| Abstract: | Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas. To elucidate the advantages conferred on tumor cells by loss of Rap1GAP, Rap1GAP expression was silenced in human colon carcinoma cells. Suppressing Rap1GAP induced profound alterations in cell adhesion. Rap1GAP-depleted cells exhibited defects in cell/cell adhesion that included an aberrant distribution of adherens junction proteins. Depletion of Rap1GAP enhanced adhesion and spreading on collagen. Silencing of Rap expression normalized spreading and restored E-cadherin, β-catenin, and p120-catenin to cell/cell contacts, indicating that unrestrained Rap activity underlies the alterations in cell adhesion. The defects in adherens junction protein distribution required integrin signaling as E-cadherin and p120-catenin were restored at cell/cell contacts when cells were plated on poly-l-lysine. Unexpectedly, Src activity was increased in Rap1GAP-depleted cells. Inhibition of Src impaired spreading and restored E-cadherin at cell/cell contacts. These findings provide the first evidence that Rap1GAP contributes to cell/cell adhesion and highlight a role for Rap1GAP in regulating cell/matrix and cell/cell adhesion. The frequent downregulation of Rap1GAP in epithelial tumors where alterations in cell/cell and cell/matrix adhesion are early steps in tumor dissemination supports a role for Rap1GAP depletion in tumor progression.Mammalian Rap proteins Rap1a/b and Rap2a/b/c are members of the Ras superfamily of small GTPases. Rap proteins are active when bound to GTP and inactive when bound to GDP. Cellular Rap activity is regulated by the concerted action of guanine nucleotide exchange factors that activate Rap (RapGEFs) and Rap-specific GTPase-activating proteins (RapGAPs) that inactivate Rap (reviewed in reference 10). The Rap1GAP family is composed of several members, including Rap1GAP, Rap1GAPII, Spa-1/SIPA1, and E6TP1/SIPA1L1. Several lines of evidence suggest that RapGAPs function as tumor and/or invasion suppressors. Downregulation of E6TP1 by human papillomavirus protein E6 contributes to cervical cancer (20, 21), and Spa-1 deficiency in mice induces a spectrum of myelodysplastic disorders similar to chronic myelogenous leukemia (26). The SPA1 gene was identified as a candidate for the metastasis efficiency modifier locus in mice (38). Although the relevance of this observation to humans is not yet clear, single-nucleotide polymorphisms in the SPA1 gene in human breast tumors have been associated with lymph node involvement and poor survival (15). Intriguingly, Spa-1 interacts with Brd4 (18) and Rrp-1b (13), the protein products of genes associated with patterns of extracellular matrix protein gene expression characteristic of metastatic tumors (14).The RAP1GAP gene maps to 1p35-36, a chromosomal region subject to copy number alterations in human tumors (36, 49). Rap1GAP protein levels are decreased in pancreatic adenocarcinomas (53), papillary thyroid carcinomas (37, 47, 57), and melanomas (56). Rap1GAP downregulation has been shown to arise as a consequence of proteasomal degradation (46), loss of heterozygosity (37, 53), and promoter methylation (56, 57). Mutations of unknown significance in RAP1GAP have been identified in breast cancer (42). Although downregulation of Rap1GAP is frequent in human tumors, the functional significance of decreased Rap1GAP expression is unknown. Up to now, studies assessing the role of Rap1GAP in tumor cells have relied exclusively on overexpression experiments. Overexpression of Rap1GAP in oropharyngeal squamous cell (54) and pancreatic (53) carcinoma lines impaired tumor formation in mouse xenograft models. In vitro, overexpression of Rap1GAP impaired tumor cell proliferation (34, 47, 53, 54, 56) and enhanced apoptosis (34, 53, 56). In some instances, overexpression of Rap1GAP inhibited tumor cell migration and invasion (3, 47, 53, 56), while in others, it enhanced invasion (34). While these studies provide insight into cellular processes that can be deregulated by overexpression, they do not assess the significance of depletion of endogenous Rap1GAP in human tumors.Colorectal cancer (CRC) is one of the leading causes of cancer deaths worldwide. The majority of CRC deaths arise as a consequence of distant metastases, most frequently to the liver. While the genetic basis of CRC is well understood (19, 48), less is known about the events that trigger the transition to metastatic disease. We report that Rap1GAP is highly expressed in normal colonic epithelium and that its expression is profoundly decreased in primary colorectal carcinomas. As one strategy to assess the significance of Rap1GAP depletion, the expression of Rap1GAP was silenced in human colon carcinoma cells. Silencing of Rap1GAP induced marked increases in Rap1 and Rap2 activity, the first evidence that Rap1GAP is an essential negative regulator of Rap GTPases in colon cancer. Rap1 regulates inside-out signaling through integrins (reviewed in references 8, 9, and 11) and is a target of outside-in signaling via cadherins (reviewed in reference 30). Downregulation of Rap1GAP induced profound alterations in cell/matrix and cell/cell adhesion. Suppressing Rap1GAP expression enhanced adhesion and spreading on collagen. Unexpectedly, based on the role of Rap1 in promoting cell/cell adhesion, silencing of Rap1GAP impaired cell/cell adhesion. These findings demonstrate a requirement for regulated Rap activity in the maintenance of epithelial cell structure and demonstrate a heretofore unappreciated role for Rap1GAP in the regulation of cell/cell adhesion. As the dissemination of tumor cells requires the weakening of cell/cell adhesion and an enhanced ability to adhere to collagen-rich interstitial matrices, our studies identify a potential mechanism through which loss of Rap1GAP contributes to tumor progression. |
