The insulin sensitivity of muscle affects the development of type 2 diabetes, but there are large gaps in knowledge on which biomolecular signalling pathways are involved when muscles develop insulin resistance. Now researchers have found a protein that appears to have a key role in developing insulin resistance and thus also in developing type 2 diabetes.
The uptake of glucose in the muscles is essential for regulating blood glucose. Otherwise, glucose accumulates in the blood and leads to elevated levels, which is associated with an increased risk of disease, including type 2 diabetes and cardiovascular disease.
In this context, insulin has a crucial role in enabling the muscles to take up glucose from the bloodstream, but when type 2 diabetes develops, the muscles often become insulin resistant and thus do not take up the glucose.
Researchers have known for 10 years that the Rac1 protein from the family of Rho GTPases is important for regulating glucose uptake in skeletal muscle. The researchers in the new study discovered a previously unknown protein that controls Rac1.
The discovery clarifies the insulin-signalling cascade that leads from insulin release to insulin resistance in the muscles of people with type 2 diabetes. The discovery also suggests an avenue for potential new drug targets for these people.
“If a drug can influence some links in this signalling cascade, it will probably be effective in preventing the development of insulin resistance with the associated elevated blood glucose. This is the potential of this very promising discovery,” explains a researcher behind the discovery, Lisbeth Møller, Postdoctoral Fellow, Department of Biomedical Sciences, University of Copenhagen.
The research, which has been published in the Proceedings of the National Academy of Sciences of the United States of America, was carried out in collaboration with researchers from the Department of Nutrition, Exercise and Sports, University of Copenhagen, from various departments of the University of Southern Denmark and researchers from Australia and the United States.
An important molecule for regulating glucose uptake
When researchers discovered Rac1 10 years ago, they found that it affects the regulation of glucose uptake in muscle cells. Insulin resistance is accompanied by lower activity of Rac1, and the muscle cells are unable to take up glucose. But why the activity of Rac1 was so low in insulinresistant muscles was unknown. The researchers wanted to determine what inhibits this very important protein.
They therefore extracted Rac1 from muscle cells together with all the molecules that bind to Rac1.
The researchers identified Rho guanine dissociation inhibitor α (RhoGDIα) as being the protein that deactivates Rac1 by binding to it – so that Rac1 does not enable the muscles to take up glucose.
“Insulin activates a long signalling cascade, which ends with RhoGDIα releasing Rac1. Rac1 is then activated and enables the muscles to take up glucose from the blood. Rac1 controls the factor that transports glucose across the cell wall of the muscle cells when Rac1 is released from RhoGDIα,” says the other main author, Lykke Sylow, Associate Professor, Molecular Metabolism in Cancer and Ageing Group, Department of Biomedical Sciences, University of Copenhagen.
Difficult to fully eliminate RhoGDIα
To confirm the finding, the researchers carried out a series of experiments with both muscle cells and muscles from live mice.
In the cell experiments, they investigated what happens to the cells when RhoGDIα is either upregulated or downregulated. This showed that upregulating RhoGDIα inhibited Rac1 and made the muscles resistant to insulin.
Conversely, the experiment also showed that removing RhoGDIα activated Rac1, making the muscle cells more sensitive to insulin and taking up glucose.
Similarly, in experiments with mice, introducing large quantities of RhoGDIα into the mice’s muscle cells made them insulin resistant.
The researchers also found that a large quantity of RhoGDIα exacerbates insulin resistance in mice fed a high-fat diet. However, removing the RhoGDIα actually made the mice more insulin resistant, which was surprising to the researchers.
“When we briefly removed RhoGDIα from muscle cells, they became more sensitive to insulin, but the mice in which RhoGDIα was downregulated over a long period of time instead became more insulin resistant. This is surprising, but it suggests that RhoGDIα not only inhibits Rac1 but also ensures that Rac1 can be reactivated. If RhoGDIα is not present for a long time in the cells, Rac1 also disappears, and this naturally leads to insulin resistance, because there is a lack of control over the transporters that must enable the cells to take up glucose,” explains Lykke Sylow.
She elaborates that tampering with essential proteins often results in the body compensating and trying to stabilise everything again. This is probably what happens when a lack of RhoGDIα leads to insulin resistance over a long period of time, although RhoGDIα plays the opposite role under physiologically normal conditions.
In collaboration with doctors from the University of Southern Denmark, the researchers confirmed that RhoGDIα probably also has the same role among people by finding RhoGDIα in muscle biopsies taken from people with type 2 diabetes and strong insulin resistance.
Potential target for novel drugs
Lykke Sylow thinks that the study contributes new and important basic knowledge about the processes controlling glucose uptake, insulin resistance and ultimately the risk of developing type 2 diabetes.
The researchers think that the signalling cascade includes insulin initially activating a kinase, which activates another kinase, which activates a third kinase and so on. Finally, RhoGDIα is presumably phosphorylated by adding a phosphate group to RhoGDIα, and that phosphate group presumably blocks the location on the protein on which Rac1 binds.
When Rac1 cannot bind to RhoGDIα, it instead settles in the cell wall and enables muscle cells to take up glucose.
According to Lykke Sylow, this insight may be an avenue for new targets for treating people with insulin resistance and type 2 diabetes.
“We cannot simply remove RhoGDIα, since this just gradually exacerbates insulin resistance. But we can probably find some targets elsewhere in the signalling pathway in muscles, and this can lead to short-term downregulation of RhoGDIα, improving insulin resistance and blood glucose. In addition, Rac1 also has an important role in developing some types of cancer, so our discoveries may also be useful there,” concludes Lykke Sylow.
