The endogenous cannabinoid anandamide inhibits human breast cancer cell proliferation

Anandamide was the first brain metabolite shwon to act as a ligand of "central" CB1 cannabinoid receptors. Here we report that the endogenous cannabinoid potently and selectively inhibits the proliferation of human breast cancer cells in vitro. Anandamide dose-dependent inhibited the proliferation of MCF-7 and EFM-19 cells with IC50 values between 0.5 and 1.5 mu M and 83-92% maximal inhibition at 5-10 mu M. The proliferation of several other nonmammary tumoral cell lines was not affected by 10 ELM anandamide. The anti-poliferative effect of anandamide was not due to toxicity or to apoptosis of cells but was accompained by a reduction of cells in the S phase of the cell cycle. A stable analogue of anandamide (R)-methanandamide, another endogenous cannabinoid, 2-arachidonoylglycerol and the synthetic cannabinoid HU-210 also inhibited EFM-19 cell proliferation, whereas arachidonic acid was much less effective. These cannabimimetic substances displaced the binding of the selective cannabinoid agonist [H-3] CP 55, 940 to EMF-19 membranes with an order potency identical to that observed for inhibition of EFM-19 cell proliferation. Moreover, anandamide cytostatic effect was inhibited by the selected CB1 receptor antagonist SR 141716A. Cell proliferation was arrested by a prolactin mAb and enhanced by exogenous human prolactin, whose mitogenic action was reverted by very low (0.1-0.5 mu M) doses of anandamide. Anandamide suppressed the levels of the long form of prolactin receptor in both EFM-19 and MCF-7 cells, as well as a typical prolactin-induced response, i.e., the expression of the breast cancer cell susceptibility gene brca1. These data suggest that anandamide blocks human breast cancer cell proliferation through CB1-like receptor-mediated inhibition of endogenous prolactin action at the level of prolactin receptor.