Uptake and 25-hydroxylation of vitamin D3 by isolated rat liver cells

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.

Steroid Hormone Vitamin D: Implications for Cardiovascular Disease

Understanding of vitamin D physiology is important because about half of the population is being diagnosed with deficiency and treated with supplements. Clinical guidelines were developed based on observational studies showing an association between low serum levels and increased cardiovascular risk. However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. For decades, administration of vitamin D has been used in rodents as a reliable experimental model of vascular calcification. Technically, vitamin D is a misnomer. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin. It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Importantly, endogenously synthesized forms are carried by a binding protein, whereas dietary forms are carried within lipoprotein particles. This may result in distinct biodistributions for sunlight-derived versus supplement-derived vitamin D hormones. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendations.

Antiproliferative action of vitamin D

During the past few years, it has become apparent that vitamin D may play an important role in malignant transformation. Epidemiological studies suggest that low vitamin D serum concentration increases especially the risk of hormone-related cancers. Experimentally, vitamin D suppresses the proliferation of normal and malignant cells and induces differentiation and apoptosis. In the present review we discuss the mechanisms whereby vitamin D regulates cell proliferation and whether it could be used in prevention and treatment of hyperproliferative disorders like cancers.

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.

Plasma carriers influence the uptake of cholecalciferol by human hepatoma-derived cells

The uptake of [3H]cholecalciferol by the human hepatoma-derived cell lines Hep G2 and Hep 3B was examined as a function of the sterol's presentation on various plasma proteins at their native concentrations. Control cultures utilized devitalized cells cross-linked with glutaraldehyde and estimated nonspecific sterol adherence to cells. With both cell lines, neither albumin nor plasma vitamin D binding protein permitted cholecalciferol uptake above control values. With Hep G2 cells, only low-density lipoprotein presentation of the sterol resulted in significant cellular uptake that had features resembling a receptor-mediated process. With Hep 3B, only high-density lipoprotein presentation of the sterol resulted in a significant uptake that was cell, carrier, and time dependent. These results support the hypothesis that lipoprotein carriers could account for the efficient hepatic accumulation of cholecalciferol in vivo.

Development of vitamin D3 25-hydroxylase activity in rat liver microsomes

The ontogeny of vitamin D3 25-hydroxylase activity has been determined in liver microsomes of rat fetuses and neonates. Production of 25-hydroxyvitamin D3 was low (0.11 pmol/g liver/h) 3 days prior to birth. Production rates were 1.2, 2.2, 1.8, and 2.8 pmol/g liver/h on Day 0, Day 2, Day 7, and Day 15, respectively. 25-Hydroxyvitamin D3 production in neonates increased sixfold from Day 15 to Day 22 to a value twice that of the mothers (17.6 pmol/g liver/h compared with 7.3 pmol/g liver/h). Activity in the maternal microsomes was constant (0.22 to 0.30 pmol/mg protein/h) except for the day of parturition (0.54 pmol/mg protein/h) and Day 22 postpartum (0.44 pmol/mg protein/h). A cytosolic factor, present as early as 3 days prior to birth, was required for vitamin D3 25-hydroxylase activity in the fetuses and stimulated the 25-hydroxylase reaction (up to 2.5-fold) in neonates and mothers. The ability of cytosol to prevent degradation of vitamin D3 was also present in the fetal stage. These data suggest that microsomal vitamin D3 25-hydroxylase activity in rat liver microsomes develops slowly and reaches full activity near the weaning stage. Since the cytosolic factor(s) is/are present in the fetal stage, the limiting component in the maturation of vitamin D3 25-hydroxylase activity in liver microsomes is the development of the cytochrome P-450 vitamin D3 25-hydroxylase.

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

A protein with high affinity for vitamin D3 and 25-hydroxyvitamin D3 in rat mesenteric lymph has been studied. Mesenteric lymph was collected after duodenal instillation of radiolabeled vitamin D3 and 25-hydroxyvitamin D3. As previously described, approximately 10% of vitamin D3 (S. Dueland, J.I. Pedersen, P. Helgerud, and C.A. Drevon, J. Biol. Chem. 257: 146-150, 1982) and 95% of 25-hydroxyvitamin D3 (S. Dueland, J.I. Pedersen, P. Helgerud, and C.A. Drevon, Am. J. Physiol. 245 (Endocrinol. Metab. 8): E463-E467, 1983) recovered in mesenteric lymph were associated with the alpha-globulin fractions. The radioactive vitamin D3 recovered in the lymph fraction with d greater than 1.006 (free of chylomicrons) coeluted with purified rat serum binding protein for vitamin D and its metabolites (DBP) from an antirat DBP column. The results obtained by immunoblotting after sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that this protein in mesenteric lymph had molecular weight and immunological properties identical with purified serum DBP. Purified serum DBP labeled with 125I was injected intravenously and mesenteric lymph was collected. in lymph, suggesting that DBP may be transferred from blood to mesenteric lymph and that plasma and lymph DBP may have a similar origin.

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

Using the isolated perfused rat liver we examined the uptake of [14C] or [3H] vitamin D3 and [14C] triglycerides or [3H] cholesterol by the liver of normal rats, from different lipoprotein fractions, as measured by disappearance from the perfusate. When incorporated into chylomicrons only 43% of the vitamin D3 remained in the perfusate at 60 min (i.e. 57% uptake) as compared to 68% of the triglycerides (i.e. 32% uptake). When added on very low density lipoproteins (VLDL) at 60 min 37 +/- 3% (n = 6) of the vitamin D3, 38 +/- 2% of the cholesterol (n = 3) (P NS), and 83 +/- 4% of the triglycerides (n = 3) remained in the perfusate (P less than 0.0005 for cholesterol:triglycerides and vitamin D3:triglycerides). For high density lipoprotein fraction (HDL) isolated perfused livers were studied with and without albumin present in the perfusate. Without albumin 19 +/- 8% (n = 3) of the vitamin D3 and 43 +/- 8% (n = 3) of the cholesterol remained in the perfusate at 60 min. The results with albumin present were 40 +/- 1% (n = 5) of the vitamin D3 and 63 +/- 4% (n = 5) of the cholesterol at 60 min (P less than 0.0005). The cholesterol:cholesterol ester ratio of the VLDL fraction was 8.8:1 and of the HDL fraction 1:1.4. There was no metabolism of vitamin D3 during the 1 h perfusion. These results suggest that vitamin D3 is in equilibrium with the lipoprotein surface, and the hepatic uptake of vitamin D is a surface phenomenon independent of lipoprotein metabolism.

Sequestration and microsomal C-25 hydroxylation of [3H]-vitamin D3 by the rat liver

A study of the vitamin D3 (D3) 25-hydroxylase was undertaken in an in vivo-in vitro model. [3H]-D3 (0.7, 1.0, 10, or 100 nmol/100 g of body weight) was injected into the portal vein and the liver was excised 18 seconds later. The liver homogenate was then submitted to differential centrifugation and the amount of [3H]-D3 incorporated in the subcellular fractions was evaluated. The microsomal fraction was also incubated in vitro and the appearance of [3H]-25-hydroxyvitamin D3 [25(OH)D3] was determined by high performance liquid chromatography (HPLC). Results showed that the fractional liver [3H]-D3 uptake varied between 37 percent and 48 percent of the dose injected. The intracellular distribution of [3H]-D3 showed that most of the vitamin was incorporated into the microsomal fraction (45% to 50% of the intracellular [3H]-D3) except at the highest dose of [3H]-D3 where the cytosolic fraction contained the highest amount (56.4%) of the incorporated vitamin. Mathematical analysis of the intracellular [3H]-D3 distribution showed that the microsomal fraction was the only subcellular fraction that was found to incorporate [3H]-D3 in relation to the total liver uptake of the vitamin. The apparent Michaelis-Menten kinetics of the [3H]-D3-25-hydroxylase showed that with substrate concentration of up to 88.5 nM, the apparent Km and Vmax were 28.2 nM and 25.8 fentomoles (fmol) X min-1 X mg microsomal pro-1, respectively, but the reaction lost considerable efficiency with higher substrate concentrations. With the in vivo-in vitro model used, the cytosolic fraction was not essential for the optimal C-25 hydroxylation of D3. These results show that the endoplasmic reticulum of rat hepatocytes possess a high capacity for D3 incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption, distribution, and transport of vitamin D3 and 25-hydroxyvitamin D3 in the rat

The absorption of [14C]vitamin D3 and 25-[3H]hydroxyvitamin D3 into the mesenteric lymph and portal vein after simultaneous instillation in rat duodenum has been studied. The appearance of both isotopes in the mesenteric lymph and portal vein during the first 2 h was negligible. After 2-4 h the isotopes appeared both in the lymph and portal vein. The appearance of 25-[3H]hydroxyvitamin D3 in the mesenteric lymph was faster than that of [14C]vitamin D3. Total recovery in the lymph during 24 h for [14C]vitamin D3 and 25-[3H]hydroxyvitamin D3 was 15.2 and 14.3%, respectively. While 80% of vitamin D3 in mesenteric lymph was recovered in the chylomicrons (d less than 1.006 g/ml), 94% of 25-hydroxyvitamin D3 was recovered in the protein fraction with d greater than 1.21 g/ml. Seventy-one percent of 25-hydroxyvitamin D3 in the protein fraction was associated with the alpha-globulins. Both vitamin D3 and 25-hydroxyvitamin D3 that were recovered in the chylomicrons (d less than 1.006 g/ml) were transferred in vitro to the lymph fraction with d greater than 1.006 g/ml. Less than 12% of 25-hydroxyvitamin D3 was recovered in the liver after intravenous injection of mesenteric lymph labeled with 25-[3H]hydroxyvitamin D3. Previously, under similar conditions 40% of administered vitamin D3 was recovered in the liver.

Plasma clearance, transfer, and distribution of vitamin D3 from intestinal lymph

The clearance of vitamin D3 from plasma after intravenous injection to rats of intestinal lymph labeled in vivo with radioactive vitamin D3 has been studied. The half-life of the injected vitamin D3 was about 6 min. In double-label experiments the half-life was the same for vitamin D3 as for vitamin A, which is removed from the circulation as chylomicron remnants. Fractionation of the blood plasma according to the density showed that 2 min after the injection of the lymph 30-95% of the radioactive vitamin D3 was recovered in plasma fractions with higher densities than chylomicron remnants (d greater than 1.019 g/ml). The percentage of vitamin D3 in the chylomicron fraction (d less than 1.006 g/ml) of the injected lymph was 66-95%. Taking this into account it could be calculated that the amount of radioactive vitamin D3 transferred from chylomicrons or chylomicron remnants to plasma with d greater than 1.019 g/ml within 2 min varied from 26 to 72%. The transferred radioactivity was recovered in a protein fraction with chromatographic mobilities identical to the binding protein for vitamin D and its metabolites. During the first 13 min after injection the amount of radioactivity recovered in the liver was less than 10%, but from 15 to 55 min the amount increased to about 50% of the injected dose. These findings suggest that the transfer of vitamin D3 from chylomicrons to the alpha-globulin fraction previously observed in vitro is of importance for the in vivo handling of vitamin D3.(ABSTRACT TRUNCATED AT 250 WORDS)