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]retino l (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 ex tracellular medium and part is degraded. Nonspecific endocytosis of I-125-R BP 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 inhibito r of protein export from the main secretory route, and is absent in HeLa ce lls, which do not synthesize REP. These data suggest that at least part of retinol taken up from exogenous holoRBP is delivered to newly synthesized R EP. H-3-ROH recycled by HeLa cells is bound to serum albumin, as is a porti on of that recycled by HepG2 cells. Transfer of H-3-ROH from REP to serum a lbumin does not occur in the absence of cells. We conclude that REP is endo cytosed through a specific pathway and that the RBP-associated retinol is t ransferred to newly synthesized RBP or to serum albumin. (C) 1999 Academic Press.