Flashcards in 9.14.16 Lecture Deck (65):
What are the 6 hallmarks of cancer?
1. Evade growth suppressors
2. Sustain proliferative signaling
3. Resist cell death
4. Induce angiogenesis
5. Enable replicative immortality
6. Activate invasion and metastasis
Cancer is a ___ disease.
The incidence of cancer increases with age. What does this suggest?
Cancers do not arise from single mutations; they reflect accumulating mutations over time.
Cancers can arise from what two types of mutations?
1. Heritable germline mutation
2. Somatic de novo mutation
What are the two main types of cancer genes?
1. Oncogenes - drive cell cycle
2. Tumor suppressor genes - block cell cycle
What is the two-hit hypothesis?
This hypothesis states that many cancers require both copies of a gene to be mutated to lead to cancer. One germline copy of a damaged gene in every cell i not sufficient to develop cancer. Loss of the good copy of the gene could occur, producing cancer.
Tumor suppressors act ___ phenotypically; the germline mutation is inherited ___.
Recessive; autosomal dominant
What are the 6 oncogene pathways that drive the cell cycle and lead to the promotion of growth and malignancy of tumors?
1. Promote cell division
2. Inhibit apoptosis
3. Promote immortality
4. Promote angiogenesis
5. Promote metastasis
6. Promote genetic instability (via #2 and #3)
What are the 5 tumor suppressor gene pathways that normally prevent cancer by restricting growth?
1. Inhibit cell division
2. Promote apoptosis
3. Inhibit immortality
4. Inhibit angiogenesis
5. Inhibit metastasis
Provide an overview of apoptosis, beginning with the 3 stimulators of the central cell death signal and ending with phagocytosis.
TNFalpha and Fas death receptors, growth factor withdrawal, and DNA damage/activation of p53 can all activate the central cell death signal. This leads to protease activation of caspases. This leads to endonuclease activity, cell surface alterations, and cytoskeletal reorganization. This leads to phagocytosis. Bcl2 can block the cell death signal and CrmA and p35 can block protease activation of caspases.
What are the two apoptotic pathways?
1. Extrinsic - death receptor pathway
2. Intrinsic - mitochondrial-dependent pathway
Where is the cross-talk between the extrinsic and intrinsic pathways?
Caspase 8 can signal tBID.
What are caspases?
Cys/Asp proteases synthesized in the cell as inactive precursors
True or false - caspases can only be activated through the extrinsic pathway.
False - caspases can be activated by signals from both pathways.
How are caspases activated?
Cytosolic procaspases exist as inactive monomers (initiator caspase). The apoptotic signal triggers adaptor proteins, which trigger dimizeration, activation, and cleavage to form the active caspase. This activates (via cleavage) executioner caspases (inactivated as a dimer), which cleave at multiple substrates, leading to apoptosis.
What are the initiator caspases?
Caspases 8 and 9
What are the executioner caspases?
Caspases 3, 6, 7
What are the two domains of the initiator caspases (in their cytosolic procaspase form)?
1. Adaptor binding domain
2. Protease domain
Describe the process of DNA fragmentation during apoptosis.
1. CAD, an endonuclease, is inactivated by iCAD, an inhibitor.
2. An active executioner caspase cleaves iCAD, activating CAD.
3. CAD cleaves DNA between nucleosomes, leading to fragmentation.
The extrinsic pathway of apoptosis is activated through ___.
Fas death receptors
Describe the death receptor-mediated pathway.
1. Fas ligands (or other tumor necrosis factor ligands) bind to Fas death receptors.
2. Several ligand-bound receptor trimers cluster together.
3. The death domains on the receptor tails are activated.
4. The death domains interact with domains on the FADD (Fas-associated death domain).
5. FADD recruits initiator caspases (caspase 8) via a death effector domain on both FADD and the caspase, forming DISC (death inducing signaling complex)
6. Within DISC, 2 adjacent initiator caspases interact and cleave one another
7. Form activated protease dimer, which leaves itself
8. Stabilizes and releases active caspase dimer into the cytosol, leading to activation of the executioner caspases
The intrinsic pathway of apoptosis depends on the ___.
Describe the intrinsic pathway of apoptosis.
1. Apoptotic stimuli causes the mitochondria to release cytochrome c.
2. Cytochrome c activates Apaf1, which unfolds partly
3. CARD domain is exposed
4. 7 active Apaf1 proteins interact at CARDs to form the apoptosome
5. Recruit and activate caspase 9
6. Caspase 9 cleaves and activates executioner caspases
Bcl-2 family members are related by regions of ___ and structural ___.
What are the three general types of Bcl-2 family members?
1. Anti-apoptotic Bcl2 family proteins (Bcl2, BclXL) - guardians
2. Pro-apoptotic effector Bcl2 family proteins (Bax, Bak) - effectors
3. Pro-apoptotic BH3-only proteins (Bad, Bim, Bid, Puma, Noxa) - initiators
Describe the differences in shared domains between the guardians, effectors, and initiators of the Bcl-2 family.
Guardians have BH4, BH3, BH1, BH2, TM. Effectors have all except BH4. Initiators only have BH3.
___ proteins regulate the intrinsic pathway of apoptosis. Describe how this occurs.
Bcl2 proteins; Inactive effector Bcl2 family proteins exist as monomers. An apoptotic stimulus leads to aggregation of effector Bcl2 family proteins on the outer mitochondrial membrane. This leads to the release of cytochrome c and other proteins.
In the absence of an apoptotic stimulus, what do anti-apoptotic Bcl2 family proteins do?
Bind to and inhibit the effector Bcl2 family proteins on the mitochondrial outer membrane.
In the presence of an apoptotic stimulus, what do BH3-only proteins do?
BH3-only proteins are activated and bind to the anti-apoptotic Bcl2 family proteins, inhibiting their inhibition. Note that some BH3-only proteins can stimulate mitochondrial protein release by directly binding to and activating the effector Bcl2 family proteins.
The activation of p53 triggers ___ via upregulation of ___ expression. Describe this process.
Apoptosis; BH3-only; DNA damage leads to kinase activation. p53 is phosphorylated, Mdm2 falls off, and p53 increases. This leads to transcription of many genes, including BH3-only genes (Puma and Noxa). BH3-only proteins bind to Bcl2, activating the intrinsic apoptotic pathway.
What are three ways in which extracellular survival factors inhibit apoptosis?
1. Increased production of anti-apoptotic Bcl2 family proteins through stimulus of these genes by survival factors.
2. Inactivation of pro-apoptotic BH3-only proteins
3. Inactivation of anti-IAPs
Describe the inactivation of pro-apoptotic BH3-only proteins.
Survival factors bind to the receptor and activate it. This activates Akt kinase. Bad is phosphorylated and inactivated. Bcl2 is freed and activated.
Describe the inactivation of anti-IAPs.
Survival factors bind to and active the receptor. This activates MAP kinase. Hid (anti-IAP protein) is phosphorylated and inactived. IAP is activated, which blocks apoptosis.
What are the two types of cancerous tumors?
1. Benign (constrained, adenoma): remain inside basal lamina that marks the boundary of the normal structure.
2. Malignant (invasive, adenocarcenoma): benign tumor that degrades the integrity of the tissue and travels from the original site to colonize other tissues (metastasis).
What are the heritable cancer cell properties?
1. Reproduce in defiance of normal cell constraints on growth and division
2. Invade and colonize territories normally reserved for other cells
What are the three classifications of cancer by cell of origin?
1. Carcinoma: arise from epithelial cells (cytokeratin positive)
2. Sarcoma: arise from connective tissue or muscle cells (vimentin positive)
3. Leukemia/lymphoma: arise from abnormal hematopoiesis (WBCs)
Most cancers derive from...
...a single abnormal cell.
Tumors are clonal - what does this mean?
They originate from a single parent cell (germline or somatic). When a cell gains a mutation, it passes it on to its progeny. Mutations continue to accumulate and are copied to descendent cells. If one cell acquires enough mutations to become cancerous, cancer cells are then derived from that one transformed cell.
Cancer cells contain ___ mutations (and perhaps ___ changes); a single mutation is not enough to change a normal cell into a cancer cell.
Carcinogenesis is linked to ___ (change in DNA sequence)
Cancers develop gradually from increasingly aberrant cells and tumor progression involves successive rounds of random inherited change followed by natural selection. Describe the example of colorectal carcinoma.
Development of colorectal carcinoma results from a series of genetic changes.
1. Normal epithelium changes to hyperplastic epithelium (loss of Apc)
2. Hyperplastic epithelium changes to early adenoma
3. Early adenoma changes to intermediate adenoma (k-RAS activation)
4. Intermediate adenoma changes to late adenoma (loss of TSG)
5. Late adenoma changes to carcinoma (loss of p53)
Invasion and metastasis occur.
What are the three major steps of clonal evolution of cancers?
1. Acquisition of increasing numbers of mutations
2. Cell proliferation
3. Invasion through the basal lamina
Clones are often ___ in that cells with different genetic compositions (mutations) arise in each tumor.
Many cancers are genetically ___. Describe this process.
Unstable; Self-renewal of epithelial cell population by repeated cell division leads to telomere shortening/uncapping. Normally, this leads to normal p53 levels and cell cycle checkpoint control. The normal senescent cell stops dividing. However, p53 can be lost and cell cycle checkpoint control fails. A mutant cell survives and proliferates. A cycle of chromosome fusion, bridging, breakage, and translocation begins. This leads to massive chromosomal damage. Most cells will die due to catastrophic genomic instability and DNA damage. However, sometimes the telomerase is reactivated. Chromosomes are partially stabilized and the cell survives with many mutations. This leads to cancer.
Cancer cell display an altered control of growth in several ways. What are the 4 major ways in which this occurs?
Transformation, altered metabolism, evasion of apoptosis, and senescence.
Describe the transformed phenotype of cancer cells.
Abnormal in shape, motility, responses to growth factors, loss of contact inhibition, continue dividing and moving even after confluence
What is the Warburg effect?
Non-proliferating cells normally oxidize glucose to ATP through oxidative phosphorylation. When deprived of oxygen, these cells use glycolysis followed by conversion of pyruvate to lactate to regenerate NAD+. Tumor cells, however, produce abundant lactate even with oxygen present due to an increased rate of glycolysis fed by a large increase in glucose import. Tumor cells then resemble rapidly proliferating embryo cells.
A decrease in apoptosis in tumors contributes to growth. Normal cells balance cell division and apoptosis. Describe what happens in abnormal cells in this respect?
Abnormal cells either increase cell division, decrease apoptosis, or both.
___ often occurs inside tumors, spilling cell contents and increasing inflammation, which drives tumor progress.
How is senescence avoided in tumor cells?
By activating or maintaining telomerase (telomeres do not shorten) and by inactivating p53 to block sensecence
The tumor microenvironment influences cancer development via what three things?
Invasion, metastasis, and EMT
The stroma provides a framework for the tumor. Explain.
Cancer cells secrete signals that alter stroma cell behavior as well as proteolytic enzymes that modify the extracellular matrix. The stromal cells respond by secreting proteins that stimulate cancer growth.
What is metastasis?
A multi-step process involving invasion of local tissues, movement through the circulatory system, leaving the blood vessels, and establishing new cellular colonies at distant sites.
Cancer cells bind to ___ in the basal lamina.
___ secreted by cancer cells allow for invasion (penetration into tissues).
What are the three steps to invasion?
1. Bind to laminin via receptors on tumor cells, inducing secretion of proteolytic enzymes
2. Digest basal lamina via type-IV collagenase
Which parts of metastasis are difficult?
Escape from parent tissue (invasiveness causes entry into vessel) and colonization of remote site (survival of cells in foreign tissue, initial growth of cells in foreign tissue, and persistence of growth); Note that cells must acquire these key properties in order to become metasti
What parts of metastasis are easy?
Travel through circulation (survival in circulation, arrest in capillary/small vessel, exit into remote tissue/organ)
Metastic cells have a ___ phenotype. In this state, the cell acquires a stem-like de-differntiated phenotype.
In order to colonize its target, the metastic cell must do what?
Reverse its phenotype by transitioning back to a more differentiated epithelial state.
Describe the process of EMT.
1. Cells lose E-cadherin in plasma membrane
2. Cells lose cell-cell and ECM contacts, round-up, and acquire motility.
3. PI3K/Akt and RAS/MAPK pathways are activated; pluripotent TF are activated.
4. Increased protease secretions (urokinase plasminogen activator, type IV procollagenase) coupled with decrease protease inhibitor and extracellular matrix secretions.
What is angiogenesis?
The ability of a tumor to stimulate new blood vessel formation; allows for tumor expansion, local invasion, and dissemination through delivery of oxygen, nutrients, and survival factors.
Describe the process of angiogenesis.
Tumor cells secrete growth factors (VEGF and FGF). These act on the vascular endothelial cell to increase expression of procollagenase and urokinase plasminogen activator (UPA), as well as inhibition of protease inhibitors.
Anti-angiogenic drugs target the ___ cells, not the cancer cells themselves.