HER2 and uPAR cooperativity contribute to metastatic phenotype of HER2-positive breast cancer

Abstract

Human epidermal growth factor receptor type 2 (HER2)-positive breast carcinoma is highly aggressive and mostly metastatic in nature though curable/manageable in part by molecular targeted therapy. Recent evidence suggests a subtype of cells within HER2-positive breast tumors that concomitantly expresses the urokinase plasminogen activator receptor (uPAR) with inherent stem cell/mesenchymal-like properties promoting tumor cell motility and a metastatic phenotype. This HER-positive/uPAR-positive subtype may be partially responsible for the failure of HER2-targeted treatment strategies. Herein we discuss and substantiate the cumulative preclinical and clinical evidence on HER2-uPAR cooperativity in terms of gene co-amplification and/or mRNA/protein co-overexpression. We then propose a regulatory signaling model that we hypothesize to maintain upregulation and cooperativity between HER2 and uPAR in aggressive breast cancer. An improved understanding of the HER2/uPAR interaction in breast cancer will provide critical biomolecular information that may help better predict disease course and response to therapy.

Keywords: HER2-positive breast cancer; HER2/ERBB2; co-amplification; co-overexpression; correlation; uPAR/PLAUR.

Figure 1. Scatter plot depicting the correlation of the relative RNA expression levels of HER2 (A) and PLAU (B) versus PLAUR, respectively

(C) Kaplan-Meier curves with respect to metastases-free survival (MFS) stratified based on low and very high PLAUR RNA expression levels in the overall collective, in the HER2 amplified and HER2 normal subset of patients. The curves were compared with the log-rank test and statistical analyses were performed with R (Version 2.15.2).

Figure 2. Protein functional interaction network for ERBB2/HER2 and PLAUR/uPAR

The proteins interacting with ERBB2 and uPAR were obtained from STITCH database 4.0. The nodes are formed by the individual proteins. The blue lines indicate the protein-protein functional interaction. Note that thicker lines indicate a stronger strength of functional interaction. Abbreviations: PLAUR, plasminogen activator, urokinase receptor; EGFR, epidermal growth factor receptor ;Src, v-src sarcoma (Schmidt-Ruppin A-2) viral oncogene homolog (avian); PRKCA, Protein kinase Cα; NF-κB (NFKB1), nuclear factor of kappa light polypeptide gene enhancer in B-cells 1).

Figure 3. Schematic representation of common transcriptional factor binding sites (indicated by matrix family) for ERBB2/HER2 and PLAUR

Genomatix analysis identified alternative promoters for PLAUR and ERBB2. Note that the V$ETSF and V$KLFS family of transcription factors are common to both the promoters. Black arrows indicate the transcription start sites (TSS).

Figure 4. A model comparative diagrammatic representation of the regulatory signaling cascade in a primary/early metastatic HER2-positive breast carcinoma condition that also co-overexpress uPAR

(A). In HER2-amplified primary BC, signals transduced from HER2 through SRC/PKCα/NF-κB leads to HER2 and PLAUR mRNA co-expression, but no frequent HER2 and PLAUR co-amplification has been observed. (B). However, in an early-stage aggressive HER2-positive BC condition, we propose that hyper-activation of HER2 transduces strong signals (bold arrows) through SRC or PKCα or NF-κB individually or in a concerted manner, leading to activation of members of ETS or KLF family. Consequently, binding of ETS or KLF family members on the promoter region of HER2 or PLAUR gene, leads to their co-amplification, thereby facilitating the high expression of uPAR and HER2 in HER2-positive BC subtype. Depending on the downstream effectors (SRC or PKCα or NF-κB) mediating the signaling pathway, one or more members of the ETS and KLF family will be involved in the regulation of HER2 and PLAUR gene amplification. According to literature, high expression of uPAR is associated with invasive potential of BC. Therefore, it can be assumed that high uPAR expression gives the invasive advantage to the early stage aggressive HER2-positive BC condition, which is reflected in the high metastatic potential of most of the HER2-positive BC subtype that co-overexpress uPAR. Also, depending on the availability and binding of endogenous uPA to uPAR, the signaling cascade initiated from uPAR in association with integrin family of receptors or GPCRs can also increase the expression of HER2 and uPAR at the cell surface following signaling mediated by SRC/PKCα/NF-κB (represented by dotted arrow), leading to the activation of ETS and KLF transcriptional factors that regulate HER2 and PLAUR gene amplification. Green box represents binding site of ETS on HER2 or uPAR promoter region. Red box represents binding site of KLF on HER2 or PLAUR promoter region.