The epidemiological data suggest that breast cancer risk decreases in women who complete full-term pregnancy at a young age. Studies on a rat breast cancer model indicate that human chorionic gonadotropin (hCG), a hormone that is present in very high levels during pregnancy, could be responsible for this decrease. These findings, as well as those demonstrating the presence of functional luteinizing hormone (LH)/hCG receptors in human breast cells, prompted us to investigate the anti-proliferative and anti-invasive effects of hCG in human breast cancer MCF-7 cells by down-regulating NF-kappaB and AP-1 transcription factors. Treatment of MCF-7 cells with highly purified hCG resulted in a modest dose-dependent and hormone-specific decrease in cell proliferation. hCG treatment also decreased cell invasion, which was more dramatic than the decrease in cell proliferation. These hCG actions were abrogated when receptor synthesis was inhibited by treatment with antisense hCG/LH receptor phosphorothioate oligodeoxynucleotide. hCG treatment prevented the tumor necrosis factor-dependent NF-kappaB and AP-1 activation, which paralleled a decrease in the phosphorylation and degradation of IkappaBalpha. The findings that hCG treatment increased cAMP synthesis and activated cAMP-dependent protein kinase, dibutyryl cAMP mimicked hCG in preventing NF-kappaB activation, and dideoxyadenosine, an adenylate cyclase inhibitor, prevented the hCG effect on NF-kappaB suggested that the hCG actions are mediated via the cAMP-dependent protein kinase A signaling pathway. In summary, our results demonstrate that hCG has anti-proliferative and anti-invasive effects in MCF-7 cells by down-regulating NF-kappaB and AP-1. These findings support the premise that hCG could be responsible for the pregnancy-induced protection against breast cancer in women.