Role as a transcription factor β-catenin can enter the nucleus and serve as a transcription factor, although the exact mechanism for this translocation into the nucleus is still under investigation. Once in the nucleus, β-catenin interacts with
T-cell factor/Lymphoid enhancer factor (TCF/LEF), switching TCF/LEF from a repressor to a transcriptional promoter. It also recruits the protein
Mediator, which then recruits
RNA polymerase II and other general transcription factors. The Wnt/β-catenin pathway activates transcription of many genes, notably those encoding the following products: • ABC multidrug transporter (ABCB1) • Survivin (BIRC5) • Cyclin D1 (CCND1) • Fibroblast growth factor 18 (FGF18) • Matrix metalloproteinase-14 (MMP14) • Myc proto-oncogene (MYC) Many of the genes activated by the β-catenin pathway are directly or indirectly involved in cell growth, multiplication, and survival. These genes are important in the processes embryogenesis and tumorigenesis, making β-catenin's role as a transcription factor of particular interest to researchers.
Regulation of degradation through phosphorylation The cellular level of β-catenin is mostly controlled by its
ubiquitination and
proteosomal degradation. The
E3 ubiquitin ligase TrCP1 (also known as β-TrCP) can recognize β-catenin as its substrate through a short linear motif on the disordered N-terminus. However, this motif (Asp-Ser-Gly-Ile-His-Ser) of β-catenin needs to be
phosphorylated on the two
serines in order to be capable to bind β-TrCP. Phosphorylation of the motif is performed by
glycogen synthase kinase 3 alpha and beta (GSK3α and GSK3β). GSK3s are constitutively active enzymes implicated in several important regulatory processes. There is one requirement, though: substrates of GSK3 need to be pre-phosphorylated four amino acids downstream (C-terminally) of the actual target site. Thus it also requires a "priming kinase" for its activities. In the case of β-catenin, the most important priming kinase is
casein kinase 1 (CKI). Once a serine-threonine rich substrate has been "primed", GSK3 can "walk" across it from C-terminal to N-terminal direction, phosphorylating every 4th serine or
threonine residue in a row. This process will result in dual phosphorylation of the aforementioned β-TrCP recognition motif as well.
The beta-catenin destruction complex For
GSK3 to be a highly effective
kinase on a substrate, pre-phosphorylation is not enough. There is one additional requirement: Similar to the
mitogen-activated protein kinases (MAPKs), substrates need to associate with this enzyme through high-affinity
docking motifs. β-Catenin contains no such motifs, but a special protein does:
axin. What is more, its GSK3 docking motif is directly adjacent to a β-catenin binding motif.
APC can interact with multiple
axin molecules at the same time as it has three
SAMP motifs (Ser-Ala-Met-Pro) to bind the
RGS domains found in
axin. In addition, axin also has the potential to oligomerize through its C-terminal DIX domain. The result is a huge, multimeric protein assembly dedicated to β-catenin phosphorylation. This complex is usually called the
beta-catenin destruction complex, although it is distinct from the
proteosome machinery actually responsible for β-catenin degradation. It only marks β-catenin molecules for subsequent destruction.
Wnt signaling and the regulation of destruction In resting cells,
axin molecules oligomerize with each other through their C-terminal DIX domains, which have two binding interfaces. Thus they can build linear oligomers or even polymers inside the cytoplasm of cells. DIX domains are unique: the only other proteins known to have a DIX domain are
Dishevelled and
DIXDC1. (The single
Dsh protein of
Drosophila corresponds to three paralogous genes,
Dvl1,
Dvl2 and
Dvl3 in
mammals.)
Dsh associates with the cytoplasmic regions of
Frizzled receptors with its
PDZ and
DEP domains. When a
Wnt molecule binds to
Frizzled, it induces a poorly known cascade of events, that result in the exposure of dishevelled's DIX domain and the creation of a perfect binding site for
axin. Axin is then titrated away from its oligomeric assemblies – the β-catenin destruction complex – by
Dsh. Once bound to the receptor complex,
axin will be rendered incompetent for β-catenin binding and GSK3 activity. Importantly, the cytoplasmic segments of the Frizzled-associated
LRP5 and
LRP6 proteins contain GSK3 pseudo-substrate sequences (Pro-Pro-Pro-Ser-Pro-x-Ser), appropriately "primed" (pre-phosphorylated) by
CKI, as if it were a true substrate of GSK3. These false target sites greatly inhibit GSK3 activity in a competitive manner. This way receptor-bound
axin will abolish mediating the phosphorylation of β-catenin. Since β-catenin is no longer marked for destruction, but continues to be produced, its concentration will increase. Once β-catenin levels rise high enough to saturate all binding sites in the cytoplasm, it will also translocate into the nucleus. Upon engaging the transcription factors
LEF1,
TCF1,
TCF2 or
TCF3, β-catenin forces them to disengage their previous partners: Groucho proteins. Unlike
Groucho, that recruit
transcriptional repressors (e.g.
histone-lysine methyltransferases), β-catenin will bind
transcriptional activators, switching on target genes.
Role in cell–cell adhesion Cell–cell adhesion complexes are essential for the formation of complex animal tissues. β-catenin is part of a
protein complex that form
adherens junctions. These cell–cell adhesion complexes are necessary for the creation and maintenance of
epithelial cell layers and barriers. As a component of the complex, β-catenin can regulate cell growth and adhesion between cells. It may also be responsible for transmitting the contact inhibition signal that causes cells to stop dividing once the epithelial sheet is complete. The E-cadherin – β-catenin – α-catenin complex is weakly associated to
actin filaments. Adherens junctions require significant
protein dynamics in order to link to the actin cytoskeleton, An important component of the adherens junctions are the cadherin proteins. Cadherins form the cell–cell junctional structures known as adherens junctions as well as the
desmosomes. Cadherins are capable of homophilic interactions through their extracellular cadherin repeat domains, in a Ca2+-dependent manner; this can hold adjacent epithelial cells together. While in the adherens junction, cadherins recruit β-catenin molecules onto their intracellular regions. β-catenin, in turn, associates with another highly
dynamic protein,
α-catenin, which directly binds to the actin filaments. This is possible because α-catenin and cadherins bind at distinct sites to β-catenin. The β-catenin – α-catenin complex can thus physically form a bridge between cadherins and the
actin cytoskeleton. Organization of the cadherin–catenin complex is additionally regulated through
phosphorylation and
endocytosis of its components.
Roles in development β-Catenin has a central role in directing several developmental processes, as it can directly bind
transcription factors and chromatin regulators, Early embryos endomesoderm specification also involves the activation of the β-catenin dependent transcriptional activity by the first morphogenetic movements of embryogenesis, though mechanotransduction processes. This feature being shared by vertebrate and arthropod bilateria, and by cnidaria, it was proposed to have been evolutionary inherited from its possible involvement in the endomesoderm specification of first metazoa. During the blastula and gastrula stages,
Wnt as well as
BMP and
FGF pathways will induce the antero-posterior axis formation, regulate the precise placement of the primitive streak (gastrulation and mesoderm formation) as well as the process of neurulation (central nervous system development). In
Xenopus oocytes, β-catenin is initially equally localized to all regions of the egg, but it is targeted for ubiquitination and degradation by the β-catenin destruction complex.
Fertilization of the egg causes a rotation of the outer cortical layers, moving clusters of the
Frizzled and
Dsh proteins closer to the equatorial region. β-catenin will be enriched locally under the influence of Wnt signaling pathway in the cells that inherit this portion of the cytoplasm. It will eventually translocate to the nucleus to bind
TCF3 in order to activate several genes that induce dorsal cell characteristics. This signaling results in a region of cells known as the grey crescent, which is a classical organizer of embryonic development. If this region is surgically removed from the embryo, gastrulation does not occur at all. β-Catenin also plays a crucial role in the induction of the
blastopore lip, which in turn initiates gastrulation. Inhibition of GSK-3 translation by injection of antisense mRNA may cause a second blastopore and a superfluous body axis to form. A similar effect can result from the overexpression of β-catenin.
Asymmetric cell division β-catenin has also been implicated in regulation of cell fates through
asymmetric cell division in the model organism
C. elegans. Similarly to the
Xenopus oocytes, this is essentially the result of non-equal distribution of
Dsh,
Frizzled,
axin and
APC in the cytoplasm of the mother cell.
Stem cell renewal One of the most important results of Wnt signaling and the elevated level of β-catenin in certain cell types is the maintenance of
pluripotency. High frequency peristaltic mechanical strains of the colon are also involved in the β-catenin dependent maintenance of homeostatic levels of colonic stem cells through processes of mechanotransduction. This feature is pathologically enhanced towards tumorigenic hyperproliferation in healthy cells compressed by pressure due genetically altered hyperproliferative tumorous cells. In other cell types and developmental stages, β-catenin may promote
differentiation, especially towards
mesodermal cell lineages.
Epithelial-to-mesenchymal transition β-Catenin also acts as a morphogen in later stages of embryonic development. Together with
TGF-β, an important role of β-catenin is to induce a morphogenic change in epithelial cells. It induces them to abandon their tight adhesion and assume a more mobile and loosely associated
mesenchymal phenotype. During this process, epithelial cells lose expression of proteins like
E-cadherin,
Zonula occludens 1 (ZO1), and
cytokeratin. At the same time they turn on the expression of
vimentin,
alpha smooth muscle actin (ACTA2), and fibroblast-specific protein 1 (FSP1). They also produce extracellular matrix components, such as
type I collagen and
fibronectin. Aberrant activation of the Wnt pathway has been implicated in pathological processes such as fibrosis and cancer. In cardiac muscle development, β-catenin performs a biphasic role. Initially, the activation of Wnt/β-catenin is essential for committing mesenchymal cells to a cardiac lineage; however, in later stages of development, the downregulation of β-catenin is required. It has been shown that β-catenin forms a complex with
emerin in cardiomyocytes at adherens junctions within intercalated discs; and this interaction is dependent on the presence of
GSK 3-beta phosphorylation sites on β-catenin. Knocking out emerin significantly altered β-catenin localization and the overall intercalated disc architecture, which resembled a
dilated cardiomyopathy phenotype. In animal models of
cardiac disease, functions of β-catenin have been unveiled. In a guinea pig model of
aortic stenosis and left ventricular
hypertrophy, β-catenin was shown to change subcellular localization from intercalated discs to the
cytosol, despite no change in the overall cellular abundance of β-catenin.
Vinculin showed a similar profile of change. N-cadherin showed no change, and there was no compensatory upregulation of
plakoglobin at intercalated discs in the absence of β-catenin. In a hamster model of
cardiomyopathy and
heart failure, cell–cell adhesions were irregular and disorganized, and expression levels of adherens junction/intercalated disc and
nuclear pools of β-catenin were decreased. These data suggest that a loss of β-catenin may play a role in the diseased intercalated discs that have been associated with cardiac muscle hypertrophy and heart failure. In a rat model of
myocardial infarction,
adenoviral gene transfer of non
phosphorylatable, constitutively-active β-catenin decreased MI size, activated the
cell cycle, and reduced the amount of
apoptosis in cardiomyocytes and cardiac
myofibroblasts. This finding was coordinate with enhanced expression of pro-survival proteins,
survivin and
Bcl-2, and
vascular endothelial growth factor while promoting the differentiation of cardiac
fibroblasts into myofibroblasts. These findings suggest that β-catenin can promote the regeneration and healing process following myocardial infarction. In a spontaneously-
hypertensive heart failure rat model, investigators detected a shuttling of β-catenin from the intercalated disc/
sarcolemma to the
nucleus, evidenced by a reduction of β-catenin expression in the membrane protein fraction and an increase in the nuclear fraction. Additionally, they found a weakening in the association between
glycogen synthase kinase-3β and β-catenin, which may indicate altered protein stability. Overall, results suggest that an enhanced nuclear localization of β-catenin may be important in the progression of
cardiac hypertrophy. Regarding the mechanistic role of β-catenin in cardiac hypertrophy, transgenic mouse studies have shown somewhat conflicting results regarding whether upregulation of β-catenin is beneficial or detrimental. A recent study using a conditional knockout mouse that either lacked β-catenin altogether or expressed a non-degradable form of β-catenin in cardiomyocytes reconciled a potential reason for these discrepancies. There appears to be strict control over the subcellular localization of β-catenin in cardiac muscle. Mice lacking β-catenin had no overt phenotype in the left ventricular
myocardium; however, mice harboring a stabilized form of β-catenin developed
dilated cardiomyopathy, suggesting that the temporal regulation of β-catenin by protein degradation mechanisms is critical for normal functioning of β-catenin in cardiac cells. In a mouse model harboring knockout of a desmosomal protein, plakoglobin, implicated in
arrhythmogenic right ventricular cardiomyopathy, the stabilization of β-catenin was also enhanced, presumably to compensate for the loss of its plakoglobin homolog. These changes were coordinate with Akt activation and
glycogen synthase kinase 3β inhibition, suggesting once again that the abnormal stabilization of β-catenin may be involved in the development of cardiomyopathy. Further studies employing a double knockout of plakoglobin and β-catenin showed that the double knockout developed cardiomyopathy,
fibrosis and
arrhythmias resulting in
sudden cardiac death. Intercalated disc architecture was severely impaired and
connexin 43-resident
gap junctions were markedly reduced.
Electrocardiogram measurements captured spontaneous lethal ventricular arrhythmias in the double transgenic animals, suggesting that the two catenins—β-catenin and plakoglobin—are critical and indispensable for mechanoelectrical coupling in cardiomyocytes. == Clinical significance ==