Synergistic induction of folate receptor beta by all-trans retinoic acid and histone deacetylase inhibitors in acute myelogenous leukemia cells: mechanism and utility in enhancing selective growth inhibition by antifolates.

The folate receptor (FR) type beta is a promising target for therapeutic intervention in acute myelogenous leukemia (AML), owing particularly to its selective up-regulation in the leukemic cells by all-trans retinoic acid (ATRA). Here we show, using KG-1 and MV4-11 AML cells and recombinant 293 cells, that the histone deacetylase (HDAC) inhibitors trichostatin A (TSA), valproic acid (VPA), and FK228 potentiated ATRA induction of FR-beta gene transcription and FR-beta mRNA/protein expression. ATRA and/or TSA did not induce de novo FR synthesis in any of a variety of FR-negative cell lines tested. TSA did not alter the effect of ATRA on the expression of retinoic acid receptor (RAR) alpha, beta, or gamma. Chromatin immunoprecipitation assays indicate that HDAC inhibitors act on the FR-beta gene by enhancing RAR-associated histone acetylation to increase the association of Sp1 with the basal FR-beta promoter. Under these conditions, the expression level of Sp1 is unaltered. A decreased availability of putative repressor AP-1 proteins may also indirectly contribute to the effect of HDAC inhibitors. Finally, FR-beta selectively mediated growth inhibition by (6S) dideazatetrahydrofolate in a manner that was greatly potentiated in AML cells by ATRA and HDAC inhibition. Therefore, the combination of ATRA and innocuous HDAC inhibitors may be expected to facilitate selective FR-beta-targeted therapies in AML.