Part 4 Flashcards
What is rapamycin?
Drug derived from bacterium Streptomyces hygroscopicus (1999)
- AKA Sirolimus and Rapamune
Antiobiotic, antifungal, immunosuppressant, inhibits mTORC1
- mTOR = mechanistic target of rapamycin
Given after transplants (immunosuppressive) or to coat stents used to support arteries open (Helps to suppress rejection of stent)
Clinical trials with rapamyacin on yeast, C elegans and drosophila and now dogs
extends lifespan via mTOR anabolic response dampening
mimics effects of calorie restriction
Goal of life and metabolism
Life: survival and replication (cellular level maintain or replace)
Metabolism: maintenance of homeostasis - excess or deficiency leads to dysregulation
mTOR role
mTOR affects downstream
When mTOR is ramped up vs. dampened
mTOR integrates intra/extracellular signals involved in cell growth
- nutrients, energy (AMPK inhibition) and growth factors (PKB/Akt activation)
- regulates cellular metabolism
- central to cell growth and proliferation
SREBP (lipid synthesis) and protein synthesis towards cell growth and proliferation
cancer ramps up mTOR, rapamycin dampens mTOR
mTORC1 stimulation pathway
End results of intracellular stimulation
1) Insulin binds and activates signaling cascade PI3K –> PIP3 –> PKB
2) PKB leads to activation (indirect) of mTORC1
- PKB stimulates movement of GLUT4 receptors to membrane, and hexokinase and citrate lyase activity
3) mTORC1 stimulates AA uptake, glucose uptake, and SREBP1 activity
mTORC1 is a connector of lipid and protein synthesis and glycolysis
Regulates metabolism, proliferation, cell survival, growth and angiogenesis
Key regulators of mTORC1
PI3K = phosphatidylinositol-3-kinase
Akt = Protein Kinase B (PKB)
mTOR = mechanistic target of rapamyacin
Regulation of HGMR activity
1) Low [AMP], glucagon and sterols activate AMPK
2) AMPK phosphorylates HGMR and inactivates cholesterol and coQ10 synthesis
3) Insulin activates HMGR phosphatase which activates HMGR conversion of HMG CoA –> mevalonate for cholesterol and coQ10 synthesis
mTOR stimulation of mRNA translation
1) Atk stimulates mTORC1
2) mTORC1 phosphorylates 4E-BP1 bound to elF4E
3) Phosphorylated 4E-BP1 releases elF4E
4) elF4E with elF4A and elF4G form elF4F complex which binds 5’ cap of mRNA for translation
5) In cancerous cells MNK1/2 phosphorylates elF4E to promote protein synthesis
mTORC1 contains
elF4E stands for and role
mTOR, mLST8, and Raptor
eukaryotic initiation factor 4E - recognizes 5’ cap mRNA structure
- inactive when bound to unphosphorylated 4E-BP1
- discovered by Nahum Sonenberg
elF4F complex includes: elF4E, elF4G, elF4A
Metabolic pathway activations in muscle
During hypertrophy: growth, anabolic pathways and activation of PI3K-Atk-mTOR pathway
During atrophy: wasting, catabolic pathways and activation of ubiquitin-proteasome pathways
Percent contributors of protein synthesis in the body
Importance of post-translational modifications in protein synthesis
Visceral: 50%
Muscle: 30%
Blood proteins (plasma and blood cells): 20%
Peptide bonds are expensive to form (4 ATP per) - post-translational modification is cheaper
2 types of protein degradation in the body
About each pathway
Helps to determine protein levels within cells
Protein damaged such as oxidation or mis-folding
Ubiquitin-proteasome pathway: major selective protein degradation pathway
- Ubiquitin is attached to lysine side chains by ligases E1/2/3 (roles not important)
- polyubiquitinated protein are recognized by proteasome complex for degradation
- ATP dependent
Lysosomal pathway:
- vesicles containing low pH and proteins which only function at low pH
- Helps to protect cell contents in the cell that produced them
- analogous to digestive enzymes that must be hydrolyzed to be active
Ubiquitin-proteasome pathway
1) activated ubiquitins are added by ligases to protein lysine side chains
2) Ubiquitin guides protein into proteasome
3) Multisubunit proteasome degrades protein marked by polyubiquitin tail
4) ubiquitin is recycled
Lysosomal protein degradation pathway
1) Vesicles released from ER must fuse with lysosomes to be degraded (non-selective under normal conditions)
- leads to gradual turnover of cytosolic proteins
2) fusion with lysosome forms phagolysosome and contents are digested by low pH active enzymes
- Controls for uncontrolled digestion of cell contents
3) During cellular starvation this process becomes more selective for specific AA sequences or to sacrifice to make other amino acids
When is lysosomal degradation selective?
During cellular starvation or when there are limiting AA in the diet
selective uptake and degradation of specific AA sequences
- AA recycled for other product synthesis
