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Department of Pharmacology & Toxicology | |
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Garold S. YostProfessor of Pharmacology and Toxicology |
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Education and TrainingB.S., 1971, Bethel College M.S., 1974, University of Hawaii Ph.D., 1977, Colorado State University, Organic Chemistry Postdoctoral Fellow 1977-78, University of California San Francisco, Pharmaceutical Chemistry Visiting Lecturer 1979-81, Johns Hopkins University, Pharmacology Research InterestsDr. Yost is internationally recognized as an authority on toxicology in the respiratory tract, with particular expertise on the cytochrome P450-mediated mechanisms of lung injury. Research in the Yost laboratory is largely focused on the elucidation of the chemical, biochemical, and cellular mechanisms of toxicity to lung tissues that are caused by exposure to environmental pollutants. There are a number of chemicals that cause selective damage to lung tissues after inhalation or from ingestion followed by systemic exposure to circulating chemicals. One of the major compounds that we have studied is 3-methylindole (3MI) that is produced in animal and human digestive systems, and is also found in significant quantities in cigarette smoke or several different foods. The toxicity caused by 3MI is specifically targeted to lung tissues and certain cells within the lungs of experimental animals. The specificity for lung damage by 3MI is observed despite the systemic circulation of this toxin to all organs, and much of the original work on the etiology of pneumotoxicity of 3MI was done in cattle where the fermentation of tryptophan produces 3MI. Mechanisms for the selective damage to lung tissues include selective bioactivation of 3MI by cytochrome P450 enzymes that are expressed only in lung cells and poor detoxication of reactive, toxic intermediates in lung cells. We have pursued both of these hypotheses in our work; our observations have led to the conclusion that both are operative, but specific P450-mediated bioactivation of 3MI to multiple reactive intermediates (Figure 1) is the most important mechanism for organ-selective toxicity of 3MI to lungs. We have used stable isotopes of hydrogen and oxygen to study the specific chemical transformations that take place in the bioactivation and detoxication of 3MI in animal and human lung tissues and specific lung cells. 3-Methylindole has proven to be a unique probe for the study of biotransformation enzymes. Much of the recent work in our lab has centered on the cytochrome P450 genes that are selectively expressed in lungs of animals and man. We have cloned, sequenced, mutated and expressed several new P450 cDNAs. These cDNAs have been expressed in E. coli expressions systems to study the structure/function relationships of related genes within several subfamilies of P450 genes. In addition, the cDNAs have been expressed in viable human lung cell lines and the mechanisms of bioactivation of toxicants like 3MI and certain halogenated hydrocarbons have been assessed using these transfected cell lines. We have also determined the mechanisms responsible for lung-selective expression of four important human P450 genes, CYP2F1, CYP3A4, CYP4B1, and CYP2S1. The results of this research have established novel pathways through which human genes are specifically expressed in lung cells through the action of novel transcription factors. Additional new studies address the mechanisms of cytochrome P450 enzymes that produce reactive electrophilic intermediates from drugs by dehydrogenation processes. Many of the reactive intermediates alkylate the active sites of the enzymes that produced them, and they therefore cause a self-catalyzed destruction of the enzymes. This area of research uses recombinant P450 enzymes and site-directed mutants of the enzymes to determine the mechanisms responsible for selective dehydrogenation and/or enzyme inactivation. Molecular models of the active sites of P450 enzymes (Figure 2) are used to determine mechanisms of dehydrogenation. We also have an extensive research initiative into the mechanisms responsible for human lung disease caused by particulate matter in air pollution. We have cloned, expressed, and characterized multiple "irritant receptors" that are expressed on human lung epithelial cells, and are activated by particulates in polluted air, and by capsaicinoids that are present in pepper sprays. This work has provided compelling evidence for the importance of these receptors in human lung disease.
Figure 1. Bioactivation of the Lung Toxicant, 3-Methylindole to Three Putative Reactive Intermediates (shown in boxes).
Figure 2. Molecular Model of CYP3A4 with Docked TNFα Inhibitor
Selected Publications1. B.A. Carr, S. Ramakanth, G.A. Dannan, and G.S. Yost, Characterization of CYP4B2, Specific Catalyst of Methyl Oxidation of 3-Methylindole, Mol. Pharm. 63, 1137-1147 (2003). 2. B.A. Carr, J. Wan, R.N. Hines and G.S. Yost, Characterization of the human lung CYP2F1 gene and identification of a novel lung-specific binding motif, J. Biol. Chem. 278, 15473-15483 (2003). 3. C.A. Reilly and G.S. Yost, Structural and Enzymatic Parameters that Determine Alkyl Dehydrogenation/Hydroxylation of Capsaicinoids by P450 Enzymes, Drug Metab. Dispos. 33, 530-536 (2005). 4. J. Wan, B.A. Carr, R.N. Hines, and G.S. Yost, Sp1 and Sp3 Regulate Basal Transcription of the Human CYP2F1 Gene, Drug Metab. Dispos. 33, 1244-53 (2005). 5. K. Kassahun, K. Skordos, I. McIntosh, D. Slaughter, G.A. Doss, T.A. Baillie, and G.S. Yost, Zafirlukast Metabolism by Cytochrome P450 3A4 Produces an Electrophilic α,β-Unsaturated Iminium Species That Results in the Selective Mechanism-Based Inactivation of the Enzyme, Chem. Res. Toxicol. 18, 1427-1437 (2005). 6. M.E. Johansen, C. A. Reilly, and G.S. Yost, TRPV1 Antagonists Elevate Cell Surface Populations and Receptor Function to Exacerbate TRPV1-Mediated Toxicities in Human Lung Epithelial Cells. Toxicol. Sci. 89, 278-86 (2006). 7. J.M. Veranth, T.A. Moss, J.C. Chow, R. Labban, W.K. Nichols, J.C. Walton, J.G. Watson, and G.S. Yost, Correlation of In Vitro Cytokine Responses with the Chemical Composition of Particulate Matter Samples, Environ. Health Perspect. 114, 341-349 (2006). 8. C.A. Reilly and G.S. Yost, Metabolism of capsaicinoids by P450 enzymes: a review of recent findings on reaction mechanisms, bio-activation, and detoxification processes, Drug Metab. Rev. 38, 685-706 (2006). 9. Z. Yan, L.M. Easterwood, N. Maher, R. Torres, N. Huebert, and G.S. Yost, Metabolism and Bioactivation of 3-Methylindole by Human Liver Microsomes, Chem. Res. Toxicol. 20, 140-148 (2007). 10. K.C. Thomas, A.S. Sabnis, M.E. Johansen, D.L. Lanza, P.J. Moos, G.S. Yost, and C.A. Reilly, TRPV1 Agonists Cause Endoplasmic Reticulum Stress and Cell Death in Human Lung Cells, J. Pharmacol. Exp. Ther. 321,830-838 (2007). 11. J.S. Biggs, J. Wan, N.S. Cutler, J. Hakkola, P. Uusimäki, H. Raunio, and G.S. Yost, Transcription Factor Binding to a Putative Double E-Box Motif Represses CYP3A4 Expression in Human Lung Cells, Mol. Pharmacol. in press (2007). 12. H. Sun, W.J. Ehlhardt, P. Kulanthaivel, D.L. Lanza, C.A. Reilly, and G.S. Yost, Dehydrogenation of Indoline by Cytochrome P450 Enzymes: a Novel "Aromatase" Process, J. Pharmacol. Exp. Ther., 322, 843-851 (2007). 13. H. Sun and G.S. Yost, Metabolic Activation of a Novel 3-Substituted Indole-Containing TNF-α Inhibitor: Dehydrogenation and Inactivation of CYP3A4, Chem. Res. Toxicol., in press (2008). 14. J.S. Kartha and G.S. Yost, Mechanism-Based Inactivation of Lung-Selective Cytochrome P450 CYP2F Enzymes, Drug Metab. Dispos., in press (2008).
Honors and AppointmentsAssociate Editor - Drug Metabolism and Disposition (2002-present) Chair - National Academy of Sciences Subcommittee on Spacecraft Water Exposure Guidelines (2002-present). Member - Environmental Health Sciences Review Committee, NIEHS, NIH (2002-2006) Secretary - International Society for the Study of Xenobiotics - elected position (2002-2006)
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