Abstract 2012: GPNMB cooperates with Neuropilin-1 and Integrin a5b1 to promote breast cancer tumorigenesis and metastasis
Authors: Maric Gordana , Annis Matthew , Rose April , Perkins Dru , Macdonald Patricia , Siegel Peter 
Published: May 2015

Abstract
Deaths attributed to breast cancer primarily occur following metastasis to distant organs such as bone, lung, liver and brain. In a screen for metastasis modulators, our lab has identified GPNMB as a gene whose overexpression in weakly metastatic breast cancer cell lines enhances primary tumor growth, promotes endothelial cell recruitment, drives lung and bone metastasis in vivo and induces the acquisition of an invasive phenotype in vitro. We have shown that elevated GPNMB levels in the breast tumor epithelium strongly correlate with decreased disease-free and overall survival, even within the basal/triple-negative breast cancer subset, associated with a high grade and poor prognosis. Based on our research findings, CDX-011, an antibody-drug conjugate that selectively targets GPNMB, was investigated in a multi-centre Phase II clinical trial for patients with metastatic breast cancer and showed promising activity in TNBCs, where treatment options are limited.
Gene expression profiling of 66cl4 murine mammary carcinoma cells and BT549 basal breast cancer cells revealed that Neuropilin-1 (Nrp1) levels are increased in response to GPNMB overexpression. Using a panel of GPNMB-domain mutants (GPNMB ΔCYT and GPNMB RGD), we show that GPNMB forms a complex with Nrp1, and that this association requires the cytoplasmic tail of GPNMB and the PDZ-binding motif of Nrp1. The presence of a GPNMB/Nrp-1 complex potentiates the VEGF signaling cascade in a manner that depends on GPNMB association with Nrp1.
We demonstrate that GPNMB enhances adhesion of breast cancer cells to fibronectin, increases the expression of α5β1 and associates with this receptor through its RGD integrin-binding domain. The recruitment of GPNMB into integrin complexes activates downstream Src and Fak signaling pathways and, reciprocally, induces phosphorylation of GPNMB on its half-ITAM motif. Using a spontaneous model of breast cancer tumor formation, we show that both the RGD domain and the cytoplasmic tail of GPNMB are required to increase primary tumor formation. However, only the GPNMB RGD mutant showed a decrease in metastatic burden when compared to WT GPNMB, indicating that GPNMB association with α5β1, but not Neuropilin-1 is required to promote lung metastasis. A survey of RNAseq datasets reveals that GPNMB is strongly correlated with Nrp1 and α5 in breast cancer and emphasizes the clinical relevance of our data. These findings outline the importance of novel GPNMB/α5β1 and GPNMB/Nrp1 complexes in potentiating breast cancer tumorigenesis and metastasis.
Citation Format: Gordana Maric, Matthew Annis, April Rose, Dru Perkins, Patricia Macdonald, Peter Siegel. GPNMB cooperates with Neuropilin-1 and Integrin a5b1 to promote breast cancer tumorigenesis and metastasis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2012. doi:10.1158/1538-7445.AM2014-2012