Chapter 26- Cancer Cells

Becker’s World of the Cell Tenth Edition, Chapter 26: Cancer Cells

Overview of Cancer

  • Definition: Cancer is a disease arising from abnormalities in cell function, often caused by gene mutations and changes in gene expression. These alterations can be influenced by environmental factors, lifestyle choices, and certain infections.

  • Causes of Cancer: Primarily gene mutations, which can be inherited or acquired over time. Research is ongoing to better understand these mechanisms.

  • Treatment Queries: Important questions include the specifics of what causes cancer and the treatment options available, emphasizing the need for improved patient outcomes.

26.1 How Cancers Arise

  • Categories of Cancers:

    • Carcinomas: Comprising about 90% of all cancers, these arise from epithelial cells and include lung, breast, and colon cancers.

    • Sarcomas: These tumors develop from supportive tissues like bone, cartilage, fat, and muscle, and are less common.

  • Types of Cancers:

    • Lymphomas: Arising from blood and lymphatic cells, typically presenting as solid masses. These can significantly affect immune function.

    • Leukemias: These proliferate mainly in the bloodstream, leading to an overproduction of abnormal white blood cells.

Tumor Characteristics

  • Tumor Definition: A tumor is a mass of tissue resulting from uncontrolled cell division, characterized by excessive proliferation.

  • Key Mechanism: Balance among cell division, differentiation, and programmed cell death is crucial in normal tissue maintenance and is often disrupted in cancer, leading to tumor formation and growth.

Concept of Cell Differentiation

  • Process: Cell differentiation is the process where cells acquire specialized properties and typically lose their ability to divide. For instance, in skin, new cells from the basal layer replace aged cells on the surface; one cell divides and the other differentiates, moving toward the surface.

Normal vs. Tumor Growth in Skin

  • Normal Growth: Involves orderly cell migration and differentiation.

  • Tumor Growth: Disruption occurs, with cells migrating and retaining their ability to divide, leading to excessive growth.

Stem Cells

  • Defining Traits: Stem cells can divide to yield more stem cells or differentiate into specialized cells, maintaining tissue homeostasis.

  • Transit-Amplifying Cells: These are specialized cells that can still divide a few times before final differentiation occurs.

Balance of Division and Differentiation in Skin

  • In healthy tissue, division and differentiation are carefully balanced, preventing excess dividing cells and tumor development.

Disruption of Balance in Cancer

  • Cancer disconnects cell division from differentiation and programmed cell death, resulting in excessive growth of tumor cells.

Tumor Types

  • Benign Tumors: Have a confined growth pattern and are generally not dangerous.

  • Malignant Tumors: Have the ability to invade surrounding tissues and may metastasize, spreading cancer throughout the body.

Characteristics of Cancer Cells

  • Anchorage Independent: Cancer cells can proliferate irrespective of attachment to a solid surface, unlike normal cells.

  • Density-Dependent Inhibition: Normal cells stop dividing when a confluent layer forms; cancer cells disregard this and pile up.

Immortalization of Cancer Cells

  • Cancer cells can divide indefinitely, contrasting with normal cells that are limited by a programmed number of divisions. HeLa cells serve as a prominent example of this phenomenon.

Telomeres and Cancer

  • Function of Telomeres: Protect chromosome ends and shorten with each division. Many cancer cells produce telomerase to maintain telomere length, bypassing limits on division.

Defects Contributing to Cancer

  • Altered Signaling Pathways: Cancer cells exhibit defects leading to uncontrolled proliferation often without growth factors.

  • Disruption of Cell Cycle Controls: Normal cell cycle checkpoints are impaired, failing to stop division under inappropriate conditions.

Apoptosis in Cancer

  • Cancer can arise not only from increased division but also from the failure to undergo programmed cell death (apoptosis), resulting in more cancerous cells.

Cancer Development Process

  • Multistep Process:

    • Initiation: Normal cells convert into precancerous states due to genetic alterations.

    • Promotion: Further exposure to cancer-promoting agents leads to tumor development.

Evidence for Initiation and Promotion

  • DMBA (dimethylbenz[a]anthracene) functions as an initiator but requires additional promoting agents, like croton oil, for tumor induction.

Tumor Progression

  • Phases: Growth and differentiation of tumors follow initiation and promotion, with certain cancer cells dominating due to enhanced growth rates.

Cancer Cell Properties

  • Tumors show intra-tumor heterogeneity, leading to variable genetic capabilities among cells, complicating treatment strategies.

Cancer Spread

  • Cancer mortality is primarily due to metastasis rather than primary tumor growth, highlighting the importance of understanding metastatic potential.

Angiogenesis and Tumor Growth

  • Angiogenesis: Tumors larger than a few millimeters require new blood vessel formation, initiated by signaling molecules from cancer cells.

Experimental Evidence for Angiogenesis

  • Studies indicate that cancer cells need angiogenesis to support growth beyond a specific size, linking tumor growth to its blood supply.

Factors in Angiogenesis

  • Activators: Molecules such as vascular endothelial growth factor (VEGF) stimulate blood vessel formation.

  • Inhibitors: Natural inhibitors like angiostatin and endostatin help regulate angiogenesis, limiting tumor growth.

Invasion and Metastasis Mechanisms

  • Invasion: Refers to the direct migration of cancer cells into adjacent tissues.

  • Metastasis: Involves cancer cells entering the bloodstream to establish secondary tumors in other organs.

Mechanisms of Metastasis

  • Successful metastasis requires cancer cells to invade tissues, survive bloodstream circulation, and establish new growth; patterns are influenced by blood flow and organ-specific factors.

"Seed and Soil" Hypothesis

  • Proposed by Stephen Paget, this hypothesis suggests that the growth of specific cancers is influenced by suitable environments in distant organs.

Immune System Influence on Cancer

  • The immune system’s role in preventing and recognizing cancer cells is crucial; immunosuppressed individuals have a higher incidence of cancer.

Overcoming Immune Defenses

  • Cancer cells may evade detection or inhibit immune responses, leading to unchecked growth and tumor progression.

Conclusion

  • A thorough understanding of cancer cell biology is essential for developing better treatments and preventative strategies, highlighting the intricate interplay of genetic, environmental, and immune factors in cancer progression.