- Email: [email protected]
STRA6, a Cell-Surface Receptor for Retinol-Binding Protein: The Plot Thickens
Cell Metabolism
Previews STRA6, a Cell-Surface Receptor for Retinol-Binding Protein: The Plot Thickens William S. Blaner1,* 1
Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA *Correspondence: [email protected] DOI 10.1016/j.cmet.2007.02.006
How retinol (vitamin A) is taken up by cells has been an area of active research since the first report of a cell-surface receptor for retinol-binding protein (RBP) in the mid-1970s. Two recent reports, by Kawaguchi et al. (2007) and Pasutto et al. (2007), provide compelling data regarding the molecular identity and properties of a RBP receptor and open new research avenues for a better understanding of retinoid biology. Retinoids (vitamin A and its metabolites and synthetic analogs) are needed for maintaining vision, immunity, barrier function, reproduction, embryogenesis, and normal cell proliferation and differentiation (Sporn et al., 1994). Most of the physiologic actions of retinoids can be accounted for by the transcriptional regulatory activity of all-trans- and 9-cis-retinoic acid. Within the nucleus, retinoic acid acts through three retinoic acid receptors (RARa, RARb, and RARg) and three retinoid X receptors (RXRa, RXRb, and RXRg), which, in conjunction with transcriptional coactivators and corepressors, regulate target gene expression (Chambon, 2005). In vision, 11cis-retinal is the chromophore for the visual pigment rhodopsin. Retinoids cannot be synthesized de novo by animals and are acquired from the diet. In the circulation, retinoids exist as retinol bound to retinol-binding protein (RBP), retinoic acid bound to albumin, or retinyl ester in lipoprotein (primarily chylomicron) particles (Figure 1 and Vogel et al., 1999). (RBP has also recently started to be referred to by its genetic designation, RBP4.) In the fasting circulation, retinol-RBP is the preponderant retinoid form, accounting for >95% of retinoids, while following intake of a retinoid-rich meal, chylomicron retinyl ester concentrations can exceed those of retinol-RBP. Circulating levels of retinoic acid are always low relative to retinol-RBP (<1%). In general, most retinoid is acquired by tissues from retinol-RBP.
Although a cell-surface receptor for RBP was first described by a number of groups in the mid-1970s, the molecular identity of this receptor remained elusive (summarized in Vogel et al., 1999). The work of Kawaguchi et al. (2007) now establishes STRA6 as a cell-surface receptor for retinolRBP that removes retinol from RBP and transports it across the plasma membrane, where it can be metabolized (see Figure 1). STRA6 is a member of a large group of ‘‘stimulated by retinoic acid’’ genes that encode transmembrane proteins and other proteins whose functions are largely unknown (Taneja et al., 1995). Kawaguchi et al. (2007) provide compelling biochemical evidence that STRA6 acts as a high-affinity cell-surface receptor for RBP and propose that STRA6 is a major physiological mediator of retinol uptake by cells. Using purified RBP derivatized with an ultraviolet crosslinking agent, these investigators were able to identify STRA6 as a protein present in membrane preparations from retinal pigment epithelial (RPE) cells that binds specifically to RBP with a high affinity (Kd = 59 nM). STRA6 expression greatly enhanced retinol uptake from RBP by COS-1 cells, while RNAi knockdown of STRA6 in WiDr cells greatly diminished retinol uptake. Immunolocalization studies showed strong STRA6 expression on the basolateral membrane of RPE cells, in the blood vessels of the retina, and in the hippocampus and spleen. Another recent report (Pasutto
164 Cell Metabolism 5, March 2007 ª2007 Elsevier Inc.
et al., 2007) indicates that homozygous mutations in the human STRA6 gene cause a pleiotropic, multisystem malformation syndrome. Nearly all STRA6 mutations are lethal in the perinatal period. One patient with longterm survival showed profound mental retardation, short stature, and a relatively large head. Since these malformations are common developmental lesions associated with maternal retinoid deficiency and ablation of RARs, RXRs, or enzymes involved in retinoic acid metabolism (Sporn et al., 1994; Chambon, 2005), the report of Pasutto et al. (2007) strongly supports the notion that STRA6 indeed mediates retinoid-dependent actions within the embryo. One of the intriguing aspects of the work of Pasutto et al. (2007) is that that the absence of RBP in humans and in mice gives rise to relatively mild phenotypes that are less severe than those observed for mutations in human STRA6. The known human mutations of RBP cause severe biochemical retinoid deficiency but otherwise only mild clinical symptoms (night blindness and a modest retinal dystrophy; Biesalski et al., 1999). RBP-deficient mice display several mild phenotypes including visual disturbances that resolve by 4–5 months of age if the mice are maintained on a controlled retinoid diet (Quadro et al., 1999). For both RBP-deficient humans and mice, it has been proposed that chylomicron (postprandial) retinyl esters are able to maintain retinoid sufficiency in
Cell Metabolism
Previews
Figure 1. Schematic Representation of Retinoid Metabolism Retinoids are present in the circulation as retinyl esters in lipoprotein particles (primarily chylomicrons) (brown box), RBP-bound retinol (dark blue box), or albumin-bound retinoic acid (teal box). During fasting, retinol-RBP is the preponderant circulating retinoid. Following a retinoid-rich meal, chylomicron retinyl ester concentrations can greatly exceed those of retinol-RBP. Circulating levels of albumin-bound retinoic acid are always low relative to retinol-RBP (<1%). Kawaguchi et al. (2007) establish STRA6 as a cell-surface receptor for retinol-RBP that removes retinol from RBP and transports it across the plasma membrane. Specific cellular processes that are needed to mediate chylomicron retinyl ester or retinoic acid uptake are not yet established. Following its uptake into cells by STRA6, retinol can be oxidized to retinal or retinoic acid. Retinal is the retinoid form needed to support vision. Retinoic acid mediates retinoid-dependent transcriptional processes. Many cells can convert retinol to retinyl ester, an inert retinoid storage form. Retinol metabolism within the cell is complex, involving the actions of several tissue-specific intracellular retinol-binding proteins, as well as retinol dehydrogenases (RDHs) and retinal dehydrogenases (RalDHs) that are needed to catalyze oxidation of retinol to retinal and retinal to retinoic acid, respectively. (Adapted from Vogel et al., 1999.)
the absence of retinol-RBP. This raises the question of whether STRA6 may have another role (or roles) in mediating retinoid actions during development beyond its role as a receptor for RBP. Whereas STRA6 is widely expressed in the murine embryo, in the adult, it is highly expressed in cells that compose blood-organ barriers in the brain (choroid plexus and the brain microvascular), eye (RPE), testis (Sertoli cells), kidney, spleen, and the female reproductive tract. Interestingly, computer analysis of the amino acid sequence for STRA6 had previously led investigators to postulate that STRA6 functions as a facilitative transporter of one or more nutrients (Bouillet et al., 1997). STRA6 has also been shown to be potentially important in cancer biology. STRA6 can be synergistically upregulated by Wnt1 and retinoids in mouse mammary epithelial
cells (Szeto et al., 2001), and STRA6 mRNA levels are also highly elevated in mammary gland tumors and human colorectal tumors. The important studies of Kawaguchi et al. (2007) and Pasutto et al. (2007) raise many intriguing questions regarding retinoid actions and metabolism. One of these centers on whether STRA6 acts in regulating insulin responsiveness. Yang et al. (2005) showed that serum RBP levels are elevated in adipose tissue of adiposeGlut4 / mice and in several insulinresistant states, thus proposing that RBP is an adipocyte-derived signal that may contribute to the pathogenesis of type 2 diabetes. Considering the very high binding affinity of RBP for STRA6, this raises the possibility that STRA6 may also have a role in mediating insulin resistance and type 2 diabetes. Since STRA6 expression is upregulated when retinoic acid levels are
high, one might ask how STRA6 acts in cells or tissues experiencing retinoid insufficiency. Will diminished expression of STRA6, owing to the absence of retinoic acid, result in downregulation of STRA6 expression and retinol uptake by the cell? Alternatively, when retinoic acid levels are high, will STRA6 levels increase, resulting in potentially excessive cellular intake of retinol? Another question that certainly deserves study arises from the reported tissue expression pattern of STRA6. STRA6 is undetectable (or is expressed at low levels) in several tissues that are highly responsive to retinoids. One of these tissues is skin (Pasutto et al., 2007), which clinically is responsive to retinoid therapies (for instance, to Accutane or Retin-A). STRA6 is not highly expressed in liver (Bouillet et al., 1997; Szeto et al., 2001; Pasutto et al., 2007), an organ that plays a central role in retinoid storage and metabolism. Does STRA6 facilitate retinol uptake into all retinoidresponsive and retinoid-metabolizing tissues, or only some? Clearly, the findings of Kawaguchi et al. (2007) and Pasutto et al. (2007) open many new avenues for research focused on RBP and retinoid actions within both the embryo and the adult, and, hence, the plot thickens. REFERENCES Biesalski, H.K., Frank, J., Beck, S.C., Heinrich, F., Illek, B., Reifen, R., Gollnick, H., Seeliger, M.W., Wissinger, B., and Zrener, E. (1999). Am. J. Clin. Nutr. 69, 931–936. Bouillet, P., Sapin, V., Chazaue, C., Messaddeq, N., De´cimo, D., Dolle´, P., and Chambon, P. (1997). Mech. Dev. 63, 173–186. Chambon, P. (2005). Mol. Endocrinol. 19, 1418–1428. Kawaguchi, R., Yu, J., Honda, J., Hu, J., Whitelegge, H., Ping, P., Wita, P., Bok, D., and Sun, H. (2007). Science 315, 820–825. Pasutto, F., Sticht, H., Hammersen, G., Gillessen-Kaesbach, G., Fitzpatrick, D.R., Nurnburg, G., Brasch, F., Schirmer-Zimmermann, H., Tolmie, J.L., Chitayat, D., et al. (2007). Am. J. Hum. Genet. 80, 550–560. Quadro, L., Blaner, W.S., Salchow, D.J., Vogel, S., Piantedosi, R., Gouras, P., Freeman, S., Cosma, M.P., Colantuoni, V., and Gottesman, M.E. (1999). EMBO J. 18, 4633–4644. Sporn, M.B., Roberts, A.B., and Goodman, D.S. (1994). The Retinoids: Biology, Chemistry, and Medicine, Second Edition (New York: Raven Press, Ltd.).
Cell Metabolism 5, March 2007 ª2007 Elsevier Inc. 165
Cell Metabolism
Previews Szeto, W., Jaing, W., Tice, D.A., Rubinfeld, B., Hollingshead, P.G., Fong, S.E., Dugger, D.L., Phan, T., Yansura, D.G., Wong, T.A., et al. (2001). Cancer Res. 61, 4197–4205. Taneja, R., Bouilett, P., Boylan, J.F., Gaub, M.-P., Roy, B., Gudas, L.J., and Chambon, P.
(1995). Proc. Natl. Acad. Sci. USA 92, 7854– 7858.
Blaner, eds. (Heidelberg, Germany: Springer Verlag), pp. 31–96.
Vogel, S., Gamble, M.V., and Blaner, W.S. (1999). Retinoid uptake, metabolism and transport. In The Handbook of Experimental Pharmacology: The Retinoids, H. Nau and W.S.
Yang, Q., Graham, T.E., Mody, N., Preiter, F., Peroni, O.D., Zabolotny, J.M., Kotani, K., Quadro, L., and Kahn, B.B. (2005). Nature 436, 356–362.
166 Cell Metabolism 5, March 2007 ª2007 Elsevier Inc.
Previews STRA6, a Cell-Surface Receptor for Retinol-Binding Protein: The Plot Thickens William S. Blaner1,* 1
Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA *Correspondence: [email protected] DOI 10.1016/j.cmet.2007.02.006
How retinol (vitamin A) is taken up by cells has been an area of active research since the first report of a cell-surface receptor for retinol-binding protein (RBP) in the mid-1970s. Two recent reports, by Kawaguchi et al. (2007) and Pasutto et al. (2007), provide compelling data regarding the molecular identity and properties of a RBP receptor and open new research avenues for a better understanding of retinoid biology. Retinoids (vitamin A and its metabolites and synthetic analogs) are needed for maintaining vision, immunity, barrier function, reproduction, embryogenesis, and normal cell proliferation and differentiation (Sporn et al., 1994). Most of the physiologic actions of retinoids can be accounted for by the transcriptional regulatory activity of all-trans- and 9-cis-retinoic acid. Within the nucleus, retinoic acid acts through three retinoic acid receptors (RARa, RARb, and RARg) and three retinoid X receptors (RXRa, RXRb, and RXRg), which, in conjunction with transcriptional coactivators and corepressors, regulate target gene expression (Chambon, 2005). In vision, 11cis-retinal is the chromophore for the visual pigment rhodopsin. Retinoids cannot be synthesized de novo by animals and are acquired from the diet. In the circulation, retinoids exist as retinol bound to retinol-binding protein (RBP), retinoic acid bound to albumin, or retinyl ester in lipoprotein (primarily chylomicron) particles (Figure 1 and Vogel et al., 1999). (RBP has also recently started to be referred to by its genetic designation, RBP4.) In the fasting circulation, retinol-RBP is the preponderant retinoid form, accounting for >95% of retinoids, while following intake of a retinoid-rich meal, chylomicron retinyl ester concentrations can exceed those of retinol-RBP. Circulating levels of retinoic acid are always low relative to retinol-RBP (<1%). In general, most retinoid is acquired by tissues from retinol-RBP.
Although a cell-surface receptor for RBP was first described by a number of groups in the mid-1970s, the molecular identity of this receptor remained elusive (summarized in Vogel et al., 1999). The work of Kawaguchi et al. (2007) now establishes STRA6 as a cell-surface receptor for retinolRBP that removes retinol from RBP and transports it across the plasma membrane, where it can be metabolized (see Figure 1). STRA6 is a member of a large group of ‘‘stimulated by retinoic acid’’ genes that encode transmembrane proteins and other proteins whose functions are largely unknown (Taneja et al., 1995). Kawaguchi et al. (2007) provide compelling biochemical evidence that STRA6 acts as a high-affinity cell-surface receptor for RBP and propose that STRA6 is a major physiological mediator of retinol uptake by cells. Using purified RBP derivatized with an ultraviolet crosslinking agent, these investigators were able to identify STRA6 as a protein present in membrane preparations from retinal pigment epithelial (RPE) cells that binds specifically to RBP with a high affinity (Kd = 59 nM). STRA6 expression greatly enhanced retinol uptake from RBP by COS-1 cells, while RNAi knockdown of STRA6 in WiDr cells greatly diminished retinol uptake. Immunolocalization studies showed strong STRA6 expression on the basolateral membrane of RPE cells, in the blood vessels of the retina, and in the hippocampus and spleen. Another recent report (Pasutto
164 Cell Metabolism 5, March 2007 ª2007 Elsevier Inc.
et al., 2007) indicates that homozygous mutations in the human STRA6 gene cause a pleiotropic, multisystem malformation syndrome. Nearly all STRA6 mutations are lethal in the perinatal period. One patient with longterm survival showed profound mental retardation, short stature, and a relatively large head. Since these malformations are common developmental lesions associated with maternal retinoid deficiency and ablation of RARs, RXRs, or enzymes involved in retinoic acid metabolism (Sporn et al., 1994; Chambon, 2005), the report of Pasutto et al. (2007) strongly supports the notion that STRA6 indeed mediates retinoid-dependent actions within the embryo. One of the intriguing aspects of the work of Pasutto et al. (2007) is that that the absence of RBP in humans and in mice gives rise to relatively mild phenotypes that are less severe than those observed for mutations in human STRA6. The known human mutations of RBP cause severe biochemical retinoid deficiency but otherwise only mild clinical symptoms (night blindness and a modest retinal dystrophy; Biesalski et al., 1999). RBP-deficient mice display several mild phenotypes including visual disturbances that resolve by 4–5 months of age if the mice are maintained on a controlled retinoid diet (Quadro et al., 1999). For both RBP-deficient humans and mice, it has been proposed that chylomicron (postprandial) retinyl esters are able to maintain retinoid sufficiency in
Cell Metabolism
Previews
Figure 1. Schematic Representation of Retinoid Metabolism Retinoids are present in the circulation as retinyl esters in lipoprotein particles (primarily chylomicrons) (brown box), RBP-bound retinol (dark blue box), or albumin-bound retinoic acid (teal box). During fasting, retinol-RBP is the preponderant circulating retinoid. Following a retinoid-rich meal, chylomicron retinyl ester concentrations can greatly exceed those of retinol-RBP. Circulating levels of albumin-bound retinoic acid are always low relative to retinol-RBP (<1%). Kawaguchi et al. (2007) establish STRA6 as a cell-surface receptor for retinol-RBP that removes retinol from RBP and transports it across the plasma membrane. Specific cellular processes that are needed to mediate chylomicron retinyl ester or retinoic acid uptake are not yet established. Following its uptake into cells by STRA6, retinol can be oxidized to retinal or retinoic acid. Retinal is the retinoid form needed to support vision. Retinoic acid mediates retinoid-dependent transcriptional processes. Many cells can convert retinol to retinyl ester, an inert retinoid storage form. Retinol metabolism within the cell is complex, involving the actions of several tissue-specific intracellular retinol-binding proteins, as well as retinol dehydrogenases (RDHs) and retinal dehydrogenases (RalDHs) that are needed to catalyze oxidation of retinol to retinal and retinal to retinoic acid, respectively. (Adapted from Vogel et al., 1999.)
the absence of retinol-RBP. This raises the question of whether STRA6 may have another role (or roles) in mediating retinoid actions during development beyond its role as a receptor for RBP. Whereas STRA6 is widely expressed in the murine embryo, in the adult, it is highly expressed in cells that compose blood-organ barriers in the brain (choroid plexus and the brain microvascular), eye (RPE), testis (Sertoli cells), kidney, spleen, and the female reproductive tract. Interestingly, computer analysis of the amino acid sequence for STRA6 had previously led investigators to postulate that STRA6 functions as a facilitative transporter of one or more nutrients (Bouillet et al., 1997). STRA6 has also been shown to be potentially important in cancer biology. STRA6 can be synergistically upregulated by Wnt1 and retinoids in mouse mammary epithelial
cells (Szeto et al., 2001), and STRA6 mRNA levels are also highly elevated in mammary gland tumors and human colorectal tumors. The important studies of Kawaguchi et al. (2007) and Pasutto et al. (2007) raise many intriguing questions regarding retinoid actions and metabolism. One of these centers on whether STRA6 acts in regulating insulin responsiveness. Yang et al. (2005) showed that serum RBP levels are elevated in adipose tissue of adiposeGlut4 / mice and in several insulinresistant states, thus proposing that RBP is an adipocyte-derived signal that may contribute to the pathogenesis of type 2 diabetes. Considering the very high binding affinity of RBP for STRA6, this raises the possibility that STRA6 may also have a role in mediating insulin resistance and type 2 diabetes. Since STRA6 expression is upregulated when retinoic acid levels are
high, one might ask how STRA6 acts in cells or tissues experiencing retinoid insufficiency. Will diminished expression of STRA6, owing to the absence of retinoic acid, result in downregulation of STRA6 expression and retinol uptake by the cell? Alternatively, when retinoic acid levels are high, will STRA6 levels increase, resulting in potentially excessive cellular intake of retinol? Another question that certainly deserves study arises from the reported tissue expression pattern of STRA6. STRA6 is undetectable (or is expressed at low levels) in several tissues that are highly responsive to retinoids. One of these tissues is skin (Pasutto et al., 2007), which clinically is responsive to retinoid therapies (for instance, to Accutane or Retin-A). STRA6 is not highly expressed in liver (Bouillet et al., 1997; Szeto et al., 2001; Pasutto et al., 2007), an organ that plays a central role in retinoid storage and metabolism. Does STRA6 facilitate retinol uptake into all retinoidresponsive and retinoid-metabolizing tissues, or only some? Clearly, the findings of Kawaguchi et al. (2007) and Pasutto et al. (2007) open many new avenues for research focused on RBP and retinoid actions within both the embryo and the adult, and, hence, the plot thickens. REFERENCES Biesalski, H.K., Frank, J., Beck, S.C., Heinrich, F., Illek, B., Reifen, R., Gollnick, H., Seeliger, M.W., Wissinger, B., and Zrener, E. (1999). Am. J. Clin. Nutr. 69, 931–936. Bouillet, P., Sapin, V., Chazaue, C., Messaddeq, N., De´cimo, D., Dolle´, P., and Chambon, P. (1997). Mech. Dev. 63, 173–186. Chambon, P. (2005). Mol. Endocrinol. 19, 1418–1428. Kawaguchi, R., Yu, J., Honda, J., Hu, J., Whitelegge, H., Ping, P., Wita, P., Bok, D., and Sun, H. (2007). Science 315, 820–825. Pasutto, F., Sticht, H., Hammersen, G., Gillessen-Kaesbach, G., Fitzpatrick, D.R., Nurnburg, G., Brasch, F., Schirmer-Zimmermann, H., Tolmie, J.L., Chitayat, D., et al. (2007). Am. J. Hum. Genet. 80, 550–560. Quadro, L., Blaner, W.S., Salchow, D.J., Vogel, S., Piantedosi, R., Gouras, P., Freeman, S., Cosma, M.P., Colantuoni, V., and Gottesman, M.E. (1999). EMBO J. 18, 4633–4644. Sporn, M.B., Roberts, A.B., and Goodman, D.S. (1994). The Retinoids: Biology, Chemistry, and Medicine, Second Edition (New York: Raven Press, Ltd.).
Cell Metabolism 5, March 2007 ª2007 Elsevier Inc. 165
Cell Metabolism
Previews Szeto, W., Jaing, W., Tice, D.A., Rubinfeld, B., Hollingshead, P.G., Fong, S.E., Dugger, D.L., Phan, T., Yansura, D.G., Wong, T.A., et al. (2001). Cancer Res. 61, 4197–4205. Taneja, R., Bouilett, P., Boylan, J.F., Gaub, M.-P., Roy, B., Gudas, L.J., and Chambon, P.
(1995). Proc. Natl. Acad. Sci. USA 92, 7854– 7858.
Blaner, eds. (Heidelberg, Germany: Springer Verlag), pp. 31–96.
Vogel, S., Gamble, M.V., and Blaner, W.S. (1999). Retinoid uptake, metabolism and transport. In The Handbook of Experimental Pharmacology: The Retinoids, H. Nau and W.S.
Yang, Q., Graham, T.E., Mody, N., Preiter, F., Peroni, O.D., Zabolotny, J.M., Kotani, K., Quadro, L., and Kahn, B.B. (2005). Nature 436, 356–362.
166 Cell Metabolism 5, March 2007 ª2007 Elsevier Inc.