Abstract 5186: Identification of novel cell-surface proteins associated with homotypic adhesion in human colon cancer cell lines
Authors: Williams Russell , Luu Truong , Lee Norman H. 
Published: February 2011

Abstract
The loss of homotypic cell adhesion is one of several prerequisites for metastasis. This study aimed to characterize this process by comparing the adhesive properties and E-cadherin expression of a panel of eight human colon cancer cell lines. Homotypic adhesion was determined by cell dissociation assays. E-cadherin is generally considered the primary mediator of cell-cell adhesion in epithelial cells and epithelial-derived tumors, but E-cadherin mRNA and protein expression did not correlate with homotypic adhesion across the cell lines. Consequently, the mediation of adhesion in these cell lines must be more complex.
To characterize this additional complexity, gene expression profiling was performed and a set of genes encoding transmembrane proteins was identified that correlated with adhesion across the cell lines. The functional relevance of these correlated genes in adhesion was confirmed by siRNA knockdown in SW480 colon cancer cells. Knockdown of either CD99 or CD99L2 led to a loss of homotypic adhesion, down-regulation of E-cadherin mRNA and protein levels and up-regulation of vimentin mRNA and protein levels. The expression patterns of E-cadherin and vimentin following gene knockdown is consistent with epithelial-mesenchymal transition (EMT). EMT is a transcriptional program active in both development and in tumor progression, which causes cells to have decreased adhesion, increased invasion, and increased motility. In contrast, knockdown of the correlated gene LRRC8B in SW480 cells led to a loss of homotypic adhesion but had no effect on the expression of either E-cadherin or vimentin, suggesting an EMT-independent mechanism.
The cell-surface molecules CD99 and CD99L2 appear to be involved in a signaling and/or transcriptional network that helps maintain an epithelial phenotype, and loss of this network may be associated with EMT and cancer progression. To characterize this network, gene expression profiling was performed in SW480 cells following siRNA-mediated knockdown of either CD99 or CD99L2. Because these two cell-surface proteins are structurally related and displayed similar functional roles in adhesion, we hypothesized that they would share overlapping downstream transcriptional effects. Analysis of the gene expression data has identified a shared set of transcription factors whose expression is affected by the knockdown of either CD99 or CD99L2.
The approach of correlating the adhesion characteristics and gene expression of a panel of colon cancer cell lines has identified a set of novel genes which are functionally involved in adhesion. Further investigation will help to identify the contributions of CD99, CD99L2 and LRRC8B to the homotypic adhesion process and clarify the downstream transcriptional networks involved in this facet of tumor progression.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5186.