Cell cycle effects of the EGFR TK inhibitor erlotinib in NSCLC cell lines

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p27KIP, a cyclin-dependent kinase (cdk) inhibitor and a putative tumor suppressor, plays a critical role in the regulation of cell proliferation and cell cycle progression. Erlotinib, an orally bioavailble EGFR tyrosine kinase inhibitor, has been shown a potential activity against cancer cell growth in vitro and in vivo. In previous work, we have reported that erlotinib exerts the inhibitory effects on EGFR high expressed human non-small cell lung cancer cell H322 through the disruption of G1 -related cell cycle regulators and blocking cell cycle progression from G1 to S phase transition. The present study shows that erlotinib arrests H322 cells at G1 phase by accumulating the p27KIP protein, and initiating a chain of events by decreasing the amounts of cyclin A and cyclin E and the suppression of cyclin A- or cyclin E-associated cdk-2 kianse activity. Erlotinib induced G1 arrest and p27KIP up-regulation in the tested sensitive cell lines (H322, H358, and A431), but to lesser degree in the resistant cell lines (A549, H596, and H1299). The G1 arrest and p27KIP up-regulation by erlotinib was sustained within 8 h of removal of drug exposure, and completely relieved at 24 h. The immunocytochemical and immunoblot studies revealed that erlotinib treatment resulted in p27KIP localization in nucleus, whereas; the p27KIP was predominately detected at cytoplasm in control cells. In addition, erlotinib treatment led to a dramatic inhibition of p27KIP phosphorylation at Thr 187, and down-regulation of Skp2 expression. As a result, the half-life of p27KIP was significantly increased by erlotinib treatment. Furthermore, the reduction of p27KIP expression using p27KIP small interfering RNA abrogated erlotinib-induced G1 phase arrest and cell growth inhibition. Taken together, the results of this study demonstrated that p27KIP as a key mediator of cell cycle plays an implant role in erlotinib-induced cell growth inhibition at G1 phase arrest. This work was supported in part by NIH grant CA50270 and OSI Pharmaceuticals.