Cell, cell cycle and origin of cancer

Marina Marjanovic, Ph.D.

Program Administrator, Strategic Initiative on ImagingBeckman Institute for Advanced Science and Technology

Adjunct Assistant Professor, College of MedicineUniversity of Illinois at Urbana-Champaign

1. Living things are highly organized.

2. Living organisms are homeostatic.

3. Living organisms reproduce themselves.

4. Living organisms grow and develop.

5. Living organisms respond to stimuli.

6. Living organisms are adapted.

7. Living organisms can take energy from the environment and

change its form.

Properties of life

Simple molecules

Macromolecules

Organelles

Cells

Tissues

Organs

Organ systems

Organism

Cell = smallest unit of life

Organizational levels of life

• Cells rely on the integration of structures and organelles in order to function.• Cell is a living unit greater than the sum of its parts.

5 µ

m

The functions of cell division: reproduction

The functions of cell division: growth and development

The functions of cell division: tissue renewal

Chromosome formation and replication

Cell cycle

Stages of mitotic cell division in an animal cell

Stages of mitotic cell division in an animal cell

Cell cycle

• The frequency of cell division varies with the type of cell• These cell cycle differences result from regulation at the molecular level

The cell cycle control system

• The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock

Control system

G2 checkpoint

M checkpoint

G1 checkpoint

G1

S

G2M

• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received.

• Both internal and external signals control the cell cycle checkpoints.• Cancer cells do not respond normally to the body’s control mechanisms and form

tumors.

G1 checkpoint

G1G1

G0

(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.

(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled.

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During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase.

4

Accumulated cyclin moleculescombine with recycled Cdk molecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.

2

Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates.

1

Cdk

CdkG2

checkpoint

CyclinMPF

Cyclin is degraded

DegradedCyclin

G 1

G 2

S

M

G1G1 S G2 G2SM MMPF activity

Cyclin

Time

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle

MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase.

3

M

• Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

• The activity of cyclins and Cdks fluctuates during the cell cycle

MPF = cyclin + cyclin-dependent kinaseMPF = ‘M-phase-promoting factor’

Control of growth

• Cytokines: cyclins, cyclin dependent kinases (CDK).• Growth factors – PDGF, FGF• Growth inhibitors• Cancer suppressor genes – p53• Oncogenes – c-onc, p-onc, v-onc, etc.

Effect of external factors on cell division

Cells anchor to dish surface anddivide (anchorage dependence).

When cells have formed a complete single layer, they stop dividing (density-dependent inhibition).

If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition).

Normal mammalian cells. The availability of nutrients, growth factors, and a surface for attachment limits cell density to a single layer. Normally,cells divide 20-50 times.

(a)

25 µm

• In density-dependent inhibition crowded cells stop dividing.

• Most animal cells exhibit anchorage dependence in which they must be attached to a surface to divide.

• Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.

25 µm

Cancer cells do not exhibitanchorage dependence or density-dependent inhibition.

Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. They can divide indefinitely (“immortal” cells).

(b)

Properties of a transformed cell:• Unusual number of chromosomes• Abnormal metabolism• Loss of normal cellular functions• Loss of density-dependent inhibition• Loss of anchorage dependence• Release of signal molecules that cause growth of blood vessels toward the tumor

Section of a malignant epithelial skin tumor (squamous cell carcinoma). An increase in the number of cells in mitosis and diversity of nuclear morphology are signs of malignancy.

Section of a fast-growing malignant epithelial skin tumor showing an increased number of cells in mitosis and great diversity of nuclear morphology.

Controlled & reversible proliferation

• Hypertrophy – size

• Hyperplasia – number

• Metaplasia – change

• Dysplasia – disordered

• loss of cell uniformity• diversity in nuclear size and shape• increased number of cells in mitosis• pre-malignant change (carcinoma in situ)

neoplasia = new growth

Uncontrolled & irreversible proliferation (neoplastic growth)

• Progressive, purposeless, pathologic proliferation of cells characterized by loss of control over cell division.

• DNA damage at growth control genes is central to development of neoplasia.

• Types:

Benign•slow growing•capsulated•non-invasive •do not metastasize •well differentiated

Malignant•fast growing •non capsulated •invasive •metastasize•poorly differentiated

Bilateral Cystadenoma Ovary

Intestinal Lipoma

Meningioma

Hepatic Adenoma

Breast Carcinoma

Lung Carcinoma

Osteosarcoma

Hepatic Adenocarcinoma

Hepatic Adenocarcinoma

Essential alterations for malignant transformation:

1. Self-sufficiency in growth signals

2. Insensitivity to growth inhibitory signals

3. Evasion of apoptosis

4. Defects in DNA repair mechanism

5. Limitless replicative potential

6. Sustained angiogenesis

7. Ability to invade and metastasize

1. Cancer cells acquire self-sufficiency in growth signals

Oncogenes: genes that promote autonomous growth

Normal Cancer

Proto-oncogene Oncogene

ONCOPROTEINS

Regulates cell replication and differentiation in

presence of mitogenic stimuli

Promotes cell growthin absence of

mitogenic stimuli

Proto-oncogene

Oncogene

Oncoproteins

Point mutation / deletion

TranslocationInsertion

Amplification

Viral oncogenes (~15% cancers) Non- viral oncogenes

Proto-oncogene

DNA

Translocation or transposition:gene moved to new locus, under new controls

Newpromoter

Gene amplification:multiple copies of the gene

Point mutationwithin a controlelement

Oncogene Oncogene

Point mutationwithin the gene

Normal growth-stimulatingprotein in excess

Normal growth-stimulatingprotein in excess Normal growth-stimulating

protein in excessHyperactive ordegradation-resistant protein

Cell cycle-stimulatingpathway

Growthfactor

G protein

Receptor

MUTATION

Protein kinases(phosphorylationcascade)

NUCLEUS

HyperactiveRas protein(product of oncogeneissues signalson its own.

Transcriptionfactor (activator)

DNA

Gene expression

Protein thatstimulatesthe cell cycle

Activeformof p53

DNADNA damagein genome

UVlight

Protein kinasesMUTATION

Defective ormissingtranscriptionfactor, such as p53, cannotactivatetranscription

Protein kinases(phosphorylationcascade)

Cell cycle-inhibitingpathway

Cell cycle-stimulatingpathway

Protein thatstimulatesthe cell cycle

NUCLEUS

DNA

Gene expression

Transcriptionfactor (activator)

Receptor

G protein

Growthfactor

MUTATION

HyperactiveRas protein(product ofoncogene)issues signalson its own

Protein thatinhibitsthe cell cycle

Protein overexpressed

EFFECTS OF MUTATIONS

Protein absent

Cell cycle notinhibited

Increased celldivision

Cell cycle overstimulate

Effects ofmutations

Activeformof p53

DNADNA damagein genome

UVlight

Protein kinasesMUTATION

Defective ormissingtranscriptionfactor, such as p53, cannotactivatetranscription

Protein kinases(phosphorylationcascade)

Cell cycle-inhibitingpathway

Cell cycle-stimulatingpathway

Protein thatinhibitsthe cell cycle

NUCLEUS

DNA

Gene expression

Transcriptionfactor (activator)

Receptor

G protein

Growthfactor

MUTATIONHyperactiveRas protein(product ofoncogene)issues signalson its own

Protein thatstimulatesthe cell cycle

Colon

Colon wall

Loss oftumor-suppressorgene APC (orother)

Normal colonepithelial cells

Small benigngrowth (polyp)

Larger benigngrowth (adenoma)

Activation ofras oncogene

Loss oftumor-suppressorgene DCC

Loss oftumor-suppressorgene p53

Additionalmutations

Malignant tumor(carcinoma)

Multi-step model for the development of colorectal cancer

• Progressive, non lethal DNA damage leading to uncontrolled cell division

2. Cancer cells are insensitive to growth inhibitory signalsNormal: tumor suppressor genes regulate cell proliferation

Abnormal: loss of function of these genes

Apoptosis = programmed cell death (removal of transformed cells, removing of damaged cells, shaping of the embryo-morphogenesis)

•cell and nucleus become compact (pyknotic nucleus)•DNA is fragmented•DNA and cytoplasm fragments are forming vesicles (‘blebes”)that are detaching•vesicles are engulfed by macrophages, but no inflammatory reaction•prevents formation of tumors

Necrosis = accidental cell death (pathological process)

•cells swell and burst•macrophages engulf the debris by phagocytosis •secrete molecules that activate other immunodefensive cells and promote inflammation

3. Cancer cells can evade apoptosis

For example:

•BCL-2 gene protects cells from apoptosis

•MYC gene stimulates cell proliferation and collaborates with BCL-2

•p53 mutations cause cells to evade apoptosis

4. Cancer cells show DNA repair defects and genomic instability

• The immense ability of normal cells to repair damaged DNA protects most of us from developing cancer.

• The abnormality of DNA repair genes allows mutations in other genes to have their carcinogenic effect.

• Three DNA repair systems:- Mismatch repair - Nucleotide excision repair - Recombination repair

5. Cancer cells have limitless replicative potential

• Normal somatic cells after a fixed number of divisions develop replicative senescence by the process of telomeric shortening.

• Cancer cells escape this by reactivating telomerase activity that is normally present in germ cells.

6. Cancer cells develop sustained angiogenesis

• Normally, oxygen can diffuse about 1-2 mm.

• For growth beyond 2 mm, tumor has to develop its own blood vessels - neovascularization.

• At some stage, early tumors develop the “angiogenic switch” with increase of angiogenic factors.

7. Cancer cells are capable of invasion and metastases

• Main feature of malignancy

• Major cause of cancer related morbidity and mortality

• During metastasis cells detach from tumor and migrate to form tumors in specific environments (soil-seed hypothesis); for example breast cancer usually spreads to bones and lungs;

• It is possible that cancer cells require certain tissue stiffness;

• HCT8 cells form islands on soft surface; on the 7th day they detach (‘metastasis’) and completely detach by 14th day; cell dissociation is not apparent in stiff environments?!?

• They proliferate fast, but occasionally they stop to divide to move/migrate; non-specific adhesion decreases;

• Question: Is the metastasis fundamentally linked to the mechanical properties of the ECM?

SUMMARY

Carcinogenesis is a complex multi-step process at both phenotypic and genetic level.

Fundamental rules:

1. Non-lethal genetic damage

2. Clonal proliferation of affected cells

3. Ability to migrate and invade other tissues

Tumor markers are biochemical indicators of presence of tumor:

• Cell surface antigens• Cytoplasmic proteins• Enzymes• Hormones

• Detected in the plasma / serum / body fluids

Tumor markers:

Prostate (bone metastases)

Small cell ca lung, neuroblastoma

Isoenzymes:

Prostatic acid phosphatase

Neuron specific enolase

Liver, germ cell tumors

Colon, pancreas, lung, stomach

Oncofetal antigens:

Alpha Fetoprotein (AFP)

Carcinoembryonic antigen (CEA)

Trophoblastic tumors

Medullary thyroid carcinoma

Pheochromocytoma

(paraneoplastic syndromes)

Hormones:

Human chorionic gonadotropin

Calcitonin

Catecholamines / metabolites

Ectopic hormones

Associated cancersMarker

Tumor markers:

Ovarian cancer

Colon, pancreas

Breast cancer

Mucins and glycoproteins:

CA-125

CA-19-9

CA-15-3

Multiple myeloma and related

Prostate

Specific proteins:

Immunoglobulins

Prostate specific antigen and

prostate specific membrane antigen

Associated cancersMarker

Why are tumor markers not primary tools for detection?

• Tumor markers cannot be used as primary modality for diagnosis of

cancer.

• Used as supportive evidence of cancer

• Not all cancers elaborate tumor markers

• Many lack sensitivity and specificity

• Used as a follow up tool for effectiveness of therapy

• Used to detect and monitor recurrences