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Retinoid receptors in bone and their role in bone remodeling.

Henning P, Conaway HH, Lerner UH - Front Endocrinol (Lausanne) (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Centre for Bone and Arthritis Research, Institute for Medicine, Sahlgrenska Academy, University of Gothenburg , Gothenburg , Sweden.

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.

No MeSH data available.


Related in: MedlinePlus

Vitamin A is provided from the food either as preformed vitamin A (retinyl esters) or as provitamin A carotenoids. Retinyl esters are hydrolyzed by pancreatic and intestinal enzymes and free retinol is taken up by the enterocytes. Half of the carotenoids is oxidized to retinal and then reduced to retinol. Retinol is esterified with long-chain fatty acids and incorporated into chylomicrons together with intact carotenoids and then carried by the lymphatics. The chylomicrons are taken up by hepatocytes in the liver where vitamin A is stored as retinyl esters. Before being released from the liver to the circulation, retinyl esters are hydrolyzed to retinol which binds to retinol-binding protein (RBP).
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Figure 1: Vitamin A is provided from the food either as preformed vitamin A (retinyl esters) or as provitamin A carotenoids. Retinyl esters are hydrolyzed by pancreatic and intestinal enzymes and free retinol is taken up by the enterocytes. Half of the carotenoids is oxidized to retinal and then reduced to retinol. Retinol is esterified with long-chain fatty acids and incorporated into chylomicrons together with intact carotenoids and then carried by the lymphatics. The chylomicrons are taken up by hepatocytes in the liver where vitamin A is stored as retinyl esters. Before being released from the liver to the circulation, retinyl esters are hydrolyzed to retinol which binds to retinol-binding protein (RBP).

Mentions: Vitamin A is obtained from the diet either as retinyl esters in eggs, liver, bottled milk or fortified cereals, or as carotenoids (e.g., β-carotene) in vegetables such as carrots or spinach. Approximately, 75% of vitamin A comes from retinyl esters. Retinyl esters and carotenoids taken up by enterocytes are incorporated in chylomicrons (Figure 1). These are transported by the lymphatics and then released into the circulatory system. Approximately, 66–75% of dietary retinoid is eventually taken up by hepatocytes, where vitamin A can be stored as retinyl esters or hydrolyzed to retinol, which binds to retinol-binding protein (RBP) before being released into the bloodstream (27). The remaining dietary retinoids are taken up by extra-hepatic tissues such as white adipose tissue, skeletal muscle, heart, lungs, and kidneys (28).


Retinoid receptors in bone and their role in bone remodeling.

Henning P, Conaway HH, Lerner UH - Front Endocrinol (Lausanne) (2015)

Vitamin A is provided from the food either as preformed vitamin A (retinyl esters) or as provitamin A carotenoids. Retinyl esters are hydrolyzed by pancreatic and intestinal enzymes and free retinol is taken up by the enterocytes. Half of the carotenoids is oxidized to retinal and then reduced to retinol. Retinol is esterified with long-chain fatty acids and incorporated into chylomicrons together with intact carotenoids and then carried by the lymphatics. The chylomicrons are taken up by hepatocytes in the liver where vitamin A is stored as retinyl esters. Before being released from the liver to the circulation, retinyl esters are hydrolyzed to retinol which binds to retinol-binding protein (RBP).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4356160&req=5

Figure 1: Vitamin A is provided from the food either as preformed vitamin A (retinyl esters) or as provitamin A carotenoids. Retinyl esters are hydrolyzed by pancreatic and intestinal enzymes and free retinol is taken up by the enterocytes. Half of the carotenoids is oxidized to retinal and then reduced to retinol. Retinol is esterified with long-chain fatty acids and incorporated into chylomicrons together with intact carotenoids and then carried by the lymphatics. The chylomicrons are taken up by hepatocytes in the liver where vitamin A is stored as retinyl esters. Before being released from the liver to the circulation, retinyl esters are hydrolyzed to retinol which binds to retinol-binding protein (RBP).
Mentions: Vitamin A is obtained from the diet either as retinyl esters in eggs, liver, bottled milk or fortified cereals, or as carotenoids (e.g., β-carotene) in vegetables such as carrots or spinach. Approximately, 75% of vitamin A comes from retinyl esters. Retinyl esters and carotenoids taken up by enterocytes are incorporated in chylomicrons (Figure 1). These are transported by the lymphatics and then released into the circulatory system. Approximately, 66–75% of dietary retinoid is eventually taken up by hepatocytes, where vitamin A can be stored as retinyl esters or hydrolyzed to retinol, which binds to retinol-binding protein (RBP) before being released into the bloodstream (27). The remaining dietary retinoids are taken up by extra-hepatic tissues such as white adipose tissue, skeletal muscle, heart, lungs, and kidneys (28).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Centre for Bone and Arthritis Research, Institute for Medicine, Sahlgrenska Academy, University of Gothenburg , Gothenburg , Sweden.

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.

No MeSH data available.


Related in: MedlinePlus