Evaluation of the calcium mobilizing action of acetaminophen and bromobenzene in rat hepatocyte cultures

Inhibition of protein phosphatase activity and changes in protein phosphorylation following acetaminophen exposure in cultured mouse hepatocytes

Protein phosphorylation was determined in cultured mouse hepatocytes exposed to an hepatotoxic concentration of acetaminophen (APAP) for selected times up to 12 h. Cultures were radiolabled with 32P-orthophosphoric acid and the cell extracts were analyzed by 2D gel electrophoresis and autoradiography. APAP exposure selectively increased the phosphorylation state of proteins of molecular weight 22, 25, 28, and 59 kDa and decreased the phosphorylation of a 26-kDa protein. Evidence is presented that these changes (1) are dependent on cytochrome P-450 activation of APAP; (2) occur well before enzyme leakage in this in vitro model; (3) are not likely attributed to GSH depletion alone; (4) are in part mimicked by okadaic acid, calyculin A, and cantharidic acid, three structurally distinct inhibitors of protein phosphatases 1 and 2A; and (5) are paralleled by a decline in protein phosphatase activity. The physiological consequences of protein phosphatase inactivation could be significant in APAP overdose since these enzymes are involved in the dephosphorylation of regulatory proteins that control many cell functions. This study also provides the first evidence for disruption in signal transduction pathways as a response to or component of APAP-induced hepatic injury.

Basal hormone levels in women who use acetaminophen for menstrual pain

OBJECTIVE

To compare basal gonadotropin and estradiol levels between women using acetaminophen versus those using no or other types of analgesics.

DESIGN

Observational study.

SETTING

Three IVF clinics in greater Boston.

PATIENT(S)

Three hundred eighty-six women accepted for their first IVF treatment who completed questionnaires recording medical history, including analgesic use for menstrual pain, and who had blood drawn during the menstrual phase of a cycle before treatment.

MAIN OUTCOME MEASURE(S)

Basal FSH, LH, and estradiol.

RESULT(S)

Basal hormone levels, especially LH, were lower for women who regularly used acetaminophen compared with women who used no medication or other types of analgesics. Lower hormone levels in acetaminophen users were consistently observed when women were subdivided by age, body mass index, smoking history, and degree of menstrual pain-features that might have influenced analgesic use or hormone levels.

CONCLUSION(S)

This study provides preliminary evidence that acetaminophen may lower gonadotropin and estradiol levels and offers a biologic basis for the epidemiologic observation that acetaminophen use may reduce the risk of ovarian cancer.

Regulation of glucuronidation by glutathione redox state through the alteration of UDP-glucose supply originating from glycogen metabolism

The effect of altered redox state of glutathione was investigated on p-nitrophenol glucuronidation in isolated mouse hepatocytes. Decrease of GSH/GSSG ratio provoked by various agents caused increased glucuronidation which was accompanied by stimulated glycogenolysis and elevated UDP-glucose content. The stimulation of glycogenolysis and glucuronidation by glutathione consumption could be prevented by the reduction of oxidized glutathione with dithiothreitol and by the glycogenolysis inhibitor fructose. In permeabilized hepatocytes glycogen metabolism, bypassed by the addition of UDP-glucose, stimulated glucuronidation which was insensitive to glutathione depletion. In liver microsomes either UDP-glucuronosyltransferase activity or UDP-glucuronic acid transport was not influenced by GSH/GSSG ratio. These results suggest that alteration of the GSH/GSSG ratio regulates glucuronidation by affecting enzymes of the glycogen metabolism via the modification of UDP-glucuronate supply.

Selective protein arylation and acetaminophen-induced hepatotoxicity

More than 20 years have passed since the early reports of acute hepatotoxicity with APAP overdose. During that period investigative research to discover the "mechanism" underlying the toxicity has been conducted in many species and strains of intact animals as well as in a variety of in vitro and culture systems. Such work has clarified the primary role of biotransformation and the protective role of GSH. Understanding the former provides explanations for the toxic interactions which may occur with alcohol or other xenobiotics, while understanding of the latter led to the development of antidotes for the treatment of acute poisoning. Acetaminophen (APAP)-induced hepatotoxicity: roles for protein arylation. Initiating events in toxicity require biotransformation of APAP to NAPQI followed by arylation of several important proteins with subsequent alteration of protein structure and function. The immediate consequence of the alterations is detectable in several organelles and these may represent multiple initiating events which are depicted as acting in concert to cause cell injury (large arrowheads). Arylation of cytosolic 58-ABP with subsequent translocation to the nucleus is depicted as a possible signaling mechanism for determining outcome at the cell or organ level (within dotted boundary). For simplicity NAPQI's potentials for oxidizing protein sulfhydryls and direct binding to DNA have been omitted. Significant light has also been shed on the biochemical and cellular events which accompany APAP-induced hepatotoxicity. However, such studies have not identified a unique mechanism of toxicity that is universally accepted. The recent identification of several protein targets which become arylated during toxicity--along with the findings that arylation of some of those target proteins results in loss of protein function--demonstrates that covalent binding does, indeed, have biological consequences and is not merely an indicator of the fleeting presence of reactive electrophiles. These observations further suggest that multiple independent insults to the cell may be involved in toxicity. it is now apparent that the concept of a multistage process that involves both initiation and progression events is appropriate for APAP toxicity, and it is unlikely that a unique initiating event will ever be identified. In light of recent findings it is more likely that a number of such cellular events occur very early after toxic overdosage, and that they collectively set in motion and perpetuate the biochemical, cellular, and molecular processes which will determine outcome. The importance of 58-ABP arylation with early, apparently selective, translocation to the nucleus remains to be elucidated. To date there is nothing to suggest that this represents an initiating event in toxicity. rather it is plausible that the translocation may play a role in signaling electrophile presence and in calling for cellular defense against electrophile insult. This is reflected in the hypothetical model presented in Fig. 3. Critical experimental testing of this model will advance our understanding of the cellular and molecular responses to toxic electrophile insult.

Glutathione depletion induces glycogenolysis dependent ascorbate synthesis in isolated murine hepatocytes

The relationship between glutathione deficiency, glycogen metabolism and ascorbate synthesis was investigated in isolated murine hepatocytes. Glutathione deficiency caused by various agents increased ascorbate synthesis with a stimulation of glycogen breakdown. Increased ascorbate synthesis from UDP-glucose or gulonolactone could not be further affected by glutathione depletion. Fructose prevented the stimulated glycogenolysis and ascorbate synthesis caused by glutathione consumption. Reduction of oxidised glutathione by dithiothreitol decreased the elevated glycogenolysis and ascorbate synthesis in diamide or menadione treated hepatocytes. Our results suggest that a change in GSH/GSSG ratio seems to be a sufficient precondition of altering glycogenolysis and a consequent ascorbate synthesis.

Bromobenzene detoxification in the human liver-derived HepG2 cell line

1. The applicability of the human hepatoma cell line, HepG2, as a cell culture model for studying xenobiotic liver toxicity has been investigated using the well-characterized hepatotoxic chemical, bromobenzene. 2. Bromobenzene caused a concentration- (0-10 mM) and time-dependent (0-180 min) decrease in HepG2 cell viability. The degree of toxicity was dependent upon the culture medium composition and the state of cell growth. Toxicity in Modified Earle's and Williams' E Media was maximal at 7 days growth compared with 3 and 10 days, and was greater in Williams' than in Earle's medium. Toxicity in Dulbecco's medium was apparent only at 10 days growth and was less than the maximum toxicity in the other media. 3. Bromobenzene was detoxified by epoxide hydrase. The question of metabolic activation by P450 remained unresolved, but any involvement of P450 was by forms not inhibited by ketoconazole. 4. The mechanism of bromobenzene toxicity did not appear to involve lipid peroxidation, depletion of reduced glutathione, calcium-mediated proteolysis or metabolic activation by prostaglandin synthetase, but may have involved direct solvent-induced cell damage. 5. This study demonstrates the potential usefulness of HepG2 cells in toxicity testing and highlights the importance of standardizing culture conditions.

Acetaminophen-induced cytotoxicity in cultured mouse hepatocytes: correlation of nuclear Ca2+ accumulation and early DNA fragmentation with cell death

Hepatotoxic doses of acetaminophen cause widespread alkylation of liver and early loss of cytosolic Ca2+ regulation. Although the precise location and target of lethal alkylation are not known, Ca2+ accumulation is viewed as a possible link between cell alkylation and cell death. We have recently shown that Ca2+ accumulates in the nucleus and that DNA fragments in vivo before the development of acetaminophen-induced necrosis in mice. The present study examined cultured hepatocytes for nuclear damage and its association with cell death in vitro. Positive results would argue for two key points. (1) Nonparenchymal cell damage does not explain DNA fragmentation induced by acetaminophen in vivo. (2) A chemical that causes necrosis can produce DNA damage considered characteristic of apoptosis. Hepatocytes from NIH Swiss mice were isolated by collagenase perfusion, cultured in Williams' E medium for 24 hr, and exposed to acetaminophen. Cytotoxicity was assessed by lactate dehydrogenase leakage and release of [3H]adenine from a prelabeled nucleotide pool. Genomic DNA fragmentation was assessed quantitatively by colorimetric analysis and qualitatively by agarose gel electrophoresis. Acetaminophen caused DNA damage from 1-4 hr onward and produced significant release of lactate dehydrogenase and [3H]adenine nucleotides at later times. Agarose gel electrophoresis revealed a "ladder" of DNA fragments characteristic of Ca(2+)-mediated endonuclease activation. Cytotoxicity correlated with nuclear Ca2+ accumulation (r greater than 0.895, p less than 0.05) and with percentage DNA fragmentation (r greater than 0.835, p less than 0.05). Nuclear changes in vitro generally reproduced those observed in vivo. Collectively, these findings demonstrate that nuclear Ca2+ accumulation and DNA fragmentation appear as early events that correlate directly with later cytotoxicity. These changes may contribute to acetaminophen-induced injury leading to cell death in vitro and necrosis in vivo.