Effects of the hepatotoxic agents retrorsine and aflatoxin B1 on hepatic protein synthesis in the rat

Alternative Cell Sources for Liver Parenchyma Repopulation: Where Do We Stand?

Acute and chronic liver failure is a highly prevalent medical condition with high morbidity and mortality. Currently, the therapy is orthotopic liver transplantation. However, in some instances, chiefly in the setting of metabolic diseases, transplantation of individual cells, specifically functional hepatocytes, can be an acceptable alternative. The gold standard for this therapy is the use of primary human hepatocytes, isolated from livers that are not suitable for whole organ transplantations. Unfortunately, primary human hepatocytes are scarcely available, which has led to the evaluation of alternative sources of functional hepatocytes. In this review, we will compare the ability of most of these candidate alternative cell sources to engraft and repopulate the liver of preclinical animal models with the repopulation ability found with primary human hepatocytes. We will discuss the current shortcomings of the different cell types, and some of the next steps that we believe need to be taken to create alternative hepatocyte progeny capable of regenerating the failing liver.

Retrorsine: a kinetic study of its influence on rat liver regeneration in the portal branch ligation model

BACKGROUND

Retrorsine, a naturally occurring pyrrolizidine alkaloid, impairs liver regeneration after partial hepatectomy by mechanisms that are still unclear.

AIM

The aim of the study was to clarify the influence of retrorsine on cell cycle progression in the regenerating liver lobes of rats after portal branch ligation (PBL).

METHODS

Liver weight, protein and DNA contents, DNA synthesis (5'-bromodeoxyuridine (BrdU) incorporation) and cellular levels of Cyclin E, CDK-2, CDK-4 and proliferating cell nuclear antigen (PCNA) were assessed before and 24, 48, 72 and 168 h after PBL.

RESULTS

Before surgery, higher levels of cyclin E, CDK-2, CDK-4 and PCNA as well as BrdU incorporation were found in the liver of retrorsine-treated rats than in untreated rats. Liver weight gain, protein and DNA synthesis as well as induction of cell cycle related proteins were all strongly impaired by retrorsine in the regenerating lobes after PBL.

CONCLUSIONS

In conclusion, retrorsine impairs liver regeneration in the PBL model not only by an S or G2/M phase block, but also by a block located before the G1/S transition of the cell cycle.

Cynoglossum officinale toxicity in calves

Six calves were given dried, ground Cynoglossum officinale daily in a dose which provided 15 (two calves) or 60 (four calves) mg per kg per day of total pyrrolizidine alkaloids. Those calves given 60 mg per kg of total pyrrolizidine alkaloids per day died following a single dose of plant material. These calves had a marked elevation of serum gamma-glutamyltransferase (GGT) and aspartate transaminase (AST) activities and serum bile acid and total bilirubin (TBili) concentrations. These four calves all had massive hepatocellular necrosis and haemorrhage of the liver. Of the two calves that were given 15 mg per kg of total pyrrolizidine alkaloids per day, one died on day 34 and the other survived until day 35 when it was painlessly killed. There were significant elevations in serum AST and GGT activities in these calves. The histological lesions of the calf surviving until 35 days were compatible with pyrrolizidine alkaloid toxicity, that is megalocytosis, karyomegaly and necrosis of hepatocytes with karyomegaly of biliary epithelium. The pyrrolizidine base present in Cynoglossum officinale (heliotridine) and its esters have a similar type of toxicity to the highly toxic and more familiar macrocyclic diester pyrrolizidine alkaloids of the pyrrolizidine base (retronecine), present in Senecio or Crotolaria species.

Time-course effects of ethanol pretreatment on hepatic necrosis and fat accumulation induced by aflatoxin B1 in the rat

Effect of ethanol pretreatment on acute hepatotoxicity and hepatic fat accumulation induced by aflatoxin B1 (AFB1) was followed up to 120 h in male Wistar rats. Pretreatment with 4 oral doses of ethanol (4.0 g/kg body wt. each) at 48, 45, 24 and 21 h prior to AFB1 (2.0 mg/kg body wt.) single intraperitoneal administration caused a significant increase in the activity of plasma glutamic oxaloacetic transaminase (PGOT, 2.4-fold), plasma glutamic pyruvic transaminase (PGPT, 2.8-fold), liver triglycerides (2.3-fold) and the severity of liver necrosis at 72 h after AFB1 administration. The effect of ethanol pretreatment on an increase in the accumulation of liver cholesterol and cholesterol esters induced by AFB1 is additive in nature. In a time-course study, it was shown that liver necrosis and triglyceride, cholesterol and cholesterol ester accumulation occurred simultaneously in both groups of rats treated with AFB1 and ethanol-AFB1. These results suggest that fat accumulation per se is not a primary cause of liver necrosis induced by AFB1 and ethanol-AFB1.

The relationship of certain blood parameters to aflatoxin resistance in Japanese quail

Blood chemistry profiles were obtained for two lines of Japanese quail selected for resistance to aflatoxin, and for a nonselected control line. Nine of the 18 plasma components measured in samples taken at 4 weeks of age changed as a result of selection. Plasma concentrations of total protein, albumin, cholesterol, and potassium, and the activities of lactic dehydrogenase, glutamic-oxalacetic transaminase, and cholinesterase were significantly elevated in aflatoxin-resistant quail compared with the nonselected line. The activities of beta-glucuronidase and alpha-amylase changed most dramatically; both activities were much lower in the resistant lines than in the control line. In another experiment, serum total protein, albumin, alpha-amylase, and beta-glucuronidase were tested as identifiers of aflatoxin-resistant individuals within a nonselected population of quail. Serum samples obtained from 150 nonselected quail immediately before and 24 hr after administration of an oral dose of aflatoxin were analyzed for each of the four components. The acute toxicosis decreased body weight, serum alpha-amylase activity, total protein, and albumin; whereas, serum beta-glucuronidase activity and the coefficients of variation for each parameter were increased. Correlations between measurements made prior to dosing and parameters reflecting aflatoxin susceptibility were not significant. However, postdose determinations of albumin, protein, and beta-glucuronidase were significantly related to susceptibility parameters. These data indicate that the four blood components tested cannot be utilized to identify resistant birds within a nonselected population of quail without an aflatoxin challenge; but albumin, protein, and beta-glucuronidase are correlated with resistance when measured during an aflatoxin stress.

Inhibition of rat liver aminoacyl-tRNA synthetases in vitro after acute and chronic aflatoxin B1 administration in vivo

In a cell-free system the influence of acute aflatoxin B1 administration on the fractions of the hepatic microsomal translation complex of female rats was examined and compared with the influence of chronic aflatoxin B1 treatment. Polypeptide synthesis determined by [14C]leucine incorporation with the postmitochondrial supernatant was inhibited by 80% 24 h after acute aflatoxin B1 administration whereas inhibition was only 17% in animals 30 weeks after chronic treatment. After acute aflatoxin B1 administration inhibition of protein synthesis was by 67%, mainly on the polysomal level, whereas inhibition with the pH 5 enzyme was only by 22%. After chronic aflatoxin B1 administration inhibition was by 18%, mainly with the pH 5 enzyme and by only 6% on the polysomes. The inhibition by pH 5 enzyme was further investigated with regard to aminoacyl-tRNA synthetases, components of pH 5 enzyme. Leucine-specific aminoacyl-tRNA synthesis was inhibited by 20% 24 h after acute and by 12% 30 weeks after chronic aflatoxin B1 treatment. Aminoacyl-tRNA synthetase activities in a crude synthetase preparation tested with 5 amino acids were found to be noncompetitively inhibited. It is concluded that the inhibition of protein synthesis after acute and chronic aflatoxin B1 administration is in part due to decreased aminoacyl-tRNA levels caused by inhibited aminoacyl-tRNA synthetases.

Sequential biochemical and histological changes in rats treated with aflatoxin B1

Thirteen biochemical parameters (viz. glucose, calcium, inorganic phosphorous, urea nitrogen, uric acid, cholesterol, albumin, total protein, total bilirubin, alkaline phosphatase, lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase) were determined in serum and partly in liver of rats 1-28 days after i.p. aflatoxin B1 (AFB) (3 mg/kg). Histological examinations of the liver were also made in parallel to the biochemical studies. In the serum, enzyme activities and total bilirubin level increased and peaked on the 2nd day, while other activities of aspartate aminotransferase and alanine aminotransferase in the liver significantly decreased and reached a minimum on the 2nd day after AFB administration. The depression of the liver enzyme activities persisted over 7 days. The liver protein content also reduced transiently during 1-1.5 days. However, all biochemical parameters returned to normal levels 2 weeks after treatment, and remained so throughout the rest of experimental period. Histological changes in the liver were very similar to those reported by other.

Effect of aflatoxin B1 on hepatic polyribosomes and protein synthesis in the rat

This study was concerned with the effect of aflatoxin B1 on protein synthesis in rat liver. Specifically, the effect of the administration of aflatoxin B1 (6.0 mg/kg body weight) on hepatic polyribosomes (free and membranebound), protein synthesis in vitro, and initiation factors activity of rats within 3-12 h was investigated. The results revealed that aflatoxin B1 rapidly led to disaggregation of free and membrane-bound polyribosomes, to inhibition of in vitro protein synthesis by both populations of polyribosomes, to little or no effect on the activities of initiation factors, and to defective ribosomes, particularly the 60S ribosomal subnits, of both types of polyribosomes. Comparative studies on the effects of aflatoxin B1 and actinomycin D revealed progressive disaggregation by both hepatic free and membrane-bound polyribosomes after aflatoxin B1 but not only of the free polyribosomes after actinomycin D.

The role of liver glutathione in the acute toxicity of retrorsine to rats

(1) Two procedures have been used to change the glutathione concentration in the livers of male rats. The glutathione level is increased to about double that of the controls, 0.5 h after the administration of cysteine (200 mg/kg, i.p.) and to about 25% that of controls, 1 h after the administration of 2-chloroethanol (30 mg/kg, i.p.). (2) The acute LD50 of retrorsine to rats (42 mg/kg) is increased by pretreatment with cysteine to 83 mg/kg and decreased by pre-treatment with chloroethanol to 23 mg/kg. In all three groups, deaths are accompanied by haemorrhagic centrilobular necrosis of the liver. (3) 2 h after the administration of retrorsine to rats (60 mg/kg), the levels of pyrrolic metabolites in the livers of animals pre-dosed with cysteine or chloroethanol are respectively about 60% and 200% those of rats given no pre-treatment. (4) Neither in the normal nor in the pre-treated rats dose retrorsine (60 mg/kg) cause a detectable fall in liver glutathione concentration 0.5-4 h after dosing. By 24 h, the glutathione concentration in the livers of the retrorsine-dosed rats is higher than those of the corresponding controls. There was no significant change in the liver weights of the treated rats relative to the controls. (5) Treatment of rats with retrorsine (60 mg/kg) causes a fall in the liver concentrations of cytochrome P-450, 24 h after dosing. This loss of cytochrome P-450 is increased in rats pre-treated with chloroethanol. The concentrations of cytochrome b5 in the same animals are not significantly reduced.

The effect of aflatoxins on the incorporation of RNA and protein precursors by isolated hepatocytes

Hepatocytes prepared by a simplified enzymatic technique were active in the incorporation of RNA and protein precursors into acid-insoluble material. The incorporation of RNA precursors was very markedly inhibited by low levels of aflatoxin B1 and G1 but not by aflatoxins B2 and G2. The activity of mixed function oxidases (MFO), the drug-metabolizing system of the endoplasmic reticulum, could be suppressed in these cells by SKF525A or stimulated by NADPH. SKF525A caused a reduction in the inhibition by aflatoxin B1 of the incorporation of RNA precursor into macromolecules. This finding suggests that a metabolite of aflatoxin B1 is the actual inhibitor of RNA synthesis in the cells. Measurement of lactate dehydrogenase activity showed these cells to be leaky on incubation at 37 degrees C and thus not suitable for studies of protein secretion.

Translational step inhibited in vivo by aflatoxin B1 in rat-liver polysomes

Aflatoxin B1 strongly inhibits protein synthesis in rat liver cells. We previously demonstrated that this inhibition could be divided into two steps: up to 5 h aflatoxin blocks protein synthesis directly and specifically at the polysome level; beyond 7 h protein synthesis inhibition appears chiefly as a consequence of transcription impairment due to drug action. This paper confirms the foregoing results and represents an attempt to localize the translational step inhibited in vivo by aflatoxin B1. We used the simulation study developed by Li, Kisilevsky, Wasan and Hammond, 1972 (Biochim. Biophys. Acta, 272, 451-462) to determine precisely the site inhibited in vivo after drug intoxication. This analysis is based on two parameters: the kinetics of polysome labeling to follow the nascent peptide synthesis, and the kinetics of supernatant labeling to follow the completed protein synthesis. Up to 5 h after dosing, aflatoxin specifically inhibits the elongation and/or termination steps during protein synthesis; after longer periods of time inhibition occurs essentially at the initiation step. When the intracellular concentration of aflatoxin is too high, particularly 2 h after dosing, each step of protein synthesis is blocked. Polypeptide synthesis by the postmitochondrial supernatants isolated from aflatoxin-treated animals is impaired in the same proportion as protein synthesis in vivo. The damage caused by aflatoxin is mostly observed on microsomes. However, purified polysomes isolated from aflatoxin-treated rats synthesize proteins in vitro to the same extent as those from controls. These results suggest that aflatoxin metabolite(s) are bound to polysomes with noncovalent bonds. These active metabolites are probably lost during polysome isolation procedures. Finally, relationships between protein metabolism and aflatoxin carcinogenesis are discussed.