Differentiation-independent retinoid induction of folate receptor type β, a potential tumor target in myeloid leukemia

Abstract

AbstractFolate receptor (FR) type β is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-β expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-β expression in vitro in myeloid leukemia cells from patient marrow. FR-β was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D3, or transforming growth factor β. ATRA did not induce FR-β expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-β expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-β promoter–luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-β promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)α, RARβ, or RARγ, it appears that the retinoid effect on FR-β expression could be mediated by ligand binding to RARs α, β, or γ, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARα and RARγ selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-β expression. Thus, blocks in the RARα-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-β expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-β expression in myeloid leukemia cells refractory to retinoid differentiation therapy.