Retinoic Acid Sensitivity of Triple-Negative Breast Cancer Cells Characterized by Constitutive Activation of the notch1 Pathway: The Role of Rarβ

Abstract

Triple-negative breast cancer (<i>TNBC</i>) is a heterogeneous disease that lacks effective therapeutic options. In this study, we profile eighteen <i>TNBC</i> cell lines for their sensitivity to the anti-proliferative action of all-trans retinoic acid (ATRA). The only three cell lines (<i>HCC-1599</i>, <i>MB-157</i> and <i>MDA-MB-157</i>) endowed with ATRA-sensitivity are characterized by genetic aberrations of the <i>NOTCH1</i>-gene, causing constitutive activation of the NOTCH1 γ-secretase product, N1ICD. N1ICD renders <i>HCC-1599</i>, <i>MB-157</i> and <i>MDA-MB-157</i> cells sensitive not only to ATRA, but also to γ-secretase inhibitors (DAPT; PF-03084014). Combinations of ATRA and γ-secretase inhibitors produce additive/synergistic effects in vitro and in vivo. RNA-sequencing studies of <i>HCC-1599</i> and <i>MB-157</i> cells exposed to ATRA and DAPT and ATRA+DAPT demonstrate that the two compounds act on common gene sets, some of which belong to the NOTCH1 pathway. ATRA inhibits the growth of <i>HCC-1599</i>, <i>MB-157</i> and <i>MDA-MB-157</i> cells via RARα, which up-regulates several retinoid target-genes, including RARβ. RARβ is a key determinant of ATRA anti-proliferative activity, as its silencing suppresses the effects exerted by the retinoid. In conclusion, we demonstrate that ATRA exerts a significant anti-tumor action only in <i>TNBC</i> cells showing constitutive NOTCH1 activation. Our results support the design of clinical trials involving combinations between ATRA and γ-secretase inhibitors for the treatment of this <i>TNBC</i> subtype.