INTERACTIONS OF TRANSTHYRETIN (TTR) AND RETINOL-BINDING PROTEIN (RBP)IN THE UPTAKE OF RETINOL BY PRIMARY RAT HEPATOCYTES

The mechanism by which cells take up retinol from retinol-binding prot ein (REP) and the role of the RBP-transthyretin (TTR) complex remain u nclear, Here we report on retinol uptake through the RBP-TTR complex b y primary cultured rat hepatocytes (parenchymal cells, PC) and nonpare nchymal cells (NPC) following incubation with [H-3]retinol-REP or the [H-3]-retinol-RBP-TTR complex under several conditions. The cellular a ccumulation of retinol was time and temperature dependent in both PC a nd NPC, Analysis by HPLC showed that the incorporated [3H]retinol in N PC was mainly converted to retinyl ester, although in PC it remained m ainly as unesterified retinol. However, the amount of retinol taken up from the RBP-TTR complex was nearly twofold greater than that from RE P alone. The uptake of [H-3]retinol from protein-bound retinol was inh ibited by an excess of either retinol-RBP or retinol-RBP-TTR complex. Moreover, retinol uptake through the RBP-TTR complex was inhibited by an excess of free TTR. From these results we postulate that TTR may ta ke part as a positive regulator in the delivery of REP-bound retinol f rom plasma, possibly by a membrane receptor, and that retinol uptake t akes place preferentially from the RBP-TTR complex into both PC and NP C. The uptake of [H-3]retinol (2 mu M) by PC was saturated, whereas up take by NPC was not. These results indicate that the physiological imp ortance of TTR in retinol delivery may be especially important to vita min A-storing stellate (Ito) cells in the NPC fraction. (C) 1997 Acade mic Press.