Regulated expression of human filaggrin in keratinocytes results in cytoskeletal disruption, loss of cell-cell adhesion, and cell cycle arrest

Filaggrin is an intermediate filament (IF)-associated protein that aggregat es keratin IFs in vitro and is thought to perform a similar function during the terminal differentiation of epidermal keratinocytes. To further explor e the role of filaggrin in the cytoskeletal rearrangement that accompanies epidermal differentiation, we generated keratinocyte cell lines that expres s human filaggrin using a tetracycline-inducible promoter system. Filaggrin expression resulted in reduced keratinocyte proliferation and caused an al teration in cell cycle distribution consistent with a post-GI phase arrest. Keratin filament distribution was disrupted in filaggrin-expressing lines, while the organization of actin microfilaments and, microtubules was more mildly affected. Evidence for direct interaction of filaggrin and keratin I Fs was seen by overlay assays of GFP-filaggrin with keratin proteins in vit ro and by filamentous filaggrin distribution in cells with low levels of ex pression. Cells expressing moderate to high levels of filaggrin showed a ro unded cell morphology, loss of cell-cell adhesion, and compacted cytoplasm. There was also partial or complete loss of the desmosomal proteins desmopl akin, plakoglobin, and desmogleins from cell-cell borders, while the distri bution of the adherens junction protein E-cadherin was not affected. No alt erations in keratin cytoskeleton, desmosomal protein distribution, or cell shape were observed in control cell lines expressing beta -galactosidase. F ilaggrin altered the cell shape and disrupted the actin filament distributi on in IF-deficient SW13 cells, demonstrating that filaggrin can affect cell morphology independent of the presence of a cytoplasmic IF network. These studies demonstrate that filaggrin, in addition to its known effects on IF organization, can affect the distribution of,other cytoskeletal elements in cluding actin microfilaments, which can occur in the absence of a cytoplasm ic IF network. Further, filaggrin can disrupt the distribution of desmosome proteins, suggesting an additional role(s) for this protein in the cytoske letal and desmosomal reorganization that occurs at the granular to cornifie d cell transition during terminal differentiation of epidermal keratinocyte s. (C) 2001 Academic Press.