patoloji-ders-notlari

Title

Serdar Balcı

The Mechanisms of Cell Death

Serdar BALCI, MD

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Cell necrosis Liver

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Cell necrosis Renal cortex

SHRINKAGE/HYPERCHROMASIA

medicalschoolpathology.com

Core Pathology , Third Edition

APOPTOSIS

Apoptosis (falling off)

A pathway of cell death

**Cells activate enzymes **

The enzymes degrade the cells’ own nuclear DNA and nuclear and cytoplasmic proteins

Fragments break off the cell

Apoptosis

• Plasma membrane stays intact
• There are “signals” on the surface of membranes, attracting phagocytes
• Cellular content does not leak out
  • There is no inflammatory response

Apoptosis Early stage

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Apoptosis Late stage

Wheater’s Basic Pathology: A Text, Atlas and Review of Histopathology , Fifth Edition

Factors controlling apoptosis

• Inhibitors __ __
  • growth factors, extracellular cell matrix, sex steroids, some viral proteins
• Inducers
  • **growth factor withdrawal, loss of matrix attachment, glucocorticoids, some viruses, free radicals, ionising radiation, DNA damage, ligand-binding at ‘death receptors’ **

Causes of Apoptosis

Physiologic

Pathologic

Physiologic Causes of Apoptosis

• during embryogenesis
  • Webs among fingers
  • Different phases of renal genesis
• Involution after hormone cessation
  • Endometrial breakdown menstrual cycle
  • Involution of breast after breastfeeding
• Skin, GI mucosa epithelial cells
  • Turnover on cells
• Neutrophils, lymphocytes
  • After inflammation is over
  • No more signals
• Lymphocytes in thymus
  • Auto reactive cells are eliminated
• Cytotoxic T-cell induced cell death
  • Cells receive “die” comment from lymphocyte when they are infected with a virus

Pathologic Causes of Apoptosis

When cell DNA changes beyond repair

It is a controlled death to avoid further damage in the cell/tissue/organism

Pathologic Causes of ApoptosisDNA damage

• Radiation, cytotoxic anticancer drugs, extremes of temperature, hypoxia
  • If mild ends into apoptosis
  • If severe necrosis
• DNA damage occurs directly or via free radicals
• To prevent malignant transformation cell goes to apoptosis
• Some antineoplastic agents cause DNA damage to force neoplastic cells into apoptosis

Pathologic Causes of ApoptosisMisfolded proteins

Misfolded proteins accumulate in ER

Induces apoptosis

Pathologic Causes of ApoptosisViruses

Adenovirus and HIV induces cell death

Viral hepatitis leads to host defense which then induces cell death

Pathologic Causes of ApoptosisPathologic Atrophy

• After duct obstruction
  • Pancreas
  • Parotid gland
  • Kidney

Chromatin condensation and aggregation

Karyorrhexis

fragmentation of DNA into nucleosome-sized pieces

cells rapidly shrink

form cytoplasmic buds

fragment into apoptotic bodies

Apoptotic bodies:

membrane-bound vesicles of cytosol and organelles

Core Pathology , Third Edition

**Mechanism of apoptosis depends on ** caspase **enzymes via ** two pathways

**There is a ** balance __ btw pro and anti-apoptotic signals__

The Mitochondrial (Intrinsic) Pathway of Apoptosis

• Responds to stimuli
• growth factors (or their withdrawal)
• biochemical stress
• DNA damage
  • **due to radiation or cytotoxic chemotherapy **

• Bcl-2
  • Anti-apoptotic
  • inhibit many factors that induce apoptosis
• Bax
  • Pro-apoptotic
  • forms Bax–Bax dimers which enhance apoptotic stimuli
• The ratio of Bcl-2 to Bax
  • determines whether a cell goes to apoptosis

• p53 is a multifunctional protein
  • induces cell cycle arrest and initiates DNA damage repair
  • if this is unsuccessful, p53 can induce apoptosis

Mitochondria contain several proteins that are capable of inducing apoptosis

The Death Receptor (Extrinsic) Pathway of Apoptosis

• Ligand-binding
• Death receptors on the cell surface
• Tumour necrosis factor receptor (TNFR) gene family
  • TNFR1 and Fas (CD95)

Ligand binding

clustering of receptor molecules on the cell surface

initiation of a signal transduction cascade

activation of caspases

Fas ligand expressed mainly on activated T lymphocytes

A caspase antagonist FLIP, block activation of caspases downstream of death receptors

Some viruses produce homologues of FLIP to keep infected cells alive

Activation and Function of Caspases

• Mitochondrial and death receptor pathways
• Activation of the initiator caspases
  • caspase-9 and -8
• Active forms of these enzymes are produced
• These cleave and thereby activate executioner caspases
• These cleave numerous targets
• Nucleases degrade DNA and nucleoproteins
• Caspases also degrade nuclear matrix and cytoskeleton
• Fragmentation of cells.

Clearance of Apoptotic Cells

• “Eat me” signals on apoptotic bodies
  • Normal cell membrane: phosphatidylserine is present on the inner leaflet
  • Apoptotic body: this phospholipid flips to outer surface
  • apoptotic bodies express adhesive glycoproteins that are recognized by phagocytes
• Tissue macrophages, phagocytosis of the apoptotic cells

No membrane damage

The content does not leak out

No inflammation is present

Apoptosis and Necrosis may occur together

• **DNA damage **
  • activates poly-ADP(ribose) polymerase
  • depletes cellular supplies of NAD
  • fall in ATP levels
  • necrosis
• In ischemia
  • Early cell death is apoptosis
  • Necrosis later as ischemia worsens

EXAMPLES OF APOPTOSIS

Growth Factor Deprivation

• Hormone-sensitive cells (hormones)
• Lymphocytes (antigens and cytokines)
• Neurons (nerve growth factor)
• Apoptosis is via the mitochondrial pathway
  • Activation of pro-apoptotic members of the Bcl-2 family
  • Decreased synthesis of Bcl-2 and Bcl-xL

DNA Damage

• Radiation, chemotherapeutic agents → DNA damage
• p53 protein accumulates in cells
  • It arrests cell cycle at the G1 phase to allow the DNA to be repaired
  • If the damage is too great to be repaired successfully, p53 triggers apoptosis
    • activate Bax and Bak
    • increasing the synthesis of pro-apoptotic members of the Bcl-2 family
• When p53 is mutated or absent (as in some cancers)
  • Cells with damaged DNA survive and replicate
  • mutations accumulate
  • neoplastic transformation occur

Accumulation of Misfolded ProteinsER Stress

• Normal protein synthesis
  • chaperones in the ER control the proper folding of newly synthesized proteins
  • misfolded polypeptides are ubiquitinated and targeted for proteolysis
• When unfolded or misfolded proteins accumulate in the ER
  • inherited mutations
  • environmental perturbations
• Unfolded protein response
  • activates signaling pathways
  • increase the production of chaperones and retard protein translation
  • reduce the levels of misfolded proteins in the cell
• When more misfolded proteins accumulate
  • ER stress
  • activation of caspases
  • apoptosis

Apoptosis of Self-Reactive Lymphocytes

• **Lymphocytes recognizing self antigens are normally produced in all individuals **
• When these lymphocytes meet with self antigens, the cells die by apoptosis
  • Both the mitochondrial pathway and the Fas death receptor pathway
• Failure of apoptosis of self-reactive lymphocytes is one of the causes of autoimmune diseases.

Cytotoxic T Lymphocyte–Mediated Apoptosis

• CTLs recognize foreign antigens presented on the surface of infected host cells and tumor cells
  • Granzymes enter the target cells
  • Granzymes activate cellular caspases
  • Kills target cells by directly inducing the effector phase of apoptosis, without engaging mitochondria or death receptors
• CTLs also express FasL on their surface and may kill target cells by ligation of Fas receptors

NECROPTOSIS

Necroptosis

Shares aspects of both necrosis and apoptosis

• Morphologically resembles necrosis
  • loss of ATP
  • swelling of the cell and organelles
  • generation of ROS
  • release of lysosomal enzymes
  • rupture of the plasma membrane
• Mechanistically resembles programmed cell death
• Triggered by genetically programmed signal transduction events
  • Programmed necrosis
• Different from necrosis driven passively by toxic or anoxic injury to the cell
• No caspase activation
  • “caspase-independent” programmed cell death

Triggered by ligation of TNFR1, and viral proteins of RNA and DNA viruses

caspase-independent but dependent on signaling by the RIP1 and RIP3 complex

RIP1-RIP3 signaling reduces mitochondrial ATP generation

Production of ROS

Permeabilizes lysosomal membranes

Cellular swelling and membrane damage

Release of cellular contents evokes an inflammatory reaction as in necrosis

• Physiologic:
  • During the formation of the mammalian bone growth plate
• Pathologic
  • Steatohepatitis
  • Acute pancreatitis
  • Reperfusion injury
  • Neurodegenerative diseases such as Parkinson disease
• Backup mechanism in host defense
  • Against certain viruses that encode caspase inhibitors (cytomegalovirus)

Pyroptosis

• Occurs in cells infected by microbes
• Activation of caspase-1
• Cleaves the precursor form of IL-1 to generate biologically active IL-1
• Caspase-1 along with closely related caspase-11 cause death of the infected cell
• Pyro: fever
  • Fever inducing cytokine IL-1

AUTOPHAGY

Autophagy: self-eating

• Lysosomal digestion of the cell’s own components
• A survival mechanism in times of nutrient deprivation
  • recycling to provide nutrients and energy
• **autophagy genes.” Autophagy has been proposed as a form of cell death in different diseases such as neurodegenerative diseases and degenerative diseases of muscle. **

Intracellular organelles and portions of cytosol are sequestered within an autophagic vacuole

The vacuole fuses with lysosomes to form an autophagolysosome

**Lysosomal enzymes digest the cellular components. **

Autophagy may also signal cell death by apoptosis.