noid-bi nd ing proteins in regenerating rat liver: demonstration of a novel cellular retinoid-binding protein

Changes in the levels of liver cellular retinoland retinoic acid-binding proteins were studied after partial (about 70%) hepatectomy for 14 days in the rat. It was found that a novel binding protein designated F-type appears transiently in liver cytosol 3 days after the operation. The appearance of this protein coincides with the peak level of the a,-fetoprotein. In contrast, cellular retinoic acid-binding protein was detected only the first day after hepatectomy, whereas no significant change was observed in the level of the cellular retinol-binding protein during the entire observation period. [3H]Retinol or [3H]retinoic acid complexed with serum retinol-binding protein injected intravenously into vitamin A-deficient rats 1 day after the hepatectomy was recovered 5 min or 20 min later bound specifically to cellular retinolor retinoic acid-binding protein, respective1y.l The results presented here strongly suggest that each of the three cellular retinoid-binding proteins plays a distinct role in cell proliferation and differentiation.-Omori, M., Y. Muto, and T. Nagao. Cellular retinoid-binding proteins in regenerating rat liver: demonstration of a novel cellular retinoid-binding protein. J . Lipid Res. 1981. 22: 899-904. Supplementary key words deficiency * a,-fetoprotein vitamin A . retinoic acid . vitamin AIt has been established that cytosols of many tissues contain two different binding proteins for vitamin A and its analogs (retinoids). T h e first one is cellular retinol-binding protein (CRBP) (1) and the second is cellular retinoic acid-binding protein (CRABP) (2). CRBP (3, 4) and CRABP (5, 6) have been purified to homogeneity. Both proteins have molecular weight of 14,600 but can be distinguished by their binding specificity either for retinol or retinoic acid. Shidoji and Muto (7) have recently reported that the fish eye cytosol contains another cellular retinoidbinding protein with a similar molecular size but which appears to have binding affinity for both retinol and retinoic acid. A similar binding protein has also been detected in the brain cytosol of developing chick embryo (8), human fetal liver, and human hepatocellular carcinoma (9). We call this cellular retinoidbinding protein F-type (CRBP (F)) as suggested previously (8, 9). The present study was undertaken to determine whether the levels of these proteins change during liver regeneration. EXPERIMENTAL PROCEDURE