Cell Cycle Regulation and Cancer

Growth factors are specific proteins released by certain cells that act as signals to stimulate other cells to divide.
- Platelet-derived growth factor (PDGF) is a notable example.
  - It is produced by blood cell fragments known as platelets.
  - PDGF is essential for the division of fibroblasts when cultured in a lab setting.
- Experiment demonstrating PDGF (Figure 12.18):
  1. A sample of human connective tissue is cut into small pieces.
  2. Enzymes are used to digest the extracellular matrix, isolating free fibroblasts into a suspension.
  3. These fibroblasts are then transferred into culture vessels.
  4. PDGF is added to half of the culture vessels, while the other half serves as a control without PDGF.
  5. Results: Fibroblasts with PDGF added divide, as observed under an SEM (scanning electron microscope) image marked with a $10 \ \mu m$ scale bar. Fibroblasts without PDGF do not divide.
Density-dependent inhibition: This is a physical factor where crowded cells stop dividing. Once a certain density is reached, cell-surface proteins of adjacent cells make contact, sending signals that inhibit further cell division.

Density-dependent inhibition (revisited): Crowded cells cease dividing.
Anchorage dependence: Most animal cells require attachment to a substratum (e.g., the extracellular matrix of a tissue or the surface of a culture dish) to successfully divide.
Together, density-dependent inhibition and anchorage dependence are crucial mechanisms that regulate the growth of normal cells, ensuring they stop dividing at an optimal population density and remain in their appropriate locations.
Cancer cells distinctly exhibit neither type of regulation over their division.
Illustration of these concepts (Figure 12.19):
- (a) Normal mammalian cells:
  - Demonstrate anchorage dependence: they need a surface ( $20 \ \mu m$ scale bar shown for cell imagery) to divide.
  - Exhibit density-dependent inhibition: they form a single, organized layer and stop dividing once contact is made with neighboring cells.
  - If a gap is introduced in this layer, cells will divide just enough to fill that gap and then cease division again due to density-dependent inhibition.
- (b) Cancer cells: Lack both anchorage dependence and density-dependent inhibition, allowing them to pile up and grow uncontrollably, forming multiple layers.

Cancer cells disregard the normal internal and external signals that regulate the cell cycle.
They continue to divide even when crucial growth factors are depleted from their environment.
Cancer cells can circumvent the need for external growth factors in several ways to grow and divide:
- They may synthesize and release their own growth factors, self-stimulating their division.
- They may possess cell-surface receptors that convey a growth factor's signal without the actual presence of the growth factor molecule.
- They may have an abnormal or faulty cell cycle control system internally.

Transformation: The process by which a normal cell acquires the ability to divide indefinitely, essentially becoming a cancer cell.
Tumors: If transformed cells are not detected and eliminated by the immune system, they proliferate to form masses of abnormal cells within otherwise normal tissue.
- Benign tumor: A tumor where the abnormal cells remain confined to the original site. Most benign tumors are not life-threatening or serious, though their location can sometimes cause problems (e.g., intracranial benign tumors).
- Malignant tumor: A tumor whose cells invade surrounding tissues and have the potential to undergo metastasis.
  - Metastasis: The process by which cancer cells spread to distant parts of the body through the circulatory or lymphatic systems, where they can form additional tumors (secondary or metastatic tumors).
Visualizing Tumor Growth and Metastasis (Figure 12.20):
1. A tumor originates from a single transformed cancer cell.
2. The cancer cells proliferate and invade adjacent normal tissue (e.g., glandular tissue, shown with a $5 \ \mu m$ scale bar for a breast cancer cell image).
3. Cancer cells enter lymph vessels and blood vessels, using these pathways to travel throughout the body.
4. A small percentage of these circulating cancer cells successfully metastasize, establishing new metastatic tumors in other organs or tissues.

Localized tumors: These can often be treated effectively with high-energy radiation.
- Mechanism: Radiation damages the DNA within the cancer cells.
- Cancer cells are frequently more susceptible to radiation damage than normal cells because they have often lost the ability to efficiently repair DNA damage.
Metastatic tumors: Due to their widespread nature, these are typically treated with chemotherapeutic drugs.
- Mechanism: Chemotherapeutic drugs target the cell cycle, interfering with cell division. They are designed to be toxic to rapidly dividing cells, which include most cancer cells.
Personalized medicine: With advancements in molecular techniques, cancer treatments are becoming increasingly