Cell Bio (pt2) Cell Division, Migration, and Apoptosis Handout

extrinsic apoptosis pathway

uses "death" ligands and "death" receptors

- death-inducing signaling complex forms

- this pathway is often used by cells of the immune system to kill targets

death-inducing signaling complex in extrinsic apoptosis

procaspase 8 contains an extra portion (pro=portion), which is removed by proteolytic cleavage

- caspase 8

extrinsic apoptosis pathway is often used by cells of the immune system to kill targets such as:

target cells are often in immune system, or are infected or "sick". most cells express TNFR

- cytotoxic T cells

- natural killer cells

- some macrophage

FasL and Fas system

FasL present in the PM of cytotoxic T-cells (and others, certain macrophage) binds Fas in the PM of the target (cancer) cells

- the DISC forms in the target cells, stimulating extrinsic apoptosis

TNF

tumor necrosis factor

TRADD

Tumor necrosis factor Receptor type 1-Associated Death Domain

FADD

Fas Associated via Death Domain

if internal "stress" is too much, what happens?

cells die via intrinsic apoptosis

stress

an abnormal state or condition that is sensed as such by the cell when it occurs

cell stress can cause intrinsic apoptosis

- DNA damage (many types)

- telomere erosion

- reactive oxygen species, free radicals

- ER, unfolded proteins

- DNA replication stress

- hypoxia

- drugs, chemicals, radiation, UV light

- nutrient deprivation, high glucose

- pH

- cell surface changes

- energy, "warburg effect"

- infection

- oncogene/tumor suppressor

- ribosomal, nucleolar

- aneuploidy

- heavy metals

- heat shock

- cytokines from immune cells

- altered adhesion

cell suicide is adaptively valuable for the good of the organism

- materials are recycled

- tissues remain healthy

intrinsic apoptosis

internal cellular damage ->

activation of pro-apoptotic BH3-only protein and subsequent activation of Bak or Bax ->

mitochondria -> MOMP ->

EndoG, SMAC/Diablo etc ->

cytochrome C is released via pores during MOMP -> apoptosome ->

executioner procaspases -> executioner caspase

mitochondrial outer membrane permeabilization (MOMP)

MOMP is coupled to cell death

- occurs all at once

- very fast

- caspase independent (zVAD-fmk)

- other molecules leak out too

MOMP is regulated

Bcl-2, Bax/Bak, and BH3-only families control MOMP

- pro-survival guardian and pro-apoptosis pore-forming proteins are involved

- they work at the level of the mitochondria

- BH3 only initiator proteins inhibit guardian proteins, allowing MOMP

- pores form via Bax/Bak, CytC leaks out

- MOMP point of no return for committment to apoptosis

pore formation in mitochondria for MOMP

- Bcl-2 binds to and inactivates Bax/Bak

- 14-3-3 binds to inactivated (phosphorylated) Bad, and helps dephosphorylate and "activate" Bad

- Bad can then bind to and inactivate Bcl-2, allowing for Bax/Bak to oligomerize and form pores

- MOMP occurs and CytC escapes into the cytoplasm

Bax/Bak

pore forming proteins

Bcl-1

guardian protein

Bad

BH3-only initiator protein

oligomerize

get together

clearance of apoptotic cells

apoptotic cells cleared by macrophages

- this prevents inflammation

- condensed, fragmented chromatin

apoptotic cells cleared by macrophages

"eat me" signal on apoptotic cells

- phosphatidyl serine (PS) flips from the inner leaflet to the outer leaflet of the plasma membrane

BH3 family profiling and mitochondrial priming

Bcl-2, Bax/Bak, and BH3-only families control MOMP

- these work at the level of the mitochondria

- BH3 only (initiator) proteins inhibit the pro-survival (guardian) proteins, allowing MOMP

- pores formed, CytC leaks out

mitochondrial priming

balance of BH3-only (initiator) vs Bcl2 (guardian) and presence of Bax/Bak

- BH3-only aid in MOMP by inhibiting anti-apoptotic proteins, thus promoting pore formation

mitochondria priming has important implications for what?

cancer and potentially aging-related pathologies

venetoclax

BH3 mimetic

- small peptide that binds up Bcl2, making it easier for MOMP to occur

- venetoclax increases mitochondrial priming, tilting the balance toward more and "easier" apoptosis

- BH3 mimetics act as initiators