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.
