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G E Neal

Publications and source records attributed to G E Neal.

At least 37 records · Page 2Linked to original sources
PubMed1996

Modulation of the cytochrome P450 system as a mechanism of chemoprotection.

The cytochrome P450-dependent mono-oxygenase system has evolved as one of our primary defences against toxic chemicals present in our environment. This multi-enzyme system functions as an adaptive response to environmental challenge in that exposure to specific toxic agents induces the expression of cytochrome P450 isozymes active in their metabolism. In most cases, such metabolism leads to an increased rate of chemical detoxification, but in certain cases it can also lead to an increased rate of chemical activation to toxic products. The induction of cytochrome P450s leading to cytoprotection is a major mechanism of chemoprotection, and it is well documented that this pathway prevents a large number of toxic reactions. Incredibly, despite the importance of this metabolic pathway, the effects of a wide range of chemoprotective agents with different mechanisms of action on the expression of specific cytochrome P450 isozymes remains poorly understood. In this chapter, we discess the ability of different chemical compounds to modulate cytochrome P450 both at the transcriptional and post-transcriptional levels, in a way which affects its ability to metabolize or detoxify chemical carcinogens.

Animals↗
PubMed1996

Regulation of glutathione S-transferases and aldehyde reductase by chemoprotectors: studies of mechanisms responsible for inducible resistance to aflatoxin B1.

A number of xenobiotics, including the synthetic antioxidant ethoxyquin, inhibit aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rat. Two detoxification enzymes that mediate ethoxyquin-induced chemoprotection against AFB1 have been identified by protein purification: a glutathione S-transferase (GST) Yc2 subunit with at least 100-fold greater activity towards AFB1-8,9-epoxide than previously studied transferases, and a unique aldehyde reductase with activity towards the dialdehydic form of AFB1-8,9-dihydrodiol. Molecular cloning has revealed that the Yc2 subunit is a class alpha GST and that the aflatoxin-metabolizing aldehyde reductase (AFAR) is a distant member of the aldo-keto reductase superfamily. Enzyme assay and western blotting have shown that many chemoprotectors, such as ethoxyquin, butylated hydroxyanisole, butylated hydroxytoluene, oltipraz and indole-3-carbinol, that inhibit AFB1-mediated hepatocarcinogenesis induce both GST Yc2 and AFAR. However, western blotting suggests that these enzymes are not always coordinately regulated, as treatment with phenobarbital and beta-naphthoflavone results in differences in the relative increase in hepatic GST Yc2 and AFAR. These findings indicate that GST Yc2 and AFAR represent important resistance mechanisms against AFB1 in the rat. This conclusion is supported by the observation that GST Yc2 and AFAR are overexpressed in rat liver preneoplastic nodules, which display pleiotropic drug resistance.

Aflatoxin B1↗
PubMed1995

Aflatoxin M1 in human breast milk samples from Victoria, Australia and Thailand.

This study examined the exposure of infants to aflatoxin M1 (AFM1) and of lactating mothers to aflatoxin B1 (AFB1), using AFM1 in breast milk as a biomarker for exposure to AFB1. An enzyme-linked immunosorbent assay was modified for the analysis of AFM1 in breast milk samples from 73 women from Victoria for comparison with breast milk samples from Thailand (n = 11). The results were compared with those obtained by HPLC. AFM1 was detected in 11 samples from Victoria and five samples from Thailand at median concentrations of 0.071 ng/ml (range 0.028 to 1.031 ng/ml) and 0.664 ng/ml (range 0.039 to 1.736 ng/ml), respectively. Levels of AFM1 in Thai milk samples were significantly higher than those in milk samples from Victoria.

Aflatoxin B1↗
PubMed1995

Genetic implications in the metabolism and toxicity of mycotoxins.

In common with many xenobiotics, metabolic activation and detoxification play crucial roles in the determination of a toxic response of animal species, including man, to exposure to mycotoxins. Control of expression of the relevant enzymes, both constitutive and inducible, is therefore a major factor in mycotoxin-induced acute or chronic toxicities. The involvement of these factors in the toxic responses to aflatoxins and ochratoxins will be briefly reviewed. In the case of the aflatoxins, the importance of secondary, conjugating metabolism has become increasingly evident. The specific control of expression of these enzymes, through sequences present in the 5' region of the gene, is evident (e.g. by the changes seen during development and differences between the sexes). The existence of these control mechanisms has made feasible the development of chemoprotective strategies. Although less detailed information is available concerning the metabolic activation and detoxification of the ochratoxins, it appears probable that future studies will reveal a role for the genetic control of expression of enzymes responsible for the target nephrotoxicity of these mycotoxins.

Aflatoxin B1↗
PubMed1995

Aflatoxin and outcome from acute lower respiratory infection in children in The Philippines.

Aflatoxin is immunosuppressive in experimental conditions. This study addressed its potentially contributory role in the poor outcome of acute lower respiratory infections (ALRI) in children in The Philippines. The catchment area included peri-urban slums and middle-class housing. One hundred and fifteen children (mean age 2.1, range 0.08-12 years) were enrolled and their serum and urine obtained at presentation with ALRI. Aflatoxins in serum and aflatoxin metabolites in urine were measured by previously validated ELISA tests. Using the 1986 WHO criteria for the severity of ALRI, 31% had mild, 12% moderate, 49% severe and 8% severe complicated pneumonia. Eighty of 97 (82%) chest radiographs were abnormal. Ninety per cent of the children were below average weight for age, using Filipino standards, with a mean of 79% (range 27-157%). Thirteen (11%) children died. Aflatoxin in their serum, reflecting recent ingestion, was detected in 33%, with a mean positive value of 462 pg/ml. Aflatoxin metabolites (reflecting chronic ingestion) were detected in 64 of 65 urines collected, with a mean value of 0.1-4.77ng/ml. None of the children with detectable serum aflatoxin died. Anorexia and impaired consciousness were strongly associated with a poor outcome (prolonged fever or death). There was a strong association between undetectable serum aflatoxin concentrations and death (p = 0.004), perhaps reflecting anorexia. There was no relationship between the concentration of urinary aflatoxin metabolites and outcome. Serum was also obtained from 29 mothers on admission and none contained detectable aflatoxin. As virtually all the children had evidence of exposure to aflatoxin, a potentially immunosuppressive role in the context of pneumonia cannot be excluded.

Aflatoxins↗
PubMed1995

Snakebite treatment at a southeastern regional referral center.

Our objective was to determine the prevalence of poisonous snakebite victims admitted to a regional trauma center in Southeastern Georgia over a 10-year period, as well as the type of snake, grade of envenomation, treatment administered, morbidity and mortality, and outcome. Records of patients admitted to the center for snakebite from a 24-county catchment area during the 10-year period (January 1984 to January 1994) were retroactively reviewed. Sixty-three (63) bites in 62 victims of venomous snakebites were treated. The snake distribution was rattlesnake: 19 (30%), copperhead: 18 (29%), cottonmouth moccasin: 8 (12%), unknown: 18 (29%). Envenomation grades were Grade I: 20 (32%), Grade II: 24 (38%), Grade III: 10 (16%), and Grade IV: 9 (14%). Fourteen of 19 (74%) Grades III and IV envenomations were from rattlesnakes. Antivenin was used in all Grade IV and half of the Grade III envenomations. Antivenin was administered within 3 hours of injury in all but one case. Five patients had surgery. Two patients (both Grade I) developed anaphylaxes from antivenin given before hospitalization. All patients recovered. An average of 6 snakebites were treated each year. Expeditious transport, attention to the type of snake inflicting the bite, and judicious use of antivenin will result in a favorable outcome for the snakebite victim.

Adult↗
PubMed1994

Cloning of cDNAs from fetal rat liver encoding glutathione S-transferase Yc polypeptides. The Yc2 subunit is expressed in adult rat liver resistant to the hepatocarcinogen aflatoxin B1.

Fetal rat liver possesses substantial levels of glutathione S-transferase (GST) activity toward aflatoxin B1-8,9-epoxide. The enzyme responsible for this activity is an Alpha-class GST heterodimer comprising Yc1 and Yc2 subunits. The cDNAs encoding these polypeptides have been cloned and shown to share approximately 91% identity over 920 base pairs, extending from nucleotide -23 to the AATAAA polyadenylation signal. GST Yc2Yc2 expressed in Escherichia coli was found to exhibit 150-fold greater activity toward aflatoxin B1-8,9-epoxide than GST Yc1Yc1. Comparison between the structures of Alpha-class GST suggests that tyrosine at residue 108 and/or aspartate at residue 208 is responsible for the high aflatoxin B1 detoxication capacity of Yc2. Immunoblotting and enzyme assays have shown that liver from adult female rats contains about 10-fold greater levels of Yc2 than is found in liver from adult male rats. This sex-specific expression of Yc2 in adult rat liver may contribute to the relative insensitivity of female rats to aflatoxin B1. Dietary administration of oltipraz, a synthetic antioxidant which protects against aflatoxin-hepatocarcinogenesis, serves as an inducer of GST Yc2.

Aflatoxin B1↗
PubMed1994

Regulation of aflatoxin B1-metabolizing aldehyde reductase and glutathione S-transferase by chemoprotectors.

Ingestion of aflatoxin B1 (AFB1) represents a major risk factor in the aetiology of human hepatocellular carcinoma. In the rat, the harmful effects of AFB1 can be prevented by the administration of certain drugs which induce hepatic detoxification enzymes. We have previously shown that treatment of rats with the chemoprotector ethoxyquin (EQ) results in a marked increase in expression of the Alpha-class glutathione S-transferase (GST) Yc2 subunit which has high activity towards AFB1-8,9-epoxide [Hayes, Judah, McLellan, Kerr, Peacock and Neal (1991) Biochem. J. 279, 385-398]. To allow an assessment of whether the increased expression of GST Yc2 represents a general adaptive resistance mechanism to chemical stress, that is invoked by both chemoprotectors and carcinogens, we have examined the effects of EQ, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), phenobarbital (PB), AFB1, 3-methylcholanthrene (3-MC) and clofibrate on the AFB1-glutathione-conjugating activity and the GST subunit levels in rat liver. In addition, the effect of these drugs on the hepatic levels of an aldehyde reductase (AFB1-AR) that metabolizes the cytotoxic dialdehydic form of AFB1 has been studied as this enzyme also appears to be important in chemoprotection. Administration of the antioxidants EQ, BHA or BHT, as well as PB, led to a marked increase in levels of the GST Yc2 subunit in rat liver, and this increase coincided with a substantial rise in the GST activity towards AFB1-8,9-epoxide; neither AFB1, 3-MC nor clofibrate caused induction of Yc2 or any of the GST subunits examined. Among the xenobiotics studied, EQ was found to be the most effective inducing agent for the Yc2 subunit as well as Yc1, Yb1 and Yf. However, PB was equally as effective as EQ in increasing levels of the Ya-type subunits, although it was not found to be as potent an inducer of the other GST subunits, including Yc2. In addition to induction of GST, EQ caused a substantial increase in the hepatic content of AFB1-AR. Both BHA and BHT were also able to induce this enzyme but, by contrast, PB was found to be a poor inducer of AFB1-AR. AFB1, 3-MC and clofibrate were unable to serve as inducers of this reductase. The presence of Alpha-class GST, including the Yc2 subunit, was examined in various rat tissues. Constitutive expression of Yc2 was found in the epididymis at levels comparable with that observed in the liver from EQ-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Aflatoxin B1↗
PubMed1994

The primary and secondary metabolism of aflatoxin B1 by rat hepatocytes cultured on matrigel.

Rat hepatocytes on culturing rapidly lose the ability to metabolize many xenobiotics including the mycotoxin aflatoxin B1 (AFB1). Rat hepatocytes cultured for 5 days on plastic retain < 10% of the initial cytochrome P450 content compared with 34% in cells grown on matrigel alone and 74% in cells cultured on matrigel plus phenobarbital (PB). Although microsomal activation of AFB1 was preserved in matrigel cultured hepatocytes and increased by PB in all the culturing conditions examined, the AFB1 macromolecular binding capacity was reduced to < 10% of that present in the initial cultures. This lack of correlation between cytochrome P450 levels, microsomal activation, and AFB1 binding involved cytosolic glutathione-S-transferase activity (GST). This increased in matrigel cultured cells and was further enhanced by PB. Western blotting of the cytosols showed that the expression of the alpha class GST subunit Yc2 correlated with AFB1-GSH conjugate formation. By contrast, expression of the alpha class GST Ya-type subunit showed no similar close correlation. The expression of pi class GST Yf subunit, which is associated with dedifferentiation in hepatocytes, was suppressed by culturing on matrigel. The results obtained using hepatocytes cultured on matrigel indicate that despite limitations, this comprises a potentially useful model for the in vivo situation.

Aflatoxin B1↗
PubMed1994

Experience with laparoscopic and open appendectomies in a surgical residency program.

Laparoscopic appendectomy represents another challenge for residency training. This retrospective study was conducted to determine the efficacy of this operation, to contrast it to conventional appendectomy, and to describe experience in teaching residents. We examined the case records of 151 patients who underwent appendectomies performed by residents between September 1990 and September 1992. Surgery was performed laparoscopically in 26% of the cases, by conventional means in 65%, and converted to an open procedure in 9% of the total patients. Acute appendicitis was confirmed in 56% of laparoscopic patients, 85% of conventional patients, and 86% of converted patients. Age, sex, signs and symptoms, complications, and operative times were similar in laparoscopic and conventional groups. Mean hospital stay was 3.5 days in the laparoscopic group, 4.5 days in the conventional group, and 8.5 days in the converted group. Operating room charges were highest in the laparoscopic group. Surgical procedure depended on the experience and bias of the attending surgeon. Laparoscopic appendectomy appears to be easily learned by surgical residents and to be associated with an outcome similar to that of conventional appendectomy.

Adult↗
PubMed1993

An ethoxyquin-inducible aldehyde reductase from rat liver that metabolizes aflatoxin B1 defines a subfamily of aldo-keto reductases.

Protection of liver against the toxic and carcinogenic effects of aflatoxin B1 (AFB1) can be achieved through the induction of detoxification enzymes by chemoprotectors such as the phenolic antioxidant ethoxyquin. We have cloned and sequenced a cDNA encoding an aldehyde reductase (AFB1-AR), which is expressed in rat liver in response to dietary ethoxyquin. Expression of the cDNA in Escherichia coli and purification of the recombinant enzyme reveals that the protein exhibits aldehyde reductase activity and is capable of converting the protein-binding dialdehyde form of AFB1-dihydrodiol to the nonbinding dialcohol metabolite. We show that the mRNA encoding this enzyme is markedly elevated in the liver of rats fed an ethoxyquin-containing diet, correlating with acquisition of resistance to AFB1. AFB1-AR represents the only carcinogen-metabolizing aldehyde reductase identified to date that is induced by a chemoprotector. Alignment of the amino acid sequence of AFB1-AR with other known and putative aldehyde reductases shows that it defines a subfamily within the aldo-keto reductase superfamily.

Aflatoxin B1↗
PubMed1993

Resistance to aflatoxin B1 is associated with the expression of a novel aldo-keto reductase which has catalytic activity towards a cytotoxic aldehyde-containing metabolite of the toxin.

Fischer 344 rats readily develop liver cancer when exposed to aflatoxin B1 (AFB1) but dietary administration of the antioxidant ethoxyquin (EQ) provides protection against hepatocarcinogenesis. Chemoprotection by EQ is accompanied by the overexpression of enzymes which detoxify activated AFB1. Aflatoxin-protein adduct formation takes place following metabolism of AFB1 to the dialdehydic form of AFB1-dihydrodiol. The dialdehyde can be detoxified by reduction to a dialcohol through the catalytic actions of an enzyme present in the hepatic cytosol from rats fed EQ-containing diets; this metabolite is essentially undetectable in reaction mixtures that use hepatic cytosol from rats fed control diets. The enzyme responsible for catalyzing the formation of dihydroxy-aflatoxin B1 has been purified from the livers of rats fed on diets supplemented with EQ. It is a soluble monomeric protein with an approximate M(r) of 36,600. Besides its activity toward AFB1 this enzyme also catalyzes the reduction of the model substrate 4-nitrobenzaldehyde. Amino acid sequencing of cyanogen bromide-derived peptides obtained from this reductase indicated that it has not been characterized hitherto, at least not a molecular level. Therefore, this inducible enzyme has been designated aflatoxin B1-aldehyde reductase (AFB1-AR). The livers of adult rats administered dietary EQ contain at least 15-fold greater levels of AFB1-AR than the livers from rats fed control diets. Aflatoxin B1-AR was also found to be present in increased amounts in livers bearing preneoplastic nodules and in rat hepatoma, both of which are known to express increased resistance to AFB1. Kidney contains high constitutive levels of AFB1-AR and the administration of EQ increases its concentration in renal cytosol about 3-fold. Although AFB1-AR is present in trace amounts in rat lung it was not detected in brain and in neither tissue was it found to be induced by EQ. Evidence suggests that AFB1-AR is a previously unrecognized enzyme that could provide protection against the cytotoxic effects of aflatoxin B1 resulting from the formation of protein adducts. The relative importance of AFB1-AR and the glutathione-S-transferase Yc2 subunit in conferring resistance to aflatoxin B1 is discussed.

Aldehyde Reductase↗
PubMed1993

A novel aldehyde reductase with activity towards a metabolite of aflatoxin B1 is expressed in rat liver during carcinogenesis and following the administration of an anti-oxidant.

In contrast with fractions from control animals, an aldehyde reductase, which catalyses the reduction of aflatoxin B1-dihydrodiol, in the dialdehyde form at physiological pH values, to aflatoxin B1-dialcohol, is expressed in cytosolic fractions prepared from rat livers bearing pre-neoplastic lesions, or following treatment with the anti-oxidant ethoxyquin. This expression parallels the development of resistance to the toxin. Unlike the aflatoxin B1-dihydrodiol, the dialcohol does not undergo binding to protein. This enzyme activity could play a mechanistic role in hepatocarcinogenesis and chemoprotection in the rat. Correlated n.m.r. and m.s. spectra are provided in Supplementary Publication SUP 50171 (3 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1993) 289, 9.

Aflatoxin B1↗
PubMed1993

In vitro metabolism of aflatoxin B1 by normal and tumorous liver tissue from Thailand.

Liver tissues were obtained from 20 liver cancer patients from Thailand, an area where the incidence of this tumour is high and where exposure to aflatoxin occurs. The expression of hepatic cytochrome P450s (P450) and glutathione S-transferase (GST) was examined and this expression was compared to the in vitro metabolism of aflatoxin B1 (AFB1). There was a > 10-fold inter-individual variation in expression of the various P450s including CYP3A4 (57-fold), CYP2B6 (56-fold) and CYP2A6 (120-fold). Microsomal metabolism of AFB1 to AFB1 8,9-epoxide (as measured by AFB1 tris-diol formation) and aflatoxin Q1 (AFQ1), the major metabolite produced, was significantly correlated with CYP3A3/4 expression (P < 0.001) and, to a lesser extent, with CYP2B6 expression (P < 0.01). There was a significantly reduced expression of major P450 proteins in microsomes from liver tumours compared to microsomes from the paired normal liver when analysed by Western immunoblot analysis. The production of AFQ1 and AFB1 tris-diol was almost uniformly reduced in tumours, but interestingly, the production of AFP1 was significantly increased. The immunoreactive expression of the major human classes of cytosolic GSTs (alpha, mu and pi) was also analyzed in normal and tumorous liver tissue. The expression of GSTA (alpha) and GSTM (mu) class proteins was markedly decreased and GSTP (pi) increased in the majority of tumour cytosols compared to normal liver. The cytosolic GST activity (1-chloro-2,4-dinitrobenzene conjugation) was significantly lower in liver tumours compared to normal liver (193 +/- 149 versus 875 +/- 299 nmol/min/mg, P < 0.0001), as was glutathione peroxidase (GPx) activity (cumene hydroperoxide) (26 +/- 23 versus 70 +/- 26 nmol/min/mg respectively, P < 0.005). Ten out of 14 individuals (71%) were homozygous null when genotyped for GSTM1. There was no detectable conjugation of AFB1 8,9-epoxide to glutathione by cytosol either from tumorous or normal liver. Thus, capacity of human cytosols to conjugate reactive AFB1 metabolites to GSH resembled AFB1-sensitive species such as rat, trout and duck rather than resistant species such as mouse and hamster. These data indicate a strong capacity of multiple forms of human hepatic P450s to metabolize AFB1 to both the reactive intermediate AFB1 8,9-epoxide and the detoxification product AFQ1. These results suggest that in view of the lack of significant GST-mediated protection against AFB1 in human liver, variations in expression of hepatic P450, due either to genetic polymorphisms or to modulation by environmental factors, may be important determinants in the risk of liver cancer development in AFB1-exposed populations.

Adolescent↗
PubMed1993

HBsAg, aflatoxins and primary hepatocellular carcinoma.

Nigeria is a very high risk area for primary hepatocellular carcinoma and this is the first study to utilize measurements of both hepatitis B virus status and aflatoxin levels in the same patients to determine the role of these factors in the causation of liver cancer in this environment. We have shown that there is a higher prevalence of hepatitis B surface antigen (P < 0.005) and higher 'pathologic' serum levels of aflatoxins (P < 0.05) in patients with primary hepatocellular carcinoma than in matched controls. It is considered that the results of this study may strengthen the hypothesis that hepatitis B virus may be an important aetiological factor in the development of primary hepatocellular carcinoma. Further work is in progress to correlate the level of aflatoxin serum albumin adducts with liver damage in order to assess the value of the albumin adduct as a marker of risk of liver cancer development.

Adult↗
PubMed1993

HBsAg and aflatoxins in sera of rural (Igbo-Ora) and urban (Ibadan) populations in Nigeria.

The purpose of this study was to screen for the presence of hepatitis B surface antigen and aflatoxins in the sera of 100 non-hospitalized individuals from the rural population of Igbo-Ora and 89 non-hospitalized individuals from the urban population of Ibadan, Nigeria. Hitherto, such a study as this has not been undertaken in this environment. The proportions of hepatitis B surface antigen carriage and serum 'pathologic' levels of aflatoxins were high (47-49%, 8.2-9.0% respectively) but varied very little between the two different populations sampled. These findings indicate that determined efforts should be instituted to reduce or eliminate hepatitis B virus infection and aflatoxin contamination of high risk foodstuffs from this environment.

Adolescent↗
PubMed1993

A comparison of the effects of aflatoxin B1 on the livers of rats and duck hepatitis B virus-infected and noninfected ducks.

A need exists for an appropriate animal model for the involvement of both hepatitis B virus infection and ingestion of aflatoxins in the etiology of liver cancer. Duck hepatitis B virus-infected ducks, on the basis of hepatoma development in the wild in China, appear to offer this possibility. The duck has been reexamined as a model system, and key metabolic processes have been assayed in comparison with the rat model for hepatocarcinogenesis. Aflatoxin B1 was found to be more actively metabolized by hepatic microsomes isolated from Pekin ducks in vitro to the aflatoxin B1-8,9-epoxide than corresponding fractions from the rat, and in vivo, higher levels of aflatoxin B1-guanine adduct were formed in hepatic DNA than in the livers of the aflatoxin B1-sensitive F344 rat. Repair of this DNA lesion in the duck and the subsequent formation of the ring-opened aflatoxin B1-FAPy adduct paralleled that in the rat. No effect of duck hepatitis B virus infection was found on any of these biochemical processes. The formation of hepatic lesions was also studied, and lesions were compared with those seen in the aflatoxin B1-treated rat. Histological analysis of necropsy specimens from ducks, 20 mo after the ducks received doses of aflatoxin B1 (25 and 50 micrograms/kg body wt), showed almost complete regression of the early acute lesions, with no evidence of neoplasia. Male F344 rats treated with aflatoxin B1 150 micrograms/kg 5 days/wk for 4 wk had extensive bile duct hyperplasia at the end of the treatment period and 100% incidence of hepatocellular carcinoma after 52 wk. The possible basis for the relative sensitivity of ducks and rats to the carcinogenic action of aflatoxin B1 is discussed.

Aflatoxin B1↗