Osteocalcin is the most abundant non-collagenous protein produced in bone. It has traditionally been regarded as a marker of bone turnover and is thought to act in the bone matrix to regulate mineralization. However, emerging knowledge regarding osteocalcin has expanded to include functions in energy metabolism, fertilization, and regulation of cognition. Fully carboxylated osteocalcin binds to hydroxyapatite, thereby modulating bone turnover, whereas undercarboxylated osteocalcin in the circulation binds to osteocalcin-sensing receptors and acts as a hormone that affects multiple physiological aspects. In this review, we summarize the current knowledge regarding the hormonal actions of osteocalcin in various organs and potential cellular downstream signaling pathway that may be involved.

Introduction
Undercarboxylated Osteocalcin
Endocrine Functions of Osteocalcin
Osteocalcin Sensing Receptor
Osteocalcin Signaling and TranscriptionalRegulation
Conclusions
References

• Kyunghwa Baek(Department of Pharmacology, College of Dentistry and Research Institute of Oral Science, Gangneung-Wonju National University) Correspondence to