AXL induces epithelial-to-mesenchymal transition and regulates the function of breast cancer stem cells

Abstract

Despite significant progress in the treatment of breast cancer particularly through the use of targeted therapy, relapse and chemo-resistance remain a major hindrance to the fight to minimize the burden of the disease. It is becoming increasingly clear that a rare subpopulation of cells known as cancer stem cells (CSC), able to be generated through epithelial to mesenchymal transition (EMT) and capable of tumor initiation and self-renewal, contributes to treatment resistance and metastases. This means that a more effective therapy should target both the chemoresistant CSCs and the proliferating epithelial cells that give rise to them in order to reverse EMT and attenuate their conversion to CSCs. Here, we demonstrate a novel function of AXL in acting upstream to induce EMT in normal and immortalized human mammary epithelial cells in an apparent positive feedback loop mechanism and regulate breast CSC (BCSC) self-renewal and chemoresistance. Downregulation of AXL using MP470 (amuvatinib) reversed EMT in mesenchymal normal human mammary epithelial cells and murine BCSCs attenuating self-renewal and restored chemosensitivity of the BCSCs. AXL expression was also found to be associated with expression of stem cell genes, regulation of metastases genes, increased tumorigenicity, and was important for BCSC invasion and migration. Inactivation of AXL also led to downregulation of NFκB pathway and reduced tumor formation in vivo. Together, our data suggest that targeted therapy against AXL, in combination with systemic therapies, has the potential to improve response to anti-cancer therapies and to reduce breast cancer recurrence and metastases.

Therapeutic Activity of Anti-AXL Antibody against Triple-Negative Breast Cancer Patient-Derived Xenografts and Metastasis

“…Indeed, AXL downregulation in breast cancer using siRNAs inhibits AKT phosphorylation and decreases cell invasion, motility, and metastasis. On the other hand, ectopic expression of AXL in human mammary epithelial cells leads to a downregulation of E-cadherin expression and increased expression of mesenchymal markers (13). Along similar lines, transfection of SLUG and SNAIL genes in MCF10A breast cancer cells induces the acquisition of mesenchymal markers and loss of epithelial characteristics together with increased AXL expression (12).…”

“…Although AXL is required to maintain the invasiveness of mesenchymal-like cells, EMT starting point is the overexpression of intracellular proteins, such as SLUG or RAS (12,37). More recently, Asiedu and colleagues described AXL as an inducer of EMT using the TKI MP470 (13). AXL inhibition led to the downregulation of EMT markers, such as SNAIL and N-cadherin.…”

“…Among the EMT cellular pathways, receptor tyrosine kinases can be inducers (for instance, c-MET), or can be activated (for instance, PDGFRa) following induction of the expression of different EMT transcription factors (35,36). In breast cancer, AXL was known as an EMT-induced regulator of cancer metastasis, under the control of SLUG (13). Although AXL is required to maintain the invasiveness of mesenchymal-like cells, EMT starting point is the overexpression of intracellular proteins, such as SLUG or RAS (12,37).…”

Biomarkers for EMT and MET in breast cancer: An update

“…On the contrary, the silencing of AXL by viral-mediated shRNA led to the upregulation of epithelial markers, while mesenchymal markers were downregulated. Inactivation of AXL also led to downregulation of the NF-κB pathway and reduced tumor formation in vivo (98). In addition, AXL is overexpressed in highly invasive breast cancer cell lines.…”

“…It plays a important role in cancer progression and metastasis (95)(96)(97). Asiedu et al (98) reported that AXL overexpression in HMLE cells downregulated E-cadherin, while the expression of mesenchymal markers such as N-cadherin, Snail and Slug was upregulated. A similar result was observed by forced expression of AXL in the normal human mammary epithelial cell line MCF10A.…”