PROLACTIN RECEPTOR ANTAGONISTS THAT INHIBIT THE GROWTH OF BREAST-CANCER CELL-LINES

We investigated the mechanism of action of the human prolactin (hPRL) receptor on four different breast cancer cell lines, T-47D, MCF-7, BT- 474, and SK-BR3, that express elevated levels of the receptor compared with normal cells. Cells treated with human growth hormone (hGH), whi ch binds and activates the hPRL receptor, exhibited bell-shaped dose-r esponse growth curves consistent with the sequential dimerization mech anism proposed for the hPRL receptor (Fuh, G., Colosi, P., Wood, W.I., and Wells, J.A. (1995) J. Biol. Chem. 268, 5376-5381). Growth stimula tion was enhanced by Zn2+ which preferentially increases the affinity of hGH for the hPRL receptor. Furthermore, receptor-selective variants of hGH that bind the hPRL receptor but not the hGH receptor were agon istic, providing additional support that specific binding to the hPRL receptor can stimulate these breast cancer cells to grow. On this basi s we produced variants of hGH and human placental lactogen (hPL) that were potential antagonists because they bind but do not dimerize the h PRL receptor. The hPL-based antagonist was less potent than the hGH-ba sed antagonist toward the growth of MCF-7 cells, consistent with the l ower affinity of hPL for hPRL receptor than for hGH. However, the hPL- based antagonist was more potent than the hGH antagonist for BT-474 ce lls. Antibodies to the hPRL receptor inhibited growth of FDC-P1 cells transfected with the hPRL receptor; these also inhibited MCF-7 cells a nd T47D cells but not BT-474 cells. A unique feature of BT-474 cells w as found when screening its cDNA revealed the presence of a novel alte rnative splice of the hPRL receptor that codes for the soluble extrace llular domain; this may explain these differential inhibitory effects. These studies provide further molecular insight into the potential ro le of the hPRL receptor in breast cancer and demonstrate that hPRL rec eptor antagonists can inhibit the growth of breast cancer cells.