Effects of dietary retinoic acid on cellular retinol- and retinoic acid-binding protein levels in various rat tissues

Retinol dehydrogenase 11 is essential for the maintenance of retinol homeostasis in liver and testis in mice

Retinol dehydrogenase 11 (RDH11) is a microsomal short-chain dehydrogenase/reductase that recognizes all-trans- and cis-retinoids as substrates and prefers NADPH as a cofactor. Previous work has suggested that RDH11 contributes to the oxidation of 11-cis-retinol to 11-cis-retinaldehyde during the visual cycle in the eye's retinal pigment epithelium. However, the role of RDH11 in metabolism of all-trans-retinoids remains obscure. Here, we report that microsomes isolated from the testes and livers of Rdh11^-/- mice fed a regular diet exhibited a 3- and 1.7-fold lower rate of all-trans-retinaldehyde conversion to all-trans-retinol, respectively, than the microsomes of WT littermates. Testes and livers of Rdh11^-/- mice fed a vitamin A-deficient diet had ∼35% lower levels of all-trans-retinol than those of WT mice. Furthermore, the conversion of β-carotene to retinol via retinaldehyde as an intermediate appeared to be impaired in the testes of Rdh11^-/-/retinol-binding protein 4^-/-(Rbp4^-/-) mice, which lack circulating holo RBP4 and rely on dietary supplementation with β-carotene for maintenance of their retinoid stores. Together, these results indicate that in mouse testis and liver, RDH11 functions as an all-trans-retinaldehyde reductase essential for the maintenance of physiological levels of all-trans-retinol under reduced vitamin A availability.

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Obesity, inflammation, and insulin resistance

White adipose tissue (WAT) is considered an endocrine organ. When present in excess, WAT can influence metabolism via biologically active molecules. Following unregulated production of such molecules, adipose tissue dysfunction results, contributing to complications associated with obesity. Previous studies have implicated pro- and anti-inflammatory substances in the regulation of inflammatory response and in the development of insulin resistance. In obese individuals, pro-inflammatory molecules produced by adipose tissue contribute to the development of insulin resistance and increased risk of cardiovascular disease. On the other hand, the molecules with anti-inflammatory action, that have been associated with the improvement of insulin sensitivity, have your decreased production. Imbalance of these substances contributes significantly to metabolic disorders found in obese individuals. The current review aims to provide updated information regarding the activity of biomolecules produced by WAT.

Biomarker and animal models for assessment of retinoid efficacy in cancer chemoprevention

Vitamin A is essential for normal growth and development. Epidemiology and laboratory studies suggest that decreased vitamin A levels and defective metabolism/ action may contribute to the genesis of certain cancers. Based on this information, natural and synthetic derivatives of vitamin A (retinoids) have been used for chemoprevention of cancer. Retinoids have had some success in the chemoprevention of leukoplakia and in the decreased incidence of second primaries in head and neck cancer. There is little information on biomarkers that can be used to assess the efficacy of the chemopreventive activity of retinoids. The ability of retinoids to induce RARb has been consistently shown to correlate with the response of cells and tissues to retinoic acid, but few other biomarkers have been certified as indicators of retinoid activity. In light of the failure of the ATBC and CARET clinical intervention trials for chemoprevention of lung cancer, greater use of animal models for chemoprevention studies is necessary. The potential combination of phytochemicals that inhibit DNA methyltransferase activity with retinoids holds promise for more effective chemoprevention of retinoid-unresponsive premalignant lesions.

Signaling pathways in retinoid chemoprevention and treatment of cancer

The Vitamin A metabolite, retinoic acid, has been shown to have chemopreventive and therapeutic activity for certain cancers such as head and neck, cervical, neuroblastoma and promyelocytic leukemia. Retinoic acid achieves these activities by inducing differentiation and/or growth arrest. A large number of studies have investigated the mechanism(s) by which retinoic acid alters the behavior of premalignant and tumor cells. Although much important data has been obtained, the exact signaling pathways required for retinoic acid to exert its biological effects remains elusive. In this review, we outline the role and function of retinoid nuclear receptors, followed by a discussion of how major signaling pathways are affected in different tumor types by retinoids. We conclude by examining the effect of retinoic acid on G1 cell cycle regulatory proteins in various tumors.

Modulation of binding characteristics of peripheral benzodiazepine receptors in vitamin A-deficient guinea pig lung

Both vitamin A and peripheral benzodiazepine receptors (PBRs) are involved in the control of cell proliferation and differentiation. The objective of this study was to determine whether vitamin A deficiency causes any modulation in the binding characteristics of the PBRs. Forty-five weanling guinea pigs were divided into three groups (control, pair-fed control, and vitamin A-deficient). Vitamin A-deficiency status was achieved after 90 days of feeding. It caused atelectasis, hyperplasia and metaplasia of alveolar epithelium, acute inflammation of the tracheobronchial tree, and a significant increase in the number of alveolar type II cells. In comparison to the control and pair-fed control groups, the lung of vitamin A-deficient animals had 40.6 and 42.8% less PBR density, respectively. There was no significant difference in the equilibrium dissociation constant (KD) of PBRs between the control and the pair-fed control groups, whereas the KD value was 77.7 and 60% higher in the vitamin A-deficient groups than in the control and the pair-fed control groups, respectively. Furthermore, vitamin A-deficiency caused a decrease in the binding capacity of PBRs in both nuclear and mitochondrial fractions. These results may suggest a close functional relationship between vitamin A and PBRs.

In vivo regulation of rat epididymal proteins by retinoids: analysis by two-dimensional electrophoresis

The role of retinoids in the regulation of epididymal fluid protein expression was investigated. We compared the patterns of two-dimensional electrophoretic gels of proteins from luminal fluids, cytosols and spermatozoa (from control rats only) of control, retinoid-depleted, retinoid-depleted retinoic acid-complemented and retinoid-depleted testosterone-supplemented rats. This study compared the luminal fluid patterns from the 4 diets and observed 13 proteins whose expression was dependent on nutritional status. Eight were either absent or very weakly expressed in retinoid-depleted animals only, while their presence was obvious in control rats and in the retinoid-deficient retinoic acid- and testosterone-complemented groups. The expression of 8 proteins was greatly enhanced in retinoid-depleted testosterone-supplemented fluids as compared to control fluids. Five of the regulated proteins seemed to be captured by spermatozoa as they were observed in sperm protein patterns of control rats. These results clearly show that the synthesis of several epididymal proteins is influenced by retinoids. Since testosterone-supplemented animals on retinoid-free diet elicited the same response as retinol and retinoic acid ones, testosterone is likely to be the mediator of retinoid action on epididymal protein synthesis. Nevertheless, the observation of one protein whose expression is stimulated by retinoic acid only and is totally independent of testosterone also favors the direct influence of this retinoid.

All-trans retinoic acid induces cellular retinol-binding protein in human skin in vivo

We examined the regulation of cellular retinol-binding protein (CRBP) mRNA and protein expression in human skin in vivo by all-trans retinoic acid and all-trans retinol. Treatment of human skin for 24 h with all-trans retinoic acid (0.1%) or all-trans retinol (1.6%) induced CRBP mRNA 5.5-fold (p < 0.01, n = 10) and 5.7-fold (p < 0.01, n = 5), respectively, compared with skin treated with vehicle or sodium lauryl sulfate (used as an irritant control). In vitro translation of poly A+ RNA from all-trans retinoic acid, all-trans retinol, sodium lauryl sulfate, and vehicle-treated human skin demonstrated that the observed increased CRBP mRNA in all-trans retinoic acid- and all-trans retinol-treated skin was able to direct increased (2.3-2.9-fold) CRBP protein synthesis. Riboprobe in situ hybridization revealed that CRBP mRNA was uniformly elevated throughout the epidermis and in dermal cells after all-trans retinoic acid treatment of human skin. Western analysis revealed that CRBP protein was elevated 3.2-fold (p < 0.01, n = 6) and 3.0-fold (p < 0.01, n = 6) after all-trans retinoic acid treatment of human skin in vivo for 24 and 96 h, respectively, compared with vehicle- and sodium lauryl sulfate-treated skin. In addition, functional CRBP levels measured by [3H]all-trans retinol binding were elevated 1.9-fold (p < 0.01, n = 6) and 3.5-fold (p < 0.01, n = 6) at 24 and 94 h, respectively, after all-trans retinoic acid treatment, compared with vehicle- or sodium lauryl sulfate-treated skin. Gel mobility shift analysis revealed that retinoid receptors in nuclear extracts from human skin formed a specific complex with a DNA probe containing the retinoic acid response element in the mouse CRBP gene. Monoclonal antibodies to nuclear retinoid receptors demonstrated that predominantly retinoic acid receptor-alpha/retinoid X receptor-alpha heterodimers bound to the CRBP retinoic acid response element. These data demonstrate that CRBP expression in human skin in vivo is regulated by exogenous all-trans retinoic acid and all-trans retinol.

Studies on the metabolism of retinol and retinol-binding protein in transthyretin-deficient mice produced by homologous recombination

Tissue needs for retinoids are believed to be satisfied through the delivery in the circulation of retinol by its specific plasma transport protein, retinol-binding protein (RBP), which circulates as a 1-to-1 protein complex with transthyretin (TTR). The binding of RBP to TTR is thought to prevent filtration of retinol-RBP in the kidney and to play a role in secretion of RBP from hepatocytes. Recently a strain of mice (TTR-) that totally lacks immunoreactive TTR was produced by targeted mutagenesis. We have explored the effects of TTR deficiency on retinol and RBP metabolism in this mutant strain. In pooled plasma from the TTR- mice retinol levels averaged 6% of those of wild type animals. Similarly, plasma RBP in the TTR- mice was found to be 5% of wild type levels. Hepatic retinol and retinyl ester levels were similar for mutant and wild type mice, suggesting that the mutation affects neither the uptake nor storage of dietary retinol. Levels of retinol and retinyl esters in testis, kidney, spleen, and eye cups from TTR- mice were normal. Plasma all-trans-retinoic acid levels for the TTR- mice were 2.3-fold higher than those of wild type (425 versus 190 ng/dl). Kidney RBP levels were similar for the mutant and wild type mice and we were unable to detect intact RBP in urine from TTR- mice. Hepatic RBP levels in the TTR- mice were 60% higher than those of wild type mice (39.8 versus 25.0 micrograms of RBP/g of tissue). These data may suggest that there is a partial blockage in RBP secretion from TTR- hepatocytes that leads to lessened plasma levels of retinol-RBP.

Cellular binding proteins for fatty acids and retinoids: similar or specialized functions?

The cellular fatty acid-binding proteins (FABP) and cellular retinoid (retinol, retinoic acid)-binding proteins (CRtBP) are structurally and functionally-defined groups within an evolutionarily conserved gene family. CRtBP are expressed in both fully differentiated and developing tissues in a manner that supports a relationship to the action of retinoic acid in morphogenesis and cellular differentiation. The FABP are, by contrast, expressed only in fully differentiated tissues in a manner compatible with a major function in the metabolism of long-chain fatty acids (LCFA) for energy production or storage. The precise function(s) of FABP and CRtBP remain imperfectly understood, while subspecialization of function(s) within the two groups is suggested by the complex diversity in both of structurally distinct members that display striking tissue and temporal specificity of expression in addition to ligand specificity. Notwithstanding this considerable apparent functional diversity among the FABP and CRtBP, available evidence supports a dual set of generic functions for both protein groups in a) promoting cellular flux of poorly water-soluble ligands and their subsequent metabolic utilization or transformation, and b) sequestration of ligands in a manner that limits their association with alternative binding sites within the cell, of which members of the steroid hormone nuclear receptor superfamily (HNR) are a potentially important category. Theoretical as well as experimental models probing diffusional fluxes of LCFA in vitro and in living cells have provided support for a function for FABP in intracellular LCFA transport. Protein-bound ligand also appears to provide the substrate for metabolic transformation of retinoids bound to CRtBP, but convincing evidence is lacking for an analogous mechanism in the direct facilitation of fatty acid utilization by FABP. An emerging relationship between FABP and CRtBP function centers on their binding of, and induction by, ligands which activate or transform specific HNR-the retinoic acid receptors and the peroxisome proliferator activated receptor in the case of CRtBP and FABP, respectively. Evidence consistent with both a 'promotive' role (provision of ligands for HNR) and a 'protective' role (limiting availability of free ligand for HNR association) has been advanced for CRtBP. Available data supports a 'protective' function for cellular retinoic acid-binding proteins (CRABP) and liver FABP (L-FABP) and points to the existence of ligand-defined, lipid-binding-protein-HNR relationships in which CRABP serve to attenuate the induction of gene expression by retinoic acid, and in which L-FABP may modulate a cellular adaptive multigene response to increased LCFA flux or compromised LCFA utilization.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular retinoic-acid-binding-protein and retinol-binding-protein mRNA expression in the cells of the rat seminiferous tubules and their regulation by retinoids

The levels of the mRNA corresponding to the intracellular binding proteins for retinoic acid and retinol (CRABP1 and CRBP1, respectively) were studied in primary cultures of somatic and germ cells of the rat seminiferous tubules. We show that the CRABP1 mRNA is expressed in Sertoli and germ cells and a single molecular species of mRNA is detected. CRBP1 mRNA is detected in Sertoli and peritubular cells. The regulation of the expression of both genes by retinoids was studied in Sertoli cells. CRABP1 mRNA levels are not affected by either retinoic acid or retinol, whereas both compounds positively regulate CRBP1 mRNA synthesis in a dose-dependent manner. A fivefold increase in CRBP1 mRNA levels was observed 32-48 h after addition of either agent. These results demonstrate that in Sertoli cells the expression of CRABP1 is not affected by retinoids, similar to the situation observed in vivo and in other in-vitro cultures. CRBP1-gene expression is, instead, induced and the variations in CRBP1-mRNA levels may regulate the intracellular concentrations of retinoids, as a response to changes in the vitamin-A nutritional status.

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Vitamin A intake and in vivo expression of the genes involved in retinol transport

Two different metabolic alterations in vitamin A status are known to cause changes in the amount of circulating retinol-binding protein (RBP) and cellular retinol-binding protein (CRBP) in experimental animals; namely vitamin A deficiency, characterized by depleted retinol-liver stores and hypervitaminosis A, characterized by hepatic accumulation of retinyl esters. We have induced vitamin A deficiency and hypervitaminosis A in two groups of rats with the aim of determining whether the expression of the genes coding for these two proteins might be directly regulated by retinol. Using human RBP and CRBP cDNAs as probes, we measured the rate of transcription of the two genes in liver nuclei from control and treated rats by run-on transcription assays, and the steady-state level of the mRNAs by Northern blot analysis of total liver RNA. The distribution profile of RBP and CRBP mRNAs on fractionated liver polysomes was also examined. We have found a threefold decrease in the hepatic level of CRBP mRNA in vitamin-A-deficient animals, while the RBP mRNA is not affected by this nutritional deprivation. The decreases does not correspond to a lower transcription rate of the gene and therefore it is likely to result from lower stability of the CRBP mRNA. In hypervitaminosis A, we do not observe any differences in both the steady-state level of the mRNAs and in the rate of transcription of the two genes. The results are discussed in terms of retinol-dependent stabilization of the mRNA coding for CRBP.

Localization of cellular retinoic acid-binding protein to amacrine cells of rat retina

Monoclonal antibodies to performic acid-oxidized cellular retinoic acid-binding protein (CRABP) from bovine retina were prepared by fusion of spleen cells from immunized mice with mouse myeloma cells. Five antibodies were studied in detail. It was established by ELISA that the antibodies react with CRABP and oxidized CRABP, but not with other oxidized or unmodified retinoid-binding proteins. Competitive ELISA demonstrated that the antibodies react with heat-denatured antigen but not with native protein. Western blotting and immunostaining, following sodium dodecyl sulfate gel electrophoresis, provided evidence for recognition of a single component in retinal supernatants whose staining is prevented by preabsorption of the antibody with heat-denatured CRABP. The insoluble fraction from a retinal homogenate contains residual CRABP and two weakly-reacting components, whose staining is not affected by preabsorption of the antibody with antigen. Each antibody produces the same staining pattern on cryostat sections of rat retina by indirect immunofluorescence. Amacrine somata on both sides of the inner plexiform layer are labeled, as well as processes forming laminae within this layer. These results suggest that retinoic acid may play a functional role in the inner retina.

Immunolocalization of cellular retinol-, retinaldehyde- and retinoic acid-binding proteins in rat retina during pre- and postnatal development

Cellular retinol-, retinaldehyde- and retinoic acid-binding proteins were localized in rat retina during pre- and postnatal development by indirect immunofluorescence. Cryostat tissue sections were prepared daily from embryonic day 11 until the day of birth (E11-22) and from postnatal days 1-32 (P1-32). Cellular retinaldehyde- and retinol-binding proteins were first detected in retinal pigment epithelium on E13 and E18, respectively, and in Müller cells at P1 and P15. Parallel studies showed that in adult retina cellular retinoic acid-binding protein is present in a subpopulation of GABAergic amacrine cells. During retinal differentiation, cellular retinoic acid-binding protein was first detected at E18 in cells sclerad to the developing inner plexiform layer, suggesting that this binding protein is expressed in amacrine cells very early during differentiation. During early ocular morphogenesis, cellular retinoic acid-binding protein was present in mesenchymal cells enveloping the eye (E12-15), in the neuroblastic layer of the retina (E13-15), in the nerve fibre layer (E14-15), and the developing optic nerve (E15). Our results suggest that retinoic acid, the natural ligand of cellular retinoic acid-binding protein, may be involved in neuronal differentiation in the inner retina. The studies further support a role for cellular retinoic acid-binding protein in mediating the effects of retinoic acid on developing neural crest cells and raise new questions about the role of cellular retinaldehyde-binding protein in the visual cycle and during development.

Levels of binding proteins for retinoids in cultured Sertoli cells: effect of medium composition

The levels of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) have been measured in Sertoli cells maintained under different cultural conditions. Sertoli cells were isolated from prepubertal rats and cultured in a chemically defined medium without or with follicle-stimulating hormone (FSH), insulin, retinol or testosterone added individually or in combinations. The additions were made at the beginning of the culture or 24 h before the cells were subjected to determinations of CRBP and CRABP by radioimmunoassay. No differences were observed either after 1 or 4 days of treatment. The results obtained indicated that the levels of the two retinoid-binding proteins were unchanged in Sertoli cells in response to hormone and/or retinol administration. To rule out the possibility that the Sertoli cells used in our study were unresponsive to the hormones, lactate production by the cells cultured in the presence of FSH or insulin was measured. The amount of lactate produced under hormonal stimulation was significantly higher than the amount produced in absence of the hormones, thus indicating the ability of our Sertoli cells to respond to the hormonal stimulation.

Effect of a high systemic background level of vitamin A on the teratogenicity of all-trans-retinoic acid given either acutely or subacutely

Groups of 12 Charles River CD virgin female rats were either supplemented with 25,000 IU/kg of vitamin A palmitate or not during the first 8 days of pregnancy and in the first experiment given a single dose of either 5 or 10 mg/kg of all-trans-retinoic acid (RA) on day 9 of pregnancy. In a second experiment, similar groups were given either 4 or 8 mg/kg RA daily from day 6 through day 15 so that each treatment with RA was given to vitamin A supplemented rats or nonsupplemented rats. The high systemic background of vitamin A increased the teratogenicity of the 10 mg/kg dose of RA given on day 9 by 50%, but reduced the teratogenicity of the 8 mg/kg dose given on days 6-15. The reasons for this paradox are discussed and related to the human propensity to self-medicate with megadoses of vitamins.

Ligand-specific and non-specific in vivo modulation of human epidermal cellular retinoic acid binding protein (CRABP)

Retinoic acid (RA) is bound intracellularly by a specific, low molecular weight protein (CRABP), that is unrelated to its nuclear receptor and whose function and regulation are still unknown. In the present study we were able to obtain an in vivo modulation of CRABP by different stimuli in one of the major target organs of RA: the human skin. We found increased CRABP after daily application during 4 days of natural or synthetic retinoids (RA, acitretin, isotretinoin, Ro137410, retinol), that have either a high affinity to CRABP or can be transformed into RA. Only Ro150778 with no affinity and no reported transformation had no effect. No macro- or microscopical changes could be observed with any of the tested compounds. Induction of inflammatory and hyperproliferative changes in the skin by topical dithranol treatment, UVB irradiation or scotch tape stripping also induced a significant increase of CRABP 3 days after exposure. Topical diflucortolone showed not only a tendancy to decrease intrinsic CRABP levels, but significantly reduced the retinoid stimulated rise of CRABP. Thus we conclude that the increase of CRABP in a fully differentiated adult tissue seems to be a biological phenomenon following processes of inflammation and proliferation with a lag of several days, while retinoids seem to be able to induce such a rise independently of, or before, the appearance of such processes. Corticosteroids seem to be inhibitors of this reaction. We discuss the hypothesis that CRABP might function as an intracellular 'buffer' in the case of RA overload.