Rexinoid-induced Expression of IGFBP-6 Requires RARβ-dependent Permissive Cooperation of Retinoid Receptors and AP-1

Abstract

The synthetic rexinoid bexarotene (Targretin, LGD1069) inhibits the formation of both estrogen receptor-negative and estrogen receptor-positive breast cancer in preclinical models and controls the expression of growth-regulatory biomarkers, such as IGFBP-6 (insulin-like growth factor-binding protein 6), RARβ, or cyclin D1. In this study, we identified a classical retinoic acid-responsive element in the first intron in the IGFBP-6 gene adjacent to a consensus AP-1 binding site, both elements essential for rexinoid-induced expression of IGFBP-6. In chromatin binding experiments, bexarotene increased the occupancy of the identified enhancer element by RXRα, RARβ, cJun, cFos, and p300. In normal mammary epithelial cells and T47D breast cancer cells, small interfering RNA-mediated knockdown of all RXR isoforms or RARβ, but not RARα or RARγ alone, blocked the induction of IGFBP-6 by bexarotene. Simultaneous knockdown of RARα and RARγ abrogated both the induction of RARβ and the up-regulation and secretion of IGFBP-6. The suppression of either RARβ or cJun by small interfering RNA blocked the recruitment of RXRα and cJun to the enhancer. These results demonstrate a novel cooperative interaction between retinoid receptors and AP-1 orchestrated by RARβ and highlight a novel mechanism by which RARβ can mediate the cancer-preventive effects of rexinoids.