Nuclear export of human β-catenin can occur independent of CRM1 and the adenomatous polyposis coli tumor suppressor

beta -Catenin is a mediator of the Wnt-signaling pathway. In many cancers, beta -catenin is stabilized and accumulates in the nucleus where it associates with lymphoid-enhancing factor II T-cell transcription factors to activate genes involved in cell transformation. Previously, we showed that adenomatous polyposis coli (APC) protein can regulate beta -catenin localization by nuclear export. In this study, we used in vitro transport assays to test whether cellular beta -catenin can exit the nucleus independent of APC and the CRM1 export receptor, In digitonin-permeabilized SW480 (APC(mut/mut)) tumor cells, nuclear beta -catenin decreased > 60% in export reactions in the absence of exogenous factors, Under similar conditions, nuclear c-ABL was only exported after the addition of cytosolic extract, and the export was blocked by the CRM1-specific inhibitor, leptomycin B, The nuclear export of beta -catenin was not blocked by leptomycin B treatment, revealing a CRM1- and APC-independent pathway. The export of beta -catenin was sensitive to lower temperatures and the removal of ATP, indicating an active process. Ectopically expressed yellow fluorescent protein-beta -catenin also displayed CRM1-independent export. Conversely, the overexpression of the CRM1 transporter moderately stimulated export of nuclear beta -catenin, confirming that beta -catenin exits the nucleus by at least two distinct pathways. The shuttling ability of tumor cell beta -catenin has implications for its regulation and its role in transferring signals between the nucleus and plasma membrane.