Binding protein for vitamin D and its metabolites in rat mesenteric lymph

Vitamin D and its binding protein in patients with leiomyomas

AIM

This study examined the levels of VitD, VitD binding protein (DBP), and free VitD in leiomyomas patients and their association with the quantity, dimensions, and site of fibroid growths. Additionally, we evaluated the potentiality of employing these factors as a biomarker tool for the diagnosis and assessment of uterine fibroid progression.

METHODS

This study involved the participation of 55 women with leiomyomas and 50 healthy women. We utilized commercial ELISA kits to measure the levels of total VitD and DBP in their serum. Additionally, we calculated the levels of free VitD and the ratio of VitD to DBP. Moreover, we determined the number, size, and location of the leiomyomas in the patients.

RESULTS

There were no significant differences in the levels of total VitD between the groups. However, patients had significantly lower levels of free VitD and higher levels of DBP compared to the control group. The size of the largest leiomyomas showed a negative relationship with free VitD and a positive relationship with DBP. Receiver operating characteristic analyses, showed that the cut-off value for free VitD was 4.47 pg/mL, with a sensitivity of 75.6% and a specificity of 74.4%. The cut-off value for DBP was 256.2 μg/mL, with a sensitivity of 86% and a specificity of 70.3%.

CONCLUSIONS

Free VitD and DBP potentially contribute to the development of leiomyomas and are linked to the size of these tumors. The measurement of serum levels of these factors could serve as additional biomarkers for the diagnosis of leiomyomas.

Vitamin D in dairy cows: metabolism, status and functions in the immune system

The function of vitamin D in calcium homoeostasis in dairy cows, such as in other vertebrates, is known for many years. In recent years, new and interesting, non-classical functions of vitamin D have been elucidated, including effects on the immune system. The major aim of this review is to provide an overview of effects of vitamin D or its metabolites on the immune system in dairy cows. The first part of the review provides an overview of vitamin D metabolism, with particular reference to the role of various proteins (25- and 1-hydroxylases, vitamin D binding protein, vitamin D receptor) in vitamin D signalling. The second part deals with the role of the concentration of 25-hydroxyvitamin D [25(OH)D] in plasma as an indicator of the vitamin D status in dairy cows, and its dependence on sunlight exposure and dietary vitamin D supplementation. In this part also the "free hormone hypothesis" is discussed, indicating that the concentration of free 25(OH)D might be a more valid indicator of the vitamin D status than the concentration of total 25(OH)D. The third part deals with classical and the non-classical functions of vitamin D. Among the non-classical functions which are based on an autocrine vitamin D signalling, particular reference is given to the effects of vitamin D and vitamin D metabolites on the immune system in bovine immune cells and in dairy cows. Recent findings provide some indication that vitamin D or its metabolite 25(OH)D could enhance the immune function in dairy cows and be useful for the prevention and therapy of mastitis. However, the number of studies reported so far in this respect is very limited. Thus, much more research is required to yield clear concepts for an optimised usage of vitamin D to improve the immune system and prevent infectious diseases in dairy cows.

Vitamin D Binding Protein: A Historic Overview

Vitamin D and all its metabolites are bound to a specific vitamin D binding protein, DBP. This protein was originally first discovered by its worldwide polymorphism and called Group-specific Component (GC). We now know that DBP and GC are the same protein and appeared early in the evolution of vertebrates. DBP is genetically the oldest member of the albuminoid family (including albumin, α-fetoprotein and afamin, all involved in transport of fatty acids or hormones). DBP has a single binding site for all vitamin D metabolites and has a high affinity for 25OHD and 1,25(OH)2D, thereby creating a large pool of circulating 25OHD, which prevents rapid vitamin D deficiency. DBP of higher vertebrates (not amphibians or reptiles) binds with very high affinity actin, thereby preventing the formation of polymeric actin fibrils in the circulation after tissue damage. Megalin is a cargo receptor and is together with cubilin needed to reabsorb DBP or the DBP-25OHD complex, thereby preventing the urinary loss of these proteins and 25OHD. The total concentrations of 25OHD and 1,25(OH)2D in DBP null mice or humans are extremely low but calcium and bone homeostasis remain normal. This is the strongest argument for claiming that the "free hormone hypothesis" also applies to the vitamin D hormone, 1,25(OH)2D. DBP also transports fatty acids, and can play a role in the immune system. DBP is genetically very polymorphic with three frequent alleles (DBP/GC 1f, 1s, and 2) but in total more than 120 different variants but its health consequences, if any, are not understood. A standardization of DBP assays is essential to further explore the role of DBP in physiology and diseases.

Uptake and degradation of filamentous actin and vitamin D-binding protein in the rat

Tissue uptake and degradation of 125I-tyramine-cellobiose-labelled filamentous actin, vitamin D-binding protein (DBP) and actin-DBP complex were studied in the rat. Actin and actin-DBP complex were cleared from plasma at a faster rate than was DBP. About 40% of injected actin was recovered in the liver between 10 and 30 min after administration. Of the total radioactivity recovered in the liver, about 35% and 40% was detected in parenchymal and endothelial cells respectively when labelled actin or DBP-actin complex was injected intravenously. When labelled DBP alone was injected, approx. 55% of the radioactivity recovered in liver was in the Kupffer cells. These results suggest that actin is targeting the DBP-actin complex to the endothelial and parenchymal liver cells. Filamentous actin was also taken up in large amounts and at a rapid rate in parenchymal as well as non-parenchymal liver cells in vitro. Our data indicate that the rat has a mechanism to clear actin and the DBP-actin complex from plasma and that both parenchymal and non-parenchymal liver cells are involved in this process.

Uptake and degradation of vitamin D binding protein and vitamin D binding protein-actin complex in vivo in the rat

We have labelled the rat vitamin D binding protein (DBP), DBP-actin and rat albumin with 125I-tyramine-cellobiose (125I-TC). In contrast with traditional 125I-labelling techniques where degraded radioactive metabolites are released into plasma, the 125I-TC moiety is trapped intracellularly in the tissues, where the degradation of the labelled proteins takes place. By using this labelling method, the catabolism of proteins can be studied in vivo. In this study we have used this labelling technique to compare the tissue uptake and degradation of DBP, DBP-actin and albumin in the rat. DBP-actin was cleared from plasma at a considerably faster rate than DBP. After intravenous injection of labelled DBP-actin complex, 48% of the radioactive dose was recovered in the liver after 30 min, compared with 14% when labelled DBP was administered. Only small amounts of DBP-actin complex were recovered in the kidneys. In contrast with the results obtained with DBP-actin complex, liver and kidneys contributed about equally in the uptake and degradation of DBP determined 24 h after the injection. When labelled DBP was compared with labelled albumin, the amount of radioactivity taken up by the liver and kidneys by 24 h after the injection was 2 and 5 times higher respectively. In conclusion, liver and kidneys are the major organs for catabolism of DBP in the rat. Furthermore, binding of actin to DBP enhances the clearance of DBP from circulation as well as its uptake by the liver.

Sex differences in the hydroxylation of cholecalciferol and of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol in rat liver

The effect of sex hormones on hydroxylation of cholecalciferol ('vitamin D3') and of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol has been investigated in female- and male-rat livers. The mitochondrial cholecalciferol 25-hydroxylase and C27-steroid 27-hydroxylase activities were respectively 4.6- and 2.7-fold higher in female- than in male-rat livers. The microsomal 1 alpha-hydroxycholecalciferol 25-hydroxylase was 2.8-fold higher in male- than in female-rat liver. No significant difference was found in the microsomal 25-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol. Liver microsomes (microsomal fractions) from male, but not from female, rats also catalysed 1-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol. Injection of testosterone into female rats decreased the mitochondrial cholecalciferol 25-hydroxylase and C27-steroid 27-hydroxylase activities, but not to a statistically significant extent. Testosterone treatment had no effect on the microsomal hydroxylases in female-rat liver. Injection of oestradiol valerate to male rats resulted in increased activities of both mitochondrial hydroxylases to the same levels as those of control females, while the microsomal enzyme activities decreased. The present results indicate that sex hormones exert a regulatory control on the mitochondrial cholecalciferol 25-hydroxylase and C27-steroid 27-hydroxylase activities.