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TOXC 207 PowerPoint Presentation

TOXC 207

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TOXC 207

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    1. TOXC 207 / PHCO 207 / ENVR 231 Advanced Toxicology Biochemistry of Liver Injury Christopher Black, Ph.D. for Edward L. LeCluyse, Ph.D. 919-545-9959x306

    2. Effect of Toxic Chemicals on the Liver The liver is the most common site of damage in laboratory animals administered drugs and other chemicals. There are many reasons including the fact that the liver is the first major organ to be exposed to ingested chemicals due to its portal blood supply. Although chemicals are delivered to the liver to be metabolized and excreted, this can frequently lead to activation and liver injury. Study of the liver has been and continues to be important in understanding fundamental molecular mechanisms of toxicity as well as in assessment of risks to humans.

    3. Zonation of Liver Microstructure

    4. Zonal Expression of P450’s

    5. Chemical-induced Hepatotoxicity Hepatotoxic response depends on concentration of toxicant delivered to hepatocytes in the liver acinus Hepatotoxicity a function of: Blood concentration of (pro)toxicant Blood flow in Biotransformation (to more or less toxic species) Blood flow out Most hepatotoxicants produce characteristic patterns of cytolethality across the acinus

    6. Types of Liver Injury or Responses Cell Death (necrosis, apoptosis) Cholestasis (disrupted transport function) Steatosis, Phospholipidosis Oxidative stress Mitochondrial dysfunction Modulation of CYP activities (inhibition, induction) Fibrosis/Cirrhosis Hepatitis

    7. Most Hepatotoxic Chemicals Cause Necrosis Result of loss of cellular volume homeostasis Affects tracts of contiguous cells Plasma membrane blebs Increased plasma membrane permeability Organelle swelling Vesicular endoplasmic reticulum Inflammation usually present

    8. Necrosis Damage occurs in different parts of the liver lobule depending on oxygen tension or levels of particular drug metabolizing enzymes. Allyl alcohol causes periportal necrosis because the enzymes metabolizing it are located there. CH2=CHCH2OH CH2 =CHCHO Carbon tetrachloride causes centrilobular necrosis - endothelial and Kupffer cells adjacent to hepatocytes may be normal - with diethylnitrosamine, endothelial cells are also killed. Due to activation by higher concentrations of cytochrome P450 in zone 3.

    10. Chemical Exposure Can Also Lead to Apoptosis Defined primarily by morphological criteria: Condensation of chromatin Gene expression, protein synthesis Ca++-dependent endonuclease activation Cleavage to oligonucleosomes Cytoplasmic organelle condensation Phagocytosis Inflammation absent Death-receptor (TNF-R1, Fas) or mitochondrial pathways Unlike necrotic cells, apoptotic cells show no evidence of increased plasma membrane permeability

    11. Chemical-induced Hepatocyte Apoptosis

    12. Apoptosis Mechanism

    13. Fate of Injured Cells

    14. LIPIDOSIS Many chemicals cause a fatty liver. Sometimes associated with necrosis but often not. Not really understood but essentially is due to an imbalance between uptake of fatty acids and their secretion as VLDL. Carbon tetrachloride can cause lipidosis by interfering in apolipoprotein synthesis as well as oxidation of fatty acids. Other chemicals can cause lipidosis by interfering with export via the Golgi apparatus. Ethanol can induce increased production of fatty acids.

    15. Consequences of Toxic Mechanisms Disruption of intracellular calcium Cell lysis Disruption of actin filaments Cholestasis Generation of high-energy reactions Covalent binding and adduct formation Adduct-induced immune response Cytolytic T cells and cytokines Activation of apoptotic pathways Programmed cell death with loss of nuclear chromatin Disruption of mitochondrial function Decreased ATP production Increased lactate and reactive oxygen/nitrogen species (ROS, RNS) Peroxidation of Membrane Lipids Blebbing of plasma membrane

    16. Mechanisms of Chemical-induced Toxicity Direct effects Toxicants can have direct surfactant effects upon plasma membranes Chlorpromazine and phenothiozines, erythromycin salts, chenodeoxycholate Effects on the cytoskeleton, resulting in plasma membrane permeability changes Phalloidin, taxol Effects upon mitochondrial membranes and enzymes Cadmium, butylated hydroxyanisole, butylated hydroxytoluene, inhibitors and uncouplers of electron transport

    17. Mechanisms of Chemical-induced Toxicity Alteration in the intracellular prooxidant-antioxidant ratio Redox cycling of toxicant (e.g., quinone) produces oxygen radicals, depletes GSH Hydroperoxides and metal ions (Fe, Cu) can produce oxidative stress and deplete GSH Lipid peroxidation, protein sulfhydryl oxidation, disruption of Ca++ homeostasis

    18. Redox Cycling and Formation of Oxygen Radicals

    19. Critical Role of Glutathione Glutathione is the major cellular nucleophile, detoxication pathway for most electrophilic chemicals Glutathione depletion generally makes cells more susceptible to electrophilic cellular toxicants, ‘threshold’ effect Glutathione depletion induced by alkylating agents , oxidative stress, substrates, biosynthetically with buthionine sulfoximine Glutathione can be increased by precursors, such as N-acetylcysteine, which is used as an antidote for toxicity

    20. Mechanisms of Chemical-induced Toxicity Disruption of Calcium Homeostasis Calcium regulates a wide variety of physiological processes Ca++ accumulation in necrotic tissue, association with ischemic and chemical toxicity Ca++ homeostasis in the cell very precisely regulated Impairment of homeostasis can lead to Ca++ influx, release, or extrusion

    21. Chemical Disruption of Ca++ Homeostasis Release from mitochondria Uncouplers, quinones, hydroperoxides, MPTP, Fe+2, Cd+2 Release from endoplasmic reticulum CCl4, bromobenzene, quinones hydroperoxides, aldehydes Influx through plasma membrane CCl4, CHCl3, dimethylnitrosamine, acetaminophen, TCDD Inhibition of efflux from the cell Cystamine, quinones, hydroperoxides, diquat, MPTP, vanadate

    22. Consequences of Disruption of Ca++ Homeostasis Alterations in the cytoskeleton Plasma membrane blebbing Ca++ regulation of polymerization Ca++-activated protease activity Alterations in plasma membrane channels Activation of phospholipases Ca++- and calmodulin-dependent Increased membrane permeability Stimulation of arachidonate metabolism

    23. Consequences of Disruption of Ca++ Homeostasis Activation of proteases Calpain: Ca++-activated, non-lysosomal Degradation of cytoskeletal and membrane proteins Activation of endonucleases DNA fragmentation, cell death Acetaminophen, SDS, uncouplers Possible mechanism of mutation induction by cytotoxic agents

    24. Mechanisms of Chemical-induced Toxicity Reactive Metabolite Formation Many compounds are metabolically activated to chemically reactive toxic species Aflatoxin, carbon tetrachloride, acetaminophen, bromobenzene, nitrosamines, pyrrolizidine alkaloids Chemically reactive metabolites (electrophiles) can covalently bind to crucial cellular macromolecules (nucleophiles) Glutathione (GSH) is the prevalent cellular nucleophile, which acts as a protective agent

    25. Covalent Binding Theory of Chemical Toxicity Metabolism of chemical to reactive metabolite Covalent binding of reactive metabolite to critical cellular nucleophiles (protein SH, NH, OH groups) Inactivation of critical cell function (e.g., ion homeostasis) Cell death

    26. Immune-mediated Hepatotoxicity

    27. Cytochromes P450 Prevalent heme-containing proteins of liver Localized in the smooth endoplasmic reticulum Many different forms with overlapping substrate specificity Biosynthesis induced by treatment with a variety of xenobiotics Induction can reduce or exacerbate hepatotoxicity

    29. Biotransformation of Toxicants: Phase II Reactions ‘Synthetic’ reactions, conjugation with hydrophilic groups Glucuronic acid, sulfate, glutathione, amino acids Generally considered detoxication, water-soluble product Can be metabolically activated to an unstable reactive product

    30. Acetaminophen Metabolism and Toxicity

    31. Acetaminophen Protein Adducts

    33. Induction of Biotransformation Reactions Two major categories of CYP inducers Phenobarbital is prototype of one group - enhances metabolism of wide variety of substrates by causing proliferation of SER and CYP in liver cells. Polycylic aromatic hydrocarbons are second type of inducer (ex: benzo[a]pyrene). Induction appears to be an environmental adaptive response to chemical insult Receptors (AhR, PXR, CAR, PPAR) are regulators of genes involved in hepatic biotransformation reactions

    34. Nuclear Receptors Involved in P450 Enzyme Induction

    35. Consequences of Cytochrome P450 Enzyme Induction Increased toxic effect Acetaminophen Alcohol, 3-MC Bromobenzene, CCl4 Phenobarbital Increased bioactivation Cyclophosphamide Macrolides, pesticides Increased tumor formation Altered disposition of endogenous substrates Altered cell function proliferation of peroxisomes and SER increased liver weight Porphyria, chloracne PCDDs, azobenzenes, biphenyls (PCBs), naphthalene

    36. CYP 1A1 biotransformation PAHs from incomplete combustion undergo oxygenation to generate arene oxides

    37. DNA adduct formation Reactive electrophiles bind covalently to DNA

    39. Efflux pumps in hepatocytes Transporters and Xenobiotic Elimination

    40. Ultrastructure of Bile Canaliculi in Hepatocytes

    41. Potential Mechanisms for Cholestasis

    42. Chemo-sensitization via Transporter Inhibition

    44. Other Agents Causing Cholestasis in Animals Lithocholic acid – action can be reversed by cholic acid suggesting a competition for transport proteins Ouabain – blocks Na+/K+ pump Phalloidin and Cytochalasin B – Both affect actin microfilaments - possibly disrupting the actin corset around the bile canaliculus Cyclosporin A – Causes symptoms of jaundice with no changes in the liver. Probably affects bile acid metabolism

    45. Summary Biochemical mechanisms of hepatoxicity are complex Some ‘classic’ cytotoxicity mechanisms and pathways Some unique mechanisms and pathways The observance of hepatoxicity is often a fine balance between multiple factors Toxicokinetic Environmental Physiological

    46. Suggested Reading Jaeschke H, Gores GJ, Cederbaum AI, Hinson JA, Pessayre D, Lemasters JJ. Mechanisms of hepatotoxicity. Toxicol Sci. 65(2):166-76, 2002. Klaassen CD, ed., Casarett and Doull’s Toxicology. The Basic Science of Poisons. 6th edition , McGraw Hill, New York, 2001. Kim JS, He L, Qian T, Lemasters JJ. Role of the mitochondrial permeability transition in apoptotic and necrotic death after ischemia/reperfusion injury to hepatocytes. Curr Mol Med. 3(6):527-35, 2003. Puga A, Xia Y and Elferink C. Role of AhR in cell cycle regulation. Chem-Biol Interact 141:117-30, 2002. Hestermann EV, Stegeman JJ and Hahn ME. Relative contributions of affinity and intrinsic efficacy to AhR ligand potency. Toxicol App Pharmacol 168: 160-72, 2000.