Abstract 2287: A sub-population of HER2+ breast carcinomas is characterized by co-amplification of the ERBB2 and RARA genes that renders cancer cells sensitive to retinoids and combinations of these agents with lapatinib
Authors: Garattini Enrico , Flora Marcella , Gardini Giorgio , Paroni Gabriela 
Published: June 2012

Abstract
In breast cancer, HER2-positivity is often the result of an amplification of the corresponding gene (ERBB2) mappping to chr 17 close to the locus of the retinoid receptor, RARα (RARA). We set up a PCR assay and a FISH method to evaluate the proportion of patients with co-amplification of ERBB2 and RARA using sections obtained from 76 HER2+ cases. We observed that 20-28% of the cases showed co-amplification of ERBB2 and RARA (ERBB2+/RARA+). To evaluate whether RARA co-amplification translated into retinoid sensitivity, we turned to a panel of 12 breast carcinoma cell lines. The growth of 3 ERBB2+/RARA+ cell lines, SKBR3, UACC-812 and AU565, was inhibited by all-trans-retinoic acid (ATRA). In response to the retinoid, growth inhibition was accompanied by cell differentiation. ERBB2+/RARA− cell lines were totally refractory to ATRA. Interestingly, SKBR3, UACC-812, and AU565 showed IC50 values to the anti-proliferative effects of ATRA falling in the low nanomolar range. These values were two orders of magnitude lower than the IC50s calculated for the control cell lines MCF-7 and T47D (ERBB2−/RARA−), representing popular models of the well known association between ERα-positivity and sensitivity to retinoids in breast carcinoma. We evaluated whether targeting the retinoid and HER2 pathways simultaneously has the potential to represent a viable therapeutic strategy for this group of patients. The anti-proliferative, differentiating and apoptotic effects of combinations between ATRA and lapatinib, a tyrosine kinase inhibitor used for the treatment of HER2+ breast carcinoma, was assessed in our panel of cell lines. In agreement with retinoid sensitivity, lapatinib and ATRA determined synergistic anti-proliferative effects only in the case of the cell lines, SKBR3, UACC-812 and AU565. Furthermore, apoptosis was observed only in ERBB2+/RARA+ cells treated with the combination of lapatinib and ATRA. In SKBR3 cells, this effect was preceded by a lapatinib-dependent enhancement of the lactogenic differentiation afforded by ATRA. The molecular mechanisms underlying the cross-talk between lapatinib and ATRA were evaluated focusing specifically on the RARα and HER2 signal transduction pathways, as well as performing whole-genome gene expression studies in SKBR3 cells. These last studies and functional approaches demonstrated that the combination produced perturbations of the TGFβ, FGF and IGF pathways and indicated a role for RIG-I and FOXO3A in the cross-talk. Our results constitute the rationale for the use of combinations between lapatinib and ATRA in the treatment of ERBB2+/RARA+ breast cancer.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2287. doi:10.1158/1538-7445.AM2011-2287