D - Translace
Regulace translace
- In 3-day fasted rats, glucose injection
- Increased relative proinsulin mRNA levels by three- to four-fold within 24 h
- Effect was blocked by pharmacological inhibition of transcription
- With actinomycin D [14]
- Glucose
- Important factor for maintaining insulin mRNA stability [14]
- Elevation of intracellular cAMP levels can prevent this reduction [14]
- Partly controlled by dephosphorylation of eukaryotic initiation factor 2a (eIF2a) via protein phosphatase 1 (PP1)
- Exposure of ß-cells to high glucose for 2 hours
- Significantly decreases the ratio of phosphorylated eIF2a to eIF2a
- The overall protein translation induced by glucose compared to the fasting state in ß-cells
- Only 3-fold compared with an up to 8-fold induction in proinsulin translation [14]
- Pancreatic ER kinase (PERK)
- Phosphorylates eIF2a
- Can be partially compensated for by other kinases
- PERK mutation = Wolcott-Rallison syndrome
- Permanent neonatal diabetes in humans [14]
- PERK-deficient mice
- Severe defects in insulin synthesis
- X ß-cell proliferation and differentiation
- Permanent neonatal diabetes as seen in humans [14]
- Not required in adults for maintaining ß-cell mass [14]
- ß-cells have evolved a mechanism to detect the amount of insulin stored and secreted
- Insulin granules
- Long distance on tubulin tracks
- Peripheral actin network
- Anchored to actin cortex via ICA512 and ß2-synthrophin
- Linked to the cytoskeleton
- Granule membrane fuses transiently to the cell membrane to release insulin
- Ca2+ levels in the meantime activate the protease µ-calpain to cleave away a cytosolic fragment from ICA512
- Free ICA512 cytosolic fragment
- In nucleus binds tr. factor STAT5
- Prevent STAT 5 from dephosphorylation
- Upregulates insulin transcription [14]
- Also bind to sumoylating enzyme PIAS
- Reverses the binding of ICA512 to STAT5 [14]
- Regulating the speed of insulin translation
- Exposure of rat islets to 25 mM glucose for 1 hour
- An induction in intracellular proinsulin levels by up to ten-fold from baseline (2.8 mM glucose)
- Whereas proinsulin mRNA quantities remained the same
- Acute glucose-stimulated insulin synthesis is independent of mRNA synthesis within the first 45 min
- Because blockage of transcription only slowed insulin accumulation after that time frame [14]
- insulin mRNA stability
- Depends on nutrient status
- Important factor that influences insulin protein synthesis
- Decreases under lower glucose concentrations
- Increases under high glucose conditions
- In the absence of glucose, insulin mRNA levels in ß-cells decrease sharply
- Reversed by elevation of intracellular cAMP levels
- Rats fasted for 3 days have only 15–20% of the levels of pancreatic insulin mRNA [14]
- Post-transcriptional regulation
- Controls the modulation of immediate insulin synthesis [14]
- Transcriptional level
- Modulation of delayed insulin synthesis [14]
- Polypyrimidine tract binding proteins (PTBPs)
- Regulate mRNA translation
- Exon repression while mRNA is undergoing
- Splicing in nuclei
- Stabilization
- Ribosome recruitment in the cytosol [14]
- Upregulate translation
- Extending mRNA viability
- Stimulating the initiation of translation [14]
- Cytosolic PTBP1
- Binds to a CU-rich sequence in the 3' UTR of proinsulin
- Stabilizes proinsulin mRNA [14]
- Upregulates translation of several insulin granule proteins
- Binding to ICA512 mRNA decreases 3' UTR decay
- Deletion of the PTB binding site
- Reduced prohormone convertase 2 (PC2) translation [14]
- Binds Insulin and insulin granule protein mRNA
- Enables their gene-specific activation by glucose [14]
