34- Healing, Repair, and Cell Aging

A 22-year-old woman with leukemia undergoes bone marrow transplantation and receives partially mismatched donor marrow. One month later, she has a scaling skin rash. Examination of a skin biopsy specimen reveals the cellular change shown in the figure. This change most likely results from which of the following biochemical reactions? (A) Activation of caspases (B) Reduction of ATP synthesis (C) Increase in glycolysis (D) Activation of lipases (E) Lipid peroxidation

*The answer A.* This cell is shrunken and has been converted into a dense eosinophilic mass. The surrounding cells are normal, and there is no inflammatory reaction. This pattern is typical of apoptosis. Caspase activation is a universal feature of apoptosis, regardless of the initiating cause. Apoptosis induced in recipient cells from donor lymphocytes occurs with graftversus-host disease. Reduced ATP synthesis and increased glycolysis occur when a cell is subjected to anoxia. These changes are reversible. Lipases are activated in enzymatic fat necrosis. Lipid peroxidation occurs when the cell is injured by free radicals.

A study of aging shows that senescent cells have accumulated damage from toxic byproducts of metabolism. There is increased intracellular lipofuscin deposition. Prolonged ingestion of which of the following substances is most likely to counteract this aging mechanism? A Antioxidants B Analgesics C Antimicrobials D Antineoplastic agents E Glucocorticoids

*The answer is A.* Antioxidants may counteract the effects of reactive oxygen species (ROS) that may accumulate acutely and chronically within cells as a consequence of environmental insults and pathologic processes. Certainly, health foodstores promote this concept with sales of products such as vitamin E. However, cellular damage is multifactorial, and proving that one compound has a significant effect is difficult. Analgesics ameliorate the perception of pain from cellular damage, but they do not prevent or diminish cell damage; they only mask it. Antimicrobials may help the body's own immune defenses against infectious agents and shorten and/or diminish tissue damage. However, longterm use of antimicrobials is discouraged because it may alter the body's own useful microbial flora, and it can promote development of drug-resistant strains that pose a serious health risk for the general population. (As Mr. Spock noted, "The needs of the many outweigh the needs of the few.") Antineoplastic agents are given for malignancies and rarely have benefit for cancer prevention. Glucocorticoids provide short-term improvement in well-being, but when used for longer periods, they have deleterious effects.

A cesarean section is performed on a 20-year-old woman to deliver a term infant, and the lower abdominal incision is sutured. The sutures are removed 1 week later. Which of the following statements best describes the wound site at the time of suture removal? (A) Granulation tissue is still present (B) Collagen degradation exceeds synthesis (C) Wound strength is 80% of normal tissue (D) Type IV collagen predominates (E) No more wound strength will be gained

*The answer is A.* At 1 week, wound healing is incomplete, and granulation tissue is still present. More collagen is synthesized in the following weeks. Wound strength peaks at about 80% by 3 months. Type IV collagen is found in basement membranes.

Which of the following is an anti apoptotic protein? a) Bcl-Xs b) Bfl 2 c) FasL d) Bax

*The answer is A.* Bcl-Xs is an anti-antipoptotic.

A 38-year-old man has a health screening examination. He has a routine chest x-ray that shows a 2 cm nodule in the right lower lobe. The nodule has focal calcifications. A wedge resection of the nodule is done. On microscopic examination the nodule shows caseous necrosis and calcification. Which of the following processes explains the appearance of the calcium deposition: a) Dystrophic calcification b) Apoptosis c) Hypercalcemia d) Metastatic calcification e) Excessive ingestion of calcium

*The answer is A.* Calcium is deposited in and around the granuloma as a reaction to injury with necrosis. Most pulmonary granulomas are the result of infection, typically tuberculosis.

A 69-year-old woman has had transient ischemic attacks for the past 3 months. On physical examination, she has an audible bruit on auscultation of the neck. A right carotid endarterectomy is performed. The curetted atheromatous plaque has a grossly yellow-tan, firm appearance. Microscopically, which of the following materials can be found in abundance in the form of crystals within cleftlike spaces? A Cholesterol B Glycogen C Hemosiderin D Immunoglobulin E Lipofuscin

*The answer is A.* Cholesterol is a form of lipid commonly deposited within atheromas in arterial walls, imparting a yellow color to these plaques and a glistening appearance if abundant. Direct damage to the atheroma can yield cholesterol emboli. Glycogen is a storage form of carbohydrate seen mainly in liver and muscle. Hemosiderin is a storage form of iron that appears in tissues of the mononuclear phagocyte system (e.g., marrow, liver, spleen), but can be widely deposited with hereditary hemochromatosis. Immunoglobulin occasionally may be seen as rounded globules in plasma cells (i.e., Russell bodies). Lipofuscin is a golden brown pigment that increases with aging in cell cytoplasm, mainly in cardiac myocytes and in hepatocytes.

A 16-year-old girl with a history of suicidal depression swallows a commercial solvent. A liver biopsy is performed to assess the degree of damage to the hepatic parenchyma. Histologic examination demonstrates severe swelling of the centrilobular hepatocytes (shown in the image). Which of the following mechanisms of disease best accounts for the reversible changes noted in this liver biopsy? (A) Decreased stores of intracellular ATP (B) Increased storage of triglycerides and free fatty acids (C) Intracytoplasmic rupture of lysosomes (D) Mitochondrial membrane permeability transition (E) Protein aggregation due to increased cytosolic pH

*The answer is A.* Hydropic swelling may result from many causes, including chemical and biologic toxins, infections, and ischemia. Injurious agents cause hydropic swelling by (1) increasing the permeability of the plasma membrane to sodium; (2) damaging the membrane sodium-potassium ATPase (pump); or (3) interfering with the synthesis of ATP, thereby depriving the pump of its fuel. The other choices are incorrect because they do not regulate concentrations of intracellular sodium. Diagnosis: Hydropic swelling, hepatotoxicity

A cellular mutation results in transcription with translation of a protein that does not fold properly. The misfolded protein remains within the cell and is not excreted. Activation of which of the following cytoplasmic enzymes is most likely to occur? a) Caspase b) Glutathione peroxidase c) NADPH oxidase d) Ribonuclease e) Telomerase

*The answer is A.* Misfolded proteins can trigger apoptosis with release of caspases that lead to nuclear and cytoplasmic fragmentation. Huntington disease is one example of a genetic disease with neuronal cell loss from protein misfolding.

A 90-year-old woman with mild diabetes and Alzheimer disease dies in her sleep. At autopsy, hepatocytes are noted to contain golden cytoplasmic granules that do not stain with Prussian blue. Which of the following best accounts for pigment accumulation in the liver of this patient? (A) Advanced age (B) Alzheimer disease (C) Congestive heart failure (D) Diabetic ketoacidosis (E) Hereditary hemochromatosis

*The answer is A.* Substances that cannot be metabolized accumulate in cells. Examples include (1) endogenous substrates that are not processed because a key enzyme is missing (lysosomal storage diseases), (2) insoluble endogenous pigments (lipofuscin and melanin), and (3) exogenous particulates (silica and carbon). Lipofuscin is a "wear and tear" pigment of aging that accumulates in organs such as the brain, heart, and liver. None of the other choices are associated with lipofuscin accumulation. Diagnosis: Aging, lipofuscin

The photomicrograph above shows a section from the heart of an otherwise healthy elderly man who died in an auto accident. A Prussian blue stain is negative. What is true of this intracellular accumulation above the arrow? A. The accumulation does not damage the cell. B. The accumulation is secondary to an inherited mutation. C. The accumulation is secondary to prior hemolysis. D. The accumulation is secondary to very early ischemic necrosis. E. The accumulation is from occupational exposure.

*The answer is A.* The accumulation is lipofuscian granules, which increase with age. This does not damage the cell.

A 54-year-old man with a chronic cough has a squamous cell carcinoma diagnosed in his right lung. While performing a pneumonectomy, the thoracic surgeon notes that the hilar lymph nodes are small, 0.5 to 1.0 cm in size, and jet black in colour throughout. Which of the following is the most likely cause for this appearance to the hilar nodes? A Anthracotic pigment B Lipochrome deposits C Hemosiderosis D Metastatic carcinoma

*The answer is A.* The black colour comes from carbon pigments in dust particles inhaled over the years, engulfed by macrophages, and sent via lymphatics to the lymph nodes. It looks bad but does not compromise lung function. Smokers will have more anthracosis.

An experiment is conducted involving cellular aspects of wound healing. Components of the extracellular matrix are analyzed to determine their sites of production and their binding patterns to other tissue components. Which of the following molecules synthesized by fibroblasts can best bind to cellular integrins and extracellular collagen and attach epidermal basal cells to basement membrane? A Dermatan sulfate B Fibronectin C Heparin D Hyaluronic acid E Procollagen

*The answer is B.* Fibronectin is a key component of the extracellular matrix and has a structure that looks like a paper clip. Fibronectin can be synthesized by monocytes, fibroblasts, and endothelium. Dermatan sulfate, a glycosaminoglycan, forms a gel that provides resilience and lubrication. Heparin that is infused has an anticoagulant function. Hyaluronic acid binds water to form a gelatinous extracellular matrix. Procollagen produced by fibroblasts is formed into ropelike strands of collagen, which provide tensile strength.

A 40-year-old chef cuts his finger with a sharp knife while chopping vegetables. He goes to an acute care facility and you discharge him after performing the procedure seen in the photo below. What will be true regarding wound healing in this patient? A. The wound will heal by first intention and will be more likely to get infected. B. The wound will heal by first intention and will be less likely to get infected. C. The wound will heal by second intention and will be more likely to get infected. D. The wound will heal by second intention and will be less likely to get infected.

*The answer is B.* First intention healing is the simplest type of healing. It occurs when the edges of the wound are brought together by sutures, staples, or other means used to close surgical incisions. Only a small amount of granulation tissue needs be formed.

A new drug is developed that binds to cellular microtubules. The function of the microtubules is diminished, so that mitotic spindle formation is inhibited. Which of the following is the most likely use for this drug? A Antimicrobial therapy B Chemotherapy C Pain management D Prevention of atherosclerosis E Weight reduction

*The answer is B.* Microtubules are cytoskeletal components required for cell movement. Mitotic spindles are required for cell division, and if cancer cells cannot divide, then the neoplasm cannot grow. Antibiotics are directed at microorganisms that do not have microtubules. Pain is produced largely through release of mediators of inflammation. Atheroma formation is affected by endothelial damage and lipid accumulation, and though there is cellular proliferation, it occurs over many years. Weight reduction is accomplished primarily via atrophy of adipocytes, not inhibition of cell proliferation.

Which of the following is characteristic of wound healing by secondary intentions? A. There are well approximated surgical edges B. It is seen most commonly in larger wounds C. Minimal granulation tissue is involved D. Wound remodeling is minimal

*The answer is B.* Secondary intentions is seen most commonly in large wounds.

An experiment analyzes cells for enzyme activity associated with sustained cellular proliferation. Which of the following cells is most likely to have the highest telomerase activity? (A) Endothelial cells (B) Stem cells (C) Neurons (D) Neutrophils (E) Erythrocytes

*The answer is B.* Stem cells have the highest telomerase activity, and the telomere length can be stabilized in these cells. This allows stem cells to retain the ability to divide throughout life. Normal somatic cells have no telomerase activity, and telomeres progressively shorten with each cell division until growth arrest occurs.

What roles in regulating the intrinsic pathway of apoptosis are played by the Bcl-2 protein family members Bax and Bcl-2? a) Bax inhibits apoptosis while Bcl-2 stimulates apoptosis. b) Bax stimulates apoptosis while Bcl-2 inhibits apoptosis. c) Both Bax and Bcl-2 inhibit apoptosis. d) Both Bax and Bcl-2 stimulate apoptosis.

*The answer is B.* The intrinsic apoptotic pathway is tightly regulated in mammalian cells by two groups of proteins, one favouring apoptosis and the other inhibiting it. They all belong to a single protein family, the Bcl-2 family. Bax and Bad are pro-apoptotic members of the family and are believed to be involved in the initial formation of pores in the mitochondrial membrane. Bcl-2 (the first discovered member) and Bcl-xL are important anti-apoptotic proteins that prevent pore formation and cytochrome c release. Whether a stimulus that causes cell stress produces cell destruction will therefore depend on the balance between the pro-apoptotic and anti-apoptotic proteins.

A 70-year-old man with hypercalcemia died suddenly. At autopsy, microscopic examination showed noncrystalline amorphous deposits of calcium salts in gastric mucosa, renal interstitium, and alveolar walls of lungs. Which of the following underlying conditions would most likely explain these findings? A Chronic active hepatitis B Diffuse parathyroid hyperplasia C Disseminated tuberculosis D Generalized atherosclerosis E Normal aging process F Pulmonary emphysema

*The answer is B.* The microscopic findings suggest metastatic calcification, with deposition of calcium salts in tissues that have physiologic mechanisms for losing acid, creating an internal alkaline environment that favors calcium precipitation. Hypercalcemia can have a variety of causes, including primary and secondary hyperparathyroidism, bone destruction secondary to metastases, paraneoplastic syndromes, and, less commonly, vitamin D toxicity or sarcoidosis. Chronic renal disease reduces phosphate excretion by the kidney, resulting in an increase in serum phosphate. Because the solubility product of calcium and phosphorus must be maintained, the serum calcium is depressed, triggering increased parathyroid hormone output to increase the calcium level, which promotes calcium deposition. Chronic hepatitis leads to hyperbilirubinemia and jaundice. The granulomas of tuberculosis have caseous necrosis with dystrophic calcification. Another form of dystrophic calcification occurs when atherosclerotic lesions calcify. Dystrophic calcification is seen more often in the elderly, but it is the result of a lifetime of pathologic changes, not aging itself. Pulmonary emphysema can lead to respiratory acidosis that is compensated by metabolic alkalosis, with the result that the serum calcium level remains relatively unchanged.

A pathologist is investigating the cytology of cells that have been infected with a particularly virulent strain of the influenza virus. The physician suspects that the virus results in cell death after viral replication in order to expedite the spread of the virus. She recalls that there are three known biochemical mechanisms of initiating programmed cellular death: WHAT ARE THEY?. Which of the following biochemical components plays a common role in all of these 3 processes? A. FAS ligand B. Caspase C. Bax D. Bcl-2 E. CD-95 protein

*The answer is B.* The three programmed cellular deaths are: (1) transmembrane receptor-mediated interaction EXTRINSIC (2) stimuli producing intracellular signals leading to mitochondrial-initiated events, INTRINSIC and (3) release of cytoplasmic granules into a cell via a perforin molecule Caspases are seen in all three processes.

A 72-year-old man died suddenly from congestive heart failure. At autopsy, his heart weighed 580 g (normal 330 g) and showed marked left ventricular hypertrophy and minimal coronary arterial atherosclerosis. A serum chemistry panel ordered before death showed no abnormalities. Which of the following pathologic processes best accounts for the appearance of the aortic valve seen in the figure? A Amyloidosis B Dystrophic calcification C Hemosiderosis D Hyaline change E Lipofuscin deposition

*The answer is B.* The valve is stenotic because of nodular deposits of calcium. The process is "dystrophic" because calcium deposition occurs in damaged tissues. The damage in this patient is a result of excessive wear and tear with aging. Amyloid deposition in the heart typically occurs within the myocardium and the vessels. Hereditary hemochromatosis is a genetic defect in iron absorption that results in extensive myocardial iron deposition (hemosiderosis). Hyaline change is a descriptive term used by histologists to describe protein deposits that are glassy and pale pink. The amount of lipofuscin increases within myocardial fibers (not valves) with aging.

An experiment introduces a knockout gene mutation into a cell line. The frequency of shrunken cells with chromatin clumping, karyorrhexis, and cytoplasmic blebbing is increased compared with a cell line without the mutation. Overall survival of the mutant cell line is reduced. Which of the following genes is most likely to be affected by this mutation? A BAX B BCL2 C C-MYC D FAS E p53

*The answer is B.* These histologic findings are typical of apoptosis. The BCL2 gene product inhibits cellular apoptosis by binding to Apaf-1. Hence, the knockout removes this inhibition The BAX gene product promotes apoptosis, and a knockout would protect against apoptosis. The C-MYC gene is involved with oncogenesis. The FAS gene encodes for a cellular receptor for Fas ligand that signals apoptosis. Activity of the p53 (TP53) gene normally stimulates apoptosis, but mutation favors cell survival.

In a clinical trial, a chemotherapeutic agent is given to patients with breast cancer metastases. Samples of the cancer cells are obtained and assessed for the presence of death of tumor cells by apoptosis. Mutational inactivation of which of the following products is most likely to render tumor cells resistant to the effects of such an agent? (A) BCL-2 (B) p53 (C) NF-κB (D) Cytochrome P-450 (E) Granzyme B

*The answer is B.* When DNA damage is induced by chemotherapeutic drugs (or other agents), normal p53 genes trigger the cells to undergo apoptosis. When p53 is inactivated, this pathway of cell death can be blocked, rendering the chemotherapy less effective. BCL-2 and NF-κB activity favor cell survival. Cytochrome P-450 does not affect apoptosis. Granzyme B can trigger apoptosis, but it is found in cytotoxic T cells and not in tumor cells.

A 40-year-old man underwent laparotomy for a perforated sigmoid colon diverticulum. A wound infection complicated the postoperative course, and surgical wound dehiscence occurred. Primary closure was no longer possible, and the wound "granulated in." Six weeks later, the wound is only 10% of its original size. Which of the following processes best accounts for the observed decrease in wound size over the past 6 weeks? (A) Increase in synthesis of collagen (B) Myofibroblast contraction (C) Inhibition of metalloproteinases (D) Resolution of subcutaneous edema (E) Elaboration of adhesive glycoproteins

*The answer is B.* Wound contraction is a characteristic feature of healing by second intention that occurs in larger wounds. Collagen synthesis helps fill the defect, but does not contract it. The inhibition of metalloproteinases leads to decreased degradation of collagen and impaired connective tissue remodeling in wound repair. Edema diminishes over time, but this does not result in much contraction. Adhesive glycoproteins such as fibronectin help to maintain a cellular scaffolding for growth and repair, but they do not contract

Which is true regarding Rb protein? A. It is the gatekeeper of the restriction point at G2-M B. Phosphorylated Rb leads to arrest of the cell cycle C. Phosphorylation of Rb leads to progression of the cell cycle. D. Rb associates with E2F; the cell is then committed to mitosis

*The answer is C.* Active Rb (Retinoblastoma protein) inactivates E2F protein. CDKs (cyclin-dependent kinases) from G1 stage of the cell cycle phosphorylate Rb, thereby inactivating it. E2F is now active and free to turn on genes required for progression of cell cycle into S stage. The brake, from Rb, is disengaged.

A 71-year-old man diagnosed with pancreatic cancer is noted to have decreasing body mass index. His normal connective tissues undergo atrophy by sequestering organelles and cytosol in a vacuole, which then fuses with a lysosome. However, the cancer continues to increase in size. Which of the following processes is most likely occurring in the normal cells but is inhibited in the cancer cells of this man? A Aging B Apoptosis C Autophagy D Hyaline change E Karyorrhexis

*The answer is C.* Autophagy is a form of cellular downsizing in response to stress, as the cell consumes itself, by upregulating Atgs genes. Lipofuscin granules are residual bodies left over from this process. Cell death may eventually be triggered by autophagy, but by a different mechanism than apoptosis, a form of single cell necrosis in which cell fragmentation occurs. Cancer cells acquire the ability to prevent autophagy, perhaps by downregulating PTEN gene expression, and maintain a survival advantage even as the patient is dying. There is slow autophagy with aging, but autophagy is accelerated with stressors such as malnutrition and chronic disease. Hyaline is a generic term for intracellular or extracellular protein accumulations appearing pink and homogeneous with H&E staining. Karyorrhexis is nuclear fragmentation in a necrotic cell.

A 55-year-old man presents to his primary care physician for his annual check-up. He says that he has generally been feeling well; however, he has noticed that he has had some intermittent very low grade fevers of unknown etiology. His past medical history is significant for well-controlled hypertension and diabetes. On physical exam he is found to have painless enlargement of a number of cervical and axillary lymph nodes. A biopsy of the enlarged nodes is obtained and genetic testing shows that part of translocation that involves the BCl-2 gene. Which of the following would most likely be observed in cells with this genetic change? A. Decreased activation of FAS B. Decreased activity of p53 C. Decreased release of cytochrome c D. Increased activation of p21 E. Increased activation of FASL

*The answer is C.* BCL-2 is an anti-apoptotic factor, which would decrease the release of cytochrome C.

The triggering of the intrinsic pathway of apoptosis involves a balance between pro-apoptotic and anti-apoptotic proteins. Which of the following is anti-apoptotic? a) Bax b) Bak c) Bcl-2 d) Cytochrome c

*The answer is C.* Bax and Bak are pro-apoptotic proteins. Bcl-2, is anti apoptotic. The key event which triggers the start of the intrinsic pathway of apoptosis is the release of cytochrome c from the mitochondrion. Cytochrome c is therefore pro-apoptotic. Bcl-2, controls the permeability of mitochondria. In healthy cells, Bcl-2 and the related protein Bcl-xL, maintain the integrity of mitochondrial membranes, in large part by holding two proapoptotic members of the family, Bax and Bak, in check. When cells are deprived of growth factors and survival signals, or are exposed to agents that damage DNA, or accumulate unacceptable amounts of misfolded proteins, a number of sensors are activated called BH3 proteins. They in turn shift this delicate, life-sustaining balance in favor of pro-apoptotic Bak and Bax. As a result, Bak and Bax dimerize, insert into the mitochondrial membrane, and form channels through which cytochrome c and other mitochondrial proteins escape into the cytosol. After cytochrome c enters the cytosol, it, together with certain cofactors, activates caspase-9. The net result is the activation of a caspase cascade, ultimately leading to nuclear fragmentation and formation of apoptotic bodies.

A 40-year-old man is pulled from the ocean after a boating accident and resuscitated. Six hours later, the patient develops acute renal failure. Kidney biopsy reveals evidence of karyorrhexis and karyolysis in renal tubular epithelial cells. Which of the following biochemical events preceded these pathologic changes? (A) Activation of Na+/K+ ATPase (B) Decrease in intracellular calcium (C) Decrease in intracellular pH (D) Increase in ATP production (E) Increase in intracellular pH

*The answer is C.* During periods of ischemia, anaerobic glycolysis leads to the overproduction of lactate and a decrease in intracellular pH. Lack of O2 during myocardial ischemia blocks the production of ATP. Pyruvate is reduced to lactate in the cytosol and lowers intracellular pH. The acidification of the cytosol initiates a downward spiral of events that propels the cell toward necrosis. The other choices point to changes in the opposite direction of what would be expected in irreversible cell injury. Diagnosis: Acute tubular necrosis

An experiment analyzes cells for enzyme activity associated with sustained cellular proliferation. Which of the following cells is most likely to have the highest telomerase activity? A Endothelial cells B Erythrocytes C Germ cells D Neurons E Neutrophils

*The answer is C.* Germ cells have the highest telomerase activity, and the telomere length can be stabilized in these cells. This allows testicular germ cells to retain the ability to divide throughout life. Normal somatic cells have no telomerase activity, and telomeres progressively shorten with each cell division until growth arrest occurs. Erythrocytes do not even have a nucleus.

In a study of viral hepatitis infection, it is observed that cytotoxic T lymphocytes (CTLs) induce death in virally infected hepatocytes. The CTLs release perforin to allow entry of their granules. Which of the following substances is found in those granules that directly activates programmed cell death? A BCL2 B Endonuclease C Granzyme B D Nitric oxide E p53

*The answer is C.* Granzyme B is a serine protease found in CTLs that can directly trigger apoptosis. CTLs express Fas ligand on their surfaces, and when contacting Fas receptors on the target cell, the ligand can induce apoptosis by the extrinsic (death receptor-initiated) pathway. BCL2 favors cell survival. Nitric oxide helps destroy phagocytized microbes. Endonucleases are generated following caspase activation and lead to nuclear fragmentation. When p53 is activated by intrinsic DNA damage during cell proliferation, apoptosis is triggered. Mutations in p53 may allow accumulation of genetic damage, a process that promotes unregulated cell growth (neoplasia).

A 62-year-old man has had increasing knee pain with movement for the past 10 years. The knee joint surfaces are eroded and the joint space narrowed. There is loss of compressibility and lubrication of articular cartilaginous surfaces. Loss of which of the following extracellular matrix components has most likely occurred in this man? A Elastin B Fibronectin C Hyaluronan D Integrin E Laminin

*The answer is C.* He has osteoarthritis, or degenerative joint disease, with loss of articular hyaline cartilage. Hyaluronan (hyaluronic acid) is a large mucopolysaccharide, one form of proteoglycan, which forms a hydrated, compressible gel contributing to the shock-absorbing function of joint surfaces. Elastin is a fibrillar protein that provides recoil in tissues such as skin, arterial walls, and ligaments that need to stretch and return to their original shape. Fibronectin is a form of glycoprotein that serves an adhesive function. Integrins are glycoproteins that serve as cellular receptors for extracellular matrix components; they can link to intracellular actin so that cells can alter their shape and mobility. Laminins are a form of glycoprotein that help to anchor epithelial surfaces in basement membranes.

In an experiment involving observations on wound healing, researchers noted that intracytoplasmic cytoskeletal elements, including actin, interact with the extracellular matrix to promote cell attachment and migration in wound healing. Which of the following substances is most likely responsible for such interaction between the cytoskeleton and the extracellular matrix? (A) Epidermal growth factor (B) Fibronectin (C) Integrin (D) Platelet-derived growth factor (E) Type IV collagen (F) Vascular endothelial growth factor

*The answer is C.* Integrins interact with the extracellular matrix proteins (e.g., fibronectin). Engagement of integrins by extracellular matrix proteins leads to the formation of focal adhesions at which integrins link to intracellular cytoskeletal elements such as actin. These interactions lead to intracellular signals that modulate cell growth, differentiation, and migration during wound healing. Epidermal growth factor stimulates epithelial cell and fibroblast proliferation. Platelet-derived growth factor (PDGF) can be produced by endothelium, macrophages, smooth muscle cells, and platelets; PDGF mediates migration and proliferation of fibroblasts and smooth muscle cells and migration of monocytes. Type IV collagen is found in basement membranes on which cells are anchored. Vascular endothelial growth factor promotes angiogenesis (capillary proliferation) through endothelial cell proliferation and migration in a healing response.

Which of the following proteins is a death receptor which triggers the extrinsic pathway of apoptosis? a) caspase-8 b) FADD c) Fas d) Fas ligand

*The answer is C.* Killer T cells, which are able to signal target cells to undergo apoptosis, carry on their surface a protein complementary to Fas, called the Fas ligand. When this binds to Fas on a target cell it stimulates the initiation of apoptosis. Fas activates death domain (FADD) binds to the cytosolic domain of the Fas receptor. Procaspase-8 then binds as a cluster to the FADD-receptor complex and is activated by cleavage. Once active caspase-8 is formed a proteolytic caspase cascade ensues, leading to activation of the same effector caspases as in the intrinsic pathway.

A 69-year-old woman has had a chronic cough for the past year. A chest radiograph shows a 6-cm mass in the left lung. A needle biopsy specimen of the mass shows carcinoma. A pneumonectomy is performed, and examination of the hilar lymph nodes reveals a uniform, dark black cut surface. Which of the following factors most likely accounts for the appearance of these lymph nodes? A Aging effects B Bleeding disorder C Cigarette smoking D Liver failure E Multiple metastases

*The answer is C.* Lung and hilar lymph nodes accumulate anthracotic pigmentation when carbon pigment is inhaled from polluted air. The tar in cigarette smoke is a major source of such carbonaceous pigment. Older individuals generally have more anthracotic pigment, but this is not inevitable with aging—individuals living in rural areas with good environmental air quality have less pigment. Resolution of hemorrhage can produce hemosiderin pigmentation, which imparts a brown color to tissues. Hepatic failure may result in jaundice, characterized by a yellow color in tissues. Metastases are mass lesions that impart a tan-to-white appearance to tissues.

A 10-year-old boy undergoes an appendectomy. Granulation tissue develops normally at the incision site. Tissue remodeling begins at this site with degradation of collagen in the extracellular matrix by which of the following proteins? (A) Cytokines (B) Lipoxygenases (C) Metalloproteinase (D) Nitric oxide (E) Plasminogen activator

*The answer is C.* Metalloproteinase degrades collagen in the extracellular matrix.

A man decides to restrict his dietary caloric content so that his body mass index is very low. He fully expects to live longer because he has read that caloric restriction prolongs life. In this man, which of the following intracellular substances will most likely mediate the effect of calorie restriction upon increased longevity? A. Caspase B. Glutathione C. mTOR D Telomerase

*The answer is C.* Reduced IGF-1 signaling lower rates of cell growth and metabolism and possibly reduced errors in DNA replication, better DNA repair and improve protein homeostasis. Calorie restriction also serves to improve immunity. And when you block mTOR, you block aging.

You are asked to present a grand rounds seminar on the role of abnormal proteins in disease. In this connection, intracellular accumulation of an abnormally folded protein plays a role in the pathogenesis of which of the following diseases? (A) AA amyloidosis (B) AL amyloidosis (C) α1-Antitrypsin deficiency (D) Gaucher disease (E) Tay-Sachs disease

*The answer is C.* Several acquired and inherited diseases are characterized by intracellular accumulation of abnormal proteins. The deviant tertiary structure of the protein may result from an inherited mutation that alters the normal primary amino acid sequence, or may reflect an acquired defect in protein folding. α1-Antitrypsin deficiency is a heritable disorder in which mutations in the gene for α1-antitrypsin yield an insoluble protein. The mutant protein is not easily exported. It accumulates in liver cells, causing cell injury and cirrhosis. Pulmonary emphysema is another complication of α1-antitrypsin deficiency. Choices A and B are amyloidoses that represent extracellular deposits of fibrillar proteins arranged in β-pleated sheet. Choices D and E are lysosomal storage diseases that represent intracellular deposits of unmetabolized sphingolipids. Diagnosis: a1-Antitrypsin defi ciency

Which cellular organelles are involved in the initiation of the intrinsic pathway of apoptosis? a) endoplasmic reticulum b) lysosomes c) mitochondria d) peroxisomes

*The answer is C.* The initiating procaspases of the intrinsic apoptosis pathway are attached to the outer membrane of the mitochondria. Thus the mitochondria are the organelles involved in the initiation of internally stimulated apoptosis. In some circumstances incorrectly folded proteins that accumulate in the endoplasmic reticulum can be transported to the cytoplasm where they trigger apoptosis directly. However, this is an unusual mode of initiation which occurs rarely.

A 4-year-old boy is brought by his mother to the emergency room for malaise, dizziness, and sleepiness. The mother owns a dry cleaning shop and found her son in the back room with an open canister of carbon tetrachloride, one of their cleaning fluids. The boy reports feeling nauseous and has a mild headache. He is otherwise healthy and takes no medications. On exam, he appears tired and drowsy but is able to answer questions. Which of the following is most likely to be affected by this patient's exposure to the dry cleaning fluid? A. Bone marrow B. Gastric mucosa C. Hepatocytes D. Lung parenchyma E. Myocardium

*The answer is C.* The patient in this vignette presents with exposure to carbon tetrachloride, an organic compound that is used in the dry cleaning industry as a cleaning solution. Carbon tetrachloride causes free radical injury to hepatocytes. Free radicals are compounds with unpaired electrons that can initiate a cascade of damage in which one free radical generates additional free radicals. This ultimately leads to cell membrane peroxidation and DNA and protein damage. Carbon tetrachloride is a known free radical generator that is used as a solvent in the dry cleaning industry. It is a potent hepatotoxin and can lead to hepatocyte damage with associated fatty liver changes. Patients who are exposed to carbon tetrachloride should undergo immediate liver testing with appropriate follow-up.

What is this wear and tear pigment called? A. anthracosis B. steatosis C. lipofuscin D. hemosiderosis

*The answer is C.* The wear and tear pigment is called lipofuscin.

A 22-year-old woman has a congenital anemia. She has required multiple transfusions of red blood cells for many years. She now has no significant findings on physical examination. Which of the following microscopic findings would most likely present in her liver? A Steatosis in hepatocytes B Bilirubin in canaliculi C Hemosiderin in hepatocytes D Glycogen in hepatocytes E Amyloid in portal triads

*The answer is C.* There is 250 mg of iron in each unit of blood. The body has no mechanism for getting rid of excess iron. A small amount of iron is lost with normal desquamation of epithelia, and menstruating women will lose a bit more. The excess iron becomes storage iron, or hemosiderin. Over time, hemosiderosis involves more and more tissues of the body, particularly the liver.

A 47-year-old man has a lung carcinoma with metastases. He receives chemotherapy. A month later, histologic examination of a metastatic lesion shows many foci in which individual tumor cells appear shrunken and deeply eosinophilic. Their nuclei exhibit condensed aggregates of chromatin under the nuclear membrane. The pathologic process affecting these shrunken tumor cells is most likely triggered by release of which of the following substances into the cytosol? A BCL2 B Catalase C Cytochrome c D Lipofuscin E Phospholipase

*The answer is C.* This histologic picture is typical of apoptosis produced by chemotherapeutic agents. The release of cytochrome c from the mitochondria is a key step in many forms of apoptosis, and it leads to the activation of caspases. BCL2 is an antiapoptotic protein that prevents cytochrome c release and prevents caspase activation. Catalase is a scavenger of hydrogen peroxide. Lipofuscin is a pigmented residue representing undigested cellular organelles in autophagic vacuoles, much like old clothes in a closet. Phospholipases are activated during necrosis and cause cell membrane damage.

An 18-year old G2 P1 woman has an amniocentesis performed at 17 weeks gestation. Fibroblasts recovered from amniocentesis are grown in culture to assess the karyotype of fetal cells. These cells are subcultured for additional experimental work, but the culture is lost after 50 doublings of the cells has occurred, and the fibroblasts no longer grow. Which of the following factors affecting these cells is most likely demonstrated by this phenomenon? A Nutrition B Mutation C Apoptosis D Aging E Oxidation

*The answer is D.* A number of factors play a role in aging, but the lack of immortality of individual cells is one feature. The lack of telomerase activity in most somatic cells prevents repeated division. Stem cells have greater replicative capacity.

Which is NOT a feature of apoptotic cell death? A. Fragmentation of DNA by endonucleases B. Caspase activation C. Lack of inflammation D. Spillage of intracellular contents into the microenvironment

*The answer is D.* Apoptotic cell death does not include spillage of intracellular contents into the microenvironment.

A 38-year-old man has had headaches and nausea for the past 2 months. Laboratory findings show hypercalcemia and hypophosphatemia and normal serum albumin. Urine microscopic analysis shows deposition of calcium salts in the renal tubular epithelium. Which of the following processes has most likely produced this change in the kidney? A. Dystrophic calcification B. Renal tubular atrophy C. Autophagocytosis D. Metastatic calcification E. Cellular aging

*The answer is D.* Deposition of calcium in normal healthy tissues as a result of prolonged hypercalcemia is called metastatic calcification. This process may occur in hyperparathyroidism. Dystrophic calcification refers to calcium deposition in injured tissues, with normal serum calcium levels. Atrophy decreases cell size but is not accompanied by calcium deposition. Autophagocytosis yields more golden-brown lipofuscin pigment in the cytoplasm, particularly in hepatocytes and myocardial fibers, a process that becomes more apparent with aging.

A 22-year-old woman from Albania has a congenital anemia requiring multiple transfusions of RBCs for many years. On physical examination, her skin has a bronze color. Liver function tests show reduced serum albumin. Which of the following findings would most likely appear in a liver biopsy specimen? A Amyloid in portal triads B Bilirubin in canaliculi C Glycogen in hepatocytes D Hemosiderin in hepatocytes E Steatosis in hepatocytes

*The answer is D.* Each unit of blood contains about 250 mg of iron. The body has no mechanism for getting rid of excess iron. About 10 to 20 mg of iron per day is lost with normal desquamation of epithelia; menstruating women lose slightly more. Any excess iron becomes storage iron, or hemosiderin. Over time, hemosiderosis involves more and more tissues of the body, particularly the liver, but also skin. Initially, hemosiderin deposits are found in Kupffer cells and other mononuclear phagocytes in the bone marrow, spleen, and lymph nodes. With great excess of iron, liver cells also accumulate iron. Amyloid is an abnormal protein derived from a variety of precursors, such as immunoglobulin light chains. Bilirubin, a breakdown product of blood, can be excreted in the bile so that a person does not become jaundiced. Glycogen storage diseases are inherited and present in childhood. Steatosis usually occurs with ingestion of hepatotoxins, such as alcohol.

Which of the following does not affect the rate and efficacy of wound healing? A. Immunosuppression B. Vitamin C deficiency C. Bacterial infection D. Gender

*The answer is D.* Gender does not affect the rate and efficacy of wound healing.

A 45-year-old man presents with increasing abdominal girth and yellow discoloration of his skin and sclera. Physical examination reveals hepatomegaly and jaundice. A Prussian blue stain of a liver biopsy is shown in the image. What is the major intracellular iron storage protein in this patient's hepatocytes? (A) Bilirubin (B) Haptoglobin (C) Hemoglobin (D) Hemosiderin (E) Transferrin

*The answer is D.* Hemosiderin is a partially denatured form of ferritin that aggregates easily and is recognized microscopically as yellow-brown granules in the cytoplasm, which turn blue with the Prussian blue reaction. In hereditary hemochromatosis, a genetic abnormality of iron absorption in the small intestine, excess iron is stored mostly in the form of hemosiderin, primarily in the liver. Hemoglobin (choice C) is the iron-containing pigment of RBCs. Bilirubin (choice A) is a product of heme catabolism that may accumulate in liver cells but does not stain with Prussian blue. Transferrin (choice E) binds serum iron. Diagnosis: Hereditary hemochromatosis

A 40-year-old woman has had chronic congestive heart failure for the past 3 years. In the past 2 months, she developed a cough productive of rust-colored sputum. A sputum cytology specimen now shows numerous hemosiderin-laden macrophages. Which of the following subcellular structures in these macrophages is most important for the accumulation of this pigment? A Chromosome B Endoplasmic reticulum C Golgi apparatus D Lysosome E Ribosome

*The answer is D.* Heterophagocytosis by macrophages requires that endocytosed vacuoles fuse with lysosomes to degrade the engulfed material. With congestive heart failure, extravasation of RBCs into alveoli occurs, and pulmonary macrophages must phagocytose the RBCs, breaking down the hemoglobin and recycling the iron by hemosiderin formation. The other listed options are components that play a role in cell synthetic functions.

At autopsy, the heart of a 63-year-old man weighs only 250 g and has small right and left ventricles. The myocardium is firm, with a dark chocolate-brown color throughout. The coronary arteries show minimal atherosclerotic changes. An excessive amount of which of the following substances would most likely be found in the myocardial fibers of this heart? (A) Melanin (B) Hemosiderin (C) Glycogen (D) Lipofuscin (E) Bilirubin

*The answer is D.* Lipofuscin is a "wear-and-tear" pigment that increases with aging, particularly in liver and myocardium. The pigment has minimal effect on cellular function in most cases. Rarely, there is marked lipofuscin deposition in a small heart, a so-called brown atrophy. Melanin pigment is responsible for skin tone: the more melanin, the darker the skin. Hemosiderin is the breakdown product of hemoglobin that contains the iron. Hearts with excessive iron deposition tend to be large. Glycogen is increased in some inherited enzyme disorders, and when the heart is involved, heart size increases. Bilirubin, another breakdown product of hemoglobin, imparts a yellow appearance (icterus) to tissues

At autopsy, the heart of a 63-year-old man weighs only 250 g (normal 330 g) and has small right and left ventricles. The myocardium is firm, with a dark chocolate-brown color throughout. The coronary arteries show minimal atherosclerotic changes. An excessive amount of which of the following substances, shown in the figure, would most likely be found in the myocardial fibers of this heart? A Bilirubin B Glycogen C Hemosiderin D Lipofuscin E Melanin

*The answer is D.* Lipofuscin is a "wear-and-tear" pigment that increases with aging, particularly in liver and myocardium. This granular golden brown pigment seen adjacent to the myocyte nucleus in the figure has minimal effect on cellular function in most cases. Rarely, there is marked lipofuscin deposition in a small heart, a so-called brown atrophy. Bilirubin, another breakdown product of hemoglobin, imparts a yellow appearance (icterus) to tissues. Hemosiderin is the breakdown product of hemoglobin that contains the iron. Hearts with excessive iron deposition tend to be large. Glycogen is increased in some inherited enzyme disorders, and when the heart is involved, heart size increases. Melanin pigment is responsible for skin tone: the more melanin, the darker the skin.

A 90-year-old woman with mild diabetes and Alzheimer disease dies in her sleep. At autopsy, hepatocytes are noted to contain golden cytoplasmic granules that do not stain with Prussian blue Which of the following mechanisms of disease best describes the pathogenesis of pigment accumulation in hepatocytes in the patient described? (A) Degradation of melanin pigments (B) Inhibition of glycogen biosynthesis (C) Malabsorption and enhanced deposition of iron (D) Peroxidation of membrane lipids (E) Progressive oxidation of bilirubin

*The answer is D.* Lipofuscin is found in lysosomes and contains peroxidation products of unsaturated fatty acids. The presence of this pigment is thought to reflect continuing lipid peroxidation of cellular membranes as a result of inadequate defenses against activated oxygen radicals. None of the other mechanisms of disease leads to the formation and accumulation of lipofuscin granules.

A CT scan of a 43-year-old woman with a parathyroid adenoma and hyperparathyroidism reveals extensive calcium deposits in the lungs and kidney parenchyma. These radiologic findings are best explained by which of the following mechanisms of disease? (A) Arteriosclerosis (B) Dystrophic calcifi cation (C) Granulomatous infl ammation (D) Metastatic calcification (E) Tumor embolism

*The answer is D.* Metastatic calcification is associated with an increased serum calcium concentration (hypercalcemia). Almost any disorder that increases serum calcium levels can lead to calcification in the alveolar septa of the lung, renal tubules, and blood vessels. The patient in this case had a parathyroid adenoma that produced large quantities of parathyroid hormone. Other examples of metastatic calcification include multiple opacities in the cornea of a child given large amounts of vitamin D and partially calcified alveolar septa in the lungs of a patient with breast cancer metastatic to bone. Breast cancer metastases to bone are often osteolytic and, therefore, accompanied by hypercalcemia. Dystrophic calcifi cation (choice B) has its origin in direct cell injury. Arteriosclerosis (choice A) is an example of dystrophic calcification.

Which of the following are killed by the extrinsic apoptosis pathway? a) Cells with damaged DNA. b) Developing nerve cells that fail to make profitable connections. c) Irradiated cells. d) Virus infected cells.

*The answer is D.* The extrinsic apoptosis pathway is triggered by signals from outside the cell. Virus infected cells are recognized by cytotoxic T-lymphocytes, which activate the extrinsic apoptosis pathway. Other signals that arise outside the cell can activate intrinsic apoptosis. For example nerve cells that fail to make profitable connections do not receive a growth factor signal. The removal of this signal allows intrinsic apoptosis to be activated. Factors that stress and damage cells, such as toxins, radiation or short term oxygen depletion, can stimulate intrinsic apoptosis even though they originate outside the cell. This may be because they damage DNA, though other more specific trigger mechanisms may be involved. Damaged DNA in a cell results in an increased amount of the protein p53. This initially increases production of DNA repair enzymes, but if the DNA is not repaired in a reasonable time p53 signals the activation of the intrinsic apoptosis pathway.

While in a home improvement center warehouse buying paint, a 35-year-old man hears 'Look out below!' and is then struck on the leg by a falling pallet rack, which strikes him on his left leg in the region of his thigh. The skin is not broken. Within 2 days there is a 5 x 7 cm purple colour to the site of injury. Which of the following substances has most likely accumulated at the site of injury to produce a yellow-brown colour at the site of injury 16 days later? A Lipofuscin B Bilirubin C Melanin D Hemosiderin

*The answer is D.* The iron in the heme pigment from the red blood cells in the hemorrhage beneath the skin is incorporated into hemosiderin granules that impart the yellow to brown colour of the healing contusion (bruise) from blunt force trauma.

A 28-year-old man with a history of radiation/bone marrow transplantation for leukemia presents with severe diarrhea. He subsequently develops septic shock and expires. Microscopic examination of the colon epithelium at autopsy reveals numerous acidophilic bodies and small cells with pyknotic nuclei. Which of the following proteins most likely played a key role in triggering radiation induced cell death in this patient's colonic mucosa? (A) Cytochrome P450 (B) β-Catenin (C) E-Cadherin (D) P-Selectin (E) p53

*The answer is E.* Apoptosis detects and destroys cells that harbor dangerous mutations, thereby maintaining genetic consistency and preventing the development of cancer. There are several means, the most important of which is probably p53, by which the cell recognizes genomic abnormalities and "assesses" whether they can be repaired. If the damage to DNA is so severe that it cannot be repaired, the cascade of events leading to apoptosis is activated, and the cell dies. This process protects an organism from the consequences of a nonfunctional cell or one that cannot control its own proliferation (e.g., a cancer cell). After it binds to areas of DNA damage, p53 activates proteins that arrest the cell in G1 of the cell cycle, allowing time for DNA repair to proceed. It also directs DNA repair enzymes to the site of injury. If the DNA damage cannot be repaired, p53 activates mechanisms that terminate in apoptosis. There are several pathways by which p53 induce apoptosis. This molecule downregulates transcription of the antiapoptotic protein Bcl-2, while it upregulates transcription of the proapoptotic genes bax and bak. Cytochrome P450 (choice A) is a member of the mixed function oxidase system. β-Catenin (choice B) is a membrane protein associated with cell adhesion molecules. Selectins (choices C and D) are cell adhesion molecules involved in leukocyte recirculation. Diagnosis: Apoptosis

A 45-year-old woman has had worsening dyspnea for the past 5 years. A chest CT scan shows panlobular emphysema. Laboratory studies show a deficiency of α1-antitrypsin (AAT). Her AAT genotype is PiZZ. A liver biopsy specimen examined microscopically shows abundant PAS-positive globules within periportal hepatocytes. Which of the following molecular mechanisms is most likely responsible for this finding in her hepatocytes? A Decreased catabolism of AAT in lysosomes B Excessive hepatic synthesis of AAT C Impaired dissociation of AAT from chaperones D Inability to metabolize AAT in Kupffer cells E Retained misfolded AAT in endoplasmic reticulum

*The answer is E.* Mutations in the AAT gene give rise to AAT molecules that cannot fold properly. In the PiZZ genotype, both alleles have the mutation. The partially folded molecules accumulate in hepatocyte endoplasmic reticulum and cannot be secreted. Impaired dissociation of the CFTR protein from chaperones causes many cases of cystic fibrosis. There is no abnormality in the synthesis, catabolism, or metabolism of AAT in patients with AAT deficiency. AAT is the major circulating alpha globulin that protects tissues such as lung from damaging proteases.

In an experiment, surgical incisions are made in a study group of laboratory rats. Observations about the wounds are recorded over a 2-week period using various chemical mediators. Which of the following steps in the inflammatory-repair response is most likely affected by neutralization of transforming growth factor β (TGF-β)? A Chemotaxis of lymphocytes B Increase in vascular permeability C Leukocyte extravasation D Migration of epithelial cells E Production of collagen

*The answer is E.* TGF-β stimulates many steps in fibrogenesis, including fibroblast chemotaxis and production of collagen by fibroblasts, while inhibiting degradation of collagen. All of the other steps listed are unaffected by TGF-β.

A 60-year-old woman developed chest pain that persisted for 4 hours. A radiographic imaging procedure showed an apparent myocardial infarction involving a 3 × 4 cm area of the posterior left ventricular free wall. Laboratory findings showed serum creatine kinase of 600 U/L. The patient received anti-arrhythmic and pressor agents to treat the decreased cardiac output while in the hospital. Which of the following pathologic findings would most likely be seen in the left ventricle 1 month later? (A) Abscess (B) Complete resolution (C) Coagulative necrosis (D) Nodular regeneration (E) Fibrous scar

*The answer is E.* The elevated creatine kinase level indicates that myocardial necrosis has occurred. The destruction of myocardial fibers precludes complete resolution. The area of myocardial necrosis is gradually replaced by a fibrous scar. Liquefactive necrosis with abscess formation is not a feature of ischemic myocardial injury. Coagulative necrosis is typical of myocardial infarction, but after 1 month, a scar would be present. Nodular regeneration is typical of hepatocyte injury because hepatocytes are stable cells.

A 24-year-old man with acute appendicitis undergoes surgical removal of the inflamed appendix. The incision site is sutured. A trichrome-stained section of the site is shown in the figure. How long after the surgery would this appearance most likely be seen? (A) 1 day (B) 2 to 3 days (C) 4 to 5 days (D) 1 weeks (E) 1 month

*The answer is E.* The figure shows dense collagen with some remaining dilated blood vessels, typical of the final phase of wound healing, which is extensive by the end of the first month. On day 1, the wound is filled only with fibrin and inflammatory cells. Macrophages and granulation tissue are seen 2 to 3 days postoperatively. Neovascularization is most prominent by days 4 and 5. By week 2, collagen is prominent, and fewer vessels and inflammatory cells are seen.

Which of the following occurs in the late stage of tissue repair ? A. Acute inflammation B. Thrombosis C. Infarction D. Granulation Tissue Formation E. Tissue Remodeling

*The answer is E.* Tissue remodeling occurs in the late stage of tissue repair.

An experiment is conducted in which cells in tissue culture are subjected to high levels of ultraviolet radiant energy. Electron microscopy shows cellular damage in the form of increased cytosolic aggregates of denatured proteins. In situ hybridization reveals that protein components in these aggregates also are found in proteasomes. Which of the following substances most likely binds to the denatured proteins, targeting them for catabolism by cytosolic proteasomes? A Adenosine monophosphate B Calcium C Caspase D Granzyme B E Hydrogen peroxide F Ubiquitin

*The answer is F.* Heat-shock proteins provide for a variety of cellular "housekeeping" activities, including recycling and restoration of damaged proteins and removal of denatured proteins. Ubiquitin targets denatured proteins and facilitates their binding to proteasomes, which then break down the proteins to peptides. ADP increases when ATP is depleted, helping to drive anaerobic glycolysis. Cytosolic calcium levels may increase with cell injury that depletes ATP; the calcium activates phospholipases, endonucleases, and proteases, which damage the cell membranes, structural proteins, and mitochondria. Caspases are enzymes that facilitate apoptosis. Granzyme B is released from cytotoxic T lymphocytes and triggers apoptosis. Hydrogen peroxide is one of the activated oxygen species generated under conditions of cellular ischemia, producing nonspecific damage to cellular structures, particularly membranes.