Cytosol and nuclear estrogen receptors (occupied and unoccupied sites) and progesterone receptors in human breast cancer

Summary Estrogen and progesterone receptor concentrations in cytosol and nucleus were measured in 21 primary breast cancer tumors. Twelve out of the 21 tumor samples were cytosol estrogen receptor positive, 8 of which contained only unoccupied estrogen binding sites in the cytosol, but 2 of the 9 estrogen receptor negative samples did contain cytosol binding sites already occupied by endogenous homone. Four other estrogen receptor negative tumors only showed nuclear binding sites. Only 3 of the 12 estrogen receptor positive tumors also contained progesterone receptors. All of these tumors also had estrogen receptor in the nucleus. However, three of the 17 progesterone receptor negative samples had progesterone receptor only in the nucleus. The present data indicate that 3 possible classes of false negative tumors can be encountered: 1) estrogen receptors occupied by endogenous hormone, 2) tumors containing only nuclear estrogen receptors, and 3) tumors having only nuclear progesterone receptors. Measurement of nuclear estrogen receptor together with the progesterone receptor provides further information on whether the estrogen receptor system is not only present but also functional, and should be of value in the prediction of hormone dependent breast cancer.