Synthesis and Secretion of Corticosteroid-Binding Globulin by Rat Liver

Abstract

Classical glucocorticoid receptors (type II) have a high affinity for synthetic and natural glucocorticoids. We have previously demonstrated an additional binding site in kidney cytosol (type III) which has a high affinity for corticosterone but a low affinity for dexamethasone. In many ways, this binder resembles plasma corticosteroid-binding globulin (CBG). The first goal of this study was to determine the organ distribution of the type III binding sites. Cytosol was prepared from isolated cells to avoid plasma contamination. Of the tissues examined, type III sites were found only in liver and kidney; sites were absent from thymocytes, IM-9 lymphocytes, adipocytes, and bone cells. The second goal of this study was to ascertain whether CBG is synthesized in liver and kidney. Liver and kidney slices were incubated in vitro and the concentration of type III sites was seen to rise in hepatic cytosol and incubating medium but not kidney. To verify the impression that liver was synthesizing and secreting CBG, the following experiments were performed: (a) To demonstrate that type III sites were CBG, steroid-binding profiles and migration on polyacrylamide gel electrophoresis were shown to be identical for hepatic type III sites and serum. (b) To indicate that the rise in type III sites was dependent on protein synthesis, it was shown that cycloheximide blocked the appearance of new type III sites. (c) To establish that the type III sites were being secreted, in situ liver perfusion experiments showed time-dependent release of new sites into the perfusate. In conclusion, liver synthesizes and secretes type III sites, a finding previously suspected but never proved. The presence of type III sites in kidney remains to be explained.