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The Biology of Cancer
Peter Curtin, M.D.
Professor of Clinical Medicine
Clinical Director, Blood and Marrow Transplantation Division
September 12, 2013
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The Hallmarks of Cancer
• A conceptual framework for understanding the biological diversity of
cancer and to understanding the multistep process of progression
from normal cells to a neoplastic state
• Hallmarks of Cancer are eight, acquired, functional capabilities that
allow cancer cells to survive, proliferate and disseminate
• First proposed by Hanahan and Weinberg in 2000
• Updated and expanded by Hanahan and Weinberg (Cell, 2011)
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The Hallmarks of Cancer
• 1. Sustaining proliferative signaling
• 2. Evading growth suppressors
• 3. Resisting cell death
• 4. Enabling replicative immortality
• 5. Inducing angiogenesis
• 6. Activating invasion and metastasis
• 7. Deregulating cellular energetics
• 8. Avoiding immune destruction
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1. Sustaining proliferative signaling
• Cancer cells need to sustain chronic proliferation
• Growth promoting signals are normally conveyed by growth factors
binding to cell surface receptors which typically contain intracellular
tyrosine kinase domains
• The tyrosine kinase domain transmits signals via downstream
signaling protein pathways to control progression through the cell
cycle, cell growth and to influence cell survival and energy metabolism
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Growth factor receptor with tyrosine kinase
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Sustaining proliferative signaling
• Mechanisms to sustain proliferative signaling:
• 1. Cancer cells produce growth factors themselves (autocrine
stimulation)
• 2. Stimulate nearby normal (stromal) cells to produce growth factors
• 3. Increased levels of cell surface receptor proteins
• 4. Structurally abnormal receptors, active in the absence of growth
factor
• 5. Constitutive activation of signaling proteins downstream from the
receptor
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2. Evading growth suppressors
• Cancer cells must overcome programs that negatively regulate cell
proliferation
• These programs depend on the action of tumor suppressor genes that
normally govern the decision of cells to proliferate or to undergo
program cell death (apoptosis)
• Tumor suppressor genes have typically been discovered when
inactivation leads to the development of cancer
• The retinoblastoma (RB) protein and p53 protein are two tumor
suppressors (among many described)
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Evading growth suppressors
• RB protein integrates extracellular and intracellular signals to decide
whether the cells should proceed thru growth and division
• Loss of RB function (by deletion or mutation) removes a gatekeeper of
cell cycle progression resulting in persistent cell proliferation
• p53 protein senses intracellular stress and abnormality
• If DNA damage is present or if growth promoting signals, oxygen or
glucose are suboptimal, p53 can stop cell cycle progression until
these conditions normalize
• If overwhelming or irreparable damage to intracellular systems occurs,
p53 can trigger program cell death (apoptosis)
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3. Resisting cell death
• Programmed cell death by apoptosis is a natural barrier to the
development of cancer
• The apoptotic machinery is composed of upstream regulator proteins
and downstream effector proteins
• Regulators
• Extracellular/extrinsic pathway: Fas ligand and receptor, Tumor
necrosis factor (TNF)/TNF receptor
• Intracellular/intrinsic pathway: senses intracellular signals
• Effectors: inactive proteases (caspases 8 and 9) are activated
initiating a proteolytic cascade leading to cellular disassembly and
consumption (apoptosis)
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Extrinsic & Intrinsic Apoptotic Pathways
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Resisting cell death:
The Bcl-2 family
• The Bcl-2 family of pro- and anti-apoptotic regulatory proteins:
• Bcl-2 (and relatives) are inhibitors of apoptosis, acting by binding to
and inactivating two pro-apoptotic proteins, Bax and Bak, that live in
the mitochondrial membrane
• Bax and Bak, when released from Bcl-2 binding (and inhibition),
disrupt the outer mitochondrial membrane, releasing cytochrome c
which activates the cascade of proteolytic capsases leading to the
cellular changes of apoptosis
• Bcl-2 interacts with Bax and Bak via BH3 interaction domains
• Other proteins that sense cellular abnormalities contain BH3 domains
(“BH3-only” proteins) can activate apoptosis by interfering with Bcl-2
or by activating Bax or Bak directly
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Resisting cell death
Sensors that trigger apoptosis:
• DNA damage sensor functions via p53
• Insufficient survival factor signaling (e.g. IL-3 for lymphocytes)
• Hyperactive signaling by some oncoproteins (e.g. Myc)
• Each of these can activate BH3-only proteins to induce apoptotic
cascade
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Resisting cell death:
• Mechanisms to limit or circumvent apoptosis:
• Loss of the p53 tumor suppressor function
• Increased expression of anti-apoptotic regulators (e.g. Bcl-2)
• Increased expression of survival signals (e.g. IL-3)
• Decreased expression of pro-apoptotic regulators (e.g. Bax and Bak)
• Interrupting the extrinsic apoptotic pathway
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4. Enabling replicative immortality
• Normal cells pass thru a limited number of cell (division) cycles before
becoming senescent (alive but non-proliferative) or undergoing crisis
leading to cell death
• In the culture, repeated cycles of cell division lead to senescence and
then crisis, resulting in death of the majority of cells.
• Rare cells that survive crisis exhibit unlimited replicative potential and
are said to be immortalized (a characteristic of established cell lines)
• Telomeres, multiple tandem copies of 6 nucleotide repeats protecting
the ends of chromosomes, are central to limiting the number of
division cycles that a normal cell can undergo and, conversely, to the
unlimited proliferation capacity of malignant cells
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Enabling replicative immortality:
Telomeres and Telomerase
• In normal cells, telomeres progressively shorten with each cell division
• Eventually lose the ability to protect the ends of chromosomes from
end-to-end fusions
• End-to-end chromosomal fusion results in scrambling of the karyotype
and leads to apoptosis
• The length of telomeric DNA in a cell dictates how many cell
generations its offspring can pass through prior to telomere erosion
and cell death
• Telomerase is a specialized DNA polymerase that adds telomere
repeats to the ends of telomeres
• Telomerase is almost absent in normal cells but is present in
immortalized cells
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Enabling replicative immortality:
Telomeres and Telomerase
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Enabling replicative immortality:
Telomeres and Telomerase
• Mechanisms to achieve replicative immortality:
• Increase expression of telomerase
• Activation of an alternative, recombination based telomeremaintenance mechanism (less common)
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5. Inducing angiogenesis
• Tumors require new blood vessel formation to survive and grow
• In adult life, normal angiogenesis is required as a part of wound
healing and during the female reproductive cycle
• During tumor development and progression the “angiogenic switch” is
activated and remains on to support crowding of new vessels stained
neoplastic growth
• The angiogenic switch is controlled by factors that either induce or
oppose angiogenesis
• Vascular endothelial growth factor (VEGF) signals thru receptor
tyrosine kinases (VEGFR) to promote angiogenesis
• Fibroblast growth factor (FGF) also is pro-angiogenic
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Inducing angiogenesis
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Inducing angiogenesis
• Thrombospondin-1 (TSP-1) and fragments of Plasmin (angiostatin)
and type 18 collagen (endostatin) are endogenous inhibitors of
angiogenesis
• These proteins serve as regulators of normal transitory angiogenesis
during healing and may also act as barriers to cancer driven
angiogenesis
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Inducing angiogenesis
• Mechanisms to induce angiogenesis:
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Increased VEGF expression due to hypoxia or oncogene signaling
(Ras and Myc)
• Increased expression of FGF and other pro-angiogenic molecules
• Leukocytes (macrophages, neutrophils, mast cells) infiltrating premalignant and malignant lesions can activate the angiogenic switch
and sustain ongoing angiogenesis
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6. Activating invasion and metastasis
• Carcinomas developed alterations in shape as well as in their
attachment to other cells and to the extracellular matrix
• E-cadherin is a cell-to-cell adhesion molecule that helps to assemble
epithelial cells into sheets and to maintain quiescence
• E-cadherin is frequently down regulated and occasionally inactivated
by mutation in carcinomas
• N-cadherin is normally expressed in migrating neurons and
mesenchymal cells during organogenesis
• N-cadherin is often upregulated in invasive carcinoma cells
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Activating invasion and metastasis
• The invasion-metastasis cascade:
• Local invasion followed by intravasation of cancer cells into nearby
blood and lymph vessels
• Transit via lymph and blood followed by escape from vessels into
distant parenchyma (extravasation)
• Formation of small tumor nodules (micrometastases)
• Growth into macroscopic tumors (colonization)
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Activating invasion and metastasis
• “Epithelial-mesenchymal transition” (EMT) is a developmental
regulatory program involved in embryonic morphogenesis and wound
healing that epithelial cancers co-opt to acquired the ability to invade,
resist apoptosis and disseminate
• EMT and related migratory processes in embryogenesis are
controlled by a set of transcriptional factors (Snail, Slug, Twist etc.)
• These factors are expressed widely in cancer and are important in
invasion and metastasis
• The factors induce loss of adherans junctions, conversion from round
to spindle shape, expression of matrix-degrading enzymes, increased
motility and resistance to apoptosis
• The EMT program in embryogenesis and cancer is influenced by
signals from neighboring stromal cells
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Activating invasion and metastasis:
EMT and Cancer
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Activating invasion and metastasis
• Crosstalk between cancer cells and nearby stromal cells is involved in
invasive growth and metastasis:
• Mesenchymal stem cells produce CCL5 in response to signals from
cancer cells; CCL5 acts on the cancer cells to stimulate invasion
• Cancer cells produce IL-4 to activate macrophages to elaborate
matrix-degrading enzymes to facilitate invasion
• Tumor associated macrophages supply epidermal growth factor to
breast cancer cells; the cancer cells release CSF-1 to stimulate the
macrophages
• Colonization (growth into macro-metastases) requires adaptation to a
new environment
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7. Deregulating cellular energetics
• Chronic cell proliferation requires adjustment of energy metabolism
• Normal cells use glucose via glycolysis under anaerobic conditions
but favor oxidative phosphorylation under aerobic conditions
• Cancer cells can reprogram glucose metabolism to favor glycolysis
even under aerobic conditions
• Glycolysis is much less efficient than oxidative phosphorylation
• Cancer cells compensate by up-regulating glucose transporters
(GLUT1) to increase glucose uptake into the cell
• Use of glycolysis is associated with activated oncogenes (RAS, MYC),
with mutant tumor suppressors (p53) and can be further increased in
the setting of hypoxia, present in many tumors
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Deregulating cellular energetics
• Why do cancer cells favor glycolysis?
• Perhaps diversion of glycolytic intermediates into biosynthetic
pathways generating nucleosides and amino acids facilitates
biosynthesis of macromolecules required for proliferation
• Similar metabolic changes are found in rapidly dividing embryonic
tissues suggesting a role in supporting active cell proliferation
• Some tumors have two populations of cells, one more hypoxic that
use glycolysis and secrete lactate, another, better oxygenated that
imports lactate and uses it as an energy source in oxidative
phosphorylation
• Oxygenation in tumors fluctuates, both in time and space, due to the
instability and disorganization of tumor vasculature
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8. Avoiding immune destruction
• Theory of Immune Surveillance:
• Suggests that the immune system recognizes and eliminates the vast
majority of incipient cancer cells
• Tumors that develop either manage to avoid immune detection or limit
immunologic killing
• Increases in certain cancers in immunocompromised individuals
supports this theory; however, most of these cancers are virally
induced, unlike the majority of cancers
• Mice engineered to lack cytotoxic T lymphocytes (CTLs), helper T
cells or natural killer (NK) cells all developed more carcinogeninduced tumors and more rapidly growing tumors
• Supports the role of cellular immunity in tumor eradication
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Avoiding immune destruction
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Avoiding immune destruction
• Transplant experiments showed that tumors arising in
immunodeficient mice can only be successfully transplanted into
immunodeficient mice while tumors arising in immunocompetent mice
can be successfully transplanted into both types of mice
• Suggests that highly immunogenic cancers are eliminated in
immunocompetent mice (“immunoediting”), leaving only weakly
immunogenic cancers to survive and be transplanted into both types
of mice
• In immunodeficient mice, highly immunogenic cancers survive and
can be transplanted into immunodeficient mice but will not survive in
immunocompetent mice
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Avoiding immune destruction
• Clinical observations supporting antitumor immune response:
• Patients with colon and ovarian tumors that are heavily infiltrated with
CTLs and NK cells have a better prognosis
• Immunosupressed organ transplant recipients have developed donorderived cancers, suggesting that in the tumor-free donor, the cancer
was held in check by an intact immune system
• Highly immunogenic tumors may have ways of disabling components
of the immune system by secreting TGF-beta or other
immunosuppresive factors or by recruiting regulatory T cells (T regs)
or myeloid-derived supressor cells which can both supress the
function of cytotoxic T lymphocytes
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Two Enabling Characteristics of Cancer
• Genome Instability and Mutation
• Tumor Promoting Inflammation
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Genome Instability and Mutation
• Acquisition of Hallmarks depends on a succession of alterations in the
genomes of neoplastic cells
• Certain mutations offer a selective advantage leading to outgrowth of
the dominant clone
• Multistep tumor progression is a series of clonal expansions each
triggered by the chance acquisition of an enabling mutation
• Cancer cells increase mutation rate by: increased sensitivity to
mutagenic agents thru breakdown in the genomic maintenance
machinery
• Mutation rate also increased by compromising the systems that
monitor genomic integrity and force damaged cells into senescence or
apoptosis
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Tumor-Promoting Inflammation
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Tumor cells are typically infiltrated by cells of the immune system
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Immune cells aiming to eradicate the tumor vs. enhancing tumor
development and progression
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Inflammatory cells can supply growth factors to sustain proliferative
signaling, survival factors limiting cell death, pro-angiogenic factors,
matrix-modifying enzymes that facilitate angiogenesis, invasion and
metastasis and induction signals leading to activation of EMT
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Inflammation sometimes demonstrable at early stages of tumor
development and may foster development from pre-malignant lesion to
cancer
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Inflammatory cells can release chemicals, particularly reactive oxygen
species that are mutagenic, hastening malignant progression
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Role of the tumor microenviroment in cancer
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The Cancer Stem Cell
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Cancers are comprised of heterogenous populations of cells comprised
of regions with various degrees of differentiation, proliferation, vascularity,
inflammation and invasiveness
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Cancer stem cells represent a further degree of cellular heterogeneity
and are likely a common constituent of most tumors
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Defined by their ability to efficiently seed new tumors in recipient host
mice
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Initially identified in hematologic malignancies but subsequently found in
solid tumors as well
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Cell surface markers and gene transcription profiles similar to those of
normal tissue stem cells
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Cancer stems cells are likely to be particularly resistant to conventional
chemotherapy and may require novel therapeutic approaches
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The Cancer Stem Cell
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The Cancer Genome Era
• The project to determine the sequence of an entire human genome
began in 1990 and was completed in 2003
• Several large sequencing labs in the US and abroad participated
• The cost of the Human Genome Project was $2.7 billion
• Subsequent improvements in technology has allow faster and
cheaper sequencing
• Currently genome sequencing may take several weeks and cost
$10,000 or less
• Identify important genetic mutations that are biologic drivers for given
cancers or subsets of cancers, improve prognostication, provide
additional targets for drug development, provide foundation for cancer
therapy that is tailored to each cancer
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Therapeutic Targets
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