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Biomedical subjects

L Samson

Publications and source records attributed to L Samson.

At least 37 records · Page 2Linked to original sources
PubMed1995

Suppression of Escherichia coli alkB mutants by Saccharomyces cerevisiae genes.

The alkB gene is one of a group of alkylation-inducible genes in Escherichia coli, and its product protects cells from SN2-type alkylating agents such as methyl methanesulfonate (MMS). However, the precise biochemical function of the AlkB protein remains unknown. Here, we describe the cloning, sequencing, and characterization of three Saccharomyces cerevisiae genes (YFW1, YFW12, and YFW16) that functionally complement E. coli alkB mutant cells. DNA sequence analysis showed that none of the three gene products have any amino acid sequence homology with the AlkB protein. The YFW1 and YFW12 proteins are highly serine and threonine rich, and YFW1 contains a stretch of 28 hydrophobic residues, indicating that it may be a membrane protein. The YFW16 gene turned out to be allelic with the S. cerevisiae STE11 gene. STE11 is a protein kinase known to be involved in pheromone signal transduction in S. cerevisiae; however, the kinase activity is not required for MMS resistance because mutant STE11 proteins lacking kinase activity could still complement E. coli alkB mutants. Despite the fact that YFW1, YFW12, and YFW16/STE11 each confer substantial MMS resistance upon E. coli alkB cells, S. cerevisiae null mutants for each gene were not MMS sensitive. Whether these three genes provide alkylation resistance in E. coli via an alkB-like mechanism remains to be determined, but protection appears to be specific for AlkB-deficient E. coli because none of the genes protect other alkylation-sensitive E. coli strains from killing by MMS.

Alkylation↗
PubMed1995

The radiologic report: a realistic approach.

The radiologic report records all of the important steps taken in the practice of radiology and should be evaluated with special interest. The literature reflects the subjectivity inherent to a field left to personal interpretation, and few prospective studies of radiologic reporting are available. To determine the characteristics of the ideal radiologic report, the authors held workshops with two groups of radiologists and residents in radiology. The following items (in order of priority) were considered of prime importance by the participants and are discussed here: expression of an opinion and presentation of a workable differential diagnosis; an answer to the referring physician's question; a list of the limitations of the examination, if any; open-ended recommendations to the referring physician; integration of the findings with the clinical context; a synthesis of the findings obtained with various imaging modalities, when pertinent; and a complete description of the findings. The radiologic report has been compared to a scientific paper, and this comparison suggests the format for bringing these items together. The resulting realistic blueprint for the ideal radiologic report can be tailored to fit every imaging situation. Moreover, it can be a useful tool for evaluating the whole imaging process.

Humans↗
PubMed1994

All four known cyclic adducts formed in DNA by the vinyl chloride metabolite chloroacetaldehyde are released by a human DNA glycosylase.

We have previously reported that human cells and tissues contain a 1,N6-ethenoadenine (epsilon A) binding protein, which, through glycosylase activity, releases both 3-methyladenine (m3A) and epsilon A from DNA treated with methylating agents or the vinyl chloride metabolite chloroacetaldehyde, respectively. We now find that both the partially purified human epsilon A-binding protein and cell-free extracts containing the cloned human m3A-DNA glycosylase release all four cyclic etheno adducts--namely epsilon A, 3,N4-ethenocytosine (epsilon C), N2,3-ethenoguanine (N2,3-epsilon G), and 1,N2-ethenoguanine (1,N2-epsilon G). Base release was both time and protein concentration dependent. Both epsilon A and epsilon C were excised at similar rates, while 1,N2-epsilon G and N2,3-epsilon G were released much more slowly under identical conditions. The cleavage of glycosyl bonds of several heterocyclic adducts as well as those of simple methylated adducts by the same human glycosylase appears unusual in enzymology. This raises the question of how such a multiple, divergent activity evolved in humans and what may be its primary substrate.

Acetaldehyde↗
PubMed1994

Urinary excretion of pyridinolines crosslinks measured by immunoassay and HPLC techniques in normal subjects and in elderly patients with vitamin D deficiency.

Hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) are specific constituents of mature skeletal collagens excreted in urine. Their measurement represents a sensitive index of bone resorption. In this study, we have measured urinary excretion of pyridinolines crosslinks by immunoassay (ELISA) and HPLC methods in 80 patients with different bone resorption rates. We chose a sample of 44 healthy adults (30 men and 14 women) and a sample of 36 elderly patients (7 men and 29 women) presenting a secondary hyperparathyroidism due to a vitamin D deficiency. The correlation between HPLC (x) and ELISA (y) was judged satisfactory (y = 0.794x + 6.947, r = 0.92). The sensitivity of pyridinolines estimation was 50 nmol/l for immunoassay and 20 nmol/l for HPLC. The intra-assay and inter-assay coefficients of variation for the two analytical methods was < 10%. The mean excretion of crosslinks (nmol/mmol of creatinine) measured by both methods in the sample of healthy adults was higher in women than in men. The amount of pyridinolines crosslinks excreted by elderly patients with vitamin D deficiency are three time higher than those of normal adults when measured by ELISA and HPLC methods. The distribution of different molecular forms of urinary pyridinoline crosslinks was investigated. Values of pyridinolines measured by HPLC in our samples of elderly patients have shown that free and peptide-bound pyridinolines with molecular weight (mol. wt.) smaller than 1000 Da represent approximately 80% of the total pyridinolines contained in urinary samples. A study on the evaluation of the antiserum used in the immunoassay for reacting with the different molecular forms isolated from urine showed a high affinity for free and peptide-bound pyridinolines with molecular weight smaller than 10,000 Da and that do not react strongly with peptide-bound with molecular weight greater than 10,000 Da. We conclude that, although this immunoassay does not measure total pyridinolines and does not distinguish between HP and LP, it seems convenient for diagnostic of metabolic bone diseases.

Adult↗
PubMed1994

The Escherichia coli AlkB protein protects human cells against alkylation-induced toxicity.

Escherichia coli can ameliorate the toxic effects of alkylating agents either by preventing DNA alkylation or by repairing DNA alkylation damage. The alkylation-sensitive phenotype of E. coli alkB mutants marks the alkB pathway as an extremely effective defense mechanism against the cytotoxic effects of the SN2, but not the SN1, alkylating agents. Although it is clear that AlkB helps cells to better handle alkylated DNA, no DNA alkylation repair function could be assigned to the purified AlkB protein, suggesting that AlkB either acts as part of a complex or acts to regulate the expression of other genes whose products are directly responsible for alkylation resistance. However, here we present evidence that the provision of alkylation resistance is an intrinsic function of the AlkB protein per se. We expressed the E. coli AlkB protein in two human cell lines and found that it confers the same characteristic alkylation-resistant phenotype in this foreign environment as it does in E. coli. AlkB expression rendered human cells extremely resistant to cell killing by the SN2 but not the SN1 alkylating agents but did not affect the ability of dimethyl sulfate (an SN2 agent) to alkylate the genome. We infer that SN2 agents produce a class of DNA damage that is not efficiently produced by SN1 agents and that AlkB somehow prevents this damage from killing the cell.

AlkB Homolog 1, Histone H2a Dioxygenase↗
PubMed1994

[Radiological changes of talc pleurodesis in cases of effusion].

This study is based on the observations of 86 pleurodesis done by talc insufflation during thoracoscopy in 82 patients suffering from benign (8%) and malignant (92%) pleural effusions. Serial chest films were obtained on every patient. Chest computed tomography was obtained in ten patients. The most frequent finding seen in the early phase and one month later was the appearance of loculations (94%) in selective areas of the thorax. Occasionally they take the appearance of airfluid levels (22%). These loculations are characteristically located in the axillary (60%), intrafissural (30%) and paramediastinal (34%) areas of the chest. In the late phase with a mean evolution time of 6 months, these loculations evolve in 77% of patients in areas of pleural thickening. CT of the chest demonstrates the presence of characteristic pleural thickening in the form of coarse (5/12) and/or fine linear densities (7/12) corresponding to talc deposits, on the pleural surface. These modifications are shown by light microscopy examination of the pleural done at the autopsy.

Female↗
PubMed1993

Protection against chloroethylnitrosourea cytotoxicity by eukaryotic 3-methyladenine DNA glycosylase.

A eukaryotic 3-methyladenine DNA glycosylase gene, the Saccharomyces cerevisiae MAG gene, was shown to prevent N-(2-chloroethyl)-N-nitrosourea toxicity. Disruption of the MAG gene by insertion of the URA3 gene increased the sensitivity of S. cerevisiae cells to N-(2-chloroethyl)-N-nitrosourea, and the expression of MAG in glycosylase-deficient Escherichia coli cells protected against the cytotoxic effects of N-(2-chloroethyl)-N-nitrosourea. Extracts of E. coli cells that contain and express the MAG gene released 7-hydroxyethylguanine and 7-chloroethylguanine from N-(2-chloroethyl)-N-nitrosourea-modified DNA in a protein- and time-dependent manner. The ability of a eukaryotic glycosylase to protect cells from the cytotoxic effects of a haloethylnitrosourea and to release N-(2-chloroethyl)-N-nitrosourea-induced DNA modifications suggests that mammalian glycosylases may play a role in the resistance of tumor cells to the antitumor effects of the haloethylnitrosoureas.

Animals↗
PubMed1993

Mechanism of inactivation of human O6-alkylguanine-DNA alkyltransferase by O6-benzylguanine.

Human O6-alkylguanine-DNA alkyltransferase was rapidly inactivated by low concentrations of O6-benzylguanine, but the alkyltransferase from the Escherichia coli ogt gene was much less sensitive and alkyltransferases from the E. coli ada gene or from yeast were not affected. O6-Benzyl-2'-deoxyguanosine was less potent than the base, but was still an effective inactivator of the human alkyltransferase and had no effect on the microbial proteins. O6-Allylguanine was somewhat less active, but still gave complete inactivation of both the human and Ogt alkyltransferases at 200 microM in 30 min, slightly affected the Ada protein, and had no effect on the yeast alkyltransferase. O4-Benzylthymidine did not inactivate any of the alkyltransferase proteins tested. Inactivation of the human alkyltransferase by O6-benzylguanine led to the formation of S-benzylcysteine in the protein and to the stoichiometric production of guanine. The rate of guanine formation followed second-order kinetics (k = 600 M-1 s-1). Prior inactivation of the alkyltransferase by reaction with a methylated DNA substrate abolished its ability to convert O6-benzylguanine into guanine. These results indicate that O6-benzylguanine inactivates the protein by acting as a substrate for alkyl transfer and by forming S-benzylcysteine at the acceptor site of the protein. The inability of O6-benzylguanine to inactivate the microbial alkyltransferases may be explained by steric constraints at this site.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

In vivo evidence for endogenous DNA alkylation damage as a source of spontaneous mutation in eukaryotic cells.

Three genes that participate in the repair of DNA alkylation damage were recently cloned from Saccharomyces cerevisiae: the MGT1 O6-methylguanine DNA methyltransferase gene, the MAG 3-methyladenine DNA glycosylase gene, and the APN1 apurinic/apyrimidinic (AP) endonuclease gene. Altering the expression levels of these three genes produced significant changes in the S. cerevisiae spontaneous mutation rate. Spontaneous mutation increased in the absence of the MGT1 DNA methyltransferase, presumably because unrepaired, spontaneously produced, O6-alkylguanine lesions mispair during replication. Moreover, changing the ratios of the MAG 3-methyladenine DNA glycosylase and the APN1 AP endonuclease had profound effects on spontaneous mutation rates. In the absence of APN1, the overexpression of MAG increased spontaneous mutation, and the underexpression of MAG decreased spontaneous mutation. We infer that the MAG glycosylase acts upon spontaneously produced 3-alkyladenine and 7-alkylguanine DNA lesions to produce mutagenic abasic sites, and that if the repair of these abasic sites is not initiated by the APN1 AP endonuclease they cause mutations during replication. Our results indicate that eukaryotic cells harbor endogenous metabolites that alkylate nuclear DNA at both oxygens and nitrogens.

Alkylation↗
PubMed1993

Effects of a physiological dose of cholecystokinin on food intake and postprandial satiation in man.

CCK-33 was infused intravenously to groups of 9 lean and 9 obese volunteers in doses that elicited plasma CCK concentrations in the physiological range. The effect of these infusions on food intake and satiety signals was compared with the effect of saline infusions in the same subjects. Food intake (486 +/- 52 g; mean +/- S.E.M.) was slightly, but not significantly decreased (553 +/- 55 g after saline), and hunger and fullness feelings after eating were unaffected, in both of the two groups. We conclude that the infusion of CCK-33 to plasma levels comparable to those observed after a fatty meal does not have a major effect on food intake and postprandial hunger feelings.

Adult↗
PubMed1993

A common element involved in transcriptional regulation of two DNA alkylation repair genes (MAG and MGT1) of Saccharomyces cerevisiae.

The Saccharomyces cerevisiae MAG gene encodes a 3-methyladenine DNA glycosylase that protects cells from killing by alkylating agents. MAG mRNA levels are induced not only by alkylating agents but also by DNA-damaging agents that do not produce alkylated DNA. We constructed a MAG-lacZ gene fusion to help identify the cis-acting promoter elements involved in regulating MAG expression. Deletion analysis defined the presence of one upstream activating sequence and one upstream repressing sequence (URS) and suggested the presence of a second URS. One of the MAG URS elements matches a decamer consensus sequence present in the promoters of 11 other S. cerevisiae DNA repair and metabolism genes, including the MGT1 gene, which encodes an O6-methylguanine DNA repair methyltransferase. Two proteins of 26 and 39 kDa bind specifically to the MAG and MGT1 URS elements. We suggest that the URS-binding proteins may play an important role in the coordinate regulation of these S. cerevisiae DNA repair genes.

Alkylation↗
PubMed1993

Bombesin reduces food intake in lean man by a cholecystokinin-independent mechanism.

This double blind study was undertaken to determine whether infusion of bombesin (BBS) inhibits the intake of a carbohydrate-rich meal in nine lean healthy subjects and whether inhibition of food intake by BBS is mediated by cholecystokinin (CCK). During infusion of BBS, the amount of food eaten was decreased compared to that after saline infusion (482 +/- 74 vs. 602 +/- 68 g; P < 0.01). Subjective criteria of satiation were also significantly affected by BBS infusion (P < 0.05). Administration of the CCK receptor antagonist loxiglumide (CR1505) to six of the subjects did not prevent the decrease in food intake due to BBS (365 +/- 69 g) or the subjective criteria for satiety. Furthermore, the amount of food eaten during loxiglumide treatment alone (537 +/- 109 g) was not different from that during control saline infusion. In conclusion, infusion of BBS inhibits the intake of a carbohydrate-rich meal by a CCK-independent mechanism.

Adult↗
PubMed1992

The Saccharomyces cerevisiae MGT1 DNA repair methyltransferase gene: its promoter and entire coding sequence, regulation and in vivo biological functions.

We previously cloned a yeast DNA fragment that, when fused with the bacterial lacZ promoter, produced O6-methylguanine DNA repair methyltransferase (MGT1) activity and alkylation resistance in Escherichia coli (Xiao et al., EMBO J. 10,2179). Here we describe the isolation of the entire MGT1 gene and its promoter by sequence directed chromosome integration and walking. The MGT1 promoter was fused to a lacZ reporter gene to study how MGT1 expression is controlled. MGT1 is not induced by alkylating agents, nor is it induced by other DNA damaging agents such as UV light. However, deletion analysis defined an upstream repression sequence, whose removal dramatically increased basal level gene expression. The polypeptide deduced from the complete MGT1 sequence contained 18 more N-terminal amino acids than that previously determined; the role of these 18 amino acids, which harbored a potential nuclear localization signal, was explored. The MGT1 gene was also cloned under the GAL1 promoter, so that MTase levels could be manipulated, and we examined MGT1 function in a MTase deficient yeast strain (mgt1). The extent of resistance to both alkylation-induced mutation and cell killing directly correlated with MTase levels. Finally we show that mgt1 S.cerevisiae has a higher rate of spontaneous mutation than wild type cells, indicating that there is an endogenous source of DNA alkylation damage in these eukaryotic cells and that one of the in vivo roles of MGT1 is to limit spontaneous mutations.

Amino Acid Sequence↗
PubMed1992

The suicidal DNA repair methyltransferases of microbes.

Virtually every organism so far tested has been found to possess an extremely efficient DNA repair mechanism to ensure that certain alkylated oxygens do not accumulate in the genome. The repair is executed by DNA methyltransferases (MTases) which repair DNA O6-methylguanine (O6MeG), O4-methylthymine (O4MeT) and methylphosphotriesters (MePT). The mechanism is rather extravagant because an entire protein molecule is expended for the repair of just one, or sometimes two, O-alkyl DNA adduct(s). Cells profit from such an expensive transaction by earning protection against death and mutation by alkylating agents. This review considers the structure, function and biological roles of a number of well-characterized microbial DNA repair MTases.

Amino Acid Sequence↗
PubMed1992

[Palliative treatment with an endoluminal prosthesis for obstruction of the esophagus and the cardia].

Between December 1984 and September 1991, a total of 68 palliative esophageal intubations were performed in 57 patients. The mean age was 64 years, there were 40 males and 17 females. The more frequent causes of esophageal obstruction were malignant tumors of the esophagus (49%), lung (23%) and cardia (19%). Total dysphagia (24.5%), dysphagia to liquids (47%) and esophagotracheal aspiration (14%) were present in these patients. In 65% of patients insertion of the prosthesis was the first attempt at palliation, in the remaining patients it followed the failure of another type of palliation. The decision to palliate esophageal obstruction was established preoperatively in 86% of patients, in the remaining 14% it was decided at the time of surgery. The intubation by pulsion (Atkinson) [1] was successful in 57% of patients and by traction (Celestin) [2] in 86% of patients. The associated morbidity was 36% for the Atkinson prosthesis and 13% for the Celestin prosthesis, although the latter required a laparotomy. The in hospital mortality was 12.5%. The mean survival was 85 days. A normal or semi-solid diet was tolerated by over 60% of patients following intubation. Palliation of esophageal obstruction by intubation improves the quality of life but is associated with a high morbidity.

Cardia↗