Melatonin and breast cancer: Investigation of a potential involvement of prolactin in the mechanism by which melatonin inhibits cell growth.

Abstract

The pineal hormone melatonin (MLT) inhibits the promotion of human breast cancer cell growth in vitro by prolactin (PRL). When MCF-7 estrogen receptor (ER) positive human breast cancer cells were grown in a monolayer culture system containing 5% charcoal-stripped fetal bovine serum (CSS) in the presence of PRL and/or MLT for up to 6-days, MLT at each of the time-points tested significantly reduced the mitogenic response to PRL. In fact, in some instances the addition of MLT further reduced growth below the levels of untreated controls. Doses of MLT within the normal nighttime physiological range (10-⁹M - 10¹²M) were most effective. However, MLT alone had no effect on growth. Additionally, MLT partially to completely blocks the "PRL-like" activity of human growth hormone and several different monoclonal antibodies (Mabs) directed against the PRL receptor (PRLR), which stimulate growth via an interaction with the PRLR. Furthermore, MLT diminishes the PRLR dimerization-induced enhancement of the Mabs activity by a second cross-linking antibody further indicating that MLT specifically blocks the PRLR-mediated growth signal in these cells. Melatonin inhibits PRL's activity in ER-positive ZR75-1, and ER-negative HS578T breast cancer cells as well. Conversely, neither MLT nor PRL has any effect on the growth of BT20 ER-negative cells, suggesting that MLT's activity depends more on the PRL-responsiveness of the cells than their ER-status. Moreover, MLT's activity was estrogen-independent since it is seen under phenol red-free (estrogen deficient) culture conditions. Under these conditions, MLT almost completely blocks PRL's 7-fold stimulation of growth. Melatonin also inhibits the modest stimulatory effect of PRL on MCF-7 cell colonies in soft agar. However, under these culture conditions sensitivity to MLT was altered. Polyamines (PA) reversed; the inhibitory effects of MLT, indicating their potential importance in MLT's mechanism of action. However, no effect of MLT was seen on ornithine decarboxylase (ODC), the rate-limiting enzyme in the synthesis of PA. Melatonin did, however, block the PA rescue of cells from difluoromethylornithine's (DFMO), suggesting that MLT may block PA action.