Processing of prolactin by human breast cancer cells in long term tissue culture

Two human breast cancer cell lines (T-47D and MCF-7) and one cell line derived from normal human milk (HBL-100) not only specifically bound but also degraded prolactin. Quantitative differences in the ability to bind and degrade prolactin among the cell lines exist, although there was a good correlation between the number of prolactin receptor sites and prolactin degradative activity. Iodo-prolactin as well as native prolactin were degraded. The prolactin molecule was processed to yield at least three small molecular weight peptides which were released into the incubation medium. These peptides neither bound to fresh receptors nor to anti-prolactin antibodies. The protease inhibitor N-alpha-p-tosyl-L-lysine chloromethyl ketone, lysosomotropic agents such as chloroquine and ammonium chloride, and metabolic inhibitors 2,4-dinitrophenol and sodium azide, all abolished prolactin degradation by the breast cancer cells. When prolactin degradation was inhibited, specific binding and the subsequent release of intact 125I-prolactin was still observable, suggesting that hormonal degradation was not a prerequisite to dissociation of prolactin. However, prolactin degradation did account for the accelerated rate of dissociation of prolactin. Studies utilizing inhibitors suggest that the receptor-bound 125I-prolactin was degraded by an energy-dependent internalization process such as pinocytosis; lysosomal enzymes are probably involved in the degradation of prolactin by human breast cancer cells.