Naloxone acts as an antagonist of estrogen receptor activity in MCF-7 cells

Estrogen promotes the growth of breast cancer via estrogen receptors (ER). Earlier studies showed that the opioid receptor antagonist naloxone inhibits MCF-7 breast cancer growth in mice. We examined the cellular and molecular mechanism of naloxone antagonism of ER alpha activity in human MCF-7 cells. Naloxone (100 nmol/L) inhibited 17 beta-estradiol (E2)-induced (10 nmol/L) MCF-7 cell proliferation by 65% and mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation. Naloxone blocked the E2-induced activation of ER alpha, with 85% inhibition after 5 minutes and 100% recovery after 60 minutes. This assay is based on quantitation of E2-activated nuclear ER alpha binding to the immobilized coactivator peptide. A significant decrease in E2-induced ER alpha transactivation was observed in the presence of naloxone in the estrogen response element-lucif erase reporter assay (P < 0.05, E2 versus E2 + naloxone). Naloxone also blocked E2-induced down-regulation of ER alpha mRNA at 30 minutes and 6 hours. Although naloxone inhibits ER alpha activity directly, it also induces a cross-talk between mu-opioid receptor (MOR) and ER alpha. Immunoprecipitates with anti-MOR antibody showed the presence of ER alpha in cells incubated with E2 in the presence of naloxone but not in cells incubated with E2 or naloxone alone. Higher amounts of ER alpha associated with MOR after 60 minutes compared with 10 minutes of incubation. Naloxone inhibited E2-bovine serum albumin-FITC binding to plasma membrane associated ER alpha and also inhibited the direct binding of [H-3]E2 to ER alpha. Thus, naloxone modulates ER alpha activity directly as well as indirectly via MOR. This study suggests that naloxone-like compounds can be developed as novel therapeutic molecules for breast cancer therapy.