C h a p t e r 2
Cellular Responses to Stress, Injury, and Aging
41
present in peroxisomes, catalyzes the reaction that forms
water from hydrogen peroxide. Nonenzymatic antioxi-
dants include carotenes (e.g., vitamin A), tocopherols
(e.g., vitamin E), ascorbate (vitamin C), glutathione, and
flavonoids, as well as micronutrients such as selenium and
zinc.
18
Nonenzymatic antioxidants often directly react
with oxidants to “disarm” them. For example, vitamin
C directly scavenges superoxide and hydroxyl radicals.
19
Oxidative damage has been implicated in many
diseases. Mutations in the gene for SOD are associ-
ated with amyotrophic lateral sclerosis (ALS; so-called
Lou Gehrig disease
).
20
Oxidative stress is thought to
have an important role in the development of can-
cer.
21
Reestablishment of blood flow following 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 devel-
opment, progression, and prognosis of cardiovascular
diseases is thought to be caused in part by oxidative
stress.
22
In addition, oxidative stress has been associated
with age-related functional declines.
23
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 dam-
age 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 the 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 of time exists between reversible and irre-
versible cell damage. A classic study found that the epi-
thelial cells of the proximal tubule of the kidney in the
rat could survive 20 but not 30 minutes of ischemia.
24
Recent work has identified a group of proteins called
hypoxia-inducible factors
(HIFs). During hypoxic condi-
tions, HIFs cause the expression of genes that stimulate
red blood cell formation, manufacture glycolytic enzymes
that produce ATP in the absence of oxygen, and increase
angiogenesis
25
(i.e., the formation of new blood vessels).
Hypoxia can result from an inadequate amount of oxy-
gen in the air, respiratory disease, ischemia (i.e., decreased
blood flow due to vasoconstriction or vascular obstruc-
tion), anemia, edema, or inability of the cells to use oxy-
gen. 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 depends on 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, the 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, physical factors such as a cold temperature
can cause severe constriction 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, oxida-
tive metabolism ceases, and the cell reverts to anaero-
bic 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 mem-
branes and cause chromatin clumping and cell volume
changes.
An important effect of reduced ATP is acute cell swell-
ing caused by failure of the energy-dependent sodium/
potassium (Na
+
/K
+
)-adenosine triphosphatase (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 dila-
tion of the endoplasmic reticulum, increased membrane
permeability, and decreased mitochondrial function.
2
To
a point, the cellular changes due to hypoxia are revers-
ible if oxygenation is restored. However, if the oxygen
supply is not restored there is continued loss of enzymes,
proteins, and ribonucleic acid through the hyperperme-
able cell membrane. Injury to the lysosomal membranes
results in the leakage of destructive lysosomal enzymes
into the cytoplasm and enzymatic digestion of cell com-
ponents. Leakage of intracellular enzymes through the
permeable cell membrane into the extracellular fluid
provides an important clinical indicator of cell injury
and death. These enzymes enter the blood and can be
measured by laboratory tests.
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 cytosolic calcium signaling. Calcium/
calmodulin–dependent kinases indirectly mediate the
effects of calcium on responses such as smooth muscle
contraction and glycogen breakdown. Normally, intracel-
lular calcium ion levels are kept extremely low compared
with extracellular levels. These low intracellular levels
are maintained by energy-dependent membrane-associ-
ated calcium/magnesium (Ca
++
/Mg
++
)-ATPase exchange
systems
2
and sequestration of calcium ions within organ-
elles such as the mitochondria and smooth endoplasmic
reticulum. Ischemia and certain toxins lead to an increase
in cytosolic calcium because of the increased influx across
the cell membrane and the release of calcium from intra-
cellular stores. The increased calcium level may inappro-
priately activate a number of enzymes with potentially
damaging effects. These enzymes include phospholipases
that can damage the cell membrane, proteases that dam-
age the cytoskeleton and membrane proteins, ATPases
that break down ATP and hasten its depletion, and endo-
nucleases 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.