Cytokeratin 8/18 protects breast cancer cell lines from TRAIL-induced apoptosis

TNF-related apoptosis inducing ligand (TRAIL) induces apoptosis by engaging its death receptors (DRs) 4 and/or 5 on targeted cells. Clinical attempts to stimulate this apoptotic pathway for cancer therapy, including the use of recombinant human TRAIL (rhTRAIL) or receptor agonistic antibodies, have been underway for over a decade. Unfortunately, these agents have only shown limited therapeutic effects due largely to tumor resistance arising from mechanisms yet to be defined. Here we show that intermediate filament proteins, keratin 8 and keratin 18 (K8/K18), negatively regulate TRAIL induced apoptosis. K8/K18 protein levels are consistently higher in TRAIL-resistant cells compared to TRAIL-sensitive cells in a panel of breast cancer cell lines. Blockade of K8 increased expression of DR5 on the surface of targeted cells and sensitized the cells to TRAIL-induced apoptosis. Conversely, ectopic expression of K8/K18 downregulated DR5 protein expression. K8/K18 appears to negatively regulate apoptosis signaling via DR5 in breast cancer cells. Our findings warrant additional studies to determine if K8/K18 could be a predictor of tumor resistance to DR5-targeted therapies.