1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the most active vitamin D metabolite, regulates proliferation, survival, and differentiation in many cell types. 1,25(OH)(2)D(3) and several less calcemic analogs are in clinical trials against various neoplasias. We studied the effects of 1,25(OH)(2)D(3) on a panel of human breast cancer cells, which show similar vitamin D receptor (VDR) content but variable transcriptional and anti-proliferative responsiveness. In MDA-MB-453 cells, one of the responsive lines, 1,25(OH)(2)D(3) increased cell and nuclear size and induced a change from a rounded to a flattened morphology. By phase contrast, laser confocal and electron microscopy, we found that 1,25(OH)(2)D(3) changed the cytoarchitecture of actin filaments and microtubules and nuclear shape, induced filopodia and lamellipodia, and promoted cell-to-cell contacts via large cytoplasmic extensions. However, although claudin-7 and occludin content in the cells increased upon exposure to 1,25(OH)(2)D(3), these proteins were not located at the plasma membrane probably due to the absence of E-cadherin expression. Additionally, 1,25(OH)(2)D(3) induced the accumulation of alpha(v)-integrin, beta(5)-integrin, focal adhesion kinase (FAK), and paxillin in focal adhesion plaques, concomitant with the increased phosphorylation of the FAK. 1,25(OH)(2)D(3) enhanced MDA-MB-453 and MDA-MB-468 cell adhesion to plastic but decreased adhesion to laminin. The expression of the mesenchymal marker N-cadherin and of the myoepithelial marker P-cadherin was down-regulated by 1,25(OH)(2)D(3) in several breast cancer cell lines. Other myoepithelial proteins such as alpha(6)-integrin, beta(4)-integrin, and smooth muscle alpha-actin (SMA) were also repressed by 1,25(OH)(2)D(3) in MDA-MB-453 and MDA-MB-468 cells. Accordingly, mice lacking VDR (Vdr(-/-)) showed abnormally high levels of SMA and P-cadherin in their mammary gland. These findings show that 1,25(OH)(2)D(3) profoundly affects the phenotype of breast cancer cells, and suggest that it reverts the myoepithelial features associated with more aggressive forms and poor prognosis in human breast cancer.