Carcinogenesis: A balance between β-catenin and APC

AbstractA protein first identified by its association with cadherin cell adhesion molecules, β-catenin, has been implicated in carcinogenesis. In a number of different types of cancer, signalling through β-catenin is upregulated either by direct mutation of β-catenin or loss of negative regulation by the APC tumor suppressor protein

Adhesion-independent mechanism for suppression of tumor cell invasion by E-cadherin

Loss of E-cadherin expression or function in tumors leads to a more invasive phenotype. In this study, we investigated whether the invasion suppressor activity of E-cadherin is mediated directly by tighter physical cell adhesion, indirectly by sequestering β-catenin and thus antagonizing β-catenin/T cell factor (TCF) signaling, or by other signaling pathways. To distinguish mechanisms, we expressed wild-type E-cadherin and various E-cadherin mutants in invasive E-cadherin–negative human breast (MDA-MB-231) and prostate (TSU-Pr1) epithelial carcinoma cell lines using a tetracycline-inducible system. Our data confirm that E-cadherin inhibits human mammary and prostate tumor cell invasion. We find that adhesion is neither necessary nor sufficient for suppressing cancer invasion. Rather, the invasion suppressor signal is mediated through the β-catenin–binding domain of the E-cadherin cytoplasmic tail but not through the p120ctn-binding domain. β-catenin depletion also results in invasion suppression. However, alteration in the β-catenin/TCF transcriptional regulation of target genes is not required for the invasion suppressor activity of E-cadherin, suggesting the involvement of other β-catenin–binding proteins

Paraxial protocadherin mediates cell sorting and tissue morphogenesis by regulating C-cadherin adhesion activity

Little is known about how protocadherins function in cell adhesion and tissue development. Paraxial protocadherin (PAPC) controls cell sorting and morphogenetic movements in the Xenopus laevis embryo. We find that PAPC mediates these functions by down-regulating the adhesion activity of C-cadherin. Expression of exogenous C-cadherin reverses PAPC-induced cell sorting and gastrulation defects. Moreover, loss of endogenous PAPC results in elevated C-cadherin adhesion activity in the dorsal mesoderm and interferes with the normal blastopore closure, a defect that can be rescued by a dominant-negative C-cadherin mutant. Importantly, activin induces PAPC expression, and PAPC is required for activin-induced regulation of C-cadherin adhesion activity and explant morphogenesis. Signaling through Frizzled-7 is not required for PAPC regulation of C-cadherin, suggesting that C-cadherin regulation and Frizzled-7 signaling are two distinct branches of the PAPC pathway that induce morphogenetic movements. Thus, spatial regulation of classical cadherin adhesive function by local expression of a protocadherin is a novel mechanism for controlling cell sorting and tissue morphogenesis

A Mode of Regulation of β-Catenin Signaling Activity in Xenopus Embryos Independent of Its Levels

AbstractThe signaling activity of β-catenin is thought to be regulated by phosphorylation of a cluster of N-terminal serines, putative sites for GSK3β. In the prevailing model in the literature, GSK3β-dependent phosphorylation of these sites targets β-catenin for ubiquitin-mediated degradation. Wnt signaling inhibits GSK3β activity and this blocks degradation, allowing β-catenin to accumulate and signal. We show here that β-catenin activity is not regulated solely by protein stability. Mutations in the putative GSK3β phosphorylation sites of β-catenin enhance its signaling activity, but this cannot be accounted for by accumulation of either total or cadherin-free protein. Instead, the mutant protein has a threefold higher specific activity than the wild type both in vivo and in an in vitro signaling assay. We conclude that the N-terminal serines convey a layer of regulation upon β-catenin signaling in addition to the effects these sites exert upon protein stability

Distinct molecular forms of β-catenin are targeted to adhesive or transcriptional complexes

β-Catenin plays essential roles in both cell–cell adhesion and Wnt signal transduction, but what precisely controls β-catenin targeting to cadherin adhesive complexes, or T-cell factor (TCF)-transcriptional complexes is less well understood. We show that during Wnt signaling, a form of β-catenin is generated that binds TCF but not the cadherin cytoplasmic domain. The Wnt-stimulated, TCF-selective form is monomeric and is regulated by the COOH terminus of β-catenin, which selectively competes cadherin binding through an intramolecular fold-back mechanism. Phosphorylation of the cadherin reverses the TCF binding selectivity, suggesting another potential layer of regulation. In contrast, the main cadherin-binding form of β-catenin is a β-catenin–α-catenin dimer, indicating that there is a distinct molecular form of β-catenin that can interact with both the cadherin and α-catenin. We propose that participation of β-catenin in adhesion or Wnt signaling is dictated by the regulation of distinct molecular forms of β-catenin with different binding properties, rather than simple competition between cadherins and TCFs for a single constitutive form. This model explains how cells can control whether β-catenin is used independently in cell adhesion and nuclear signaling, or competitively so that the two processes are coordinated and interrelated

Cadherin-mediated cell sorting not determined by binding or adhesion specificity

Cadherin adhesion molecules play important roles in the establishment of tissue boundaries. Cells expressing different cadherins sort out from each other in cell aggregation assays. To determine the contribution of cadherin binding and adhesion specificity to the sorting process, we examined the adhesion of cells to different purified cadherin proteins. Chinese hamster ovary cell lines expressing one of four different cadherins were allowed to bind to the purified cadherin extracellular domains of either human E-cadherin or Xenopus C-cadherin, and the specificity of adhesion was compared with cell-sorting assays. None of the different cadherin-expressing cells exhibited any adhesive specificity toward either of the two purified cadherin substrates, even though these cadherins differ considerably in their primary sequence. In addition, all cells exhibited similar strengthening of adhesion on both substrates. However, this lack of adhesive specificity did not determine whether different cadherin-expressing cells would sort from each other, and the tendency to sort was not predictable by the extent of sequence diversity in their extracellular domains. These results show that cadherins are far more promiscuous in their adhesive-binding capacity than had been expected and that the ability to sort out must be determined by mechanisms other than simple adhesive-binding specificity

Characterization of a 60S complex of the adenomatous polyposis coli tumor suppressor protein

AbstractThe tumor suppressor protein adenomatous polyposis coli (APC) is a multifunctional protein with a well characterized role in the Wnt signal transduction pathway and roles in cytoskeletal regulation and cell polarity. The soluble pool of APC protein in colon epithelial tumor cells exists in two distinct complexes fractionating at ∼20S and ∼60S in size. The 20S complex contains components of the β-catenin destruction complex and probably functions in the Wnt pathway. In this study, we characterized the molecular nature of the 60S APC- containing complex by examining known potential binding partners of APC. 60S APC did not contain EB1 or diaphanous, proteins that have been reported to interact with APC and are implicated in microtubule plus end stabilization. Nor did the two other microtubule associated proteins, MAP4 or KAP3, which is thought to link APC to kinesin motor proteins, associate with the 60S complex. Minor fractions of α-tubulin, γ-tubulin and IQGAP1, a Rac1 and CDC42 effector that interacts with APC, specifically associated with APC in the 60S fraction. We propose that 60S APC is a discrete high molecular weight complex with a novel function in cytoskeletal regulation in epithelial cells apart from its well established role in targeting catenin destruction or its proposed role in microtubule plus end stabilization