Halocarbon hepatotoxicity is not initiated by Ca2+-stimulated endonuclease activation

Evidence That 1,1-Dichloroethylene Induces Apoptotic Cell Death in Murine Liver

1,1-Dichloroethylene (DCE) causes dysfunction of hepatic mitochondria. As mitochondria have been implicated in apoptosis through opening of the permeability transition pore (PTP), we have undertaken studies to test the hypothesis that DCE induces apoptosis, in addition to necrosis, in murine liver. Our primary objective was to identify the biochemical events associated with DCE-induced apoptosis. Female CD-1 mice were treated with a mildly hepatotoxic dose of DCE (125 mg/kg, i.p.). Using the fluorescent dye JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide), decreased hepatic mitochondrial membrane potential was detected at 2 h. Western blotting of liver cytosolic proteins showed greater immunoreactivity for cytochrome c in fractions from mice treated with DCE for 4 h than in controls. Furthermore, caspase-9 activity was significantly increased 6 h after DCE exposure. Immunohistochemical studies with an antibody to activated caspase-3 and terminal deoxynucleotidyl transferase dUTP nick-end labeling staining were used to detect apoptotic cells. In both experiments, positive reactivities were observed in centrilobular hepatocytes 12 and 24 h after DCE. Additionally, centrilobular hepatocytes showing morphological criteria of apoptosis were observed at 24 h. Apoptosis and all apoptotic events were inhibited by pretreatment for 20 min with cyclosporine A (CyA) (50 mg/kg), a specific inhibitor of the mitochondrial PTP. To determine a major role for mitochondrial permeability transition (MPT) in DCE hepatotoxicity, serum alanine aminotransferase (ALT) activity was evaluated. ALT activity was significantly elevated 2 to 24 h after DCE, and CyA failed to inhibit this activity. These data suggested that DCE produces apoptosis by inducing MPT, causing release of cytochrome c into the cytosol and caspase activation.

The protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methyl-piperzine (H-7) prevents and reverses Ca(2+)-mediated injury in isolated rat hepatocyte couplets

We have recently shown that protein kinase C (PKC) activation induces similar morphological and functional alterations in couplets to that caused by increments of intracellular Ca(2+). Since certain PKC isoforms are activated by Ca(2+), we tested whether the PKC inhibitor H-7 can counteract the alterations induced by this ion in couplets. The Ca(2+) ionophore A23187, which can mobilize Ca(2+) from extracellular and intracellular sources, decreased, in a dose-dependent manner, the percentage of couplets accumulating the fluorescent bile acid analogue cholyl-lysyl-fluorescein (CLF) in their canalicular vacuoles, i.e., in the canalicular vacuolar accumulation test (cVA of CLF), a measure of the overall capability of the couplets to secrete and retain CLF. To a similar extent, A23187 also decreased the percentage of couplets retaining CLF once secreted, i.e., in the canalicular vacuole retention test (cVR of CLF), a measure of tight junctional integrity. ATP (50 microM), another Ca(2+)-elevating compound, altered canalicular function in a similar extent to A23187. All these functional changes were prevented by H-7 in a dose-dependent manner. Canalicular dysfunction was accompanied by bleb formation and extensive redistribution of F-actin from the pericanalicular area to the cell body, which was also fully prevented by H-7; the intracellular Ca(2+) chelator, 1, 2-bis(o-aminophenoxy)-ethene-N,N,N',N'-tetraacetate tetrakis-(acetomethylester), (BAPTA/AM) (20 microM) had virtually the same preventive effects as H-7. Both H-7 and BAPTA/AM not only prevented but also reversed the decrease in cVA of CLF and blebbing induced by A23187. Thus, H-7 can both prevent and reverse Ca(2+)-mediated hepatocellular injury.

Apoptosis and necrosis in toxicology: a continuum or distinct modes of cell death?

Mounting evidence indicates that apoptosis rather than necrosis predominates in many cytolethal toxic injuries. Associated cell death models of apoptosis and necrosis are either: (1) totally separate death modes, (2) a continuum whereby they are extremes of biochemically overlapping death pathways, or (3) essentially distinct processes with only limited molecular and cell biology overlap. We conclude that the current balance of evidence favours the third of these options. The established axiom that, even when considering the same toxicant, injury amplitude (dose) is a primary determinant of whether cells die via active cell death (apoptosis) or failure of homeostasis (necrosis) remains valid. Tissue selectivity of toxicants can stem from the apoptotic or necrotic thresholds at which different cells die, as well as targeting factors such as toxicokinetics, receptor recognition, bioactivation, and cell-specific lesions.

Extracellular calcium is not required for gliotoxin or dexamethasone-induced DNA fragmentation: a reappraisal of the use of EGTA

The immunomodulating agent gliotoxin and related toxins cause apoptotic cell death in a variety of cell types including macrophages and thymocytes [Waring et al. (1988) J. biol. Chem., 263, 18,493-18,499; Waring et al. (1990) Int. J. Immunopharmac., 12, 445-457]. The mechanism of induction of apoptosis by gliotoxin is not yet known, although it does not require protein synthesis [Waring (1990) J. biol. Chem., 14,476-14,480], unlike dexamethasone-induced apoptosis in thymocytes. Because of the reported requirement for extracellular calcium in apoptosis induced by dexamethasone, we studied the effects of extracellular calcium on gliotoxin-induced apoptosis in macrophages. Initial experiments using calcium-depleted media showed no inhibition of apoptotic DNA fragmentation by gliotoxin. By measuring residual 45Ca2+ remaining in cells pulsed with labelled calcium over the time period required for DNA fragmentation, we could demonstrate some uptake of extracellular calcium into treated cells as assessed by residual, slowly exchanging calcium. When cells were treated with the calcium chelator EGTA at 0.5-2 mM, calcium uptake was abolished but DNA fragmentation was unaffected. EGTA at higher concentrations, up to 8 mM, did inhibit DNA fragmentation without any additional inhibition of calcium uptake. Similar results were found for dexamethasone-treated thymocytes. Thymocytes treated with 8 mM EGTA, however, were not rescued from apoptosis but died by necrosis. These results indicate that extracellular calcium is not essential for apoptosis induced by these agents and that the use of high concentrations of EGTA to establish a requirement for extracellular calcium in apoptosis should be treated with caution.

The role of the nucleus and other compartments in toxic cell death produced by alkylating hepatotoxicants

Hepatocellular necrosis occurs under a wide range of pathological conditions. In most cases, toxic cell death takes place over a finite span of time, delayed from the point of initial injury and accompanied by homeostatic counterresponses that are varied and complex. The present strategies for discovering critical steps in cell death recognize that (1) different toxins produce similar morphologic changes that precede killing in widely varied cell types, and that (2) lethal events are likely to involve one or more compartmentalized functions that are common to most cells. Investigations of the plasma membrane, endoplasmic reticulum, cytoplasm, mitochondrion, and nucleus have greatly advanced our understanding of acute hepatocellular necrosis. This report examines each compartment but emphasizes molecular changes in the nucleus which may explain cell death caused by alkylating hepatotoxicants. Accumulating knowledge about two distinct modes of cell death, necrosis and apoptosis, indicates that loss of Ca2+ regulation and subsequent damage to DNA may be critical steps in lethal damage to liver cells by toxic chemicals.

Extensive alteration of genomic DNA and rise in nuclear Ca2+ in vivo early after hepatotoxic acetaminophen overdose in mice

Hepatotoxic doses of acetaminophen cause early impairment of Ca2+ homeostasis. In this in vivo study, 600 mg/kg acetaminophen caused total nuclear Ca2+ and % fragmented DNA to rise in parallel from 2-6 hr, followed by large later increases mirroring frank liver injury. Agarose gel electrophoresis revealed substantial loss of large genomic DNA from 2 hours onward, with accumulation of DNA fragments in a ladder-like pattern resembling apoptosis. Extensive late cleavage of DNA probably resulted from cell death, whereas degradative loss of large genomic DNA at 2 hours arose at an early enough point to contribute to acetaminophen-induced liver necrosis in mice.

Platelet-derived growth factor blocks the increase in intracellular free Ca2+ caused by calcium ionophores and a volatile anesthetic agent in Swiss 3T3 fibroblasts without altering toxicity

Platelet-derived growth factor (PDGF) produced an almost complete block of the increase in intracellular free Ca2+ concentration ([Ca2+]i) in Swiss 3T3 fibroblasts caused by the Ca2(+)-selective ionophores 4-bromo-A23187 and ionomycin, and by the volatile anesthetic agent halothane. The effect of PDGF was similar to the decreased [Ca2+]i response to Ca2(+)-ionophores produced by phorbol 12-myristate 13-acetate, an activator of protein kinase C. There was no effect of PDGF or PMA on the acute or delayed toxicity of the Ca2(+)-ionophores to Swiss 3T3 cells, suggesting that the increase in [Ca2+]i is not the direct cause of toxicity of these agents.

Lipopolysaccharide induces double-stranded DNA fragmentation in mouse thymus: protective effect of zinc pretreatment

Intraperitoneal injection of female NAW/W1 mice with 5 mg of Salmonella typhimurium lipopolysaccharide/kg results in decreased body and thymus weight. Reduced thymic weight is accompanied by fragmentation of DNA into multimers of about 200 bp size. This effect is consistent with the induction of intranucleosomal cleavage of double-stranded DNA in thymus. Maximal fragmentation of DNA occurs between 18 and 24 h after treatment; by 48 h post lipopolysaccharide treatment, there is little evidence of thymic DNA fragmentation. Pretreatment of mice with Zn protects against lipopolysaccharide-induced DNA fragmentation. This effect is maximal at about 72 h after Zn treatment (24 h after lipopolysaccharide treatment) and persists until about 96 h after Zn treatment. At 72 h after pretreatment, the antagonism of thymic DNA fragmentation by Zn is dose-dependent. To examine the role of the acute phase inflammatory response elicited by lipopolysaccharide treatment in the production of changes in thymic weight and DNA integrity, the effects of treatment with casein, a well-characterized inducer of the acute phase inflammatory response in mice, were examined. In contrast to the effect of lipopolysaccharide, casein treatment did not produce a similar pattern of DNA fragmentation in thymus. Taken together, these data suggest that lipopolysaccharide induces DNA fragmentation in thymus by a mechanism which does not occur during the pathophysiological changes which accompany the casein-induced acute phase response. Further, the antagonism by Zn of lipopolysaccharide-induced fragmentation of thymic DNA is consistent with earlier findings that Zn can prevent dexamethasone-induced DNA fragmentation in vitro.

Early loss of large genomic DNA in vivo with accumulation of Ca2+ in the nucleus during acetaminophen-induced liver injury

Hepatotoxic doses of acetaminophen cause early impairment of Ca2+ homeostasis in the liver. This in vivo study considers the nucleus as a possible site of lethal Ca2+ action by evaluating whether acetaminophen raises Ca2+ in this compartment, whether DNA becomes altered, and whether DNA changes occur early enough during injury to contribute causally to necrosis. Fed Swiss mice were treated with 600 mg/kg acetaminophen ip and livers and blood samples were collected over time. Total nuclear Ca2+ accumulation and fragmentation damage to DNA showed modest parallel increases between 2 and 6 hr, followed by greater than 200% rises at 12 hr mirroring the appearance of frank liver injury (ALT greater than 10,000 U/liter). However, agarose gel electrophoresis revealed extensive loss of large genomic DNA from 2 hr onward, accompanied by the appearance of periodic DNA fragments. Thus, acetaminophen raised nuclear Ca2+ concentrations and promoted DNA fragmentation in vivo. The considerable cleavage of DNA seen at late times probably resulted from cell death, whereas loss of large genomic DNA from 2 hr onward appeared at an early enough point in time to be a contributing factor in acetaminophen-induced liver necrosis.