2 Lecture Outline, 11/30/05 Review the Cell cycle Cancer is a genetic diseaseOncogenes and proto-oncogenesNormally promote cell growth.Become oncogenic after point mutations, duplications, deletion of silencer, etcTumor Suppressor genesNormally inhibit cell growth.Allow cell growth when damaged or deleted.Mutator genesThe multi-step model of cancer
3 Cancer is caused by mutant genes Mutations in regulatory genes lead to uncontrolled cell growthUnderstanding gene regulation is one key to understanding cancerAll cancers trace back to single cellMust accumulate multiple mutations, all in the same cell lineage
8 Free E2F is a transcription activator Rb represses E2FCyclin/Cdk inactivates Rb by phosphorylation
9 P53 is a transcription factor that activates p21, an inhibitor of cyclin/cdk2
10 Signaling pathways that regulate cell division 1Growthfactor2ReceptorpGTPRas3G proteinHyperactiveRas protein(product ofoncogene)issues signalson its own4Protein kinases(phosphorylationcascade)5Transcriptionfactor (activator)NUCLEUSDNAGene expressionProtein thatstimulatesthe cell cycleUVlightDNA damagein genome1Activeformof p53Defective ormissingtranscriptionfactor, such asp53, cannotactivateMUTATIONinhibitsEFFECTS OF MUTATIONSProteinoverexpressedCell cycleoverstimulatedIncreased celldivisionCell cycle notinhibitedProtein absent(a) Cell cycle–stimulating pathway.This pathway is triggered by a growthfactor that binds to its receptor in theplasma membrane. The signal is relayed toa G protein called Ras. Like all G proteins, Rasis active when GTP is bound to it. Ras passesthe signal to a series of protein kinases.The last kinase activates a transcriptionactivator that turns on one or more genesfor proteins that stimulate the cell cycle. If amutation makes Ras or any other pathwaycomponent abnormally active, excessive celldivision and cancer may result.(b) Cell cycle–inhibiting pathway. In thispathway, DNA damage is an intracellularsignal that is passed via protein kinasesand leads to activation of p53. Activatedp53 promotes transcription of the gene for aprotein that inhibits the cell cycle. Theresulting suppression of cell division ensuresthat the damaged DNA is not replicated.Mutations causing deficiencies in anypathway component can contribute to thedevelopment of cancer.(c) Effects of mutations. Increased cell division,possibly leading to cancer, can result if thecell cycle is overstimulated, as in (a), or notinhibited when it normally would be, as in (b).MUTATIONFigure 19.12
11 Stimulation versus inhibition of G1 progression Stimulation of G1 progressionInhibition of G1 progressionmitogensDNA-damageanti-mitogen (TGF-b)mycrasPKBp53p15?cyclin D / cdk4p27p21cyclin D / cdk4p16PPpRBpRBpRBpRBPPPPInhibitionS-phase entryInhibitionS-phase entryS-phase entryallowedS-phase entryallowedoncogenestumor suppressor genes
12 OncogenesAll are involved in positive control of cell growth and division.About 100 different oncogenes have been identifiedCan be various kinds of proteins:Growth factors, regulatory genes involved in the control of cell multiplication.Protein kinases, add phosphate groups to target proteins, important in signal transduction pathways.“Proto-oncogenes”Normal form of the gene that is involved in positive regulation of the cell cycle
13 Genetic changes that can turn proto-oncogenes into oncogenes DNATranslocation or transposition:gene moved to new locus,under new controlsGene amplificationPoint mutationwithin a controlelementwithin the geneOncogeneNormal growth-stimulatingprotein in excessHyperactive ordegradation-resistant proteinNewpromoterFigure 19.11
14 Translocation puts abl under the control of a different promoter
16 Ras Proto-oncogene Mutated in 30% of all cancers. A “molecular switch” in the signal transduction pathway leading from growth factors to gene expression controlling cell proliferation:GF receptor Ras TF target genes growth.A single amino acid change in Ras protein can cause constant stimulation of the pathway, even in the absence of growth factors.
17 Receptor tyrosine kinases can activate ras ras is a monomeric G-protein“molecular switch”You’ve seen RAS before . . .
18 Mitogen Activated Protein Kinases MAPKs Ras activation sets off a phosphorylation cascadeMitogenActivatedProteinKinasesMAPKs10100MAPKKKMAPKK1,000Controls:-TranscriptionFactors-Translation-Cell DivisionMAPK100,000
19 - broken ras – won’t shut off PROBLEMS IN CANCER:- broken ras – won’t shut offBroken receptor – thinks ligand thereeven when it isn’tbroken MAPK – on all the time, even whennot phosphorylatedRESULT: continuous signal for cell to divide
23 p53 Gene Detects DNA damage The “Last Gatekeeper” Involved in 50% of cancersOften not malignant despite other cancer-causing mutations until p53 is inactivated by mutation.Two possible responses to DNA damage:1) Acts as a Transcription Factor to activate expression of p21, which inhibits CDK/G1 cyclin to halt the cell cycle; then activates DNA repair.2) Triggers Apoptosis (programmed cell death) if damage can’t be repaied.
24 Apoptosis = controlled cell death “executioner” proteins (caspases) break down the cellReduced cell death can also lead to cancer
27 Mutator genesCancer is caused by mutations, so factors that increase mutation rate will increase cancer rate.What kinds of genes would increase mutation rate?Example: BRCA1 and BRCA2Many environmental factors (carcinogens) also cause DNA damage or mutations, that can lead to cancer
28 A multistep model for the development of colorectal cancer (1) The clonal origin of tumors: each individual cancer is a clone that arises from a single cell.The progeny cells have growth advantage over the surrounding normal cells.Colon(2) Cancer development is a multi-step process. Multiple mutations accumulated over periods of many years ----“multi-hit” model.1 Loss oftumor-suppressorgene APC (orother)2 Activation ofRas oncogene4 Loss oftumor-suppressorgene p53Colon wall3 Loss oftumor-suppressorgene DCC5 AdditionalmutationsNormal colonepithelial cellsSmall benigngrowth (polyp)Larger benigngrowth (adenoma)Malignant tumor(carcinoma)Figure 19.13