Abstract Background: Much is known about the mechanisms involved in the response to anti-hormonal treatment or those involved in the response to antiangiogenic therapy. It is also known the association between angiogenesis and hormonal status, both in physiological and pathological settings. However, the molecular and cellular mechanisms contributing to the efficacy of combined antiangiogenic-antihormonal therapy in breast cancer are still unknown. This combination is currently in clinical trials, but unfortunately there are scarce preclinical studies contributing to the rationale of combining antiangiogenic and antihormonal therapies. Aims: To define the mechanisms involved in the response to combined antiangiogenic-antihormonal treatments in breast cancer cells. Methods: Breast cancer cell lines with different estrogen dependence (MCF-7, BT-474, MDA-MB-231) were subjected to an estrogen gradient (estradiol), and treated with fulvestrant (antiestrogen), plus bevacizumab (anti-angiogenic). Cellular proliferation and apoptosis were determined using the corresponding kits. Proliferation and survival intracellular signaling pathways, estrogen alpha and VEGF receptors activation and COX-2 expression were analyzed by western-blot using specific antibodies. Results: In estrogen-dependent breast cancer cells (MCF-7 and BT-474) the pro-proliferative effect of estradiol decreased after bevacizumab treatment. Furthermore, the combination of the antiangiogenic with an antiestrogen enhanced this antiproliferative effect, that was also related to the reduction in the levels of VEGF-A in the culture medium and to diminished ER alpha phosphorylation. The combined treatment also altered the phosphorylation of Akt and Erk1/2 signaling kinases. Interestingly, bevacizumab treatment augmented COX-2 activity in MCF-7. Conclusions: Our results suggest that in estrogen dependent breast cancer cells the anti-proliferative effect of bevacizumab depends on estradiol concentration, that in turn affects VEGF production levels, using a different mechanism to apoptosis. The combination of bevacizumab with antiestrogens enhances this antitumoral effect, altering intracellular signaling pathways of proliferation and survival. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-05-07.