The breast cancer resistance protein ABCG2 effluxes a variety of drugs and is believed to play an important role in multidrug resistance to chemotherapy. We show here for the first time that dexamethasone (DEX) and progesterone (PROG) are able to strongly inhibit ABCG2 expression in progesterone receptor (PR)-positive MCF7 and PR-negative MDA-MB-231 breast cells. In contrast, in the latter cells stably-transfected with progesterone receptor isoforms A and B, ABCG2 expression was strongly up-regulated by DEX and PROG. In addition, two other ligands of Pregnane X Receptor (PXR) and/or Glucocorticoid Receptor (GR) were also able to clown-regulate ABCG2 expression in PXR- and GR-positive MCF7 cells. ABCG2 expression regulation by DEX likely resulted from the activation of PR-, PXR-, and/or GR-signaling pathways. ABCG2 expression inhibition by DEX was associated with increased sensitivity to mitoxantrone, a known ABCG2 substrate. The findings suggest that DEX may be useful in improving drug efficacy under certain conditions. (C) 2008 Elsevier Inc. All Fights reserved.