Multiple post-translational modifications regulate E-cadherin transport during apoptosis
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abstract
E-cadherin is synthesized as a precursor and then undergoes cleavage by proprotein convertases. This processing is essential for E-cadherin maturation and cell adhesion. Loss of cell adhesion causes detachment-induced apoptosis- anoikis. Anoikis can be inhibited despite loss of cell-matrix interactions by preserving E-cadherin mediated cell-cell adhesion. Conversely, acute loss of E-cadherin sensitizes cells to apoptosis by unknown post-translational mechanisms. In response to drug treatment of breast cancer cells, our analysis revealed that two independent modifications of E-cadherin inhibit its cell surface transport. Firstly, O-linked beta-N-acetylglucosamine (O-GlcNAc) modification of the cytoplasmic domain retains E-cadherin in the endoplasmic reticulum. Secondly, incomplete processing by proprotein convertases arrests E-cadherin transport late in the secretory pathway. We demonstrated these E-cadherin modifications (detected by specific lectins and antibodies) do not affect binding to α-catenin, β-catenin or γ-catenin. However, E-cadherin binding to Type I gamma phosphatidylinositol phosphate kinase (PIPKIγ), a protein required for recruitment of E-cadherin to adhesion sites, was blocked by O-GlcNAc glycosylation (O-GlcNAcylation). Consequently, E-cadherin trafficking to the plasma membrane was inhibited. However, deletion mutants that cannot be O-GlcNAcylated continued to bind PIPKIγ, traffic to the cell surface and delayed apoptosis, confirming the biological significance of the modifications and PIPKIγ binding. Thus, O-GlyNAcylation of E-cadherin accelerated apoptosis. Furthermore, cell stress induced inactivation of proprotein convertases, inhibited E-cadherin maturation further exacerbating apoptosis. The modifications of E-cadherin by O-GlcNAcylation and lack of pro-region processing represent novel mechanisms for rapid regulation of cell surface transport of E-cadherin in response to intoxication.
