Proteomic Analysis of the E3 Ubiquitin-Ligase Hakai Highlights a Role in Plasticity of the Cytoskeleton Dynamics and in the Proteasome System.

Carcinoma, the most common type of cancer, arises from epithelial cells. The transition from adenoma to carcinoma is associated with the loss of E-cadherin and, in consequence, the disruption of cell-cell contacts. E-cadherin is a tumor suppressor, and it is down-regulated during epithelial-to-mesenchymal transition (EMT); indeed, its loss is a predictor of poor prognosis. Hakai is an E3 ubiquitin-ligase protein that mediates E-cadherin ubiquitination, endocytosis and finally degradation, leading the alterations of cell-cell contacts. Although E-cadherin is the most established substrate for Hakai activity, other regulated molecular targets for Hakai may be involved in cancer cell plasticity during tumor progression. In this work we employed an iTRAQ approach to explore novel molecular pathways involved in Hakai-driven EMT during tumor progression. Our results show that Hakai may have an important influence on cytoskeleton-related proteins, extracellular exosome-associated proteins, RNA-related proteins and proteins involved in metabolism. Moreover, a profound decreased expression in several proteasome subunits during Hakai-driven EMT was highlighted. Since proteasome inhibitors are becoming increasingly used in cancer treatment, our findings suggest that the E3 ubiquitin-ligase, such as Hakai, may be a better target than proteasome for using novel specific inhibitors in tumor subtypes that follow EMT.

Role of the microtubule-targeting drug vinflunine on cell-cell adhesions in bladder epithelial tumour cells.

BACKGROUND: Vinflunine (VFL) is a microtubule-targeting drug that suppresses microtubule dynamics, showing anti-metastatic properties both in vitro and in living cancer cells. An increasing body of evidence underlines the influence of the microtubules dynamics on the cadherin-dependent cell-cell adhesions. E-cadherin is a marker of epithelial-to-mesenchymal transition (EMT) and a tumour suppressor; its reduced levels in carcinoma are associated with poor prognosis. In this report, we investigate the role of VFL on cell-cell adhesions in bladder epithelial tumour cells. METHODS: Human bladder epithelial tumour cell lines HT1376, 5637, SW780, T24 and UMUC3 were used to analyse cadherin-dependent cell-cell adhesions under VFL treatment. VFL effect on growth inhibition was measured by using a MTT colorimetric cell viability assay. Western blot, immunofluorescence and transmission electron microscopy analyses were performed to assess the roles of VFL effect on cell-cell adhesions, epithelial-to-mesenchymal markers and apoptosis. The role of the proteasome in controlling cell-cell adhesion was studied using the proteasome inhibitor MG132. RESULTS: We show that VFL induces cell death in bladder cancer cells and activates epithelial differentiation of the remaining living cells, leading to an increase of E-cadherin-dependent cell-cell adhesion and a reduction of mesenchymal markers, such as N-cadherin or vimentin. Moreover, while E-cadherin is increased, the levels of Hakai, an E3 ubiquitin-ligase for E-cadherin, were significantly reduced in presence of VFL. In 5637, this reduction on Hakai expression was blocked by MG132 proteasome inhibitor, indicating that the proteasome pathway could be one of the molecular mechanisms involved in its degradation. CONCLUSIONS: Our findings underscore a critical function for VFL in cell-cell adhesions of epithelial bladder tumour cells, suggesting a novel molecular mechanism by which VFL may impact upon EMT and metastasis.

Hakin-1, a New Specific Small-Molecule Inhibitor for the E3 Ubiquitin-Ligase Hakai, Inhibits Carcinoma Growth and Progression.

The requirement of the E3 ubiquitin-ligase Hakai for the ubiquitination and subsequent degradation of E-cadherin has been associated with enhanced epithelial-to-mesenchymal transition (EMT), tumour progression and carcinoma metastasis. To date, most of the reported EMT-related inhibitors were not developed for anti-EMT purposes, but indirectly affect EMT. On the other hand, E3 ubiquitin-ligase enzymes have recently emerged as promising therapeutic targets, as their specific inhibition would prevent wider side effects. Given this background, a virtual screening was performed to identify novel specific inhibitors of Hakai, targeted against its phosphotyrosine-binding pocket, where phosphorylated-E-cadherin specifically binds. We selected a candidate inhibitor, Hakin-1, which showed an important effect on Hakai-induced ubiquitination. Hakin-1 also inhibited carcinoma growth and tumour progression both in vitro, in colorectal cancer cell lines, and in vivo, in a tumour xenograft mouse model, without apparent systemic toxicity in mice. Our results show for the first time that a small molecule putatively targeting the E3 ubiquitin-ligase Hakai inhibits Hakai-dependent ubiquitination of E-cadherin, having an impact on the EMT process. This represents an important step forward in a future development of an effective therapeutic drug to prevent or inhibit carcinoma tumour progression.

Hakai overexpression effectively induces tumour progression and metastasis in vivo.

At early stages of carcinoma progression, epithelial cells undergo a program named epithelial-to-mesenchymal transition characterized by the loss of the major component of the adherens junctions, E-cadherin, which in consequence causes the disruption of cell-cell contacts. Hakai is an E3 ubiquitin-ligase that binds to E-cadherin in a phosphorylated-dependent manner and induces its degradation; thus modulating cell adhesions. Here, we show that Hakai expression is gradually increased in adenoma and in different TNM stages (I-IV) from colon adenocarcinomas compared to human colon healthy tissues. Moreover, we confirm that Hakai overexpression in epithelial cells drives transformation in cells, a mesenchymal and invasive phenotype, accompanied by the downregulation of E-cadherin and the upregulation of N-cadherin, and an increased proliferation and an oncogenic potential. More importantly, for the first time, we have studied the role of Hakai during cancer progression in vivo. We show that Hakai-transformed MDCK cells dramatically induce tumour growth and local invasion in nude mice and tumour cells exhibit a mesenchymal phenotype. Furthermore, we have detected the presence of micrometastasis in the lung mice, further confirming Hakai role during tumour metastasis in vivo. These results lead to the consideration of Hakai as a potential new therapeutic target to block tumour development and metastasis.

Clinical implications of epithelial cell plasticity in cancer progression.

In the last few years, the role of epithelial cell plasticity in cancer biology research has gained increasing attention. This concept refers to the ability of the epithelial cells to dynamically switch between different phenotypic cellular states. This programme is particularly relevant during the epithelial-to-mesenchymal transition (EMT) in cancer progression. During colonization, epithelial cells first activate the EMT programme to disseminate from a primary tumour to reach a distant tissue site. During this process, cells are transported into the circulation and are able to escape the immune system of the host. Then, a reverse process called mesenchymal-to-epithelial transition (MET) occurs on cells that settle in the distant organs. Although epithelial cell plasticity has an important impact on tumour biology, the clinical relevance of this concept remains to be recapitulated. In this review, we will update the current state of epithelial cell plasticity in cancer progression and its clinical implications for the design of therapeutic strategies, the acquisition of multidrug resistance, and future perspectives for the management of cancer patients.