Distribution of desmosomal proteins in F9 embryonal carcinoma cells and epithelial cell derivatives.

In diverse epithelia, cytoskeletal keratin intermediate filaments (IFs) associated with the cytoplasmic face of intercellular junctional desmosomes. The processes underlying desmosome formation and keratin IF interactions remain unclear. We have examined F9 embryonal carcinoma (EC) cell differentiation as a model for embryonic development of epithelial surface desmosomes. As determined by immunofluorescence microscopy and biochemical protein techniques, F9 EC cells, which lack surface desmosomes and keratin IFs, express the desmosomal proteins desmoplakins I and II (DP I/II), desmoglein I (DG I) and plakoglobin (PK). DP I/II are present at low level and are relatively soluble in buffer containing Triton X-100. Immunofluorescence localizes DP I/II to the juxtanuclear, centrosomal region. Species of DG I and PK are detected in both the Triton X-100-soluble and -insoluble protein fractions. DG I appears dispersed throughout the cell while PK resides at cell-cell boundaries. In epithelial cell cultures induced by retinoic acid (RA) treatment, each of the desmosomal proteins is organized into punctate desmosome-like structures with the appearance of simple epithelial K8/K18 IFs. The steady-state levels of DP I/II and PK increase with a partitioning of the majority of the desmosomal components into the insoluble fraction. In epithelial cells which lack distinct surface desmosomes, an intracellular association of keratin bundles with DP I/II is observed, suggesting that keratin filaments may facilitate the translocation of these desmosomal components to the cell surface. Parietal endoderm-like cells, derived by treatment with RA and dibutyryl cAMP, are analogous to F9 EC cells in that the cells express desmosomal components and do not display surface desmosomes. Moreover, K8 and K18 do not form distinct filaments, and the protein and RNA levels of K8 are low relative to epithelial cells induced by RA alone. The F9 system appears to be a relevant model for studies of desmosome assembly and the potential interactions of desmosomal proteins and keratin IFs in embryonic epithelial cell types.