Finally, we provide evidence that the RAR-dependent induction of RAR plays an important role in the anti-proliferative action exerted by ATRA in sensitive cells. 2. cancer (cell lines for their sensitivity to the anti-proliferative action of all-trans retinoic acid (ATRA). The only three cell lines (and and cells sensitive not only to ATRA, but also to -secretase inhibitors (DAPT; PF-03084014). Combinations of ATRA and -secretase inhibitors produce additive/synergistic effects in vitro and in vivo. RNA-sequencing studies of and 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 and cells via RAR, which up-regulates several retinoid target-genes, including RAR. RAR 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 cells showing constitutive NOTCH1 activation. Our results support the design of clinical trials involving combinations between ATRA and -secretase inhibitors for the treatment of this subtype. cells share common features such as a high proliferation index and a basal-like gene expression signature, this tumor type is very heterogeneous and lacks effective therapeutic strategies [1,2]. NOTCH1 is a transmembrane receptor and its constitutive activation is observed in approximately 3% of all instances [3,4]. Normally, NOTCH1 activation requires binding to a membrane tethered ligand on neighboring cells, which causes a series of proteolytic events [5,6]. The final -secretase-dependent cleavage of NOTCH1 causes the release and nuclear translocation of Miltefosine the receptor intracellular website (N1ICD), which is definitely part of an active transcriptional complex controlling the manifestation of various target genes [7,8]. Among the known target genes, users of the HES and HEY family members, CyclinD1 and cMyc stand out . Some of these genes, with particular reference to cMyc, are involved in the proliferative effects induced from the activation of the NOTCH pathway in certain types of leukemia and solid tumors. All this supports the development Miltefosine of strategies based on NOTCH focusing on providers, with particular reference to -secretase inhibitors, for the treatment of cases characterized by constitutive NOTCH1 activation [9,10]. However, the active dosages of -secretase inhibitors are characterized by systemic toxicity , assisting the necessity of identifying pharmacological agents improving the activity and reducing the toxicity of these compounds. All-trans retinoic-acid (ATRA) is the active metabolite of vitamin A and a non-conventional anti-tumor agent endowed with cyto-differentiating properties [12,13]. In combination with chemotherapy or arsenic trioxide, ATRA is used in the treatment of acute promyelocytic Miltefosine leukemia with exceptional results, inducing long-term remission in the majority of individuals . The restorative activity observed in this type of acute leukemia has raised interest in the use of ATRA and derived synthetic retinoids for the customized management of solid tumors, including breast cancer . With this last context, a substantial quantity of pre-clinical in vitro and in vivo results indicate that ATRA is definitely a encouraging agent in the treatment/chemo-prevention of mammary tumors [12,16]. Recently, we offered data supporting the idea that the majority of luminal breast cancers are sensitive to the anti-tumor action of ATRA [17,18]. In contrast, only a small fraction of basal or tumors are likely to be responsive to the retinoid. In breast tumor cells, the anti-tumor action of ATRA is definitely mainly due to a growth-inhibitory effect . However, we recently demonstrated that challenge of mammary tumor cells with the retinoid reactivates endogenous retroviruses causing a response . The process may be at least partially the consequence of epigenetic CACNLB3 effects, including perturbations in the DNA methylation process [20,21]. Activation of may have significant restorative ramifications, as the process results in interferon-dependent immune reactions that are likely to sensitize the neoplastic cell to immune-checkpoint inhibitors and additional immune-therapeutics. The biological action of ATRA is generally mediated from the activation of RARs and RXRs, which are users of the nuclear receptor family [12,22]. Nuclear receptors are ligand-activated transcription factors which control the activity of numerous target genes. ATRA is definitely a pan-RAR agonist, activating.