Effects of Oral Administration of Non-genotoxic Hepato-hypertrophic Compounds on Metabolic Potency of Rat Liver

It remains uncertain why non-genotoxic compounds that result in liver hypertrophy cause liver tumors. In an effort to resolve this issue, we examined whether liver post-mitochondrial fraction (S9) prepared from rats treated with non-genotoxic compounds affected the genotoxicity of pro-mutagens. Known hepatotoxic compounds, such as piperonyl butoxide (PBO), decabromodiphenyl ether (DBDE), beta-naphthoflavone (BNF), indole-3-carbinol (I3C) and acetaminophen (AA), were orally administered to male and female F344 rats at doses sufficient to cause liver hypertrophy. Rats received diets containing each test compound for 3 days, 4 weeks or 13 weeks, and were then kept for 4 weeks without the test chemical. S9 prepared from the livers of each group was used for the Ames test with 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), benzo[a]pyrene (BaP) and N-nitrosodimethylamine (NDMA). In both sexes, liver hypertrophy was observed following administration of all test compounds, and was then reversed to the control state when administration ceased. The mutagenicity of MeIQx, BaP and NDMA increased with the use of S9 derived from rats treated with non-genotoxic compounds other than AA. DBDE administration had a marked effect on the mutagenicity of BaP (over a 30-fold increase in females) and NDMA (about a 20-fold increase in males). To estimate the involvement of metabolic enzymes in the alteration of mutagenicity, we measured the activity of ethoxyresorufin-O-deethylase (EROD) and methoxyresorufin-O-demethylase (MROD) (phase I enzymes), and UDP-glucuronosyltransferase (UGT) and glutathione S-transferase (GST) (phase II enzymes) in each S9 sample. The activity of phase I enzymes increased, even at the 3rd day following administration, and then decreased gradually, except in the case of AA, while the activity of phase II enzymes increased slightly. These results suggest that non-genotoxic hepato-hypertrophic compounds may be partly involved in carcinogenesis by modulating the metabolism of pre-carcinogens incorporated from the environment, in a manner that is dependent on sex and pre-incorporated chemicals

Genetic Regulation of Fluxes: Iron Homeostasis of Escherichia coli

Iron is an essential trace-element for most organisms. However, because high concentration of free intracellular iron is cytotoxic, cells have developed complex regulatory networks that keep free intracellular iron concentration at optimal range, allowing the incorporation of the metal into iron-using enzymes and minimizing damage to the cell. We built a mathematical model of the network that controls iron uptake and usage in the bacterium Escherichia coli to explore the dynamics of iron flow. We simulate the effect of sudden decrease or increase in the extracellular iron level on intracellular iron distribution. Based on the results of simulations we discuss the possible roles of the small RNA RyhB and the Fe-S cluster assembly systems in the optimal redistribution of iron flows. We suggest that Fe-S cluster assembly is crucial to prevent the accumulation of toxic levels of free intracellular iron when the environment suddenly becomes iron rich.Comment: 8 pages, 4 figure

Freeing Pseudomonas putida KT2440 of its proviral load strengthens endurance to environmental stresses

2.6% of the genome of the soil bacterium Pseudomonas putida KT2440 encodes phage-related functions, but the burden of such opportunistic DNA on the host physiology is unknown. Each of the four apparently complete prophages borne by this strain was tested for stability, spontaneous excision and ability to cause lysis under various stressing conditions. While prophages P3 (PP2266-PP2297) and P4 (PP1532-1584) were discharged from the genome at a detectable rate, their induction failed otherwise to yield infective viruses. Isogenic P. putida KT2440 derivatives bearing single and multiple deletions of each of the prophages were then subjected to thorough phenotypic analyses, which generally associated the loss of proviral DNA with an increase of physiological vigour. The most conspicuous benefit acquired by prophage-less cells was a remarkable improvement in tolerance to UV light and other insults to DNA. This was not accompanied, however, with an upgrade of recA-mediated homologous recombination. The range of tolerance to DNA damage gained by the prophage-free strain was equivalent to the UV resistance endowed by the TOL plasmid pWW0 to the wild-type bacterium. While the P. putida's prophages are therefore genuinely parasitic, their detrimental effects can be offset by acquisition of compensatory traits through horizontal gene transfer.This study was supported by the BIO Program of the Spanish Ministry of Science and Innovation, the ST-FLOW and ARYSIS Contracts of the EU, the ERANET-IB Program and the PROMT Project of the CAM. The work in MK Laboratory is supported by Estonian Science Foundation, grant number 9114 to MK, by Estonian Ministry of Research Targeted Financing Project SF0180031s08.Peer reviewe

A cluster of constitutive mutations affecting the C-terminus of the redox-sensitive SoxR transcriptional activator.

Activation of Escherichia coli oxidative stress regulon genes (sodA, zwf, fumC, nfo, etc.) is mediated by a two-stage regulatory system: the redox-sensitive SoxR protein transcriptionally activates the soxS gene, whose product then stimulates transcription of the regulon genes. Previous experiments showed that limited 3' truncation of soxR gene causes constitutive soxRS expression. DNA sequence analysis of the soxR genes from the soxRS-constitutive strains isolated originally (Greenberg et al. (1990) Proc. Natl. Acad. Sci. USA 87, 6181-6185) revealed that three alleles encode amino acid substitutions or a chain termination clustered near the C-terminus of SoxR. Two other single-amino-acid substitutions in constitutive alleles mapped to the helix-turn-helix motif and to a region of unknown function in the center of the polypeptide, respectively. No constitutive mutation was found within the region encoding the cysteines of the SoxR FeS center, in the soxR or soxS promoters, or in the soxS structural gene. Since an in-frame deletion of just nine SoxR residues (136-144; full-length SoxR = 154 residues) gave rise to a powerful constitutive allele, it appears that a small segment of the SoxR C-terminus maintains the protein in the inactive state. Conservely, an intact C-terminus is evidently not required for gene activation by SoxR