Uptake of [ 3 H]Vitamin D 3 from Low and High Density Lipoproteins by Cultured Human Fibroblasts

Abstract

The plasma distribution and cellular uptake of [³H]vitamin D₃ was studied in vitro using cultured human fibroblasts. Incubation of [³H]vitamin D₃ (cholecalciferol) with plasma followed by sequential ultracentrifugal fractionation of the lipoproteins indicated that 2–4% of the radioactivity associated with the very low density lipoprotein (VLDL), 12% with low density lipoprotien (LDL), and approximately 60% with the high density lipoprotein (HDL). The remaining radioactivity, 25%, was associated with the sedimented plasma fractions. By comparison, an average of 86% of the radioactivity from [³H] 1,25-dihydroxycholecalciferol associated with the sedimented plasma fractions. The uptake of [³H]vitamin D₃ from plasma, LDL, or HDL was studied in cultured human cells; uptake by normal fibroblasts was greatest from LDL and least from plasma. The cellular association of vitamin D₃ was time, concentration, and temperature dependent. At a concentration of 50 μg LDL/ml of medium, the uptake of [³H]vitamin D₃ from LDL at 37°C was rapid and reached a maximum at approximately 4 hr; it was slower from HDL but continued to increase slowly up to 24 hr. The significance of these in vitro findings is uncertain since much of the vitamin D₃ absorbed from the intestine reportedly associates with chylomicrons and is rapidly taken up by the liver.