cancer lecture 3
Cancer Biology: Cell Division, Invasiveness, and Genomic InstabilityDensity-Dependent Inhibition & Contact Inhibition
Density-dependent inhibition, also known as contact inhibition, is a fundamental process that regulates cell division in normal cells.
Normal Cell Behavior
When normal cells are introduced into a petri dish at low numbers, they begin to divide and proliferate.
As cells begin to touch one another, their rate of division slows down.
Once the cells fill the bottom of the dish, forming a single layer, the rate of cell division slows further and is balanced by the rate of cell death.
This state, where the total cell number remains constant and the dish is covered by a single layer of cells, is called confluence.
Contact inhibition ensures that normal cells create a layer only one cell thick, known as a monolayer.
Cancer Cell Behavior
The behavior of cancer cells is significantly different.
If a cancer cell is seeded among normal cells, all cells will initially proliferate.
However, once confluence is reached, normal cells will regulate their growth, while cancer cells continue to divide in an unregulated manner.
This unregulated division leads to the formation of a clump of cells, often referred to as a focus, as cancer cells ignore the cues that signal overcrowding.
Demonstrating Contact Inhibition (In Vitro)
Contact inhibition can be demonstrated by removing cells from a confluent monolayer:
Cells are removed by scratching the monolayer with a needle, creating a "wound."
The surviving cells at the edge of the wound exhibit two key behaviors:
They begin to proliferate more rapidly because they are no longer fully contact inhibited.
They migrate into the empty area of the wound, attempting to fill it up.
Cancer cells, in contrast, have lost these regulatory signals and will continue to divide even when fully surrounded.
Cancer Cell Characteristics: Invasiveness and Metastasis
Invasiveness and metastasis are critical hallmarks of malignant cancer.
Stages of Cervical Cancer Progression (Example)
This example illustrates the progression of cervical cancer through different stages:
Normal Epithelium:
Dividing cells are confined to a specific basal layer.
Cells above this layer differentiate and are not actively dividing.
Low-Grade Intraepithelial Neoplasia:
Cells begin to look different and are no longer strictly confined to the single basal layer.
They have not yet fully penetrated through the entire epithelium.
High-Grade Intraepithelial Neoplasia:
Abnormal cells are present throughout the entire tissue layer.
However, they are still confined within the original tissue and have not broken through the underlying structures.
Carcinoma (Invasive):
Cells break free of their home tissue.
This is often due to the cancer cells gaining the ability to express enzymes that can cut through the basal lamina and other extracellular matrix structures that normally contain them.
Once cells can leave their home tissue, the cancer is considered invasive.
Tumor Progression: Benign to Metastatic
The progression of a tumor from a localized mass to a metastatic disease involves several steps:
Benign Tumor:
Cells have a proliferative advantage over their neighbors (dividing faster, dying slower).
However, they are still contained within their original tissue and have not acquired all characteristics for uncontrolled division and spread.
Invasion:
Cancer cells break through the basal lamina, becoming invasive.
They gain the ability to leave their home tissue.
Intravasation (Entering Circulation):
Invasive cells can invade nearby blood vessels (capillaries) or lymph vessels.
Capillaries in tumors are often poorly constructed, making invasion easier.
Lymph vessels are generally easier to enter than intact blood vessels.
Once in the circulation (blood or lymph), cells can travel throughout the body.
Extravasation (Exiting Circulation) & Colonization:
The circulating cancer cells exit the blood or lymph vessel at a distant site.
They then adhere to the new tissue and begin to proliferate, forming a secondary tumor (metastasis).
An example given is cancer cells adhering to and colonizing the liver.
Factors Influencing Metastasis
Metastasis is a complex and often inefficient process for cancer cells:
Blood Flow Pattern: The direction of blood flow from the primary tumor can influence where cancer cells might travel.
Mechanical Trapping: Blood vessels at a secondary site must be able to mechanically trap the tumor cells.
Adhesive Proteins: Tumor cells may possess specific adhesive proteins on their surface that allow them to bind to cells in a new region.
Supportive Microenvironment: The microenvironment of the secondary site must be hospitable and supportive for the cancer cells to survive and proliferate.
Cancer cells manipulate their local microenvironment in the primary tumor to get necessary signaling molecules.
Finding a similarly hospitable environment at a distant site is rare, which is why metastasis is not easy and has a low success rate for individual cells.
Genomic Instability in Cancer
One of the properties of cells capable of cancerous growth is their inherent genomic instability.
Karyotype Analysis
Karyotypes visualize the complete set of chromosomes in a cell, revealing chromosomal abnormalities characteristic of cancer:
Normal Karyotype:
Shows a diploid set of chromosomes (e.g., 46 chromosomes in humans).
Each chromosome has a homologous pair (one from each parent), appearing even and organized.
Example: A normal male karyotype would show an X and a Y sex chromosome.
Tumor Cell Karyotype:
Exhibits significant abnormalities, indicating genomic instability.
Translocation Events: Chromosomes may be broken and fused with non-homologous chromosomes.
Example: A chromosome #2 might be fused with parts of chromosome #8 and #12, appearing as a hybrid chromosome.
These are events that should not happen in normal cells.
Aneuploidy: Abnormal numbers of chromosomes (e.g., missing entire chromosomes or having extra copies).
These chromosomal rearrangements and numerical changes contribute to the uncontrolled growth and altered behavior of cancer cells.
Chromosomal Abnormalities in Tumor Cells (Continued)
Messed Up Karyotype: Tumor cells exhibit profoundly abnormal karyotypes, indicative of severe genomic instability.
All of these breakage and fusion events, that's not normal.
Multiple Duplication Events: Tumor cells are often no longer diploid.
Example: 4 copies of chromosome number 10 (instead of 2).
Example: 3 copies of chromosome number 13 (instead of 2).
Chromosomal Deletion and Duplication: Both losses and gains of chromosomal material are common.
Chromosomal Breakage and Fusion Events: Non-homologous chromosomes break and fuse, creating abnormal, often very long, chromosomes.
Example: A small piece of one chromosome fused with a larger piece of another.
These events lead to incredible instability in the karyotype.
Consequences of Breakage and Fusion:
Chromosomes will not behave normally.
Genes can be moved from their normal control elements, leading to:
Overexpression
Non-expression
Expression in inappropriate contexts
A gene can be cut within its transcription unit, causing severe problems.
Genes Critical for Cancer Development
Uncontrolled cell division in cancer is driven by mutations in specific classes of genes that regulate the cell cycle and genomic integrity.
There are three main classes of genes whose mutations contribute to this uncontrolled cell division:
Genes that maintain genomic stability
Genes that trigger cell growth and division (Proto-oncogenes)
Genes that stop the cell cycle from progressing (Tumor Suppressor Genes)
1. Genes Maintaining Genomic Stability
Role: These genes are responsible for preventing chromosomal breakage, fusion, and duplication events. They ensure the integrity of the genome.
Impact of Mutation: When these genes are mutated, the cell loses its ability to maintain genomic stability.
This leads to the observed chromosomal abnormalities (e.g., breakage, fusion, duplication) seen in tumor cells.
Clearly to get to that place, then the genes that are involved in maintaining genomic stability have been mutated, right, because this should not happen.
2. Proto-Oncogenes and Oncogenes
Proto-oncogenes:
Normal Function: Genes that normally trigger cell growth and division. They initiate specific phases in the cell cycle (e.g., progression from G1 to S, or G2 to M).
Analogy: Considered the "gas pedal" of the cell (telling the cell to go forward, divide).
Examples: Genes encoding components of cell signaling pathways that promote cell division.
Oncogenes:
Formation: A proto-oncogene that has been mutated in a way that makes it hyperactive or overexpressed.
Type of Mutation: Gain-of-function mutation. This means the gene gains a new, enhanced, or uncontrolled function it didn't previously have (e.g., becoming more active).
Analogy: A "stuck gas pedal" – even if you take your foot off, the cell continues to "go."
Consequence: Leads to uncontrolled cell division.
Examples of Proto-oncogenes in Signal Transduction Pathways:
Any protein involved in a signal transduction pathway that ultimately stimulates the cell to progress through the cell cycle can be encoded by a proto-oncogene and become an oncogene through a gain-of-function mutation.
Secreted Signaling Molecules (Ligands): Genes for these molecules (e.g., growth factors).
Transmembrane Receptors: Genes for receptors that bind signaling molecules (e.g., Receptor Tyrosine Kinases).
GTP-Binding Proteins (G-proteins): Genes for proteins like Ras, which act as molecular switches.
Example: Ras Protein:
Normally, Ras is active when bound to GTP and inactive when GTP is hydrolyzed to GDP.
A mutation that causes Ras to lose its ability to hydrolyze GTP to GDP means it would always stay "on."
This hyperactive Ras would continuously stimulate downstream pathways, leading to uncontrolled cell proliferation.
Relay Proteins: Genes for proteins involved in intracellular signaling cascades (e.g., protein kinases in phosphorylation cascades).
Regulatory Proteins / Transcription Factors: Genes for proteins that regulate gene expression, promoting cell cycle progression.
3. Tumor Suppressor Genes
Normal Function: Genes that normally stop the cell cycle from progressing. They regulate cell growth in a negative way, acting as "policemen" to halt division when necessary.
Analogy: The "brake pedal" of the cell cycle.
Impact of Mutation: When these genes are mutated, they typically undergo a loss-of-function mutation.
The cell loses its ability to put the "brakes" on the cell cycle.
This removes critical checkpoints and regulatory mechanisms, leading to uncontrolled cell division.
Conclusion: Cancer Critical Genes
DNA maintenance genes, tumor suppressor genes, and proto-oncogenes are collectively referred to as cancer critical genes. These genes are crucial for ensuring that a cell divides only when it is supposed to, and their malfunction is central to cancer development.