Retinoids are in clinical trials in the prevention and treatment of different types of cancer. We observed that a group of retinoids called retinyl ethers exhibit tissue-specific chemopreventive activity against carcinogen-induced mammary tumors in rats. We also observed that the active retinyl ethers specifically bind to cellular retinol binding protein (CRBP) even though they lack an active OH group. We are now focusing, through a multidisciplinary approach, on the development of two novel series of retinyl ethers and elucidation of their mechanism of action in cancer prevention. The new retinoic acid receptor (RAR and RXR)-selective all-trans- and 9-cis-retinoids will be evaluated for their chemopreventive potential against methylnitrosourea-induced mammary cancer in rats. We will determine the binding affinity of the ethers for CRBP and serum retinol binding protein (RBP), and assess whether a correlation exists between these affinities and their biological potency in different assays. We will evaluate, in MCF-cells as well as in a reconstituted system, whether the retinyl ethers act as such, or as pro-drugs that are metabolized to an active form subsequent to CRBP-binding. We will also study, analogous to retinoic acid (RA) import, the possible existence of a CRBP-dependent channeling for transmigration of the ethers/retinol to the nuclei of MCF cells. We will further elucidate the role, if any, for the ethers, albeit poor binders of RARs/RXRs, in RAR/RXR homo- and hetero-dimerization, as well as transcriptional complex formation with the RA-response elements. By using gel mobility shift assays and glutathione S-transferase assays, we will also evaluate if the antagonistic action of retinyl ethers is via a modulation of co-activators/co-repressors of RARs/RXRs. An understanding of the metabolic fate and the mode of action of retinyl ethers is critical for the rational development of the new retinoids in the control of breast cancer.
