Cancer and Cell Cycle

The normal cell cycle is a highly regulated process by which a cell divides and duplicates itself. It consists of several distinct phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). Here’s a detailed explanation of each phase:

  1. G1 Phase (Gap 1):
    • This is the first phase of the cell cycle.
    • The cell is actively growing, performing its normal functions, and preparing for DNA replication.
    • It checks for any DNA damage or errors in the genetic code. If found, the cell can repair the damage or trigger apoptosis (cell death) if the damage is irreparable.
    • It’s a crucial phase for quality control before DNA replication.
  2. S Phase (Synthesis):
    • During this phase, DNA synthesis or replication takes place.
    • The cell’s DNA is duplicated to ensure that each daughter cell receives an identical set of genetic information.
    • At the end of the S phase, the cell has two complete sets of chromosomes.
  3. G2 Phase (Gap 2):
    • After DNA replication, the cell enters the G2 phase.
    • It continues to grow and prepare for cell division.
    • Another round of checks is performed to ensure that DNA replication was accurate and without errors.
  4. M Phase (Mitosis):
    • The M phase is when actual cell division occurs.
    • It consists of several sub-phases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis.
    • During mitosis, the cell’s duplicated chromosomes are separated into two identical sets, and the cell divides into two daughter cells.

Now, let’s discuss the fundamental principles of cancer in relation to the cell cycle:

Cancer is characterized by uncontrolled cell growth and division, which can be attributed to disruptions in the normal cell cycle. Here’s how cancer relates to the cell cycle:

  1. Loss of Cell Cycle Control:
    • In cancer cells, the normal regulatory mechanisms that control the cell cycle are disrupted.
    • Mutations or alterations in genes responsible for regulating cell cycle checkpoints can lead to uncontrolled cell division.
  2. Continuous Proliferation:
    • Cancer cells often divide rapidly and continuously without entering the resting phase (G0) that normal cells may enter.
    • This results in the accumulation of a large number of abnormal cells.
  3. Avoidance of Apoptosis:
    • Cancer cells can evade programmed cell death (apoptosis) even when they have genetic mutations or damage.
    • This allows damaged or abnormal cells to survive and contribute to the growth of tumors.
  4. Genetic Instability:
    • Cancer cells frequently exhibit genetic instability, leading to further mutations and the generation of diverse cell populations within a tumor.
    • This genetic heterogeneity can make cancer challenging to treat.
  5. Metastasis:
    • Cancer cells can break away from the primary tumor and invade nearby tissues or spread to distant organs through the bloodstream or lymphatic system.
    • This ability to metastasize is a hallmark of malignancy.

In summary, cancer is a complex disease that arises due to disruptions in the normal cell cycle, resulting in uncontrolled cell growth, evasion of regulatory mechanisms, and the potential for metastasis. Understanding these fundamental principles is crucial for developing effective cancer treatments.


Alterations involved in malignant transformation

Malignant transformation, the process by which normal cells become cancerous, involves a series of essential alterations at the genetic, molecular, and cellular levels. These changes enable cells to proliferate uncontrollably, evade the body’s natural defense mechanisms, and invade surrounding tissues. Here are the key alterations involved in malignant transformation:

  1. Genetic Mutations: Mutations in the DNA of a cell are the primary drivers of malignant transformation. These mutations can occur spontaneously or be induced by external factors like radiation or chemicals. Mutations can affect oncogenes (genes that promote cell growth) or tumor suppressor genes (genes that inhibit cell growth). Activation of oncogenes and inactivation of tumor suppressor genes are common events.
  2. Chromosomal Aberrations: Large-scale genetic changes, such as chromosomal translocations, amplifications, and deletions, can lead to abnormal gene expression patterns. For example, the fusion of two different genes can create an oncogenic fusion protein, which drives uncontrolled cell growth.
  3. Epigenetic Modifications: Epigenetic changes, which do not alter the DNA sequence but affect gene expression, play a significant role. DNA methylation and histone modifications can silence tumor suppressor genes and activate oncogenes, contributing to cancer development.
  4. Telomere Maintenance: Cancer cells often acquire the ability to maintain their telomeres, the protective caps at the ends of chromosomes, which allows them to divide indefinitely. In contrast, normal cells undergo senescence or apoptosis when their telomeres shorten.
  5. Sustained Angiogenesis: Tumor cells stimulate the formation of new blood vessels (angiogenesis) to ensure a constant supply of nutrients and oxygen. This sustains their growth and metastasis.
  6. Immune System Evasion: Cancer cells can evade immune system surveillance by inhibiting the recognition and attack of immune cells. They may express molecules that suppress immune responses or develop mutations that make them less visible to the immune system.
  7. Loss of Contact Inhibition: Normal cells stop dividing when they come into contact with neighboring cells due to contact inhibition. Cancer cells lose this control mechanism and continue to proliferate, leading to the formation of tumors.
  8. Invasion and Metastasis: Malignant cells acquire the ability to invade surrounding tissues and enter the bloodstream or lymphatic system. They can then travel to distant sites in the body, where they form secondary tumors (metastases).
  9. Self-Sufficiency in Growth Signals: Cancer cells become self-sufficient in receiving growth signals. They can produce their growth factors or alter signaling pathways to promote their own growth, independent of external signals.
  10. Resistance to Apoptosis: Cancer cells often acquire resistance to programmed cell death (apoptosis). This enables them to survive and proliferate despite DNA damage or other stressors that would normally trigger cell death.

These alterations are not mutually exclusive, and different types of cancer can involve various combinations of these changes. Additionally, the specific genetic and molecular alterations can vary widely among different cancer types and even within individual tumors. Understanding these essential alterations is crucial for developing targeted therapies and improving the diagnosis and treatment of cancer.


Cell Proliferation Steps


Proto-oncogenes, Growth Factors and Receptors

  1. Proto-oncogenes:
    • Proto-oncogenes are normal genes present in our cells that play a crucial role in regulating cell growth, division, and differentiation. They encode proteins involved in various cellular processes.
    • These genes are essential for normal cell function and are tightly regulated to prevent uncontrolled cell growth.
    • Mutations or alterations in proto-oncogenes can turn them into oncogenes, which promote abnormal cell proliferation and can lead to cancer.
    • Some common examples of proto-oncogenes include RAS, MYC, and RAF.
  2. Growth Factors:
    • Growth factors are proteins that stimulate cell growth, proliferation, and differentiation. They act as signaling molecules in various cellular processes.
    • They play a critical role in embryonic development, tissue repair, and immune responses.
    • Growth factors can be grouped into different families based on their functions, including epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF).
    • These molecules typically bind to specific receptors on the cell surface to initiate a cascade of intracellular signaling events.
  3. Receptors for Growth Factors:
    • Cell surface receptors are proteins located on the cell membrane that bind to specific ligands, such as growth factors. These receptors transmit signals from the extracellular environment to the interior of the cell.
    • There are several types of receptors for growth factors, including receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). Here, we’ll focus on RTKs since they are commonly associated with growth factor signaling.
    • RTKs have three main components: an extracellular ligand-binding domain, a transmembrane domain, and an intracellular kinase domain.
    • When a growth factor binds to the extracellular domain of an RTK, it causes the receptor to dimerize (pair up), leading to the activation of the kinase domain.
    • The activated kinase domain phosphorylates tyrosine residues on the receptor itself (autophosphorylation) and downstream signaling molecules. This phosphorylation initiates a cascade of intracellular signaling events, ultimately affecting gene expression and cell behavior.
    • Aberrant activation of RTKs due to mutations or overexpression can contribute to cancer development, as they can promote uncontrolled cell growth.

In summary, proto-oncogenes are normal genes that, when mutated, can become oncogenes and promote cancer. Growth factors are signaling proteins that regulate cell growth, and they interact with specific receptors on the cell surface, such as receptor tyrosine kinases, to transmit signals that influence cellular behavior. Dysregulation of these processes can contribute to cancer development.


Two-Hit Hypothesis of Knudson


Tumor Suppressor Genes

Tumor suppressor genes, also known as anti-oncogenes, are a crucial component of the body’s defense against cancer. These genes play a pivotal role in regulating cell growth, preventing the formation of tumors, and maintaining genomic stability. Here’s a detailed explanation of tumor suppressor genes:

  1. Function: Tumor suppressor genes act as “brakes” on cell division and growth. They ensure that cells only divide when necessary and that damaged or abnormal cells are either repaired or undergo programmed cell death (apoptosis). This regulation is essential to prevent the uncontrolled cell division characteristic of cancer.
  2. Mutation and Cancer: Mutations in tumor suppressor genes can disrupt their normal function, leading to the development of cancer. Unlike oncogenes (genes that promote cell growth), which are often activated by mutations, tumor suppressor genes are typically inactivated or “switched off” by mutations.
  3. Two-Hit Hypothesis: Many tumor suppressor genes follow a concept called the “two-hit hypothesis.” This means that for cancer to develop, both copies (alleles) of a tumor suppressor gene in a cell must be mutated or inactivated. The first hit can be an inherited mutation, while the second hit is often acquired during a person’s lifetime due to environmental factors or spontaneous mutations.
  4. Examples of Tumor Suppressor Genes:
    • p53 (TP53): Perhaps the most well-known tumor suppressor gene, p53 plays a central role in DNA repair and cell cycle regulation. Mutations in p53 are found in a wide range of cancers.
    • BRCA1 and BRCA2: Mutations in these genes significantly increase the risk of breast and ovarian cancers.
    • PTEN: This gene is involved in regulating cell growth and division. Mutations are associated with several types of cancer, including prostate cancer.
    • RB1: This gene helps control the cell cycle by regulating the transition from G1 to S phase. Mutations in RB1 are linked to retinoblastoma and other cancers.
  5. Tumor Suppression Mechanisms: Tumor suppressor genes exert their anti-cancer effects through various mechanisms, including:
    • Cell Cycle Regulation: They control checkpoints in the cell cycle, ensuring that cells only divide when conditions are favorable and DNA is intact.
    • DNA Repair: Tumor suppressor genes help repair damaged DNA, preventing the accumulation of mutations.
    • Apoptosis: They trigger programmed cell death in cells with irreparable damage, eliminating potential cancerous cells.
    • Cell Adhesion: Some tumor suppressors promote cell adhesion to prevent the spread of cancer cells (metastasis).
  6. Cancer Therapy: Understanding tumor suppressor genes is critical for developing cancer therapies. Targeted therapies, such as drugs that reactivate specific tumor suppressors, are under development to treat various cancers.

In summary, tumor suppressor genes are guardians of genomic integrity and play a vital role in preventing cancer. Mutations that inactivate these genes can lead to uncontrolled cell growth and contribute to the development of tumors. Studying and targeting these genes is essential for advancing our understanding of cancer and developing more effective treatments.


Cellular changes in tumor cells

Cellular changes in tumor cells, also known as cancer cells, involve a complex series of genetic, molecular, and structural alterations that distinguish them from normal cells. These changes collectively enable uncontrolled growth and proliferation, a hallmark of cancer. Here’s a detailed explanation of the key cellular changes in tumor cells:

  1. Genetic Mutations:
    • Tumor cells often harbor mutations in critical genes that regulate cell growth, division, and repair. Mutations can be inherited or acquired over time due to various factors like exposure to carcinogens or spontaneous errors in DNA replication.
    • Proto-oncogenes, which normally promote cell growth, can become oncogenes when mutated, leading to uncontrolled cell division.
    • Tumor suppressor genes, which inhibit cell growth, can be inactivated when mutated, allowing cells to bypass growth control mechanisms.
  2. Aberrant Signaling Pathways:
    • Tumor cells frequently exhibit dysregulated signaling pathways. For example, the mitogen-activated protein kinase (MAPK) pathway, which controls cell growth, can become overactive.
    • The phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway, involved in regulating cell survival and metabolism, may also be hyperactivated.
  3. Genomic Instability:
    • Tumor cells often have increased genomic instability, leading to a higher mutation rate. This instability can result from defects in DNA repair mechanisms, such as mutations in DNA repair genes like BRCA1 and BRCA2.
  4. Loss of Cell Cycle Control:
    • Normal cells have strict cell cycle regulation, with checkpoints that ensure proper DNA replication and repair. Tumor cells frequently lose control over these checkpoints, leading to unregulated cell division.
    • The G1/S and G2/M checkpoints may be compromised, allowing cells with damaged DNA to progress through the cell cycle.
  5. Resistance to Apoptosis:
    • Apoptosis, or programmed cell death, is a natural mechanism that eliminates damaged or abnormal cells. Tumor cells often develop resistance to apoptosis, allowing them to survive and proliferate despite genetic damage.
  6. Altered Metabolism:
    • Tumor cells often undergo metabolic reprogramming, favoring glycolysis (the Warburg effect) even in the presence of oxygen, which is less efficient but provides necessary building blocks for rapid growth.
  7. Immune Evasion:
    • Tumor cells can develop mechanisms to evade the immune system, making it challenging for the body to recognize and destroy them. This can involve changes in cell surface markers or the production of immunosuppressive factors.
  8. Invasion and Metastasis:
    • Tumor cells may acquire the ability to invade nearby tissues and enter the bloodstream or lymphatic system, facilitating metastasis to distant organs. This process often involves changes in cell adhesion molecules and the extracellular matrix.
  9. Angiogenesis:
    • Tumor cells can induce the formation of new blood vessels (angiogenesis) to ensure a continuous supply of nutrients and oxygen, enabling their growth and survival.
  10. Heterogeneity:
  • Tumors are often heterogeneous, meaning they contain a mix of cells with different genetic and phenotypic characteristics. This heterogeneity can make treatment challenging as some cells may be resistant to therapy.

Understanding these cellular changes is crucial for developing targeted therapies and interventions to combat cancer. Research in this field continues to uncover new insights into the complexities of tumor cell biology, offering hope for more effective cancer treatments in the future.


DNA Repair Defects


Homing of tumor cells

Homing of tumor cells, also known as metastasis, is a complex process by which cancer cells spread from the primary tumor site to other parts of the body. This process involves several intricate steps:

  1. Local Invasion: Cancer cells initially break away from the primary tumor mass through a process called local invasion. They lose their adhesion to neighboring cells and begin to degrade the extracellular matrix (ECM) surrounding the tumor using enzymes like matrix metalloproteinases (MMPs). This allows them to penetrate nearby tissues.
  2. Intravasation: Once cancer cells have invaded the local tissue, they can enter the bloodstream or lymphatic vessels. This is called intravasation. The tumor cells need to adapt to survive in the circulation, as they encounter shear forces and immune system surveillance.
  3. Transport Through Blood or Lymph: Cancer cells can be carried by the bloodstream or lymphatic system to distant sites in the body. Their journey is perilous, and most circulating tumor cells die due to various stressors, including immune attacks and lack of suitable microenvironments.
  4. Arrest and Extravasation: Eventually, some cancer cells arrest in small blood vessels or capillaries at distant sites. Here, they adhere to the vessel walls and extravasate, or exit the bloodstream. This process requires the tumor cells to interact with specific adhesion molecules on the vessel walls.
  5. Microenvironment Adaptation: After extravasation, cancer cells need to adapt to the foreign microenvironment of the new tissue. This may involve interacting with local cells, recruiting blood vessels (angiogenesis) to provide nutrients, and evading the immune system.
  6. Formation of Micro-Metastases: Once adapted, a small cluster of cancer cells can proliferate, forming micro-metastases in the secondary organ or tissue. These micro-metastases may remain dormant for some time before actively growing.
  7. Macro-Metastasis Formation: In favorable conditions, micro-metastases can grow into macro-metastases, which are clinically detectable and contribute to the progression of the disease. This involves sustained growth, angiogenesis, and interaction with the surrounding tissue.

The homing of tumor cells is a highly regulated and intricate process that relies on numerous factors, including genetic mutations in the cancer cells, the tumor microenvironment, immune responses, and vascular structures. It is a critical factor in the spread and progression of cancer and a major challenge in cancer treatment. Targeting the steps involved in metastasis is a key focus of cancer research to develop more effective therapies for preventing or treating metastatic disease.


Development of sustained angiogenesis

Sustained angiogenesis refers to the continuous development and maintenance of new blood vessels from pre-existing ones. This process is crucial for various physiological functions, including wound healing, tissue repair, and the growth of solid tumors. Here’s a detailed explanation of the development of sustained angiogenesis:

  1. Initiation of Angiogenesis: Angiogenesis typically begins when the body detects a need for increased blood supply in a specific area. This can be due to factors like tissue injury, hypoxia (low oxygen levels), or the release of growth factors and cytokines. The key factor in initiating angiogenesis is usually vascular endothelial growth factor (VEGF).
  2. Endothelial Cell Activation: Endothelial cells, which line the inner walls of blood vessels, play a central role in angiogenesis. When stimulated by pro-angiogenic factors like VEGF, these cells become activated and start to proliferate and migrate.
  3. Matrix Degradation and ECM Remodeling: The extracellular matrix (ECM) surrounding blood vessels needs to be restructured to make way for new vessels. Enzymes called matrix metalloproteinases (MMPs) are released to break down components of the ECM, allowing endothelial cells to move through and create new vessel structures.
  4. Endothelial Cell Proliferation: Activated endothelial cells multiply, forming a cluster at the site where new blood vessels are needed. These cells organize themselves into a structure called a “tip cell” and “stalk cell.” Tip cells lead the way, extending long filopodia to guide the formation of the new vessel.
  5. Tube Formation: Endothelial cells, guided by tip cells, align and connect to form a tubular structure. This initial tube serves as the foundation for the new blood vessel.
  6. Recruitment of Supporting Cells: To stabilize and mature the newly formed vessel, pericytes and smooth muscle cells are recruited to surround and support the endothelial cells. This process is essential for the long-term maintenance of the vessel.
  7. Blood Vessel Maturation: As the new vessel matures, it becomes more stable and functional. The basement membrane is established, and the vessel gains the ability to regulate blood flow through the release of vasoactive molecules.
  8. Vessel Remodeling and Pruning: The vascular network continuously adapts to meet the tissue’s changing needs. This involves both the formation of new vessels and the regression or pruning of unnecessary ones to maintain an efficient circulatory system.
  9. Feedback Mechanisms: Various feedback mechanisms, including oxygen and nutrient sensing, ensure that angiogenesis is appropriately regulated. If the tissue’s oxygen and nutrient demands are met, angiogenesis is suppressed to prevent excessive vessel growth.
  10. Sustained Maintenance: Once the new blood vessels are established, they require ongoing maintenance to ensure their functionality. This involves ongoing communication between endothelial cells, supporting cells, and the surrounding tissue.
  11. Pathological Angiogenesis: In some cases, sustained angiogenesis can become dysregulated and contribute to disease. For example, in cancer, tumors can promote excessive and abnormal blood vessel growth to nourish their growth.

Sustained angiogenesis is a highly regulated and dynamic process crucial for tissue homeostasis and repair. Understanding the intricate mechanisms involved in angiogenesis is vital for developing therapeutic strategies to target angiogenesis-related diseases or promote tissue regeneration.