Pedro Fernandez-Salguero

Targeted disruption of the alpha isoform of the peroxisome proliferator-activated receptor gene in mice results in abolishment of the pleiotropic effects of peroxisome proliferators.

To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in rodents, we disrupted the ligand-binding domain of the alpha isoform of mouse PPAR (mPPAR alpha) by homologous recombination. Mice homozygous for the mutation lack expression of mPPAR alpha protein and yet are viable and fertile and exhibit no detectable gross phenotypic defects. Remarkably, these animals do not display the peroxisome proliferator pleiotropic response when challenged with the classical peroxisome proliferators, clofibrate and Wy-14,643. Following exposure to these chemicals, hepatomegaly, peroxisome proliferation, and transcriptional-activation of target genes were not observed. These results clearly demonstrate that mPPAR alpha is the major isoform required for mediating the pleiotropic response resulting from the actions of peroxisome proliferators. mPPAR alpha-deficient animals should prove useful to further investigate the role of this receptor in hepatocarcinogenesis, fatty acid metabolism, and cell cycle regulation.

Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor

The aryl hydrocarbon (Ah) receptor (AHR) mediates many carcinogenic and teratogenic effects of environmentally toxic chemicals such as dioxin. An AHR-deficient (Ahr-/-) mouse line was constructed by homologous recombination in embryonic stem cells. Almost half of the mice died shortly after birth, whereas survivors reached maturity and were fertile. The Ahr-/- mice showed decreased accumulation of lymphocytes in the spleen and lymph nodes, but not in the thymus. The livers of Ahr-/- mice were reduced in size by 50 percent and showed bile duct fibrosis Ahr-/- mice were also nonresponsive with regard to dioxin-mediated induction of genes encoding enzymes that catalyze the metabolism of foreign compounds. Thus, the AHR plays an important role in the development of the liver and the immune system.

Role of CYP2E1 in the Hepatotoxicity of Acetaminophen

CYP2E1, a cytochrome P-450 that is well conserved across mammalian species, metabolizes ethanol and many low molecular weight toxins and cancer suspect agents. The cyp2e1 gene was isolated, and a mouse line that lacks expression of CYP2E1 was generated by homologous recombination in embryonic stem cells. Animals deficient in expression of the enzyme were fertile, developed normally, and exhibited no obvious phenotypic abnormalities, thus indicating that CYP2E1 has no critical role in mammalian development and physiology in the absence of external stimuli. When cyp2e1 knockout mice were challenged with the common analgesic acetaminophen, they were found to be considerably less sensitive to its hepatotoxic effects than wild-type animals, indicating that this P-450 is the principal enzyme responsible for the metabolic conversion of the drug to its active hepatotoxic metabolite.

Genomic instability in Gadd45a- deficient mice

Gadd45a-null mice generated by gene targeting exhibited several of the phenotypes characteristic of p53-deficient mice, including genomic instability, increased radiation carcinogenesis and a low frequency of exencephaly. Genomic instability was exemplified by aneuploidy, chromosome aberrations, gene amplification and centrosome amplification, and was accompanied by abnormalities in mitosis, cytokinesis and growth control. Unequal segregation of chromosomes due to multiple spindle poles during mitosis occurred in several Gadd45a –/– cell lineages and may contribute to the aneuploidy. Our results indicate that Gadd45a is one component of the p53 pathway that contributes to the maintenance of genomic stability.

Targeted Genomic Disruption of H-ras and N-ras, Individually or in Combination, Reveals the Dispensability of Both Loci for Mouse Growth and Development

ABSTRACT Mammalian cells harbor three highly homologous and widely expressed members of the ras family (H-ras, N-ras, and K-ras), but it remains unclear whether they play specific or overlapping cellular roles. To gain insight into such functional roles, here we generated and analyzed H-ras null mutant mice, which were then also bred with N-ras knockout animals to ascertain the viability and properties of potential double null mutations in both loci. Mating among heterozygous H-ras+/− mice produced H-ras −/− offspring with a normal Mendelian pattern of inheritance, indicating that the loss of H-rasdid not interfere with embryonic and fetal viability in the uterus. Homozygous mutant H-ras −/− mice reached sexual maturity at the same age as their littermates, and both males and females were fertile. Characterization of lymphocyte subsets in the spleen and thymus showed no significant differences between wild-type and H-ras −/− mice. Analysis of neuronal markers in the brains of knockout and wild-type H-ras mice showed that disruption of this locus did not impair or alter neuronal development. Breeding between our H-ras mutant animals and previously available N-ras null mutants gave rise to viable double knockout (H-ras −/−/N-ras −/−) offspring expressing only K-ras genes which grew normally, were fertile, and did not show any obvious phenotype. Interestingly, however, lower-than-expected numbers of adult, double knockout animals were consistently obtained in Mendelian crosses between heterozygous N-ras/H-ras mice. Our results indicate that, as for N-ras, H-ras gene function is dispensable for normal mouse development, growth, fertility, and neuronal development. Additionally, of the three ras genes, K-ras appears to be not only essential but also sufficient for normal mouse development.

Targeted Disruption of the Microsomal Epoxide Hydrolase Gene MICROSOMAL EPOXIDE HYDROLASE IS REQUIRED FOR THE CARCINOGENIC ACTIVITY OF 7,12-DIMETHYLBENZ[a]ANTHRACENE

Microsomal epoxide hydrolase (mEH) is a conserved enzyme that is known to hydrolyze many drugs and carcinogens, and a few endogenous steroids and bile acids. mEH-null mice were produced and found to be fertile and have no phenotypic abnormalities thus indicating that mEH is not critical for reproduction and physiological homeostasis. mEH has also been implicated in participating in the metabolic activation of polycyclic aromatic hydrocarbon carcinogens. Embryonic fibroblast derived from the mEH-null mice were unable to produce the proximate carcinogenic metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens. They were also resistant to DMBA-mediated toxicity. Using the two-stage initiation-promotion skin cancer bioassay, the mEH-null mice were found to be highly resistant to DMBA-induced carcinogenesis. In a complete carcinogenesis bioassay, the mEH mice were totally resistant to tumorigenesis. These data establish in an intact animal model that mEH is a key genetic determinant in DMBA carcinogenesis through its role in production of the ultimate carcinogenic metabolite of DMBA, the 3,4-diol-1,2-epoxide.

Expression of CYP2A genes in human liver and extrahepatic tissues.

Members of the human cytochrome P450 2A (CYP2A) subfamily are known to metabolize several promutagens, procarcinogens, and pharmaceuticals. In this study, the expression of the three genes found in the human CYP2A gene cluster was investigated in the liver and several extrahepatic tissues by gene-specific reverse transcriptase-polymerase chain reaction (RT-PCR). All three transcripts (CYP2A6, CYP2A7, and CYP2A13) were found to be present in liver. Quantitative RT-PCR analysis showed that CYP2A6 and CYP2A7 mRNAs were present at roughly equal levels in the liver, while CYP2A13 was expressed at very low levels. Two putative splicing variants of CYP2A7 were found in the liver. Nasal mucosa contained a low level of CYP2A6 and a relatively high level of CYP2A13 transcripts. Kidney, duodenum, lung, alveolar macrophages, peripheral lymphocytes, placenta, and uterine endometrium were negative for all transcripts. This survey gives a comprehensive picture of the expression pattern of CYP2A genes in liver and extrahepatic tissues and constitutes a basis for a search for functional CYP2A forms and their roles in chemical toxicity in liver and nasal mucosa.

The Cyp2 a gene subfamily: species differences, regulation, catalytic activities and role in chemical carcinogenesis

The CYP2A subfamily has been characterized in several mammalian species including mouse, rat, rabbit, hamster, cattle and human. Marked species differences have been demonstrated in the catalytic activities and regulation of this subfamily. In humans, the CYP2A genes are found as a cluster on the long arm of chromosome 19 with the CYP2B and CYP2F genes. Marked interindividual differences in expression of the CYP2A6 gene was found in livers analyzed in vitro and in humans phenotyped in vivo by using coumarin, a specific substrate for the enzyme. Efforts are underway to determine the existence of mutant and variant CYP2A6 alleles in the human population. Since CYP2A6 is able to metabolically-activate chemical carcinogens and is expressed in extrahepatic tissue, it will be of interest to determine whether genetic differences in expression of the gene is associated with cancer risk.

Organization and evolution of the cytochrome P450 CYP2A-2B-2F subfamily gene cluster on human chromosome 19

Cytochrome P450 genes from the CYP2A, CYP2B, and CYP2F subfamilies form a tight cluster which we have localized on the detailed physical map of human chromosome 19. The corresponding three gene subfamilies are also clustered in the mouse genome, on the region of chromosome 7 known to be syntenic to human chromosome 19. One hundred eight cosmid clones from the human P450 region were assembled into a single contig of 350 kb, restriction mapped, and probed with cDNAs from the three gene subfamilies. A total of 11 genes were identified in humans, including five from the 2A subfamily, three from the 2B subfamily, and three from the 2F subfamily; at least six of the 11 are pseudogenes. The organization of the genes, with members of the three subfamilies intermixed, indicates that the evolution of this gene cluster has been complex. The modern gene arrangement in humans is probably the result of a series of tandem duplications, plus at least one inverted duplication. The identification of all genes and pseudogenes in this cluster also makes it possible to determine the origins of some previously known variant P450 transcripts.

Proteasome Inhibition Induces Nuclear Translocation and Transcriptional Activation of the Dioxin Receptor in Mouse Embryo Primary Fibroblasts in the Absence of Xenobiotics

ABSTRACT The aryl hydrocarbon receptor (AHR) is a transcription factor that is highly conserved during evolution and shares important structural features with the Drosophila developmental regulatorsSim and Per. Although much is known about the mechanism of AHR activation by xenobiotics, little information is available regarding its activation by endogenous stimuli in the absence of exogenous ligand. In this study, using embryonic primary fibroblasts, we have analyzed the role of proteasome inhibition on AHR transcriptional activation in the absence of xenobiotics. Proteasome inhibition markedly reduced cytosolic AHR without affecting its total cellular content. Cytosolic AHR depletion was the result of receptor translocation into the nuclear compartment, as shown by transient transfection of a green fluorescent protein-tagged AHR and by immunoblot analysis of nuclear extracts. Gel retardation experiments showed that proteasome inhibition induced transcriptionally active AHR-ARNT heterodimers able to bind to a consensus xenobiotic-responsive element. Furthermore, nuclear AHR was transcriptionally active in vivo, as shown by the induction of the endogenous target gene CYP1A2. Synchronized to AHR activation, proteasome inhibition also induced a transient increase in AHR nuclear translocator (ARNT) at the protein and mRNA levels. Since nuclear levels of AHR and ARNT are relevant for AHR transcriptional activation, our data suggest that proteasome inhibition, through a transient increase in ARNT expression, could promote AHR stabilization and accumulation into the nuclear compartment. An elevated content of nuclear AHR could favor AHR-ARNT heterodimers able to bind to xenobiotic-responsive elements and to induce gene transcription in the absence of xenobiotics. Thus, depending on the cellular context, physiologically regulated proteasome activity could participate in the control of endogenous AHR functions.