PTEN inactivation in lung cancer cells and the effect of its recovery on treatment with epidermal growth factor receptor tyrosine kinase inhibitors

To understand the mechanisms of PTEN inactivation, which is reported to be involved in tumor progression and drug resistance in lung cancer, we analyzed the expression levels of PTEN at mRNA and protein levels, along with the genetic and epigenetic status of the PTEN gene, in a panel of lung cancer cell lines. Western blot analysis showed that six out of 25 (24%) cell lines displayed low expression of PTEN protein. The level of PTEN mRNA correlated well with corresponding protein expression in each of these six cell lines. In two of the six cell lines genomic analysis revealed homozygous deletions of the PTEN gene. Another two of the six cell lines displayed hypermethylation of the PTEN gene promoter assessed by methylation-specific PCR. The levels of PTEN mRNA and protein expression in PC9/f9 and PC9/f14 cells, which are gefitinib-resistant derivatives of the gefitinib-sensitive cell line, PC9, were reduced compared to the parental line. After treatment with the demethylating agent 5-aza-2'deoxycytidine (5-AZA) and the histone deacetyltransferase (HDAC) inhibitor Trichostatin A (TSA), the expression levels of PTEN mRNA and protein in these four cell lines (PC9/f9, PC9/f14, PC10 and PC14) were actually restored. In summary, reduction in PTEN protein expression was regulated by histone deacetylation and hypermethylation of the gene promoter, as well as homozygous deletion. In addition, we demonstrated that the combination treatment of gefitinib and TSA induced significant growth inhibition in gefitinib-resistant PC9/f9 and PC9/f14 cells. These findings suggest that the combination of the epidermal growth factor receptor tyrosine kinase inhibitor gefitinib with the demethylating agent 5-AZA and the HDAC inhibitor TSA may be a useful strategy for the treatment of some lung cancers.