Major study reveals how exercise improves insulin sensitivity and action

Exercise is well known for increasing how insulin acts on skeletal muscle, thereby improving its uptake of glucose. Thus, physical activity protects against the development of insulin resistance and ultimately type 2 diabetes.

The mechanisms underlying both insulin resistance and the positive effects of exercise are still largely unknown, but that is now changing because of a new technology that has enabled researchers to determine the fundamental molecular changes in the signalling pathways of skeletal muscle.

According to a researcher behind the study, the results identify many potential targets for new drugs.

“The major pharmaceutical companies have long focused on the pancreas and liver when developing drugs for people with insulin resistance and type 2 diabetes, but they have overlooked the skeletal muscles. The muscles carry out 70–80% of total glucose uptake and have a key role in metabolising glucose. In this study, we showed locations at the molecular level in skeletal muscle in which drugs can potentially improve insulin sensitivity, thereby reducing the risk of developing type 2 diabetes. The interesting thing will be if we can treat people and prevent the development of insulin resistance and type 2 diabetes before the insulin-producing beta cells in the pancreas fail,” explains Jørgen Wojtaszewski, Professor, August Krogh Section for Human Physiology and August Krogh Section for Molecular Physiology, Department of Nutrition, Exercise and Sports, University of Copenhagen, Denmark.

The research has been published in Cell Metabolism.

The study was carried out by Jørgen Wojtaszewski with colleagues from the University of Copenhagen and the University of Sydney and Pfizer Global Research and Development.

Skeletal muscle absorbs glucose from the blood

Insulin resistance is a precursor of type 2 diabetes that develops when the skeletal muscle (and other tissues) no longer responds to the signals from insulin to absorb more glucose from the blood. To compensate for the reduced insulin action, the beta cells in the pancreas produce more and more insulin.

For many this is unsustainable in the long term; at some point the pancreas loses the ability to respond adequately and type 2 diabetes develops.

Exercise is well known for increasing insulin sensitivity in skeletal muscle, but the precise mechanisms behind it have been unknown until now.

In this study, Jørgen Wojtaszewski and colleagues aimed to determine how exercise affects insulin sensitivity and the associated signalling pathways.

“Studying the mechanisms behind the increase in insulin action after exercise may enable us to identify new molecular targets for treatment by discovering what causes insulin resistance and what malfunctions in the cells of the skeletal muscles. This could be described as developing a pill to mimic the effects of exercise,” says Jørgen Wojtaszewski.

Investigated how exercise affects insulin action

The researchers recruited 20 healthy volunteers; half had normal insulin sensitivity and the other half insulin resistance. On experimental days, the participants were asked to perform one-legged knee-extensor exercise while the other leg rested.

The researchers then took blood samples and muscle biopsies from both legs to analyse the difference in insulin sensitivity between the legs but also between the groups of participants.

Jørgen Wojtaszewski explains that the muscles remain sensitised to insulin for many hours after a single training session.

This means that exercising in the morning (such as cycling to work) will enable insulin sensitivity and glucose uptake to continue to be elevated during the rest of the day.

The researchers used a very special analysis that involved examining protein phosphorylation in the muscle biopsies. Phosphorylation tells something about protein action or activity.

Phosphorylation can either increase or decrease protein activity, and since phosphorylation can be added to or removed from proteins very rapidly, this enables cells to adapt to changing conditions.

By examining differences in protein phosphorylation between the participants, researchers can identify signalling pathways and proteins that affect insulin action in skeletal muscle.

Jørgen Wojtaszewski and colleagues have described this technology, which they call personalised phosphoproteomics.

Participants vary greatly

The advanced experimental set-up revealed great differences in participants’ insulin sensitivity – not only between groups but also within the groups, even though the participants were matched across many parameters.

One difference was that the muscles of some participants responded up to five times as strongly to insulin stimulation as others, and this applied to both the exercised and the non-exercised leg.

In addition, the study found great diversity in how a single exercise session affected insulin sensitivity in the exercised leg, with some participants doubling insulin sensitivity and others only increasing it by about 10%. This effect was present whether the participants were insulin resistant or not.

“This is the first study to show that insulin action improves when people with insulin resistance are physically active. This has always been assumed but has not been shown in human trials until now. The large range of both insulin action and effect of physical activity among the participants is remarkable, precisely because they were well matched within the two groups (age, sex, health status and fitness). This variation has previously presented challenges in this type of data analysis, such as identifying clear differences between groups, but we have succeeded in turning it to a great advantage for the subsequent analysis of molecular causation,” says Jørgen Wojtaszewski.

Thousands of proteins altered

The research also revealed several interesting facts based on the researchers identifying and quantifying more than 12,000 phosphorylations in the muscle biopsies. About 750 differed between the insulin-sensitive and insulin-resistant participants. Insulin regulated about 1,800 phosphorylations.

The researchers linked many phosphorylations with the insulin action of individual participants, and exercise likewise changed the phosphorylation status of many proteins.

Since the researchers had biopsies from both legs, and because they knew the insulin response of all the participants, they isolated the phosphorylations that were potentially involved in altering insulin action in connection with exercise.

“Until now, we have had very limited knowledge of what happens to the activity of various proteins that affect insulin sensitivity. In this study, we identified several molecular differences between insulin-sensitive and insulin-resistant muscle and several possible actors that obviously have a role in improving insulin sensitivity following exercise. We did not know about the vast majority of these differences beforehand and did not have any idea that they would be involved. This is amazing,” explains Jørgen Wojtaszewski.

Previous conclusions rejected

The study also revealed interesting findings about individual proteins. For example, researchers have studied mammalian target of rapamycin (mTOR) for many years and have found that activating mTOR leads to the development of insulin resistance.

However, the new study suggests that mTOR activity is downregulated in insulin resistance.

“This is very interesting for several reasons. It indicates completely different activity of mTOR than we had thought and it shows that we often cannot extrapolate the results from animal and cell studies to humans. We need to involve people, as in this study. The research also demonstrates the value of our personalised phosphoproteomics technology. One of our current projects involves silencing the mTOR activity of humans using drugs, so we can learn more about the effects of this protein,” notes Jørgen Wojtaszewski.

A protein as a possible drug candidate

Another interesting finding was that phosphorylation of the protein MINDY1 was linked to improved insulin sensitivity during exercise.

To investigate the effects of MINDY1, the researchers performed a follow-up cell experiment in which they removed MINDY1 from the cells, and this increased their insulin sensitivity.

Although MINDY1 worsens insulin action, the study also showed that this negative effect of MINDY1 lessens the moment MINDY1 is phosphorylated – which is precisely what happens during exercise.

“This indicates that MINDY1 may be a useful drug target because targeting MINDY1 and reducing its action may trigger an increase in insulin sensitivity and therefore mimic the effect of exercise. This could be preventive treatment to reduce the risk of developing type 2 diabetes,” explains Jørgen Wojtaszewski.

Proof of concept

Jørgen Wojtaszewski explains that the researchers created a proof of concept that studying phosphorylation in skeletal muscle can elucidate the mechanisms behind human insulin sensitivity and resistance.

He elaborates that pharmaceutical companies are already interested in using this method to identify new drug targets not just in type 2 diabetes but also in several other disease areas.

Jørgen Wojtaszewski continues to focus on insulin resistance and type 2 diabetes: “Regarding insulin resistance and type 2 diabetes, the study suggests interesting targets in skeletal muscle that have not been used in treatment yet. The aim may be to maintain insulin sensitivity so that the insulin-producing beta cells in the pancreas do not fail and people do not develop type 2 diabetes,” he concludes.