Abstract B11: HER2 regulates PARP-1 expression via the let-7a microRNA in HER2+ breast cancer

Abstract

Abstract Background: HER2+ breast cancers are sensitive to PARP inhibition and express elevated levels of the PARP-1 protein. However, the mode of regulation of PARP-1 expression by HER2 is not well understood. In this study, we report that the let-7a microRNA is regulating PARP-1 expression at the post-transcriptional level in HER2+ breast cancer cells. Methods: Human HER2+ breast cancer cell lines BT-474 and SKBR3 were used in this study. MDA-MB-231 (non-HER2 expressing breast cancer cells) were also stably transfected with a HER2 wild-type plasmid (231 HER2) or the vector control (231 NEO). Further, to identify candidate microRNAs regulated by HER2 overexpression we used the nCounter miRNA Expression Assay. MicroRNA and mRNA expression were validated via qRT-PCR analysis in breast cancer cell lines or patient primary tumors. Cells were also transfected with a HER2 siRNA, a let-7a mimic, and an inhibitor of let-7a as well as their respective controls. Western blot analysis and firefly luciferase assays were used to determine whether the 3'UTR of PARP1 was being directly targeted by the let-7a microRNA. Results: HER2 did not regulate PARP-1 at the mRNA level but increased PARP-1 protein in HER2+ breast cancer cells. Specifically, ectopic HER2 overexpression correlated with increased PARP-1 protein levels in the 231 HER2 cell line. Conversely, silencing HER2 reduced PARP-1 protein levels in the BT-474 and SKBR3 cell lines. NanoString nCounter analysis revealed that the HER2+ breast cancer cell lines expressed low levels of the let-7a microRNA. Further, let-7a expression was upregulated after HER2 knockdown in the two native HER2+ breast cancer cell lines. The let-7a mimic also reduced both PARP1 protein expression and luciferase activity in the 231 HER2 and BT-474 cell lines, whereas the let-7a inhibitor reversed these effects in the 231 NEO cell line. Importantly, HER2+ breast tumors expressed higher levels of PARP-1 and lower levels of let-7a; whereas, the HER2- breast tumors expressed lower levels of PARP-1 and higher levels of let-7a. Conclusions: These results suggest that let-7a regulates PARP-1 expression in HER2+ breast tumors. Let-7a may also be a potential therapeutic target and predictive biomarker of PARPi sensitivity in HER2+ breast cancer patients. Citation Format: Monicka Wielgos, Rajani Rajbhandari, Susan Nozell, Shi Wei, Eddy S. Yang. HER2 regulates PARP-1 expression via the let-7a microRNA in HER2+ breast cancer. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr B11.