Keratins Modulate c-Flip/Extracellular Signal-Regulated Kinase 1 and 2 Antiapoptotic Signaling in Simple Epithelial Cells

Abstract

Among the large family of intermediate filament proteins, the keratin 8 and 18 (K8/K18) pair constitutes a hallmark for all simple epithelial cells, such as hepatocytes and mammary cells. Functional studies with different cell models have suggested that K8/K18 are involved in simple epithelial cell resistance to several forms of stress that may lead to cell death. We have reported recently that K8/K18-deprived hepatocytes from K8-null mice are more sensitive to Fas-mediated apoptosis. Here we show that upon Fas, tumor necrosis factor alpha receptor, or tumor necrosis factor alpha-related apoptosis-inducing ligand receptor stimulation, an inhibition of extracellular signal-regulated kinase 1 and 2 (ERK1/2) activation sensitizes wild-type but not K8-null mouse hepatocytes to apoptosis and that a much weaker ERK1/2 activation occurs in K8-null hepatocytes. In turn, this impaired ERK1/2 activation in K8-null hepatocytes is associated with a drastic reduction in c-Flip protein, an event that also holds in a K8-null mouse mammary cell line. c-Flip, along with Raf-1, is part of a K8/K18-immunoisolated complex from wild-type hepatocytes, and Fas stimulation leads to further c-Flip and Raf-1 recruitment in the complex. This points to a new regulatory role of simple epithelium keratins in the c-Flip/ERK1/2 antiapoptotic signaling pathway.

We thank H. Baribault for the gift of the MGT cell line; R. Kemler for the TROMA-1, -2, and -3 hybridomas; J. L. Lessard for the anti-actin monoclonal antibody; and J. Tschopp, U. R. Rapp, and J. Charron for the plasmids encoding HA-tagged c-Flip, Raf-1, and MEK1 (dominant negative), respectively. We are grateful of S. Champetier for producing the K8-containing retrovirus and D. Savoie for helping with the MGT cell line. We also thank J. Huot and A. Anderson for helpful discussions and critical reading of the manuscript.

This study was supported by a grant from Canadian Institutes of Health Research and a grant from The Cancer Research Society.