Lecture 5 - Basic Mechanisms of Toxicants 3

what is oxidative stress?

imbalance of cellular oxidants and antioxidants in favor of oxidants

what causes direct generation of ROS/RNS?

a) Xenobiotic bioactivation (carbon tetrachloride, benzene)

b) Redox cycling (paraquat, MPP+)

c) Transition metals (Fe2+, Cu2+)

d) inhibition of mitochondrial electron transport (many phytochemicals)

what things cause indirect generation of ROS/RNS?

increased Ca2+

what is the result of increased Ca2+ that causes ROS/RNS indirectly?

a) activates dehydrogenases in citric acid cycle and increases electron output (NADH and FADH2), leads to increase in O2- (superoxide) by the electron transport chain

b) Ca2+-activated proteases convert xanthine dehydrogenase to xanthine oxidase, the by-products of which are O2- and H2O2

c) Neurons and endothelial cells constitutively express NOS that is activated by Ca2+ increase NO production which reacts with O2- to produce highly reactive ONOO- (peroxynitrite)

what are examples of reactive species that are created when you have superoxide?

a. initially - peroxynitrite (ONOO-) and hydrogen peroxide (HOOH)

b. then peroxynitrite picks up CO2 but then divides into nitrogen dioxide (NO2) and Carbonate anion radical (CO2-)

c. hydrogen peroxide goes through fenton reaction then divides into hydroxyl radical (HO)

what are consequences of ROS/RNS?

a. can directly oxidize and affect protein function and can mutate DNA leading to cellular dysfunction

b. oxidatively inactivate Ca2+/ATPases and elevate Ca2+

c. drain ATP reserves

d. ONOO- induces DNA single-strand breaks, which activate poly(ADP-ribose) polymerase (PARP)

e. lipid peroxidation, cell swelling, and cell rupture

when ROS/RNS drain ATP reserves what happens?

a. NO is reversible inhibitor of cytochrome oxidase

b. ONOO- irreversibly inactivates complexes I/II/III and aconitase

c. ROS can disrupt mitochondrial membranes and dissipate the electrochemical gradient needed for ATP synthase

what does PARP transfer?

PARP transfers ADP-ribose units from NAD+ to PARP; consumption of NAD+ compromises ATP synthesis

what are the steps of Lipid Peroxidation?

1. Free radicals can initiate peroxidative degradation of lipids by hydrogen abstraction from fatty acids

2. the lipid radical (L) formed is converted to the lipid peroxyl radical (LOO) by oxygen fixation

3. lipid hydroperoxide (LOOH) is then formed by hydrogen abstraction from another lipid

what are mutagens?

cause changes to cell DNA that are passed on when cell divides

what are carcinogen?

if mutagen produces neoplastic cell

what are two major classes of gene involved in carcinogenesis?

• proto-oncogenes

• tumor-suppressor genes

what are proto-oncogenes?

promote cell cycle progression (most active when growing)

what is an example of proto-oncogenes?

constitutive activity of growth factor tyrosine-kinase receptors can cause neoplastic transformation

what are tumor-suppressor genes?

inhibit cell cycle progression

what is an example of tumor-suppressor genes?

mutations in tumor suppression gene product p53

what are the overall steps of carcinogenesis?

Cause DNA damage → if repaired cell survives, if not cell death → if survives goes through DNA replication → repairs gets to survive, if not dies → Mutation → sometimes silent sometime deadly → then moves on and you get activation of oncogenic proteins or inactivation of tumor suppression proteins → neoplastic cell transformation → clonal expansion → tumor

what does toxicant cause of proto-oncogene?

mutation to oncogene then transcription/translates to oncogene protein (permanently activated) which causes mitosis and then uncontrolled proliferation

what does toxicant cause of tumor suppressor gene?

mutation of tumor suppressor gene to mutant tumor suppressor gene → transcription/translation → causes the mutant tumor suppressor protein to be inactive

what is teratogenesis?

the creation of birth defects during fetal development

what are teratogens?

substances that induce birth defects (interferes with development)

what happens to embryo if something interferes at the embryonic development stage?

most likely death b/c you are often affecting develop stages and processes

what happens to fetus if something interferes at the fetal development stage?

if early in fetal development limits chance of survival but later in fetal development probably lives because it has developed more

what do Lupines cause?

Teratogen that causes arthrogryposis or twisted legs; bloom in years of relatively high moisture on the prairies (“crooked calf disease”)

what are examples of Teratogenic alkaloids?

• Conium maculatum (hemlock or poison hemlock)

• Nicotiana glauca (species of wild tobacco known by common name tree tobacco) - cattle, sheep, goats

• Nicotiana tabacum or cultivated tobacco

• Veratrum californicum (cyclopamine)

what does Veratrum californicum cause?

inhibits the action of the hedgehog signaling pathway, which is involved in the formation of the neural system (now investigated as anti-cancer potential therapy)

what are the two forms of the Thalidomide compound?

• R-enantiomer = Sedative

• S-enantiomer = teratogen

what are the manifestions of structural neurotoxicity?

• neuronopathies

• axonopathies

• myelinopathies

manifestations of functional neurotoxicity?

neurotransmission-associated abnormalities

what do neuronopathies cause?

• injury or death to neurons, targets cell bodies

• irreversible loss

• initial injury followed by apoptosis or necrosis

what is the primary site of toxicity for axonopathies?

the axon

what happens with axonopathies?

• degeneration of axon, also known as Wallerian degeneration

  • loss of axon distal to lesion

  • loss of surrounding myelin, and cell body remains intact

  • chromatolysis and margination of Nissl substance

where are axonopathies reversible and irreversible?

irreversible in CNS and reversible in PNS

what do myelinopathies affect?

schwann cells and myelin

what happens as a result of myelinopathies?

• intramyelinic edema

• demyelination by affecting myelin or myelin-producing cells

• remyelination in CNS occurs to a limited extent - oligodendrocytes

• remyelination in PNS done by Schwann cells

what is the result of neurotransmission-associated abnormalities?

• interruption of impulse transmission

• blockade of trans-synaptic communication

• inhibition of neurotransmitter uptake

• interference with second-messenger systems