Abstract P3-01-06: Decorin-mediated suppression of tumorigenesis and skin invasion in inflammatory breast cancer via inhibition of the E-cadherin/EGFR axis
Authors: Hu Xiaoding , Villodre Emilly Schlee , Larson Richard , Rahal Omar M. , Wang Xiaoping , Krishnamurthy Savitri , Tripathy Debu , Ueno Naoto T , Woodward Wendy A , Debeb Bisrat Godefay 
Published: March 2020

Abstract
Background: Inflammatory breast cancer (IBC) is a clinically distinct and highly aggressive form of primary breast cancer. Although considered rare, IBC accounts for 10% of breast cancer-related deaths owing to its rapid proliferation and strong propensity to metastasize. The molecular mechanisms underlying the aggressiveness and metastatic propensity of IBC remain elusive. Through transcriptome profiling we identified Decorin (DCN) as being significantly altered in metastatic IBC cell sublines. DCN is a secreted, small leucine-rich proteoglycan known to function as a tumor/metastasis suppressor by inhibiting several signaling pathways including EGFR, TGFβ, and c-MET. However, whether or how DCN regulates IBC tumorigenesis or metastasis is unknown. The aim of this study was to investigate the function and mechanism of DCN in IBC tumorigenesis and metastasis.
Methods: Three IBC cell lines [ER-/HER2+ (MDA-IBC3, SUM190) and ER-/HER2- (SUM149)] were used. DCN gene expression in clinical samples was analyzed from publically available datasets and the IBC Consortium dataset. DCN was stably expressed in IBC cell lines by using lentiviral vectors. For in vivo studies, DCN-overexpressing stable cell lines were injected into cleared mammary fat pads of SCID/Beige mice and tumor growth was monitored via caliper measurements. Tumor-skin involvement was assessed visually during primary tumor growth and tumor excision. Reverse phase protein array analysis was used for proteomic profiling. Protein-protein interactions were analyzed by reciprocal immunoprecipitation of exogenous or endogenous proteins.
Results: DCN expression was significantly lower in breast cancer samples than in normal breast (p&lt;0.0001), in more aggressive breast cancer subtypes (p&lt;0.0001), and in metastatic tumors relative to primary tumors (p&lt;0.0001). High DCN expression correlated with improved overall survival (p&lt;0.0001) and relapse-free survival (p=0.0003). In vitro, DCN overexpression in IBC cell lines inhibited colony formation (MDA-IBC3, p=0.0115; SUM149, p=0.0068), migration (SUM149, p=0.0165), invasion (SUM149, p=0.0159), and primary and secondary mammosphere formation (primary: MDA-IBC3, p=0.0209; SUM149, p=0.0232; secondary: MDA-IBC3, p=0.01; SUM149, p=0.0031). In vivo, DCN overexpression in MDA-IBC3 cells inhibited primary tumor growth (p=0.0092) and reduced skin invasion (89.9% DCN control vs 33.3% DCN-overexpressed, p=0.017, Chi-square test). Our proteomics data in DCN-overexpressing SUM149 and MDA-IBC3 cells showed downregulated EGFR and E-cadherin protein levels. Mechanistically, DCN reduced expression of E-cadherin and EGFR and reduced phosphorylation of EGFR in IBC cells. Exogenous and endogenous co-immunoprecipitation experiments showed direct physical binding between DCN and E-cadherin proteins in all IBC cell lines. Moreover, overexpression of DCN downregulated E-cadherin via protein instability rather than decreased E-cadherin mRNA expression. E-cadherin knockdown in IBC cells decreased, whereas its overexpression increased, activation of EGFR signaling without affecting DCN expression. Finally, restoring E-cadherin in DCN-overexpressing IBC cell lines rescued the inhibitory effect of DCN on EGFR signaling.
Conclusions: We found that DCN inhibited tumorigenesis and skin invasion in IBC via its direct interaction with and stabilization of E-cadherin and its suppression of EGFR signaling. Our findings provide new insights and a novel mechanism for IBC pathobiology that may be therapeutically targetable. Future studies will determine the role of DCN in IBC metastasis and the detailed mechanism of DCN-mediated suppression of tumorigenesis and metastasis in IBC.
Citation Format: Xiaoding Hu, Emilly Schlee Villodre, Richard Larson, Omar M. Rahal, Xiaoping Wang, Savitri Krishnamurthy, Debu Tripathy, Naoto T Ueno, Wendy A Woodward, Bisrat Godefay Debeb. Decorin-mediated suppression of tumorigenesis and skin invasion in inflammatory breast cancer via inhibition of the E-cadherin/EGFR axis [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-01-06.