Mechanisms of Cell Injury.
The mechanisms by which injurious agents cause cell injury and death are complex. Some agents, such as heat, produce direct cell injury. Other factors, such as genetic derangements, produce their effects indirectly through metabolic disturbances and altered immune responses. There seem to be at least three major mechanisms whereby most injurious agents exert their effects:
• Free radical formation
• Hypoxia and ATP depletion
• Disruption of intracellular calcium homeostasis (Fig. 5.7)
Free Radical Injury
Many injurious agents exert damaging effects through reactive chemical species known as free radicals. Free radicals are highly reactive chemical species with an unpaired electron in the outer orbit (valence shell) of the molecule. In the literature, the unpaired electron is denoted by a dot, for example, •NO. The unpaired electron causes free radicals to be unstable and highly reactive, so that they react nonspecifically with molecules in the vicinity. Moreover, free radicals can establish chain reactions consisting of many events that generate new free radicals. In cells and tissues, free radicals react with proteins, lipids, and carbohydrates, thereby damaging cell membranes; inactivate enzymes; and damage nucleic acids that make up DNA. The actions of free radicals may disrupt and damage cells and tissues.
Reactive oxygen species (ROS) are oxygen-containing molecules that include free radicals such as superoxide _ (O2) and hydroxyl radical (OH•) and nonradicals such as hydrogen peroxide (H O ). These molecules are produced endogenously by normal metabolic processes or cell activities, such as the metabolic burst that accompanies phagocytosis. However, exogenous causes, including ionizing and UV radiation, can cause ROS production in the body. Oxidative stress is a condition that occurs when the generation of ROS exceeds the ability of the body to neutralize and eliminate ROS. Oxidative stress can lead to oxidation of cell components, activation of signal transduction pathways, and changes in gene and protein expression. DNA modification and damage can occur as a result of oxidative stress. Although ROS and oxidative stress are clearly associated with cell and tissue damage, evidence shows that ROS do not always act in a random and damaging manner. Current studies have found that ROS are also important signaling molecules that are used in healthy cells to regulate and maintain normal activities and functions such as vascular tone and insulin and vascular endothelial growth factor signaling. Oxidative damage has been implicated in many diseases. Mutations in the gene for SOD are linked with amyotrophic lateral sclerosis (ALS; so-called Lou Gehrig disease). Oxidative stress is thought to play an important role in the development of cancer. Reestablishment of blood flow after loss of perfusion, as occurs during heart attack and stroke, is associated with oxidative injury to vital organs. The endothelial dysfunction that contributes to the development, progression, and prognosis of cardiovascular disease is thought to be caused in part by oxidative stress.
In addition to the many diseases and altered health conditions associated with oxidative damage, oxidative stress has been linked with the age-related functional declines that underlie the process of aging.
Antioxidants are natural and synthetic molecules that inhibit the reactions of ROS with biologic structures or prevent the uncontrolled formation of ROS. Antioxidants include enzymatic and nonenzymatic compounds. Catalase can catalyze the reaction that forms water from hydrogen peroxide. Nonenzymatic antioxidants include carotenes (e.g., vitamin A), tocopherols (e.g., vitamin E), ascorbate (vitamin C), glutathi-one, flavonoids, selenium, and zinc.
Hypoxic Cell Injury
Hypoxia deprives the cell of oxygen and interrupts oxidative metabolism and the generation of ATP. The actual time necessary to produce irreversible cell damage depends on the degree of oxygen deprivation and the metabolic needs of the cell. Some cells, such as those in the heart, brain, and kidney, require large amounts of oxygen to provide energy to perform their functions. Brain cells, for example, begin to undergo permanent damage after 4 to 6 minutes of oxygen deprivation. A thin margin can exist between the time involved in reversible and irreversible cell damage. During hypoxic conditions, hypoxia-inducible factors (HIFs) cause the expression of genes that stimulate red blood cell formation, produce ATP in the absence of oxygen, and increase angiogenesis (i.e., the formation of new blood vessels).
Hypoxia can result from an inadequate amount of oxygen in the air, respiratory disease, ischemia (i.e., decreased blood flow due to vasoconstriction or vascular obstruction), anemia, edema, or inability of the cells to use oxygen. Ischemia is characterized by impaired oxygen delivery and impaired removal of metabolic end products such as lactic acid. In contrast to pure hypoxia, which depends on the oxygen content of the blood and affects all cells in the body, ischemia commonly affects blood flow through limited numbers of blood vessels and produces local tissue injury. In some cases of edema, the distance for diffusion of oxygen may become a limiting factor in the delivery of oxygen. In hypermetabolic states, cells may require more oxygen than can be supplied by normal respiratory function and oxygen transport. Hypoxia also serves as the ultimate cause of cell death in other injuries. For example, a physical agent such as cold temperature can cause severe vasoconstriction and impair blood flow.
Hypoxia causes a power failure in the cell, with wide-spread effects on the cell’s structural and functional components. As oxygen tension in the cell falls, oxidative metabolism ceases and the cell reverts to anaerobic metabolism, using its limited glycogen stores in an attempt to maintain vital cell functions. Cellular pH falls as lactic acid accumulates in the cell. This reduction in pH can have adverse effects on intracellular structures and biochemical reactions. Low pH can alter cell membranes and cause chromatin clumping and cell shrinkage.
One important effect of reduced ATP is acute cell swelling caused by failure of the energy-dependent sodium/ potassium (Na+/K+)–ATPase membrane pump, which extrudes sodium from and returns potassium to the cell. With impaired function of this pump, intracellular potassium levels decrease and sodium and water accumulate in the cell. The movement of water and ions into the cell is associated with multiple changes including widening of the endoplasmic reticulum, membrane permeability, and decreased mitochondrial function. In some instances, the cellular changes due to ischemia are reversible if oxygenation is restored. If the oxygen supply is not restored, however, there is a continued loss of enzymes, proteins, and ribonucleic acid through the hyperpermeable cell membrane. Injury to the lysosomal membranes results in the leakage of destructive lysosomal enzymes into the cytoplasm and enzymatic digestion of cell components. Leakage of intra-cellular enzymes through the permeable cell membrane into the extracellular fluid provides an important clinical indicator of cell injury and death.
Impaired Calcium Homeostasis
Calcium functions as an important second messenger and cytosolic signal for many cell responses. Various calcium-binding proteins, such as troponin and calmodulin, act as transducers for the cytosolic calcium signal. Calcium/calmodulin–dependent kinases indirectly mediate the effects of calcium on responses such as smooth muscle contraction and glycogen breakdown. Normally, intracellular calcium ion levels are kept extremely low compared with extracellular levels. The low intracellular calcium levels are maintained by membrane-associated calcium/magnesium (Ca2+/Mg2+)–ATPase exchange systems. Ischemia and certain toxins lead to an increase in cytosolic calcium because of increased influx across the cell membrane and the release of calcium from intracellular stores. The increased calcium level may inappropriately activate a number of enzymes with potentially damaging effects. These enzymes include the phospholipases, responsible for damaging the cell membrane; proteases that damage the cytoskeleton and membrane proteins; ATPases that break down ATP and hasten its depletion; and endonucleases that fragment chromatin. Although it is known that injured cells accumulate calcium, it is unknown whether this is the ultimate cause of irreversible cell injury.