Transfer of retinol from parenchymal to stellate cells in liver is mediated by retinol-binding protein.

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Abstract
Newly absorbed chlomicon remnant retinyl ester is endocytosed by parenchymal liver cells, and retinol is subsequently transferred to perisinusoidal stellate cells in liver. In the present study we have approaches to elucidate the mechanism for the paracrine transfer of retinal between liver parenchymal and stellate cells. In one series of experiments, chlyomicrons labeled with [3H]retinyl palmitate or with retinyl [3H]palmitate were injected intravenously into rats. It was shown that the retinol as well as the palmitate moiety were initially taken up in parenchymal liver cells. However, only the retinol moiety was detected in stellate cells indicating that the retinyl ester is hydrolyzed before retinol is transferred to stellate cells. It is well known that parenchymal liver cells secrete retinol bound to retinol-binding protein (RBP), and we have recently found that stellate cells do have RBP receptors. Here we report that antibodies against RBP completely block the transfer of retinol from parenchymal to stellate cells. These findings indicate that following uptake of chylomicron remnat retinyl ester in parenchymal cells, the retinyl ester is hydrolyzed, and retinol secreted from parenchymal cells on RBP is taken up by stellate cells by means of RBP receptors.