Vitamin D3 uptake by the isolated perfused rat liver from lipoprotein fractions is separate from cholesterol and triglyceride uptake

Vitamin D toxicity related to its physiological and unphysiological supply

Vitamin D is defined as a nutrient despite its rare occurrence in food. Vitamin D status is determined mainly by solar UV light action in skin. However, the strategy to combat vitamin D deficiency has been to increase oral intake of vitamin D in greater amounts than could be obtained from food. Persistent large intakes of vitamin D can cause hypercalcaemic toxicity. Although the amounts recommended to prevent deficiency are far less than those causing such toxicity, the possibility of other toxic actions from increased intake of vitamin D has been ignored. Animal experiments have demonstrated that moderate amounts of oral vitamin D over time result in atherosclerosis. Differences in the fate of vitamin D could explain this angiotoxicity.

Vitamin D bioavailability in cystic fibrosis: a cause for concern?

Despite the inclusion of extra vitamin D in their regimen of fat-soluble vitamin supplementation, cystic fibrosis patients remain chronically depleted of vitamin D. The persistence of suboptimal vitamin D status is often blamed on the maldigestion and malabsorption of fat. However, the mitigated success of recent clinical trials with high-dose vitamin D supplementation suggests that vitamin D bioavailability might be impaired in these patients. Given the growing understanding of the importance of this vitamin in the regulation of multiple biological functions beyond skeletal health, the present review analyzes the current literature by addressing each step of vitamin D metabolism and action in the context of this life-limiting pathology. In addition, it highlights the importance of vitamin D in relation to organs and or conditions affected by cystic fibrosis.

Investigation of the potential association of vitamin D binding protein with lipoproteins

BACKGROUND

Vitamin D binding protein (DBP) acts as a vitamin D carrier and an actin scavenger. We have previously reported a correlation between serum DBP and lipid parameters in a cystic fibrosis population. In the present study, the relationship between serum DBP, lipoprotein fractions and vitamin D is investigated.

METHODS

The presence of DBP in lipoprotein fractions was examined using precipitation, gel permeation chromatography and ultracentrifugation. The association between DBP and lipids was investigated in a cohort study of 211 men. Total and actin-free DBP concentrations were assessed by immunonephelometry and enzyme-linked immunosorbent assay. Serum 25(OH)- and 1.25(OH)(2)-vitamin D(3) were assayed by radioimmunoassay. Total cholesterol, high-density lipoprotein cholesterol (HDL), triglycerides and insulin were measured using routine methods. Low-density lipoprotein-cholesterol (LDL) was calculated according to Friedewald's formula.

RESULTS

DBP was found to be present in very-low-density lipoprotein (VLDL). Gel permeation chromatography revealed a bimodal DBP distribution with a lipid-bound fraction besides the known free fraction. Ultracentrifugation confirmed the presence of DBP and 25(OH)-vitamin D(3) in the VLDL particle. Total serum DBP concentration and the actin-bound DBP/DBP ratio correlated significantly with total cholesterol, LDL-cholesterol, triglycerides and albumin. The 25(OH)-vitamin D(3)/DBP ratio correlated negatively with serum triglyceride concentration and body mass index (BMI). The actin-bound DBP complex was identified with Western blot.

CONCLUSIONS

The lipid-bound DBP fraction may be of greater importance than initially thought. The present findings may have clinical consequences in view of the important physiological role of DBP.

Behaviour of phospholipase modified-HDL towards cultured hepatocytes. II. Increased cell cholesterol storage and bile acid synthesis

Human total HDL (hydrated density 1.070-1.210), HDL2 (1.070-1.125), HDL3 (1.125-1.210) or HDL separated by heparin affinity chromatography were treated with or without purified phospholipase A2 from Crotalus adamanteus. Control and treated HDL were reisolated and were then incubated with cultured hepatocytes. 1. Mass measurements evidenced a time-dependent cholesterol enrichment in hepatocytes cultured in the absence of lipoproteins. Addition of HDL2 still enhanced by 25% the cell cholesterol content and down-regulated endogenous sterol synthesis in similar proportions. Conversely, HDL3 slightly decreased the amount of free cholesterol in hepatocytes (-12%). 2. Incubations with phospholipase A2-treated HDL resulted in a 35%-50% increase of both the cellular cholesterol esterification and the cholesterylester accumulation, when compared to cells cultured in the presence of control-HDL. This effect was observed with HDL2, HDL3 and combining the data with all subfractions. 3. Cultured hepatocytes secreted cholic and beta-muricholic acids as major bile acids and HDL2 showed a tendency to stimulate their secretion. Phospholipase treatment of HDL again induced an increased production by hepatocytes of those two bile acids. Thus, whereas HDL2 and HDL3 display different behaviours with respect to cell cholesterol content, neosynthesis and bile acid secretion, their modifications by phospholipases always orientate the cell sterol metabolism in the same direction: increased cholesterylester accumulation and bile acid production.

The uptake and metabolism of 25-hydroxyvitamin D3 and vitamin D binding protein by cultured porcine kidney cells (LLC-PK1)

1. Uptake of 3H-25OHD3, 3H-25OHD3-DBP, 125I-holo-DBP and 125I-apo-DBP by LLC-PK1 cells was linearly related to the concentration of each in the culture media. The presence of DBP in the medium significantly reduced the amount of 3H-25OHD3 taken up by cells. 2. Free 25OHD3 and 25OHD3 bound to DBP were both metabolized by the cells to 24,25(OH)2D3 and an unidentified product of apparent lower polarity than 25OHD3. 3. A significant amount of DBP taken up by the LLC-PK1 cells was metabolized to a TCA-soluble form. 4. Uptake of DBP was similar to horseradish peroxidase, but higher than inulin, indicative of a non-specific endocytic mechanism with an adsorptive component. 5. It is suggested that both free circulating 25OHD3 and that derived from lysosomal degradation of 25OHD3-DBP are available for hydroxylation by the kidney.

Characterization of steroid hormone association with human plasma lipoproteins

Partition coefficient analysis, equilibrium dialysis, and computer simulation were used to evaluate associations of twelve steroid hormones (androstanediol, androstenediol, androstenedione, androsterone, dehydroepiandrosterone, dihydrotestosterone, estradiol, estriol, estrone, hydroxyprogesterone, progesterone, and testosterone) with human plasma high density lipoproteins (HDL), low density lipoproteins (LDL), and very low density lipoproteins (VLDL). It was determined that partitioning of steroid hormones (SH) between the aqueous medium and the surfaces of lipoproteins (LP) was the initial (first order) SH-LP interaction. For some SH, especially dehydroepiandrosterone, significant second order interactions, which may involve chemical conversions, were detected. The first order binding values of the twelve SH with three LP were combined with the corresponding binding values of SH with sex hormone-binding globulin, corticosteroid-binding globulin, and albumin in a 6 X 12 matrix. The computer program TRANSPORT was used to analyze the matrix and determine the distribution of each SH among six different binding agents in the "normal" male. It was concluded that LP are important vehicles for SH conveyance in plasma and may also be important for SH entry into cells.