Receptor-mediated uptake and processing of vitamin D-binding protein in human B-lymphoid cells

Vitamin D-binding protein (DBP), a member of a multigene family including alpha-fetoprotein (AFP) and albumin, is a serum glycoprotein that reversibly binds and transports vitamin D and its metabolites to target cells. In this work, we demonstrate that normal and malignant human B-lymphocytes specifically bind and internalize DBP. Radioiodinated DBP (125I-DBP) was used to follow the uptake of the protein by Raji cells, a human pre-B-lymphoma cell line. Time course studies of DBP uptake by these cells exhibited a saturable profile at both 4 and 37 degrees C. The binding saturation curve obtained by incubating Raji cells at 4 degrees C with different concentrations (1.5 nM to 1.5 microM) of 125I-DBP showed two saturation plateaus; Scatchard analysis showed the presence of two groups of receptor sites with a Kd1 of 2.04 x 10(-7) M (n1 = 42,161 +/- 4,336 sites/cell) and a Kd2 of 1.01 x 10(-6) M (n2 = 198,000 +/- 48,000 sites/cell). After incubation of Raji cells at 37 degrees C with both fluorescein isothiocyanate (FITC) and horseradish peroxidase conjugates, DBP was internalized and could be localized in the cytoplasm. DBP-horseradish peroxidase conjugates were used to follow the uptake and to determine the endocytic pathway of the protein in Raji cells. The initial steps, contrary to those observed for AFP, did not apparently involve coated pits and vesicles. Small vesicles (approximately 50-60 nm) with electron-dense DBP-horseradish peroxidase reaction products were observed that could fuse with large endosomes. These endosomes appeared dispersed in the cytoplasm with some preferential localization in the Golgi centrosphere region. Pulse-chase experiments showed that only 10% of the uptaken protein was released in a nondegraded form. Accordingly, most DBP molecules accumulated in endosomes should be degraded in lysosomes, instead of being recycled back to the surface, as in the case of AFP. Contrary to malignant B-cells (Raji), the uptake ability for DBP of normal quiescent B-lymphocytes was very low. Specific binding and internalization of DBP-FITC by these cells were observed following mitogen-induced activation. Significant values of uptake were obtained at 37 degrees C after 72 h of incubation in the presence of pokeweed mitogen. The binding of DBP-FITC was partially inhibited in the presence of an excess of unlabeled protein. Taken together, the actual results suggest that DBP receptors are constitutively expressed by malignant B-cells and in a transitory form by normal B-lymphocytes undergoing mitogen-induced activation.(ABSTRACT TRUNCATED AT 400 WORDS)