Endocrine functions of bone

Abstract

Significant discoveries made within the past 15 years have revealed that bone, as an organ, is not only a target of several endocrine signals but by itself acts as an unpredicted but nevertheless important endocrine tissue. Indeed, bone cells are secreting at least two hormones, FGF23 and osteocalcin, which are implicated in the regulation of phosphate homeostasis and of energy metabolism, respectively. FGF23 is secreted specifically by the cells of the osteoblast-osteocyte lineage and inhibits phosphate reabsorption in the kidney proximal tubule mainly by reducing the expression of sodium-phosphate co-transporters. FGF23 also suppresses 1,25-dihydroxyvitamin D3 synthesis in the proximal tubule and thereby impacts calcium and phosphate absorption as well. Osteocalcin is expressed and secreted by differentiated osteoblasts in bone and acts as a blood glucose-lowering hormone by stimulating insulin secretion by β-cells and by favoring insulin sensitivity in muscle, liver, and white adipose tissue. As we shall see in this chapter, the discovery that bone, like many other organs, possesses endocrine functions has opened new and exciting areas of research.