Early-stage cutaneous squamous cell carcinoma (cSCC) has a favourable prognosis. Metastatic disease is probably associated with chemoresistance mediated through the activation of pro-survival phosphatidylinositol 3-kinase/AKT signalling. Inhibition of activated AKT partially increases chemosensitivity but induces autophagy, the principal lysosomal mechanism for the bulk degradation and recycling of proteins and damaged organelles. The aim of the current study was to test the hypothesis that combined inhibition of AKT signalling and autophagy by the lysosomal inhibitor chloroquine increases the susceptibility to docetaxel-induced apoptosis of cSCC cells isolated from a lymph-node metastasis. Combined AKT inhibition and chloroquine treatment of MET 4 cSCC cells resulted in significantly enhanced inhibition of cell viability and apoptosis induced by clinically achievable concentrations of docetaxel (P < 0.001). Inhibition of both autophagy and AKT thus represents an effective and viable therapeutic strategy to increase the cytotoxicity of docetaxel for the treatment of advanced cSCC.

Triple negative breast cancer (TNBC) is a molecularly heterogeneous, clinically aggressive disease group that is highly prevalent among African-Americans and younger patients. Standard chemo/radio therapy often produces clinical responses, but recurrence and metastasis are unfortunately common. Metastatic disease is generally incurable. Chemo/radiotherapy has been shown to induce EMT and enrich a chemo-resistant cancer stem-like cell (CSC) population in TNBC. CSCs are thought to drive disease recurrence. Notch signaling is critical for maintenance of TNBC CSC. Expression of Notch1 and its ligand Jagged1 are correlated with poor prognosis. Efforts to pharmacologically target Notch with Gamma Secretase Inhibitors (GSIs) have been impaired by the systemic toxicity of the GSIs, and by the fact that Notch1 also plays a key role in anti-tumor adaptive immunity. Therapeutic agents that indirectly and selectively target Notch signaling in breast cancer cells would be a potentially attractive strategy. However, no such agents have been identified to date. We have found that the MAPK5-ERK5 kinase pathway, which contains at least two druggable targets, functions as a master regulator of Notch signaling in TNBC cells. ERK5 knockout TNBC cells have dramatically decreased expression of Notch receptors, ligands and transcriptional targets. In vivo, these cells form barely detectable tumors that do not metastasize and express lower levels of Notch1 and its ligand Jagged1. Using in silico screening, we identified a class of compounds that selectively target MAP2K5 (MEK5) and decrease the phosphorylation of MAPK7 (ERK5). We selected compound SC-181 for further study. Consistent with ERK5KO cells, pharmacological suppression of ERK5 phosphorylation with SC-181 decreased Notch1 and Jagged1 mRNAs and proteins. SC-181 reversed EMT and reduced the CD44hi/CD24lo CSC population in TNBC cells, but had no effect on T-cell proliferation. SC-181 decreased the number and size of mammospheres in a concentration-dependent manner. Overexpression of the Notch1 intracellular domain (N1IC) in ERK5KO cells rescues their phenotype, dramatically increasing the CSC fraction and promoting EMT. Our results suggest that targeting the MEK5-ERK5 pathway is a promising new strategy to selectively modulate Notch signaling in TNBC CSC without compromising tumor immunity. Citation Format: Ucar DA, Matossian MD, Hoang-Barnes VT, Hossain FM, Gupta M, Burks HE, Wright TD, Cavanaugh J, Flaherty P, Burow ME, Miele L. A novel druggable target upstream of Notch: MEK5/ERK5 signaling regulates Jagged-1 and Notch1 expression in triple negative breast cancer stem cells [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-03-04.

The organization of cell cytoskeleton is altered in events of epithelial to mesenchymal transition (EMT), promotion of cell motility, and cancer metastases. EMT is associated with decreased cell-cell adhesion, downregulation of epithelial markers like E-Cadherin, cytokeratins, and occludins, and upregulation of mesenchymal markers such as N-cadherin, vimentin, and various transcription factors such as slug and ZEB. Epithelial to mesenchymal transition is also a consequence of drug resistance and is responsible for cancer metastases. Triple negative breast cancer is highly aggressive cancer and patients show poor prognosis and disease-free survival due to the lack of targeted therapy. Mitogen activated protein kinase pathway, including extracellular activated kinase ERK1/2 and ERK5, and phosphoinositide 3-kinase (PI3K) pathway are known to alter the cytoskeleton through the downstream activation of oncogenes such as FRA-1 and loss of focal adhesions. Of these pathways, the MEK5-ERK5 pathway is understudied in triple negative breast cancer TNBC, and there are few research tools available to selectively inhibit this pathway. The diphenylamine analogs were derived from the parent molecule Mekinist, a FDA approved MEK1/2 inhibitor for melanoma, and modified to gain selectivity towards MEK5. SC-1-151, a type-III allosteric inhibitor of MEK5 is a dual MEK1/2 (98.6%) and MEK5 (59%) inhibitor; the molecule inhibits cell viability and colony formation, and attenuates tumor growth. SC-1-151 was serendipitously identified as a mesenchymal to epithelial transition activator in TNBC cell line MDA-MB-231. E-cadherin protein expression and cell morphology were examined to study MET after the treatment of MDA-MB-231 cells with different structural analogs of SC-1-151 after treatment for 5 days. The compound was further found to induce E-cadherin expression and epithelial phenotype in tamoxifen resistant estrogen positive MCF-7 cell line that underwent EMT. The compound is identified to promote this activity by targeting at least the ERK-FRA1-ZEB1 axis. Alkyl or N-Methyl piperazine substituents on the amide of ring 1 produced similar result as SC-1-151, and substituting the amide group with acid or ester also induced MET. In contrast, ortho-fluoro, para-iodo functional groups of the arene ring 2, when replaced with a meta-bromo substituent did not induce MET. We aim to test the compounds on EGF treated MDA-MB-468 cells to observe the attenuation of EGF induced EMT. Future studies will be performed to determine the specific protein interactions of the promising compounds. Citation Format: Bhatt AB, Wright TD, Anna K, Gupta M, Chakrabarty S, Flaherty PT, Hoang V, Burow M, Cavanaugh JE. Study of diphenylamine analogs as inducers of mesenchymal to epithelial transition in breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-08-07.

Aberrations in the MAPK/extracellular signal-regulated kinase (MEK/ERK) and phosphoinositide-3-kinase (PI3K) pathways have been linked to increased proliferation and survival in triple negative breast cancer (TNBC) cells. It has been proposed that these survival characteristics are enhanced through compensatory signaling and crosstalk mechanisms. Promising combinations of MEK and PI3K inhibition have been evaluated in phase I clinical trials for various cancer types. However, these clinical trials have had limited efficacy and have yet to encompass the MEK5/ERK5 pathway, which has been shown to promote cell survival. The goal of this study was to examine the crosstalk between the MEK1/2, MEK5, and PI3K pathways and determine the most promising combination of the MEK1/2, ERK5, and PI3K inhibitors, U0126, XMD8-92, and LY294002, respectively, in a diverse panel of triple negative breast cancer cell lines: BT549, MDA-MB-231, and MDA-MB-468. Our results indicate that dual inhibition of the MEK5 and PI3K pathways significantly reduced proliferation (45.53%) in MDA-MB-231 TNBC cells. Also, the combination of MEK5 and PI3K inhibition was shown to be synergistic. In contrast, inhibition of ERK1/2 alone or in combination with PI3K or ERK5 inhibition yielded mixed responses. Additionally, treatment with LY294004 in MDA-MB-231 (ERK 5 KO) was more potent (IC50= 2.5 uM) than treatment in the native MDA-MB-231 cell line (IC50= 13.7 uM). These data suggest that crosstalk between these kinases occurs and dual inhibition of PI3K and ERK5 may be a novel therapeutic approach for treating TNBC. Citation Format: Wright TD, Raybuck C, Wendekier K, Cavanaugh JE. Dual inhibition of the MEK5 and PI3K pathways synergistically reduces proliferation and viability in triple negative breast cancer cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-07-19.