In vitro modulation of prolactin binding to human mammary carcinoma cells by steroid hormones and prolactin.

This study demonstrates the specific binding of human (h) PRL to mammary carcinoma cells of the newly established line EFM-19. Under saturating conditions, [125I]hPRL bound to these cells with high affinity (Ka = 4.3 X 10(10) M-1) and low capacity (4080 binding sites/cell). In serum-free medium, stimulation of cell proliferation by PRL was found, suggesting a biological role in the growth of human breast cancer. hPRL was more effective in binding to and promoting the growth of EFM-19 cells than ovine PRL or other lactogenic hormones. Up- and down-regulation of [125I]hPRL binding occurred after pretreatment of EFM-19 cell cultures with subphysiological or higher hPRL concentrations, respectively. Physiological concentrations of 17 beta-estradiol or dihydrotestosterone increased the cellular capacity of hPRL binding. Pharmacological concentrations of dihydrotestosterone or physiological concentrations of progesterone reduced the binding of [125I]hPRL. The results provide evidence for complex regulatory mechanisms of PRL binding by human mammary carcinoma cells involving steroid hormones.