Structural and biological consequences of increased vimentin expression in simple epithelial cell types

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Abstract
Cytoskeletal intermediate filaments (IFs) constitute a diverse family of proteins whose members are expressed in tissue-specific patterns. Although vimentin IFs are normally restricted to mesenchyme, a variety of cell types express vimentin alone or together with cell-specific IFs during growth, differentiation, and neoplasia. In this study, we have investigated the influence of increased vimentin expression on the simple epithelial cell phenotype. An expression vector encoding a human vimentin cDNA was transfected into the murine HR9 endoderm and F9 embryonal carcinoma cell lines, which serve as models for early extraembryonic epithelial differentiation. Stable clones that expressed varying levels of the human vimentin were characterized by immunofluorescence and biochemical analysis. A relatively high level of vimentin expression in HR9 and differentiated F9 epithelial cells resulted in aberrant vimentin structures with a co-collapss of keratin K8/K18 filaments and lowered amounts of keratin protein. In F9 epithelial cells, the desmosomal proteins DP I/II did not appear to localize to cell surface desmosomes but rather co-aggregated with the perturbed IFs. Although overall cell morphology was not dramatically altered, individual nuclei were distorted by excess intracellular vimentin. Furthermore, cell proliferation as well as the cell spreading response time were slowed. There appears to be a threshold effect regarding overall vimentin levels as cells that expressed lower amounts of the human vimentin exhibited no obvious structural nor biological effects. Our results demonstrate that wild-type vimentin can act as a “mutant” protein when present at high intracellular levels, inducing a variety of phenotypic changes.