Structural and functional studies of vitamin A-binding proteins

Assessment of protein energy malnutrition in older persons, Part II: Laboratory evaluation

A large proportion of chronic diseases affecting older persons can be either prevented or significantly improved by improving nutrition. This places an increased burden on health care professionals caring for older persons. Screening for malnutrition at an early stage allows the intervention to be most successful. History, physical examination, and anthropometric measurements are essential parts of any nutritional evaluation. However, these tools can be highly subjective and rely heavily on the knowledge and experience of the evaluator. Incorporating biochemical measurements in the routine nutritional assessment provides an often-needed objective dimension. Interpreting these measurements must take into consideration the normal biological changes seen with aging. In this article, we review many of the biochemical parameters used in nutritional assessment and their relation to morbidity and mortality, with a special focus on normal changes seen with aging.

A 25 kDa alpha 2-microglobulin-related protein is a component of the 125 kDa form of human gelatinase

Besides the monomeric mammalian 95 kDa progelatinase, two additional forms, a disulfide-bridged 220 kDa dimer and a 125 kDa form were isolated from human PMN leukocytes. The 125 kDa progelatinase was identified as a covalently linked, disulfide-bridged heterodimer formed of the monomer with a 25 kDa protein. This 25 kDa protein was isolated from gelatinase bound to the affinity support of gelatin-Sepharose and eluted by DTE-containing buffer. The amino acid sequence of tryptic peptides of this protein revealed homology with an alpha 2-microglobulin-related protein from rats, a protein so far unknown in humans.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Purification of the isoforms of tear specific prealbumin

A chromatographic method for separating tear specific prealbumin (TSP) into six isoelectric forms is described. Size exclusion high performance liquid chromatography (SE-HPLC) was used to isolate TSP from whole tears, followed by chromatofocusing fast protein liquid chromatography (FPLC) of the SE-TSP fraction on a Mono P column. This yielded six fractions varying in isoelectric point (pI) between 5.3 and 4.6. Subsequent anion exchange FPLC (Mono Q column) allowed a slight further purification of each Mono P fraction and removed Polybuffer from the Mono P fractions. Isoelectric focusing (IEF) of the TSP isoforms verified that the heterogeneity was based on pI, and confirmed that the chromatofocusing separation was in many respects the same as an IEF separation. Purity of TSP isoforms was determined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), IEF, enzyme-linked immunosorbent assay (ELISA) and immunoblotting of samples separated by SDS-PAGE and IEF. Amino acid analysis and N-terminal amino acid sequencing revealed subtle differences between the TSP isoforms. The entire purification procedure was conducted both with and without the addition of reducing agents and protease inhibitors to tear samples and all buffers used in the purification process. Relatively little difference was seen in the TSP isoform profile under these two sets of conditions. However, the tendency of isolated TSP to aggregate and precipitate was dramatically decreased under reducing conditions, resulting in significantly higher protein recoveries. This chromatographic purification procedure provides a basis for further study of the nature of the heterogeneity of TSP and characterization of the properties of this protein.

Identification of alpha-lactalbumin and beta-lactoglobulin in cynomolgus monkey (Macaca fascicularis) milk

1. An electrophoretic analysis of whey protein from cynomolgus monkey milk revealed that its constituents are more similar to bovine milk than human milk, i.e. cynomolgus monkey milk whey contains, besides alpha-lactalbumin-like protein (LaP), another predominant component similar to bovine beta-lactoglobulin (LgP), in its electrophoretic behavior on both disc- and SDS-polyacrylamide gel electrophoreses. 2. The amino acid composition of LaP shows close similarity to that of human alpha-lactalbumin, and LaP forms an immunoprecipitin line with anti-human alpha-lactalbumin rabbit antiserum. The homology between LaP and alpha-lactalbumin was further confirmed by an analysis of the N-terminal amino acid sequence. 3. LgP is not immunologically identical to bovine beta-lactoglobulin, but its amino acid composition is similar. The result of the N-terminal amino acid sequence analysis of LgP (up to the 26th residue) strongly suggests homology between this protein and beta-lactoglobulin.

Plasma retinol-binding protein response to vitamin A administration in infants susceptible to bronchopulmonary dysplasia

We hypothesized that changes in plasma retinol-binding protein (RBP) concentration in response to vitamin A administration might be useful for evaluating vitamin A status of very low birth weight infants susceptible to bronchopulmonary dysplasia. We prospectively studied 24 consecutively admitted neonates (birth weight less than 1350 gm, gestational age less than 31 weeks, ventilator dependent for greater than 24 hours after birth), who were eligible to receive 2000 IU supplemental vitamin A by intramuscular injection on postnatal day 1 and on alternate days thereafter for 28 days. In addition to serial assessment of vitamin A status, we measured plasma RBP just before and 1, 3, and 6 hours after an intramuscular injection of vitamin A (2000 IU/kg retinyl palmitate) on days 1 and 28. The percent increase in plasma RBP (delta-RBP) was high (mean +/- SD: 61 +/- 37%) and plasma vitamin A and RBP values were low on day 1, indicative of vitamin A deficiency. Supplemental vitamin A improved vitamin A status of all infants as shown by low delta-RBP (mean +/- SD: 8 +/- 9%) and normal plasma vitamin A and RBP values on day 28. Bronchopulmonary dysplasia was diagnosed in 12 of 24 infants. Infants with bronchopulmonary dysplasia had a higher mean (+/- SD) delta-RBP on day 28 than those without bronchopulmonary dysplasia (13 +/- 10% vs 3 +/- 3%, p less than 0.01), indicative of persistence of low vitamin A status in infants with lung disease despite supplementation. We conclude that the plasma RBP response to vitamin A is a useful indicator of vitamin A status in very low birth weight infants. Although vitamin A supplementation at the dosage used in this study normalizes conventional plasma indexes of vitamin A in very low birth weight infants, the plasma RBP response to vitamin A may continue to reflect persistence of low vitamin A status in the more immature infants with significant lung disease. We suggest that the plasma RBP response to vitamin A may be a useful functional test in such infants.

Crystallographic refinement of human serum retinol binding protein at 2A resolution

Human serum retinol binding protein (RBP) in complex with retinol has been crystallographically refined to an R-factor of 18.1% with 2A resolution data. The protein topology results in an anti-parallel beta-barrel that encapsulates the retinol ligand. A detailed description of the protein and the binding site is provided. Our structural work has helped to define a family of proteins, many of which are carrier proteins for smaller ligand molecules. We describe the structural basis for the conservation of sequence within the family.

Rapid repression of quiescence-specific gene expression by epidermal growth factor, insulin, and pp60v-src

We isolated a cDNA for p20K, a secreted protein preferentially synthesized in nonproliferating cells. p20K mRNA and protein levels declined rapidly following treatment with various mitogens. DNA sequence analysis of the p20K cDNA predicted a novel protein distantly related to alpha 2 mu-globulin and plasma retinol-binding protein.

Rapid repression of quiescence-specific gene expression by epidermal growth factor, insulin, and pp60v-src

We isolated a cDNA for p20K, a secreted protein preferentially synthesized in nonproliferating cells. p20K mRNA and protein levels declined rapidly following treatment with various mitogens. DNA sequence analysis of the p20K cDNA predicted a novel protein distantly related to alpha 2 mu-globulin and plasma retinol-binding protein.

Purification and properties of an 18-kilodalton, 1,25-dihydroxyvitamin D3 modulated protein from embryonic chick intestine

An 18,000-dalton protein (pI = 5.1) shown previously to be modulated by 1,25-dihydroxyvitamin D3 was purified to allow its further characterization. This protein from embryonic chick intestine was shown to comigrate during two-dimensional electrophoresis with an abundant protein from the intestine of 4-week-old chickens. The protein was purified from 4-week chick intestine and analyzed for amino acid composition, and 28 amino acids of its N-terminal sequence were determined. The N-terminal amino acid sequence had significant homology to cellular retinol binding protein II, an intestinal protein that has been recently sequenced. The purified 18-kilodalton protein was shown to bind retinol by fluorescence spectrophotometry. This 18-kilodalton protein is dramatically changed by 1,25-dihydroxyvitamin D3 in the chick embryonic organ culture system. Therefore, further study of it may lead to a better understanding of vitamin A and D interaction and how 1,25-dihydroxyvitamin D3 acts through proteins to stimulate intestinal calcium and phosphate transport.

beta-Lactoglobulin identified in marsupial milk. The primary structure, binding site and possible function of beta-lactoglobulin from eastern grey kangaroo (Macropus giganteus)

beta-Lactoglobulin has been isolated in the milk of the Eastern Grey Kangaroo (Macropus giganteus). This is the first time this protein has been reported to be in the milk of marsupials. The complete amino-acid sequence has been determined by spinning cup and pulsed liquid phase microsequencing of the protein and peptides after enzymatic or cyanogen bromide cleavages. The 155-residue protein is the shortest beta-lactoglobulin so far sequenced. When the kangaroo protein is included in a comparison of the members of the beta-lactoglobulin family, the percentage of residues common to all members is reduced from 33% to 13%. Despite the large number of accumulated amino-acid exchanges the protein exists as a dimer and shows higher homology to the usually very conservative dimeric, ruminant beta-lactoglobulins than to the monomeric protein from monogastrics. Half-cystine residues that form disulphide bridges are conserved. The Eastern Grey Kangaroo beta-lactoglobulin possesses significant homology in several characteristic segments thought to be important for a functional trait common to the beta-lactoglobulin family and retinol-binding proteins. Structural similarity to the retinol-binding protein is indicated by 22% of identical residues. Homology to the beta-lactoglobulins and retinol-binding proteins, the binding site and possible function based on comparative structural studies are discussed.

Human interstitial retinol-binding protein (IRBP): cloning, partial sequence, and chromosomal localization

A cloned 2184-bp cDNA coding for human interstitial retinol-binding protein (IRBP) has been isolated and sequenced. The probe hybridized to a 5.2-kb poly(A) RNA from human retinas. Nineteen tryptic peptides (363 amino acids) sequenced and purified from bovine IRBP could be aligned with 86-88% homology to the translated sequence. Two segments approximately 200 amino acids long were found to have a 41% residue identity, suggesting an internal duplication event. This cloned cDNA was used to probe DNA samples from a panel of 29 rodent-human somatic cell hybrids, mapping the structural gene for IRBP to chromosome 10. In situ hybridization suggested a regional localization near the centromere (p11.2----q11.2), although a secondary site of hybridization at q24----25 was also observed.

The complete amino acid sequence of human serum retinol-binding protein

The complete amino acid sequence of human serum Retinol-binding protein (RBP) including the distribution of its three disulfide bridges, has been determined. The protein consists of 182 amino acid residues, the order of which was determined following the isolation of five CNBr-fragments. Direct amino acid sequence analysis in an automatic liquid phase sequencer provided almost the entire sequences of the five CNBr-fragments. Several sets of enzymatically derived peptides of RBP were also used to elucidate the primary structure. RBP displays significant homology to bovine beta-lactoglobulin, human alpha 1-microglobulin and rat alpha 1-microglobulin. RBP contains an internal homology. Thus, residues 36 to 83 display statistically significant homology with residues 96 to 141.

Vitamin A and beta-carotene concentrations at different depths of the epidermis: a preliminary study in the cow snout

Vitamin A (retinol) is an anti-keratinizing agent essential for normal epithelial differentiation. In order to examine the epidermal distribution of vitamin A and provitamin A (beta-carotene), we took advantage of the extraordinarily thick snout epidermis of the cow which can be cut horizontally into at least 6 layers, representing keratinocytes at different stages of maturation. Extracts of saponified samples were analyzed for retinol and beta-carotene by reversed phase high-performance liquid chromatography. The highest retinol concentration (0.8 micrograms/g protein; n = 3) was recorded closest to the dermis; progressively decreasing amounts of retinol were found in the upper parts of epidermis. Maximum values of beta-carotene (1.0 micrograms/g; n = 7) were found in the lower parts of epidermis; substantially lower levels were seen at the dermal transition zone and in the upper parts of epidermis. The results suggest that the endogenous concentration of vitamin A in snout epidermis is inversely related to the degree of cellular differentiation.

Molecular dynamics simulations of the holo and apo forms of retinol binding protein. Structural and dynamical changes induced by retinol removal

The effects of removing retinol from the X-ray structure of holo-retinol binding protein are studied using the molecular dynamics technique. Structural and dynamical properties emerging from an 80 ps simulation of the apo form, for which no crystallographic structure is available, are compared with the results of a 70 ps trajectory of the holo-protein. Dynamical stationarity is attained after roughly 30 ps, and the resulting average structure is proposed as a reasonable model of the apo-protein. Conformational changes are observed for the loops at the beta-barrel entrance during the non-equilibrium part of the apo-trajectory. Tryptophan labelling experiments and retinoid reconstitution experiments point towards this part of the molecule as being involved in prealbumin binding. Structural changes in this region may therefore explain the differences in prealbumin affinity between the apo and holo forms. Furthermore, a change in the position of the alpha-helix, corresponding to a pivot around its C terminus, is observed for the apo-protein. The resulting conformation of the alpha-helix is found to be similar to that in apo-beta-lactoglobulin, which also can bind retinol and for which a crystal structure exists. The results from the holo simulation are compared to the crystallographic data and show good agreement. The dynamics of the secondary and tertiary structural elements are analysed and compared for the two forms. The beta-barrel is found to be extremely cooperative in its atomic motions in both simulations, and the top and bottom beta-sheets perform collective fluctuations with respect to each other in the low-frequency limit of the simulations. The dynamics of the alpha-helical region presents clear differences between the two forms; while the holo-protein has a well-defined spectrum for the longitudinal stretching mode, the apo form displays a fairly large bending of the alpha-helix at several points of the trajectory.

Structural changes in retinol binding protein induced by retinol removal. A molecular dynamics study

Relationships between structure and function for retinol binding protein (RBP) are elucidated with help of a 2.0 A resolution X-ray structure of the holo-protein and an average molecular dynamics (MD) structure of the apo-form. Comparisons between MD simulations of both the apo- and holo-forms with the X-ray holo-structure show conformational changes in apo-RBP that may be functionally significant. The average three dimensional structure obtained for apo-RBP is compared to the related protein apo-beta-lactoglobulin. Available biochemical information is consistent with structure/function relationships derived here.

Psoriasis and vitamin A. Plasma transport and skin content of retinol, dehydroretinol and carotenoids in adult patients versus healthy controls

The vitamin-A status of 107 patients with psoriasis and 37 healthy controls was investigated. The mean serum level of retinol-binding protein (RBP) was normal in the 79 patients with chronic plaque psoriasis covering 25% or less of the skin surface. In the 28 patients with more extensive plaque lesions or pustular/erythrodermic psoriasis, the mean serum RBP level was significantly lower than in the controls (P less than 0.05). The cutaneous concentrations of retinol (vitamin A1), dehydroretinol (vitamin A2) and carotenoids were measured in extracts of saponified shave-biopsy specimens of uninvolved and involved skin from 33 patients with plaque psoriasis. Their retinol values did not differ significantly from those found in control skin (mean, 252 ng/g), whereas the carotenoid levels in both uninvolved and involved skin were 25%-50% lower. In contrast, the dehydroretinol concentration was higher in the patients' involved skin (mean, 237 ng/g) than in their uninvolved skin (94 ng/g) and healthy control skin (70 ng/g; P less than 0.01). Although the origin of increased dehydroretinol levels in involved psoriatic skin is unknown, similar increments were observed in control epidermis in which proliferation had been induced by tape stripping. In 7 patients treated with oral etretinate (aromatic retinoid) for 2-3 weeks, the median retinol and dehydroretinol levels in involved skin increased by 107% and 212%, respectively; the vitamin-A concentrations in uninvolved skin did not change significantly. Oral treatment with beta-carotene/canthaxanthin raised the median carotenoid levels in uninvolved and involved skin by 170% and 610%, respectively, without significantly affecting the vitamin-A composition.

Cellular retinoid binding proteins

The cellular retinol-binding protein (CRBP) and the cellular retinoic acid binding protein (CRABP) have similar physicochemical characteristics. The amino acid sequences of rat CRBP and bovine CRABP have been elucidated and they display 40% sequence identity. Both protein sequences appear to be evolutionarily highly conserved. The amino acid sequence of human CRBP, deduced from a cDNA-clone, is 96% identical to the rat CRBP sequence. CRBP and CRABP are members of a protein family, all members of which may bind hydrophobic ligands and interact with membrane components. All members of the protein family are probably related in tertiary structure and might interact with membrane components through two regions with a high probability for alpha-helix. The tissue distribution of CRBP and CRABP, together with their relation to lipid transporting proteins suggests that CRBP and CRABP are cellular transporting proteins for retinol and retinoic acid, respectively.

Analyzing the structures, functions and evolution of two abundant gastrointestinal fatty acid binding proteins with recombinant DNA and computational techniques

The structures of intestinal and liver fatty acid binding proteins (FABPs) have been determined from an analysis of the nucleotide sequences of cloned cDNAs. The primary translation product of intestinal FABP mRNA contains 132 residues (Mr = 15 124). Liver FABP mRNA encodes a 127 amino acid polypeptide (Mr = 14 273). In vitro co-translational cleavage and translocation assays showed that neither sequence has a cleavable signal peptide or signal peptide equivalent - suggesting that the FABPs do not enter the secretory apparatus but rather are targeted to the cytoplasm. A variety of computational techniques were used to compare the two FABP sequences. The results indicate that liver and intestinal FABP are paralogous homologues. A superfamily of proteins was defined which includes the FABPs, the cellular retinol and retinoic acid binding proteins, the P2 protein of peripheral nerve myelin, and a polypeptide known as 422 whose synthesis is induced during differentiation of 3T3-L1 cells to adipocytes. No sequence homologies were noted between any of these small molecular weight cytosolic proteins and nonspecific lipid transfer protein (sterol carrier protein 2), phosphatidylcholine transfer protein, serum albumin or apolipoprotein AI. The FABPs may have structural features responsible for lipid-protein interactions that are not present in these non-homologous sequences. The distribution of intestinal and liver FABP mRNAs in adult rat tissues and the changes in FABP gene expression which occur during gastrointestinal development support the notion that these proteins are involved in fatty acid uptake, transport and/or compartmentalization. However, differences in tissue distribution and periods of non-coordinate expression during gastrointestinal ontogeny suggest that the two FABPs have distinct functions. The relationship between intestinal and liver FABPs and similar sized cytosolic FABPs isolated from brain, skeletal and cardiac muscle remains unclear. Recombinant DNA techniques combined with comparative sequence analyses offer a useful approach for defining unique as well as general structure-function relationships in this group of fatty acid binding proteins.

Homology of beta-lactoglobulin, serum retinol-binding protein, and protein HC

The milk protein beta-lactoglobulin has been extensively studied but its function has not been identified. A clue regarding the function of a protein can be obtained by discovering a genetic relationship with a protein of known function through comparisons of amino acid sequence. Such comparisons revealed that beta-lactoglobulin is similar to human serum retinol-binding protein and to another human protein of unknown function known as complex-forming glycoprotein heterogeneous in charge (protein HC). beta-Lactoglobulins from several species have been found to bind retinol, while the absorption and fluorescence properties reported for the unidentified heterogeneous prosthetic group of protein HC are retinoid-like. The role of serum retinol-binding protein in vitamin A transport in the circulation suggests that the other two homologous proteins may function in the binding and transport of retinoids; beta-lactoglobulin may facilitate the absorption of vitamin A from milk and protein HC may mediate the excretion of retinol-derived metabolites.

Plasma and cellular retinoid-binding proteins and transthyretin (prealbumin) are all localized in the islets of Langerhans in the rat

The immunohistochemical localization of plasma retinol-binding protein (RBP), cellular retinol-binding protein (CRBP), and transthyretin (TTR) was studied in rat pancreas. The studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method. Specific immunostaining for each of these three proteins was found localized to the islets of Langerhans. Both RBP and CRBP were localized in cells that were peripherally distributed within the islets, with an anatomic distribution that resembled that of the glucagon-containing A cells. Immunoreactive TTR was localized in cells that were more centrally distributed in the islets, with an anatomic distribution that resembled that of the insulin-containing B cells. These findings were confirmed by radioimmunoassay of a homogenate of isolated rat islets. By using sensitive and specific radioimmunoassays for each antigen, unusually high levels of CRBP, RBP, TTR, and cellular retinoic acid-binding protein (CRABP) were found in rat islets. The physiological significance of the localization of RBP, CRBP, CRABP, and TTR in the islets is not known. The findings suggest that retinoids and their binding proteins may play important metabolic roles within islet cells, and hence that they may be involved in some way in the biological, endocrine, function of the islets.

Ultrastructural changes in the organ of Corti and in the ganglion spiral cochleae after vitamin A deficiency

Guinea pigs were fed a vitamin A free diet and the cochleae of these animals were studied with the light and electron microscope. No changes could be found after ten weeks of vitamin A deficiency, but after 19 weeks the experimental animals showed a slight hypertrophy of the otic capsule, an accumulation of lysosomes in the sensory cells and a disruption and disintegration of the Hensen body. In the ganglion spiral cochleae ganglion cells appeared, filled with different myelin figures or honeycomb-like inclusions. Furthermore, the myelin sheath was split and sometimes filled with circular lamellar complexes and glycogen. These morphological findings together with the evidence of high concentrations of vitamin A in the organ of Corti support the hypothesis that vitamin A may be important for the function of the n. acusticus.

Retinoid suppression of transglutaminase activity and envelope competence in cultured human epidermal carcinoma cells. Hydrocortisone is a potent antagonist or retinyl acetate but not retinoic acid

Growth of SCC-13 squamous carcinoma cultures in the presence of retinoids considerably reduced the expression of two differentiation markers, the cellular capability to form cross-linked envelopes, and the enzyme transglutaminase required for cross-linking. A limited survey of retinoids showed that all-trans retinoic acid, 13-cis retinoic acid, and arotinoid Ro 13-6298 were highly effective in the absence of hydrocortisone and were only slightly antagonized by its presence in the medium. In contrast, retinyl acetate, retinol, and retinol bound to its plasma binding protein were quite active in the absence of hydrocortisone but were essentially inactive in its presence. Dexamethasone was also highly effective in antagonizing the suppressive action of retinyl acetate on envelope formation, while the corticosteroid antagonists cortexolone and progesterone were inactive. These results suggest that there are separate pathways, which are differentially regulated by hydrocortisone, for either the metabolism or action of retinol and retinoic acid in SCC-13 cells.

Purification and characterization of a basic 23 kDa cytosolic protein from bovine brain

A soluble basic protein has been purified from bovine brain. It is constituted by a single polypeptide chain with a molecular weight of about 23 kDa and an isoelectric point of about 8.6. The protein was further characterized by its amino-acid composition and by a 39 amino-acid-long N-terminal sequence. Sequence homologies were demonstrated with some other cytosolic proteins. Ligand-binding assays revealed a significant affinity of the 23 kDa protein for bromosulfophthalein. Immunochemical analysis using a rabbit anti 23 kDa brain protein antiserum demonstrated its simultaneous presence in bovine liver as well as in soluble extracts from different origins (mouse and rat brain; human platelets).

Crystallization of human plasma apo-retinol-binding protein

Crystals of three forms of human plasma apo-retinol-binding protein have been obtained using the procedure described for the holoprotein. The apoprotein was prepared by a novel method, which uses hydrophobic interaction and immobilized dye chromatography. The three forms were separated by fast protein liquid chromatography. All of the crystals are isomorphous and diffract to 2.5 A resolution. These crystals will be useful for studies of the mechanism of binding of retinol to its carrier using X-ray diffraction techniques.

Amino-terminal amino acid sequence of the nonspecific phospholipid exchange protein from bovine liver

The amino-terminal amino acid sequence of the nonspecific phospholipid exchange protein from bovine liver has been determined. The first 52 amino-terminal residues in the sequence were identified. The sequence determined failed to show statistically significant homology to any previously published protein sequence. However, a stretch of 12 amino acids at the end of the sequence displays homology to the phosphatidylcholine-specific phospholipid exchange protein.

Structure and tissue distribution of some retinoid-binding proteins

Vitamin A has, apart from its function in the visual pigments, general effects on several organs. Early signs of vitamin A deficiency include keratinization of epithelia and hyperkeratosis of the skin. To elucidate a generalized function for vitamin A, we have taken the approach of tracing the vitamin from its storage site in the liver via its blood transport by the retinol-binding protein (RBP) to its uptake by susceptible cells. We have also examined the intracellular occurrence of vitamin A as regards its binding to specific receptor proteins. Here we summarize data on the amino acid sequences of several vitamin A-binding proteins. The finding that CRBP and CRABP, the two intracellular proteins, are homologous to each other, to a myelin protein, and to a fatty acid-binding protein may shed light on the functions of these proteins. Retinoic acid, which binds to CRABP but not CRBP, induces differentiation of teratocarcinoma cells. This is accompanied by a lowering of the CRABP concentration, an increase of the CRBP level, and an increase in the uptake of retinol from RBP. The epidermis contains both CRBP and CRABP, and their distributions are rather similar. However, in contrast to CRBP, CRABP is most abundant in cells lining the hair follicles. CRBP occurs in greatest relative amounts in the outer layers of the epidermis. Since techniques have been developed to measure CRBP and CRABP, normal and disease-affected skin may now be explored as to quantity and cellular distribution of the retinoid-binding proteins.

Mutually antagonistic effects of hydrocortisone and retinyl acetate on envelope competence in cultured malignant human keratinocytes

Serially propagated SCC-13 keratinocytes, derived from a human squamous cell carcinoma, are greatly influenced by culture conditions in their ability to form ionophore-inducible cross-linked envelopes. Supplementation of the growth medium with fetal bovine serum at concentrations ranging from 0.5 to 20 percent had little effect on competence to form envelopes in confluent cultures. At each serum concentration, however, addition of hydrocortisone to the medium led to an increase in competence of almost fourfold, from approximately 20 to nearly 80 percent. With the serum supplementation held at 5 percent, addition of retinyl acetate to the medium suppressed competence in a concentration-dependent manner over the range of 1 to 100 ng/ml. At the highest concentration employed, competence was reduced over fourfold in the presence of hydrocortisone and virtually eliminated in its absence. When the cells were grown using serum depleted of endogenous vitamin A, a majority were competent in the absence of hydrocortisone. Under this condition, retinyl acetate suppressed competence over fivefold in the absence of hydrocortisone, but not at all in its presence. We conclude that hydrocortisone stimulates envelope competence primarily by antagonizing the suppressive effect of vitamin A. The SCC-13 cell line may prove valuable in studying mechanisms of retinoid and corticosteroid therapeutic action on diseased human keratinocytes.

Ligand-dependent regulation of intracellular protein transport: effect of vitamin a on the secretion of the retinol-binding protein

As a model of ligand-dependent protein secretion the biosynthesis, intracellular transport, and release of the retinol-binding protein (RBP) were studied in primary cultures of rat hepatocytes pulse-labeled with [35S]methionine. After various periods of chase RBP was isolated by immunoprecipitation and identified by SDS PAGE. Both normal and vitamin A-deficient hepatocytes synthesized RBP. The normal cells secreted the pulse-labeled RBP within 2 h. RBP synthesized by deficient cells was not secreted, and intracellular degradation of the protein appeared to be slow. Deficient cells could be induced to secrete RBP on the addition of retinol to the culture medium. This occurred also after protein synthesis had been blocked by cycloheximide. Since retinol induces the secretion of RBP, accumulated in the endoplasmic reticulum (ER), it seems reasonable to conclude that the transport of RBP from the ER to the Golgi complex is regulated by retinol.

The influence of retinoids on cultivated human keratinocytes

Cultured keratinocytes afford an excellent opportunity to study the influence of retinoids on the behavior of a stratified squamous epithelium and the interaction of keratinocytes with substrate. We have found that all-trans-retinoic acid retards the formation of colonies, dose not influence attachment, and causes increased shedding of cells from the cultures. Retinoids do not influence the relative abundance of the keratin polypeptides. Our observations are on human neonatal foreskin-derived keratinocytes grown in Dulbecco's modified Eagle medium containing 20% fetal bovine serum. Because fetal bovine serum contains vitamin A, our findings represent differences between low and high levels of vitamin A.