Studies on the cellular uptake of retinol binding protein and retinol

The uptake and release of I-125-RBP and of holoRBP labeled with [H-3]retinol (H-3-ROH) were studied in two cell lines which synthesize and secrete REP, the HepG2 hepatocarcinoma cell line and the Caki-1 kidney adenocarcinoma cell line, and in HeLa cells that do not express the endogenous REP gene In all three cell lines a part of endocytosed I-125-RBP is recycled to the extracellular medium and part is degraded. Nonspecific endocytosis of I-125-RBP was estimated to be approximately 10% of total endocytosed I-125-RBP. In HepG2 cells the H-3-ROH from the [H-3]retinol-RBP complex (H-3-ROH-RBP) is recycled bound to REP into serum-free chase medium. This H-3-ROH recycling is blocked in HepG2 cells by cyclohexymide and by brefeldin A, an inhibitor of protein export from the main secretory route, and is absent in HeLa cells, which do not synthesize REP. These data suggest that at least part of retinol taken up from exogenous holoRBP is delivered to newly synthesized REP. H-3-ROH recycled by HeLa cells is bound to serum albumin, as is a portion of that recycled by HepG2 cells. Transfer of H-3-ROH from REP to serum albumin does not occur in the absence of cells. We conclude that REP is endocytosed through a specific pathway and that the RBP-associated retinol is transferred to newly synthesized RBP or to serum albumin. (C) 1999 Academic Press.