Endocrinology, Vol 121, 1288-1298, Copyright © 1987 by Endocrine Society

ARTICLES

Characterization and hormonal regulation of estrogen binding proteins in the MTW-9B transplantable rat mammary tumor

DA Scala and MM Ip
Grace Cancer Drug Center, Roswell Park Memorial Institute, New York State Department of Health, Buffalo 14263.

A two-component estrogen (E2)-binding system has been characterized in the E2-independent MTW-9B rat mammary tumor by Scatchard analysis, sucrose gradient analysis, and isoelectric focusing. One cytosol receptor protein (type I) conforms to the classical estrogen receptor with a high affinity (Kd = 0.45 nM) for E2 and limited binding capacity [maximum binding (Bmax) = 53 fmol/mg protein]. The second component (type II) demonstrates a high number of sites (Bmax = 164 fmol/mg protein) and low E2-binding affinity (Kd = 22.3 nM). The type I and type II E2-binding components were shown to sediment on sucrose gradients at 9.6S and 4.5S, respectively, and to focus at isoelectric points of 6.6 and 8.0, respectively. The addition of 0.4 M KCl to the homogenization buffer converted the high affinity receptor species to a form that cosedimented and cofocused with the low affinity E2-binding protein. When tumors were grown in intact male rats, the ratio of the type II to the type I protein, as assessed by sucrose gradient analysis, increased 4.5-fold relative to that in tumors from intact females. Concomitantly, the Kd values of the type I and type II proteins were increased 9- and 2-fold, respectively, and the Bmax of the type I protein was decreased. No changes in the ratios of the E2- binding proteins were observed in tumors grown in ovariectomized female or castrated male rats; however, the Kd for both proteins was increased in tumors from the latter group. Relative to that in intact females, tumor growth was retarded in intact male rats, but was unaffected by ovariectomy or castration. These studies demonstrate that the presence of the low affinity E2-binding protein does not necessarily predict E2 responsiveness. While the role of the type II cytosolic protein has not yet been established, it is possible that it could act as a reservoir for E2, which is required to activate specific biochemical functions such as progesterone receptor synthesis. Alternatively, it could be a nonactivated, perhaps norphosphorylated, form of the E2 receptor which binds E2 with only a very low affinity.