Characterization of the human plasma binding protein for vitamin D and its metabolites synthesized by the human hepatoma-derived cell line, Hep 3B

Vitamin D binding protein is produced by human monocytes

The expression of the DBP (vitamin D binding protein) gene was investigated in monocytes and in peripheral blood lymphocytes. DBP message was amplified through 35 cycles of PCR amplification using specific oligonucleotide primers. PCR products of the expected size were further identified by Southern blotting using a specific DBP probe. No expression of the DBP gene could be detected in peripheral blood lymphocytes, nor in the monocyte-derived U 937 cell line. In contrast, message for DBP was identified in monocytes activated with lipopolysaccharide when analyzed between 6 and 10 h following stimulation. These results suggest that the temporal expression of the DBP gene could play a major role in the activation of monocytes by 1-25(OH)2D3.

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.

Purification of human serum vitamin D-binding protein by 25-hydroxyvitamin D3-Sepharose chromatography

25-Hydroxyvitamin D3-Sepharose was prepared by coupling 25-hydroxyvitamin D3-3 beta-(1,2-epoxypropyl)-ether to thio-activated Sepharose CL-6B, forming a protease-resistant linkage between the sterol and the matrix. Vitamin D-binding protein from human plasma was obtained 85-92% pure after ligand affinity chromatography. Subsequent hydroxylapatite chromatography provided homogeneous protein. The purified vitamin D-binding protein was fully active in regard to 25-hydroxyvitamin D3 and actin binding capabilities.