Factors affecting the concentration of free holo retinol-binding protein in human plasma

The Retinol Circulating Complex Releases Hormonal Ligands During Acute Stress Disorders

Intensive care workers actively participate in very hot debates aiming at defining the true metabolic, hormonal and nutritional requirements of critically ill patients, the contributory roles played by thyroid and retinoid ligands being largely underestimated. The present article makes up for redressing the balance on behalf of these last hormonal compounds. The retinol circulating complex is transported in the bloodstream in the form of a trimolecular edifice made up of transthyretin (TTR), retinol-binding protein (RBP) and its retinol ligand. TTR reflects the size of the lean body mass (LBM) and is one of the 3 carrier-proteins of thyroid hormones whereas RBP is the sole conveyor of retinol in human plasma. In acute inflammatory disorders, both TTR and RBP analytes experience abrupt cytokine-induced suppressed hepatic synthesis whose amplitude is dependent on the duration and severity of the inflammatory burden. The steep drop in TTR and RBP plasma values releases thyroxine and retinol ligands in their physiologically active forms, creating free pools estimated to be 10-20 times larger than those described in healthy subjects. The peak endocrine influence is reached on day 4 and the freed ligands undergo instant cellular overconsumption and urinary leakage of unmetabolized fractions. As a result of these transient hyperthyroid and hyperretinoid states, helpful stimulatory and/or inhibitory processes are set in motion, operating as second frontlines fine-tuning the impulses primarily initiated by cytokines. The data explain why preexisting protein malnutrition, as assessed by subnormal LBM and TTR values, impairs the development of appropriate recovery processes in critically ill patients. These findings have survival implications, emphasizing the need for more adapted therapeutic strategies in intensive care units.

The role of CYP26 enzymes in retinoic acid clearance

Retinoic acid (RA) is a critical signaling molecule that regulates gene transcription and the cell cycle. Understanding of RA signaling has increased dramatically over the past decades, but the connection between whole body RA homeostasis and gene regulation in individual cells is still unclear. It has been proposed that cytochrome P450 family 26 (CYP26) enzymes have a role in determining the cellular exposure to RA by inactivating RA in cells that do not need RA. The CYP26 enzymes have been shown to metabolize RA efficiently and they are also inducible by RA in selected systems. However, their expression patterns in different cell types and a mechanistic understanding of their function is still lacking. Based on preliminary kinetic data and protein expression levels, one may predict that if CYP26A1 is expressed in the liver at even very low levels, it will be the major RA hydroxylase in this tissue. As such, it is an attractive pharmacological target for drug development when one aims to increase circulating or cellular RA concentrations. To further the understanding of how CYP26 enzymes contribute to the regulation of RA homeostasis, structural information of the CYP26s, commercially available recombinant enzymes and good specific and sensitive antibodies are needed.

The concentration of free holo-retinol binding protein is higher in vitamin A-sufficient than in deficient Nepalese women in late pregnancy

Free holo-retinol binding protein (RBP) [i.e., unbound to transthyretin (TTR)] plays a role in transporting vitamin A across the placenta during pregnancy. In a cross-sectional study of clinically healthy urban women, we assessed the association among clinical and biochemical factors on estimated concentrations of free holo-RBP during the last trimester of pregnancy. Serum samples obtained from a subsample of women (n = 259), who had participated in the Night Vision Threshold Test study in Nepal, were analyzed for determinations of retinol by HPLC, and RBP, TTR, and alpha-1 acid glycoprotein by radial immunodiffusion. Free holo-RBP concentrations were calculated using dissociation constants for free holo- and apo-RBP. Among these women, 30% were vitamin A deficient based on either the RBP:TTR index < or = 0.36 or serum retinol < 1.05 micromol/L. Using stepwise regression analyses, the RBP:TTR index explained 75% of the variance in free holo-RBP concentrations, whereas retinol explained only 14%. Women were classified as vitamin A sufficient (n = 185) or deficient (n = 74) using the RBP:TTR index and were stratified into 3 gestational groups (I: 24-28 wk, II: 29-33 wk, III: >33 wk). Concentrations of free holo-RBP were higher in vitamin A-sufficient women than in vitamin A-deficient women (mean +/- SEM, 48.1 +/- 1.2 vs. 27.6 +/- 0.8 nmol/L; P < 0.001), and in a 3 x 2 factorial analysis, the interaction between gestational group and vitamin A status was significant. These results demonstrate that the RBP:TTR index is a useful proxy for free holo-RBP concentration and that vitamin A status affects its distribution.

Transcytosis of retinol-binding protein across renal proximal tubule cells after megalin (gp 330)-mediated endocytosis

Plasma retinol-binding protein (RBP) combined with vitamin A (retinol) is partially filtered through the glomerulus and then absorbed by proximal tubule cells, leading to recycling of retinol to the circulation. Recently, it was shown that reabsorption of RBP-retinol complexes by proximal tubule cells is mediated by megalin (gp 330), an apical endocytic receptor. It was proposed that RBP is transported by megalin to lysosomes, where it is degraded, thus liberating retinol, which then combines with newly synthesized RBP to be secreted into the bloodstream. This study shows that passage of RBP through immortalized rat renal proximal tubule (IRPT) cells occurs by transcytosis after megalin-mediated endocytosis, which provides an alternative pathway for recycling of retinol. IRPT cells cultured as polarized monolayers with tight junctions were used on permeable filters in the upper chamber of dual-chambered devices, with megalin expression exclusively on the upper surface. After addition of RBP to the upper chamber and incubation at 37 degrees C, intact RBP was found in fluids that were collected from the lower chamber. In contrast, control substances (mannitol, lysozyme, albumin, and glutathione-S-transferase) were not appreciably transported across IRPT cells, indicating that passage of RBP was by transcytosis and not by paracellular leakage. Confocal microscopy analysis of IRPT cells after addition of RBP to the upper chamber revealed RBP-containing granules at the apical membrane, subapically, and also at basolateral membranes. When RBP was added to IRPT cells together with megalin competitors, the amount of transcytosed RBP was markedly reduced. We also found that some RBP was internalized and degraded by IRPT cells, but this process was not appreciably affected by megalin competitors, indicating that RBP endocytosed by megalin was not transported to lysosomes and degraded but rather transcytosed across IRPT cells.

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

Effect of low-calorie diets on plasma retinol-binding protein concentrations in overweight women

The concentrations of total protein, albumin and retinol-binding protein, a major transport protein for vitamin A, are significantly decreased by protein-calorie malnutrition. Weight-loss diets, sometimes involving severe energy deficits over prolonged periods of time, are common in the United States. The effect, if any, of prolonged low calorie weight-loss diets with normal intakes of protein on albumin, total protein and retinol-binding protein concentrations (and potentially on vitamin A metabolism) had not been extensively studied. We measured total protein, albumin, apo + holo retinol-binding protein and holo-free- and holo-transthyretin-bound retinol-binding protein concentrations during the course of a nutritionally adequate weight-loss diet (50% calorie restriction). We found that this type of dieting did not affect total protein, albumin or apo + holo, holo-free or holo-transthretin-bound retinol-binding protein concentrations significantly. This suggests that protein intake is more critical than caloric intake for retinol-binding protein status.

Plasma retinol transport system and taste acuity in patients with obstructive jaundice

A study was conducted to elucidate the correlation between the plasma retinol transport system and taste acuity in patients with obstructive jaundice (OJ). Plasma levels of retinol, retinol-binding protein (RBP), transthyretin (TTR) and holo-RBP (retinol-RBP complex unbound to TTR), as well as the threshold of taste acuity, were determined in 8 cases with OJ (6 cases with common bile duct cancer and 2 cases with pancreas head cancer) and in 20 apparently healthy volunteers. These parameters were also measured serially in patients with OJ before and after percutaneous transhepatic biliary drainage (PTBD). Plasma levels of retinol, RBP and TTR were significantly decreased in every patients with OJ as compared with healthy controls, whereas changes in holo-RBP levels were not consistent. Taste acuity was found to be reduced in patients with decreased holo-RBP levels, while the acuity was preserved well in patients whose holo-RBP levels remained normal. Impaired taste acuity was rapidly improved in every patients after treatment with PTBD, significantly correlating with the recovery of plasma RBP and holo-RBP levels, and with the reduction of plasma total bilirubin levels. These results suggest that taste acuity is affected by the plasma retinol transport system, including serum levels of holo-RBP, from which retinol is delivered, presumably through a receptor-mediated manner, to the taste buds.

Behavior of prealbumin in the acute phase of polymyalgia rheumatica treated with 6-methylprednisolone

Serum levels of prealbumin, fibronectin, fibrinogen, alpha 1-acid glycoprotein, C-reactive protein, immunoglobulins, and white blood cell count were prospectively studied in 33 patients affected by polymyalgia rheumatica during the first 45 days of treatment with 6-methylprednisolone. Almost all parameters considered, except for fibronectin and IgM, settled within the normal range fairly quickly, while prealbumin showed a specular course compared with the other reactants. This behavior reflected the improvement of clinical symptoms registered in all patients after steroid treatment. Finally, the genesis of the low baseline prealbumin levels found in polymyalgia rheumatica/giant cell arteritis and their behavior during treatment are discussed.

Cellular retinol-binding protein in mucosa, in benign and malignant non-squamous cell tumours of the head and neck

The measurement and localisation of cellular retinol-binding protein (CRBP), in samples of normal oral mucosa, larynx papilloma and malignant lymphoma of the oropharynx, was performed with a radioimmunoassay and immunolocalisation techniques. As compared with CRBP concentration in normal mucosa, those in laryngeal papilloma were significantly higher, but those of malignant lymphoma were similar. CRBP concentrations were highest in maturing keratinocytes within the prickle cell layers of normal mucosa and in laryngeal papillomas, as estimated on the basis of immunoreactivity to CRBP. The retinyl palmitate concentrations in extracts of oral mucosa correlated to the retinol concentrations, both in plasma and mucosal extracts, but not to the CRBP content in mucosal extracts. The immunolocalisation of a cellular retinoic acid-binding protein (CRABP) like antigen in normal oral mucosa showed much the same picture with strongest staining of the maturing keratinocytes of the prickle cell layers.

Urinary excretion of albumin and retinol-binding protein during normal pregnancy

In a cross sectional study of 88 pregnant women urinary excretion of albumin, when expressed as a ratio to creatinine concentration, was not significantly different from that in a non-pregnant control group of similar age (p greater than 0.05) and did not change significantly during pregnancy. Only when albumin excretion was expressed as a fractional clearance was the urinary excretion significantly increased in the third trimester compared with the first trimester (p less than 0.05), although it was still not significantly different from that in the non-pregnant control group. Excretion of retinol-binding protein was significantly increased during all three trimesters of pregnancy (p less than 0.01 in each case) and more so in the second and third trimesters than in the first. It is concluded that the increased total protein excretion that has been described during pregnancy is not explained by an increased excretion of albumin which remains essentially normal. In contrast, the tubular absorption of proteins is decreased.

Quantitation and localization of cellular retinol-binding protein in squamous-cell carcinomas of the cervix uteri and of the oral cavity

The concentration of cellular retinol-binding protein (CRBP) was determined by radioimmunoassay in biopsies of normal mucosa and squamous-cell carcinomas of cervix uteri from 30 women. The tumour tissues contained significantly higher concentrations of CRBP (median = 120 micrograms/g protein) than normal mucosa (median = 32 micrograms/g protein). The distribution of CRBP in normal mucosa and squamous-cell carcinomas from cervix uteri and from oral cavity was evaluated by immunohistochemical techniques. In tissue sections of normal epithelium from the cervix uteri and the oral cavity, the maturing keratinocytes in the prickle-cell layers were moderately or strongly stained when antiserum against CRBP was used, while the proliferating cells in the basal-cell layer were stained only lightly if at all. Squamous-cell carcinomas of the cervix uteri and the oral cavity presented much the same picture. The observed difference in CRBP concentration between squamous-cell carcinomas and normal squamous-cell mucosa may therefore be more quantitative than qualitative.

Effect of renal insufficiency on the concentration of free retinol-binding protein in urine and serum

The concentration of retinol-binding protein (RBP) in urine was determined in 20 healthy individuals and 119 patients with various renal diseases involving tubular or glomerular dysfunction. The sera from 4 healthy individuals and 33 patients were chromatographed on Sephadex G-75 to measure the concentration of free (i.e. not prealbumin-bound) RBP. In healthy individuals, the mean concentration of free RBP in serum was 5.8 mg/l and represented 14% of total RBP; the renal clearance and the fractional tubular uptake of free RBP averaged 0.032 ml/min and 99.97%, respectively. In patients, the concentration of free RBP and the percentage of free RBP in serum were on logarithmic scales inversely correlated with the endogenous creatinine clearance (r = -0.80 and -0.76) and increased in parallel with the serum creatinine (r = 0.67 and 0.66) and beta 2-microglobulin concentrations (beta 2-m, r = 0.76 and 0.89). The semi-logarithmic plot of urine versus serum concentration of free RBP suggests a renal threshold for the tubular reabsorption of this protein at a concentration of about 25 mg/l in serum. The existence of this threshold is confirmed by the relationship between urinary RBP and serum beta 2-m showing that urinary excretion of RBP is invariably high when the serum level of beta 2-m exceeds 5 mg/l. The latter value corresponds precisely to the renal threshold for the tubular reabsorption of beta 2-m. The corresponding value for free RBP derived from the relationship between both proteins is 24 mg/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the spontaneous transfer of retinol from the retinol:retinol-binding protein complex to unilamellar liposomes

The transfer of retinol from its complex with the retinol-binding protein to cell surfaces was studied using unilamellar liposomes as a cell surface model. The transfer of retinol to liposomes at 37 degrees C was rapid and reached an apparent equilibrium within 60 min. The amount of retinol transferred to the liposomes at equilibrium was directly proportional to the starting concentration of retinol:retinol-binding protein over a wide range of retinol:retinol-binding protein concentrations and also directly proportional to the concentration of liposomal phospholipid in the system, when the concentration of retinol:retinol-binding protein was held constant. The transfer increased slightly with temperature. Transfer was increased by a factor of 1.8 at pH 4.5 compared to pH around 7. Prealbumin in amounts sufficient to complex all retinol:retinol-binding protein, decreased retinol transfer to liposomes indicating that prealbumin increases the affinity of retinol-binding protein for retinol. Addition of apo retinol-binding protein to the system decreased the transfer of retinol to liposomes considerably probably through competition with the liposomes for retinol. In similarly designed experiments delipidated bovine serum albumin competed much less with liposomes for retinol. The results show that spontaneous transfer of retinol from the retinol:retinol-binding protein complex to liposomal membranes occurs in vitro and suggests that a similar transfer may occur in vivo from retinol:retinol-binding protein to cell surface membranes.

Low plasma selenium as a risk factor for cancer death in middle-aged men

In a population study, 10,000 men (aged 46-48 years) were invited to a health screening program. At follow-up, which was up to eight years later, 61 subjects had died from cancer; from 35 of these subjects, plasma samples were available that were obtained at the initial screening. These samples, together with samples from two living controls for each case, were analyzed for selenium, retinol, cholesterol, triglyceride, and a number of plasma proteins. Plasma selenium was significantly lower (p less than 0.05) in cases than in controls (means: 1.06 vs. 1.12 mumol/l). The proportion of cases increased significantly from the highest to the lowest quintile of plasma selenium, and the relative risk for cancer death was 3.8 times higher in the lowest quintile compared with the highest. Mean plasma retinol was similar in cases (2.53 mumol/l) and controls (2.56 mumol/l). Cases and controls also had similar values for plasma cholesterol, triglyceride, uric acid, apolipoprotein B, orosomucoid, prealbumin, retinol-binding protein, and beta 2-microglobulin. Apolipoprotein AI in plasma was lower among cases (p less than 0.025). Cases smoked significantly more than controls did (p less than 0.05). Data indicate that low plasma selenium was a risk factor for cancer death in middle-aged men who lived in the same area. Further studies are necessary to establish whether differences in selenium intake, selenium metabolism, or other factors related to selenium are responsible for the relations observed. At present, the available data do not justify selenium supplementation programs in the whole population.

Studies of cellular retinol-binding protein (CRBP) in squamous-cell carcinomas of the head and neck region

Cellular retinol-binding protein (CRBP) concentration was determined by radioimmunoassay in biopsies from normal mucosa and squamous-cell cancers of the head and neck region in 26 patients. The plasma concentrations of retinol, retinol-binding protein (RBP), prealbumin, albumin, orosomucoid and alfa1-antitrypsin were also determined. In all patients the tumours contained significantly higher concentrations of CRBP (median = 185 micrograms/g tissue protein; range: 27-1,017 micrograms/g) than normal mucosa (median = 14 micrograms/g tissue protein; range 6-97 micrograms/g). CRBP could not be detected in patient plasma. The tumor/normal mucosa CRBP concentration ratio showed a significant inverse correlation to a histopathological malignancy grade score evaluating the tumour-host relationship, suggesting that tumours with high CRBP relative to normal mucosa are biologically less aggressive. Tumour CRBP showed no correlation either to CRBP concentration in normal mucosa, or to plasma retinol or plasma retinol-binding protein concentration. CRBP concentration in normal mucosa, however, showed a significant correlation to plasma retinol-binding protein concentration. Most of the patients had low levels of plasma retinol and retinol-binding protein compared to matched controls. Whether this has any relationship to the development of the tumour, or whether the active inflammation induced by the cancer leads to a low plasma retinol concentration, is unknown.

Vitamin A in skin and serum--studies of acne vulgaris, atopic dermatitis, ichthyosis vulgaris and lichen planus

The concentrations of vitamin A and total carotenoids were measured in serum and skin of 61 patients with acne vulgaris, atopic dermatitis, ichthyosis vulgaris or lichen planus, and compared with those in 37 healthy subjects. The mean serum concentrations of retinol and retinol-binding protein were significantly decreased in patients with acne (P less than 0.01) and slightly increased in those with ichthyosis (P less than 0.05), but were otherwise normal. Serum carotenoid levels did not differ between patients and controls. Superficial shave biopsies from both involved and uninvolved skin were examined for the presence of retinol (vitamin A1), dehydroretinol (vitamin A2) and total carotenoids. The mean retinol concentration was increased in lichen planus lesions (P less than 0.05) and decreased in both acne skin (involved and uninvolved) and in lesions of atopic dermatitis (P less than 0.05). The mean dehydroretinol concentration was markedly increased in lesions of atopic dermatitis and lichen planus (P less than 0.01). No consistent abnormalities were found in skin of patients with ichthyosis vulgaris. The mean carotenoid concentration in the patients' skin did not differ significantly from that in the controls. The reduced retinol level in the skin of acne patients is probably explained by diminished supply of vitamin A from the blood. The abnormal ratio of retinol to dehydroretinol in lesions of lichen planus and atopic dermatitis is possibly due to changes in cutaneous vitamin A metabolism associated with epidermal hyperproliferation and inflammation.

Rates of synthesis of prealbumin and retinol-binding protein during acute inflammation in the rat

The rates of synthesis of prealbumin (PA), retinol-binding protein (RBP), and other plasma proteins were measured in primary monolayer cultures of rat hepatocytes isolated from normal rats and from rats 18 h after induction of an inflammatory reaction by subcutaneous injection of croton oil. The inflammatory pattern of protein synthesis seemed to persist in the isolated hepatocytes for 1-2 days. This pattern included significantly decreased rates of synthesis of PA. The rate of synthesis of RBP was probably also decreased, but significantly less than the rate of PA synthesis. The results support the idea that it is mainly the decreased rate of PA synthesis which is responsible for the decreased plasma concentration of PA, and its ligand RBP and retinol during inflammation.