519-2024 lecture #2 doron

Apoptosis

  • Definition: Programmed Cell Death (PCD) is a genetically controlled process of cell suicide.

Functions of Apoptosis

  • Structuring Development: Helps shape body structures like hand digits.

  • Eliminating Unnecessary Cells: Removes cells that have outlived their usefulness, such as the tail of a tadpole.

  • Cancer Prevention: Acts as an anti-cancer mechanism by removing pre-malignant cells.

  • Immune Response: Functions in the immune system to eliminate harmful or damaged cells.

Characteristics of Apoptosis

  • Cells die without causing damage to neighboring cells; process is neat without cell lysis.

  • No leakage of cytoplasmic material, minimizing inflammation and immune response.

  • Apoptotic cells are typically engulfed by macrophages or neighboring cells.

  • Changes in cell surface act as signals for recognition by phagocytes.

  • Often requires new gene expression to execute the process effectively.

Apoptotic Pathways

  • Intrinsic Pathway: Regulated by internal signals often linked to mitochondrial function.

  • Extrinsic Pathway: Triggered by external signals; involves death receptors on cell surfaces.

Bcl-2 Family Proteins

  • Overview: A critical group of proteins that regulate apoptosis through pro-apoptotic and anti-apoptotic mechanisms.

  • BH3-only Proteins: Serve as regulators of apoptosis that can trigger cell death when activated.

  • Role in Cancer: Dysregulation of Bcl-2 family members can lead to cancer; targeting them can be a strategy for anti-cancer drugs.

Mechanisms of Apoptosis Regulation

  • Transcriptional Regulation: Genes such as PUMA and Noxa are transactivated by p53 and E2F1, contributing to apoptosis.

  • Phosphorylation: Proteins like Bad can be phosphorylated by AKT, sequestering them from pro-apoptotic action.

  • Cleavage and Sequestration: Proteins like Bid can be activated by cleavage, which enhances pro-apoptotic signaling.

p53 and Apoptosis

  • p53: Frequently mutated in cancer, stabilizes and activates to induce growth arrest or apoptosis in response to cellular stress.

  • Importance in Cancer: Approximately 50% of human tumors have mutations in the p53 gene, making p53 a target for therapeutic intervention.

Clinical Implications

  • BH3-mimetics: Compounds that mimic BH3-only proteins to induce apoptosis in cancer cells may serve as potential treatments.

  • Bcl-2 Inhibition: Drugs like ABT-737 and Venetoclax (ABT-199) selectively inhibit anti-apoptotic Bcl-2 proteins, showing promise for treating hematological cancers.

  • Safety and Side Effects: Treatments may lead to adverse effects like tumor lysis syndrome, necessitating careful monitoring.

Summary of Key Proteins in Apoptosis

  • Bcl-2 Family: Anti-apoptotic proteins (like Bcl-2) promote cell survival, while pro-apoptotic proteins (like Bax and Bak) drive cell death.

  • Variability in Action: Different BH3-only proteins exhibit varying strengths in promoting apoptosis, influenced by their tissue-specific expression.

Apoptosis

Definition

Apoptosis, also known as Programmed Cell Death (PCD), is a genetically controlled and regulated process that leads to the systematic and orderly death of cells. It serves as a critical mechanism for maintaining cellular homeostasis and plays a fundamental role in health and disease.

Functions of Apoptosis

  • Structuring Development: Apoptosis is essential during the embryonic development of multicellular organisms. It assists in shaping body structures, such as the separation of fingers in hand development, by removing cells in a precise manner.

  • Eliminating Unnecessary Cells: The process of apoptosis effectively removes cells that are no longer needed, such as the tail of a tadpole, and helps to modulate cell populations during periods of rapid growth.

  • Cancer Prevention: Apoptosis acts as a protective mechanism against cancer by eliminating cells that exhibit pre-malignant characteristics, thereby preventing the proliferation of potentially cancerous cells.

  • Immune Response: Apoptosis plays a vital role in the immune system by eliminating harmful or damaged cells, such as those infected with viruses, thereby preventing the spread of infection and contributing to overall immune system health.

Characteristics of Apoptosis

  • Non-Inflammatory Process: Apoptosis is characterized by cells dying in a controlled manner without causing damage to surrounding tissues. This neat process lacks cell lysis, which minimizes inflammation and subsequent immune responses.

  • Cell Engulfment: After apoptosis, the dying cells often undergo phagocytosis, where they are engulfed and removed by macrophages or neighboring cells, further contributing to the non-inflammatory nature of the process.

  • Surface Changes: As cells undergo apoptosis, they exhibit changes in their surface markers that signal phagocytes to recognize and eliminate them.

  • Gene Expression: The effective execution of apoptosis frequently necessitates the expression of new genes that regulate the apoptotic process.

Apoptotic Pathways

  • Intrinsic Pathway: This pathway is regulated by internal cellular signals, most notably those related to mitochondrial stress. Mitochondrial dysfunction can lead to the release of pro-apoptotic factors into the cytoplasm, activating the caspase cascade.

  • Extrinsic Pathway: Triggered by external signals, this pathway involves the interaction of death ligands with specific death receptors on the cell surface, which leads to cell death through a series of signaling events that also activate caspases.

Bcl-2 Family Proteins

  • Overview: The Bcl-2 family of proteins is a crucial group involved in the regulation of apoptosis, containing both pro-apoptotic and anti-apoptotic members that modulate mitochondrial outer membrane permeability.

  • BH3-only Proteins: These proteins act as key regulators of apoptosis and can trigger cell death when activated, often serving as the initiators of the intrinsic apoptotic pathway.

  • Role in Cancer: Dysregulation of Bcl-2 family members is a significant contributor to cancer development; thus, targeting these proteins is a strategic approach for the development of anti-cancer therapies.

Mechanisms of Apoptosis Regulation

  • Transcriptional Regulation: Key genes such as PUMA and Noxa, which promote apoptosis, are transactivated by tumor suppressors like p53 and transcription factors such as E2F1 in response to cellular stress or damage.

  • Phosphorylation: The phosphorylation of pro-apoptotic proteins like Bad by survival signals (e.g., AKT) can sequester these proteins away from their apoptotic functions, promoting cell survival instead.

  • Cleavage and Sequestration: Pro-apoptotic proteins may be activated by cleavage, such as Bid, leading to enhanced pro-apoptotic signaling through mitochondrial pathways.

p53 and Apoptosis

  • p53: Commonly referred to as the "guardian of the genome," p53 plays a pivotal role in cellular response to stress. It stabilizes and activates pathways leading to cell cycle arrest or apoptosis in reaction to DNA damage or other forms of cellular stress.

  • Importance in Cancer: Approximately 50% of human tumors harbor mutations in the p53 gene, highlighting its critical role in tumor suppression and making it a prime target for innovative cancer treatments.

Clinical Implications

  • BH3-mimetics: These are therapeutic compounds designed to mimic the function of BH3-only proteins, inducing apoptosis in cancer cells. They show promise as potential treatments in various malignancies.

  • Bcl-2 Inhibition: Some drugs, like ABT-737 and Venetoclax (ABT-199), selectively inhibit anti-apoptotic Bcl-2 proteins and have displayed encouraging results in treating hematological cancers such as chronic lymphocytic leukemia.

  • Safety and Side Effects: While these treatments offer new avenues for cancer therapy, they also come with risks, including tumor lysis syndrome, necessitating careful monitoring of patients during treatment.

Summary of Key Proteins in Apoptosis

  • Bcl-2 Family: Comprising both anti-apoptotic proteins (like Bcl-2) that promote cell survival and pro-apoptotic proteins (like Bax and Bak) that drive cells toward death, this family plays a crucial role in balancing cell life and death.

  • Variability in Action: Different BH3-only proteins exhibit various strengths in promoting apoptosis, influenced by their specific expression patterns in different tissues, underscoring the complexity of apoptotic regulation.