A hormone that is secreted by bone can suppress appetite in mice, a study in Nature reports. The discovery, which expands the repertoire of known bone-secreted hormones, reveals a previously unknown mechanism of appetite regulation.

Bone has recently emerged as an organ of the endocrine system. It is known to secrete at least two hormones, FGF23 and osteocalcin, which help to regulate kidney function and glucose homeostasis. Stavroula Kousteni and colleagues have now identified another such hormone. Lipocalin 2 (LCN2) is a protein that is secreted by osteoblasts, the cells that make bone. It induces insulin secretion, and improves glucose tolerance and insulin sensitivity in mice. The authors also find that LCN2 can cross the blood-brain barrier, bind with melanocortin 4 receptor (MC4R) in the hypothalamus and then activate an appetite-suppressing pathway, inhibiting food intake.

The study highlights a previously unknown role for LCN2. Previously, it was presumed to be an adipokine, a small-signalling molecule secreted by adipose tissue, but its levels are ten times higher in osteoblasts than in adipose tissue. At present, it is unclear why bone would suppress food intake, but it could represent a mechanism that helps to maintain bone mass and skeletal growth.