Tamoxifen prevents the recurrence of breast cancer and is also beneficial against bone demineralization and arterial diseases. It acts as an ER (estrogen receptor) α antagonist in ER-positive breast cancers, whereas it mimics the protective action of 17β-estradiol in other tissues such as arteries. However, the mechanisms of these tissue-specific actions remain unclear. Here, we tested whether tamoxifen is able to accelerate endothelial healing and analyzed the underlying mechanisms. Using 3 complementary mouse models of carotid artery injury, we demonstrated that both tamoxifen and estradiol accelerated endothelial healing, but only tamoxifen required the presence of the underlying medial smooth muscle cells. Chronic treatment with 17β-estradiol and tamoxifen elicited differential gene expression profiles in the carotid artery. The use of transgenic mouse models targeting either whole ERα in a cell-specific manner or ERα subfunctions (membrane/extranuclear versus genomic/transcriptional) demonstrated that 17β-estradiol-induced acceleration of endothelial healing is mediated by membrane ERα in endothelial cells, while the effect of tamoxifen is mediated by the nuclear actions of ERα in smooth muscle cells. Whereas tamoxifen acts as an antiestrogen and ERα antagonist in breast cancer but also on the membrane ERα of endothelial cells, it accelerates endothelial healing through activation of nuclear ERα in smooth muscle cells, inviting to revisit the mechanisms of action of selective modulation of ERα.Read less <