Abstract
Rat mammary tumors contain a unique class of cryptic cellβsurface prolactin receptors that can be unmasked by depleting the cells of energy. These cryptic receptors, which are found in mammary tumors and nonlactating normal mammary cells but not in differentiated mammary tissue, are continuously inserted and rapidly removed from the cell surface. In this report we demonstrate that prolactin regulates the level of cryptic receptors. Treatment of primary cultures of rat mammary tumor cells with prolactin at concentrations between 0.1 and 0.5 ng/ml caused cryptic receptor levels to increase within 24 h, and this increase was maintained for up to 6 days. At prolactin concentrations of 10β50 ng/ml, receptor levels were the same as in cells incubated without hormone, while a decrease in the steadyβstate level of cryptic receptors was induced within 24 h by 100β500 ng prolactin/ml. Concentrations of 1,000β5,000 ng prolactin/ml caused a rapid, doseβdependent down regulation of cryptic receptor sites. Down regulation at 5,000 ng prolactin/ml was (1) complete (84 Β± 5% reduction) in 1 h; (2) specific for lactogenic hormones; (3) completely reversed within 10 h after prolactin removal; (4) energy dependent; and (5) not blocked by the cytoskeleton active agents cytochalasin B and colchicine or by NH~4~CI, which inhibits hormone degradation. We conclude that rat mammary tumor cells have the capacity to autoβregulate cryptic prolactin receptors, a property that supports our notion that such receptors play a role in regulating prolactin responsiveness. The observed pattern of cryptic receptor autoregulation in response to prolactin concentration and time of exposure suggests that a pool of cryptic sites provides these cells with the capacity to respond to prolactin concentrations from pg to ΞΌg/ml, a range well beyond the K~d~ for the receptor itself. Since prolactin receptors in mammary tumors are not down regulated unless prolactin concentrations are well beyond the saturation point, these cells may have a selective growth advantage over cells in normal mammary tissue.