Vitamin D endocrine system and osteocytes.

The physiological role of the osteocyte, the most numerous of the three bone cell types, was significantly underestimated until recently. It is now known that they not only coordinate bone remodeling but also have an endocrine function as part of the regulatory network for calcium and phosphate homeostasis. Vitamin D and osteocytes interact in numerous ways to accomplish these activities. The major source of active vitamin D (1,25(OH)2D3) is the kidney but there is evidence that osteocytes can produce it as well. Renal 1,25(OH)2D3 regulates osteocyte production of fibroblast growth factor 23 (FGF23), a powerful phosphaturic factor with far-reaching physiological effects. The function of 1,25(OH)2D3 produced by osteocytes themselves is poorly understood and is an area of active research. Osteocytes affect local bone remodeling by producing regulatory factors for osteoblasts and osteoclasts in response to mechanical loading and to endocrine signals such as serum 1,25(OH)2D3. In addition, 1,25(OH)2D3 may inhibit mineralization in osteocyte lacunae. Whether 1,25(OH)2D3 has a role in osteocytic perilacunar remodeling is currently unknown. This short review presents the current state of our knowledge about the physiologically and clinically significant roles of vitamin D signaling in osteocytes.