- 1α,25-dihydroxyvitamin D3 (1,25D) is a powerful differentiation agent, which has potential for treatment of acute myeloid leukemia (AML), but induces severe hypercalcemia at pharmacologically active doses. We have previously shown that carnosic acid (CA), the polyphenolic antioxidant from rosemary plant, markedly potentiates differentiation induced by low concentrations of 1,25D in human AML cell lines. Here, we demonstrated similar enhanced differentiation responses to the 1,25D/CA combination in primary leukemic cells derived from patients with AML, and determined the role of the Nrf2/antioxidant response element (Nrf2/ARE) pathway in these effects using U937 human monoblastic leukemia cells as the model. CA strongly transactivated the ARE-luciferase reporter gene, induced the ARE-responsive genes, NADP(H)-quinone oxidoreductase and the γ-glutamylcysteine synthetase heavy subunit, and elevated cellular glutathione levels. Interestingly, 1,25D potentiated the effects of CA on these activities. Stable transfection of wild-type (wt) Nrf2 resulted in the enhancement, while transfection of dominant-negative (dn) Nrf2 produced suppression of differentiation induced by the 1,25D/CA combination and, surprisingly, by 1,25D alone. These opposite effects were associated with a corresponding increase or decrease in vitamin D receptor and retinoid X receptor-α protein levels, and in vitamin D responsive element transactivation. Cells transfected with wtNrf2 and dnNrf2 also displayed opposing changes in the levels of the AP-1 family proteins (c-Jun and ATF2) and AP-1 transcriptional activity. Pretreatment with AP-1 decoy oligodeoxynucleotide markedly attenuated the differentiation in wtNrf2-transfected cells, suggesting that the pro-differentiation action of Nrf2 is mediated by functional AP-1. Our findings suggest that the Nrf2/ARE pathway plays an important part in the cooperative induction of myeloid leukemia cell differentiation by 1,25D and a plant polyphenol.