THE ENDOCRINE FUNCTION OF THE BONE TISSUE

This review discusses the recent evidence showing that the skeleton itself produces at least two hormones: fibroblast growth factor 23 (FGF23) and osteocalcin. FGF23 is secreted by osteocytes in bone and acts on the kidney to inhibit 1-alpha-hydroxilation ofvitamin D and promote phosphorous excretion. The affinity of FGF23 to FGF receptor is low, but FGF23 binds to FGF receptor-Klotho complex with more affinity. Therefore, Klotho determines the kidney-specific action of FGF23. Increase in FGF23 or Klotho levels due to genetic defects or ectopic production results in low serum phosphorous levels in humans. Contrary to this, low FGF23 or Klotho levels lead to hypophosphatemia and ectopic calcification. Mouse genetics studies revealed that osteoblast product, osteocalcin, in its undercarboxylated stage acts on the pancreatic beta-cells to enhance insulin production and on peripheral tissues to increase glucose utilization as a result of increased insulin sensitivity and to reduce visceral fat. In addition to this, undercarboxylated osteocalcin may also have another hormonal role, this time as a mediator of testosterone secretion. Osteocalcin was shown to induce testosterone production in Leydig cells of the testes both in ex vivo and in vivo studies. In both localizations, at the pancreas and at the testes osteocalcin acts through the GPCR6A receptor, this activates the cAMP response element-binding protein signaling pathway. Thus, this review reports the recent studies indicating bone ’s role as an endocrine organ.