Abstract
Vitamin A (retinol) is a necessary and important constituent of the body which is provided by food intake of retinyl esters and carotenoids. Vitamin A is known best for being important for vision, but in addition to the eye, vitamin A is necessary in numerous other organs in the body, including the skeleton. Vitamin A is converted to an active compound, all-trans-retinoic acid (ATRA), which is responsible for most of its biological actions. ATRA binds to intracellular nuclear receptors called retinoic acid receptors (RARα, RARβ, RARγ). RARs and closely related retinoid X receptors (RXRα, RXRβ, RXRγ) form heterodimers which bind to DNA and function as ligand-activated transcription factors. It has been known for many years that hypervitaminosis A promotes skeleton fragility by increasing osteoclast formation and decreasing cortical bone mass. Some epidemiological studies have suggested that increased intake of vitamin A and increased serum levels of retinoids may decrease bone mineral density and increase fracture rate, but the literature on this is not conclusive. The current review summarizes how vitamin A is taken up by the intestine, metabolized, stored in the liver, and processed to ATRA. ATRA’s effects on formation and activity of osteoclasts and osteoblasts are outlined, and a summary of clinical data pertaining to vitamin A and bone is presented.
Highlights
Retinoid receptors in bone and their role in bone remodelingReviewed by: Dongxing Zhu, Roslin Institute, UK Bronwen Evans, Cardiff University, UK Carsten Carlberg, University of Eastern Finland, Finland
It was reported by Hopkins in 1906 that no animal can survive on a mixture of pure protein, fat, carbohydrates, water, and salt [1]
EFFECTS BY VITAMIN A ON OSTEOCLAST FORMATION IN CELL CULTURES Experiments in organ-cultured bones suggest that vitamin A stimulates osteoclastogenesis by increasing the differentiation of mononuclear progenitors present in periosteum/endosteum by enhancing periosteal/endosteal RANKL, which is in agreement with observations made at cortical periosteal surfaces in vivo in murine animal models
Summary
Reviewed by: Dongxing Zhu, Roslin Institute, UK Bronwen Evans, Cardiff University, UK Carsten Carlberg, University of Eastern Finland, Finland. Vitamin A is known best for being important for vision, but in addition to the eye, vitamin A is necessary in numerous other organs in the body, including the skeleton. Vitamin A is converted to an active compound, all-transretinoic acid (ATRA), which is responsible for most of its biological actions. ATRA binds to intracellular nuclear receptors called retinoic acid receptors (RARα, RARβ, RARγ). RARs and closely related retinoid X receptors (RXRα, RXRβ, RXRγ) form heterodimers which bind to DNA and function as ligand-activated transcription factors. It has been known for many years that hypervitaminosis A promotes skeleton fragility by increasing osteoclast formation and decreasing cortical bone mass. ATRA’s effects on formation and activity of osteoclasts and osteoblasts are outlined, and a summary of clinical data pertaining to vitamin A and bone is presented
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