E-Cadherin Suppresses Cellular Transformation by Inhibiting β-Catenin Signaling in an Adhesion-Independent Manner

Abstract

E-cadherin is a tumor suppressor protein with a well-established role in cell–cell adhesion. Adhesion could contribute to tumor suppression either by physically joining cells or by facilitating other juxtacrine signaling events. Alternatively, E-cadherin tumor suppressor activity could result from binding and antagonizing the nuclear signaling function of β-catenin, a known proto-oncogene. To distinguish between an adhesion- versus a β-catenin signaling–dependent mechanism, chimeric cadherin constructs were expressed in the SW480 colorectal tumor cell line. Expression of wild-type E-cadherin significantly inhibits the growth of this cell line. Growth inhibitory activity is retained by all constructs that have the β-catenin binding region of the cytoplasmic domain but not by E-cadherin constructs that exhibit adhesive activity, but lack the β-catenin binding region. This growth suppression correlates with a reduction in β-catenin/T cell factor (TCF) reporter gene activity. Importantly, direct inhibition of β-catenin/TCF signaling inhibits the growth of SW480 cells, and the growth inhibitory activity of E-cadherin is rescued by constitutively activated forms of TCF. Thus, the growth suppressor activity of E-cadherin is adhesion independent and results from an inhibition of the β-catenin/TCF signaling pathway, suggesting that loss of E-cadherin expression can contribute to upregulation of this pathway in human cancers. E-cadherin–mediated growth suppression was not accompanied by overall depletion of β-catenin from the cytosol and nucleus. This appears to be due to the existence of a large pool of cytosolic β-catenin in SW480 cells that is refractory to both cadherin binding and TCF binding. Thus, a small pool of β-catenin that can bind TCF (i.e., the transcriptionally active pool) can be selectively depleted by E-cadherin expression. The existence of functionally distinct pools of cytosolic β-catenin suggests that there are mechanisms to regulate β-catenin signaling in addition to controlling its level of accumulation.

E-cadherin inhibits tumor cell growth by suppressing PI3K/Akt signaling via β-catenin-Egr1-mediated PTEN expression

“…Recent studies have suggested that E-cadherin suppresses cell growth by inhibiting b-catenin signaling (Gottardi et al, 2001;Stockinger et al, 2001;Maher et al, 2009), we report here that short hairpin RNA (shRNA)-mediated depletion of E-cadherin resulted in relocalization of b-catenin from the membrane to the nucleus and activation of b-catenin-TCF signaling, which in turn regulates cell growth in ovarian cancer cells. Theoretically, translocation of b-catenin to the nucleus leads to its association with TCFs and results in regulated transactivation of genes containing the LEF-1/TCF-4 binding sequence near their promoter, such as cyclin D1 (Morin, 1999;Shtutman et al, 1999;Lin et al, 2000).…”

“…These data strongly indicate that loss of E-cadherin promotes the growth of SKOV-3 cells by activating the PI3K/Akt signaling pathway. Because E-cadherin has been shown to inhibit bcatenin signaling (Gottardi et al, 2001;Stockinger et al, 2001;Conacci-Sorrell et al, 2003), we therefore examined the effects of E-cadherin loss on b-catenin signaling. Downregulation of E-cadherin in SKOV-3 cells resulted in the loss of b-catenin from sites of cell-cell contact, as assessed by immunocytochemistry ( Figure 3D).…”

“…Taken together, these results suggest that E-cadherin, by the assembly of adherens junctions, regulates Egr1 and PTEN expression by modulating b-catenin signaling. Discussion E-cadherin is known to suppress tumor cell growth and invasion, and re-expression of E-cadherin in E-cadherindeficient carcinomas reverts cells to a less invasive, less aggressive phenotype (St Croix et al, 1998;Gottardi et al, 2001;Yanagisawa and Anastasiadis, 2006;Soto et al, 2008). On the other hand, the expression of E-cadherin supports cohesive, collective cell migration/ invasion (Friedl and Gilmour, 2009).…”

“…E-cadherin-containing adherens junctions ensure that the cytoplasmic pool of b-catenin is maintained at a low level. Thus, E-cadherin could antagonize b-catenin signaling and induce growth inhibition (Shtutman et al, 1999;Gottardi et al, 2001). Indeed, b-catenin has been shown to localize to the nucleus, following the loss of E-cadherin expression (Gottardi et al, 2001;Onder et al, 2008).…”

“…Thus, E-cadherin could antagonize b-catenin signaling and induce growth inhibition (Shtutman et al, 1999;Gottardi et al, 2001). Indeed, b-catenin has been shown to localize to the nucleus, following the loss of E-cadherin expression (Gottardi et al, 2001;Onder et al, 2008). However, the detailed mechanism by which the loss of E-cadherin contributes to enhanced b-catenin signaling is not well understood.…”