Retinol-binding protein mediates uptake of retinol to cultured human keratinocytes

Research Progress in Vitamin A and Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a highly heterogeneous neurodevelopmental disorder. Over the past few decades, many studies have investigated the effects of VA supplementation in ASD patients and the relationship between vitamin A (VA) levels and ASD. VA is an essential micronutrient that plays an important role in various systems and biological processes in the form of retinoic acid (RA). Recent studies have shown that serum VA concentration is negatively correlated with the severity of ASD. The lack of VA during pregnancy or early fetal development can affect brain development and lead to long-term or even permanent impairment in the learning process, memory formation, and cognitive function. In addition, VA deficiency has been reported to have a major impact on the gastrointestinal function of children with ASD, while VA supplementation has been shown to improve the symptoms of ASD to a certain extent. This paper provides a comprehensive review of the relationship between VA and ASD.

RETINOIDS IN DERMATOLOGY:FUNCTIONAL ACTIVITY AND PROSPECTS OF SYNTHETIC ANALOGUES

The aim of the work was to analyze and summarize information on the functional activity of known retinoids, as well as to determine the directions of their modification and prospects for the use of their synthetic analogues in dermatology. The literature on retinoids metabolism stages in the human body, the mechanisms of their action and the properties that determine their use in various medical fields was considered and analyzed. Information on dermatological diseases treatment with natural retinoids and their synthetic analogues was summarized. The modern classification of retinoids based on the peculiarities of their chemical structure and functional activity is given, the relationship between these characteristics and the direction of use of such drugs in dermatology and cosmetology is shown. Prospects for the practical application of synthetic retinoids represented mainly by polyaromatic compounds are due to their higher stability and efficiency, as well as reduced irritant effect due to selectivity for nuclear receptors.

The Role of Vitamin A in Wound Healing

Vitamin A is an essential micronutrient that comes in multiple forms, including retinols, retinals, and retinoic acids. Dietary vitamin A is absorbed as retinol from preformed retinoids or as pro-vitamin A carotenoids that are converted into retinol in the enterocyte. These are then delivered to the liver for storage via chylomicrons and later released into the circulation and to its biologically active tissues bound to retinol-binding protein. Vitamin A is a crucial component of many important and diverse biological functions, including reproduction, embryological development, cellular differentiation, growth, immunity, and vision. Vitamin A functions mostly through nuclear retinoic acid receptors, retinoid X receptors, and peroxisome proliferator-activated receptors. Retinoids regulate the growth and differentiation of many cell types within skin, and its deficiency leads to abnormal epithelial keratinization. In wounded tissue, vitamin A stimulates epidermal turnover, increases the rate of re-epithelialization, and restores epithelial structure. Retinoids have the unique ability to reverse the inhibitory effects of anti-inflammatory steroids on wound healing. In addition to its role in the inflammatory phase of wound healing, retinoic acid has been demonstrated to enhance production of extracellular matrix components such as collagen type I and fibronectin, increase proliferation of keratinocytes and fibroblasts, and decrease levels of degrading matrix metalloproteinases.

STRA6: role in cellular retinol uptake and efflux

Distribution of vitamin A throughout the body is important to maintain retinoid function in peripheral tissues and to ensure optimal vision. A critical step of this process is the transport of vitamin A across cell membranes. Increasing evidence indicates that this process is mediated by a multidomian membrane protein that is encoded by the stimulated by retinoic acid 6 (STRA6) gene. Biochemical studies revealed that STRA6 is a transmembrane pore which transports vitamin A bidirectionally between extra- and intracellular retinoid binding proteins. Vitamin A accumulation in cells is driven by coupling of transport with vitamin A esterification. Loss-of-function studies in zebrafish and mouse models have unraveled the critical importance of STRA6 for vitamin A homeostasis of peripheral tissues. Impairment in vitamin A transport and uptake homeostasis are associated with diseases including type 2 diabetes and a microphthalmic syndrome known as Matthew Wood Syndrome. This review will discuss the advanced state of knowledge about STRA6's biochemistry, biology and association with disease.

Physiological Role of Retinyl Palmitate in the Skin

The skin is similar to other organs in how it absorbs, stores, and metabolizes vitamin A. However, because of the anatomical location of skin and the specialized physiological roles it plays, there are ways in which the skin is rather unique. The stratified structure of the epidermis results from the orchestration of retinoid-influenced cellular division and differentiation. Similarly, many of the physiological responses of the skin, such as dermal aging, immune defense, and wound healing, are significantly affected by retinoids. While much is known about the molecular events through which retinoids affect the skin's responses, more remains to be learned. Interest in the effects of retinol, retinyl palmitate, and other retinoids on the skin, fueled in part by the promise of improved dermatologic and cosmetic products, will undoubtedly make the effects of retinoids on skin a subject for continued intense investigation.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

The C-terminal region of cis-retinol/androgen dehydrogenase 1 (CRAD1) confers ER localization and in vivo enzymatic function

Retinoic acid is generated from retinol (vitamin A) by the sequential actions of two different classes of enzymes, retinol dehydrogenases and retinal dehydrogenases. Several enzymes implicated in this process have been identified and characterized in vitro. However, our understanding of the cell biological function and regulation of this process is limited. To get further knowledge regarding the regulation of RA biosynthesis, we have determined possible regulatory mechanisms at the transcriptional and post-transcriptional levels for the prototypic microsomal retinol dehydrogenase cis-retinol/androgen dehydrogenase 1 (CRAD1). We note that the expression and stability of the enzyme are only moderately controlled by the retinoid status. Instead, we find that the cytosolic tail dramatically affects the activity of the enzyme, and we have mapped the structural elements required for ER retention and in vivo functional activity, respectively. Although inactive tail-deletion mutants display an abnormal subcellular localization, restoration of ER localization per se is not sufficient for enzymatic activity suggesting that additional trans-acting components interacting with, or modifying, the cytosolic tail are required for controlling the activity of the enzyme in vivo.

Development of a versatile reporter assay for studies of retinol uptake and metabolism in vivo

The two isomers of retinoic acid (RA), all-trans RA and 9-cis RA, are produced in several tissues in order to allow specific control of target gene transcription. Given the high potency of these receptor ligands, it seems likely that the cellular uptake and metabolic activation of the precursor, retinol (vitamin A), should be a highly regulated process. Several retinol dehydrogenases and components involved in the downstream events have been identified and partially characterized. However, less is known about the cellular uptake of retinol, and the isomerase activity giving rise to the 9-cis and 11-cis branches of the pathway. In this work, we show that the 9-cis RA biosynthesis pathway can be fully reconstituted in cultured HEK293A cells expressing a reporter system, including an endogenous isomerase activity converting all-trans retinol into 9-cis retinol. This assay allows for functional studies of known components, as well as screening for yet unidentified genes involved in the pathway. In addition to free all-trans retinol, we find that these cells can take up retinol from plasma retinol binding protein (RBP) by a mechanism that can be efficiently inhibited by blocking antibodies, suggesting that the uptake may involve a cellular receptor. We also demonstrate that overexpression of CRBPI can drive the accumulation of intracellular retinol from unbound retinol added to the medium. Thus, this versatile cellular assay can be used to study several aspects of retinol uptake and metabolism in vivo.

Signaling hierarchy downstream of retinoic acid that independently regulates vascular remodeling and endothelial cell proliferation

We previously demonstrated that during vascular morphogenesis, retinoic acid (RA) is required for the control of endothelial cell proliferation and capillary plexus remodeling. Herein, we investigate the mechanisms by which RA regulates these processes in the yolk sac. We found that although the enzyme required for RA production during early embryogenesis, retinaldehyde dehydrogenase-2 (Raldh2), was expressed in the visceral endoderm, RA receptors alpha1 and alpha2 were expressed in endothelial cells in the mesoderm, indicating that they are direct targets of RA. In Raldh2(-/-) embryos, there was down-regulation of TGF-beta1, fibronectin (Fn) and integrin alpha5, which was associated with decreased visceral endoderm survival and production of VEGF-A, Indian hedgehog (IHH), and bFGF. Exogenous provision of RA or Fn to Raldh2(-/-) explants in whole mouse embryo culture restored vascular remodeling, visceral endoderm survival, as well as integrin alpha5 expression and its downstream signaling that controls endothelial growth. Exogenous provision of visceral endoderm-derived factors (VEGF-A, IHH, and bFGF) failed to rescue endothelial cell proliferative control but collectively promoted vascular remodeling, suggesting that these processes are independently regulated via a signaling hierarchy downstream of RA.

RPE65 of Retinal Pigment Epithelium, A Putative Receptor Molecule for Plasma Retinol-Binding Protein, is Expressed in Human Keratinocytes

Retinoids are important modulators for cell growth and differentiation of normal skin. In plasma, retinol is transported coupled to plasma retinol-binding protein. In this study, we investigated gene and protein expression of RPE65, a putative receptor for plasma retinol-binding protein in human epidermal keratinocytes. We performed real-time PCR analysis to evaluate expression of RPE65 mRNA in proliferating and differentiating keratinocytes. Immunoblotting with anti-RPE65 antibody shows distinct reactivity to a 61-kDa protein. Indirect immunofluorescence on normal human epidermis reveals cell surface labeling of keratinocytes. Laser scan microscopy exhibits colocalization of plasma retinol-binding protein and RPE65 on cultured keratinocytes. Internalization experiments with [3H]retinoic acid-retinol-binding protein complex in the presence and absence of excess of retinol-binding protein indicates receptor-dependent uptake of retinoids. We further show isolation of RPE65 protein by affinity chromatography from lysates of keratinocytes using a retinol-binding protein-matrix gel column. In summary, we demonstrate mRNA and protein expression of RPE65 in epidermal keratinocytes. Colocalization of plasma retinol-binding protein with RPE65 and affinity binding suggest a direct interaction of RPE65 with plasma retinol-binding protein in cultured human keratinocytes that might be involved in retinoid uptake of keratinocytes.

Comparison of the metabolism of retinol delivered to human keratinocytes either bound to serum retinol-binding protein or added directly to the culture medium

Retinol (vitamin A) circulates in the blood bound to retinol-binding protein (RBP), which is thought to be responsible for the delivery of the vitamin to target cells, including the basal cells of the skin (keratinocytes). The process by which keratinocytes acquire retinol from RBP remains controversial. A mechanism for retinol delivery to keratinocytes involving cell surface RBP receptors has been proposed, while other studies support an RBP receptor-independent process. To further explore retinol uptake we have used a model system of human foreskin keratinocytes cultured in serum-free media to compare the metabolism of [3H]retinol delivered to the cells either bound to RBP or added directly to the culture medium. The majority of the cell-associated radioactivity found in keratinocytes incubated for 0.5 to 24 h with either free or RBP bound [3H]retinol was present as [3H]retinyl ester irrespective of the mode of delivery. In keratinocytes incubated for 24 h with [3H]retinol added directly to the culture medium or bound to RBP, [3H]retinyl ester comprised 76 and 80%, respectively, of the total cell-associated radioactivity. Also, the relative cellular levels of the different retinyl esters species synthesized by the keratinocytes were the same whether the [3H]-retinol was delivered free or bound to RBP. Finally, the kinetics of loss (turnover) of cell-associated [3H]-retinol and [3H]retinyl esters from keratinocytes pre-labeled with [3H]retinol delivered free or bound to RBP was the same. Overall, this study demonstrates that the rate and extent of retinol esterification by keratinocytes and the types of esters synthesized are the same whether the vitamin is delivered to the cells free or bound to RBP and argues against RBP receptor-mediated delivery of retinol to specific sites on the plasma membrane that influence overall retinol metabolism.

Identification of a mechanism to localize generation of retinoic acid in rat embryos

Vitamin A (retinol) is essential for normal mammalian development. However, its biological activity depends upon its conversion to retinoic acid (RA), a local mediator of cellular proliferation and differentiation. Previous studies have shown that embryonic RA is found specifically in tissues known to depend upon vitamin A for normal development and that its production follows uptake of maternal retinol. The aim of this study was to identify the mechanism for tissue-specific generation of RA in developing rat embryos. Here we show immunohistochemical localization of the retinol binding protein receptor, cellular retinol binding protein, retinol dehydrogenase and retinal dehydrogenase in rat embryos (presomitic to the 25-30 somite pair stage). These proteins are proposed to be responsible for cellular uptake of retinol, its intracellular transport and its conversion to RA. Thus, they potentially constitute the entire metabolic pathway from vitamin A to RA. All four proteins were detected specifically in tissues that are known to depend upon vitamin A for normal development including the yolk sac, heart, gut, notochord, somites, sensory placodes and the limb. Furthermore, our previous studies have demonstrated that uptake of retinol into the yolk sac depends upon a retinol binding protein receptor. Here we provide evidence that this mechanism functions also in the heart. Colocalization of cellular retinol binding protein, retinol and retinal dehydrogenase with the retinol binding protein receptor in tissues dependent upon vitamin A for normal development suggests that coordinate functioning of these proteins is responsible for cellular uptake of circulating retinol and its metabolism to RA. This is the first evidence of a tissue-specific mechanism for generation of RA from its precursor retinol in the developing embryo.

Class IV alcohol/retinol dehydrogenase localization in epidermal basal layer: potential site of retinoic acid synthesis during skin development

Vitamin A (retinol) plays a signaling role in the development of skin and other epithelial tissues. This is accomplished by a two-step metabolic pathway in which the rate-limiting step is oxidation of retinol to retinal, followed by oxidation of retinal to retinoic acid, which serves as the active ligand to activate nuclear retinoic acid receptors. Previous studies in mouse skin have shown that retinol oxidation is catalyzed by a cytosolic retinol dehydrogenase that may be a member of the alcohol dehydrogenase (ADH) enzyme family. Analysis of the ADH family has shown that class IV ADH is the most efficient isozyme for retinol oxidation but that other isozymes can catalyze this reaction. Here we have examined mouse skin for the expression of genes encoding class I ADH and class IV ADH, the only ADH isozymes in this species able to function as retinol dehydrogenases in vitro. In situ hybridization analysis of mouse skin revealed that class I ADH mRNA was absent, whereas class IV ADH mRNA was abundant and localized in the epidermal basal layer, providing evidence that the skin retinol dehydrogenase previously identified was class IV ADH. Immunohistochemical studies indicated that class I ADH protein was absent in the mouse skin, but class IV ADH protein was detected primarily in the basal layer of the epidermis, with less detection in the spinous layer and no detection in the cornified layer. This apparent down-regulation of class IV ADH expression during keratinocyte terminal differentiation provides evidence that the basal layer of the epidermis may be the primary site of local retinoic acid synthesis needed for retinoid signaling in the skin.