Evidence for a transcortin-like component in human breast cyst fluid

Abstract The occurrence of a specific glucocorticoid binding activity was investigated in breast cyst fluid (BCF) samples aspirated from 481 women under treatment for breast cystic disease. [ 3 H]cortisol was incubated with BCF at 4°C with and without 100-fold molar excess of non-radioactive steroid in order to account for non-specific binding. Corticosterone, progesterone and dexamethasone-binding activities were also investigated in about 50% of the specimens. Consistent amounts of a specific cortisol-, corticosterone- and progesterone-binding component were observed in about 30% of samples. Apparent binding values ranged from 4.31 to 80.78 nmol/l for cortisol, from 3.94 to 75.72 nmol/l for corticosterone, and from 1.18 to 16.45 nmol/1 for progesterone. No specific binding for dexamethasone was detected. Scatchard analyses for [ 3 H]cortisol and [ 3 H]progesterone were made in pools of ‘positive’ BCF, human serum and whey of human milk, respectively. The mean values of the apparent equilibrium association constant ( K a ) were compatible with the existence of high affinity protein-hormone interactions in each considered medium. Data obtained in ligand competition experiments for different steroid molecules suggest the similarity of the glucocorticoid- and progesterone-binding component found in 30% of BCF samples examined with the plasma corticosteroidbinding globulin (CBG, transcortin) and especially with the transcortin-like component detected in the whey of the human milk. It is suggested that the transcortin-like component may play an important role in controlling kinetics of transport and in regulating the effective levels of cortisol and progesterone in numerous breast cyst fluids.