title: Retinoid receptors in bone and their role in bone remodeling.
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.
authors: Centre for Bone and Arthritis Research, Institute for Medicine, Sahlgrenska Academy, University of Gothenburg , Gothenburg , Sweden.
authors: Petra
authors: P
authors: Henning
authors: Department of Physiology and Biophysics, University of Arkansas for Medical Sciences , Little Rock, AR , USA.
authors: H Herschel
authors: HH
authors: Conaway
authors: Centre for Bone and Arthritis Research, Institute for Medicine, Sahlgrenska Academy, University of Gothenburg , Gothenburg , Sweden ; Department of Molecular Periodontology, Umeå University , Umeå , Sweden.
authors: Ulf H
authors: UH
authors: Lerner
keywords: osteoblast; osteoclast; osteoporosis; retinoids; vitamin A