| Research Abstract |
The selective epidermal growth factor receptor-tyrosine kinase-(EGFR, TK)-inhibitor, gefitinib, blocks several signal transduction pathways involved in tumor cell proliferation, anti-apoptosis and angiogenesis. We demonstrate for the first time that human telomerase reverse transcriptase (hTERT) is a molecular target of gefitinib through repression of the classical mitogen-activated protein kinase (MAPK)-Ets pathway. We used four human esophageal cancer cell lines (TE-1, TE-3, TE-4, and TE-5) that strongly express EGFR to evaluate the effects of gefitinib treatment on the phosphorylation status of EGFR/mitogen-activated protein kinase (MAPK), hTERT mRNA/protein expression, transcriptional activation of the hTERT gene promoter, and telomerase activity. Phosphorylation of EGFR tyrosine residues PY1086 and PY1173, and of ERK 1/2, as well as hTERT mRNA/protein expression were markedly repressed by gefitinib treatment. A reporter assay demonstrated that exposure to gefitinib markedly down-r
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egulated the transcriptional activity (0.28-to 0.45-fold decrease) of the Ets site of the hTERT core promoter. Telomerase activity was down-regulated by gefitinib treatment in a dose-dependent manner. The concentration necessary for 50% inhibition of telomerase activity (0.12-0.71 μM) was lower than that necessary for inhibition of growth (6.35-18.54 μM). Long-term and relatively low-dose exposure to gefitinib significantly (P< 0.01) increased the proportion of cells positive for β-galactosidase in 3 cell lines (TE-1, TE-4, and TE-5), but not in TE-3. The mean TRF lengths of the 3 susceptible cell lines, but not of TE-3, were reduced in comparison with those of no-treatment controls. These results suggest that gefitinib targets telomerase activity as an anti-cancer effect through inhibition of direct EGF-induced transcriptional activation of the hTERT gene. The ability of gefitinib to inhibit telomerase at low doses may provide an approach to avoid severe adverse effects such as pulmonary fibrosis. Less
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