Characterization of Thyroxine-Binding Globulin Secreted by a Human Hepatoma Cell Line*

T4-binding globulin (TBG) is a glycoprotein synthesized by the liver and is the principal carrier of T4 and T3 in serum. In this report, we demonstrate that the Hep G2 cell line, derived from a human hepatoblastoma, synthesizes and secretes TBG, the properties of which were characterized. Hep G2 cells secreted TBG into the medium after more than 100 transfers in tissue culture conditions. At confluency and after changing to serum-free culture conditions, TBG accumulation into the medium was linear for 3 days and constituted approximately 0.16% of the proteins synthesized over 24 h. Its abundance relative to albumin is 10-fold greater than that found in normal human serum. TBG secreted by the Hep G2 cells was indistinguishable from native normal human serum TBG, as determined immunologically, by electrophoresis on polyacrylamide gel in denaturing and nondenaturing conditions, and by isoelectric focusing. It also specifically bound T4 and T3, albeit with slightly reduced affinity, and had increased heat lability. Although slightly different from normal serum TBG in caucasians, the physical and biological properties of the Hep G2-derived TBG are similar to those of the variant TBG found in the serum of some healthy Australian Aborigines.