The human pharyngeal epithelial cell line FaDu (ATCC HTB-43), the human colon epithelial cell line Caco-2 (ATCC HTB-37) and the human cervical epithelial cell line ME180 (ATCC HTB-33) were cultured in DMEM + GlutaMAX (Invitrogen) supplemented with 10% heat-inactivated fetal bovine serum (Sigma Aldrich). The human gastric epithelial cell line AGS (ATCC CRL-1739) was cultured in RPMI-1640 + GlutaMAX (Invitrogen) supplemented with 10% heat-inactivated fetal bovine serum. All cell lines were maintained at 37°C and 5% CO2. The cells were seeded into 24-well tissue culture plates the day before the experiment to form a monolayer overnight. Before each experiment, the cells were washed twice with cell culture medium without serum.