The present invention provides methods and reagents for treating cancer cells for therapeutic purposes, by contacting with a sumoylation inhibitor in a dose effective to block sumoylation of TFAP2A. In breast cancer cells the sumoylation inhibitor induces a basal to luminal shift in phenotype. Sumoylation inhibitors also reduce the number of cancer stem cells in a cancer cell population. Inhibition of sumoylation makes cancer cells more responsive to conventional chemotherapeutic therapy and radiation therapy and decreases recurrence or development of metastases.
