Muscle Hormone Irisin Stimulates Fat Browning?

Exercise is by far one of the most powerful medicines – except that we know very little about how it actually works.

So, how does exercise induce all of its many health benefits, especially in tissues and organs that are not even directly involved in this activity?

Surprising new insights into how exercise works come from a paper by Pontus Boström and colleagues from the Dana-Farber Cancer Institute and Harvard Medical School, published in a recent issue of NATURE, reporting the discovery of a new hormone secreted by skeletal muscle in response to exercise.

In their experiments in mice and humans, the researchers show that exercise induces the production and secretion of a ‘myokine’ they call irisin.

They also show that irisin (encoded by the FNDC5 gene), surprisingly, acts on white adipose cells in culture and in vivo to stimulate expression of uncoupling protein 1 (UCP1), a molecule characteristic of brown fat and non-activity thermogenesis. In mice, increased levels of irisin did indeed result in higher energy expenditure with no changes in movement or food intake.

“Nanomolar levels of this protein increase UCP1 in cultures of primary white fat cells by 50 fold or more, resulting in increased respiration. Perhaps more remarkable, viral delivery of irisin that causes only a moderate increase (~3 fold) in circulating levels stimulates a 10–20 fold increase in UCP1, increased energy expenditure and an improvement in the glucose tolerance of mice fed a high fat diet.”

As the authors do not fail to note,

“Irisin could be therapeutic for human metabolic disease and other disorders that are improved with exercise.”

Certainly, the notion that exercise can stimulate the secretion of a hormone from skeletal muscle, that appears to have multiple metabolic benefits, is a big step forward in our understanding of exactly why exercise has the many benefits that it does.

AMS
Washington, DC

Boström P, Wu J, Jedrychowski MP, Korde A, Ye L, Lo JC, Rasbach KA, Boström EA, Choi JH, Long JZ, Kajimura S, Zingaretti MC, Vind BF, Tu H, Cinti S, Højlund K, Gygi SP, & Spiegelman BM (2012). A PGC1-α-dependent myokine that drives brown-fat-like development of white fat and thermogenesis. Nature, 481 (7382), 463-8 PMID: 22237023

.