Synthesis and secretion of a novel binding protein for retinol by a cell line derived from Sertoli cells

An established cell line (TM-4) derived from murine Sertoli cells, the major supportive cell type of the testes, secretes a protein that binds retinol when grown in serum-free chemically defined medium. The protein that binds retinol is trypsin-sensitive and has an apparent Kd for retinol of 54 nM. Cholesterol, retinyl acetate, or UV-irradiated retinol at levels 100-fold in excess of retinol are poor competitors of [3H]retinol binding. Retinoic acid at a 100-fold molar excess inhibited [3H]retinol binding by 71%. In contrast, excess unlabeled retinol completely inhibits [3H]retinol binding. More than 80% of the total retinol-binding activity in confluent cultures is found in the culture medium. Prior to incubation with retinol, the protein that binds retinol has an apparent Mr of less than 150,000 by column chromatography; however, after incubation with retinol the protein that binds retinol exhibits an apparent Mr of 2 X 10(6) or greater and a sedimentation coefficient greater than 4 S. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that the major iodinatable component of the aggregated protein that binds retinol has an apparent Mr of 70,000. The secreted protein that binds retinol is not immunologically cross-reactive with either serum or cellular retinol-binding protein or transferrin. These findings suggest that Sertoli cells may secrete a protein that binds retinol. Such a protein could be involved in the transport of retinol either to the lumen of the seminiferous tubules or to the developing germ cells themselves.