Cellular responses to stress and toxic insults: adaptation, injury and death

CELLULAR RESPONSES TO STRESS & NOXIOUS STIMULI

Cell adaptation & injury

The adaptive response of a cell may consists of an increase in the size of cells (hypertrophy) and functional activity, an increase in their number (hyperplasia), a decrease in the size and metabolic activity of cells (atrophy), or change in the phenotype of cells (metaplasia). Cell injury is reversible up to a certain point, but if the stimulus persists or is severe enough from the beginning, the cell suffers irreversible injury and ultimately cell death. There are two principal pathways of cell death, necrosis and apoptosis.

ADAPTATION OF CELLULAR GROWTH AND DIFFERENTIATION

Hypertrophy

Hypertrophy refers to an increase in the size of cells, resulting in an increase in the size of the organ. The increased size of the cells is due to synthesis of more structural components of the cells. Cells capable of division may respond to stress by undergoing both hyperplasia and hypertrophy, whereas nondividing cells (e.g. myocardial fibers) increased tissue mass is due to hypertrophy.

Example of hypertrophy
Physiologic: muscles - bodybuilding, uterus during pregnancy (hormone induced hypertrophy)
Pathologic: heart enlargement

Hyperplasia

Hyperplasia is an increase in the number of cells in an organ or tissue, usually resulting in increased mass of the organ or tissue.

Physiologic hyperplasia can be divided into (1) hormonal hyperplasia, which increases the functional capacity of a tissue when needed, and (2) compensatory hyperplasia, which increases tissue mass after damage or partial resection.

Examples 
Hormonal hyperplasia: female breast during puberty
Compensatory hyperplasia: growth after partial hepatectomy

Most forms of pathologic hyperplasia are caused by excessive of hormones or growth factors acting on target cells. Examples of pathologic hyperplasia caused by hormones are endometrial hyperplasia, benign prostatic hyperplasia.

! Hyperplasia is distinct from cancer, but pathologic hyperplasia constitutes a fertile soil in which cancerous prolideration may arise.

Atrophy

Atrophy is reduced size of an organ or tissue resulting from a decrease in cell and number. Physiologic atrophy is common during normal development, some embryonic structures undergo atrophy during fetal development.

The common causes of pathologic atrophy

• Decreased workload (atrophy of disuse)
• Loss of innervation (denervation atrophy)
• Diminished blood supply
• Inadequate nutrition
• Loss of endocrine stimulation
• Pressure

In many situations atrophy is accompanied by increased autophagy. Autophagy (selfeating) is the process in which the starved cell eats its own components in an attempt to find nutrients and survice.

Metaplasia

Columnar to squamous metaplasia

Metplasia is a reversible change in which one differentiated cell type is replaced by another cell type.

The most common epithelia metaplasia is columnar to sqamous. The influences that predispose to metaplasia, if persistent, may initiate malignant transformation in metplastic epithelium.

CAUSES OF CELL INJURY

The causes of cell injury range from the external gross physical violence of an automobile accident to subtle internal abnormalities, such as a genetic mutation causing lack of a vital enzyme that impairs normal metabolic function.

Oxygen deprivation

Hypoxia is a deficiency of oxygen, which causes cell injury by reducing aerobic oxidative respiration. Hypoxia is an extremly important and common cause of cell injury and cell death. Causes of hypoxia incl reduced blood flow (ischemia), inadequate oxygenation of the blood due to cardiorespiratory failure, and decreased oxygen-carrying capacity of the blood, as in anemia or carbon monoxide poisoning or after severe blood loss.

Physical agents

Physical agents capable of causing cell injury incl. mechanical trauma, extremes of temperature (burns), sudden changes in atmospheric pressure, radiation and electric shock.

Immunologic reactions

The immune system serves an essential function in defense against infectious pathogens, but immune reactions may also cause cell injury. Injurious reactions to endogenous self-antigens are responsible for several autoimmune diseases.

Genetic derangements

Genetic defects may cause cell injury because of deficiency of functional proteins, such as enzyme defects in inborn errors of metabolism or accumulation of damaged DNA or misfolded proteins, both of which trigger cell death when are beyond repair.

Other causes of cell death are chemical agents/drugs, nutritional imbalances and infectious agents.

MORPHOLOGIC ALTERAIONS IN CELL INJURY

Reversible injury is characterized by generalized swelling of the cell and its organelles: blebbling of the plasma membrane; detachment of ribosomes from the ER; and clumping of nuclear chromatin. Two features of reversible cell injury can be recognized under the licht microscope: cellular swelling and fatty change. Within limits, the cell can repair these derangements and if the injurious stimulus abates, will return to normalcy. Persistent or excessive injury causes cell to pass to point of no return into irreversible injury and cell death. Different injurious stimuli may induce death by necrosis or apoptosis.

Reversible injury - Necrosis - Apoptosis

Necrosis

Necrotic cells are unable to maintain membrane integrity and their contents often leak out, a process that may elicit inflammation in the surrounding tissue. The enzymes that digest the necrotic cell are derived from the lysosomes of the dying cells themselves and from the lysocomes of leukocytes that are called in as part of the inflammatory reaction.

Patterns of tissue necrosis

• Coagulative necrosis is a form of necrosis in which the architecture of dead tissues is preserved for a span of at least some days - infarct
• Liquefactive necrosis is characterized by digestion of the dead cells, resulting in transformation of the tissue into a liquid viscous mass - infection
• Gangrenous necrosis is not a specific pattern of cell death, but term is commonly used. It is usually applied to a limb that has lost its blood supply and has undergone necrosis (typically coagulative necrosis). When bacterial infection is superimposed there is more liquefactive necrosis (wet gangrene)
• Caseous (cheese-like) necrosis is encountered most often in tuberculous infection
• Fat necrosis is a form of necrosis characterized by the action upon fat by digestive enzymes. Typically resulting from released of activated pancreatic lipases into the pancreas and peritoneal cavity - acute pancreatitis 
• Fibroid necrosis is a special form of necrosis usually seen in immune reactions involving blood vessels. This pattern of necrosis typically occurs when complexes of antigens and antibodies are deposited in the walls of arteries. Deposits of these immune complexes together with fibrin (leaked out of vessels) result in fibrinoid.

In the living patient most necrotic cells and their contents disappear by phagocytosis of the debris and enzymatic digestion by leukocytes. If necrotic cells and cellular debris are not promptly destroyed and reabsorbed, they tend to attract calcium salts and other minerals and to become calcified (dystrophic calcification)

MECHANISMS OF CELL INJURY

Mechanisms of cell injury

Depletion of ATP

Reduction in ATP levels is fundamental cause of necrotic cell death. The major causes of ATP depletion are reduced supply of oxygen and nutrients, mitochondrial damage, and the actions of some toxins (e.g. cyanide). High-energy phosphate in the form of ATP is required for virtually all synthetic and degradative process within the cell. These incl. membrane transport, protein synthesis, lipogenesis and the dacylation-reacylation reactions necessary for phospholipid turnover.

Mitochondrial damage

Mitochondria supply life-sustaining energy by producing ATP. Mitochondria can be damaged by increases of cytosolic calcium, reactive oxygen species, oxygen deprivation and so they are sensitive to virtually all types of injurious stimuli, including hypoxia and toxins.

Influx of calcium and loss of calcium homeostasis

Calcium ions are important mediators of cell injury. In keeping with this, depleting calcium protects cells from injury induced by a variety of harmful stimuli. Cystosolic free calcium is normally maintained at very low concentration complared with extracellular levels. Ischemia and certain toxins cause an increase in cystosolic calcium concentration leading to increased mitochondrial permeability and activation of multiple cellular enzymes.

Accumulation of oxygen-derived free radicals (oxidative stress)

Cell injury induced by free radicals, particularly reactive oxygen species (ROS), is an important mechanism of cell damage in many pathologic conditions, such as chemical and radiation injury, ischemia-reperfusion injury, cellular aging and microbial killing by phagocytes. Free radicals are chemical species that have a single unpaired electron in an outer orbit. Unpaired electons are highly reactive and "attack and modify adjacent molecules, such as - proteins, lipids, carbohydrates, nuclei acids - many of which are key components of cell membranes and nuclei.

Defects in membrane permeability

Early loss of selective membrane permeability, leading ultimately to overt membrane damage is a consistent feature of most forms of cell injury (except apoptosis) Membrane damage may affect the functions and integrity of all cellular membranes.

• Mitochondrial membrane damage results in opening of the mitochondrial permeability transition pore, leading to decreased ATP generation and release of proteins that trigger apoptotic death.
• Plasma membrane damage results in loss of osmotic balance and influx of fluids and ions, as well as cellular contents. The cells may also leak metabolites that are vital for the reconstitution of ATP, thus further depleting energy stores
• Injury to lysosomal membranes results in leakage of their enzymes into the cytoplasm. Activation of these enzymes leads to enzymatic digestion of proteins, RNA, DNA and glycogen and the cells die by necrosis.

Damage to DNA and proteins

Cells have mechanisms that repair damage to DNA but if DNA damage is too severe to be corrected the cell intiates a suicide progran that results in death by apoptosis. A similar reaction is triggered by improperly folded proteins, which may be result of inherited mutations or acquired triggers such as free radicals.

SELECTED EXAMPLES OF CELL INJURY AND NECROSIS

Ischemia and hypoxic injury

Ischemia is the most common type of cell injury in clinical medicine and it results from hypoxia induced by reduced blood flow, most commonly due to a mechanical arterial obstruction. In contrast to hypoxia, during which energy production by anaerobic glycolysis can continue, ischemia compromises the delivery of substrates of glycolysis. For this reason, ischemia tends to cause more rapid and severe cell and tissue injury than does hypoxia in the absence of ischemia.

Mechanisms of ischemic cell injury

As the oxygen tension within the cell falls, these is loss of oxidative phophorylation and decreased generation of ATP. The depletion of ATP results in failure of the sodium pump, leading to efflux of potassium, influx of sodium and water and cell swelling. There is also influx of calcium with its many deleterious effects. There is progressive loss of glycogen and decreased protein systhesis. The cytoskeleton disperses, resulting in the loss of ulatrastructural features such as microvilli and the formation of "blebs" at the cell surface. At this time the mitochondria are usually swollen, the ER remains dilated.

If ischemia persists, irrversible injury and necrosis ensue. Irrecersible injury is associated morphologically with severe swelling of mitochondria, extensive damage to plasma membranes (giving rise to myeling figures) and swelling of lysosomes. Large flocculent amorphous densities develop in the mitochondrial matrix. In myocardium these are the indication of irreversivle injury. Death is mainly by necrosis, but apoptosis also contributes.

Ischemia-reperfusion injury

Restoration of bloodflow to ischemic tissue can promote recovery of cells if they are reversibly injured, but can also paradoxically exacerbate the injury and cause cell death. This process called ischemia-reperfusion injury, is clincally important because it contributes to issue damage during myocardial and cerebral infarction and following therapies to restore blood flow.

How does reperfusion injury occur?

• Oxidative stress. New damage may be initiated during reoxygenation by increased generation of reactive oxygen and nitrogen species.
• Intracellular calcium overload. Calcium overload is exacerbated during reperfusion resulting from cell membrane damage and ROS mediated injury to sarcoplasmic recticulum.
• Inflammation. Cytokines secreted by resisdent immune cells (macrophages) and increased expression of adhesion molecules by hypoxic parenchymal and endothelial cells, all of which act to recruit circulating neutrophils to reperfused tissue. The inflammation causes additional tissue injury.
• Activation of the complement system.  Some IgM antibodies have a propensity to deposit in ischemic tissues, and when blood flow is resumed, complement proteins bind to the deposited antibodies and cause more cell injury and inflammation.

Chemical (toxic) injury

Chemical injury remains a frequent problem in clinical medicine and is a major limitation to drug therpay. Many drugs are metabolized in the liver, this organ is frequent target of drug toxicity. Chemicals induce cell injury by one or two general mechanisms:

• Direct toxicity. Some chemicals can injure cells directly by combining with critical molecular components.
• Conversion to toxic metabolies. Most toxic chemicals are not biologically active in their native form but must be converted to reactive toxic metabolies, which then act on target molecules.

APOPTOSIS

Apoptosis is a pathyway of cell death that is induced by a tightly regulated suicide progran in which cells destined to die activate intrinsic enzymes that degrade the cells own nuclear DNA and nuclear cytoplasmic proteins. Apoptotic cells break up in fargments (apoptotic bodies). The dead cell and it fargments are rapidly devoured by phagocytes, before the contents have leaked out, and therefore the cell death by this pathway does not elicit an inflammatory reaction in the host.

Causes of apoptosis

Apoptosis in physiologic situation
Death by apoptosis is a normal phenomenon that serves to eliminate cells that are no longer needed, and to maintain a steady number of various cell populations in tissues.

• The destruction of cells during embryogenesis
• Involution of hormone dependent tissues upon hormone withdrawal e.g. menstrual cycle
• Cell loss in proliderating cell population e.g. immature lymphocytes in the bone marrow and thymus that fail to express useful entigen receptors.
• Elimination of potentially harmful self-reactive lymphocytes
• Death of host cells that have served their useful purpose e.g. neutrophils at the end of an acute inflammatory response.

Apoptosis in pathologic conditions

• DNA damage
• Accumulation of misfolded proteins
• Cell death in certain infections, partcularly viral.
• Pathologic atrophy in parenchymal organ after duct obstruction

Mechanisms of apoptosis

Apoptosis results from the activation of enzymes called capases. Capases must undergo enzymatic cleavage to become active.  Two distinct pathways converge on capase activation: the mitochondrial pathway and the death receptor pathyway. 

NECROPTOSIS

Necroptosis is a hybrid form of cell death that shares aspects of both necrosis and apoptosis. The following features characterize necroptosis:

• Morphologically, it resenbles necrosis, characterized by loss of ATP, swelling of the cell and organelles, generation of ROS, release of lysosomal enzymes and ultimately rupture of the plasma membrane.
• Mechanistically it is triggered by genetically programmed signal transduction events that culminale in cell death. In this respect it resembles programmed cell death, which is considered the hallmark of apoptosis.

INTRACELLULAR ACCUMULATIONS

Abnormal intracellular accumulation

One of the manifestations of metabolic derangements in cells is the intracellular accumulation of abnormal amounts of various substances that may be harmless or associated with varying degree of injury. There are four main pathways of abnormal intracellular accumulations.

• Inadequate removal of a normal substance secondary to defects in mechanisms of packaging and transport as it fatty change (steatosis) in the liver
• Accumulation of an abnormal endogenous sustance as a result of genetic or acquired defects in its folding, packagin, transport, or secretion as with certain mutated forms of alpha1-antitrypsin.
• Failure to degrade a metobolite due to inherited enzyme deficiencies - storage diseases
• Deposition and accumulation of an abnormal exogenous substance when the cell has neither the enzymatic machinery to degrade the substance nor the ability to transport it to other sites.

Lipids

All major classes of lipids can accumulate in cells: triglycerides, cholestrol/cholesterol esters, and phospholids.

Steatosis (fatty change)

The terms steatosis and fatty change describe abnormal accumulations of triglycerides within parenchymal cells. Fatty change is often seen in the liver because it is the major organ involved in fat metabolism. The causes of steatosis incl, toxins, protein malnutrition, DM, obesity, and anorexia.  In developed nations the most common causes of fatty liver are alcohol abuse and obesity/DM.

Cholesterol and cholesterol esters

The cellular metabolism of cholesterol is tightly regulated such that most cells use cholesterol for the synthesis of cell membranes without intracellular accumulation of cholesterol or cholesterol esters. Accumulations are in several pathologic processes.

• Atherosclerosis. In atherosclerotic plaques, smooth muscle cells and macrophages within the intimal layer of aorta and large arteries are filled with lipid vacuoles, most of which are made up of cholesterol and cholesterol esters.
• Xanthomas. Intracellular accumulation of cholesterol within machrophages is also characteristic of acquired and hereditary hyperlipidemic states.
• Cholesterolosis. This refers to the focal accumulations of cholesterol-laden macrophages in the lamina propria of the gallbladder.
• Niemann-Pick disease type C. This lysosomal storage disease is caused by mutations affecting an enzyme involved in cholesterol trafficking, resulting in cholesterol accumulation in multiple organs.

Hyaline change

The term hyaline usually refers to an alteration within cells or in the extracelullar space that gives a homogeneous, glassy, pink apperance in routine hostologic sections stained with hematoxylin and eosin. It is widely used as a descriptive histologic term rather than a specific marker for cell injury. 

Glycogen

Glycogen is readily available energy source stored in the cytoplasm of healthy cells. Excessive intracellular deposits of glycogen are seen in patients with an abnormality in either glucose or glycogen metabolism. Diabetes Mellitus is the prime example of a disorder of glucose matebolism, In this disease glycogen is found in renal tubular epithelial cells, as well as withing liver cells, beta cellas of the islets of Langerhands and heart muscle cells. 

PATHOLOGIC CALCIFICATION

Pathologic calcfication is the abnormal tissue deposition of calcium salts, together with smaller amounts of iron, magnesium and other mineral salts. There are two forms of pathologic calcification, When the deposition occurs locally in dying tissues it is known as dystrophic calcifications; it occurs despite normal serum levels of calcium and in the absence of derangements in calcium metabolism. In contrast, the deposition of calcium salts in otherwise normal tissues is known as metatastic calcification, and it almost always results from hypercalcemia secondary to some disturbance in calcium metabolism.

Dystrophic calcification

Dystophic calcification is encountered in areas of necrosis, whether they are coagulative, caseous, or liquefactive type, any in foci enzymatic necrosis of fat. 

Although dystrophic calcification may simply be a telltale sign of previous cell injury, it is often a cause of organ dysfunction, such is the case in calcific valvular disease and atherosclerosis.

Metastatic calcification

Metastatic calcification may occur in normal tissues whenever there is hypercalcemia. Hypercalcemia also accentuates dystrophic calcification. There are four principal causes of hypercalcemia

• Increased secretion of parathyroid hormone (PTH) with subsequent bone resorption.
• Resorption of bone tissue e.g. secondary/ primary tumours of bone marrow.
• Vitamin D-related disorders e.g. vit D intoxication, sarcoidosism, idiopathic hypercalcemia
• Renal failure, which causes retention of phosphate, leading to secondary hyperparathyroidism.

CELLULAR AGING

Cellular aging is the result if a progressive decline in cellular function and viability caused by genetic abnormalities and the accumulation of cellular and molecular damage due to the effects of exposure to exogenous influences. 

Cellular aging

DNA damage

A variety of exogenous (physical, chemical and biological) agents and endogenous factors such as ROS threaten the integrity of nuclear and mitochondrial DNA. Although most DNA damage is repaired by DNA repair enzymes, some persists and accumulates as cells age. Patients with Werner syndrome show premature aging, and the defective genes product is DNA helicase. A defect in this enzyme causes rapid accumulation of chromosomal damage that may mimic the injury that normally accumulates during cellular aging.

Cellular senescence. 

All normal cells have a limited capacity for replication and after a fixed number of divisions cells become arrested in a terminally nondividing state. Two mechanisms are believed to underlie cellular senescence:

• Telomere attrition. One mechanism of replicative senescence involves progressive shortening of telomeres, which ultimately results in cell cycle arrest.
• Activation of tumor suppressor genes. By controlling G1 to S phase progression during the cell cycle, p16 protrects the cells from uncontrolled mitogenic signals and pushes cells along the senescence pathway.

Defective protein homeostasis

Protein homeostasis involves two mechanisms: those that maintain proteins in there correctly folded conformations and others that degrade misfolded proteins. There is evidence that both normal folding and degradation of misfolded proteins are impaired with aging. 

Deregulated nutrient sensing

Paradoxical though it may seem, eating less increases longevity. Caloric restriction increases life span in all eukaryotic species in which it has been tested. Mediaters maybe be reduced IGF-1 signaling and increases in sirtuins.