The megalin/gp330 receptor is involved in the capability of hepatic stellate cells to bind and internalize vitamin D binding protein (Gc-globulin)

Aims: Vitamin D binding protein (DBP) is a highly expressed, multifunctional and polymorphic serum protein, which also serves as the major transporter for vitamin D sterols. The present study was performed to analyze the pathophysiological interaction between DBP and hepatocytes (PC) or hepatic stellate cells (HSC), the most important fat-/retinol- storing cells in the liver that spontaneously transdifferentiate to myofibroblasts (MFB) in culture.

Methods: HSC and PC were isolated by the pronase/collagenase reperfusion method. DBP expression was monitored by immunocytochemistry, Western blots, RT-PCR, and metabolic labelling with [³⁵S]-methionine. Cytoskeletal staining of a-smooth-muscle actin of HSC and identification of megalin/gp330-receptor was performed by confocal immunocytochemistry or immunoblotting.

Results: PC synthesize DBP as shown by PCR and [³⁵S]-methionine labelling, which could be suppressed by cycloheximide, a protein synthesis inhibitor. HSC stained intensively for DBP. In particular, a linkage between DBP and actin filaments of MFB was seen. In addition, a strong signal for DBP was detected in the immunoblot of native HSC lysates. However, neither mRNA for DBP nor metabolic labelling of DBP was found in this cell type, which suggests no active synthesis by HSC. Incubation with EDTA for complexing Ca²⁺ removed most of cell-associated DBP. Confocal microscopy showed a co-localization of DBP with the multifunctional megalin/gp330 receptor on HSC.

Conclusion: DBP, synthesized by PC, is attached to HSC and internalized, probably via interaction with the megalin/gp330 receptor. The functional role of DBP in HSC activity, in the uptake of fat-soluble vitamin D derivates or in myofibroblastic transdifferentiation is currently elucidated.