Exam 4- Cell Renewal, Death, and Cancer - Comprehensive Notes

Cell Renewal, Death, and Cancer

Stem Cells and the Maintenance of Adult Tissues

• Cell Renewal Outline
  • Stem cells
  • Maintenance of adult tissues
  • Embryonic stem cells

Introduction

• Cell death and cell proliferation are balanced throughout the life of multicellular organisms.
• Animal development involves cell proliferation, differentiation, and cell death.
• Most cell death occurs by a normal physiological process of programmed cell death (apoptosis).

Introduction: Early Development vs. Adult Cell Dynamics

• Early Development
  • Rapid proliferation of embryonic cells.
  • Differentiation into specialized cells for adult tissues and organs.
• Adult Cell Dynamics:
  • Most adult cells are arrested in the G0 stage.
  • Cell loss is balanced by regulated cell proliferation.
• Tissue Maintenance
  • Proliferative cells replace those lost due to injury or programmed cell death.
  • Some tissues have continuous cell division for high turnover and proper maintenance.

Stem Cells and the Maintenance of Adult Tissues

• In early development, cells divide rapidly and differentiate into specialized cells of adult tissues and organs.
• To maintain a constant number of cells in adult tissues, cell death must be balanced by cell proliferation.
• Most differentiated cells in adult animals are no longer capable of proliferation.
• To maintain tissues, rely on proliferation of cells derived from self-renewing stem cells.
• Adult tissues experience damage, and some differentiated cells can proliferate to repair damaged tissue.

Stimuli for Proliferation

• Fibroblasts respond to platelet-derived growth factor (PDGF) after injury.
• Endothelial cells proliferate in response to vascular endothelial growth factor (VEGF) triggered by low oxygen (hypoxia).
• Liver cells can rapidly regenerate after partial removal, showcasing tissue repair capacity.

Skin Fibroblasts

• Skin fibroblasts are stem cells involved in tissue repair.
• Fibroblasts in G0 in connective tissue can proliferate quickly in response to platelet-derived growth factor (PDGF) released at the site of a wound.
• They are involved in healing and scarring.

Endothelial Cells

• Endothelial cells that line blood vessels can proliferate.
• They form new blood vessels for repair and regrowth of damaged tissue.
• Blood vessels deliver oxygen, nutrients, and growth factors required to keep tissues alive.
• Endothelial cell proliferation is triggered by vascular endothelial growth factor (VEGF).
• VEGF is produced by cells that lack oxygen (hypoxic).

Stem Cells in Liver Tissue

• The epithelial cells of some internal organs can proliferate to replace damaged tissue.
• Liver cells, normally arrested in the G0 phase of the cell cycle, are stimulated to proliferate if large numbers of liver cells are lost (e.g., by surgical removal).

Stem Cell Proliferation and Differentiation

• Key Property of Stem Cells:
  • Stem cells divide, producing one stem cell and one differentiating daughter cell.
  • Differentiation leads to various specialized cells, ensuring tissue renewal.
• Self-Renewal and Differentiation:
  • Stem cells are self-renewing; they produce new stem cells and differentiated cells.
  • Examples: Blood cells, sperm, skin epithelial cells, digestive tract lining cells.

Stem Cells That Commonly Maintain Adult Tissues

• Many types of cells have short lifespans and must be continually replaced by proliferation of stem cells.
• These include blood cells, sperm, and epithelial cells of the skin and lining the digestive tract.
• These are the tissues most impacted by chemotherapy.

Examples of Stem Cells in Action

• Hematopoietic Stem Cells:
  • Identified in the blood-forming system.
  • Produce diverse blood cell types, ensuring a constant supply of functional blood cells.
• Skin and Hair Renewal:
  • Stem cells in skin, hair follicles, and sebaceous glands continuously renew tissues.
  • Responsive to external environment and injuries, ensuring effective regeneration.
• Muscle Regeneration:
  • Satellite cells in skeletal muscle enable rapid regeneration in response to injury or exercise.
  • Satellite cells proliferate, differentiate, and fuse to form new muscle fibers.
• Challenges and Niches:
  • Identifying and understanding stem cells within microenvironments (niches).
  • Niches crucial for providing signals for stem cell maintenance, self-renewal, and differentiation.

Hematopoietic Stem Cells

• Hematopoietic (blood-forming) stem cells were the first to be identified.
• From one stem cell, blood differentiates into distinct types of blood cells with specialized functions:
  • Erythrocytes (RBC) carry $O<em>2$ and $CO</em>2$.
  • Granulocytes & Macrophages fight general infections.
  • Platelets aid in clotting reactions.
  • Lymphocytes are trained to fight specific infections (immune response)

Intestinal Epithelium

• Epithelial cells lining the intestines have a short lifespan.
• Critical for nutrient absorption, these cells undergo apoptosis.
• Continuous replacement through slow division of stem cells located at the base of intestinal crypts.
• Renewal of the intestinal epithelium starts with stem cells in the crypts (folds).
• Surface epithelium cells have differentiated functions.

Skin, Hair, and Oil Glands

• Skin and hair are also renewed by stem cells.
• The epidermis, hair follicles, and sebaceous glands are all maintained by their own stem cells.

Skeletal Muscle

• Stem cells also play a role in the repair of damaged tissue.
• Skeletal muscle can regenerate rapidly in response to injury or rebuild after exercise.
• Regeneration is mediated by proliferation of satellite cells (stem cells of adult muscle).
• Satellite cells divide, fuse, and then form new muscle fibers.
• Skeletal muscle is multi-nucleate large fused cells, and they cannot divide.

Stem Cell Niches

• Most adult tissues have stem cells, which reside in distinct microenvironments or niches.
• Niches provide the environmental signals that maintain stem cells throughout life and control the balance between self-renewal and differentiation.

Future Perspectives in Stem Cell Biology

• Wnt Pathway in Stem Cells:
  • Wnt pathway regulates proliferation of intestinal stem cells.
  • Signaling by Wnt polypeptides from adjacent cells maintains intestinal stem cells.
• Role of TGF-β and Notch Pathways:
  • TGF-β and Notch pathways contribute to stem cell regulation.
  • Interactions with the extracellular matrix also play essential roles.
• Ongoing Challenges:
  • Precise identification of stem cells in various tissues.
  • Understanding complex signaling networks within stem cell niches.
• Future Research Directions:
  • Unraveling stem cell dynamics and regulatory mechanisms for potential therapeutic applications.

Medical Application - Hematopoietic Stem Cell Transplantation

• Adult stem cells have uses in clinical medicine.
• Hematopoietic stem cell transplantation (or bone marrow transplantation) plays an important role in the treatment of a variety of cancers.
• Chemotherapy is very toxic to rapidly dividing cells and stem cells. Bone marrow donation replaces needed stem cells.

Regenerative Medicine

• Epithelial stem cells are also used in the form of skin grafts to treat burns, wounds, and ulcers.

Cell Death

• Necrosis:
  • Results from damage by an external agent, e.g., infection or injury.
  • Causes rupturing of cells and leakage of their contents into surrounding tissues.
  • As a result, the non-specific immune response occurs and leads to inflammation.
• Apoptosis:
  • A continuous, controlled process.
  • In an adult, apoptosis maintains cell numbers, balancing the production of new cells by mitosis and the death of older cells.
  • Cells are destroyed in a way that does not cause leakage of their contents into surrounding tissues.
  • So the non-specific immune response does not occur and there is no inflammation.

Apoptosis vs. Necrosis

• Apoptosis:
  • Cell shrinkage
  • Plasma membrane blebbing
  • Formation of apoptotic bodies
• Necrosis:
  • Increase in cell volume
  • Plasma membrane disruption
  • Leakage of cellular contents

Programmed Cell Death - Apoptosis

• Responsible for balancing cell proliferation and maintaining constant cell numbers in tissues undergoing cell turnover.
• Examples of Apoptosis
  • About $5 \times 10^{11}$ blood cells are eliminated daily in humans by programmed cell death.
  • Embryogenesis and fetal development – Development of fingers in hand and feet.
  • Development of mammalian nervous system.
  • Elimination of abnormal, nonfunctional or dangerous cells, such as infected cells, cells with damaged DNA, cells over-producing oxygen radicals.

Events of Apoptosis

• Proceeds by a distinguished series of cellular changes.
• The chromatin condenses and the nucleus then breaks up into small pieces.
• The cell itself shrinks and breaks up into membrane-enclosed fragments called apoptotic bodies.
• Apoptotic cells are eliminated by phagocytosis.
• Gel electrophoresis of DNA from apoptotic cells, showing its degradation to fragments corresponding to multiples of 200 base pairs (the size of nucleosomes) at 0–3 hours following induction of apoptosis.

Phagocytosis of Apoptotic Cells

• Apoptotic cells and cell fragments are recognized and engulfed by phagocytic cells.
• “Eat me signal“ One of the signals recognized by phagocytes is phosphatidylserine on the cell surface.
• In normal cells, phosphatidylserine is restricted to the inner leaflet of the plasma membrane, but it becomes expressed on the cell surface during apoptosis.

Studies on Programmed Cell Death in C.elegans

• During normal nematode development, 131 somatic cells out of a total of 1090 are eliminated by programmed cell death, yielding the 959 somatic cells in the adult worm.
• Genetic analysis identified three genes that play key roles in programmed cell death during development of C. elegans.
  • Two genes, ced-3 and ced-4, are required for cell death.
  • ced-9 inhibits cell death. The Ced-9 protein inhibits Ced-4, which activates Ced-3.

Caspases: The Executioners of Apoptosis

• Caspases are a family of proteases (i.e., protein-digesting enzymes).
• Proteases with cysteine in the active site. Cleave their target proteins at specific aspartic acids.
• They are synthesized in a cell’s cytoplasm as inactive procaspases.
• Once activated, caspase molecules have several effects, for example, they:
  • activate more procaspase to caspase
  • hydrolyze structural proteins in the cytoplasm, e.g., actin
  • hydrolyze structural proteins in the nucleus, e.g., lamins
  • activate DNA-hydrolyzing enzymes
• As structural proteins are hydrolyzed:
  • the nucleus breaks up
  • the cell shrinks and forms extensions, called blebs
  • Small membrane-bound fragments (apoptotic bodies) bud off from these blebs and are engulfed by phagocytic white blood cells
• Ced-3 is prototype of proteases - caspases

Caspase Targets

• Inhibitor of DNase (ICAD): DNA fragmentation.
• Nuclear lamins: Fragmentation of nucleus.
• Cytoskeletal proteins (actin, myosin, α-actinin, tubulin, vimentin): Cytoskeletal disruption, cell fragmentation, membrane blebbing.
• Golgi matrix proteins: Fragmentation of Golgi.
• Scramblase: Translocation of phosphatidylserine to cell surface.

What Activates Caspase Initially

• Intrinsic Pathway:
  • An intracellular signal in response to stress.
  • Mitochondria release substances into the cytosol of the cell.
  • One of these is cytochrome c, one component of the electron transport chain of aerobic respiration.
  • In the cytosol, cytochrome c causes hydrolysis of inactive procaspase to active caspase.
• Extrinsic Pathway:
  • A signal from a different cell, often a killer T cell.
  • Each cell has so-called ‘death receptors’ on its cell surface membrane.
  • When bound to a complementary protein on the cell-surface membrane of another cell, the death receptor recruits adaptor proteins inside the cytoplasm that attract procaspase molecules and hydrolyze them to active caspase.

Central Regulators of Apoptosis - The Bcl-2 Family

• The 3rd gene ced-9 was found to be closely related to a mammalian gene called bcl-2 (Oncogene – Human B cell Lymphomas).
• Bcl-2 is found to inhibit apoptosis.
• The fate of the cell—life or death—is determined by the balance of activity of three groups of proapoptotic and antiapoptotic Bcl-2 family members, which act to regulate one another.
  • Antiapoptotic (Bcl-2, Bcl-xl)
  • Proapoptotic (Bax, Bak)
  • Proapoptotic BH3-only (Bid, Bad, Noxa, Puma, Bim)

Regulatory Interaction Between Bcl-2 Family Members

• In normal cells, the proapoptotic regulatory proteins are inactive.
• Proapoptotic effector proteins are inhibited by interaction with antiapoptotic regulatory proteins.
• Cell death signals activate the proapoptotic regulatory proteins, which then antagonize the antiapoptotic proteins as well as activating the proapoptotic effector proteins directly, leading to cell death.

The Mitochondrial Pathway of Apoptosis – Intrinsic

• In Mammalian cells – Bcl-2 acts on mitochondria-controls programmed cell death.
• In mammalian cells, many cell death signals induce apoptosis as a result of damage to mitochondria.
• When active, the proapoptotic effector proteins Bax and Bak form oligomers in the outer membrane of mitochondria, resulting in the release of cytochrome c from the intermembrane space.
• Release of cytochrome c leads to the formation of apoptosomes containing Apaf-1 and caspase-9 in which caspase-9 is activated.
• Caspase-9 then activates downstream caspases, such as caspase-3, by proteolytic cleavage.
• Caspases are regulated by a group of proteins called IAP (inhibitor of apoptosis)

Signaling Pathway That Regulates Apoptosis

• Role of p53 in DNA damage-induced apoptosis
  • DNA damage is one of the principal triggers of programmed cell death, leading to the elimination of cells carrying potentially harmful mutations.
  • Several cell cycle checkpoints halt cell cycle progression in response to damaged DNA, allowing time for the damage to be repaired.
  • In mammalian cells, a major pathway leading to cell cycle arrest in response to DNA damage is mediated by the transcription factor p53.
  • DNA damage leads to activation of the ATM and Chk2 protein kinases, which phosphorylate and stabilize p53, resulting in rapid increases in p53 levels.
  • The p53 protein then activates transcription of genes encoding the proapoptotic regulatory proteins PUMA and Noxa, leading to cell death.

The PI 3-Kinase Pathway and Cell Survival

• Growth factor binds to receptor tyrosine kinase.
• PI 3-kinase is activated, converting PIP2 to PIP3.
• PIP3 activates PDK1 and Akt.
• Akt phosphorylates and inhibits proapoptotic regulatory proteins like Bad, FOXO, and p53.
• Akt activates mTORC1, promoting translation of antiapoptotic regulatory proteins like Mcl-1.

Cell Death Receptors - Extrinsic Pathway

• TNF and other cell death receptor ligands (FAS) consist of three polypeptide chains, so their binding to cell death receptors induces receptor trimerization.
• Caspase-8 is recruited to the receptor and activated by adaptor molecules.
• Once activated, caspase-8 can directly cleave and activate effector caspases.
• In addition, caspase-8 cleaves the proapoptotic regulatory protein Bid, which activates the mitochondrial pathway of apoptosis, leading to caspase-9 activation.

Extrinsic Pathway Mechanisms

• Mechanism 1: Caspase-8 cleaves procaspase-3 directly and activates it.
• Mechanism 2:
  • Caspase-8 cleaves Bid (protein that stimulates apoptosis) and its COOH-terminal part translocates to mitochondria.
  • The C-terminus of Bid triggers the release of cytochrome c which binds to apoptotic protease activating factor-1 (Apaf-1).
  • Together, cytochrome c and Apaf-1 bind to procaspase-9, activating caspase-9. Caspase-9 cleaves procaspase-3 and activates caspase-3.
• For both mechanisms activation of caspase-3 leads to the induction of apoptosis.

Cancer

• Cancer results from a breakdown of the regulatory mechanisms that govern normal cell behavior.
• Cancer cells grow and divide in an uncontrolled manner, spreading throughout the body and interfering with the function of normal tissues and organs.

Types of Cancer

• There are more than 100 types of cancer.
• A tumor is any abnormal proliferation of cells.
• Benign tumors: remain confined to the original location, neither invading surrounding normal tissue nor spreading to distant body sites.
• Malignant tumor: can invade surrounding normal tissue and spread throughout the body via the circulatory or lymphatic systems (metastasis).

Main Groups of Cancer

• Carcinomas: malignancies of epithelial cells (about 90% of human cancers).
• Sarcomas: solid tumors of connective tissue such as muscle, bone, cartilage, and fibrous tissue (rare in humans).
• Leukemias and lymphomas: arise from the blood-forming cells and immune system cells, respectively.

Further Classification of Tumors

• Tumors are further classified according to tissue of origin and type of cell involved.
• The 4 most common cancers are those of the prostate, breast, lung, and colon/rectum.

The Development of Cancer

• A fundamental feature of cancer is tumor clonality—tumors develop from single cells that begin to proliferate abnormally.
• The single-cell origin has been demonstrated by analysis of X chromosome inactivation patterns.
• Additional mutations lead to the selection of cells with progressively increasing capacities for proliferation, survival, invasion, and metastasis.

Tumor Clonality

• Clonal selection is the process by which random mutations in tumor cells confer a selective advantage to certain cell lines. Those cells (clones) eventually dominate the tumor population.
• Normal tissue is a mosaic of cells in which different X chromosomes (X1 and X2) have been inactivated.
• Tumors develop from a single initially altered cell, so each tumor cell displays the same pattern of X inactivation (X1 inactive, X2 active).

Multistep Process of Cancer Development

• Cells gradually become malignant through a progressive series of alterations.
• One indication of this is that most cancers develop late in life.
• Most cancers develop as a consequence of multiple abnormalities, which accumulate over many years.

Stages of Tumor Development

1. Tumor initiation: mutation leads to abnormal proliferation of a single cell, which grows into a population of clonal tumor cells.
2. Tumor progression: continues as additional mutations occur within cells of the tumor population.
3. Clonal selection: Descendants of these cells become dominant.

Colon Carcinoma

• Colon carcinoma is an example of tumor progression.
• Proliferation of colon epithelial cells give rise to a small benign neoplasm (an adenoma or polyp).
• Clonal selection leads to growth of adenomas of increasing size and proliferative potential.

Causes of Cancer

• Agents including radiation, chemicals, and viruses have been found to induce cancer in both experimental animals and humans.
• Radiation and many chemical carcinogens act by damaging DNA and inducing mutations.
• Carcinogens are substances that cause cancer.

Examples of Carcinogens

• Solar ultraviolet radiation—the major cause of skin cancer.
• Carcinogenic chemicals in tobacco smoke (benzopyrene, dimethylnitrosamine, and nickel compounds are identified as potential agents for causation of cancer).
• Aflatoxin - a chemical produced by some molds that contaminate peanuts and stored grains.

Tumor Promoters

• Other carcinogens are tumor promoters that stimulate cell proliferation.
• Hormones, particularly estrogens, are tumor promoters in some human cancers.
• Exposure to excess estrogen significantly increases the likelihood that a woman will develop uterine cancer.

Properties of Cancer Cells

• A primary distinction between cancer cells and normal cells in culture is that normal cells display density-dependent inhibition and contact inhibition of cell proliferation and movement.
• Normal cells proliferate until they reach a finite cell density, then cease proliferating and become quiescent, arrested in the G0 stage of the cell cycle.
• Most cancer cells are not sensitive to density-dependent inhibition.

Growth Factors and Cancer Cells

• Many cancer cells can grow in the absence of growth factors required by normal cells.
• Some cancer cells produce growth factors that stimulate their own proliferation (autocrine growth stimulation).
• Reduced growth factor dependence can also result from abnormalities in intracellular signaling systems. Example: unregulated activity of growth factor receptors or other proteins (e.g., Ras proteins or protein kinases).

Autocrine Growth Stimulation

• A cell produces a growth factor to which it and neighboring cells also respond, resulting in continuous stimulation of cell proliferation.

Cell-Cell and Cell-Matrix Interactions

• Cancer cells are less regulated by cell - cell and cell-matrix interactions.
• Most cancer cells are less adhesive than normal cells, due to reduced expression of cell surface adhesion molecules.
• Loss of E-cadherin is important in the development of carcinomas (epithelial cancers).

Secretion of Proteases

• Cancer cells secrete proteases that digest extracellular matrix components, allowing them to invade adjacent normal tissues. Example: Proteases that digest collagen allow carcinomas to penetrate the basal laminae and invade underlying connective tissue.

Angiogenesis

• Cancer cells secrete growth factors that promote the formation of new blood vessels (angiogenesis).
• When a tumor reaches about a million cells, new blood vessels are needed to supply oxygen and nutrients.
• The new capillaries are easily penetrated by tumor cells, contributing to metastasis.

Apoptosis and Telomerase

• Many cancer cells fail to undergo programmed cell death or apoptosis and have longer lifespans than normal cells.
• Tumor cells are often able to survive in the absence of growth factors required by normal cells.
• Normal cells have limited amounts of telomerase and gradually lose telomeres, leading to cessation of replication.
• Cancer cells express high levels of telomerase, allowing them to maintain chromosome ends for an indefinite number of divisions.

Summary: Normal Cells vs. Cancer Cells

Tumor Viruses

• Hepatitis B and C viruses: cause liver cancer in humans.
• Herpesviruses: cause cancer in several species, including humans (Kaposi’s sarcoma associated herpesvirus and Epstein-Barr virus).

Small DNA Tumor Viruses

• Papillomaviruses infect epithelial and cause a variety of tumors, including cervical carcinoma in humans.
• Some cause benign tumors (such as warts); others cause malignant carcinomas, particularly cervical and other anogenital cancers.
• Merkel cell polyomavirus causes a rare human skin cancer.
• Transforming proteins of these viruses frequently interact with the cellular Rb and p53 tumor suppressor proteins.

Retroviruses

• Retroviruses cause cancer in many animals, including humans.
• Human T-cell lymphotropic virus type I (HTLV-I) causes adult T-cell leukemia.
• AIDS is caused by the retrovirus HIV. HIV does not cause cancer directly, but AIDS patients have a high incidence of malignancies, particularly lymphomas and Kaposi’s sarcoma.
• Rous sarcoma virus (RSV) is the prototype of these highly oncogenic retroviruses.

Oncogenes and Tumor Suppressor Genes

• Oncogene: a gene that, when mutated or expressed at abnormally high levels, contributes to converting a normal cell into a cancer cell.
• Proto-oncogene: the “normal” cellular progenitors of oncogenes that function to promote the normal growth and division of cells.

Tumor Suppressor Genes

• Genes which normally suppress cell growth and/or motility which are frequently downregulated in tumor cells, allowing for unchecked growth/motility.
• Oncogenes are specific genes that can induce cell transformation.
• Studies of viral oncogenes also led to identification of cellular oncogenes involved in the development of non-virus-induced cancers.