CpG hypermethylation contributes to decreased expression of PTEN during acquired resistance to gefitinib in human lung cancer cell lines

• PTEN overexpression restored the sensitivity to EGFR-TKI in resistant cells.
• EGR-1 did not have a critical role in decreased expression of PTEN in gefitinib-resistant cells.
• Demethylation of the PTEN promoter by 5AZA-CdR upregulated PTEN expression.

ObjectivesWe have previously reported that decreased expression of PTEN in lung cancer PC9 cells harboring an EGFR-activating mutation (del E746–A750) results in acquisition of resistance to EGFR-TKIs, gefitinib and erlotinib, accompanied by enhanced phosphorylation of Akt and decreased nuclear translocation of a transcription factor EGR-1 [8]. In the present study, PTEN promoter methylation accounted for the decreased expression of PTEN in our gefitinib-resistant mutant.Material and methodsDNA methylation status of the PTEN promoter in PC9 and gefitinib-resistant cells were examined using methylation-specific PCR. The effect of DNA methylation on PTEN expression was evaluated by treatment of lung cancer cell lines with 5-aza-2′-deoxycytidine (5AZA-CdR).ResultsWe observed the characteristics of two gefitinib-resistant sublines, GEF1-1 and GEF2-1, derived from PC9 as follows. (1) PTEN overexpression suppressed AKT phosphorylation and restored the sensitivity to gefitinib and erlotinib in GEF1-1 cells. (2) EGR-1 siRNA mediated knockdown suppressed the expression of cyclin D1 and ICAM-1 genes but not of PTEN gene in PC9 cells. (3) Transfection of EGR-1 cDNA into a drug-resistant subline induced the expression of cyclin D1 and ICAM-1 but not of PTEN. (4) Treatment with 5AZA-CdR induced the expression of PTEN in resistant sublines but not in the parental line PC9. (5) A CpG site near the translational start point of the 5′-regulatory region was methylated in GEF1-1 and GEF2-1 but not in PC9.ConclusionOur results strongly suggest that CpG hypermethylation of the PTEN gene contributes to the decreased expression of PTEN during acquired resistance to gefitinib or erlotinib.