Senescence, Immortalization, and Tumorigenesis Notes

Learning Objectives

  • Differentiate between Cell Senescence and Crisis/Apoptosis.

  • Describe the types of senescence and the characteristics of senescent cells.

  • Provide a broad overview of the mechanisms of senescence.

  • Enlist some of the biomarkers of senescence.

  • Understand how physiologic stress and telomere length impose limitations on replication.

  • Explain the mechanisms underlying telomere maintenance (Telomerase and ALT).

  • Functions of telomeres (BFB cycles etc).

  • Functions and structure of telomerase holoenzyme.

  • Explain how telomerase activity/inactivity can suppress/promote tumour growth.

Cell Senescence vs Crisis/Apoptosis

  • Cell Senescence: A stable, irreversible arrest of cell proliferation in response to various stresses (e.g., DNA damage, oxidative stress).

  • Crisis/Apoptosis: A state of cellular failure stemming from significant DNA damage or excessive replication stress leading to cell death or neoplastic transformation.

Types of Senescence

  • Identified types include:

    • Oncogene-Induced Senescence: Triggered by activated oncogenes.

    • Replicative Senescence: Age-related ceasing of division due to telomere shortening.

    • Stress-Induced Senescence: Resulting from oxidative stress, DNA damage, etc.

Characteristics of Senescent Cells

  • Permanent Growth Arrest: Cannot be reversed by physiological stimuli.

  • Morphological Changes: Increased cell size, flat appearance, and increased cytoplasmic granularity.

  • Biochemical Markers: Express senescence-associated beta-galactosidase (SA-β-gal).

Biomarkers of Senescence

  • Telomere Length: Shortened telomeres indicative of senescence.

  • SA-β-gal: A marker for senescent cells, revealing increased lysosomal activity.

  • SASP: Senescence-associated secretory phenotype outputs (e.g., cytokines).

Mechanisms of Senescence

  • Triggered by:

    • Oxidative Stress: Leads to DNA damage.

    • Telomere Shortening: Critical for limiting cellular replication.

    • Oncogenic Signaling: Activation of certain pathways leading to senescence.

    • CDKN2A De-repression: Activation of cyclin-dependent kinase inhibitors (p21, p16Ink4a).

Hayflick Limit

  • Concept introduced by Leonard Hayflick, indicating that normal somatic cells divide only a limited number of times (approximately 40-60 divisions).

Telomeres Structure and Function

  • Composition: Repeating hexanucleotide sequences 5'-TTAGGG-3', associated with shelterin complex proteins.

  • Function: Protect chromosome ends from degradation and fusion, preventing genomic instability.

Telomere Shortening

  • Each cell division results in a loss of 50-100 ext{ bp} of telomeres.

  • Leads to crisis—a state of severe genomic damage, leading to apoptosis or senescence.

Telomerase and its Role in Cancer

  • Human Telomerase Holoenzyme: Features hTERT and hTR, necessary for telomere extension.

  • Telomerase Reactivation: Observed in ~90% of cancers, crucial for maintaining telomere length and promoting cell immortality.

  • Telomerase as a Cancer Target: Due to its role in continuous cellular replication, potential therapeutic target for cancer treatments.

Conclusion

  • Understanding the mechanisms of senescence and telomere dynamics is crucial for insights into cancer biology and potential treatment pathways.

The document addresses all the stated learning objectives:

  1. Differentiate between Cell Senescence and Crisis/Apoptosis: This is clearly explained in the section comparing cell senescence to crisis/apoptosis.

  2. Describe the types of senescence and the characteristics of senescent cells: The types of senescence and their characteristics are effectively outlined.

  3. Provide a broad overview of the mechanisms of senescence: Various mechanisms triggering senescence are detailed.

  4. Enlist some of the biomarkers of senescence: Several biomarkers are identified and explained.

  5. Understand how physiologic stress and telomere length impose limitations on replication: The impact of both stress and telomere length is covered.

  6. Explain the mechanisms underlying telomere maintenance (Telomerase and ALT): Mechanisms related to telomerase and its maintenance role are discussed.

  7. Functions of telomeres (BFB cycles etc): Functions of telomeres are described, including their role in maintaining genomic stability.

  8. Functions and structure of telomerase holoenzyme: The structure and function of telomerase are reviewed.

  9. Explain how telomerase activity/inactivity can suppress/promote tumour growth: The document addresses the implications of telomerase activity on tumor growth.
    Overall, all learning objectives are effectively addressed in this document, providing a comprehensive understanding of cell senescence and telomere dynamics.