Expression of the multiligand receptor Megalin in hepatic stellate cells

Background: The LDL receptor-related protein–2 (Megalin) is the largest membrane receptor found in mammals and mediates the endocytic uptake of vitamin/protein binding complexes. Megalin is known to be expressed in the proximal tubules of the kidney and to play an important role during the embryonic development of the brain. The aim of this study was to investigate the expression of Megalin in liver and to assess its pathobiological function in vivo and in vitro. Methods: During CCl4-induced liver fibrosis in mice and after bile duct ligation in rats, hepatic Megalin expression was studied by RT-PCR and immunohistochemistry. Megalin expression in isolated rat hepatic stellate cells (HSC) was determined using RT-PCR, immunocytochemistry, and confocal laser scanning microscopy (CLSM). To analyze a potential role of Megalin in retinol transport, HSC were incubated with FITC-labeled retinol binding protein and the recombinant Megalin chaperone RAP (receptor associated protein), and fluorescein incorporation was determined by fluorescence-activated cell analysis. Results: In both rodent models of liver fibrosis, Megalin immunoreactivity correlates with the stage of fibrosis. Isolated HSC express Megalin, and CLSM indicates that Megalin co-localizes with retinol-storing multivesicular bodies of HSC. Upon treatment of HSC with RAP, Megalin expression increases markedly (2.8–7.6 x). Flow cytometry demonstrates that this induction is associated with an augmented incorporation of FITC-labeled retinol-binding protein. Conclusions: Megalin is a novel marker for activated HSC, and retinol uptake of HSC depends on the association of retinol-binding protein with Megalin. Megalin might regulate the retinol metabolism of HSC and could represent a molecular target for antifibrotic drug design.