Week 5 (apoptosis + Cancer)

What is apoptosis?

programmed cell death 

• Controlled cellular suicide 

• Dehydration leading to cell shrinkage (allows for other cells to digest the remains) 

• Not the only way for cell death: 

• Autophagy, non-apoptotic programmed cell death/caspase-independent, Anoikis, cornification, excitotoxicity, & necrosis 

What are the changes related to apoptosis

Include morphological & chemical changes 

• Morphological = membrane blebbing/shrinkage & condensation (cells break apart with intact membranes to form apoptotic bodies), cytoskeletal collapse, disassembly of nuclear envelope (condensed nucleus), & chromatin fragmentation 

• Chemical = cell surface is altered for phagocytic recognition & intracellular proteins act as signals and activators of cell destroyers 

What causes apoptosis

Causes: stressed out cells die when there is... 

• Radiation 

• Glucocorticoid binding of nuclear receptors (cortisol = stressful environment) 

• Heat 

• Nutrient deprivation 

• Viral infection 

• Hypoxia (lacking enough oxygen) 

• High intracellular calcium 

What is necrosis

cell death in response to acute issue 

• Explosion like 

• Not controlled 

What is caspase

protease with a cysteine at their active site that cleave target substrates at a specific aspartic acid 

• Plays a role in both intrinsic and extrinsic pathway 

What is the intrinsic signaling pathway (simple)

Intrinsic signaling pathway = internal response to injury or hypoxia 

• Depend on cytochrome c release 

• Requires Bcl2 

• Sometimes recruited by the extrinsic pathway 

What is the extrinsic signaling pathway (simple)

Extrinsic signaling pathway = extracellular signal protein binds to death receptor on cell surface  

• Sometimes recruits intrinsic pathway 

What is a death receptor

Death receptor = activate caspase apoptosis (extrinsic pathway) 

• Have a ligand-binding region, 1 transmembrane domain, & a death domain 

• Belong to the TNF receptor family 

• Binding ligands belong to the TNF signal protein family 

What is DISC

DISC = death-inducing signaling complex 

• Complex in which initiator caspases interact and are activated after extracellular ligands bind to cell-surface death receptors in the extrinsic pathway of apoptosis 

What is cytochrome c

 water soluble molecule that can trigger apoptosis 

• Released by the mitochondria  

• Trigger intrinsic pathway 

• Can induce apoptosis, independent of its electron-transport activity 

What is BH123 protein

BH123 protein = pro-apoptotic protein (Bak & Bax) (BH3-only similar) 

• Regulator of cell death 

What is Bcl2 protein

Bcl2 protein = anti-apoptotic protein located on organelle membranes that acts as a membrane guardian 

• Regulator of cell death 

• Block cell death 

What are survival factors

Survival factor = extracellular signals from other cells to avoid apoptosis 

• Cells with no signals cannot import nutrients. Therefore, they eat themselves (autophagy) but eventually starve to death (not apoptosis) 

• Can stay alive by 

  • Increasing anti-apoptotic proteins (Bcl2) 

  • Inactivating pro-apoptotic proteins (BH3-only and BH123) 

What is autophagy

Autophagy = chewing up the cell insides (can be a form of cell death) 

• Cell eats itself 

Why is programmed cell death needed

Why programmed cell death is needed... 

• Quality control during development for abnormal cells 

  • Development = more cells than needed are made just in case they have to be eliminated 

• Organ/tissue sculpting  

• Maintenance of short-lived cell supply 

• Organelle damage  

• Elimination of bad immune system cells, unneeded activated lymphocytes 

What is an example of normal apoptosis

elimination of cells between developing fingers and toes in a human embryo

What is the importance of phosphatidylserine flipping out in an apoptotic cell

Chemical changes with apoptosis:  

1. DNA fragmented by endonuclease at the linker regions between nucleosomes 

2. Membrane phospholipid phosphatidylserine changes conformation and moves to outer leaflet of lipid bilayer  

3. Release of mitochondrial cytochrome c into cell cytosol 

4. Vacuole formation related to organelle autophagy  

 

What is caspase and its role

Caspases = proteases with a cysteine at their active site and that cleave target substrates at a specific aspartic acid  

• Located in cytoplasm as inactive procaspases  

• Activated procaspases become either initiator caspases or executioner caspases 

  • They cleave specific proteins (like lamins, endonucleases, cell-cell proteins, & cytoskeletal proteins) to either start or stop their function 

What is caspases role in activation (steps)

Activation:  

• 2 signal pathways (extrinsic or intrinsic) 

• Steps: 

1. Receive a death signal 

2. Procaspase domains with recruitment domain allows them to assemble with adaptor proteins into activation complexes 

3. Proximity of initiator procaspases activates them (they cleave each other) BEGINS THE CASCADE 

4. Executioner procaspases are activated and death signal is amplified 

What is the intrinsic pathway (detailed)

Intrinsic = internal response to injury or hypoxia triggers apoptosis 

• Dependent upon release of cytochrome c and other proteins from mitochondria which activate a caspase cascade 

  • Proteins bind to adaptor protein Apaf1 which becomes an apoptosome which activates procaspases 

• Sometimes recruited by the extrinsic pathway 

• Requires Bcl2 (anti-apoptotic protein) 

  • BH3-only first, then Bcl2 anti-apoptotics, then effector proteins 

• Can be activated by p53 (tumor suppressor) 

  • If cellular DNA damage accumulates 

  • Will activate other proteins & trigger the intrinsic pathway 

What is the extrinsic pathway (steps detailed)

Extrinsic = extracellular signal proteins bind to death receptors on a cell surface 

• Binding initiates cascade of events within the cell 

What is cancer’s role with apoptosis

Cancer = tumorigenic cells regulate the apoptotic pathway abnormally 

• Bcl2 production increases after chromosome translocation -> leading to apoptotic inhibition of the cells (lymphocytes) 

• P53 gene mutated so there’s no cell death or cell-cycle arrest => tumors (very common in many cancers) 

• Cells lose adherence to neighbor cells 

 image shows translocation examples

What is angiogenesis

Angiogenesis = Growth of new blood vessels by sprouting from existing ones 

What is benign mean

Benign = mass of cells/tumor that is not invading 

What is cancer stem cells

Cancer cells lose their differentiation, therefore, they look like stem cells 

What is a carcinoma

Carcinoma = cancers arising from epithelial cells 

• Most of our cells are these so 80% of cancers are these 

• Adenoma = benign tumor arising from epithelial cells in a gland 

What is a carcinogen

a substance or agent that can cause cancer (chemical, viruses, UV damage)

What is cancer

a collection of disorders that share 2 properties: cell reproduction & division despite various restrains and controls & invasion 

• metastasis

What is neoplasm

Neoplasm = mass of cells or tumor 

• No invasion when benign 

What is a malignant neoplasm

Malignant neoplasm = made up of cells from a single ancestor 

• Monoclonal in origin and form the “primary tumor” 

• Invade nearby tissues

What is leukemia (Blood)/lymphoma (immune system)

WBCs and immature precursors proliferate (blood cancer) 

What is a sarcoma

Sarcoma = cancers arising from connective tissue or muscle cells 

What is metastasis

Metastasis = when cells are malignant and invade (THIS IS CANCER) 

• Spread to more distant sites in the body to form secondary tumors 

• Fewer than 1 in 1000 cells will survive and metastasis successfully 

What is a tumor suppressor

Gene that appears to help prevent formation of a cancer. Loss-of-function mutations in such genes favor the development of cancer

What is an oncogene

An altered gene whose product can act in a dominant fashion to help make a cell cancerous. Typically, an oncogene is a mutant form of a normal gene (proto-oncogene) involved in the control of cell growth or division 

• Originate from proto-oncogenes that encode protein products that control cell growth and differentiation 

• When activated by mutation, these act as dominant gain-of-function mutations that lead to the deregulation of cell cycle control 

  • A growth promoting effect 

  • Ex: Myc, K-ras, Wnt-3, her-2/neu, cyclin E 

• Still a normal protein, it is just being over produced 

ex: p53, CDK inhibitors, BRCA1

What is a proto-oncogene

Proto-oncogene = Normal gene, usually concerned with the regulation of cell proliferation, that can be converted into a cancer-promoting oncogene by mutation 

What is the basic cause of cancer

Basic cause of cancer = Damage to specific genes (mutations) that accumulate in somatic cells (somatic mutations) over time until a cell loses a critical number of growth-controlled mechanisms and initiates a tumor 

• Mutations involved in regulating cell growth & differentiation may occur and can lead to deregulation of growth and cell cycle madness 

  • If only one mutation were able to convert healthy cells into cancerous cells, we would not be viable organisms (need multiple mutations for cancer to develop) 

• At least 5 mutations are required for onset of clinically observable tumors: 

1. Gene amplification: extra copies of a single gene are transcribed 

2. Nonsense mutations 

3. Gene deletion: loss of a sequence of nucleotides within an exon or splice site 

4. Gene rearrangements: in Ig-producing cells (T and B) can produce massive clonal populations 

5. Point mutations: replacement of a single nucleotide 

• Other causes include genetic alterations of specific molecules such as integrins or chromosome issues 

• Specific genetic alterations are associated with specific caners 

What are tumors

Tumors: comprised of genetically identical cells and are clonal in nature 

• Single aberrant cell starts it all 

• Alterations in cellular DNA accumulate over time in succeeding generations of daughter cells (clonal expression) 

• Daughter cells with several mutations replace the cells previously comprising the tumor 

• Induce angiogenesis 

What are cancer cell characteristics

Cancer cells: are genetically unstable & continue to accumulate more point mutations or deletions or develop crazy chromosome issues more than normal cells 

• Have abnormal growth control 

• Lose their differentiation (look like stem cells) 

• Have defective death control 

• Utilize glucose at a much higher level (suck energy of neighboring cells) 

• Continue dividing despite DNA damage  

• Avoid replicative senescence through p53 mutation or telomerase maintenance 

• Must survive in a foreign environment 

• Critical genes = proto-oncogenes/oncogenes, tumor suppressor genes, & DNA maintenance genes 

What is angiogenesis (detailed)

Angiogenesis: 

• Cell aggregates need oxygen to survive, without oxygen, hypoxia sets in 

  • Hypoxia activates an angiogenic switch that increases hypoxia-inducible factor that activates transcription of genes that encode proteins that attract endothelial cells (via VEGF) and formation of new blood vessels 

    • Tumors may function like this 

What are tumor suppressor genes (detailed)

Tumor suppressor genes = Gene that appears to help prevent formation of a cancer. Loss-of-function mutations in such genes favor the development of cancer 

• Encode for protein products that suppress tumor formation by controlling cell growth 

• Loss-of-function mutation = results in inactivation of the tumor suppressor protein and can lead to uncontrolled cellular proliferation 

  • Ex: p53, BRCA1, E3 ubiquitin protein, RB, p16, CDK inhibitors 

  • Inactivation of these genes occurs through mutation or deletion of sequences over a large coding region (exon) 

    • Causes uncontrolled cellular proliferation because of loss of negative regulation 

• Like the brakes on the bus 

What is p53 and it’s role with DNA damage

p53 = a cellular stress sensor that reacts to various stresses and produces specific responses that stop damaged cells from dividing (guardian of the genome) 

• Tumor suppressor gene & DNA repair gene 

• Only functions in certain circumstances, limiting the harm done by DNA damage 

  • Can set apoptosis in motion 

  • Stops cell division and cell cycle 

  • Induces transcription of p21 

  • Loss of p53 promotes cancer 

• mice without p53 are normal but develop cancer before reaching 10 months old 

  • Promotes cancer by: 

    • Allowing DNA-damaged cells to divide 

    • Allows damaged cells to escape apoptosis 

    • Leads to genetic instability 

    • Makes cells resistant to anti-cancer drugs

What are Cdk inhibitors

Cyclin-dependent kinase inhibitors = one type of protein coded for by tumor suppressor genes 

• Inactive cyclin-dependent kinases and prevent them from phosphorylating (therefore controlling cell cycle) 

What are DNA repair genes

 DNA repair genes = genes that assure accurate replication of DNA 

• Ex: p53 

• If mutated, these genes that encode specific proteins causes genomic instability (widespread mutations, chromosome breaks, aneuploidy) 

• When these mutations affect pathways that regulate cellular proliferation, a tumor may arise 

What are proto-oncogenes (detailed)

Proto-oncogenes = Normal gene, usually concerned with the regulation of cell proliferation, that can be converted into a cancer-promoting oncogene by mutation 

• Necessary for normal cellular proliferation (originally obtained from viruses) 

• Become activated by point mutations to become an oncogene 

• Like the accelerator on a bus (destination = cell division) 

What causes cancer cells to be immortal

Cancer cell immortality:  

• Tumor cells overcome replicative cell senescence (cell death after a set number of divisions) by activating telomerase which replaces the telomere segments that are lost during each cell division 

  • Changes in cancer cells disable checkpoint controls so cell cycle never stops 

  • Telomerase activity is maintained