Effect of retinoic acid and apo-RBP on serum retinol concentration in acute renal failure

Vitamin A: potential misclassification of vitamin A status among patients with type 2 diabetes and hypertension in urban Ghana

BACKGROUND

Sub-Saharan Africa is facing a double burden of malnutrition: vitamin A deficiency (VAD) prevails, whereas the nutrition-related chronic conditions type 2 diabetes (T2D) and hypertension are emerging. Serum retinol—a VAD marker—increases in kidney disease and decreases in inflammation, which can partly be attributed to alterations in the vitamin A-transport proteins retinol-binding protein 4 (RBP4) and prealbumin. Kidney dysfunction and inflammation commonly accompany T2D and hypertension.

OBJECTIVE

Among urban Ghanaians, we investigated the associations of T2D and hypertension with serum retinol as well as the importance of kidney function and inflammation in this regard.

DESIGN

A hospital-based, case-control study in individuals for risk factors of T2D, hypertension, or both was conducted in Kumasi, Ghana (328 controls, 197 with T2D, 354 with hypertension, and 340 with T2D plus hypertension). In 1219 blood samples, serum retinol, RBP4, and prealbumin were measured. Urinary albumin and estimated glomerular filtration rate (eGFR) defined kidney function. C-reactive protein (CRP) >5 mg/L indicated inflammation. We identified associations of T2D and hypertension with retinol by linear regression and calculated the contribution of RBP4, prealbumin, urinary albumin, eGFR, and CRP to these associations as the percentages of the explained variance of retinol.

RESULTS

VAD (retinol <1.05 μmol/L) was present in 10% of this predominantly female, middle-aged, overweight, and deprived population. Hypertension, but not T2D, was positively associated with retinol (β: 0.12; 95% CI: 0.08, 0.17), adjusted for age, sex, socioeconomic factors, anthropometric measurements, and lifestyle. In addition to RBP4 (72%) and prealbumin (22%), the effect of increased retinol on individuals with hypertension was mainly attributed to impaired kidney function (eGFR: 30%; urinary albumin: 5%) but not to inflammation.

CONCLUSIONS

In patients with hypertension, VAD might be underestimated because of increased serum retinol in the context of kidney dysfunction. Thus, the interpretation of serum retinol in sub-Saharan Africa should account for hypertension status.

Hypervitaminosis A is prevalent in children with CKD and contributes to hypercalcemia

BACKGROUND

Vitamin A accumulates in renal failure, but the prevalence of hypervitaminosis A in children with predialysis chronic kidney disease (CKD) is not known. Hypervitaminosis A has been associated with hypercalcemia. In this study we compared dietary vitamin A intake with serum retinoid levels and their associations with hypercalcemia.

METHODS

We studied the relationship between vitamin A intake, serum retinoid levels, and serum calcium in 105 children with CKD stages 2-5 on dialysis and posttransplant. Serum retinoid measures included retinol (ROH), its active retinoic acid (RA) metabolites [all-trans RA (at-RA) and 13-cis RA] and carrier proteins [retinol-binding protein-4 (RBP4) and transthyretin (TTR)]. Dietary vitamin A intake was assessed using a food diary.

RESULTS

Twenty-five children were in CKD 2-3, 35 in CKD 4-5, 23 on dialysis and 22 posttransplant; 53 % had vitamin A intake above the Reference Nutrient Intake (RNI) value. Children receiving supplemental feeds compared with diet alone had higher vitamin A intake (p = 0.02) and higher serum ROH (p < 0.001). Notably, increased ROH was seen as early as CKD stage 2. For every 10 ml/min/1.73 m(2) fall in estimated glomerular filtration rate (eGFR), there was a 13 % increase in ROH. RBP4 levels were increased in CKD 3-5 and dialysis patients. The lowest ratios of ROH:RBP4 were seen in dialysis compared with CKD 2-3 (p = 0.03), suggesting a relative increase in circulating RBP4. Serum ROH, RBP4 and at-RA were associated with serum calcium. On multivariable analysis RBP4 levels and alfacalcidol dose were significant predictors of serum calcium (model R (2) 32 %) in dialysis patients.

CONCLUSIONS

Hypervitaminosis A is seen in early CKD, with highest levels in children on supplemental feeds compared with diet alone. Serum retinoid levels significantly predict hypercalcemia.

Characterization of the vitamin A transport in preterm infants after repeated high-dose vitamin A injections

BACKGROUND/OBJECTIVES

Preterm infants have low vitamin A stores at birth, and parenteral administration of high-dose vitamin A reduces pulmonary morbidity. The aim was to characterize vitamin A transport and status.

SUBJECTS/METHODS

Prospective study of 69 preterm infants (median birth weight 995 g, gestational age 28 weeks), in which 51 received 5000 IU vitamin A three times per week intramuscular (i.m.) for 4 weeks and 18 infants without i.m. vitamin A served as controls. Serum retinol, retinyl palmitate, total retinol-binding protein 4 (RBP4), retinol-unbound RBP4 (apo-RBP4) and transthyretin concentrations were determined at days 3 (D3) and 28 (D28) of life.

RESULTS

D3 retinol concentrations were low for the entire group (382 (285/531) nmol/l; median/interquartile range) and unrelated to gestational age. D28 retinol was unchanged in controls (382 (280/471) nmol/l), but increased in the vitamin A group (596 (480/825) nmol/l; P<0.001). A similar pattern was observed for RBP4. The calculated retinol-to-RBP4 ratio rose in vitamin A infants (D3: 0.81 (0.57/0.94), D28: 0.98 (0.77/1.26); P<0.01) but not in controls. In the vitamin A group, the retinol-to-RBP4 ratio was >1 in 15% of all infants on D3 and in 45% of infants on D28, but was ⩽1 in all, but one, controls on D28.

CONCLUSIONS

In preterm infants receiving a 4-week course of high-dose i.m. vitamin A, serum retinol concentrations increased by 55%, with molar concentrations of retinol exceeding those of RBP4 in 45% of the infants suggesting transport mechanisms other than RBP4.

Inhibition of retinoic acid biosynthesis by the bisdichloroacetyldiamine WIN 18,446 markedly suppresses spermatogenesis and alters retinoid metabolism in mice

Knowledge of the regulation of testicular retinoic acid synthesis is crucial for understanding its role in spermatogenesis. Bisdichloroacetyldiamines strongly inhibit spermatogenesis. We reported previously that one of these compounds, WIN 18,446, potently inhibited spermatogenesis in rabbits by inhibiting retinoic acid synthesis. To understand how WIN 18,446 inhibits retinoic acid synthesis, we characterized its effects on human retinal dehydrogenase ALDH1A2 in vitro as well as its effects on retinoid metabolism in vivo using mice. WIN 18,446 strongly and irreversibly inhibited ALDH1A2 in vitro. In vivo, WIN 18,446 treatment completely abolished spermatogenesis after 4 weeks of treatment and modestly reduced adiposity in mice fed a chow diet. Effects of WIN 18,446 on retinoid concentrations were tissue-dependent. Although lung and liver retinyl ester concentrations were lower in WIN 18,446-treated animals, adipose retinyl ester levels were increased following the treatment. Interestingly, animals treated with WIN 18,446 had significantly higher circulating retinol concentrations compared with control mice. The effect on spermatogenesis by WIN 18,446 was not prevented by simultaneous treatment with retinoic acid, whereas effects on other tissues were partially or completely reversed. Cessation of WIN 18,446 treatment for 4 weeks reversed most retinoid-related phenotypes except for inhibition of spermatogenesis. Our data suggest that WIN 18,446 may be a useful model of systemic acquired retinoic acid deficiency. Given the effects observed in our study, inhibition of retinoic acid biosynthesis may have relevance for the treatment of obesity and in the development of novel male contraceptives.

Vitamin A metabolism is changed in donors after living-kidney transplantation: an observational study

Background

The kidneys are essential for the metabolism of vitamin A (retinol) and its transport proteins retinol-binding protein 4 (RBP4) and transthyretin. Little is known about changes in serum concentration after living donor kidney transplantation (LDKT) as a consequence of unilateral nephrectomy; although an association of these parameters with the risk of cardiovascular diseases and insulin resistance has been suggested. Therefore we analyzed the concentration of retinol, RBP4, apoRBP4 and transthyretin in serum of 20 living-kidney donors and respective recipients at baseline as well as 6 weeks and 6 months after LDKT.

Results

As a consequence of LDKT, the kidney function of recipients was improved while the kidney function of donors was moderately reduced within 6 weeks after LDKT. With regard to vitamin A metabolism, the recipients revealed higher levels of retinol, RBP4, transthyretin and apoRBP4 before LDKT in comparison to donors. After LDKT, the levels of all four parameters decreased in serum of the recipients, while retinol, RBP4 as well as apoRBP4 serum levels of donors increased and remained increased during the follow-up period of 6 months.

Conclusion

LDKT is generally regarded as beneficial for allograft recipients and not particularly detrimental for the donors. However, it could be demonstrated in this study that a moderate reduction of kidney function by unilateral nephrectomy, resulted in an imbalance of components of vitamin A metabolism with a significant increase of retinol and RBP4 and apoRBP4 concentration in serum of donors.

Serum retinol binding protein 4 level is related with renal functions in Type 2 diabetes

AIM

We aimed to evaluate the metabolic parameters and diabetes complications which would probably affect the serum retinol-binding protein 4 (RBP4) levels in Type 2 diabetic individuals. In addition to serum RBP4 concentration, the levels of its ligands, serum retinol and transthyretin (TTR) were also considered in this evaluation.

SUBJECTS AND METHODS

Serum RBP4, retinol, and TTR levels were measured in 53 Type 2 diabetic subjects and 30 body mass index (BMI)- matched controls. The molar ratios of RBP4 to retinol and RBP4 to TTR were compared.

RESULTS

While the RBP4 values were similar to those in the control group in Type 2 diabetic patients, the molar ratio of RBP4 to TTR was found to be higher than that of the control group. The serum RBP4 levels in patients who had retinopathy and macrovascular disease were similar to those in patients who did not. However, the RBP4 levels, molar ratios of RBP4 to retinol and RBP4 to TTR in micro- macroalbuminuric patients were found to be significantly higher than in normoalbuminuric subjects and controls. There was no correlation between the RBP4 levels and the patients' age, BMI, duration of diabetes, LDL, triglyceride, serum creatinine, and glycated hemoglobin values. Micro-macroalbuminuria and estimated glomerular filtration rate were independent determinants for increased serum RBP4 levels.

CONCLUSION

According to the data obtained from this study, diabetic retinopathy and cardiovascular complications do not affect the serum RBP4 level in Type 2 diabetes. Renal functions rather than the metabolic factors of diabetes determine the RBP4 level and its relation with its ligands.

The effects of postnatal retinoic acid administration on nephron endowment in the preterm baboon kidney

Administration of retinoic acid (RA), the active metabolite of vitamin A, is linked to the stimulation of nephrogenesis. The aim of this study was to determine whether early postnatal administration of RA could enhance ongoing nephrogenesis in a baboon model of premature birth. Unbiased stereological methods were used to estimate kidney volume, renal corpuscle volume, and nephron number. The percentage of abnormal glomeruli and the number of glomerular generations was also determined in the kidneys of preterm control (n = 6) and preterm +RA (n = 6) animals that received 500 microg/kg/d of all-trans RA after premature delivery. There was no significant difference between the preterm control and the preterm +RA groups in kidney size, nephron number (preterm control: 329,924 +/- 41,752; preterm +RA: 354,041 +/- 52,095; p = 0.59), renal corpuscle volume, number of glomerular generations, or the percentage of abnormal glomeruli. The proportion of abnormal glomeruli did not appear to be linked to any elements of postnatal care examined. The results of this study indicate that early postnatal administration of RA is unable to stimulate nephrogenesis in the kidney of the preterm baboon. Encouragingly, it does not appear to have any adverse effects on kidney development.

Isoforms of retinol binding protein 4 (RBP4) are increased in chronic diseases of the kidney but not of the liver

Background

The levels of retinol-binding protein 4 (RBP4) – the carrier protein for Vitamin A in plasma – are tightly regulated under healthy circumstances. The kidney, the main site of RBP4 catabolism, contributes to an elevation of RBP4 levels during chronic kidney disease (CKD) whereas during chronic liver disease (CLD) RBP4 levels decrease. Little is known about RBP4 isoforms including apo-RBP4, holo-RBP4 as well as RBP4 truncated at the C-terminus (RBP4-L and RBP4-LL) except that RBP4 isoforms have been reported to be increased in hemodialysis patients. Since it is not known whether CLD influence RBP4 isoforms, we investigated RBP4 levels, apo- and holo-RBP4 as well as RBP4-L and RBP4-LL in plasma of 36 patients suffering from CKD, in 55 CLD patients and in 50 control subjects. RBP4 was determined by ELISA and apo- and holo-RBP4 by native polyacrylamide gel electrophoresis (PAGE). RBP4-L and RBP4-LL were analyzed after immunoprecipitation by mass spectrometry (MALDI-TOF-MS).

Results

RBP4 isoforms and levels were highly increased in CKD patients compared to controls (P < 0.05) whereas in CLD patients RBP4 isoforms were not different from controls. In addition, in hepatic dysfunction RBP4 levels were decreased whereas the amount of isoforms was not affected.

Conclusion

The occurrence of RBP4 isoforms is not influenced by liver function but seems to be strongly related to kidney function and may therefore be important in investigating kidney function and related disorders.

Microalbuminuria is a major determinant of elevated plasma retinol-binding protein 4 in type 2 diabetic patients

Plasma retinol-binding protein 4 (RBP4) may be a new adipokine linked to obesity-induced insulin resistance and type 2 diabetes. The impact of diabetic nephropathy on plasma RBP4 levels, however, is not known. We tested the hypothesis that microalbuminuria is associated with elevated plasma concentrations of RBP4 in type 2 diabetic subjects. Retinol, its binding protein and transthyretin (TTR) were measured in the plasma and urine of 62 type 2 diabetic subjects, 26 of whom had microalbuminuria. The results were compared to 35 healthy control subjects. Despite no differences in plasma retinol, concentrations of the RBP4 were significantly elevated in plasma of diabetic patients and significantly higher in those with microalbuminuria. The higher plasma levels of the binding protein in subjects with microalbuminuria were accompanied by both significantly elevated plasma TTR and increased urinary levels of RBP4. There were no correlations of plasma-binding protein levels and parameters of insulin resistance. Our study suggests that plasma RBP4 levels in type 2 diabetic patients are affected by incipient nephropathy. Therefore, further studies evaluating RBP4 as a regulator of systemic insulin resistance and type 2 diabetes will need to take renal function into consideration.

Megalin-mediated reuptake of retinol in the kidneys of mice is essential for vitamin A homeostasis

The reuptake of retinol (ROH) and retinol-binding protein (RBP) in the kidneys is mediated by the endocytic receptor megalin, suggesting an important role for this receptor in vitamin A (VA) metabolism. We examined the extent to which megalin deficiency may affect urinary ROH excretion, levels of ROH and RBP in plasma, as well as storage of VA in liver and kidney. For this purpose, mice with a kidney-specific megalin gene defect (megalin(lox/lox); apoE(Cre)) and control mice (megalin(lox/lox)) were fed either a basal diet containing 4500 retinol equivalents (RE)/kg diet or a diet without VA during experimental periods of 42 and 84 d. Urinary ROH excretion was observed only in megalin(lox/lox); apoE(Cre) mice (P < 0.0001, 2-way ANOVA) and not in the controls. Plasma ROH and RBP differed only by diet (P < 0.05), but not genotype (P = 0.615). A major effect of megalin deficiency, however, was evident in retinyl ester levels in the liver (P < 0.05), which were approximately 37% lower than those in megalin(lox/lox) controls (P < 0.05, Student's t test) during the 84-d period of dietary VA deprivation. Kidney levels of VA were not affected by the receptor gene defect. The findings demonstrate that urinary ROH excretion caused by megalin deficiency requires accelerated mobilization of hepatic VA stores to maintain normal plasma ROH levels, which suggests that megalin plays an essential role in systemic VA homeostasis.

New aspects in vitamin a metabolism: the role of retinyl esters as systemic and local sources for retinol in mucous epithelia

Vitamin A and its active metabolites are important for growth and differentiation of a variety of cells, mainly in mucosa-associated epithelia, where they exhibit a wide spectrum of activities. Vitamin A, stored as retinyl esters (REs), is delivered from liver stores into the bloodstream as retinol bound to retinol binding protein. This process is regulated homeostatically, ending up in a more or less constant plasma retinol level. In situations of a high vitamin A demand (e.g., inflammation, diseases, prenatal period), this supply can be insufficient because of delayed production of retinol binding protein, leading to local deficiencies and impairment of structure and function in the respective tissues. This delay may be overcome by cellular RE stores. Several cell types, including buccal mucosa cells, can take up RE. Retinyl palmitate is taken up when it is applied topically to either metaplastically mutated rat vaginal epithelium (as a gel) or to human meta- and dysplastic bronchial epithelia (via inhalation) that have a vitamin A deficiency. In rats and humans, the modified epithelia can be normalized, at least in part. In conclusion, topically applied retinyl esters may be a promising therapy for local retinol deficiencies and may reverse the morphological alterations of the epithelium in tissues that are vitamin A deficient.

Retinol binding protein isolated from acute renal failure patients inhibits polymorphonuclear leucocyte functions

BACKGROUND

Protein factors accumulating in sera of patients with end-stage renal disease (ESRD) that interfere with the nonspecific immune response by inhibiting essential functions of polymorphonuclear leucocytes (PMNLs) have previously been described. No such factor has been isolated from acute renal failure (ARF) patients to date.

MATERIALS AND METHODS

Using a three-step chromatographic procedure involving ion exchange, size exclusion and hydrophobic interaction chromatography we purified the apo- and holo-form of retinol binding protein (RBP) from high-flux dialyser (polyacrylonitrile; AN69) ultrafiltrates of patients with ARF. Their effect on the chemotaxis of PMNLs isolated from healthy donors was determined by the under-agarose method. Whole-blood assays applying flow cytometry were used to assess phagocytosis and the oxidative metabolism of PMNLs. Apoptosis was assessed by determining the DNA content using propidium iodide.

RESULTS

Isolated apo- and holo-forms of RBP were truncated on their C-terminus as determined by mass spectrometry. All isolates significantly inhibited the chemotactic movement of PMNLs obtained from healthy donors and the PMNL oxidative metabolism stimulated by E. coli. These effects were concentration dependent. Retinol binding protein had no influence on the PMNL oxidative metabolism stimulated by PMA and on PMNL phagocytosis. Commercially available RBP isolated from urine influenced PMNL functions in the same way. Inhibition of p38 mitogen-activated protein kinase (MAPK) by SB203580 significantly attenuated the phagocytosis-induced respiratory burst and RBP did not lead to a further decrease. Polymorphonuclear leucocyte apoptosis was significantly inhibited by RBP.

CONCLUSIONS

The apo- and holo-forms of RBP isolated from the ultrafiltrate of ARF patients inhibit PMNL chemotaxis, oxidative metabolism and apoptosis. Therefore, RBP may be considered a uraemic toxin contributing to a disturbed immune defence.

Effects of chronic renal disease on the transport of vitamin A in plasma and urine of dogs

OBJECTIVE

To determine plasma and urine concentrations of retinol, retinyl esters, retinol-binding protein (RBP), and Tamm-Horsfall protein (THP) in dogs with chronic renal disease (CRD).

ANIMALS

17 dogs with naturally developing CRD and 21 healthy control dogs.

PROCEDURE

A diagnosis of CRD was established on the basis of clinical signs, plasma concentrations of creatinine and urea, and results of urinalysis. Concentrations of retinol and retinyl esters were measured by use of reverse-phase high-performance liquid chromatography. Concentrations of RBP and THP were measured by use of sensitive ELISA systems.

RESULTS

Dogs with CRD had higher plasma concentrations of retinol, which were not paralleled by differences in plasma concentrations of RBP. Calculated ratio of urinary total vitamin A (sum of concentrations of retinol and retinyl esters to creatinine concentration) and ratio of the concentration of urinary retinyl esters to creatinine concentration did not differ between groups. However, we detected a significantly higher retinol-to-creatinine ratio in the urine of dogs with CRD, which was paralleled by a higher urinary RBP-to-creatinine ratio. Thus, in dogs with CRD, the estimated fractional clearance of total vitamin A, retinol, and RBP was increased. Furthermore, dogs with CRD had a reduced urinary THP-to-creatinine ratio.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this study documented that CRD affects the concentrations of retinol in plasma and urine of dogs. Analysis of the data indicates that measurement of urinary RBP and urinary THP concentrations provides valuable information that can be helpful in follow-up monitoring of dogs with CRD.

Evidence for an essential role of megalin in transepithelial transport of retinol

Transepithelial transport of retinol is linked to retinol-binding protein (RBP), which is taken up and also synthesized in a number of epithelia. By immunocytochemistry of human, rat, and mouse renal proximal tubules, a strong staining in apical endocytic vacuoles, lysosomes, endoplasmic reticulum, Golgi, and basal vesicles was observed, in accordance with luminal endocytic uptake as well as a constitutive synthesis and basal secretion of RBP. Analysis of mice with target disruption of the gene for the major endocytic receptor of proximal tubules, megalin, revealed no RBP in proximal tubules of these mice. Western blotting and HPLC of the urine of the megalin-deficient mice instead revealed a highly increased urinary excretion of RBP and retinol, demonstrating that glomerular filtered RBP-retinol of megalin-deficient mice escapes uptake by proximal tubules. A direct megalin-mediated uptake of purified RBP-retinol was indicated by surface plasmon resonance analysis and uptake in immortalized rat yolk sac cells. Uptake was partially inhibited by a polyclonal megalin antibody and the receptor-associated protein. The present data show that the absence of RBP-binding megalin causes a significantly increased loss of RBP and retinol in the urine, demonstrating a crucial role of megalin in vitamin A homeostasis.

Retinoids regulate expression of the retinol-binding protein gene in hepatoma cells in culture

The expression of the gene coding for retinol-binding protein has been studied in a system of cultured human hepatoma cells exposed to retinoids. We report that the gene is positively modulated by retinol and retinoic acid in a time- and dose-dependent fashion. The stimulation at the mRNA level is paralleled by an increase of the corresponding protein that is secreted in the presence of the physiological ligand. An RBP-CAT chimeric gene, introduced by transfection, is also responsive to the treatment, showing the gene dose-dependency as the endogenous gene. These results demonstrate that retinoids up-regulate the RBP gene and that the control takes place at transcriptional level.