Retinoic acid (RA) exhibits chemotherapeutic and chemopreventive activities in a number of cancers, including breast cancer. RA therapy is however confounded by a marked toxicity of this agent at pharmacological doses and by development of RA-resistance in tumors. The biological activities of RA are mediated by the nuclear hormone receptors retinoic acid- and retinoid X- receptors (RARs and RXRs). In addition, RA binds in cells to two intracellular proteins, cellular RA binding proteins I and II (CRABP-I and CRABP-II). We previously found that CRABP-II (but not CRABP-I) facilitates the delivery of RA to the receptor that is activated by it, namely RAR. We found further that this activity of CRABP-II enhances the transcriptional activity of RAR, suggesting that the presence of the binding protein could sensitize cells to the biological activities of RA. Our recent data indeed indicate that CRABP-II expression enhances the responsiveness of cultured mammary carcinoma cells to RA-induced growth inhibition, and suppresses tumor development in two mouse models of breast cancer. Hence, CRABP-II may comprise a novel target for therapy and prevention of some cancers. To better understand the mechanisms underlying the activities of RA and CRABP-II in mammary carcinomas, this application has the following specific aims: (1) To identify target genes that are regulated by RA and CRABP-II in MCF-7 mammary carcinoma cells. Affymetrix expression arrays will be utilized to compare gene expression profiles of MCF-7 cells that express different levels of CRABP-II, in the absence of or upon treatment with RA. (2) To study the effects of CRABP-II and RA on tumor growth in transgenic breast cancer mouse models. The mouse model MMTV-neu/Erb-B2, in which breast tumors arise spontaneously and progress under immune surveillance, will be used. Two mouse models will be generated: MMTV-neu mice with disrupted expression of CRABP-II, and MMTV-neu mice that over-express CRABP-II in mammary tissue. Characterization of tumors that develop in mice expressing different levels of CRABP-II will allow us to assess the ability of the protein to inhibit tumorigenesis and to sensitize breast tumors to RA-induced growth inhibition in vivo. In addition, Affymetrix arrays will be used to analyze genes that are involved in the effects of CRABP-II expression and RA administration on tumor growth in MMTV-neu mice.
