| Folate receptor (FR) type β is a promising target for therapeutic intervention in acute myelogenous leukemia (AML) owing particularly to its specific upregulation in AML cells by all-trans retinoic acid (ATRA) (Wang et al., 2000; Pan et al., 2002). Here we identify functional elements in the FR-β gene and examine the molecular mechanism of transcriptional induction of FR-β by ATRA. The basal promoter activity of FR-β resulted from synergistic interaction between Sp1 and Ets binding (EBS) elements and repression by upstream AP-1-like elements, whose action required EBS. A minimal promoter containing the Sp1 and Ets elements was ATRA-responsive. The repressor elements bound Fos family proteins; association of the proteins with the repressor elements correlated negatively with FR-β expression in peripheral blood neutrophils and monocytes and also in KG-1 (AML) cells grown in the absence or in the presence of ATRA. Furthermore, downregulation of FR-β in KG-1 cells treated with O-tetradecanoylphorbol 13-acetate (TPA) was accompanied by increased AP-1 binding to the repressor elements. From chromatin immunoprecipitation (ChIP) assays, the nuclear receptor RARα associated with the Sp1 region and RARs β and γ associated with the AP-1 and Sp1 regions; treatment of KG-1 cells with ATRA did not alter Sp1 binding but increased the association of RARα and decreased the association of RARs β and γ. ATRA also decreased RAR expression levels. The results suggest that the FR-β gene is a target for multiple coordinate actions of nuclear receptors for ATRA directly and indirectly acting on a transcriptional complex containing activating Sp1/Ets and inhibitory AP-1 proteins. The multiple mechanisms favor the prediction that ATRA will induce FR-β expression in a broad spectrum of AML cells. Further, optimal FR-β induction may be expected when all three RAR subtypes bind agonist. |
