Effects And Mechanisms Of E-cadherin Mediated Cell-cell Adhesion On The Regulation Of PTEN In Human Mammary Carcinoma Cells

E-cadherin is one of the most important adhesion molecules in cell-cell adhesion in epithelial cells. Besides its role in cell-cell adhesion, it also mediates the communications between cells. This highly conserved gene can play critical roles in malignant cell transformation and espically in tumor development and progression. The loss of E-cadherin protein correlates with increased invasiveness, metastasis, and cell proliferation, resulting in it being referred to as the "tumor suppressor" gene. The recognization and binding of E-cadherin dimmers transduce signals into the cytoplasmic signaling pathways such as PI3K/Akt pathway. PTEN, the negative regulator of PI3K/Akt pathway, acts in direct antagonism to growth factor stimulated PI3K. Although PTEN has multiple functions in the cell cycle arrest, cell proliferation and migration inhibition, suppression of invasion, and apoptosis induction, the regulation of PTEN is still under investigation. The aim of this investigation is to find if E-cadherin activate PI3K/Akt and regulate PTEN expression and activity in human mammary carcinoma cells and try to find the molecular mechanism.In this study, we built up an E-cadherin mediated cell-cell adhesion restoration model in the E-cadherin null human mammary carcinoma cell line MDA-MB-435. Further investigation showed that E-cadherin expressed cells growth was inhibited due to aβ-catenin transcriptional activity independent and cyclin D1 and p27 dependent pathway. The PI3K/Akt pathway was regulated by the E-cadherin mediated cell -cell adhesion because of the up-regulation of PTEN expression and the reduction of Akt activity.To make certain the reason of PTEN proteins up-regulation, the transcriptional levels of PTEN in these cells were compared in E-cadhein expressed Ecad6 cells with E-cadherin null Neo1 cells. We found that there were no significant changes, implying that E-cadherin expression has no effect on the PTEN transcription activity. Chx pulse-chase experiement showed that expression of E-cadherin prolonged PTEN half-life and increased PTEN stability. Further investigation demonstrated that the proteasome-dependent PTEN degradation pathway, but not lysosome-dependent degradation pathway, was delayed in E-cadherin expressed cells. The ubiquition of PTEN was decreased.PTEN was found colocalized and interacted with E-cadherin in the E-cadherin expressed MDA-MB-435 cells. E-cadherin mutant EC81, which lost the ability of binding withβ-catenin could also up-regulate PTEN. Decreasing the totalβ-catenin expression by RNAi partly impaired the PTEN protein levels. These results suggested thatβ-catenin were involved in the E-cadherin induced PTEN up-regulation, but not through the direct binding. We also found the regulation of PTEN by E-cadherin was cell-cell adhesion dependent and cell type specific.In conclusion, E-cadherin could up-regualte PTEN protein and stability in MDA-MB-435 cells. This up-regulation was cell-cell adhesion dependent and mediated by the delayed proteasome degradation pathway through the interation of E-cadherin with PTEN.