The number of people with diabetes mellitus is increasing in Germany. It is now estimated that 7% to 8% of adults in this country suffer from type 2 diabetes, the most frequent and lifestyle-related form of diabetes mellitus. Every year, 500,000 new cases are diagnosed (German Health Report Diabetes 2018). A large number of drugs are available for the treatment of diabetes, but "Most drugs for diabetes only combat the symptoms of the disease. They have no effect on the cause of the high blood sugar," explains Alex Vegiopoulos from the German Cancer Research Center (DKFZ).

Together with his colleagues, Vegiopoulos is investigating the mechanism of action of one class of drugs, which is an exception: Glitazones are able to fundamentally improve the lipid and sugar metabolism and restore the sensitivity of body cells to the hormone insulin.

In healthy people, insulin transfers sugar from the blood into the body cells, where it is used to generate energy. In people with type 2 diabetes, the body cells no longer respond to insulin, and the sugar no longer enters the cells and accumulates in the blood instead. The result is the elevated blood sugar level characteristic of diabetes. Glitazones cause the body cells to react to insulin again and the sugar from the blood can enter the body cells.

But there is a catch: Glitazones can have serious side effects and are therefore not suitable for a broad application. Nevertheless, there is great interest among researchers in the mode of action of these drugs—they not only improve the sensitivity of body cells to insulin, but also promote the conversion of white to beige adipose tissue.

This second aspect of glitazone plays an important role in the treatment of type 2 diabetes, which is often caused by obesity. Unlike white adipose tissue, which only serves to store fat, beige adipose tissue burns energy and excess nutrients. Its function is similar to that of brown adipose tissue, which is often called a "slimming agent."

In mouse studies, DKFZ scientists discovered that glitazone treatment activates the gene Cited4 in the fatty tissue of the animals. It turned out that the effect of glitazone was dependent on the function of this gene.

"However, this only applied to female mice, but not to males," reports Vegiopoulos. When the scientists switched off Cited4 in male mice, the glitazones continued to function normally in the animals. This is different in female mice—without the gene, glitazones did not improve the insulin sensitivity of the body cells, which was due to a reduced promotion of the conversion of white to beige adipose tissue.

"In men, the mechanism of action of glitazones may function independently of the gene, whereas in women the gene is absolutely necessary," concludes Vegiopoulos. "For the development of new diabetes therapies based on the mechanism of action of glitazones, it is important to know that the metabolism of men and women differs.

The results from the DKFZ researchers also show how important it is to research new drugs in both female and male volunteers and patients, not only at an advanced stage of development, but also in early animal studies. So far, new active substances are still mainly being investigated in male animals.

The study is published in EMBO Molecular Medicine.