Protein zero, a nervous system adhesion molecule, triggers epithelial reversion in host carcinoma cells.

Abstract

Abstract. Protein zero (Po) is the immunoglobulin gene superfamily glycoprotein that mediates the selfadhesion of the Schwann cell plasma membrane that yields compact myelin. HeLa is a poorly differentiated carcinoma cell line that has lost characteristic morphological features of the cervical epithelium from which it originated. Normally, HeLa cells are not self-adherent. However, when Po is artificially expressed in this line, cells rapidly aggregate, and Po concentrates specifically at cell-cell contact sites. Rows of desmosomes are generated at these interfaces, the plasma membrane localization of cingulin and ZO-1, proteins that have been shown to be associated with tight junctions, is substantially increased, and cytokeratins coalesce into a cohesive intracellular network. Immunofluorescence patterns for the adherens junction proteins N-cadherin, et-catenin, and vinculin, and the desmosomal polypeptides desmoplakin, desmocollin, and desmoglein, are also markedly enhanced at the cell surface. Our data demonstrate that obligatory cell-cell adhesion, which in this case is initially brought about by the homophilic association of Po molecules across the intercellular cleft, triggers pronounced augmentation of the normally sluggish or sub-basal cell adhesion program in HeLa cells, culminating in suppression of the transformed state and reversion of the monolayer to an epithelioid phenotype. Furthermore, this response is apparently accompanied by an increase in mRNA and protein levels for desmoplakin and N-cadherin which are normally associated with epithelial junctions. Our conclusions are supported by analyses of ten proteins we examined immunochemically (Po, cingulin, ZO-1, desmoplakin, desmoglein, desmocollin, N-cadherin, a-catenin, vinculin, and cytokeratin-18), and by quantitative polymerase chain reactions to measure relative amounts of desmoplakin and N-cadherin mRNAs.Po has no known signaling properties; the dramatic phenotypic changes we observed are highly likely to have developed in direct response to Po-induced cell adhesion. More generally, the ability of this "foreign" membrane adhesion protein to stimulate desmosome and adherens junction formation by augmenting wellstudied cadherin-based adhesion mechanisms raises the possibility that perhaps any bona fide cell adhesion molecule, when functionally expressed, can engage common intracellular pathways and trigger reversion of a carcinoma to an epithelial-like phenotype.

The Minimal Essential Unit for Cadherin-mediated Intercellular Adhesion Comprises Extracellular Domains 1 and 2

“…(27,46). Our data support the idea that EC3-5 domains serve to make more accessible the minimal essential adhesion unit in the extracellular milieu by distancing it further out from the cell surface, where it can more readily encounter a cognate partner molecule protruding from an apposing cell surface.…”

“…It is not the case that NEC1 is adhesive, but the cell surfaces cannot approximate close enough contact to engage these short molecules across the extracellular space, because NEC1 is not adhesive either with itself or with wild type N-cadherin molecules that presumably protrude at a greater distance from a cell surface. Furthermore, small adhesion molecules, like Po, can induce competent adhesive contacts across a very small intercellular gap (4.6 nm) (27,46).…”

“…We chose Po to replace the native N-cadherin cytoplasmic domain, because Po is a transmembrane calcium-independent adhesion molecule with a single immunoglobulin-like extracellular domain and a cytoplasmic region with no catenin-binding sites (27). By substituting the Po cytoplasmic domain for the cadherin wild type domain, we effectively "neutralize" the contribution to cadherin cell adhesion mediated by an intracellular regulator domain.…”

L-MPZ, a Novel Isoform of Myelin P0, Is Produced by Stop Codon Readthrough

“…In the HeLa cell culture system, forcibly expressed P0 protein is localized to the plasma membrane, especially at sites of close apposition between two P0-expressing cells (26). To investigate the cellular localization of L-MPZ and P0, we performed double immunostaining of full-length P0 cDNA-transfected HeLa cells using anti-L-MPZ and anti-P0 antibodies.…”

Truncated DCC Reduces N-Cadherin/Catenin Expression and Calcium-Dependent Cell Adhesion in Neuroblastoma Cells

“…Regulatory effects of NCAM on other cell-cell interactions have been suggested by demonstrations that abrogation of NCAM-mediated adhesion can block or delay the formation of neuromuscular and gap junctions (Grumet et al, 1982;Keane et al, 1988;Rutishauser et al, 1983), and Michalides et al (1994) have shown that NCAM can augment adherens junction formation in mesenchymal cells by redistributing cadherins to sites of cell-cell contact. Protein zero (Doyle et al, 1995) and Ep-CAM (Litvinov et al, 1997), immunoglobulin superfamily proteins that mediate calcium-independent cell adhesion, have also been shown to modulate cadherin function. The demonstration that overexpression of truncated DCC can modulate N-cadherin/catenin expression and possibly adhesion function in neuroblastoma cells suggests that an important role of the DCC protein may be to mediate such adhesion molecule "cross talk."…”