Brown. Fat.

Well that doesn’t sound particularly lovely, or interesting in the least bit. But to your average obesity researcher this is one of the hottest topics on the board right now. Brown is the new black, so to speak.

We’ve known for years that there are a couple different types of fat- your average lazy storage site white fat, and the active, heat-producing brown fat. Brown fat has a unique uncoupling protein (UCP1) that gets upregulated every time the sympathetic nervous system starts cranking out the catecholamines (epinephrine and norepinephrine). But there has turned out to be another player, a second hand in the brown fat pot.

Natriuretic Peptides. Yes I can hear your concern from here. It seems that most people know them as the proteins in control of your blood pressure and fluid/salt release in the kidneys. But not anymore. It seems much has come to light on the role of blood pressure on brown fat function.

NP’s originate from the heart itself, which lends a complexity to what was formerly known as a simple pump, to being in and of itself a real endocrine organ. There are two receptors that tightly control the response to the NP’s- the signaling form of the receptor (NPRA) and the clearance form of the receptor (NPRC). These NP receptors were unexpectedly found in adipose tissue.

Mice lacking NPRC are exceedingly lean and full of brown fat. Mice lacking NPRA tended towards obesity. So it is a ratio of these two receptors that becomes important to us and to how white fat undergoes “browning”.

Collins and Bordicchia (2013) conducted an experiment involving fasting (decreases NPRC) and exposure to cold (raises blood pressure) to test the effect on the NR receptors and fat deposits. They found that the expression of NPRA increased and NPRC decreased, and lipolysis abounded. The brown fat production gears started turning. So why then if high blood pressure helps upregulate NPRA does obesity and hypertension often come together hand in hand?

Some earlier work suggests that the two receptors live very separate lives indeed. Some NP peptides bound selectively with one and not the other. It appears to increase brown fat and thermogenesis. Future research will have to take both receptors and the individual NP peptides into account. Of course there is a delicate homeostasis with any hormone; while injecting NP’s at first sounds like a promising idea, high circulating levels of NP’s in the body is a well known characteristic of heart failure. Guess we should probably wait until there’s a little more research done first, unless you’re up for ice baths and fasting, that is.

Resources

• Heart Hormones Fueling A Fire In Fat, Sheila Collins and Marica Bordicchia, Adipocyte, April 2013.