PTB1b and its effects in Metabolism and Body Weight Regulation
Date: November 3rd, 2005
Title: “The Role of PTB1b in Metabolism and Body Weight Regulation”
Speaker: Börk Balkan, Ph.D., The Institute for Diabetes Discovery
Protein Tyrosine Phosphatase 1b (PTP1b) and insulin sensitivity
Dr. Börk Balkan presented data to support a role for protein tyrosine phosphatase 1b (PTP1b) inhibition in improving insulin sensitivity in Type II diabetics.
The predominant metabolic defects in type II diabetes can be characterized as early insulin resistance, and a decline and ultimate failure of beta cell function. In addition, both fasting and post-meal glucose levels in the blood are high. Compared to controls, type II diabetics have reduced insulin-stimulated glucose disposal in all insulin sensitive tissues, particularly skeletal muscle, liver, and fat.
Early interventions, through diet and exercise, which improve insulin sensitivity before patients completely lose beta cell function, are an important therapy because they allow many prediabetics to delay or prevent the progression to overt diabetes. Unfortunately, it has remained difficult for many patients to achieve and sustain weight loss and to increase their levels of physical exercise. This increases the need for pharmacological intervention.
PTP1b dephosphorylates the insulin receptor and downstream signaling elements, such as insulin receptor substrates (IRS). It thereby acts as a negative regulator of insulin action. Consistent with this view, small molecular inhibitors of PTP1b increase the activation of the insulin signaling pathway in rodents. When studied in the hyperinsulinemic, euglycemic clamp, these inhibitors augment insulin’s ability to stimulate glucose uptake and ameliorate the insulin resistance in obesity.
The following data support a potential therapeutic role for PTP1b inhibition in management of insulin resistance. PTP1b inhibition enhances insulin signaling (insulin receptor and AKT phosphorylation) in insulin sensitive tissues (muscle, liver, and epidydimal fat) when compared to tissues treated with vehicle or pioglitazone.
Initial experiments testing the effects of PTP1b inhibition in animal models further suggest this as a pharmacological agent to improve insulin sensitivity. In a 40 day study using 10 ob/ob mice, PTP1b inhibition reduced plasma insulin levels and improved oral glucose tolerance relative to control animals.
In addition, PTP1b inhibition lowered HBA1c levels, reduced plasma triglycerides, and reduced hepatic steatosis, compared to control animals. However, administration of a PTP1b inhibitor did not lower body weights or alter average food consumption, when compared to controls.
This is in contrast to the effects of the PTP1b inhibitor in animals on a high fat diet. Animals in this study were either maintained on chow or switched to a high fat diet and treated daily with PTP1b inhibitor or vehicle for 9 weeks. Results showed that animals switched to the high-fat diet that received a PTP1b inhibitor gained less weight over time than animals maintained on high-fat diet plus vehicle or pioglitazone.
The treatment group that received the PTP1b inhibitor had a weight gain pattern similar to animals kept on chow. PTP1b inhibition appeared to prevent adiposity and insulin resistance in high-fat fed DIO animals.
The authors of this study attributed the anti-obesity action of PTP1b inhibition to enhanced leptin action because ob/ob mice are leptin deficient whereas the animals of high-fat diet have a functioning albeit resistant leptin system. Indeed, PTP1b has been shown to reduce leptin signaling and administration of a PTP1b inhibitor enhances leptin signaling.
To substantiate whether PTP1b inhibition augments leptin’s ability to reduce body weight, mice were dosed with either PTP1b inhibitor or vehicle, and after one week, all animals received daily low-dose leptin injections (b.i.d.). The results revealed that administration of PTP1b augmented leptin’s effect on body weight, but the drop in epididymal fat pad weight (5%) did not reach statistical significance.
In summary, PTP1b inhibition might have a potentially therapeutic role in the management of insulin resistance in type II diabetics, provided that trials in humans reproduce what has been seen in animals.
The studies presented in this talk demonstrate that PTP1b inhibition improves insulin action and glycemic control, improves leptin signaling, and increases fat oxidation, leading to a gradual decrease in adiposity. At present, the potential effects on food intake are inconclusive.
Q. What do you mean by “relative function” on your graph?
A. There is a double failure of beta-cell function in the progression of diabetes. The initial defect one of kinetics, early in a meal too little insulin released leading to excessive glucose excursions (impaired glucose tolerance) and elevated insulin levels in the later prandial period. In the later stages of type 2 diabetes there is an absolute deficiency in insulin leading to basal hyperglycemia.
Q. Is there any evidence that a therapeutic intervention at lower levels of insulin resistance can help prevent diabetes from occurring?
A. Yes, this is definitely true, that’s why it’s important to intervene early.
Q. What tissues does PTB1b act on (to dephosphorylate)?
A. Data suggest that it may be active in all insulin sensitive tissues.
Q. Why is the magnitude of the increase in insulin signaling higher in the liver than in the muscle (in reference to tissues treated with a PTB1b inhibitor)?
A. My thought is that the drugs hit the liver first.
Q. Can you explain the hyperinsulinemic euglycemic clamp?
A. This refers to a system where glucose blood levels are maintained at a constant level (“clamped”), while insulin is infused into the system for a period of time, typically 2 hours. This system allows for the calculation of insulin’s effect on glucose metabolism.
Q. How does PTB1B work?
A. It associates with the phosphorylated tyrosine residues of the substrate molecules and de-phosphorylates them.
Q. Was there much variability in animals body weight gain when administered the PTB1b inhibitor?
A. We found them to be quite uniform in body weight gain. However, other groups, particularly Barry Levin, found that they could predict how much weight animals gained in DIO (diet induced obesity) animals simply by looking at leptin levels.
Q. Are the intake measures you have oral intake?
A. Yes, animals were orally gavaged.
Q. Are leptin levels measured before body weights start changing?
A. No, after.
Q. Is this a downstream effect due to change in insulin activity or a direct effect on fat cells (referring to PTB1b inhibition and the effect to reduce enzymes for lipogenesis)?
A. It is probably due to an increase in leptin action.
Q. Couldn’t it also be due to the fact that the animals were losing weight?
A. Yes, we cannot account for that.
Q. Do you have food intake data for your animals?
Q. Are you willing to call this drug an insulin sensitizer?
A. Yes, definitely, I think it has the potential to be that.
Q. Would you consider this a treatment for type II diabetes, where animals/humans are both insulin and leptin resistant?
A. Yes, because it improves insulin signaling.
Q. Does this drug cross over into the brain?
A. I will get to that in a moment.
Q. How big is the compound (PTB1b inhibitor)?
A. Fairly small-500 bp.
Q. Is the molecule fat soluble?
Q. Are you sure the weight loss in the epidydimal fat pad you see isn’t just due to water loss?
A. We are considering that, yes. However, given that this is just one fat pad, if loss across the entire body was comparable, we would be seeing quite dramatic weight losses (whole body).
Q. If you took animals off the drug, would you expect them to rebound and gain weight back?
A. I don’t know, but we are planning to do that study. My hypothesis is that obese animals will not gain this weight back, but that lean animals will.
Q. It would seem to be more important to target individuals who are prone to diabetes and obesity, instead of using this therapy as treatment?
A. Yes, but there might be safety or toxicity issues that we cannot yet answer.
Q. Where is PTB1b found?
A. In all insulin sensitive tissues.
Q. Is it assumed that insulin receptors in the brain also become resistant during diabetes?
A. There are a number of studies that have shown this.
Q. What is the development status of this drug?
A. It’s still pretty early.
Q. What about the effect of this drug on homocysteine?
A. That has not been measured yet.