8 and -10 was highest in the soluble cytosolic fraction, while that for caspase-9 was highest in the nucleus. We also found that caspase-3 and -6 were present at high levels and caspase-2, -8 and -9 at moderate levels in the nerve endings fraction as well as in the soluble cytosolic fraction. These results suggest that rat brain caspases are differentially expressed in the subcellular fractions of the rat brain, and that caspases not only contribute to the regulation of neuronal death, but also to synaptic plasticity.

C/EBPbeta phosphorylation by RSK creates a functional XEXD caspase inhibitory box critical for cell survival

Upon activation by liver injury, hepatic stellate cells produce excessive fibrous tissue leading to cirrhosis. The hepatotoxin CCl(4) induced activation of RSK, phosphorylation of C/EBPbeta on Thr(217), and proliferation of stellate cells in normal mice, but caused apoptosis of these cells in C/EBPbeta-/- or C/EBPbeta-Ala(217) (a dominant-negative nonphosphorylatable mutant) transgenic mice. Both C/EBPbeta-PThr(217) and the phosphorylation mimic C/EBPbeta-Glu(217), but not C/EBPbeta-Ala(217), were associated with procaspases 1 and 8 in vivo and in vitro and inhibited their activation. Our data suggest that C/EBPbeta phosphorylation on Thr(217) creates a functional XEXD caspase substrate/inhibitor box (K-Phospho-T(217)VD) that is mimicked by C/EBPbeta-Glu(217) (KE(217)VD). C/EBPbeta-/- and C/EBPbeta-Ala(217) stellate cells were rescued from apoptosis by the cell permeant KE(217)VD tetrapeptide or C/EBPbeta-Glu(217).

Subcellular redistribution of BAX during apoptosis induced by anticancer drugs

BAX is the 192-amino acid, 21-kDa protein which is ubiquitously distributed in normal tissues and is regarded as a tumor suppressor sensitizing malignant cells to anticancer drugs. In spite of many studies, the molecular mechanism of BAX action is still obscure. In the present study subcellular BAX translocations in human colon adenocarcinoma COLO 205 cells exposed to various anticancer drugs [camptothecin (CPT), etoposide (ETO), staurosporine (STP), 2-chloro-2'-deoxyadenosine (2CdA) and nimesulide (NIM)] was examined. Cells were grown on coverslips under optimal conditions (10% FCS/DMEM) or were stimulated to apoptosis with the drugs examined. Laser scanning cytometry was applied for the quantitative analysis of BAX expression, and distribution in the cytoplasmic (BAX Cf) and nuclear (BAX Nf) area. BAX maximal pixel (BAX MP), the parameter corresponding to aggregation of BAX in the cell, was also measured. All examined drugs increased the number of cells with high BAX MP, reaching the peak at 60 min after drug administration. The most pronounced effect was in the case of 2CdA, CPT and STP. The increase in BAX MP was observed only when antibody recognizing the 43-61 amino acid sequence was used. When antibody binding the N-terminal epitope (11-30 amino acid sequence) was applied, the number of cells expressing high BAX MP significantly decreased. These results indicate that apoptotic stimuli delivered by anticancer drugs led to aggregation of BAX in cancer cells, which is dependent on BAX activation by its cleavage at the N-terminal epitope and exposure of the BH3 domain. It was shown that BAX Nf increased in cells treated with CPT, STP, ETO, 2CdA and NIM, whereas BAX Cf rose after STP and NIM. The increase in BAX Nf and, occurring in most treatments, the increase in the BAX Nf:Cf ratio indicates a BAX shift from the cytoplasm to the nucleus. Furthermore, staining with different antibodies showed that only the activated form of BAX was translocated to the nucleus. Immunoelectron microscopy revealed that CPT-induced apoptosis was associated with translocation of BAX from the cytosol to organellar membranes (mitochondrial, Golgi apparatus and endoplasmic reticulum) and via nuclear envelope pores to the nucleus, occurring within 60-180 min of cell exposure to the drug. The subcellular translocations of BAX preceded in time the appearance of morphological symptoms of apoptosis. In conclusion, (i) in spite of different molecular mechanisms of apoptosis induction by the anticancer drugs examined, BAX remains a common link in the chain of reactions leading to cell death, and (ii) BAX activation and subcellular translocations from the cytosol to organellar membranes and nucleus are key cellular responses to drugs bearing proapoptotic properties.

Detection of caspases activation by fluorochrome-labeled inhibitors: Multiparameter analysis by laser scanning cytometry

BACKGROUND

The fluorochrome-labeled inhibitors of caspases (FLICA) were recently used as markers of activation of these enzymes in live cells during apoptosis (Bedner et al.: Exp Cell Res 259:308-313, 2000). The aims of this study were to (a) explore if FLICA can be used to study intracellular localization of caspases; (b) combine the detection of caspase activation with analysis of the changes with cell morphology detected by microscopy and laser scanning cytometry (LSC); and (c) adapt the assay to fixed cells that would enable correlation, by multiparameter analysis, of caspase activation with the cell attributes that require cell permeabilization in order to be measured.

METHODS

Apoptosis of human MCF-7, U-937, or HL-60 cells was induced by camptothecin (CPT) or tumor necrosis factor-alpha (TNF-alpha) combined with cycloheximide (CHX). Binding of FLICA to apoptotic versus nonapoptotic cells was studied in live cells as well as following their fixation and counterstaining of DNA. Intensity of cell labeling with FLICA and DNA-specific fluorochromes was measured by LSC.

RESULTS

Exposure of live cells to FLICA led to selective labeling of cells that had morphological changes characteristic of apoptosis. The FLICA labeling withstood cell fixation and permeabilization, which made it possible to stain DNA and measure its content for identification of the cell cycle position of labeled cells. When fixed cells were treated with FLICA, both apoptotic and nonapoptotic cells became strongly labeled and the labeling pattern was consistent with the localization of caspases as reported in the literature. A translocation of the FLICA binding targets from mitochondria to cytosol was seen in the MCF-7 cells treated with CPT. FLICA binding was largely (> 90%) prevented by the substrates of the caspases or by the unlabeled caspase inhibitors having the same peptide moiety as the respective FLICA.

CONCLUSIONS

The detection of caspase activation combined with cell permeabilization requires exposure of live cells to FLICA followed by their fixation. Cell reactivity with the respective FLICA, under these conditions, identifies the activated caspases and makes it possible to correlate their activation with the cell cycle position and other cell attributes that can be measured only after cell fixation/permeabilization. FLICA can also be used to study intracellular localization of caspases, including their translocation.

Characterization of a novel isoform of caspase-9 that inhibits apoptosis

We have identified a novel isoform of rat caspase-9 in which the C terminus of full-length caspase-9 is replaced with an alternative peptide sequence. Casp-9-CTD (where CTD is carboxyl-terminal divergent) is expressed in multiple tissues, with the relative highest expression observed in ovary and heart. Casp-9-CTD was found primarily in the cytoplasm and was not detected in the nucleus. Structural predictions suggest that in contrast to full-length caspase-9, casp-9-CTD will not be processed. Our model is supported by reduced protease activity of casp-9-CTD preparations in vitro and by the lack of detectable processing of casp-9-CTD proenzyme or the induction of cell death following transfection into cells. Both neuronal and non-neuronal cell types transfected with casp-9-CTD were resistant to death evoked by trophic factor deprivation or DNA damage. In addition, cytosolic lysates prepared from cells permanently expressing exogenous casp-9-CTD were resistant to caspase induction by cytochrome c in reconstitution assays. Taken together, our observations indicate that casp-9-CTD acts as a dominant-negative variant. Its expression in various tissues indicates a physiological role in regulating cell death.