Mesenchymal–epithelial transitions: Spontaneous and cumulative syntheses of epithelial marker molecules and their assemblies to novel cell junctions connecting human hematopoietic tumor cells to carcinomatoid tissue structures

Using biochemical as well as light‐ and electron‐microscopic immunolocalization methods, in cultures of unicellular human blood tumor cells, we have studied the phenomenon of spontaneous and cumulative syntheses of certain epithelial proteins and glycoproteins and their assemblies to two major kinds of novel cell–cell junctions, adhering junctions (AJs) and junctions based on the epithelial cell adhesion molecule (EpCAM). More than two decades, we have selected and characterized clonal sublines of multipotential hematopoietic K562 cells, which are enriched in newly formed AJs based on cis‐clusters of desmoglein Dsg2, in some sublines accompanied by desmocollin Dsc2. Both desmosomal cadherins can be anchored in a submembranous plaque containing plakoglobin and plakophilins Pkp2 and Pkp3, with or without other armadillo proteins and desmoplakin. Also, these cells are often connected by an additional, extended junction system, in which the transmembrane epithelial glycoprotein EpCAM is associated with a cytoplasmic plaque rich in several actin‐binding proteins such as afadin, α‐actinin, ezrin and vinculin. Both kinds of junctions contribute to connections of K562 cells into epithelioid monolayers or even three‐dimensional, tissue‐like structures, thus markedly changing the cell biological nature and behavior of the resulting tumor subforms (mesenchymal–epithelial transitions). We discuss molecular mechanisms involved in the formation and function of these junctions, also with respect to tumor spread and metastasis, as well as diagnostic and therapeutic consequences.