Nucleo-cytoplasmic distribution of beta-catenin is regulated by retention

Beta-catenin signaling in fibroproliferative disease

BACKGROUND

Beta-catenin has been historically recognized as both an intermediate in the "canonical Wnt signaling pathway" and as a component of functional adherens junctions.

MATERIALS AND METHODS

Cellular accumulation of beta-catenin levels can result in transactivation of gene transcription and cellular proliferation during normal cellular and disease development. Recent evidence has identified beta-catenin in an additional role as a component of cutaneous wound healing.

RESULTS

This finding is in keeping with previous observations that post-translational modifications of beta-catenin that are associated with its cytoplasmic accumulation are frequently observed in fibroproliferative diseases with characteristics of dysregulated wound healing. These diseases include hypertrophic scar formation, aggressive fibromatoses, Lederhose disease, and Dupuytren's contracture (DC).

CONCLUSIONS

While its precise roles in disease initiation and progression remain to be explored, this review highlights our current knowledge of beta-catenin regulation and describes some potential upstream mediators of beta-catenin accumulation and signaling in fibroproliferative disease.

Unwinding a Path to Nuclear β-Catenin

In this issue of Cell, Yang et al. (2006b) show that PDGF, a growth factor that induces the transition of epithelial cells to mesenchymal cells, stimulates the c-Abl kinase-dependent phosphorylation of p68 RNA helicase. Phosphorylated p68 dissociates beta-catenin from the Axin destruction complex, thereby promoting nuclear beta-catenin signaling independent of Wnt activation.

Transcription under the control of nuclear Arm/beta-catenin

The Wingless/Wnt pathway controls cell fates during animal development and regulates tissue homeostasis as well as stem cell number and differentiation in epithelia. Deregulation of Wnt signaling has been associated with cancer in humans. In the nucleus, the Wingless/Wnt signal is transmitted via the key effector protein Armadillo/beta-catenin. The recent identification and functional analysis of novel Armadillo/beta-catenin interaction partners provide new and exciting insights into the highly complex mechanism of Wingless/Wnt target gene activation.

Aberrant Wnt/beta-catenin signaling can induce chromosomal instability in colon cancer

Chromosomal instability (CIN), a hallmark of most colon tumors, may promote tumor progression by increasing the rate of genetic aberrations. CIN is thought to arise as a consequence of improper mitosis and spindle checkpoint activity, but its molecular basis remains largely elusive. The majority of colon tumors develop because of mutations in the tumor suppressor APC that lead to Wnt/beta-catenin signaling activation and subsequent transcription of target genes, including conductin/AXIN2. Here we demonstrate that Wnt/beta-catenin signaling causes CIN via up-regulation of conductin. Human colon tumor samples with CIN show significantly higher expression of conductin than those without. Conductin is up-regulated during mitosis, localizes along the mitotic spindles of colon cancer cells, and binds to polo-like kinase 1. Ectopic expression of conductin or its up-regulation through small interfering RNA-mediated knock-down of APC leads to CIN in chromosomally stable colon cancer cells. High conductin expression compromises the spindle checkpoint, and this requires localized polo-like kinase 1 activity. Knock-down of conductin by small interfering RNA in colon carcinoma cells or gene ablation in mouse embryo fibroblasts enforces the checkpoint.

Adenomatous polyposis coli (APC) differentially regulates beta-catenin phosphorylation and ubiquitination in colon cancer cells

Most colorectal cancers have mutations of the adenomatous polyposis coli (APC) gene or the beta-catenin gene that stabilize beta-catenin and activate beta-catenin target genes, leading ultimately to cancer. The molecular mechanisms of APC function in beta-catenin degradation are not completely known. APC binds beta-catenin and is involved in the Axin complex, suggesting that APC regulates beta-catenin phosphorylation. Some evidence also suggests that APC regulates beta-catenin nuclear export. Here, we examine the effects of APC mutations on beta-catenin phosphorylation, ubiquitination, and degradation in the colon cancer cell lines SW480, DLD-1, and HT29, each of which contains a different APC truncation. Although the current models suggest that beta-catenin phosphorylation should be inhibited by APC mutations, we detected significant beta-catenin phosphorylation in these cells. However, beta-catenin ubiquitination and degradation were inhibited in SW480 but not in DLD-1 and HT29 cells. The ubiquitination ofbeta-catenin in SW480 cells can be rescued by exogenous expression of APC. The APC domains required for beta-catenin ubiquitination were analyzed. Our results suggest that APC regulates beta-catenin phosphorylation and ubiquitination by distinct domains and by separate molecular mechanisms.