Osteocalcin: A Protein Hormone Connecting Metabolism, Bone and Testis Function

During the last decade, the disclosure of systemic effects of osteocalcin (OCN) in its undercarboxylated form contributed to switch the concept of bone from a merely structural apparatus to a fully endocrine organ involved in the regulation of systemic functions. Since that time, the role of OCN as osteokine has been more and more widened appreciated and detailed by the major use of animal models, starting from the original function in the bone extracellular matrix as Gla-protein and spanning from the protective effects towards weight gain, insulin sensitivity and glucose homeostasis, to the anabolic and metabolic roles in skeletal muscle, to the stimulating effects on the testis endocrine function and male fertility, to the most recent preservation from anxious and depressive states through a direct activity on the central nervous system. In this review, experimental data supporting the inter-organ communication roles of this protein are discussed, together with the available data supporting the consistency between experimental data obtained in animals and those reported in humans. In addition, a specific session has been devoted to the possible significance the OCN as a template agonist on its receptor GPRC6A, for the development of novel therapeutic and pharmacological approaches for the treatment of dismetabolic states and male infertility.