The present thesis demonstrates an association between a protein member of the nuclear receptor family, estrogen receptor alpha (ERα), and an integral membrane protein, beta 1 integrin (β1-integrin), in breast tumor cells. This association takes place at the plasma membrane and regulates ERα subcellular localization, its degradation rate and both its genomic and nongenomic activities. Conversely, ERα affects β1-integrin activity by modulating its internalization and recycling dynamics and its degradation rate. This dynamic reciprocity system is involved in key processes for tumor cell biology. These include alterations in cellular adhesion and motility triggered by estrogens and tamoxifen resistance induced by the extracellular matrix. Tamoxifen is an antagonist of ERα in the breast and represents the endocrine therapy of choice for 70% of breast cancer patients. The results presented in this thesis show a novel mechanism of β1-integrin and ERα endocytosis induced by estrogens and, moreover, suggest that this internalization is the first step for triggering the non-genomic and genomic signaling cascades typically induced by ERα, essential for breast cancer cell growth and dissemination.