Our Science – Gonzalez Website

Frank J. Gonzalez, Ph.D.

Laboratory of Metabolism Head, Nucleic Acids Section Laboratory Chief Building 37, Room 3106 NCI-Bethesda Bethesda, MD 20892 Phone:   301-496-9067 Fax:   301-496-8419 E-Mail:   fjgonz@helix.nih.gov

Biography

Dr. Gonzalez received his Ph.D. from the University of Wisconsin in Madison and was a staff fellow at the National Institute of Child Health and Human Development prior to joining the NCI. A member of the Senior Biomedical Research Service, he received the Rawls Palmer Progress in Medicine Award from the American Society for Clinical Pharmacology, the John J. Abel Award and the Bernard B. Brodie Award in Drug Metabolism from the American Society of Pharmacology and Experimental Therapeutics, the George Scott Award from the Toxicology Forum, the North American Achievement Award from the International Society for the Study of Xenobiotics and an honorary D.Sc. from Mahidol University, Thailand. He is a six-time recipient of the Federal Technology Transfer Award. He has been ranked in the top 5 of the Thompson ISI citation index in the field of Pharmacology & Toxicology for the past several years. He is the co-inventor of 14 patents. His studies are largely focused on the role of cytochromes P450 and xenobiotic receptors in drug metabolism and chemical carcinogenesis.

Research

In mammals, a large number of enzymes exist that metabolize drugs and other xenobiotics. Cytochromes P450 (P450) are among the most important of these enzymes involved in metabolism of most therapeutically-used drugs, toxicants and carcinogens. P450s catalyze the inactivation or metabolic-activation of chemical carcinogens. The P450s involved in xenobiotic metabolism are found in the CYP1, CYP2 and CYP3 families. Each of these families consists of two or more subfamilies. The fact that P450s can metabolically-activate toxicants and procarcinogens in vitro implies that they are involved in toxicity and cancer. Studies on the P450-null mice (Cyp1a2-, Cyp1b1- and Cyp2e1-null mice) developed in the NAS demonstrated, in an intact animal model, that P450s mediate the toxicity and carcinogenicity of chemicals and thus have a role in cancer susceptibility. Prior to studies with the null mice, the only experiments suggestive of a role for P450s in cancer etiology were indirect chemically-induced transformation assays in cell culture. No direct genetic evidence had been available to establish that P450s are necessary for carcinogenesis in an intact animal model system. Mice lacking expression of other carcinogen metabolizing enzymes including microsomal epoxide hydrolase (mEH) and the quinone oxidoreductase (NQO1) were also made; these enzymes have a role in metabolizing metabolites produced by the P450 enzymes and studies by the NAS and other laboratories have also established that these enzymes mediate chemical carcinogenesis. The null mice have firmly established that P450s and other xenobiotic-metabolizing enzymes affect not only the sensitivity to chemical carcinogens but also chemical toxicity. For example, CYP1B1-null mice are resistant to the carcinogenic effects of polycyclic aromatic hydrocarbons in skin, mammary and other tissue target sites. Mice lacking mEH are also sensitive to this class of carcinogens thus indicating that the diol-epoxide route of carcinogen activation, catalyzed by CYP1B1and mEH, is the most important or carcinogenesis in vivo. Mice lacking CYP2E1 are resistant to the colon carcinogen azoxymethane; interestingly CYP2E1 is primarily expressed in the liver indicating the importance of transport of metabolites, or pproxiamte carcinogens, between tissues and the potential organotropism of carcinogens. A large number of studies using the Cyp2e1-null mice have revealed that this P450 is required for the hepatotoxicity numbersou low molecular weight chemicals including acetaminophen, carbon tetrachloride, chloroform, acrylonitrile (converted to cyanide) and the hematotoxicity of benzene. In order to produce mouse models that can be used to more accurately predict human drug and carcinogen metabolism, P450-humanized mice are being prepared using bacterial artificial chromosomes and P1 phage artificial chromosomes. In some cases, transgenic mice carrying the human genes are bred with P450 null-mice to produce humanized mice. Mice expressing human CYP1A1, CYP1A2, CYP2E1, CYP2D6, CYP3A4, and CYP3A7 have been generated and characterized. These new mouse lines have been of great value in the study of chemical carcinogenesis. For example, a recent study with CYP1A2-humanized mice found that human CYP1A2 produces more of the N2-hydroxy metabolite of the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) than does the corresponding mouse P450. The P450 humanized mice have also revealed a potential physiological function of human P450s. For example, studies with the CYP3A4-humanized (hCYP3A4) mouse revealed that CYP3A4, under certain circumstances, can alter the serum levels of estrogen. These mice were also used to establish the mechanisms of potential gender differences in CYP3A4 expression (higher in female and male) that could account for gender differences in drug metabolism and response. Future studies will continue in the development of P450-humanized mouse models and to use these models to study the endogenous function of P450s and the roles of these enzymes in drug and carcinogen metabolism, and in the process of chemical carcinogenesis.

Xenobiotic receptors mediate the response of organisms to their chemical environment. The general paradigm is that chemicals enter the cell and bind receptors leading to the activation of genes that encode enzymes that metabolize the chemical. This serves is a means to eliminate chemicals from the body. However in many cases, stimulation of xenobiotic receptors can lead to toxic and carcinogenic responses as a result of abnormal target gene activation. Several xenobiotic receptors are under study. These include (1) the aryl hydrocarbon receptor (AHR) and its heterodimerization partner ARNT that mediate the toxic response to dioxins and polyhalogenated hydrocarbons, (2) the peroxisome proliferator activated receptors (PPAR) alpha, beta and gamma that are mainly involved in control of fatty acid metabolism, transport, and storage and 3) the farnesoid X receptor (FXR) that is responsible for control of bile acid metabolism and transport. The function of these receptors is being evaluated using gene knockout and humanized transgenic mice. As with the P450 program, we will work with the null and conditional null mouse models and with new humanized models, in particular the PPARalpha-humanized (hPPARalpha) mice. The PPARalpha-null mice were used to demonstrate that peroxisome proliferators-induced liver cancer is dependent on PPARalpha. The hPPARalpha mice demonstrated that the species differences in response to peroxisome proliferators are due to the PPARalpha protein. PPARbeta-null and PPARgamma-conditional null mice were used to demonstrate that PPARbeta and PPARgamma can act as tumor suppressors in a variety of experimentally-induced cancer models. Future studies aim to determine the mechanism of protection of PPARbeta and PPARgamma on colon carcinogenesis and the mechanism of hepatocarcinogenesis of PPARalpha activation. The Arnt conditional mull mice will be used to determine the role of Ahr in mammalian development and xenobiotic signaling in the gut and other tissues and the influence of Hif1alpha on tumorigenesis.

During the last year, metabolomics has been inserted into our analytical arsenal as a new method to characterize the P450 and nuclear receptor null and humanized mice. This high power methodology will be used in an effort to find chemical biomarkers for the expression of individual P450s, for nuclear receptor activation, and for the study of chemical toxicity and carcinogenicity of drugs and other chemicals Historically, metabolomics has been carried out largely by use of 1H-NMR. However, recently a new more power LC-MS technology has been developed. This technology not only offers promise in the translational biomarker research but can also be used to understand mechanisms of toxicity and carcinogenicity.

This page was last updated on 6/11/2008.