Normal metabolism: After carbohydrate ingestion, plasma glucose increases which stimulates insulin release from the beta cells in the pancreas. Insulin then signals for glucose uptake in the muscle, and suppression of liver glucose production.

Type 2 diabete: Insulin can neither induce glucose uptake by muscle, nor inhibit glucose production by the liver. This makes sense given that metformin is a common initial drug prescribed for prediabetes that will inhibit the liver’s ability to make glucose thereby reducing fasting blood sugar levels.

The therapies we use to treat type 2 diabetes, treat the symptom hyperglycemia(insulin injections, metformin, TZDs, SGLT2 inhibitors) not the root cause which Dr. Shulman believes to be the mechanism governing insulin resistance.

Under normal conditions, glucose in the muscle cell can undergo:

• Oxidation
• Non-oxidative glycolysis (lactate)
• Or it can be stored as glycogen 

Dr. Shulman and his team used NMR to measure glucose metabolism in a non-invasive manner. In the control group the vast majority of glucose consumed was converted into muscle glycogen. For patients with T2D muscle glycogen synthesis was impaired. 

Using NMR they were able to see the biochemical block that prevents glucose entry into the cell is rate controlled at GLUT4 glucose transport into the cell membrane because glucose-6-phosphate was reduced. This reduction in glucose entry into the cell is responsible for the glycogen synthesis. 

Used the HNMR method to measure intramyocellular fat to see if the amount of fat inside the cell was responsible for insulin resistance. They found that there is a positive correlation with muscle insulin resistance and intramyocellular lipid content. Fat inside the muscle cell is the best predictor of muscle insulin resistance across all populations.

How fat causes insulin resistance – rise in intracellular fatty acid concentration leads to decrease in intracellular glucose concentration. Successful blockage of glucose entry into the cell is achieved when the increased intracellular fatty acid concentrations promote insulin resistance by inhibiting insertion of GLUT4 into the cell membrane. Studies were conducted by increasing plasma fatty acid concentrations and seeing reduced glucose-6-phosphate concentrations inside cells, which means there is a reduction of glucose being brought into the cell. 

Normal insulin activity: An insulin molecule will bind the insulin receptor which will dimerize and autophosphorylate the receptor and turn into a kinase that will phosphorylate downstream enzymes.

Emphasis is placed on the mechanism of the phosphorylation of IRS1 which leads to activation of PI3 kinase which is required for GLUT4 insertion. When this mechanism is inhibited it results in decreased insulin-stimulated glucose transport into the cell. 

Dr. Shulman believed that a fatty acid intermediate interferes with insulin signaling of GLUT4 insertion into the cellular membrane OR it might lead to inhibition of insulin signaling. 

In order to figure out this problem an insulin infusion was administered to patients and a subsequent 4 fold increase in PI3K activity was noted, but when lipids were added to the insulin infusion there was no increase in PI3K activity from basal levels. The presence of lipids completely nullified the effect of insulin causing a decrease in glucose uptake in the muscles of the healthy volunteers.

There are many hypotheses about what causes insulin resistance. There is a strong case that DAG is the main player in insulin resistance. DAG will activate PKC-theta and will inhibit IRS1 which in turn inhibits PI3K from initiating glucose transport. It is believed that ceramides and triglycerides have been excluded from this process of PKC-theta inhibition. 

The possible causes listed were:

• ER stress
• DAGs
• BCAAs
• Inflammation
• Ceramides
• RBP4

Intramyocellular lipid accumulation is the first sign of insulin resistance in children of parents with type 2 diabetes.

Insulin resistance plays a part in the following conditions:

• Type 2 diabetes
• Hypertension
• Impaired glucose tolerance
• Decreased HDL
• Polycystic Ovarian disease
• Hyperuricemia
• Inflammation
• Decrease fibrinolytic activity(increased fibrosis)
• Hypertriglyceridemia
• Atherosclerosis

Muscle insulin resistance is the precursor for atherogenic dyslipidemia and non-alcoholic fatty liver disease(NAFLD) because ingested carbohydrates will ultimately be stored as fat, not glycogen.

A study was conducted looking at the insulin sensitivity in young, healthy, lean, non-smoking, SEDENTARY participants. The top and bottom quartiles were noted for being insulin sensitive and insulin resistant respectively. The question was asked, “How can the sedentary participants be sensitive to insulin when we know that exercise is so important for keeping sensitivity up?” 

The participants were given a carbohydrate heavy meal and the glucose and insulin levels were measured afterward. The two groups were found to have nearly identical plasma glucose measurements, but the insulin resistance group had approximately double the insulin response that the insulin sensitive group demonstrated. This phenomenon is otherwise known as hyperinsulinemia(high serum insulin level).

Then they saw that the insulin resistant group will not store carbohydrate in muscle because muscle glycogen synthesis is decreased. Instead they will store the glucose in the liver as hepatic glycogen synthesis is normal. 

Upon further investigation they noted that fat synthesis in the liver is increased by 1.4 times, and de novo lipogenesis of glucose to fat increases 2.2 times in the insulin resistant group.

The hepatic de novo lipogenesis is associated with an 80% increase in plasma triglycerides and a 20% decrease in plasma HDL.

Insulin sensitive participants store ingested carbohydrates as glycogen in liver.and muscle. In the bottom quartile the result of ingesting carbohydrates will result in increase in de novo lipogenesis resulting in increased plasma triglycerides, and decreased HDL and premature cardiovascular disease. This lipodystrophy in the liver will also increase the likelihood of developing NAFLD. NAFLD could potentially lead to NASH, ESLD, and hepatocarcinoma

Dr. Shulman presented studies that showed exercise can overcome the defective muscle insulin transport mechanism.  Exercise can overcome the defective PI3K mechanism to correct the decrease in muscle glycogen synthesis. There is an insulin independent pathway that is stimulated by muscle contraction that will reduce the plasma glucose levels by inserting GLUT4 into the surface of the  muscle cell. It was noted that a single 45 minute bout of leg exercise can lead to 2 fold increase in muscle glycogen synthesis which lead to decreased liver de novo lipogenesis and dyslipidemia

Dr. Shulman’s thoughts on how fat in liver might cause hepatic insulin resistance the normal pathway: 

IRS2 will phosphorylate PI3K which will activate AKT2 and stimulate glycogen synthesis and inhibit gluconeogenesis 

The insulin resistant pathway: Is believed that DAG will stimulate PKC-epsilon, which will inhibit the IRS2 complex, which downregulates PI3K. This PKC-epsilon role was confirmed through a rat study. 

He talks about a threonine phosphorylation in the IRS complex that binds PKC-epsilon in the liver cells and discusses how it has been conserved throughout time and across many different species. If this threonine is phosphorylated by PKC-epsilon, that will induce insulin resistance. 

DAG induces PKC-epsilon which binds and inhibits the IRS complex to inhibit glycogen synthesis and stimulate gluconeogenesis.  

Dr. Shulman talked about mice that have no fat cells will store fat in tissues where fat does not belong, specifically liver and muscle cells. This lipodystrophy inside the hepatic and muscle cells is ultimately what causes insulin resistance for those tissues. So basically the overflow of fat from the fat cells causes diabetes. With this in mind, it is easy to understand how Thiazolidinediones come into play as a therapeutic. The thiazolidinediones work to increase the amount of fat that adipose cells can hold by acting as a PPAR-gamma receptor agonist. This will help reduce the lipodystrophy and increase the insulin resistance of liver and muscle cells. Transplanting fat into the fatless mice normalizes the insulin resistance.  

Discusses a young woman with leptin deficiency, insulin resistance, hyperlipidemia, and symptomatic lipodystrophy specifically in her liver. Leptin replacement helped normalize her glucose metabolism. This is an indication that insulin resistance and obesity are not necessarily related. Lipodystrophic patients have increased muscle insulin resistance and hepatic insulin resistance. These patients were treated with leptin and it normalized the glucose handling in the patients. This leptin treatment was related to decreased triglycerides in muscle and hepatic cells.

Ectopic Lipid promotes muscle and liver insulin resistance. Leptin lead to the reversal of insulin resistance and reduces the liver and muscle fat content.  Ectopic fat had a similar role in insulin resistance in T2D.  

Can glucose resistance be reduced by weight loss? Turns out the patients only had to lose 10kg (or 10% of their body weight) to normalize in fasting serum glucose and glucose metabolism. The fat inside the patients with fatty livers can easily decrease that concentration with exercise and weight loss.  

The optimal location to store fat is in adipose tissue, not liver or muscle.  

In t2d pts of tzds, better diabetes control, increase in body weight.

MItochondria – activity decreases in oxidation can predispose patients to ectopic fat deposits and insulin resistance – decreased mitochondrial functioning naturally occurs in aging.

Insulin regulates hepatic gluconeogenesis:

How it goes wrong in diabetes insulin resistance occurs prior to the dev of hyperglycemia 

Transition from insulin resistance normal glycemic state to fasting hyperglycemia 

Increases in hepatic gluconeogenesis is responsible for fasting hyperglycemia in T2D

OLD MODEL Insulin binds to its receptor –  increases glycogen synthesis and inhibits glycogen breakdown. Phosphorylation of FOXO1 and decreases protein expression of gluconeogenic enzymes.

New finding — The expression of gluconeogenic protein expression doesn’t matter because insulin will stop it regardless of the expression occurring. 

Protein expression in tissue samples between non-diabetes and diabetes individuals show no relationship of expression in the gluconeogenic enzyme concentrations. 

Acetyl-CoA is an allosteric activator of pyruvate carboxylase regulation, increasing hepatic gluconeogenesis. The more the acetyl-CoA, the more the hepatic gluconeogenesis. 

Dr. Shulman’s working model for the direct and indirect hepatic insulin mechanisms:

Indirect, insulin reduces lipolysis – long-term fast – reduction in acetyl-CoA – reduction in gluconeogenesis

Direct, insulin increase glycogen synthesis – short-term fast – decreased glucose production

In patients with T2D, adipose tissue will increase  IL6 and TNF-alpha, which will increase lipolysis and increase intracellular DAG concentration. This increase in DAG will increase PKC-epsilon and decrease serine phosphorylation on the IRS 2 complex, and increase glycogenolysis. Similarly, this increase in lipolysis will increase in acetyl-CoA concentrations and will increase the gluconeogenesis and increase glucose production. So when you get an increase in lipolysis you’ll see an increase in glycogenolysis and gluconeogenesis.  

If we can find a way to reduce DAG and acetyl-CoA, we can find a way to reverse diabetes.

So we need to boost mitochondrial fat oxidation to eliminate DAG and acetyl-CoA from the hepatocytes 

Increase rates of mitochondrial fat oxidation by promoting mitochondrial uncoupling has been shown(in animal models) to safely: 

• increase fat oxidation
• increase hepatic insulin sensitivity 
• Decrease gluconeogenesis 
• Lowers vldl production 
• Decrease plasma triglycerides
• Increases muscle insulin sensitivity