Cellular responses to stress (Adaptations, injury and death) (3 of 5) Ali Al Khader, MD Faculty of Medicine Al-Balqa’ Applied University Email: ali.alkhader@bau.edu.jo

Differences in responses Glycogen in hepatocytes make them withstand ischemia more Striated muscle cell…2-3 hours before irreversible injury … but cardiac myocyte…20-30 minutes Genetic polymorphisms in response to toxins/drugs…e.g., P-450 differences …the study of these is called -------

Essential biochemical abnormalities in cell injury ATP depletion Mitochondrial damage Influx of calcium Accumulation of oxygen-derived free radicals (oxidative stress) Cellular membranes, esp. plasma and lysosomal membranes DNA damage & misfolding of proteins

ATP depletion Sodium-potassium pump…swelling Anaerobic glycolysis lactic acid accumulation pH activity of enzymes Ca2+ pump failure influx of calcium into cellular compartments Detachment of ribosomes and dissociation of polysomes

Mitochondrial dysfunction More in ATP because of oxidative phosphorylation Formation of reactive oxygen species because of abnormal oxidative phosphorylation Formation of mitochondrial permeability transition pore When its proteins get out to the cytoplasm, they induce apoptosis

Influx of calcium Normally: low in the cytosol, due to: -pumping outside the cell -sequestration in cellular compartments When in the cytosol: -activates enzymes: -phospholipases -proteases -endonucleases -ATPases -activates apoptosis, due to: -activation of caspases -increasing mitochondrial permeability

Oxidative stress Phagocyte when activated in inflammation Normal cell in oxidative respiration

Some causes of increased free radicals -Ionizing radiation -Catabolism of exogenous chemicals…e.g., carbon tetrachloride -Inflammation -Toxins -Aging -Oxygen toxicity

Protective: Glutathione (GSH) peroxidases Especially GSH peroxidase 1 Protective from free radicals 2GSH + H2O2 GSSG (oxidized GSH) + 2H2O More GSSG/GSH means: good activity of GSH peroxidase

Catalase…also protective 2H2O O2 + 2H2O = peroxidase (in peroxisomes)

Some vitamins…also protective Beta-carotene **By blocking synthesis of free radicals or by scavenging them

Effects of ROS Attacking double bonds of membrane polyunsaturated lipids into: Peroxides (Lipid peroxidation) On proteins: -Sulfhydryl-mediated mediated cross-linking -Direct fragmentation On DNA: React with thymine single-strand breaks

Membrane defects phospholipid synthesis…due to ATP …it can also exacerbate ATP depletion, How? degradation of phospholipids…by phospholipases due to increased Ca+2

Membrane defects…cont’d ROS and lipid peroxidation Cytoskeleton damage by proteases due to increased Ca+2 Effect of phospholipids breakdown products -un-esterified free fatty acids -acyl carnitine -lysophospholipids

DNA & proteins damage The cell typically respond to this by apoptosis

Examples of some injury mechanisms Ischemia & hypoxia…Which does have faster and more severe injury? Ischemia-reperfusion injury -The mitochondria is injured so incomplete reduction of O2 -Actions of oxidases in WBCs, endothelial cells and parenchymal cells -Antioxidant mechanisms are still compromised in injured cells -Increased WBCs (increased inflammation) and complements …injurious

Chemical (toxic) injury Directly toxic (examples): Mercuric chloride …binds to sulfhydryl groups in membrane proteins (membrane damage) Some chemotherapeutic agents

Some toxins are not intrinsically active and need modification CCl4 after processing by cytochrome P-450 in smooth ER in hepatocytes and others: -direct damage or -free radicals…CCl3.