Approaches to studying toxicological mechanisms in marine mammals Direct exposure? Semi-field studies (feeding studies) Extrapolation Biomarkers of exposure, effect, susceptibility Field associations (chemicals and effects) in vitro studies - tissues and subcellular fractions - cloned, in vitro expressed proteins - tissue/cell culture
Dose-Response shapes of curves; thresholds timing of exposure and effects (acute vs chronic) (algal toxins versus POPs) (exposure and effects separated in time) low-dose extrapolation
Distribution/toxicokinetics hydrophobicity and lipid content protein binding effect of physiological condition (fasting, pregnancy) compartmental analysis physiologically based pharmacokinetic models
Cytochrome P450 (CYP) multiple forms (57 in humans) mostly in endoplasmic reticulum (microsomal) hemoproteins require NADPH and O 2 tissue-, sex-, and stage- specific expression broad substrate specificity (endogenous and xenobiotic) some inducible nomenclature (family-subfamily-gene: e.g. CYP1A1)
Rob Letcher, Univ. of Windsor Reactions - PCB metabolism Differential susceptibility to biotransformation: Preferential loss of 3,4-unsubstituted congeners 2,2’,5,5’-TCB 2,2’,4,5,5’-PCB 2,2’,4’,5,5’,6-HCB 2,2’,3,4,4’,5’-HCB 2,2’,4,4’,5,5’-HCB
Rob Letcher, Univ. of Windsor Reactions - PCB metabolism
Rob Letcher, Univ. of Windsor Reactions - PCB metabolism CYP FMO NAT GST CYP2B MeT -lyase
OH-PCBs Formed by CYP1A and CYP2B Less hydrophobic than parent PCBs Most readily excreted; some persist in blood (m- and p-hydroxy w/ o-Cl) Poor substrates for conjugation (glucuronidation and sulfation) Multiple effects - displace T4 from transthyretin - inhibit sulfotransferase (T4, E2, 3-OH-BaP) - inhibit glucuronosyl transferase (3-OH-BaP) - agonists for estrogen receptors PCB Hydroxy PCB
Brouwer et al 1998 OH-PCBs as inhibitors of T4 transport by transthyretin (TTR)
Methylsulfonyl-PCBs Formed by sequential enzymatic reactions Less hydrophobic than parent PCBs but still persistent Bioaccumulate and persist in tissues (m- and p-MeSO 2 w/ 2,5,(6)-Cl) (liver, lung > fat) - likely role for CYP2B epoxidation as initial step adipose [MeSO 2 -PCB]/[PCB] =.01-.25 (highest in Baltic ringed and grey seal) Protein interactions - uteroglobin (progesterone-binding protein) - glucocorticoid receptor antagonist - estrogen receptor antagonist? Induce CYP2B,C and CYP3A enzymes
Biotransformation in marine mammals What is the capacity for xenobiotic metabolism in MM? Are there species differences in xenobiotic-metabolizing enzymes? - diversity - expression - inducibility - catalytic function (rates and specificity) Direct measurement of metabolites Inferences from contaminant patterns in MM tissues Direct assessment in vitro - immunochemical detection - in vitro catalytic assay (model substrates; correlations; ± inhibitors) - cloning, expression, characterization
Tanabe et al (1988) Capacity and mode of PCB metabolism in marine mammals Biotransformation capacity inferred from patterns of PCB congeners (Dall’s porpoise vs human) o-m unsub (CYP1A) m-p unsub (CYP2B) o-m unsub m-p unsub
Boon et al (1997) Relative ratios (R rel ) vs food for PCB congeners harbor seal common dolphin harbor porpoise otter 0 m,p H 2 o Cl 0 m,p H 1 o Cl (CYP1A) 1 m,p H 2-3 o Cl (CYP2B)
Immunochemical characterization of hepatic microsomal cytochromes P450 in beluga antibody to CYP formsband in beluga hepatic microsomes MAb fish 1A1+ PAb rodent 1A1/2+(1) PAb fish “2B”- PAb rat 2B1- MAb rat 2B1- PAb rabbit 2B4+ PAb dog 2B11+ PAb rat 2E1+ PAb rat 2E1+(2) White, et al. (1994) Catalytic and immunochemical characterization of hepatic microsomal cytochromes P450 in beluga whales (Delphinapterus leucas). Toxicol. Appl. Pharmacol. 126: 45-57.
Letcher, et al (1996) Immunoquantitation and microsomal monooxygenase activities of hepatic cytochromes P4501A and P4502B and chlorinated hydrocarbon contaminant levels in polar bear (Ursus maritimus). Toxicol Appl Pharmacol 137: 127-140.
CYPs in marine mammals Immunochemical evidence and cDNA cloning
White, et al. (1994) Catalytic and immunochemical characterization of hepatic microsomal cytochromes P450 in beluga whales (Delphinapterus leucas). Toxicol. Appl. Pharmacol. 126: 45-57. Catalytic characterization of hepatic microsomal cytochromes P450 in beluga
White et al. (2000) Compar. Biochem Physiol. 126, 267 Rates of PCB metabolism by hepatic microsomes (pmol/min/mg protein)
Fig. 9. (White et al. (2000)) Proposed pathways for the metabolism of 3,3',4,4'-TCB in beluga whale liver microsomes. The thickness of the arrows reflects the significance of an indicated pathway. The 4-hydroxy- 3,3',4',5-TCB reflects a positional shift of a Cl.
R.J. Letcher, et al. (2000). Methylsulfone PCB and DDE metabolites in beluga whale (Delphinapterus leucas) from the St. Lawrence river estuary and western Hudson Bay, Canada. Environ. Toxicol. Chem. 19(5), 1378-1388. StL HB
Molecular mechanisms of toxicity covalent binding to protein or DNA oxidative stress (e.g. via Reactive Oxygen Species) - lipid peroxidation - oxidative DNA damage - oxidative damage to proteins (-SH) enzyme inhibition (e.g. OP pesticides & AChE) interference with ion channels - e.g. saxitoxin, brevetoxin interference with receptor-dependent signaling - membrane bound receptors (neurotransmitter) - intracellular receptors (hormone)
Definitions Receptor (P. Erlich, 1913; J.N. Langley, 1906) A macromolecule with which a hormone, drug, or other chemical interacts to produce a characteristic effect. Two essential features: – chemical recognition – signal transduction Ligand: A chemical that exhibits specific binding to a receptor.
Definitions Specific binding (SB): High-affinity, low capacity binding of ligand to receptor Non-specific binding (NSB): Low-affinity, high capacity binding of ligand to other proteins Agonist: A ligand that binds to a receptor, increasing the proportion of receptors that are in an active form and thereby causing a biological response. Antagonist: A ligand that binds to a receptor without producing a biological response, but rather inhibits the action of an agonist. Partial agonist: An agonist that produces less than the maximal response in a tissue, even when all receptors occupied. Partial agonists have properties both of agonists and of antagonists.
Definitions Potency: The concentration or amount of a chemical required to produce a defined effect. Location along the dose axis of dose-response curve (property of ligand and tissue). Efficacy: The degree to which a ligand can produce a response approaching the maximal response for that tissue (property of ligand and tissue). Affinity: The tenacity with which a ligand binds to its receptor (property of ligand). Intrinsic Efficacy: Biological effectiveness of the ligand when bound to the receptor; e.g. ability to “activate” receptor once bound (property of ligand).
INTRINSIC EFFICACY TISSUE COUPLING AFFINITY K d EFFICACY K E Ligand- Receptor I Ligand- Receptor A Ligand + Receptor I RESPONSE POTENCY EC50 Affinity, Efficacy, and Potency Hestermann et al. 2000
Evidence for role of Ah receptor in effects of dioxins / planar PCBs Genetics inbred strains of mice (responsive and “non-responsive”) Pharmacology Structure-activity relationships for AHR binding and toxicity Cell Biology Mouse hepatoma cell mutants Molecular biology AHR-null mice
Structure-activity relationships The toxic potencies of many halogenated aromatic hydrocarbons are related to their AHR-binding affinities. Data from Safe, S. (1990) CRC Crit. Rev. Toxicol. 21: 51-88.
3D Structure of PCBs: Calculated Dihedral Angle Hans-Joachim Lehmler, Univ. of Iowa
post-AHR mechanisms of dioxin/PCB toxicity induction of CYP1A (metabolism of endogenous compound; release of ROS) altered expression of other target genes (cell proliferation/differentiation) recruitment of AHR away from endogenous function competition for factors required for other signaling pathways (ARNT, coactivators; HIF, SIM) cross-talk with other signaling pathways (estrogen, progesterone)
TCDD toxic equivalency (TEQ) approach using toxic equivalency factors (TEFs) Calculated TEQs versus Bioassay-derived TEQs
TEQ approach: Assumptions compounds act via common mechanism additivity (no synergism, antagonism) no differences in intrinsic efficacy (all full agonists) similar structure-activity relationships for endpoints of concern and endpoints used to generate TEF values similar structure-activity relationships for species of concern and species used to generate TEF values
Receptor-dependent mechanisms of toxicity in marine mammals Species differences in receptor characteristics? - diversity - expression - function (affinity, SAR, target genes)
Differential Sensitivity to Dioxin (2,3,7,8-TCDD) Mammals - laboratory species: 5000-fold variability (lethality) - humans: ? - marine mammals: ? Birds: up to 1000-fold variability among species Reptiles: ? Amphibians - anurans: 1000-fold less sensitive than fish - other amphibians: ? Bony fishes: 40-fold variability among species
Ligand-binding assays High affinity, low capacity binding (Specific Binding) Total [ 3 H]-TCDD Free (loosely bound)Bound (Total) Non-specific binding Specific binding
AHR + [ 3 H]TCDDAHR + [ 3 H]TCDD + TCDF (100x) Fractions Incubate Spin for 2 hours Fractionate Count 30% sucrose 10% sucrose Analysis of AHR specific binding on sucrose density gradients Total bindingNon-specific binding
Equilibrium Dissociation Constants (K d ) for in vitro-expressed AHR proteins *p<0.05 versus human AHR **p<0.01 versus human AHR Beluga express a high-affinity (low K d ) AHR mean K d (n=4) beluga AHR0.43 ± 0.16 nM ** mouse AHR0.68 ± 0.23 nM * human AHR1.63 ± 0.64 nM
In vitro binding affinity vs. In vivo tissue burdens K D for TCDD: 0.43 nM in vitro TCDD-Eqs in liver of St. Lawrence beluga: 0.13 nM (adult male) (Muir et al. 1996 Environ. Pollut.) Result: 23% AHR occupancy (% Maximum response depends on receptor concentration) Jensen & Hahn (2001)
Relative Potencies or Toxic Equivalency Factors (TEFs) for dioxin-like compounds in wildlife Source: van den Berg, et al. (1998) Environ. Health Persp. 106: 775-792. congenerIUPACrodentmarine PCDD/PCDF#mammals 2,3,7,8-TCDD 11 2,3,7,8-TCDF 0.1? non-ortho PCB 3,3’,4,4’,5-PeCB126 0.1? 3,3’,4,4’,5,5’-HCB169 0.01? 3,4,4’,5-TCB81 0.0001? 3,3,’4,4’-TCB77 0.0001? mono-ortho PCB 2,3,3’,4,4’-PeCB105 0.0001? 2,3’4,4’,5-PeCB118 0.0001? 2,3,3’,4,4’,5-HCB156 0.0005? TEF values
IC 50 : One-site competition model (Prism) K I : From IC 50, [ 3 H]TCDD (Cheng and Prusoff) Competitive binding of PCB congeners using in vitro expressed AHRs and [ 3 H]TCDD Jensen & Hahn (2001)
Weight of evidence approach for assessing impact of contaminants on marine mammals Epidemiological and observational studies in wildlife species Comparative mechanistic studies Mechanistic studies in laboratory animals