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Show EDITORIAL Retinol and Retinol- Binding Protein in Cerebrospinal Fluid: Can Vitamin A Take the " Idiopathic" Out of Idiopathic Intracranial Hypertension? Jenny Libien, MD, PhD and William S. Blaner, PhD diopathic intracranial hypertension ( IIH), by definition of unknown etiology, may be in need of a new name. The article by Warner et al in this issue of the Journal ofNeuro- Ophthalmology ( 1) offers evidence that abnormal retinol ( vitamin A) transport and metabolism is involved in the pathogenesis of the disease. There is a significant elevation of the retinol to retinol- binding protein ( RBP) ratio with less RBP and more unbound retinol in the cerebrospinal fluid ( CSF) of patients with IIH than of control subjects. To grossly simplify and to extrapolate, the findings suggest that IIH results from vitamin A toxicity localized to the CSF. Vitamin A toxicity has long been acknowledged as a cause of the severe headaches and papilledema associated with intracranial hypertension. The classic tale is of the Eskimo ( Inuit) who for centuries have hunted polar bears for fur and meat but do not eat the liver for fear of the headaches and blurred vision that result from ingestion. Polar bears, as carnivores at the top of the Arctic food chain, have very high hepatic vitamin A levels. Human consumption of their livers would induce acute vitamin A toxicity. Although polar bear liver is not part of our modern day diet, chronic excessive consumption of foods rich in vitamin A has been linked to intracranial hypertension. Case reports of a " carrot craver" ( 2) and several " liver lovers" ( 3) ( of cow- not polar bear- liver) describe elevated intracranial pressure with complete resolution after removal of the offending foods from the diet. Intracranial hypertension and/ or papilledema associated with hypervitaminosis A has also been reported in a number of " vitamin junkies" ( added to the lexicon in the spirit of carrot cravers and liver lovers) and in children and teenagers given vitamin A for treatment of acne and other ailments in decades past. The doses leading to elevated intracranial pressure have ranged from 25,000 to 200,000 IU of vitamin A per day and were taken for months to years before diagnosis ( 4,5). ( The recommended daily allowance of vitamin A is 2,300- 3,000 IU). Intracranial hypertension is also a well- documented side effect of therapeutic doses of the synthetic vitamin A derivative 13- cw- retinoic acid ( generic name isotretinoin; brand name Accutane) ( 6) used for treatment of acne and of the vitamin A metabolite all- trans-retinoic acid used in the treatment of acute promyelocytic leukemia ( 7,8). In an analysis of adverse ocular events after 13- cw- retinoic acid treatment that were reported to the Food and Drug Administration, the World Health Organization Spontaneous Reporting System, and the National Registry of Drug- Induced Ocular Side Effects at the Casey Eye Institute, Fraunfelder et al ( 9) identified 179 cases of intracranial hypertension associated with 13- cM- retinoic acid, with 6 of the cases having a positive rechallenge, a mean time of 2.3 months between initial exposure and diagnosis, and documented resolution of 86 cases within weeks to a few months after cessation of the drug. Department of Pathology ( JL), State University of New York Downstate Medical Center, Brooklyn, New York; and Department of Medicine ( WSB), College of Physicians and Surgeons, Columbia University, New York, New York. Address correspondence to Jenny Libien, MD, PhD, Department of Pathology, SUNY Downstate Medical Center, 450 Clarkson Ave., Box 25, Brooklyn, NY 11203; E- mail: jenny. libien@ downstate. edu J Neuro- Ophthalmol, Vol. 27, No. 4, 2007 253 J Neuro- Ophthalmol, Vol. 27, No. 4, 2007 Editorial The hypothesis that abnormal vitamin A metabolism is a cause of IIH in patients with normal vitamin A consumption has been previously investigated and the published findings are summarized in Table 1. In the first reported clinical study, Jacobson et al ( 10) measured serum retinol concentrations from 16 women with IIH and 70 control women. The investigators reported a median serum retinol concentration of 75.2 | jLg/ dL in the IIH group compared with 53.0 | Jig/ dL in the control group. Although the median serum retinol level was higher in patients with IIH than in the control subjects, there was considerable overlap between the two groups with only two of patients with IIH having serum retinol concentrations above the highest control value. Warner et al ( 1) similarly found high serum retinol levels in patients with IIH compared with anesthesia control subjects. Other researchers have found no significant difference in serum retinol concentrations between patients with IIH and control subjects ( 11,12) but have identified different indicators of abnormal retinol transport and metabolism in IIH. In a previous publication from Warner et al ( 13), retinol levels were higher in the CSF in patients with IIH than in patients with normal intracranial pressure but were not elevated in patients with intracranial hypertension due to venous outflow obstruction, tetracycline use, or systemic disease. Tabassi et al ( 12) also found higher CSF retinol levels in patients with IIH than in control subjects. The association of IIH with RBP, the transport protein for retinol, was first reported by Selhorst et al ( 11), who found high serum RBP in 7 of 30 patients with IIH but in none of the control subjects. The results published by Warner et al in this issue ( 1) also showed high serum RBP in patients with IIH compared with control subjects when controlling for body mass index ( BMI). Their novel finding of low CSF RBP levels and a higher retinol/ RBP ratio in CSF will hopefully be replicated by other investigators and will lead to greater understanding of the link between vitamin A and IIH. The mechanisms by which vitamin A and its synthetic derivatives lead to a reversible elevation in intracranial pressure are not known. Vitamin A is involved in numerous physiologic processes, including vision, cellular differentiation, and embryonic development The function of vitamin A in the adult brain is only beginning to be understood, but there is evidence for involvement in synaptic plasticity ( 14,15), memory formation ( 16,17), cortical synchrony during sleep ( 18- 20), and adult neurogenesis ( 21- 23). Vitamin A acts primarily via its metabolites, which are called retinoids. 11- cw- retinal is the visual chromophore. Outside of the visual system, the major biologically active derivative is all- trans- retinoic acid. Formation of all- trans- retinoic acid depends on highly regulated processes of retinoid absorption from the diet, transport, and metabolism ( 24). A model of vitamin A transport and metabolism is given in Figure 1. All retinoids are derived from the diet, with most dietary retinoid in the form of retinyl ester and a minority as retinol and beta-carotene. Retinyl esters are hydrolyzed to retinol within the intestinal lumen by pancreatic triglyceride lipase ( 25) or by phospholipase B at the intestinal brush border. Retinol is then absorbed from the intestinal lumen into enterocytes and re- esterified by lecithin: retinol acyl transferase ( LRAT). The newly formed retinyl esters are incorporated into chylomicrons and secreted by the enterocyte into the lymphatic system and then into the circulation. A minority of the absorbed retinyl esters are delivered to target tissues directly by chylomicrons. However, retinyl ester transport by chylomicrons is an unlikely mechanism of vitamin A delivery involved in the pathogenesis of IIH, as no retinyl esters have been detected in the brain or CSF ( 26). Most retinyl esters are delivered to the liver where they are hydrolyzed to retinol within hepatocytes. Retinol is then either secreted back into the circulation as retinol bound to RBP or re- esterified by LRAT and stored as retinyl ester in hepatic stellate cells ( 27). The retinol- RBP complex is then bound by the tetrameric form of transthyretin ( TTR) soon after secretion into the circulation. Retinoid delivery to target tissues occurs predominantly via the retinol- RBP- TTR complex. The liver synthesizes most of the RBP and TTR in the circulation, although adipose tissue also secretes a small percentage of serum RBP. Stra6, an RBP receptor, has recently been identified as a potential facilitator of retinol uptake into some tissues, including brain ( 28,29). It has been shown to be expressed in the choroid plexus ( 30). After retinol uptake into the cell, retinol may be oxidized by one of the approximately 15 identified retinol dehydrogenases ( RDHs) and then by one of the three known retinaldehyde dehydrogenases ( RALDH1- 3) to form retinoic acid. In the nucleus, retinoic acid can bind to one of three retinoic acid receptors ( RARa,( 3,7) and to one of three retinoid X receptors ( RXRa,( 3,7). The RARs and RXRs are members of the steroid/ thyroid/ retinoid superfamily of ligand-dependent transcription factors that regulate gene transcription via retinoic acid response elements ( RAREs) present in approximately 500 genes ( 31). The concentration of retinoic acid within tissues is generally very low, being usually 100 to 1000 times less than that of retinol ( 32). Retinoid transport from the circulation to the CSF and brain is not well characterized. RBP and TTR are synthesized by choroid plexus in levels exceeding hepatic synthesis as a percentage of total protein. TTR comprises approximately 5- 25% and RBP 0.5- 2.5% of total CSF protein. Choroid plexus- derived RBP and TTR are presumably responsible for retinol transport into CSF. However, it is also possible that TTR does not bind the 254 © 2007 Lippincott Williams & Wilkins Editorial JNeuro- Ophthalmol, Vol. 27, No. 4, 2007 Intestine Liver Retinyl in diet FIG. 1. Simplified scheme for the metabolism and transport of vitamin A in the intestine, liver, brain, and choroid plexus epithelium. Vitamin A is derived from the diet and absorbed across the intestinal mucosa as retinyl ester. Retinyl esters are then packaged into chylomicrons with other lipids and enter the circulation. Chylomicron remnants are taken up by the liver, and the retinyl ester is hydrolyzed to retinol within the hepatocyte. The retinol may then be re- esterified by lecithimretinol acyl transferase ( LRAT) and stored in lipid droplets within hepatic stellate cells or secreted bound to retinol- binding protein ( RBP). In the circulation, transthyretin binds the retinol- RBP complex. Retinol is then delivered to tissues throughout the body. Delivery of retinol to brain is thought to involve transport across the vasculature of the choroid plexus into the CSF. RBP secreted by the choroid plexus into the CSF binds retinol and may deliver retinol back to the choroid plexus or to the meninges. Within cells, retinol is hydrolyzed by retinol dehydrogenase ( RDH) and then by retinyl aldehyde dehydrogenase ( RALDH) to retinoic acid. Retinoic acid binds to its nuclear receptors, retinoic acid receptor ( RAR), and retinoid X receptor ( RXR), which bind to a retinoic acid receptor element ( RARE) and influence gene transcription of one of the approximately 500 retinoic acid- responsive genes. retinol- RBP complex in CSF, as the primary role of TTR in serum retinol transport is to add bulk to prevent renal filtration ( 25). CSF retinol ( free, bound to RBP, or bound to RBP- TTR) is most likely delivered to meninges and choroid plexus, both of which express RALDH and have been described as the primary sites of retinoic acid synthesis in postnatal brain ( 33,34). Interestingly, these primary sites of retinoic acid synthesis are also locations that influence intracranial pressure. Vitamin A could theoretically lead to intracranial hypertension via enhanced transcription of genes involved in CSF secretion by the choroid plexus or in CSF absorption by arachnoid villi. Aquaporins were proposed as candidate genes linking vitamin A to IIH by Fishman ( 35) in a commentary on a previous article by Warner et al ( 13). Aquaporins are membrane water channels that facilitate the transport of water and some small solutes across the membrane ( 36). Aquaporin 1 is expressed on choroid plexus epithelial cells. Studies of knockout mice show that it influences CSF production and intracranial pressure ( 37). There is evidence that retinoic acid induces expression of aquaporin- 1 in human erythroleukemia HEL cells ( 38) and of aquaporin- 5 in mouse lung epithelial cells ( 39). It is possible that altered aquaporin expression is a common pathway in the pathogenesis of IIH and in intracranial hypertension secondary to medications and systemic diseases. Steroid hormones and thyroid hormone, which have also been associated with intracranial hypertension, bind nuclear receptors and influence gene expression in a manner similar to that of retinoic acid. 255 J Neuro- Ophthalmol, Vol. 27, No. 4, 2007 Editorial TABLE 1. Summary of published retinol and retinol- binding protein ( RBP) hypertension as compared to control subjects Publication Warner et al, 2007 ( 1) Tabassi et al, 2005 ( 12) Warner et al, 2002 ( 13) Selhorst et al, 2000 ( 11) Jacobson et al, 1999 ( 10) Serum retinol High High* Normal Normal Normal High CSF, cerebrospinal fluid; NM, not measured; " Controlled for body mass index. Serum RBP Trend High High* NM NM High in some NM NR, not reported. levels in patients with idiopathic Serum retinol/ RBP High Normal* NM NM NR NM CSF retinol High Normal* High High in some NM NM CSF RBP Low Low* NM NM NM NM intracranial CSF retinol/ RBP High High* NM NM NM NM Minocycline and other tetracycline derivatives, which are also linked to intracranial hypertension, have been shown to block poly( ADP- ribose) polymerase family, member 1 ( PARP- 1) ( 40), which also plays a role in gene transcription ( 41). Further studies are needed to determine whether retinoic acid, steroid and thyroid hormones, and drugs such as minocycline can alter aquaporin expression in choroid plexus, arachnoid villi, or ependymal cells and thereby increase intracranial pressure. An alternative but intriguing explanation for the association of IIH with abnormal vitamin A transport and metabolism centers on the recent recognition that RBP may act as a signaling molecule ( 42- 44). Adipose tissue-derived RBP, referred to as RBP4 in the diabetes and obesity literature, has been shown to act as an adipokine and modulate insulin sensitivity ( 43). It is more highly expressed in visceral than in subcutaneous fat and is a marker of intra- abdominal fat mass ( 45). The increase in serum RBP in some patients with IIH reported by Selhorst et al ( 11) and the higher serum RBP when controlling for BMI reported by Warner et al in this issue ( 1) is, therefore, of considerable interest. Could adipose tissue- derived RBP be the missing link between obesity and IIH? Obesity is present in 90% of women, 60% of men, and 30% of children with IIH ( 46). Furthermore, IIH resolves more rapidly in patients with significant weight loss ( 47). These findings are consistent with the idea that a signal from adipose tissue ( such as RBP) might somehow be involved in triggering elevated intracranial pressure. Because most serum RBP is secreted by the liver and only approximately 15- 20% is from adipose tissue, it is possible that a small increase in RBP from adipose tissue would be difficult to detect. This phenomenon may account for why serum RBP was elevated in only some of the patients with IIH in the study by Selhorst et al ( 11). Warner et al ( 1) also described low levels of RBP in CSF. One could imagine that choroid plexus- derived RBP also acts as a signaling molecule and alters CSF secretion or absorption. The major argument against the idea that RBP in serum or CSF directly affects intracranial pressure is that it cannot explain the intracranial hypertension caused by 13- cis- and all- fraTM- retinoic acid. It is more likely that low RBP levels in the CSF in relation to the CSF retinol concentration lead to greater retinoic acid production and then increased transcription of retinoic acid- regulated genes. In an article from Smith and Goodman ( 5) in 1976, toxicity was proposed to occur when retinol is in excess of RBP and is delivered to tissues by lipoproteins. It was suggested that retinol delivered by lipoproteins disrupts cell membrane integrity ( 5). Recent research, especially studies of RBP- deficient mice ( 48), suggests that this early hypothesis put forward to explain biochemical effects underlying vitamin A toxicity is probably an oversimplification and incorrect. It is now thought that vitamin A toxicity arises primarily through changes in transcription rates of essential vitamin A- dependent genes and that these changes adversely influence cellular processes. The study by Warner et al ( 1), the first to measure retinol and RBP in serum and CSF, represents a major contribution to our understanding of the role of vitamin A in IIH. 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