Intracellular acidification during apoptosis can occur in the absence of a nucleus

Dual Host-Intracellular Parasite Transcriptome of Enucleated Cells Hosting Leishmania amazonensis: Control of Half-Life of Host Cell Transcripts by the Parasite

Enucleated cells or cytoplasts (cells whose nucleus is removed in vitro) represent an unexplored biological model for intracellular infection studies due to the abrupt interruption of nuclear processing and new RNA synthesis by the host cell in response to pathogen entry. Using enucleated fibroblasts hosting the protozoan parasite Leishmania amazonensis, we demonstrate that parasite multiplication and biogenesis of large parasitophorous vacuoles in which parasites multiply are independent of the host cell nucleus. Dual RNA sequencing of both host cytoplast and intracellular parasite transcripts identified host transcripts that are more preserved or degraded upon interaction with parasites and also parasite genes that are differentially expressed when hosted by nucleated or enucleated cells. Cytoplasts are suitable host cells, which persist in culture for more than 72 h and display functional enrichment of transcripts related to mitochondrial functions and mRNA translation. Crosstalk between nucleated host de novo gene expression in response to intracellular parasitism and the parasite gene expression to counteract or benefit from these host responses induces a parasite transcriptional profile favoring parasite multiplication and aerobic respiration, and a host-parasite transcriptional landscape enriched in host cell metabolic functions related to NAD, fatty acid, and glycolytic metabolism. Conversely, interruption of host nucleus-parasite cross talk by infection of enucleated cells generates a host-parasite transcriptional landscape in which cytoplast transcripts are enriched in phagolysosome-related pathway, prosurvival, and SerpinB-mediated immunomodulation. In addition, predictive in silico analyses indicated that parasite transcript products secreted within cytoplasts interact with host transcript products conserving the host V-ATPase proton translocation function and glutamine/proline metabolism. The collective evidence indicates parasite-mediated control of host cell transcripts half-life that is beneficial to parasite intracellular multiplication and escape from host immune responses. These findings will contribute to improved drug targeting and serve as database for L. amazonensis-host cell interactions.

Acidic pH promotes oligomerization and membrane insertion of the BclXL apoptotic repressor

Solution pH is believed to serve as an intricate regulatory switch in the induction of apoptosis central to embryonic development and cellular homeostasis. Herein, using an array of biophysical techniques, we provide evidence that acidic pH promotes the assembly of BclXL apoptotic repressor into a megadalton oligomer with a plume-like appearance and harboring structural features characteristic of a molten globule. Strikingly, our data reveal that pH tightly modulates not only oligomerization but also ligand binding and membrane insertion of BclXL in a highly subtle manner. Thus, while oligomerization and the accompanying molten globular content of BclXL is least favorable at pH 6, both of these structural features become more pronounced under acidic and alkaline conditions. However, membrane insertion of BclXL appears to be predominantly favored under acidic conditions. In a remarkable contrast, while ligand binding to BclXL optimally occurs at pH 6, it is diminished by an order of magnitude at lower and higher pH. This reciprocal relationship between BclXL oligomerization and ligand binding lends new insights into how pH modulates functional versatility of a key apoptotic regulator and strongly argues that the molten globule may serve as an intermediate primed for membrane insertion in response to apoptotic cues.

Alpha-tocopheryl succinate induces rapid and reversible phosphatidylserine externalization in histiocytic lymphoma through the caspase-independent pathway

Phosphatidylserine (PS) externalization is a key feature of apoptotic cell death and plays an important role in clearance of apoptotic cells by phagocytes. PS externalization during apoptosis is generally an irreversible event mediated by caspase activation and is accompanied by other apoptotic events. We report here that an apoptosis inducer alpha-tocopheryl succinate (TOS) can induce PS externalization that is independent of apoptosis and reversible in the absence of fetal bovine serum (FBS) in histiocytic lymphoma U937 cells. In the presence of FBS, TOS induced PS externalization via a caspase-dependent mechanism accompanied by mitochondrial depolarization, cell shrinkage, increase of caspase-3 activity, and chromatin condensation. In contrast, in the absence of FBS, TOS induced the rapid PS externalization which was not accompanied by other apoptotic events. The PS externalization was reversible by removing TOS and was not involved in Ca(2+)-dependent scramblase activation and thiol oxidation of aminophospholipid translocase. A similar PS externalization was also induced by cholesteryl hemisuccinate (CS), the other succinate ester. These results suggested that the mechanism of TOS- and CS-induced PS externalization in the absence of FBS was different from it occurring during typical apoptosis.

Co-clustering of Golgi complex and other cytoplasmic organelles to crescentic region of half-moon nuclei during apoptosis

Early apoptosis is defined by stereotypic morphological changes, especially evident in the nucleus, where chromatin condenses and compacts, and assumes a globular, half-moon or crescent-shaped morphology. Accumulating evidence suggests that cytoplasmic organelles such as mitochondria and the Golgi complex are major sites of integration of pro-apoptotic signaling. In this study, cytoplasmic organelles including Golgi complex, mitochondria, endosomes, lysosomes, and peroxisomes were shown to condense at the same unique region adjacent to the crescentic nucleus during a relatively early stage of apoptosis induced by staurosporine or other agents. The co-clustering phenomenon may be caused by shrinkage of cytoplasm during apoptosis although cytoskeletal markers actin and tubulin were not condensed and appeared excluded. These data suggest the co-clustering of cytoplasmic organelles plays an interesting role during the progression of the apoptotic process. It is possible that modification of pro-apoptotic proteins may arise as a result of the interplay of these cytoplasmic organelles.

Water spin dynamics during apoptotic cell death in glioma gene therapy probed by T1rho and T2rho

Longitudinal and transverse relaxations in the rotating frame, with characteristic time constants T1rho and T2rho, respectively, have potential to provide unique MRI contrast in vivo. On-resonance spin-lock T1rho with different spin-lock field strengths and adiabatic T2rho with different radiofrequency-modulation functions were measured in BT4C gliomas treated with Herpes Simplex Virus thymidine kinase (HVS-tk) gene therapy causing apoptotic cell death. These NMR tools were able to discriminate different treatment responses in tumor tissue from day 4 onward. An equilibrium two-site exchange model was used to calculate intrinsic parameters describing changes in water dynamics. Observed changes included increased correlation time of water associated with macromolecules and a decreased fractional population of this pool. These results are consistent with destructive intracellular processes associated with cell death and the increase of extracellular space during the treatment. Furthermore, association between longer exchange correlation time and decreased pH during apoptosis is discussed. In this study, we demonstrated that T1rho and T2rho MR imaging are useful tools to quantify early changes in water dynamics reflecting treatment response during gene therapy.

Nuclear mechanisms of hypoxic cerebral injury in the newborn: the role of caspases

Our laboratory has been actively engaged in investigating mechanisms of activation of initiator caspase-9 during hypoxia in the developing newborn and fetal brains. The present review has been organized as follows: (a) the effect of hypoxia on the expression and activation of caspase-3, -8, and -9 in the newborn brain; (b) the role of nitric oxide in caspase-9, and caspase-3 activation during hypoxia in the newborn brain; (c) the role of nuclear Ca(2+)-influx in caspase-9 and caspase-3 activation during hypoxia in the newborn brain; (d) the effect of caspase-9 inhibition during hypoxia on preventing downstream events including caspase-3 activation. The results of our research investigations presented in (b), (c), and (d) elucidate mechanisms of caspase activation during hypoxia in the newborn brain. These studies provide the fundamental framework for developing neuroprotective strategies in the hypoxic newborn.

Colony-stimulating factor-1 increases osteoclast intracellular pH and promotes survival via the electroneutral Na/HCO3 cotransporter NBCn1

Colony-stimulating factor-1 (CSF-1) promotes the survival of osteoclasts, short-lived cells that resorb bone. Although a rise in intracellular pH (pH(i)) has been linked to inhibition of apoptosis, the effect of CSF-1 on pH(i) in osteoclasts has not been reported. The present study shows that, in the absence of CO(2)/HCO(3)(-), CSF-1 causes little change in osteoclast pH(i). In contrast, exposing these cells to CSF-1 in the presence of CO(2)/HCO(3)(-) causes a rapid and sustained cellular alkalinization. The CSF-1-induced rise in pH(i) is not blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, an inhibitor of HCO(3)(-) transporters but is abolished by removing extracellular sodium. This inhibition profile is similar to that of the electroneutral Na/HCO(3) cotransporter NBCn1. By RT-PCR, NBCn1 transcripts are present in both osteoclasts and osteoclast-like cells (OCLs), and by immunoblotting, the protein is present in OCLs. Moreover, CSF-1 promotes osteoclast survival in the presence of CO(2)/HCO(3)(-) buffer but not in its absence. Preventing the activation of NBCn1 markedly attenuates the ability of CSF-1 to 1) block activation of caspase-8 and 2) prolong osteoclast survival. Inhibiting caspase-3 or caspase-8 in OCLs prolongs osteoclast survival to the same extent as does CSF-1. This study provides the first evidence that osteoclasts express a CSF-1-regulated Na/HCO(3) cotransporter, which may play a role in cell survival.

Mitochondria are not required for death receptor-mediated cytosolic acidification during apoptosis

In addition to cell shrinkage, membrane blebbing, DNA fragmentation and phosphatidylserine exposure, intracellular acidification represents a hallmark of apoptosis. Although the mechanisms underlying cytosolic acidification during apoptosis remained largely elusive, a pivotal role of mitochondria has been proposed. In order to investigate the involvement of mitochondria in cytosolic acidification during apoptosis, we blocked the mitochondrial death pathway by overexpression of Bcl-2 and subsequently activated the death receptor pathway by anti-CD95 or TRAIL or the mitochondrial pathway by staurosporine. We show that Bcl-2 but not caspase inhibition prevented staurosporine-induced intracellular acidification. Thus, intracellular acidification in mitochondrial apoptosis is a Bcl-2-inhibitable, but caspase-independent process. In contrast, Bcl-2 only slightly delayed, but did not prevent intracellular acidification upon triggering of death receptors. The Na(+)/H(+) exchanger NHE1 was partially degraded during apoptosis but only to a small extent and and at a delayed time point when cytosolic acidification was almost completed. We therefore conclude that cytosolic acidification is mitochondrially controlled in response to mitochondria-dependent death stimuli, but requires additional caspase-dependent mechanisms during death receptor-mediated apoptosis.

Molecular imaging of apoptosis in cancer

Apoptosis plays an important role in cancer. Mechanisms hindering its action are implicated in a number of malignancies. Also, the induction of apoptosis plays a pivotal role in non-surgical cancer treatment regimes such as irradiation, chemotherapy, or hormones. Recent advanced in imaging science have made it now possible for us to detect and visualize previously inaccessible and even unrecognized biological phenomena in cells and tissue undergoing apoptosis in vivo. Not only are these imaging techniques painting an intriguing picture of the spatiotemporal characteristics and metabolic and biophysical of apoptosis in situ, but they are expected to have an ever increasing impact in preclinical testing and design of new anticancer agents as well. Rapid and accurate visualization of apoptotic response in the clinical settings can also be of significant diagnostic and prognostic worth. With the advent of molecular medicine and patient-tailored treatment options and therapeutic agents, such monitoring techniques are becoming paramount.

Alkaline stress-induced apoptosis in human pulmonary artery endothelial cells

The effect of alkaline stress, or an increase in extracellular pH (pHext), on cell viability is poorly defined. Human pulmonary artery endothelial cells (HPAEC) were subjected to alkaline stress using different methods of increasing pHext. Viability and mode of cell death following alkaline stress were determined by assessing nuclear morphology, ultrastructural features, and caspase-3 activity. Incubation of monolayers in media set to different pHext values (7.4-8.4) for 24-h induced morphological changes suggesting apoptosis (35-45% apoptotic cells) following severe alkaline stress. The magnitude of apoptosis was related to the severity of alkaline stress. These findings were confirmed with an assessment of ultrastructural changes and caspase-3 activation. While there was no difference in the intracellular calcium level ([Ca(2+)](i)) in monolayers set to pHext 7.4 versus 8.4 following the first hour of alkaline stress, blockade of calcium uptake with the chelator, EGTA, potentiated the magnitude of apoptosis under these conditions. Potentiation of apoptosis was reduced by calcium supplementation of the media. Finally, alkaline stress was associated with an increase in intracellular pH. This is the first report of apoptosis following alkaline stress in endothelial cells in the absence of other cell death stimuli.

Effect of hypoxia on caspase-3, -8, and -9 activity and expression in the cerebral cortex of newborn piglets

Caspases play an important role in programmed cell death. Caspase-3 is a key executioner of apoptosis, whose activation is mediated by the initiator caspases, caspase-8 and caspase-9. The present study tested the hypothesis that cerebral hypoxia results in increased activation and expression of caspases-3, -8, and -9 in the cytosolic fraction of the cerebral cortex of newborn piglets. To test this hypothesis the activity and expression of caspases-3, -8, and -9 were determined in newborn piglets divided into normoxic and hypoxic groups. Caspase activity was determined spectrofluorometrically using enzyme specific substrates. The expression of caspase protein was assessed by Western blot analysis using enzyme specific antibody. Caspases-3, -8, and -9 activity and expression was significantly higher in the hypoxic group than in the normoxic group. These results demonstrate that hypoxia induces activation and increased expression of both the initiator caspases and the executioner caspase in the cerebral cortex of newborn piglets. We conclude that hypoxia results in stimulation of both the pathways of caspase-3 activation.

Early gene therapy-induced apoptotic response in BT4C gliomas by magnetic resonance relaxation contrast T1 in the rotating frame

The design and evaluation of therapeutic gene transfection protocols and vectors are under extensive development. Magnetic resonance imaging (MRI) techniques can aid considerably in the development of experimental treatment approaches, as well as in determining treatment response by observing gross tissue morphology. However, through a unique set of contrast parameters, namely T1, T2, and diffusion, more information about tissue status can be obtained while delineating and classifying tumor characteristics in more detail. We show here that T1 relaxation in the rotating frame, T1rho, provides unique in vivo MRI contrast. Ganciclovir treatment of HSV-tk+BT4C gliomas, which effectively eradicates these tumors, resulted in significantly prolonged T1rho relaxation times in MRI already after 3 days of treatment, whereas conventional contrast parameters were elevated after 6-8 days of therapy. Interestingly, the prolonged T1rho values were observed while an increase in tumor volume was still taking place. The regions of elevated T1rho relaxation coincided with high apoptotic activity as determined by histology, suggesting that T1rho MRI contrast could be used as a novel early indicator of cytotoxic cell damage in gliomas.

Transient expression of wild-type or mitochondrially targeted Bcl-2 induces apoptosis, whereas transient expression of endoplasmic reticulum-targeted Bcl-2 is protective against Bax-induced cell death

Bcl-2 protein family members function either to promote or inhibit programmed cell death. Bcl-2, typically an inhibitor of apoptosis, has also been demonstrated to have pro-apoptotic activity (Cheng, E. H., Kirsch, D. G., Clem, R. J., et al. (1997) Science 278, 1966-1968). The pro-apoptotic activity has been attributed to the cleavage of Bcl-2 by caspase-3, which converts Bcl-2 to a pro-apoptotic molecule. Bcl-2 is a membrane protein that is localized in the endoplasmic reticulum (ER) membrane, the outer mitochondrial membrane, and the nuclear envelope. Here, we demonstrate that transient expression of Bcl-2 at levels comparable to those found in stably transfected cells induces apoptosis in human embryonic kidney 293 cells and in the human breast cell line MDA-MB-468 cells. Furthermore, we have targeted Bcl-2 specifically to either the ER or the outer mitochondrial membrane to test whether induction of apoptosis by Bcl-2 is dependent upon its localization within either of these membranes. Our findings indicate that Bcl-2 specifically targeted to the mitochondria induces cell death, whereas Bcl-2 that is targeted to the ER does not. The expression of Bcl-2 does result in its cleavage to a 20-kDa protein; however, mutation of the caspase-3 cleavage site (D34A) does not inhibit its ability to induce cell death. Additionally, we find that transiently expressed ER-targeted Bcl-2 inhibits cell death induced by Bax overexpression. In conclusion, the ability of Bcl-2 to promote apoptosis is associated with its localization at the mitochondria. Furthermore, the ability of ER-targeted Bcl-2 to protect against Bax-induced apoptosis suggests that the ER localization of Bcl-2 may play an important role in its protective function.

Mitochondria-dependent apoptosis and cellular pH regulation

Mitochondria play a critical role in apoptosis induction in response to myriad stimuli. These organelles release proteins into the cytosol which trigger caspase activation or perform other functions relevant to apoptosis, including cytochrome c (cyt-c), caspases, AIF, and SMAC (Diablo). The mechanisms by which these proteins escape from mitochondria remain enigmatic. Moreover, it is unclear whether release of these proteins versus disturbances in core mitochondrial functions represents the cell death commitment mechanism. In this regard, suppression of apoptosis using broad-spectrum caspase inhibitory compounds has been reported in many circumstances to prevent the morphological and biochemical manifestations of apoptosis, and yet not protect cells from death and not preserve clonigenic survival. Thus, while mitochondrial damage can be coupled to caspase activation pathways, cell death commitment often occurs upstream of caspase activation when mitochondria-dependent cell death pathways are invoked. Here, we review evidence implicating dysregulation of cellular pH as a component of the cell death mechanism involving mitochondria. Cell Death and Differentiation (2000) 7, 1155 - 1165

Review: nuclear events in apoptosis

Initial apoptosis research characterized this form of cell death based on distinct nuclear morphology that was subsequently shown to be associated with the appearance of oligonucleosomal DNA fragments. More recent evidence has indicated that apoptosis depends upon a tightly regulated cellular program for its successful initiation and execution. Molecular participants in this program are present in different subcellular compartments, including the plasma membrane, cytosol, mitochondria, and nucleus. The interplay among these compartments and the exchange of specific signaling molecules are critical for the systematic progression of apoptosis. While numerous reports have described a key role for caspase activity in the signaling and executive steps of apoptotic cell death, there are some instances where well-established nuclear changes, characteristic of this form of cell death, can occur independently of caspase activity. Moreover, evidence indicates that certain nuclear events, including chromatin condensation and DNA fragmentation, are controlled separately and depend upon a persistent supply of energy in vivo. In this review, we discuss our current understanding of the role and regulation of nuclear events in the apoptotic process with an emphasis on protease and endonuclease activities as well as the ability of certain Bcl-2 family proteins to influence this process.

Caspase-8-mediated intracellular acidification precedes mitochondrial dysfunction in somatostatin-induced apoptosis

Activation of initiator and effector caspases, mitochondrial changes involving a reduction in its membrane potential and release of cytochrome c (cyt c) into the cytosol, are characteristic features of apoptosis. These changes are associated with cell acidification in some models of apoptosis. The hierarchical relationship between these events has, however, not been deciphered. We have shown that somatostatin (SST), acting via the Src homology 2 bearing tyrosine phosphatase SHP-1, exerts cytotoxic action in MCF-7 cells, and triggers cell acidification and apoptosis. We investigated the temporal sequence of apoptotic events linking caspase activation, acidification, and mitochondrial dysfunction in this system and report here that (i) SHP-1-mediated caspase-8 activation is required for SST-induced decrease in pH(i). (ii) Effector caspases are induced only when there is concomitant acidification. (iii) Decrease in pH(i) is necessary to induce reduction in mitochondrial membrane potential, cyt c release and caspase-9 activation and (iv) depletion of ATP ablates SST-induced cyt c release and caspase-9 activation, but not its ability to induce effector caspases and apoptosis. These data reveal that SHP-1-/caspase-8-mediated acidification occurs at a site other than the mitochondrion and that SST-induced apoptosis is not dependent on disruption of mitochondrial function and caspase-9 activation.