Secretion of N-(4-hydroxyphenyl) retinamide-retinol-binding protein from liver parenchymal cells: evidence for reduced affinity of the complex for transthyretin.

The synthetic retinoid 4-HPR has been shown to markedly lower the plasma concentration of both retinol and RBP in rats and humans. We have studied the effect of 4-HPR on the secretion of retinol-RBP from liver cells in vivo and in vitro. In rats maintained with a normal diet, a vitamin A-deficient diet or a normal diet supplemented with 4-HPR, chylomicrons [3H]retinyl esters were rapidly cleared from the plasma. The secretion of chylomicron-derived [3H]retinol from tissues to the circulation, however, was different. In control rats, the lymph-derived [3H]retinol peaked after about 2 hr, whereas 4-HPR treatment effectively reduced this peak of [3H]retinol. Our results suggest that 4-HPR inhibits secretion of retinol-RBP from the liver. Therefore, we decided to study the effect of 4-HPR on the secretion of RBP using the human hepatoma cell line HepG2. Retinol and 4-HPR were found to induce the secretion of RBP. The medium from cells treated with 4-HPR was immunoprecipitated with antibodies against human RBP. HPLC analysis of the precipitated RBP revealed the presence of 4-HPR. When the medium from cells incubated with either 4-HPR or retinol was applied to a TTR affinity column, we found that RBP from cells incubated with 4-HPR had a considerably reduced affinity for TTR. We conclude that 4-HPR binds RBP and thereby induces secretion of RBP in HepG2 cells, and that the secreted 4-HPR-RBP complex has a reduced affinity for TTR. This observation may explain the 4-HPR-induced reduction of plasma retinol and RBP observed in in vivo studies.