Insulin biosynthesis and secretion Flashcards
Name the 2 processes concerning glucose metabolism that are fired by insulin.
How can glucose in the liver reach WAT?
- Glucose is metabolized in the liver through glycogenesis or fat biosynthesis (glucose -> pyruvate -> acetylCoA -> building block for FA lipogenesis).
- Glucose is distributed and uptaken by tissues (insulin mediated uptake), particularly the brain (neural function), adipose tissue (FA lipogenesis, glycerol synthesis, cell division), muscle (glycogen store replenishment), and pancreas (catabolized to make ATP and stimulate insulin release)
Glucose can reach WAT by directly being distributed as glucose in the bloodstream (under insulin’s effect) or as VLDL, where the lipids were synthesized from glucose (also under the effect of insulin).
What are hormonal responses to a blood glucose concentration of 5mM, 3.9-4.9 mM, 3.3-4.9 mM and 6.8-7.4 mM? To what states can all of these values be associated?
5mM = normal fasting blood glucose concentration
No significant secretion of insulin or glucagon
3.9-4.9mM = overnight fast
No insulin secretion, glucagon secretion
3.3-4.9mM = prolonged fasting
No insulin secretion, glucagon secretion
6.8-7.4mM = after a meal
Insulin secretion, no glucagon secretion.
What is the metabolic response for carbohydrate metabolism to insulin secretion? What is the net result?
Increased:
-glucose uptake by cells
-glycolysis
-glycogen synthesis
Decreased:
-glycogen catabolism
-gluconeogenesis
Net result: decrease blood glucose concentration, increase in glucose utilization and glycogen storage. Net glucose uptake, rather than release
What is the metabolic response for lipid metabolism to insulin secretion? What is the net result?
Increased TG synthesis
*Decreased endothelial cell lipoprotein lipase
Net result: decrease in plasma concentrations of glycerol and free FA. Net fat storage and decreased utilization of fat for energy.
What is the metabolic response for protein metabolism to insulin secretion? What is the net result?
Why does insulin promote protein and lipid biosynthesis and glycolysis?
Increased AA uptake by cells and protein synthesis
Decreased protein catabolism
Net result: decreased plasma concentration of AA. Net protein anabolism
Insulin stimulates protein biosynthesis, lipid biosynthesis and glycolysis, which are all pathways requrired for cell division. Insulin supports the body’s need for growth, repair and energy storage.
Compare and contrast the pancreas’ alpha and beta cells.
In the islets of Langerhans (clusters of endocrine cells), there are alpha and beta cells.
Beta cells:
-activated by high glucose
-release insulin
-stimulate glucose clearance and storage, fat biosynthesis, cell proliferation and growth.
Alpha cells:
-activated by low glucose
-release glucagon
-stimulate glucose mobilization, gluconeogenesis and fat degradation.
What are the exact metabolic responses of insulin in the brain, the muscle, the WAT and the liver?
What is the overall effect?
What is the liver’s role concerning glucose and fat availability to the body?
**Come back to this slide to include enzymes of metabolic pathways, effect of insulin on allosteric inhibitors, etc…
Brain:
increased glycolysis, ATP production and satiety signalling
Muscle:
Increased glycogenesis (glycogen synthase), glycolysis and GLUT
WAT:
Increased fat biosynthesis, TG biosynthesis, lipoprotein lipase (uptake of free FA in bloodstream), cell proliferation and growth, GLUT4.
Decreased lipolysis
Liver:
Increased glycogenesis, glycolysis, fat biosynthesis, lipoprotein synthesis (to put fat into storage), cell proliferation and growth.
Decreased glycogenolysis, gluconeogenesis
Overall effect: glucose utilization, storage and lipid storage.
Liver’s key role: modulating glucose and fat availability to the body (through glucose secretion by glycogenolysis and lipoprotein packaging).
Briefly describe the structure of insulin.
How and where does proper insulin protein folding occur?
Insulin is a small peptide, with alpha and beta chains. It contains cysteine, which binds alpha and beta chains to one another through disulfide bridges, contributing to the protein’s proper folding.
For proper folding of insulin (disulfide bridge formation), cysteine must be oxidized in the ER (very oxidizing region compared to the rest of the cell.)
What is the role of glucose in insulin gene expression and biosynthesis?
How does it occur?
What is the next step after proinsulin mRNA translation, and what is the emerging issue?
Glucose activates insulin gene expression as well as proinsulin mRNA translation.
- Glucose enters beta cell through GLUT2
- In the cell cytoplasm, glucose activates several transcription factors, that will activate insulin gene transcription in the cell nucleus.
- Proinsulin mRNA will be produced from gene transcription in the nucleus and emerges from the nucleus into the cell cytoplasm.
- Translational factors in the cell cytoplasm are also activated by glucose (elF4 and elF2 complex formation) and bind to the proinsulin mRNA translational machinery to start proinsulin mRNA translation.
The next step is proper folding of preproinsulin, which requried cysteine oxidation in the ER. How does proinsulin mRNA, with all the bulky machinery get from the cell cytoplasm to the ER?
What is cotranslation transport?
What is proinsulin V.S. preproinsulin V.S. insulin?
What is a signal peptide?
Cotranslational transport: process by which newly synthesized proteins are targeted to specific cellular locations during their translation on ribosomes.
Preproinsulin: incomplete peptide, not properly folded yet, with reduced cysteines (hasn’t passed by the ER yet, still in cytoplasm).
Proinsulin: incomplete peptide, properly folded, with oxidized cysteines (passed by the ER)
Insulin: completed, properly folded peptide, ready for secretion.
Signal peptide: short sequence of amino acids located at the N-terminus (the beginning) of a nascent protein that directs the protein to specific locations within the cell, such as the endoplasmic reticulum (ER).
How does insulin biosynthesis occur through cotranslational transport?
After activation of transcription and translational factors by glucose in beta cells, and the beginning of proinsulin mRNA translation, preproinsulin will need to enter ER to become proinsulin
- During translation in cytoplasm, ribosome will react to signal peptide in nascent peptide sequence (located at N terminus) and come a to stop in translation.
- Signal peptide will call for recruitment of protein called SRP (signal recognition particle).
- SRP binds to N terminus of nascent peptide and escorts translational complex to surface of ER, where it will bind to SRP receptor.
- Bound SRP to its receptor activates opening up of Sec61 channel, through which ribosome will continue translation.
- Peptide directly translated into ER lumen, where cysteines are oxidized, allowing formation of disulfide bridges.
- Signal peptidase (protein in ER lumen) removed signal peptide from oxidized proinsulin, releasing mRNA and ribosome back into cytoplasm
- proinsulin stored in vesicles (in ER) until needed for secretion.
How is insulin secretion from beta cells mediated?
- High glucose after a meal is sensed by pancreatic beat cells, facilitated by transport of glucose through GLUT2 into cytoplasm.
- Glucose metabolized (glycolysis, pyruvate, mitochondrial metabolism) = increased ATP production by mitochondria.
- Increased ATP serves as signalling molecule, inhibiting ATP-gated potassium channel. (K+ stops exiting cell, making the membrane potential of the cell more positive AKA depolarized).
- depolarized beta cell will activate voltage gated Ca2+ channels, allowing import of calcium in beta cell and increasing calcium concentration in the cytoplasm.
- Ca2+ triggers fusion of insulin-enriched vesicles with the plasma membrane (exocytosis), releasing insulin into the bloodstream.
