Lecture 31/32 - Neoplasia and Apoptosis

What is apoptosis and what are characteristics?

Tightly regulated cellular suicide

No inflammation

Can be physiologic or pathologic

Produces apoptotic bodies

Characteristics of apoptotic bodies

Intact plasma membranes

No DAMP/PAMP release

How are apoptotic bodies discarded?

Phagocytized by macrophages

Examples of physiologic apoptosis

▪ Embryogenesis

▪ Involution with hormone withdrawal

▪ Cell loss in proliferating cell

populations

▪ Self-reactive lymphocytes (thymus)

▪ Inflammatory cells

Examples of pathologic apoptosis

▪ DNA damage

▪ Accumulation of misfolded

proteins

▪ Certain viral infections

▪ Atrophy/disuse

What are the phases of apoptosis?

Initiation phase

Execution phase

What are the 2 possible pathways of the initiation phase of apoptosis?

Intrinsic/mitochondrial pathway

Extrinsic/death receptor pathway

What happens in the intrinsic/mitochondrial pathway of the initiation phase of apoptosis?

Increased mitochondrial permeability and release of pro-apoptotic molecule cytochrome c into the cytoplasm (regulated by Bcl proteins, pro and anti apoptotic)

Formation of apoptosome (cytochrome c and Apaf-1)

Procaspase-9 → Caspase-9

What happens in the extrinsic/death receptor pathway of the initiation phase of apoptosis?

Death receptors (TNF receptor family, FAS and FASL on cytotoxic T-lymphocytes)

Formation of FADD

Procaspase-8 → Caspase-8

What happens in the execution phase of apoptosis?

▪ Mainly caspase 3, also 6 and 7

▪ Cleaves DNA into small pieces

▪ Degrade structural components

▪ Fragmentation of nucleus

Result of the initiation and execution phases of apoptosis is...

Result is apoptotic bodies that are eventually removed by macrophages

What activates the extrinsic apoptotic pathway?

Death ligand external to the cell

What is the role of FADD in the extrinsic apoptotic pathway?

Second messenger that activates caspase 8

What must be present in sufficient levels to activate effector caspases in the extrinsic pathway?

Caspase 8

What activates the intrinsic apoptotic pathway?

Irreversible cell damage

Which family of proteins regulates the response in the intrinsic apoptotic pathway?

Bcl-2 family proteins

What is the result of the intrinsic apoptotic pathway activation?

Mitochondrial outer membrane permeabilization (MOMP) and release of cytochrome C

What role does cytochrome C play in apoptosis?

It promotes the formation of the apoptosome and activation of caspase 9, then effector caspases

FADD

Fas-associated Death Domain

Bcl-2 gene family proteins

Group of both pro and anti apoptotic proteins that regulate response to cell damage and mitochondrial membrane permeability

MOMP

Mitochondrial outer membrane permeabilization

Apoptosome

Caspase-activating complex promoted by release of cytochrome C

Relationship between immortal/neoplastic cells and apoptosis

They disrupt/inactivate/evade apoptosis

What is p53?

Tumor suppressor gene (AKA TP53)

Guardian of the genome

Acts as a molecular policeman that prevents propagation of genetically damaged cells

p53 protein is activated in response to...

Activated in response to cellular stress/damage, such as DNA damage, shortened telomeres, or hypoxia

How does p53 prevent neoplastic transformation?

Through cellular quiescence, senescence, or apoptosis

Quiescence

Temporary cell cycle arrest to allow for DNA repair

Senescence

Permanent cell cycle arrest if repair fails; think cellular arrest due to aging

Apoptosis

Programmed cell death if repair fails

Ultimate protective mechanism against neoplastic transformation

Apoptosis is a common pathway to...

Neoplasia

What is a common mutation in cancer that is present in approximately half of human cancers?

Loss of p53 activity from mutation

Factors involved at controlling apoptosis

Proto oncogenes, tumor suppressor genes, mutations

Possible therapeutic option for cancer relating to the apoptosis pathway

Procaspase-3 activation to initiate execution phase of apoptosis in neoplastic cells

Essentially getting around the block that is inhibiting apoptosis

Without new blood vessels, tumors are limited in...

Size to 2 mm diameter

So, they make their own with angiogenesis

Angiogenesis

Vessel formation from existing vessels

What is vasculogenesis?

Vessel formation from mesodermal blast cells

What does neovascularization encompass?

It is a collective term that includes both angiogenesis and vasculogenesis

Mechanism of angiogenesis in tumors

▪ Endothelial cell recruitment from preexisting vessels, proliferation, and migration through ECM

▪ Maturation and differentiation of capillary sprout

▪ Balance of angiogenic and anti-angiogenic factors

Angiogenic factors

Up-regulation

Vascular endothelial growth factor (VEGF)

Fibroblast growth factor (FGF)

Anti-angiogenic factors

Down-regulation

Thrombospondin

Function of angiogenesis in tumors

▪ Supply oxygen and nutrients to tumor

▪ Endothelial cells produce growth factors (PDFG, IL1)

▪ Allow for metastasis: grow both blood and lymphatic vessels

Morphology of tumor vessels

Leaky vessels

Disorganized

Weaker pericyte coverage, things can get in and out much easier

Why is the morphology of tumor vessels important?

Therapeutic target

Looking into suffocating/infarct tumors so they cannot grow

Marker in studies for tumor growth

Immunosurveillance

Identifying and destroying tumor cells

What is used to ID the cell of origin of tumors?

Tumor antigens

Examples of tumor antigens

Proteins, glycoproteins, glycolipids, carbohydrates on surface of tumor cells

Types of tumor antigens

Tumor-specific antigens

Tumor-specific shared antigens

Tissue-specific antigens

What are tumor-specific antigens?

Antigens that are unique to tumor cells and not found on normal cells

What are tumor-specific shared antigens?

Antigens that are present in many types of tumors but are not exclusive to a single tumor type

What are tissue-specific antigens?

Antigens that are shared by tumors and the normal tissues from which they arise

What are differentiation antigens?

Both normal and tumor tissues but occur at a specific point in differentiation of normal tissues

Type of tissue-specific antigen

Example of tumor-specific antigen

Antigens derived from oncogenic viruses

Proteins produced by papillomaviruses

Example of tumor-specific shared antigen

Cell products absent/rare in normal adult tissue but common in many different types of neoplasia

MAGE family proteins

Example of tissue-specific antigen

Lymphocyte cell receptors

CD3 (T lymphocytes), CD79a (B lymphocyte)

Example of differentiation antigen

Some B lymphocyte receptors

CD20 is only present in normal mature B cells and mature (fully differentiated) B cell neoplasms, not present in immature B cell neoplasms

Most common antigens used in determining cell of origin of a neoplasm

Tissue-specific antigen

Liquid biopsy

Detection of neoplastic cells and/or tumor antigens in blood allowing for earlier detection of cancer

Characteristics of innate immune system

▪ First line of defense

▪ No antigen-specific priming

▪ No lasting immunity

▪ NK cells and macrophages

What type of cells are Natural Killer (NK) Cells?

A type of lymphocyte involved in the immune response

What do NK Cells recognize?

MHC molecules and stress-induced ligands

What is the role of NK Cells?

They kill cells that lack MHC class I

What is formed during the interaction between NK Cells and target cells?

An immunologic synapse that leads to cytolysis

Granules released by NK cells

Perforin: pore forming protein

Granzymes: apoptosis

What are the primary mechanisms by which macrophages kill pathogens?

By releasing reactive oxygen intermediates, lysosomal enzymes, nitric oxide, and tumor necrosis factor (TNF)

What stimulates macrophages to become activated?

Interferon-γ (IFN-γ) released by lymphocytes and NK cells

What type of interaction is required for macrophage activity?

Direct contact is required for macrophage activity

Independent of MHC

How can macrophages contribute to cancer progression?

They can promote angiogenesis, tumor invasion, and metastasis

Blocking macrophages may be useful in cancer therapy

Characteristics of adaptive immune system

▪ Antigen specific

▪ Cells must be primed

▪ MHC class I and II

Types of adaptive immunity

Cell mediated: CD8/CTL cells and CD4 T helper cells

Humoral: plasma cells

What are the primary effector cells of the adaptive antitumor immune response?

Cytotoxic T Lymphocytes (CTL)

Which markers are associated with Cytotoxic T Lymphocytes?

CD8 and MHC class I

How do Cytotoxic T Lymphocytes kill target cells?

By cytolysis

Which markers are associated with T helper lymphocytes?

CD4 and MHC class II

Function of T helper lymphocytes

Secrete IL2 and IFN-γ to stimulate CTLs

What is the role of Regulatory T Cells (T reg) in relation to tumors?

They protect tumors by inducing tolerance

Most effective anti tumor defense in an organism

Cell-mediated immunity by cytotoxic T lymphocytes

Function of B lymphocytes

- Involved in humoral immune response

- Antibodies bind to tumor cells, activate complement cascade and membrane attack complex

- Involved in antibody dependent cell-mediated cytotoxicity (ADCC)

How do tumors evade the immune response?

Selective outgrowth of antigen-negative variants

Altered MHC expression, antigen masking, tolerance, or immunosuppression

Altered MHC expression in tumors

▪ Downregulation of MHC

▪ However, cells with no MHC class 1 susceptible to NK cell killing

Antigen masking in tumors

Antigens complexed with other molecules, fibrin, etc

Tolerance in tumors

- Tumor antigens that are shared with normal tissue

- T reg cells

Immunosuppression by tumors

- TGF alpha production by tumor cells

- FAS ligand expression by tumor cells induces apoptosis of lymphocytes

Process of metastasis

Primary tumor formation

Local invasion

Intravasation

Survival in circulation

Arrest at a distant organ site

Extravasation

Micrometastasis formation

Metastatic colonization

Clinically detectable macroscopic metastases

Only what tumors have the ability to metastasize?

Malignant

Characteristics of metastasis

Very inefficient

Many cells die along the way

Responsible for 90% of human cancer deaths

Mechanisms of metastasis

Adhesion

Migration

Stromal invasion

Intravasation

Tumor emboli

Extravasation

What occurs during adhesion?

▪ Loss of intercellular adhesion structures (desmosomes, adhesion junctions, cadherins)

▪ Interaction with ECM by integrins and other receptors

What does metastatic migration rely on?

Autocrine growth factors (HGF, scatter factor)

What occurs during stromal invasion?

Basement membrane degradation

Increased protease activity (neoplastic or surrounding cells)

What does intravasation rely on?

Chemotactic factors, tumor-associated

macrophages

What does extravasation rely on?

Adhesion molecules on endothelium

Suitable microenvironment is necessary

Tumor cells mostly die where when metastasizing?

In the blood circulation

Characteristics of organs that more commonly have metastasis

Very well vascularized and have many capillaries/small vessels

Pathways of metastasis

Lymphatic spread

Hematogenous spread

Transcoelomic spread

Characteristics of lymphatic metastasis

Associated with carcinomas

Start in regional lymph node, become widespread

Characteristics of hematogenous metastasis

Associated with sarcomas

Seen first in capillary beds, then veins

Characteristics of transcoelomic metastasis

Neoplasm in a body cavity and it can spread to mesentery, etc

Spread by contact

Surface of organs

Carcinomas, especially in chickens