Inhibition of epidermal growth factor receptor kinase induces protease-dependent apoptosis in human colon cancer cells

Background & Aims: The epidermal growth factor receptor (EGFR) is under investigation as a therapeutic target for cancers. Colon cancer cell lines are variably dependent on autocrine stimulation of EGFR. We therefore examined the effects of a selective EGFR tyrosine kinase inhibitor, PD 153035, on proliferation and survival of five colon cancer cell lines whose autonomous proliferation is either EGFR ligand dependent or EGFR ligand independent. Methods: Effects of inhibitors were screened by MTS growth assays, [H-3]thymidine incorporation, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assay, fluorescence microscopy, immunoblotting, and in vitro protease assays. Results: PD 153035 caused dose-dependent cytostasis (200 nmol/L to 1 mu mol/L) and apoptosis (>10 mu mol/L) in ligand-dependent cell lines and caused variable apoptosis (>10 mu mol/L) but no cytostasis in ligand-independent cell lines. Apoptosis induced by 10 mu mol/L PD 153035 was not associated with induction of p53 protein expression but was accompanied by activation of caspases that cleave poly(ADP-ribose) polymerase, lamin B-1, and Bcl-2. Inhibition of caspase 3-like protease activity by DEVD-fluoromethylketone significantly delayed the onset of PD 153035-induced apoptosis. Conclusions: The EGFR tyrosine kinase inhibitor PD 153035 induces cytostasis and caspase-dependent apoptosis in EGFR ligand-dependent colon cancer cell lines. These observations encourage further investigation of EGFR tyrosine kinase inhibitors for treatment of colorectal neoplasms.