HER2 in Breast Cancer Stemness: A Negative Feedback Loop towards Trastuzumab Resistance

Abstract

HER2 receptor tyrosine kinase that is overexpressed in approximately 20% of all breast cancers (BCs) is a poor prognosis factor and a precious target for BC therapy. Trastuzumab is approved by FDA to specifically target HER2 for treating HER2+ BC. However, about 60% of patients with HER2+ breast tumor develop de novo resistance to trastuzumab, partially due to the loss of expression of HER2 extracellular domain on their tumor cells. This is due to shedding/cleavage of HER2 by metalloproteinases (ADAMs and MMPs). HER2 shedding results in the accumulation of intracellular carboxyl-terminal HER2 (p95HER2), which is a common phenomenon in trastuzumab-resistant tumors and is suggested as a predictive marker for trastuzumab resistance. Up-regulation of the metalloproteinases is a poor prognosis factor and is commonly seen in mesenchymal-like cancer stem cells that are risen during epithelial to mesenchymal transition (EMT) of tumor cells. HER2 cleavage during EMT can explain why secondary metastatic tumors with high percentage of mesenchymal-like cancer stem cells are mostly resistant to trastuzumab but still sensitive to lapatinib. Importantly, many studies report HER2 interaction with oncogenic/stemness signaling pathways including TGF-β/Smad, Wnt/β-catenin, Notch, JAK/STAT and Hedgehog. HER2 overexpression promotes EMT and the emergence of cancer stem cell properties in BC. Increased expression and activation of metalloproteinases during EMT leads to proteolytic cleavage and shedding of HER2 receptor, which downregulates HER2 extracellular domain and eventually increases trastuzumab resistance. Here, we review the hypothesis that a negative feedback loop between HER2 and stemness signaling drives resistance of BC to trastuzumab.

Keywords: EMT; HER2/ERBB2; breast cancer; cancer stem cell; metalloproteinase; p95HER2; signaling; stemness; trastuzumab.

Figure 1

Crosstalk between HER2 and BC stemness signaling pathways. Active TGF-β/Smad induces EMT by direct regulation of transcription of EMT genes and indirectly by activating DNMT1, inactivating Id1/2/3 and GSK3 through AKT and MAPK. HER2 regulates TGF-β/Smad-mediated induction of EMT through activating AKT, MAPK, and the p53 inhibitor MDM2. Active Wnt/β-catenin signaling regulates EMT by β-catenin-mediated transcriptional regulation of EMT genes through activating c-Fos. HER2-mediated activation of AKT and MAPK inhibits β-catenin through inhibition of GSK3. Notch signaling induces EMT directly via transcriptional regulation of EMT genes by NICD. NICD also up-regulates LOX and ALDH. HER2-mediated activation of MAPK inhibits cleavage of Notch receptor by inhibition of γ-secretase. In addition, HER2 itself positively affects the activation of Notch by activating PKCα that is an inhibitor of Mib1. Inhibition of Mib1 causes up-regulation of Jagged-1 a ligand for Notch receptor. Active STAT proteins up-regulate expression of EMT genes directly. STATs also up-regulate EMT by three indirect paths including inhibiting mir-200 via activating Lin-28 that inhibits Let-7. Let-7 in turn inhibits HMGA2, an EMT marker. STATs also activate p21 that down-regulates CDK2. HER2 interacts with these axes by phosphorylating and activating STATs through Src and by activating p21 through AKT. Direct and indirect interactions are not distinguished in the figure. NF: nuclear factor.

Figure 2

Negative feedback relationship between HER2 and BC stemness leading to trastuzumab resistance. HER2+ BC cell responds to trastuzumab and lapatinib in epithelial-like context. In this context, HER2 increases expression of stemness markers resulting in down-regulation of epithelial marker E-cadherin and up-regulation of EMT phenotype including MPs. In mesenchymal context MPs cleave HER2 receptor from juxtamembrane of the receptor that results HER2 shedding and expression of p95HER2. p95HER2 maintain its kinase activity and is able to activate AKT and MAPK pathways and to translocate to the nucleus where acts as transcription co-factor. Mesenchymal-like p95HER2+ cell respond to lapatinib but is resistant to trastuzumab.