There is growing evidence that insulin resistance results from chronic inflammation and that macrophages may be a key cell type responsible for the induction of insulin resistance, which is a precursor to Type II diabetes. Researchers from the University of California at San Diego and colleagues have affirmed that hypothesis, and gone a step further by revealing two new targets - CAP and macrophage - to prevent and treat diabetes.

Cbl-associated protein (CAP) is expressed in fat and skeletal muscle tissue and helps those cells with glucose uptake. Thus, the researchers hypothesized that knocking out Sorbs1, the gene that codes for CAP, would result in insulin resistance.

Instead, their recent paper in Nature Medicine showed that the knockout mice remained insulin sensitive, even on a high-fat diet (HFD). At the same time, the animals' adipose tissue showed an 80% reduction in macrophage content, leading the researchers to consider that CAP expression in the macrophage might be essential to the macrophages' role in insulin resistance. Normally, macrophages accumulate in fat tissue following HFD.

"CAP's role in macrophage function is complicated and two-fold," lead author Jerrold Olefsky, co-chair of the division of endocrinology and metabolism at UCSD, told BioCentury. "If you knock out CAP from macrophages, then macrophages can't move into fat cells and you see insulin sensitivity after HFD. If you knock out CAP from adipose tissue, then the fat cells can't recruit macrophages and you see insulin sensitivity."

Because macrophages develop from bone marrow stem cells, the researchers transplanted bone marrow from the knockout mice into wild-type mice, essentially creating a macrophage-specific CAP knockout (BMT-Sorbs1-/- mice). The recipient mice gained insulin sensitivity after HFD, while wild-type animals exhibited insulin resistance.

"Clearly, the macrophage is conferring the phenotype," said Olefsky.

Furthermore, they found that CAP is necessary for macrophage chemotaxis into adipose tissue. "When you knock out CAP, macrophages stay in the circulatory system and can't move into tissue," said Olefsky. "We looked in fat tissue after HFD and there were 80% fewer macrophages."

Olefsky said the study opens a whole field of targeting macrophages to develop therapies to restore and maintain insulin sensitivity. "If you can develop an inhibitor anywhere along the path of macrophage uptake - transmigration out of blood, chemotaxis into tissue or activation in tissue - then you will increase insulin sensitivity," he said.

Olefsky noted that although macrophages are fully functional in CAP knockout mice, inhibiting them does raise concerns about side effects related to the immune system. However, he said that so far the mice haven't had any problems with infections. "Macrophages could probably still get to sites of infection where blood gets to. But we haven't injected the mice with bacteria or anything like that to really test it. We'll leave that up to the pharma companies."

The researchers also performed reverse bone marrow transplants, putting wild-type bone marrow into the Sorbs1-/- mice. If deleting Sorbs1 from macrophage were solely responsible for the interruption of HFD-induced insulin resistance, then these recipient mice (BMT-WT) would regain insulin sensitivity after HFD.

However, insulin resistance was only partially restored in the BMT-WT mice. Indeed, the macrophage content of fat tissue remained low while circulating counts were normal. The authors concluded that deleting Sorbs1 from adipose tissue decreased the ability to signal the recruitment of macrophages.

Olefsky noted a second therapeutic strategy could be to inhibit CAP's interaction with Cbl, which forms a complex that is involved in macrophage recruitment into fat cells. This would be challenging, he suggested, because the Cbl-CAP complex functions intracellularly. "You would need a cell-permeable small molecule to block a protein-protein interaction, which is considered pharmaceutically difficult," he said.

The researchers have run additional studies to confirm macrophages' responsibility in inducing insulin resistance. "Anytime you knock out an enzyme that would impair the inflammatory pathway, you get insulin sensitivity. If a knockout stimulates the immune system then you get insulin resistance. It's all about the macrophage," Olefsky said.