Abstract 1717: Combinations of retinoids and lapatinib in the treatment of Her2/Neu-positive breast carcinomas with co-amplification of the <i>ERBB2</i> and <i>RARA</i> genes
Authors: Enrico Garattini, Valeria Guarnaccia, Cristina Bassani, Giorgio Gardini, Adriana Zanetti, Gabriela Paroni
Journal: Cancer Research (2010)

Abstract The current study represents proof of principle for the use of all-trans retinoic acid (ATRA) alone or in combination with the tyrosine kinase inhibitor, lapatinib, in the management of a subset of Her2/Neu-positive breast carcinomas. First-line treatment of Her2/Neu-positive breast carcinoma is based on the monoclonal antibody, trastuzumab, while specific tyrosine kinase inhibitors, such as lapatinib, are used as second-line agents. Novel and more effective therapeutic strategies are called for this type of cancer. ATRA and derivatives (retinoids) have been proposed in the chemoprevention and treatment of breast carcinoma. Retinoids activate specific nuclear receptors (RARs and RXRs), which mediate the therapeutic effects of these agents. The beneficial activity of retinoids is limited to ER-positive breast cancer cells, possibly as a consequence of RARα induction by estrogens. A significant fraction of Her2/Neu-positive breast carcinomas are ER-negative, hence this type of cancer is predicted to be refractory to retinoids. Her2/Neu-positivity is very often the result of an amplification of the corresponding gene (ERBB2), which maps to chromosome 17, approximately 650 kilobases upstream of the locus encoding RARα (RARA). Given this background, we evaluated whether a proportion of Her2/Neu-positive and ER-negative breast carcinomas are characterized by co-amplification of ERBB2 and RARA, using real-time PCR on genomic DNA extracted from paraffin sections from 60 patients. We observed that 28% of the cases showed co-amplification of ERBB2 and RARA, while the amplicon did not contain the latter in the remaining fraction of patients. RARA amplification was associated with increased levels of the corresponding RARα protein, as determined by immuno-histochemistry. To evaluate whether RARA co-amplification and consequent RARα over-expression was associated with retinoid sensitivity, we turned to a panel of representative breast carcinoma cell lines. We observed that the growth of two cell lines, SKBR3 and UACC812, characterized by ERBB2 and RARA co-amplification, was inhibited by ATRA. Growth inhibition was accompanied by differentiating and pro-senescence responses to the retinoid. All these phenomena were mediated by RARα, as demonstrated by the use of selective retinoid agonists and by silencing of the receptor by siRNAs. Similar effects were not observed in MDA-MB-453 and HCC1569 cells, which carry a simple ERBB2 amplicon and are totally refractory to the various effects of ATRA. In SKBR3 and UACC812, inhibition of Her2/Neu tyrosine kinase activity by lapatinib, led to a dramatic enhancement of all the retinoid responses, demonstrating strong synergism between the two compounds. Gene expression profiling and mechanistic studies in SKBR3 and UACC812 indicated a two-way cross-talk between Her2/Neu and RARα. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1717.