biochemistry _ cell growth
DESCRIPTION
biochemistry cell growth pptTRANSCRIPT
Lecture 27
Cell growth and carcinogenesis
Cell Growth• Limits to growth– Stress on DNA – Difficulty moving nutrients/wastes
across membrane• Ratio of surface area to volume• Volume increases at a faster rate than
surface area
Cell Division
• Before a cell becomes too large, it divides, producing 2 daughter cells– Each daughter cell is an exact replica of the parent
cell
• Before the cell divides, the DNA is replicated, so each new cell will have the same genetic information as the parent cell
• 2 stages (eukaryotes)– Mitosis- division of the cell nucleus– Cytokinesis- division of the cytoplasm
The Cell Cycle
• 2 major phases– Interphase– Mitosis
Interphase
• 3 phases– G1 phase= cells do most
of their growing• Increase in size and
synthesize new proteins and organelles
– S phase= chromosomes are replicated and the synthesis and DNA molecules takes place
• Usually if a cell enters S phase and begins replication, it completes the rest of the cycle
– G2 phase= many of the organelles and molecules required for cell division are produced
• Shortest of the 3 phases of interphase
Mitosis
• Divided into 4 phases– Prophase– Metaphase– Anaphase– Telophase
• Followed with Cytokinesis• Depending on cell- may last a few
minutes to several days
Cell cycle regulation is necessary for healthy growth.
Regulating Cell Growth
• Cyclins- proteins that regulate the timing of the cell cycle in eukaryotic cells– Internal regulators: proteins that respond to
events inside the cell• i.e. make sure all chromosomes have been
replicated; make sure all chromosomes are attached to the spindle before entering anaphase
– External regulators: proteins that respond to events outside the cell
• i.e. embryonic development; wound healing
Cell cycle control
Animal cells have built-in “stop” signals that halt the cell cycle at checkpoints until overridden by “go” signals
To function properly checkpoint signals have to percept “reports” from crucial cellular processes:
have it been completed correctly and should the cell cycle proceed.
Checkpoint also register signals from outside the cell
Cell cycle control
The most important decision to make is: to continue the cell division after the exit from M phase or not.Cells that do not receive the “go” signal at the G1 checkpoint, switch into a nondividing state called the G0 phase.
Cyclins and cyclin-dependent kinases (Cdks)
Regulatory molecules of the cycle transition are proteins of two main types: protein kinases and cyclins.
Protein kinases are proteins that regulate the activity of the others by phosphorylating them.
Cyclins and cyclin-dependent kinases (Cdks)
“Go” signal at the G1 and G2 checkpoints is regulated by particular protein kinases.
To be active, such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell
This kinases are called cyclin-dependent kinases – Cdks.
Control at the G2 checkpoint
The stepwise processes of the cell cycle are timed by rhythmic fluctuations in the activity of protein kinases.
There is a direct correlation between cyclin accumulation and the three major cell cycle checkpoints.
Control at the G2 checkpointCdk-cyclin complex called MPF (maturation promotion factor), acts at the G2 checkpoint to trigger mitosis.
MPF activity fluctuates with the level of cyclin in the cell.
The cyclin level rises throughout interphase (G1, S, and G2 phases), then falls abruptly during mitosis (M phase). The Cdk itself is present at a constant level.
Cdc2 (cdk1) is phosphorylated by Cyclin B at 3 regulatory positions
MPF promotes mitosis by phosphorylating various proteins
One effect of MPF is to the breakdown of its own cyclin.
The Cdk component of MPF is recycled. Its kinase activity will be restored by association with new cyclin.
Internal regulationInternal signals
Anaphase, the separation of sister chromatids, does not begin until all the chromosomes are properly attached to the spindle at the metaphase plate.
Certain associated proteins trigger a signalling pathway that keeps an anaphase promoting complex (APC) in an inactive state.
M-phase checkpoint is the gatekeeper. Only when all the kinetochores are attached to the spindle does the “wait” signal cease.
External regulation
External signals: growth factors.
Most of mammalian cells divide in culture only if the growth medium includes specific growth factors.
PDGF – platelet-derived growth factor – is required for the division of fibroblasts.
External regulation
Density-dependent inhibition of cell division, a phenomenon in which crowded cells stop dividing.
Cultured cells normally divide until they form a single layer of cells on the inner surface of the culture container.
Density-dependent inhibition
• Effects of controlled cell growth can be seen by placing some cells in a petri dish containing nutrient broth
• Cells grow until they form a thin layer covering the bottom of the dish
• Cells stop growing when they come into contact with other cells
• If cells are removed, the remaining cells will begin dividing again
Uncontrolled Cell Growth
• Cancer- disorder in which some of the body’s own cells lose the ability to control growth
• Cancer cells do not respond to the signals that regulate the growth of most cells
• P53 gene halts the cell cycle until all chromosomes have been properly replicated– A damaged or defective p53 gene causes the cells to
lose the information needed to respond to signals that would normally control their growth
Cancer results from genetic changes that affect cell cycle control
• The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development
Cancer is not a single disease but rather a name applied to a great variety of malignant tumor that are formed by the same basic process of uncontrolled growth.
Cancer is one of the most common and severe problem of clinical medicine.
Cancer has emerged as a major public health problem in developing countries for the first time, matching its effect in industrialized nations. This is a global problem.
Cancer
Cancer may affect people at all ages, even fetuses but risk for the more common varieties tends to increase with age.
Early diagnosis and early treatment are vital, and identification of persons at increased risk of cancer before its development is an important objective of cancer research.
Cancer
Self-limited in their growth.
Do not invade or metastasize (although some benign tumor types are capable of becoming malignant).
Benign Tumors
Excessive growth and division without respect to normal limit, Invasive, invade and destroy adjacent tissues, and sometime,Distant metastasis spread to other locations in the body.
Malignant Neoplasm or Tumors (Cancer)
Old age Unhealthy lifestyle (Western lifestyle), Poor diet,
lack of physical activity, or being overweight. Environmental factors, defined broadly to include tobacco use, diet, sunlight and infectious diseases.
Occupational carcinogens Radiation Family history of cancer (Genetic susceptibility) Alcohol Chemicals and other substance
Risk Factors
Cancer Type Risk FactorLung Cancer Tobacco smoke
Asbestos and other substancesAir pollution
Breast RadiationGenetic changes (Inherited mutation)
Colorectal Genetic alterationDiet
Cigarette smokingUlcerative colitis or chon's disease
Prostate DietCertain prostate changesRace Africans Americans
Risk Factor/ Cancer Type
Cancer Type Risk FactorLiver Hepatitis viruses (HCV.HBV)
Pancreas SmokingDiabetes
Being maleChronic pancreatitis
Kidney Tobacco smokingHigh blood pressure
Von-Hippel-Lindau syndrome (VHL)
Leukemia RadiationChemotherapy
Certain disease (Down syndrome)Human T cell leukemia virus
Myelodysplatic syndrome
Risk Factor/ Cancer Type
Cancer Type Risk Factor
Bladder OccupationCertain infection
Tobacco smokingRace Twice as often as Africans Americans
Treatment with cyclophosphamide or arsenic
Uterine Endometrial hyperplasiaRace Africans Americans
Hormonal replacement therapyObesity
Melanoma Dysplastic neviFair skin
Weakened immune systemSever blistering/Sunburn
UV irradiation
Risk Factor/ Cancer Type
Microorganism CancerHuman papilloma virus Cervical cancer
Helicobacter pylori Stomach cancer
Hepatitis B and hepatitis C viruses Liver cancer
Human T-cell leukemia/lymphoma virus
Lymphoma and leukemia
Human immunodeficiency virus Lymphoma and a rare cancer called Kaposi's sarcoma
Epstein-Barr virus Lymphoma
Human herpes virus 8 Kaposi's sarcoma
Certain viruses or bacteria may increase the risk of developing cancer
The etiology of cancer is multifactorial, with genetic, environmental, medical, and lifestyle factors interacting to produce a given malignancy.
Agents (molecules, radiation or viruses) that cause cancer are called carcinogens. This causal process is called carcinogenesis.
In humans, it is generally accepted that most epithelial cancers are caused by environmental exposure to certain kinds of chemicals. However, carcinogenesis is clearly influenced by large numbers of genes and non-carcinogenic environmental factors (notably diet, and underlying inflammation).
Etiology
Etiology
Most cancer is caused by genetic mutations often, by a series of mutations.
•New perceptions of previously known carcinogens: •Combined effects of multiple exposures
Decreases time available for DNA repairDecreases time available for DNA repair Converts repairable DNA damage into non-repairable Converts repairable DNA damage into non-repairable
mutationsmutations Necessary for chromosomal aberrations, insertions, Necessary for chromosomal aberrations, insertions,
deletions and gene amplificationdeletions and gene amplification Clonally expands existing cell populationsClonally expands existing cell populations
Cell Replication is Essential for Cell Replication is Essential for Multistage CarcinogenesisMultistage Carcinogenesis
THOUSANDS OF KNOWN CARCINOGENIC AGENTS ARE IN THE ENVIRONMENT
• Organic molecules (aromatic amines)• Inorganic molecules (vinyl chloride)• Heavy metals (lead, arsenic, chromium[VI]) • Viruses (HBV, HCV, HPV, HIV)• Radiation (gamma, X-ray, high energy beta)• Inert substances (asbestos)
Proportion Percentage
Chemicals tested in both rats and mice 350/590 59%
Naturally occurring chemicals 79/139 57%
Synthetic chemicals 271/451 60%
Chemicals tested in rats and/or mice
Chem. in Carcinogen. Potency Database 702/1348 52%
Natural pesticides 37/71 52%
Mold toxins 14/23 61%
Chemicals in roasted coffee 21/30 70%
Physician’s desk reference
Drugs with reported cancer tests 117/241 49%
FDA database of drug submissions 125/282 44%
Proportion of chemicals evaluated as Proportion of chemicals evaluated as carcinogeniccarcinogenic
Ames and Gold Ames and Gold Mutat ResMutat Res 447:3-13, 2000 447:3-13, 2000
Classification of Carcinogens According to the Mode Classification of Carcinogens According to the Mode of Actionof Action
GENOTOXICGENOTOXIC:: DNA-reactive or DNA-reactive metabolitesDNA-reactive or DNA-reactive metabolites Direct interaction to alter chromosomal Direct interaction to alter chromosomal
number/integritynumber/integrity May be mutagenic or cytotoxicMay be mutagenic or cytotoxic Usually cause mutations in simple systemsUsually cause mutations in simple systems
DNA AdductDNA Adduct MutationMutation CancerCancer
GENOTOXIC NON-GENOTOXIC NON-GENOTOXICGENOTOXIC
Mechanism of Carcinogenesis:Mechanism of Carcinogenesis:GenotoxicGenotoxic Carcinogens Carcinogens
1. Carcinogen activation1. Carcinogen activation 2. DNA binding2. DNA binding 4. Gene4. Gene mutationmutation
Chemical "Activated“carcinogen
3. Cell proliferation3. Cell proliferation(fix mutation)(fix mutation)
“inactivated“carcinogen
CYP450s
DNA Repair APOPTOSIS
the mechanism of causing cancer by exposure to polycyclic aromatic hydrocarbonsthe mechanism of causing cancer by exposure to polycyclic aromatic hydrocarbons
Classification of Carcinogens According to the Mode Classification of Carcinogens According to the Mode of Actionof Action
NON-GENOTOXICNON-GENOTOXIC:: Do not directly cause DNA mutation Mechanism of action is not completely
understood Difficult to detect - requires rodent
carcinogen bioassay
?? MutationMutation CancerCancer
Mechanisms of Non-Genotoxic Mechanisms of Non-Genotoxic CarcinogenesisCarcinogenesis
““black box” black box”
Increased cell proliferationIncreased cell proliferation
Decreased apoptosisDecreased apoptosis
Changes in gene expression Changes in gene expression
Induction of metabolizing enzymesInduction of metabolizing enzymes
Activation of receptors (signaling)Activation of receptors (signaling)
Oxidative stressOxidative stress
??????
Oxidative StressOxidative Stress
Indirect DNA damageIndirect DNA damage
Induction of cell proliferation/apoptosis signaling Induction of cell proliferation/apoptosis signaling
cascadescascades
Non-Genotoxic CarcinogensNon-Genotoxic Carcinogens
1)1) Mitogens: Mitogens: • stimulation of proliferationstimulation of proliferation• mutations may occur secondarily to cell proliferationmutations may occur secondarily to cell proliferation• may cause preferential growth of preneoplastic cellsmay cause preferential growth of preneoplastic cells
2) 2) Cytotoxicants: Cytotoxicants: • cytolethalcytolethal• induce regenerative growthinduce regenerative growth• mutations maymutations may occur secondarily to cell proliferation occur secondarily to cell proliferation
Mechanism of Carcinogenesis:Non-Genotoxic Carcinogens
Cell proliferation (to fix “spontaneous” mutation)Cell proliferation (to fix “spontaneous” mutation)
APOPTOSIS
CANCER
X
Apoptosis
• Apoptosis is a tightly regulated form of cell death, also called the programmed cell death.
• Morphologically, it is characterized by chromatin condensation and cell shrinkage in the early stage. Then the nucleus and cytoplasm fragment, forming membrane-bound apoptotic bodies which can be engulfed by phagocytes.
• In contrast, cells undergo another form of cell death, necrosis, swell and rupture. The released intracellular contents can damage surrounding cells and often cause inflammation.
Cancer: Pathogenesis
• General mechanisms:General mechanisms:– Acquired capabilities (Self-maintained replication, Acquired capabilities (Self-maintained replication,
longer survival, genetic instability, longer survival, genetic instability, neoangiogenesis, invasion and metastasis)neoangiogenesis, invasion and metastasis)
– Activation of oncogenes, inactivation of tumor-Activation of oncogenes, inactivation of tumor-suppressor genes, non-effective DNA repairsuppressor genes, non-effective DNA repair
– Caretaker and gatekeeper pathwaysCaretaker and gatekeeper pathways
• Caretaker genes encode products that stabilize the genome. Mutations in caretaker genes lead to genomic instability.
• Gatekeeper genes encode gene products that act to prevent growth of potential cancer cells and prevent accumulation of mutations that directly lead to increased cellular proliferation.
Error in DNA replication (randomly acquired).
Effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents.
Inheritance, and thus present in all cells from birth.
Abnormalities in the genetic material due to:
Series Mutation can Lead to Cancer
The Multistep Model of Cancer Development
• Multiple mutations are generally needed for full-fledged cancer; thus the incidence increases with age
• At the DNA level, a cancerous cell is usually characterized by at least one active oncogene and the mutation of several tumor-suppressor genes
Colon
Normal colonepithelial cells
Lossof tumor-suppressorgene APC(or other)
1
2
3
4
5Colon wall
Small benigngrowth(polyp)
Activationof rasoncogene
Lossof tumor-suppressorgene DCC
Lossof tumor-suppressorgene p53
Additionalmutations
Malignanttumor(carcinoma)
Largerbenign growth(adenoma)
Initiating
Event
Cell Proliferation
(clonal expansion)
Progression
Cell Proliferation
Cell Proliferation
Malignancy
Second Mutating
Event
"N" Mutating Event
Initiation
Promotion
Stages of CarcinogenesisStages of Carcinogenesis
Cellular and Molecular Mechanisms in Multistage Carcinogenesis: INITIATION
Initiating event involves cellular genome – MUTATIONS
Target genes: - oncogenes/tumor suppressor genes
- signal transduction
- cell cycle/apoptosis regulators“ Simple” genetic changes
SOURCES OF SOURCES OF MUTATIONSMUTATIONS
ENDOGENOUS DNA DAMAGEENDOGENOUS DNA DAMAGE EXOGENOUS DNA DAMAGEEXOGENOUS DNA DAMAGE
DepurinatioDepurinationn
DNA REPAIRDNA REPAIR
MUTATIONMUTATION
LifLifeeStyleStyless
EnvironmentaEnvironmentallAgentAgentss
FreFreee
RadicalsRadicals
PolymerasPolymerasee
ErrorsErrors
CELL REPLICATIONCELL REPLICATION
Chemical ExposureChemical Exposure (air, water, food, etc.) (air, water, food, etc.)
Internal ExposureInternal Exposure
Metabolic ActivationMetabolic Activation
Macromolecular BindingMacromolecular Binding DetoxicationDetoxication
DNADNA RNARNA ProteinProtein
Biologically Effective DoseBiologically Effective Dose
Efficiency of MispairingEfficiency of Mispairing
Cell ProliferationCell Proliferation
XX
XXInitiationInitiation
(Biomarker)(Biomarker)
Epigenetic alterations – changes induced in cells that alter the expression of the information on transcriptional, translational, or post-translational levels without changes in DNA sequence
EPIGENETICS
SAM SAH
DNMT1DNMT3aDNMT3b
Methylation of DNA
Modifications of histones
RNA-mediated modifications
• RNA-directed DNA methylation
• RNA-mediated chromatin remodeling
• RNAi, siRNA, miRNA …
A
Me
P
U
- acetylation
- methylation
- phosphorylation
- ubiquitination
P UMe
A
GENETIC AND EPIGENETIC MODELS OF THE CANCER INITIATION
Epigenetically reprogrammed cells
Mutator phenotype cells
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ALTERATIONS IN CELLULAR EPIGENOME
Normal cells
Cancer cells
Clonal selection and expression of initiated cells
Mutator phenotype cells
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ACQUISITION OF ADDITIONAL RANDOM MUTATIONS
Normal cells
Cancer cells
Cellular and Molecular Mechanisms in Multistage Carcinogenesis: PROMOTION
Reversible enhancement/repression of gene expression:
- increased cell proliferation
- inhibition of apoptosis
No direct structural alteration in DNA by agent or its metabolites
Cellular and Molecular Mechanisms in Multistage Carcinogenesis: PROGRESSION
• Irreversible enhancement/repression of gene expression
• Complex genetic alterations (chromosomal translocations, deletions, gene amplifications, recombinations, etc.)
• Selection of neoplastic cells for optimal growth genotype/ phenotype in response to the cellular environment
“ Complex”
genetic changes
• Immortalization
• Transformation
• Loss of contact growth inhibition
• Autonomy of proliferation
• Avoidance of apoptosis
• Aberrant differentiation
• Induction of angiogenesis
Phenotypic characteristics of cancer cells:
Tumor Cells and the Onset of Cancer
Hallmarks of Cancer Cells
• Self-maintained replication
• Longer survival• Genetic instability• Capable of inducing
neoangiogenesis• Capable of invasion
and metastasis
– Apoptosis down-regulation
– Lack of response to inhibitory factors
– Self-sustained proliferation
Hallmarks of Cancer Cells
• Self-maintained replication
• Longer survival• Genetic instability• Capable of inducing
neoangiogenesis• Capable of invasion
and metastasis
– Apoptosis down-regulation
– Telomerase reactivation
Hallmarks of Cancer Cells
• Self-maintained replication
• Longer survival• Genetic instability• Capable of inducing
neoangiogenesis• Capable of invasion
and metastasis
– Cooperative genetic damage
– Mutagenic agents
– Defective repair systems
Hallmarks of Cancer Cells
• Self-maintained replication
• Longer survival• Genetic instability• Capable of inducing
neoangiogenesis• Capable of invasion
and metastasis
Multistep Tumorigenesis
Stages of tumour development
Malignant cell
Dissemination of other organs
Invasion
Proliferation Angiogenesis
Neovascular endothelial
maintenance
Invasion
CytotoxicsEndocrineEGFR inhibitorsHER2 antibodies
Anti-angiogenics
Vasculartargeting agents Novel
agentsNovelagents
MetastaticCancer
of other organs
Types of Genes Associated with Cancer
• Cancer can be caused by mutations to genes that regulate cell growth and division
• Tumor viruses can cause cancer in animals including humans
Alteration of Gene ExpressionAlteration of Gene Expression
Nuclear (hormone-like) receptors Nuclear (hormone-like) receptors
Kinase cascadesKinase cascades
Calcium-, nitric oxide-mediated signalingCalcium-, nitric oxide-mediated signaling
Transcription factorsTranscription factors
Gene methylation status (hypo -> enhanced gene Gene methylation status (hypo -> enhanced gene
expression; hyper -> gene silencing)expression; hyper -> gene silencing)
What types of genes get mutated in cancer?
• Oncogenes are activated– Normal function: cell growth, gene transcription
• Tumor suppressor genes are inactivated– Normal function: DNA repair, cell cycle control,
cell death
• Oncogenes are cancer-causing genes• Proto-oncogenes are the corresponding normal
cellular genes that are responsible for normal cell growth and division
• Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle
• Proto-oncogenes can be converted to oncogenes by
– Movement of DNA within the genome: if it ends up near an active promoter, transcription may increase
– Amplification of a proto-oncogene: increases the number of copies of the gene
– Point mutations in the proto-oncogene or its control elements: cause an increase in gene expression
Proto-oncogene
DNA
Translocation ortransposition: genemoved to new locus,under new controls
Gene amplification:multiple copies ofthe gene
New promoter
Normal growth-stimulatingprotein in excess
Normal growth-stimulatingprotein in excess
Point mutation:within a control
elementwithin
the gene
Oncogene Oncogene
Normal growth-stimulatingprotein inexcess
Hyperactive ordegradation-resistantprotein
Tumor-Suppressor Genes
• Tumor-suppressor genes help prevent uncontrolled cell growth
• Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset
• Tumor-suppressor proteins
– Repair damaged DNA
– Control cell adhesion
– Inhibit the cell cycle in the cell-signaling pathway
Interference with Normal Cell-Signaling Pathways
• Mutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancers
• Mutations in the ras gene can lead to production of a hyperactive Ras protein and increased cell division
Cancer Molecular Pathways
TUMOR SUPPRESSOR GENES
Disorders in which gene is affected
Gene (locus) Function Familial Sporadic
DCC (18q) cell surface unknown colorectal interactions cancer
WT1 (11p) transcription Wilm’s tumor lung cancer
Rb1 (13q) transcription retinoblastoma small-cell lung carcinoma
p53 (17p) transcription Li-Fraumeni breast, colon, syndrome & lung cancer
BRCA1(17q) transcriptional breast cancer breast/ovarian tumors
BRCA2 (13q) regulator/DNA repair
Inherited Predisposition and Other Factors Contributing to Cancer
• Individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes
• Inherited mutations in the tumor-suppressor gene adenomatous polyposis coli （ APC ） are common in individuals with colorectal cancer
• Mutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers, and tests using DNA sequencing can detect these mutations
Multiple Stages of Human Colon Cancer• It is estimated that by age 70, 50% of the population at large have acquired pre-
cancerous adenomas in the colon; 10% of this group will progress to malignancy in the following 10 years.
• Familial Adenomatous Polyposis (FAP) is linked to the APC gene whose protein is involved in ß-catenin signaling. The gene acts as a tumor suppressor, and the loss of function mutation causes development of hundreds to thousands of adenomas, with a consequent high risk of progression to malignancy.
Multiple Stages of Human Colon Cancer
APC protein (Adenomatous Polyposis Coli) is normally expressed in colorectal epithelial cells, a site of relatively high natural proliferation rates. The epithelium is convoluted into deep recesses called crypts and projections called villi. Crypts contain stem cells for tissue replacement, and the base of the crypt is a site of high mitotic activity. As cells age, they progress up the villus to the tip.
Polyps Adenomas Progression to cancer
Germline APC+/–FAP
>90% by age 20 >90% by age 30 50% by age 40
Germline APC+/+normal
50% by age 70 5%
Cell accumulation and dysplasia
hyperplasia, aneuploidy
proliferating, anti-apoptotic, metastatic, angiogenic
Growthfactor
1
2
3
4
5
1
2
Receptor
G protein
Protein kinases(phosphorylationcascade)
NUCLEUSTranscriptionfactor (activator)
DNA
Gene expression
Protein thatstimulatesthe cell cycle
Hyperactive Ras protein(product of oncogene)issues signals on itsown.
(a) Cell cycle–stimulating pathway
MUTATION
Ras
Ras
GTP
GTP
P
P
P P
P
P
(b) Cell cycle–inhibiting pathway
Protein kinases
UVlight
DNA damagein genome
Activeformof p53
DNA
Protein thatinhibitsthe cell cycle
Defective or missingtranscription factor,
such asp53, cannot
activatetranscription.
MUTATION
EFFECTS OF MUTATIONS
(c) Effects of mutations
Proteinoverexpressed
Cell cycleoverstimulated
Increased celldivision
Protein absent
Cell cycle notinhibited
3
• Suppression of the cell cycle can be important in the case of damage to a cell’s DNA; p53 prevents a cell from passing on mutations due to DNA damage
• Mutations in the p53 gene prevent suppression of the cell cycle
p53 in apoptosis
Following DNA damage, e.g. by radiation, p53 levels rise, and proliferating cells arrest in G1. This allows time for DNA repair prior to the next round of replication. This arrest is mediated by stimulation of expression of p21CIP1, the cyclin kinase inhibitor. Very high p53 levels, or susceptible cell types, e.g. lymphocytes, are triggered to undergo apoptosis. Bcl-2 acts between p53 and the caspase.
P53 can bind to DNA
DNA
p53
P53 as a transcription factor which exerts its effect by regulating other genes
Allow cells time to repair the damaged DNA
Induce DNA repair enzymes
Li-Fraumeni Syndrome(hereditary p53 mutation)High risk of getting mutation of the second copy of the gene
DNA damaging chemicals and radiation
Tobacco---Lung cellsUVB --- Skin cells
p53 upregulated modulator of apoptosis
P53 and the cell cycle
P53 arrests the cell cycle primarily by upregulating p21 (Cip1/Waf-1), which inactivates CDK/cyclin
P53 can also activate apoptosis
P21 is a kinase inhibitor
Inhibition of p53 functions
Genetic Instability in Tumors
• (+) Oncogenes• (-) Tumor
suppressor genes• Telomere shortening
• Mismatch repair (MMR) genes
• Chromosomal Instability
• Microsatellite Instability
Aneuploidy in Human Hepatocellular Carcinoma Cell Line
Hsr = homogeneously staining region due to endoreduplication of chromosomal segments resulting in gene amplification
Figure 1.11b The Biology of Cancer (© Garland Science 2007)
Fluorescent in situ hybridization (FISH) of normal metaphase human chromosomes
using chromosome specific DNA probes with different fluorescent dyes
Aneuploid karyotype of human breast cancer cell.
Note “scrambling” of colors demonstrating chromosomal reciprocal translocations
Intra-chromosonal inversion by M-band fluorescent in situ hybridization(mFISH)
Telomeres and Cell Senescence
1800 HumanGenes
mRNA’s From 142 different human tumors
Red = elevated expression
Green = diminished expression
Gene Expression DNA Array Analysis
Tumor Immunity
• General Principles– Tumors not entirely self
• Express non-self proteins
– Immune-mediated recognition of tumor cells may be “positive mechanism of eliminating transformed cells
• Immune surveillance
Tumor Antigens
• Tumor Specific Antigens– Present only on Tumor cells– Recognized by cytotoxic T cells
• Bound by class I MHC
– Several antigens in humans found that are not unique for tumor, however are generally not expressed by normal tissue
• Melanoma-associated antigen-1 (MAGE-1): – Embryonal protein normally expressed in testis
» Melanomas, breast ca, lung ca
Tumor Antigens
• Tumor Associated Antigens– Not unique to tumors, shared by normal
cells• Differentiation- specific antigens
– CALLA (CD10) in early B cells– Prostate specific antigen PSA
Antitumor Effector Mechanisms
• Cytotoxic T-cells– MHC restricted CD-8 cells (viruses)
• NK cells– Destroying tumor cells without prior sensitization
• Macrophages– Ifn-gamma
• Humoral Mechanisms– Via complement and NK cells
Antitumor Effector Mechanisms
Cytotoxic T-cell NK cell
MacrophageHumoralMechanisms
IMMUNOSURVAILLANCE
• Argument for:– Increased cancer in immunodeficient hosts
• 200x increase in immunodeficiencies (lymphoma)– X-linked lymphoproliferative disorder (XLP
» EBV related
• Escape Mechanism Theories– Selective outgrowth of antigen-negative variants– Loss or reduction of HLA (escape T-cells)– Immunosuppression (Tumors secrete factors TGF-
b)
IMMUNOTHERAPY
• Replace suppressed components of immune system or stimulate endogenous responses– Adoptive Cellular Therapy
• Incubation of lymphocytes with IL-2 to generate lymphokine activated killer (LAK) cells with potent antitumor activity
– Enriched tumor specific cytotoxic T cells» Tumor infiltrating lymphocytes (TIL)
Cytokine Therapy
– Activate specific and nonspecific (inflammatory) host defenses.
• Interferon-a, TNF-a, Il-2, IFN-g– IFN-a activates NK cells, increase MHC expression
on tumor cells» Used for hairy cell leukemia
Antibody-Based Therapy
– Antibodies as targeting agents– Direct use of antibodies to activate host
immune system• Her-2/neu in advance breast cancer
Sample questions
• The event(s) which does not occur during interphase, is/are
• A. Chromatin condenses• B. Protein Synthesis• C. Organelles replication• D. DNA replication
• At the center of the cell cycle control system is Cdk, a protein that
• A. is phosphorylated to become active• B. binds to different cyclins• C. is only active during mitosis• D. manufactures growth factors
Sample questions
• An oncogene is
• A. a viral gene with no relation to the host cell's genes.
• B. a mutated form of a proto-oncogene.• C. a bacterial gene that causes cancer in
the host.• D. a gene that turns off cellular reproduction.