In obese mice, macrophages make up 40% of the fat tissue, compared with 10% in lean mice. Similar numbers are likely in human tissues.
In a recent study, a previously unknown type of macrophage has been discovered that reacts directly on fat breakdown.
Body fat (called adipose tissue) is not a simple bucket of calories. It is a complex organ that produces many hormones that have effects throughout the body. It responds to hormones such as insulin, and even to hormones it produces itself, such as leptin. It also has nerves that communicate with the sympathetic nervous system, to control appetite and digestion.
The newly discovered immune cells interact with the nerve cells in the fat. These nerve cells produce the hormone norepinephrine. In fat cells, norepinephrine acts as the signal to produce glycerin and free fatty acids from the stored fats. The free fatty acids are released into the blood to fuel muscles and other tissues.
The new macrophages have been named SAMs, for Sympathetic neuron-Associated Macrophages. SAMs clear out norepinephrine. Obese mice were found to have more SAMs attached to nerve cells than lean mice had. Thus, SAMs make the mice fat by preventing the fat from being broken down for energy.
Among immune cells, only SAMs have the transporter for norepinephrine that allows them to clear it out of the adipose tissue. When the researchers blocked this transporter, fat breakdown was boosted, along with energy dissipation, and the mice lost weight. They then went further, and found that the same mechanisms are in place in human cells, suggesting that blocking this transporter with a drug might be effective an effective weight loss target in humans.
This new information adds to our knowledge of how immune cells in the fat tissue affect our health. They are responsible for the chronic inflammation that accompanies obesity and type-2 diabetes, insulin resistance, high cholesterol, as well as the toxic breakdown products of gut bacteria known as lipopolysaccharides. This new study adds another item to the list — interfering with the breakdown of fats for energy.