Insulin, Receptor Tyrosine Kinases

Question 0

What are the four different types of cells found in the pancreas?

Answer 0

1. Alpha cells: secrete glucagon
2. Beta cells: secrete insulin.
3. Delta cells: secrete somatostatin
4. PP cells: secrete pancreatic polypeptide.

Question 1

Pancreas

Answer 1

Pancreas, together with the gallbladder and the liver, are involved in processing of nutrients. Pancreas is both
an exocrine gland and an endocrine gland, which is unusual. Acinar cells of the pancreas are exocrine and are involved in the secretion of digestive enzyme things like chymotrypsin, amylase, and lipase are secreted by the acinar cells. Embedded in the acinar cell regions are islets which contain the endocrine tissue whichs excrete and synthesis peptide hormones insulin and glucagon. Some also secrete somatostatin and pancreatic polypeptide.

Question 2

What are the 3 major energy sources of the body?

Answer 2

Glycogen, triglycerides, protein.

Question 3

Glycogen

Answer 3

Polymer of glucose that has alpha 1,4 and alpha 1,6 linkages. Storage supply of carbohydrates (glucose) that can easily be broken down to obtain glucose.

Question 4

Triglycerides

Answer 4

Energy storage molecule composed of glycerol head and 3 fatty acid tails.

Question 5

What is the relative distribution of glycogen in the body. Does the muscle share glycogen? Why?

Answer 5

about 80% of the glycogen is in the muscle. Muscle wont share its glycogen with anyone else. So glycogen broken down in the muscle
is only used for the muscle and no one else. This is because the glycogen is not broken down in a way that could cross the plasma membrane.
Glycogen is broken down by phosphorylase into glucose-1-phosphate and further enzymatically changed to glucose-6-phosphate. In other cells, such as the liver cells, it can be changed to glucose. Once its in the form of just glucose it can be transported across the plasma membrane.

Question 6

Where does blood glucose come from?

Answer 6

The muscle has most of the glycogen but can’t share it. The rest of the glycogen has to come from the liver where it can be broken down into glucose and transported to the rest of the body. Glycogen in the liver can be broken down very fast during exercise.
During fasting the glycogen is broken down slower and able to be
replenished by liver glucose.

Question 7

What is gluconeogenesis?

Answer 7

The process of making glucose from non-carb sources such as fat and protein.

Question 8

What do acinar cells do?

Answer 8

Secrete digestive enzymes.

Question 9

What do alpha cells do?

Answer 9

Secrete glucagon in response to low blood glucose (hypoglycemia). Glucagon stimulates glycogenolysis. Also stimulates gluconeogenesis. Net result is an increase in blood glucose. Alpha cells mainly stimulate catabolic reactions by secreting glucagon. Starving state stimulates glucagon release.

Question 10

What do beta cells secrete?

Answer 10

Insulin in response to elevated blood glucose (hyperglycemia). Insulin inhibits glycogenolysis and gluconeogenesis. This leads to decreased blood glucose levels. Fed state stimulates insulin release.

Question 11

Outline the process of hormone release and effects after a carb rich meal.

Answer 11

In response to increased blood glucose levels, insulin is secreted so it can start metabolizing blood glucose and making glycogen, fats, and proteins. After insulin secretion, blood glucose levels start to fall. Glucagon levels go down after being fed to prevent the degradation of fats and proteins. Happens over a couple of hours.

Question 12

Outline the process of hormone release and effects during fasting.

Answer 12

In response to fall of blood glucose, alpha cells secrete glucagon and insulin secretion by beta cells falls. This stabilizes the fall of blood glucose. Glucogenolysis increases to raise blood glucose levels. Happens over a period of days.

Question 13

What do insulin and glucagon work towards?

Answer 13

The rise and fall of both hormones serves to stabilize the levels of glucose to normal levels.

Question 14

What happens to glucose in the blood that is taken up into the liver after a meal.

Answer 14

Glucose is converted to glucose-6-phosphate which can be used as fuel or stored as glycogen as well as used in the synthesis of fatty acids which can then be moved to adipose tissue.

Question 15

What happens in the liver when glucagon is released?

Answer 15

Glycogen is broken down to glucose-6-phosphate which is converted to glucose and sent into the blood. Fatty acids from adipose tissue are brought to the liver and used as fuel. So glucagon stimulates catabolic reactions, stimulating the breakdown of glycogen, protein, and triglycerides for the production of glucose. Epinephrine and norepinephrine also stimulate these effects.

Question 16

Even though there isn’t much therapeutic use for glucagon, what is one thing it can be used for?

Answer 16

There’s not much therapeutic use of glucagon, but it is used in some cases of hypoglycemia
due to too much insulin. So for instance if a diabetic individual takes too much insulin
glucagon can help to correct. Glucagon works through the Gs pathway and protein
kinase A. PKA can then phosphorylate 2 important substrates: 1. Phosphorylase kinase,
which is involved in breaking down glycogen.So phosphorylation increases its activity. Phosphorylase is then going to breakdown glycogen to glucose-1-phosphate. 2. Phosphorylation
of glycogen synthase, which is going to inhibit its activity and so glycogen synthesis is inhibited. Basically the same pathway that epinephrine and norepinephrine use to raise blood
glucose levels.

Question 17

Describe the structure of insulin and the conversion to insulin from the preproinsulin form.

Answer 17

Insulin is a small protein. MW is 6 kDa. Has two separate chains on it: A chain and B chain. The chains are connected by disulfide groups. There is a signal sequence on the N-terminus that gets cleaved off in the pre stage. The C chain gets cleaved off in the pro stage,so there’s 2 cleavages that take place. Insulin is stored in vessicles inside cells and is often complexed with zinc to yield multimers of insulin which makes it more stable. Many of the therapeutically prepared solutions of insulin are also complexed with zinc to make it longer lasting and more stable. Insulin is released from the cells by calcium mediated vessicle secretion just like neurotransmiter release. But the cells that they are secreted from are not neurons.

Question 18

Is insulin stored for a long time? Is it free in the plasma?

Answer 18

About 20% of insulin is secreted daily, so it had to be resynthesized and is not stored for a long time. It’s a protein, so it doesn’t require a binding protein and is free in the plasma when traveling through the blood. Only circulates for about 10 min.

Question 19

How is insulin released from the beta cells?

Answer 19

Pancreatic beta cells are electrically excitable cells (like neurons, muscle). 1. Blood glucose rises. 2. Glucose is taken up by cell via a GLUT2 glucose transporter. 3. Intracellular ATP/ADP ratio increases as ATP increases and ADP decreases. 4. ATP dependent potassium channels close. 5. Membrane depolarizes, giving increased action potentials. 6. Voltage gated calcium channels open. 7. Calcium causes insulin secretion. 8. Insulin lowers blood glucose.

Question 20

Why does closing the ATP-dependent potassium channels lead to depolarization?

Answer 20

The channels are inhibited by high levels of ATP or low levels of ADP. The channel would normally hold te membrane potential stable and negative by allowing potassium ions to flow out of the cell. So by inhibiting the stabilization of the membrane potential, it depolarizes the membrane and action potentials can fire. This activates calcium voltage gated channels letting calcium come in. Calcium is involved in vesicle secretion and allows for insulin to be secreted.

Question 21

What is different about the insulin receptor compared to other receptor tyrosine kinases?

Answer 21

Insulin receptor looks different than the
other receptors. The other receptors are single pass transmembrane
proteins with an extracellular domain
that binds the ligand and an intracellular catalytic domain that has
tyrosine kinase activity. Insulin looks
like its already been dimerized (the other receptors work through dimerization as well but start out as monomers until they bind ligand). Since its pre-dimerized, it doesnt require dimerization after ligand
binding. Insulin receptor has a couple of different subunits in it while the other receptors are singly polypeptide chains. When the insulin receptor is made in cells, it starts off as one
long polypeptide and is proteolytically cleaved so that theres the extracellualr subunit and the intracellular subunit. So developmentally, the receptor starts out as a single pass TM protein
and is then cleaved into two parts followed by dimerization by disulfide bridges. It’s an alpha2beta2 heterodimer.

Question 22

What are receptor tyrosine kinases involved in?

Answer 22

Cell proliferation, growth, differentiation or migration. They work by dimerization and then cross-auto phosphorylation of the catalytic domains which phosphorylate tyrosine residues on the cytoplasmic region.

Question 23

How are typical RTKs activated?

Answer 23

Signal molecule binds to undimerized receptor. Dimerization occurs, transautophosphorylation (cross-phosphorylation) of receptor occurs on tyrosine residues on the cytoplasmic C terminus. Phosphorylated tyrosines serve as docking sites to activate other proteins.

Question 24

How is insulin receptor activated?

Answer 24

Already a heterodimer (alpha2beta2) prior to signal binding. Upon binding, receptor phsophorylates itself and also a special docking protein IRS-1 (insulin receptor substrate 1). IRS-1 phosphotyrosines also serve as docking sites to activate other proteins as do the phsophorylates tyrosines on the RTK.

Question 25

What are SH2 domains?

Answer 25

Domains in proteins that recognize particular phosphorylated tyrosine residues on the RTK cytoplasmic side.

Question 26

What are some effects of activation of the insulin receptor?

Answer 26

Increased glucose transport into cells, ion transport, amino acid uptake, lipid metabolism, phosphorylation/dephosphorylation cascades leading to protein synthesis and degradation as well as glycogen synthesis and metabolism along with nuclear events.

Question 27

What does the activated PDGF receptor do?

Answer 27

Phosphorylated tyrosines on the receptor serve as a binding site for SH2 domains on downstream target proteins and activate these proteins. These include PI 3-kinase, Guanine Nucleotide Exchange Factor (GEF), and Phospholipase C-gamma.

Question 28

What are the 3 primary downstream signaling pathways for RTKs?

Answer 28

1. PI 3-kinase: catalyzes PIP2+ATP=PIP3. PIP3 can then serve as a docking site to recruit downstream effectors. Activates Akt (protein kinase B). Causes insertion of membrane glucose transporter (GLU4) into the plasma membrane to allow for glucose to be taken up in peripheral tissues. Uptake of glucose via GLUT4 is a rate limiting step in muscle and adipose tissue.
2. Ras/MAP kinase: adapter->Ras GEF->Ras->MAPKKK(Raf)->MAPKK (Mek)->MAPK (Erk)-> activates transcription, protein synthesis, and cell proliferation. So the adapter and Ras GEF are both recruited and activated allowing for the exchange of GDP for GTP leading to Ras activation.
3. Phospholipase C gamma: PIP2->DAG+IP3 (not a major pathway for insulin).

Question 29

Activation of PI-3K

Answer 29

Auto phosphorylation of tyrosines on RTKs recruits and activates PI 3-kinase which binds to the phosphorylated tyrosines via it’s SH2 domain. PI 3-kinase can then phosphorylate PI(4,5)-P2 to PI(3,4,5)P3 which can then interact with PKB leading to activation of Akt (PKB). Coupling protein activation to membrane recruitment is very important to signal transduction.

Question 30

Describe how activation of Akt (PKB) works in more detail.

Answer 30

In in stimulated cells, both Alt (PKB) and PKD1 are cytosolic and inactive. PI 3-kinase is activated by binding to phosphorylated tyrosines on RTKs via it’s SH2 domain. Activation of PI 3-kinase causes PIP2->PIP3. PIP3 receuits Akt and PDK1 to the membrane via their PH domains. PDK1 then phosphorylates and activates Akt. Akt then can dissociate from the membrane and phosphorylate downstream substrates. So a docking site brings PDK1 and Akt together. Akt stimulates insertion of GLUT4 receptors into plasma membrane.

Question 31

Explain how coupling effector proteins to the membrane occurs in activation of Ras in the MAP kinase pathway.

Answer 31

The small G protein Ras is membrane bound due to covalent lipid modifications. Activation of Ras requires a GEF called Sos to exchange GDP to GTP. In unstimulated cells the Ras activator Sos is associated with the adaptor Grb2 in the cytosol. Activation of a RTK and auto phosphorylation creates binding sites for the SH2 domain of Grb2. Grb2 and Sos are recruited to the membrane where Sos activates Ras.

Question 32

What is the normal refulation of plasma glucose in response to insulin?

Answer 32

Hepatic insulin response cuts down glucose production and allows glucose to enter the blood to keep normal plasma glucose levels. Muscle/fat insulin response involves taking up glucose and metabolizing it as it enters the peripheral tissues.

Question 33

What’s the action of insulin in muscle and fat cells?

Answer 33

Mobilization of GLUT4 to the cell surface. Insulin binds to insulin receptors, intracellular cascade ensues, leading to GLUT4 vesicle mobilization to the plasma membrane and insertion of the transporters. Presence of glucose transporter is key to glucose utilization in the periphery.

Question 34

MAP kinase pathway in brief

Answer 34

Insulin binding to alpha subunit increases tyrosine kinase activity of beta subunit. Auto phosphorylation of beta subunit. Phosphorylation of IRS-1. Activation of MAP kinase cascade. Transcriptional regulation. Increased GLUT4 expression and insertion into membrane.