Abstract B32: The tumor-suppressor function of Erbin in colon cancer

Most human cancers are derived from epithelial origins. Establishing polarity in these epithelial cells is crucial for the maintenance of homeostasis in epithelial tissues. The proper establishment of epithelial polarity allows cells to sense and to respond to signals that arise from the microenvironment in a spatiotemporally controlled manner. Increasing evidence has suggested that disruption of polarity promotes the malignant progression of cancer cells. The LAP (leucine-rich repeat (LRR) and PDZ domain) family protein, ERBIN, is a basolateral-localized scaffolding protein. Erbin was initially identified as an ERBB2 binding protein and subsequently found to inhibit RAS/RAF signaling by disrupting RAS-mediated activation of RAF1. Here, we have discovered that Erbin plays an important role in suppressing epithelial-mesenchymal transition (EMT) of colorectal cancer (CRC) cells by controlling cell polarity as well as inhibiting CRC migration and metastasis. Analysis of gene expression datasets reveal that Erbin mRNA expression is significantly downregulated in cancer specimens when compared to normal controls in CRC patient samples. Additionally, using a tissue microarray, we demonstrate that the expression of Erbin protein is decreased and mislocalized in immunohistochemically stained patient tumor specimens. To further determine the functional importance of Erbin in CRC, we created stable knockdown of Erbin in multiple CRC cell lines using lentiviral sh-RNA infections. Through the use of immunofluorescent staining and confocal microscopy, we show that silencing Erbin in both SW480 and Caco2 cell lines results in the disruption of cell polarity. Loss of Erbin promotes the formation of multiple lumens and disrupts the apical/basolateral distribution of multiple proteins in 3D culture, and the loss of polarity is accompanied by an increase in cell proliferation as shown by increased numbers of EdU positive cells in 3D tumor spheroids. Furthermore, the activation of both Akt and ERK signaling are elevated and prolonged in Erbin knockdown CRC cells as determined by Western blotting analysis. As a consequence of prolonged signaling, knockdown of Erbin induces an EMT-like morphology and promotes cell migration in CRC cells. Western blotting analysis of EMT markers in both 2D and 3D cultures shows a marked increase in mesenchymal proteins in the SW480 cells lacking Erbin expression. Moreover, significant increases in cellular motility were observed in both SW480 and LIM2405 Erbin knockdown cells using both Transwell migration assays as well as live single-cell microscopy. Taken together, these results suggest that similar to other LAP protein family members, the scaffolding function of Erbin could provide the required protein-protein interactions to allow for the necessary spatial-temporal control to suppress oncogenic Akt and RAS/RAF signaling. Our study provides evidence for a novel role of Erbin in suppressing colorectal cancer progression by negatively regulating EMT and cell motility. Citation Format: Payton D. Stevens, Yang-An Wen, Tianyan Gao. The tumor-suppressor function of Erbin in colon 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 B32.