Step 3: Cellular Dysfunction and Resultant Toxicities
Impaired function of integrated systems
- Hemostasis Bleeding
Impaired Internal maintenance
Impairment of Internal cellular Maintenance: Mechanisms of Toxic Cell Death
ATP plays a central role in cellular maintenance both as a chemical for biosynthesis and as the major source of energy.
ATP is utilized in numerous biosynthetic reactions, and is incorporated into cofactors as well as nucleic acids. It is required for muscle contraction and polymerization of the cytoskeleton, fueling cellular motility, cell division, vesicular support and the maintenance of cell morphology.
Chemicals that inhibit ADP phosphorylation acting on:
1. ATP synthase: oligomycin, cyhexatin, DDT, chlordecone
2. Adenine nucleotide translocator: atractyloside, DDT free fatty acids, lysophospholids
3. Phosphate transporter: N-ethylmaleimide, mersalyl, p-benzoquinone
4. Chemicals dissipating the mitochondrial membrane potential
a.) Cationophores: pentachlorophenol, benzonitrile, salicylate, amiodarone, gramicidin, valinomycin, calcimycin
b.) Chemicals permeabilizing the mitochondrial inner membrane: PCBD-cys, chlordecone
Chemicals causing mitochondrial DNA damage thereby impairing synthesis of specific proteins encoded by the mitochondrial genome are:
1. Antiviral drugs: zidovudine, zalcitabine, didanosine, fialuridine
2. Chloramphenicol (when overdosed)
3. Ethanol (When chronically consumed)
1. Depletion of cellular ATP reserves deprives the endoplasmic and plasma membrane Ca2+ pumps of fuel, causing elevation of Ca2+ in the cytoplasm
2. Intracellular hypercalcemia facilitates formation of ROS and RNS, which oxidatively inactivates the Ca2+ pump aggravating the hypercalcemia
3. ROS and RNS can also drain the ATP reserves. NO is a reversible inhibitor of cytochrome oxidase. NO+ inactivates glyceraldehyde 3-phosphate dehydrogenase and impairs glycolysis, whereas ONOO- irreversibly inactivates several components of the electron transport chain, inhibiting cellular ATP synthesis.
4. Furthermore, ONOO can induce DNA single strand breaks, which activate poly(ADP-ribose) polymerase (PARP)
MPT causes depletion of cellular ATP, and culminating in cell lysis or necrosis
Reaction of toxicants with a target molecule may result in impaired cellular function as the third step in the development of toxicity.
ATP availability Determines the form of cell death
Role of the Target molecule
Many xenobiotics can cause both apoptosis and necrosis. Toxicants tend to induce apoptosis at low exposure levels or early after exposure at high levels, whereas they cause necrosis later at high exposure levels.
Recent findings suggest that the availability of ATP is critical in determining the form of cell death.
Each cell in a multicellular organism carries out defined programs, some of which determine whether cells undergo vision, differentiantiation, or apoptosis.
For survival, all cells must synthesize endogenous molecules, assemble macromolecular complexes, membranes, and cell organelles, maintain the intracellular environment, and produce energy for operation.
Agents that disrupt these functions jeopordize survival. There are three critical biochemical disorder that chemicals inflicting cell death initiate, namely..
Mitochondrial Permeability Transition (MPT) and the worst outcome: Necrosis
Mitochondrial Ca2+ uptake, decreased mitochondrial membrane potential, generation of ROS and RNS, depletion of ATP and consequences of the primary metabolic disorders are all considered as causative factors of an abrupt increase in the mitochondrial inner-membrane permeability, termed MPT.
Cell regulation
(Signaling)
Cell maintenance
On binding together with ATP to an adapter protein, cyt c can induce proteolytic cleavage of proteins called caspases or cysteine proteases that cleave cytoplasmic proteins into fragments.
Some caspases activate procaspases
The decisive mitochondrial events of cell death are controlled by the Bcl-2 family proteins ,which includes members that facilitate and those that inhibit these processes.
EFFECTS:
An Altenative Outcome of MPT: Apoptosis
EFFECTS
Nuclear and cytoplasmic materials condense, then it breaks into membrane-bound fragments that are phagocytosed.
The routes to apoptosis are ordered, involving cascade-like activation of catabolic processes that finally disassembled the cell.
Chemicals that adversely affect cellular energy metabolism, calcium homeostasis, and redox state and ultimately cause necrosis may also induced apoptosis
While the necrotic cell swells and lyses, the apoptotic cells shrinks
ATP Depletion
sustained rise in intracellular Ca2+
Overproduction of ROS (reactive oxygen species) and RNS (reactive Nitrogen species)
A number of xenobiotics can directly generate ROS and RNS, such as the redox cyclers and transition metals. Overproduction of ROS and RNS can be secondary to intracellular hypercalcemia, as Ca2+ helps generate ROS and RNS by activating dehydrogenases in the citric acid cycle leading to increased activity in the electron transport chain and increased formation of O2 and HOOH and by activating nitric oxide synthase which leads to formation of ONOO.
Impaired external maintenance
Dysregulation of gene expression
Dysregulation of ongoing cell function
Interplay between the primary metabolic Disorders
Spells Cellular Disaster
Impaired
- ATP synthesis
- Ca2+ regulation
- Protein synthesis
- Microtubular function
- Membrane function
Cell Injury/Death
Inappropriate
-Cell division neoplasia, teratogenesis
- Apoptosis tissue involution, teratogenesis
- Protein synthesis peroxisome proliferation
E.g., Inappropriate neuromuscular activity
- Tremor, convulsion, spasm, cardiac arrythmia
- Narcosis. paralysis, paresthesia
For regulation of these cellular programs, cells possess signaling networks that can be activated and inactivated by external signaling molecules.
Toxicant-induced Cellular Dysregulation
OVERPRODUCTION OF ROS AND RNS
A number of xenobiotics can directly generate ROS and RNS, such as the redox cyclers and transmition metals
O2
NO
O2
ONOO-
HOOH
O2
CO2
2H+
Fe (II), Cu(I), Cr(V), Ni(II)
Fenton reaction
Fe(III), Cu(II), Mn(III), Cr(VI), Ni(III)
[HOOH]-
ONOOCO2-
NO2
HO
CO3
OH
Cells are regulated by signaling molecules that activate specific cellular receptors linked to signal-transducing networks that transmit the signals to the regulatory regions of genes and/or functional proteins.
Three Mechanisms by which sustained elevations in intracellular Ca2+ levels influence the cellular energy balance.
3rd – high Ca2+ level may lead to activation of hydrolytic enzymes that degrade proteins, phospholipids and nucleic acids.
Dysregulation of Gene expression
may occur at elements that are directly responsible for transcription, at components of the intracellular signal-transduction pathway, and at the synthesis, storage, or release of the extracellular signaling molecules
Three Mechanisms by which sustained elevations in intracellular Ca2+ levels influence the cellular energy balance.
2nd – an uncontrolled rise in cytoplasmic Ca2+ causes cell injury by microfilamental dissociation.
Dysregulation of Transcription
Dysregulation of signal Transduction
Transcription of genetic information from DNA to mRNA is controlled largely by interplay between transcription factors (TFs) and the regulatory or promoter region of genes. Xenobiotics may interact with the promoter region of the gene, the TFs, or other components of the transcription initiation comples.
Extracellular signaling molecules, such as growth factors, cytokines, hormones, and neurotransmitters can ultimately activate TFs utilizing cell surface receptors and intracellular signal transducing networks.
Sustained Rise of Intracellular Ca2+
Intracellular Ca2+ levels are highly regulated and maintained by the impermeability of the plasma membrane to Ca2+ and by transport mechanisms that remove Ca2+ from the cytoplasm. Ca2+ is actively pumped from the cytosol across the plasma membrane and is sequestered in the endoplasmic reticulum and mitochondria.
Sustained elevation of intracellular Ca2+ is harmful because of :
Depletion of energy reserves by inhibiting the ATPase used in oxidative phosphorylation
Dysfunction microfilaments
Activation of hydrolytic enzymes
Generation of ROS and RNS
Pertubation of this circuit adversely affects pituitary hormone secretion and, in turn, the peripheral glands.
Decreased secretion of pituitary hormone produces apoptosis followed by involution of the peripheral target gland.
Chemically Altered Signal Transduction with Proliferative Effect.
Xenobiotics that facilitate phosphorylation of signal transducers often promote mitosis and tumor formation.
The phorbol esters and fumonisin B activate protein kinase C (PKC) mimicking diacylglycerol (DAG), one of the physiologic activators of PKC.
Hormones of the anterior pituitary exert mitogenic effects on endocrine glands in the periphery by acting on cell surface receptors.
Pituitary hormone production is under negative feedback control by hormones of the peripheral glands.
Dysregulation of Extracellular Signal Production.
Ca2+ : other PKC activator, and is mimicked by Pb2+
Activated PKC promotes mitogenic signaling by starting a cascade that activates other kinases and allows certain TFs to bind to DNA.
Protein kinases may also be activated by interacting proteins that had been altered by xenobiotics.
Chemically Altered Signal Transduction with Antiproliferative Effect.
Inhibition of Raf diminished degradation of IkB diminished binding of NF-kB to DNA diminished expression of c-Myc mRNA
Down- regulation of a normal mitogenic signal is a step away from survival and toward apoptosis.
Substrates: Glucose Fatty acids O2 Pi ADP
Products: 2H2O ATP
Pyruvate Fatty acyl-CoA
ANT
PDH
βOX
Acetyl-CoA
Dysregulation of Ongoing Cellular activity
Three Mechanisms by which sustained elevations in intracellular Ca2+ levels influence the cellular energy balance.
1st - high cytoplasmic Ca2+ levels cause increased mitochondrial Ca2+ uptake by the Ca2+ “uniporter”
NAD+
Citrate
cycle
NADH + H+
+
+
+
+
+
+
-
-
-
-
-
-
Electron
H+ e-
O4-2
4H+
transport chain
ANT
SYN
Inhibitors:
A
C
D
B
Figure 3-8. ATP synthesis (oxidative phosphorylation) in mitochondria
Toxicants can adversely affect ongoing cellular activity in specialized cells by disrupting any step in signal coupling.
Sustained Rise of Intracellular Ca2+
Dysregulation of electrically excitable cells
Many xenobiotics influence cellular activity in excitable cells, such as neurons, skeletal, cardiac, and smooth muscle cells. Release of neurotransmitters and muscle contraction are controlled by transmitters and modulators synthesized and released by adjacent neurons.