Abstract 2099: Cellular and molecular determinants of retinoic acid sensitivity in breast cancer

Abstract All-trans-retinoic acid (ATRA) is the primary metabolite of vitamin A and a promising agent in the treatment/prevention of solid tumors, including breast cancer. Breast cancer is a very heterogeneous disease that can be classified according to the global gene-expression profiles into various subgroups. A rational use of ATRA in the clinics requires definition of the breast cancer subtypes characterized by sensitivity/natural-resistance to this compound and derived retinoids. In addition, it is fundamental to define the cellular and molecular mechanisms underlying the sensitivity/resistance to ATRA. As a first step in this direction, 44 breast cancer cell lines, belonging to the Cancer Cell Line Encyclopedia (CCLE) of the Broad Institute and recapitulating the heterogeneity of this tumor, were evaluated for their sensitivity to the anti-proliferative activity of ATRA. Individual cell lines were challenged with increasing concentrations of the retinoid for 3, 6 and 9 days. The concentrations of ATRA causing 50% of the maximal growth inhibition (IC50) and the maximal effect observed were calculated. The two parameters, along with the duplication time of the cell lines, were used to define a novel algorithm (ATRA SCORE). The ATRA score is a continuous parameter which defines the sensitivity of the cell lines to ATRA in a quantitative fashion. The lower the ATRA score the higher the sensitivity of the cell to the anti-proliferative action of ATRA. Using this score, an enrichment in cell lines characterized by a luminal phenotype and/or estrogen receptor (ER)-positivity was observed in the group showing sensitivity to ATRA. In contrast an enrichment in cell lines with a basal phenotype and/or ER-negativity was evident in the group characterized by natural resistance to the retinoid. Measurement of the mRNAs corresponding to various isoforms of the retinoid receptors belonging to the RAR and RXR family in the 44 cell lines allowed us to establish correlations between the expression of these transcripts and ATRA sensitivity. With respect to this, we found a significant and direct correlation with the expression levels of RARalpha1. This indicates that the receptor represents a determinant and a predictor of ATRA- sensitivity in breast cancer cells. The correlation was confirmed also at the protein level using specific RARalpha antibodies. Most of these findings were validated with the use of short-term cultures of breast cancer tissue slices obtained from surgical specimens. The availability of the global gene-expression profiles of each cell line permitted the construction of a gene fingerprint consisting of approximately 130 elements that has the potential to predict sensitivity to ATRA not only in breast cancer cell lines, but also in tumor samples. In addition, the identified gene fingerprint allowed the identification of various and unexpected molecular determinants of sensitivity which do not belong to the classic retinoid pathway. Citation Format: Enrico Garattini, Floriana Centritto, Silvio K. Garattini, Gabriela Paroni, Marco Bolis, Maddalena Fratelli, Mineko Terao. Cellular and molecular determinants of retinoic acid sensitivity in breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2099. doi:10.1158/1538-7445.AM2014-2099