Abstract 5475: Endoglin (CD105) is a target for ovarian cancer cell-specific therapy through induction of DNA damage.
Authors: Ziebarth Angela J. , Nowsheen Somaria , Steg Adam D. , Shah Monjri M. , Katre Ashwini A. , Dobbin Zachary C. , Sood Anil K. , Conner Michael G. , Yang Eddy S. , Landen Charles N. 
Published: August 2014

Abstract
Background: Endoglin (ENG, CD105) is a protein markedly overexpressed in tumor-associated endothelial cells, and a target for anti-angiogenic therapy. Recently we have noted it to be overexpressed in chemoresistant ovarian cancer cells as well. Our objective was to evaluate the effects and mechanisms of targeting endoglin specifically in ovarian cancer cells.
Methods: Endoglin expression was assessed in multiple ovarian cancer lines by Western blot, immunohistochemistry, and flow cytometry. Anti-endoglin siRNAs were used to downregulate expression in ES2 and HeyA8MDR. In vitro, the effects of endoglin-knockdown individually and with chemotherapy were evaluated by MTT assay, cell-cycle analysis, alkaline comet assay, γ-H2AX foci formation and qPCR array for mediators of DNA damage and repair. In an orthotopic murine model, mice inoculated with ES2 or HeyA8MDR cell lines were administered chitosan-encapsulated anti-ENG siRNA or control siRNA with and without carboplatin. PCNA, γH2AX, and 53BP1 IHC and the TUNEL assay were performed to examine biologic effects of endoglin knockdown. Results: Endoglin is expressed to varying degrees by multiple ovarian cancer cell lines. Expression was on the cell surface, consistent with its recognized role as a cofactor with TGF-beta receptor, in only 5% of cells, with most other cells having cytoplasmic expression. In ES2 and HeyA8MDR cell lines, endoglin inhibition decreased cell viability, increased apoptosis, induced double-stranded DNA damage, and increased cisplatin sensitivity. Endoglin downregulation led to a decrease in expression of several DNA repair genes, including NBN, NTHL1, BARD1, H2AFX and SIRT1, and an increase in expression of DDIT3 and PPP1R15A. BARD1-specific downregulation, in turn, led to a significant decrease in BRCA1 expression, likely through ubiqutinylation. In vivo, anti-endoglin treatment decreased tumor weight in both ES2 and HeyA8MDR models when compared to control (35-41% reduction, p&lt;0.05). Combination treatment with anti-Endoglin siRNA and carboplatin was associated with even greater inhibitory effect compared to control (58-62% reduction, p&lt;0.001). Conclusions: Targeting endoglin improves platinum sensitivity of ovarian cancer cells both in vivo and in vitro. Endoglin downregulation induces DNA damage at multiple levels, one of which is through decreased expression of BARD1 and BRCA1. Anti-endoglin therapies should be pursued further as their development would allow dual treatment of both tumor angiogenesis and an aggressive subset of chemoresistant tumor cells.
Citation Format: Angela J. Ziebarth, Somaria Nowsheen, Adam D. Steg, Monjri M. Shah, Ashwini A. Katre, Zachary C. Dobbin, Anil K. Sood, Michael G. Conner, Eddy S. Yang, Charles N. Landen. Endoglin (CD105) is a target for ovarian cancer cell-specific therapy through induction of DNA damage. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5475. doi:10.1158/1538-7445.AM2013-5475
Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.