Characterization of a specific binding protein for 2,3,7,8-tetrachlorodibenzo-p-dioxin in human thymic epithelial...

Specific toxic and biochemical responses elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in human thymic epithelial (TE) cells in culture are mediated by the TCDD receptor protein. Characterization of the physicochemical properties of the TCDD receptor in cytosol fractions from cultured human TE cells indicates that this protein is similar, but not identical, to the extensively studied receptor species present in mouse and rat liver. The human TCDD receptor sediments at 9.1 S on sucrose density gradients at 0 degrees, undergoes a temperature-dependent conversion at 20 degrees to a species sedimenting at 10.7 S, and partially dissociates in the presence of 0.4 M KCl, as judged by the appearance of a peak sedimenting at 5 S. Both the temperature- and salt-dependent changes in the observed physical properties of the human TCDD receptor are inhibited by sodium molybdate. Two molybdate-stabilized binding species can be resolved from TCDD specific binding isotherms to human TE cytosol. Under identical conditions, only a single TCDD specific binding component was detected in cytosol fractions from both mouse and rat liver. Mixing cytosol prepared from human TE cells with hepatic cytosol fractions from C57BL/6 mice revealed the presence of a heat-labile, trypsin-sensitive factor in human TE cells that inhibits TCDD specific binding to the murine hepatic receptor. Molybdate stabilized the mouse receptor against the actions of this human inhibitory factor. Molybdate also may stabilize the human TCDD receptor, as indicated by the 2- to 3-fold increase in total TCDD specific binding measured in human TE cytosol fractions in the presence of this metallo-oxyanion.