Differential regulation of sodium/iodide symporter gene expression by nuclear receptor ligands in MCF-7 breast cancer cells

Abstract

The sodium/iodide symporter (NIS) mediates iodide uptake in lactating breast tissue and is expressed in some breast cancers. We have previously demonstrated that all-trans retinoic acid (tRA) stimulates NIS gene expression and the selective cytotoxic effect of β-emitting radioiodide-131 ( 131I) in both in vitro and in vivo MCF-7 breast cancer cell systems. We studied the ability of natural and synthetic retinoids, in combination with other nuclear receptor ligands, to achieve greater and more sustained induction of NIS in MCF-7 cells and enhance 131I-mediated cytotoxicity. Selective stimulation of retinoic acid receptor (RAR) β/γ produced marked NIS induction; and selective stimulation of RARα, RARγ, or retinoid X receptor produced more modest induction. Maximal NIS induction was seen with 9-cis retinoic acid and AGN190168, a RAR β/γ-agonist. Dexamethasone (Dex), but not the other nuclear receptor ligands, in combination with tRA synergistically induced iodide uptake and NIS mRNA expression, predominantly by prolonging NIS mRNA half-life. The addition of Dex reduced the EC50 of tRA for NIS stimulation to approximately 7%, such that 10-7 M tRA with addition of Dex enhanced iodide uptake and selective cytotoxicity of 131I greater than 10-6MtRA alone. AGN190168 combined with Dex synergistically increased iodide uptake and significantly prolonged induction (5 d) of iodide uptake compared with that induced by the combination of tRA/Dex or 9-cis retinoic acid/Dex. The addition of Dex reduced the effective dose of retinoid and prolonged the induction of NIS, especially with AGN190168, suggesting higher efficacy of 131I after combination treatment. Copyright © 2005 by The Endocrine Society.