CANCER’S MOLECULAR BIOLOGY: KEY FACTS YOU SHOULD KNOW

Normal Cell Division Cycle

The normal cell division cycle, also known as the cell cycle, is a series of events that cells go through as they grow and divide. This process is crucial for growth, development, and tissue repair in multicellular organisms. The cell cycle consists of several distinct phases: interphase (which includes G1, S, and G2 phases) and the mitotic phase (M phase).

1. Interphase

Interphase is the longest part of the cell cycle, during which the cell prepares for division. It is divided into three sub-phases:

• G1 Phase (Gap 1): In this phase, the cell grows in size and synthesizes various proteins necessary for DNA replication. The cell also performs its regular functions and monitors its environment to ensure conditions are favorable for division.
• S Phase (Synthesis): During this phase, DNA replication occurs. Each chromosome is duplicated to produce two sister chromatids that are held together at a region called the centromere. This ensures that when the cell divides, each daughter cell will receive an identical set of chromosomes.
• G2 Phase (Gap 2): After DNA synthesis is complete, the cell enters G2 phase where it continues to grow and produces proteins required for mitosis. The cell also checks for any DNA damage that may have occurred during replication and makes necessary repairs.

2. Mitotic Phase (M Phase)

The M phase includes both mitosis and cytokinesis:

• Mitosis: This process is further divided into several stages:
  • Prophase: Chromosomes condense and become visible under a microscope. The nuclear envelope begins to break down.
  • Metaphase: Chromosomes align at the metaphase plate in the center of the cell.
  • Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.
  • Telophase: Chromatids reach the poles, de-condense back into chromatin, and new nuclear envelopes form around each set of chromosomes.
• Cytokinesis: This is the final step where the cytoplasm divides to form two separate daughter cells. In animal cells, this occurs through a process called cleavage furrow formation; in plant cells, a new cell wall forms between the two daughter cells.

After cytokinesis is complete, each daughter cell enters its own interphase, thus beginning a new round of the cell cycle.

3. Regulation of Cell Cycle

The progression through these phases is tightly regulated by various checkpoints that monitor cellular conditions and ensure proper division. Key regulatory proteins include cyclins and cyclin-dependent kinases (CDKs), which work together to control transitions between phases.

In summary, the normal cell division cycle consists of interphase (G1, S, G2) followed by mitosis (M) and cytokinesis, allowing for growth and maintenance of healthy tissues.

Apoptosis: An Overview

Apoptosis, also known as programmed cell death, is a highly regulated and controlled process that allows cells to die in a manner that does not elicit an inflammatory response. This mechanism is crucial for maintaining homeostasis in multicellular organisms, allowing for the removal of damaged, unnecessary, or potentially harmful cells without affecting surrounding tissues. Apoptosis plays a vital role in various physiological processes such as development, immune system regulation, and tissue remodeling.

Intrinsic Pathway of Apoptosis

The intrinsic pathway of apoptosis is primarily triggered by internal cellular stress signals. This pathway is often activated in response to various stimuli such as DNA damage, oxidative stress, or growth factor deprivation. The key components involved in the intrinsic pathway include:

1. Mitochondrial Involvement: The mitochondria play a central role in the intrinsic pathway. When a cell experiences stress, pro-apoptotic proteins from the Bcl-2 family (such as Bax and Bak) are activated and promote mitochondrial outer membrane permeabilization (MOMP). This leads to the release of cytochrome c into the cytosol.
2. Caspase Activation: Once cytochrome c is released into the cytosol, it binds to Apaf-1 (apoptotic protease activating factor 1), forming a complex known as the apoptosome. This complex activates initiator caspase-9, which subsequently activates effector caspases (such as caspase-3 and caspase-7), leading to cellular dismantling.
3. Cellular Changes: The activation of these caspases results in characteristic morphological changes associated with apoptosis, including chromatin condensation, DNA fragmentation, and membrane blebbing.
4. Regulation: The intrinsic pathway is tightly regulated by anti-apoptotic proteins (like Bcl-2 and Bcl-xL) that inhibit MOMP and prevent apoptosis under non-stressful conditions.

Extrinsic Pathway of Apoptosis

The extrinsic pathway of apoptosis is initiated by external signals that bind to specific death receptors on the cell surface. This pathway is often triggered by ligands such as tumor necrosis factor (TNF) or Fas ligand (FasL). Key components involved in this pathway include:

1. Death Receptors: These are transmembrane proteins that belong to the tumor necrosis factor receptor superfamily. When ligands bind to these receptors, they undergo conformational changes that recruit adaptor proteins like FADD (Fas-associated death domain).
2. Caspase Activation: The recruitment of FADD leads to the formation of a death-inducing signaling complex (DISC), which facilitates the activation of initiator caspase-8 or caspase-10. These initiator caspases then activate downstream effector caspases (such as caspase-3).
3. Cellular Changes: Similar to the intrinsic pathway, once effector caspases are activated, they execute apoptosis through cleavage of various substrates leading to morphological changes typical of apoptotic cells.
4. Cross-talk Between Pathways: There can be cross-talk between intrinsic and extrinsic pathways; for instance, active caspase-8 can cleave Bid (a pro-apoptotic member of the Bcl-2 family), which then translocates to mitochondria and promotes MOMP similar to what occurs in the intrinsic pathway.

Key Differences Between Intrinsic and Extrinsic Pathways

1. Triggering Mechanism:
   • Intrinsic Pathway: Triggered by internal cellular stress.
   • Extrinsic Pathway: Triggered by external signals binding to death receptors.
2. Key Components:
   • Intrinsic Pathway: Involves mitochondrial release of cytochrome c and activation of Apaf-1.
   • Extrinsic Pathway: Involves death receptors and formation of DISC with adaptor proteins like FADD.
3. Caspase Activation:
   • Intrinsic Pathway: Primarily involves initiator caspase-9.
   • Extrinsic Pathway: Primarily involves initiator caspase-8 or 10.
4. Regulatory Proteins:
   • Intrinsic Pathway: Regulated by Bcl-2 family proteins.
   • Extrinsic Pathway: Regulated by receptor-ligand interactions but less dependent on intracellular regulatory mechanisms.

In summary, both pathways converge at common executioner caspases but differ significantly in their initiation mechanisms and regulatory controls.

Differentiating Between Oncogenes and Tumor Suppressor Genes

Definition and Function

Oncogenes are mutated forms of proto-oncogenes, which are normal genes that promote cell growth and division. When these genes undergo mutations or alterations, they can become permanently activated, leading to uncontrolled cell proliferation. Essentially, oncogenes act like a gas pedal stuck in the “on” position, driving excessive cell division.

In contrast, tumor suppressor genes are responsible for slowing down cell division, repairing DNA mistakes, or initiating apoptosis (programmed cell death) when necessary. These genes function as brakes on the cell cycle. When tumor suppressor genes are mutated or lost, their ability to control cell growth is compromised, allowing cells to divide uncontrollably.

Mechanisms of Action

Oncogenes can be activated through various mechanisms such as:

1. Gene Mutations: Changes in the DNA sequence that lead to a gain of function.
2. Gene Amplification: An increase in the number of copies of a gene.
3. Chromosomal Rearrangements: Structural changes in chromosomes that place an oncogene next to a strong promoter.

Tumor suppressor genes typically require two “hits” or mutations for their function to be lost—one inherited mutation and one acquired mutation during a person’s life. The loss of function can occur through:

1. Point Mutations: Small changes in the DNA sequence that render the gene inactive.
2. Deletions: Loss of part or all of the gene.
3. Epigenetic Changes: Modifications that silence the gene without altering its DNA sequence.

Examples

A well-known example of an oncogene is the RAS gene family (e.g., KRAS), which plays a critical role in cell signaling pathways that control growth and division. Mutations in RAS can lead to continuous signaling for cell proliferation.

An example of a tumor suppressor gene is TP53, which encodes for the p53 protein—a crucial regulator of the cell cycle and apoptosis. Mutations in TP53 are associated with various cancers due to its failure to prevent damaged cells from dividing.

Role in Cancer Development

Both oncogenes and tumor suppressor genes play pivotal roles in cancer development:

• Oncogenes contribute to cancer by promoting excessive cellular proliferation and survival.
• Tumor suppressor genes contribute by failing to inhibit abnormal growth or repair damaged DNA.

The interplay between these two types of genes is essential for maintaining normal cellular functions; when both types are altered, it significantly increases the risk of cancer progression.

In summary, while oncogenes drive uncontrolled cell growth akin to a malfunctioning accelerator pedal, tumor suppressor genes serve as critical regulators that ensure proper cellular function by acting like brakes on this process.

Cancer Cell Properties

1. Differences Between Cancer Cells and Normal Cells

Cancer cells exhibit several distinct properties that differentiate them from normal body cells. Understanding these differences is crucial for studying cancer biology and developing effective treatments.

• Uncontrolled Growth and Division: Unlike normal cells, which stop dividing when they reach a certain density or when signals indicate that enough cells are present, cancer cells continue to grow and divide uncontrollably. This leads to the formation of tumors, which can be benign (non-cancerous) or malignant (cancerous). In cases like leukemia, cancerous blood cells proliferate without forming solid tumors but still disrupt normal blood function.
• Disregard for Chemical Signals: Normal cells respond to chemical signals that regulate growth and division. Cancer cells often ignore these signals, leading to unchecked proliferation. This disruption in communication is a hallmark of cancer progression.
• Loss of Adhesion: Cancer cells can lose the adhesion molecules that help them stick together and remain in their designated locations within tissues. This loss allows them to detach from the primary tumor and invade surrounding tissues or spread to distant sites in the body through the bloodstream or lymphatic system.
• Lack of Specialization: Healthy cells undergo differentiation, where they mature into specific cell types with defined functions (e.g., muscle cells, nerve cells). Cancer cells often remain undifferentiated, meaning they do not develop specialized functions and instead continue to proliferate rapidly.
• Failure to Repair DNA Damage: Normal cells have mechanisms for repairing damaged DNA or undergoing apoptosis (programmed cell death) if damage is irreparable. In cancer cells, these repair mechanisms are often faulty due to mutations in critical genes such as p53. As a result, cancer cells accumulate further genetic mutations that promote their aggressive behavior.

2. Biomechanical Properties of Cancer Cells

The biomechanical characteristics of cancer cells also play a significant role in their behavior:

• Stiffness and Deformability: Research indicates that cancer cells tend to be stiffer than normal cells due to changes in their cytoskeleton—the structural framework within the cell. This increased stiffness can influence how they interact with their microenvironment and may facilitate invasion into surrounding tissues.
• Viscoelasticity: Cancer cells exhibit viscoelastic properties, meaning they can deform under stress but also return to their original shape when the stress is removed. This property aids in their ability to migrate through tight spaces during metastasis.
• Cell Morphology Changes: Under microscopic examination, cancer cells often display abnormal shapes and sizes compared to normal counterparts. These morphological changes are associated with alterations in cellular mechanics and contribute to their invasive capabilities.

3. Interaction with Microenvironment

Cancer cell behavior is heavily influenced by interactions with their surrounding microenvironment:

• Mechanoreciprocity: The relationship between tumor cells and the extracellular matrix (ECM) is dynamic; as tumor cells exert forces on the ECM, this interaction can lead to changes in both the tumor’s mechanical properties and its invasive potential.
• Local Invasion Mechanisms: The mechanical properties of both cancerous tissue and surrounding healthy tissue affect how easily cancer can invade nearby structures. For instance, collagen reorganization at tumor-stromal interfaces facilitates local invasion by providing pathways for tumor cell migration.

In summary, studying cancer cell properties involves understanding both biological differences from normal cells—such as uncontrolled growth, lack of specialization, failure of apoptosis—and biomechanical features like stiffness and interaction with the microenvironment that contribute significantly to tumor progression and metastasis.