Inducible expression of filaggrin increases keratinocyte susceptibility toapoptotic cell death

Filaggrin is an intermediate filament associated protein that aids the pack ing of keratin filaments during terminal differentiation of keratinocytes. Premature aggregation of keratin filaments is prevented by filaggrin expres sion as the inactive precursor, profilaggrin, which is localized in keratoh yalin granules in vivo. We have previously shown that filaggrin constructs, when transiently transfected into epithelial cells, lead to a collapsed ke ratin cytoskeletal network and dysmorphic nuclei with features of apoptosis , The apparent transfection rate is low with filaggrin constructs, supporti ng their disruptive role but hindering further study. To bypass this proble m, we generated stable keratinocyte cell lines that express mature human fi laggrin using a tetracycline-inducible promoter system. We found that cell lines expressing filaggrin, but not control cell lines, exhibited increased sensitivity to multiple apoptotic stimuli as measured by morphologic and b iochemical criteria. None of the cell lines showed an increase in endogenou s expression of filaggrin in response to the same stimuli. Filaggrin expres sion alone was insufficient to induce apoptosis in these keratinocyte cell lines. We conclude that filaggrin, due to its keratin binding ability, prim es cells for apoptosis, Because filaggrin is expressed at a level of the ep idermis where keratinocytes are in transition between the nucleated granula r and the anucleate cornified layers, we hypothesize that filaggrin aids in the terminal differentiation process by facilitating apoptotic machinery,