Abstract 3365: Epigenetic inactivation of PTPRT and sensitivity to STAT3 inhibition in lung cancer

Abstract

Abstract Signal Transducers and Activators of Transcription-3 (STAT3), a potent oncogenic transcription factor is persistently activated in several malignancies including lung cancer. Protein Tyrosine Phosphatase Receptor-T (PTPRT) is an endogenous inhibitor of STAT3 transcriptional activity, and is methylated in many types of human cancer. Loss-of-function of PTPRT by promoter methylation contributes to STAT3 mediated growth and survival and represent one of the plausible mechanism of STAT3 hyperactivation. We report identification of hypermethylation in CpG islands of the PTPRT promoter which correlates with its transcriptional silencing as well as expression of pSTAT3 protein and STAT3 target genes in both lung cancer cell lines and primary human tumors. Analyses of the TCGA (The Cancer Genome Atlas) data revealed that PTPRT promoter is frequently hypermethylated in a subset of lung squamous cell carcinoma and approximately 25% of adenocarcinoma tumors in association with downregulation of PTPRT mRNA expression. This observation was verified in vitro using lung cancer cell lines. Out of the 7 lung cancer cell lines examined, one lung cancer cell line showed lack of methylation (only unmethylated sequences) of the PTPRT promoter region and revealed mRNA expression (H520). All 6 cell lines which showed PTPRT promoter methylation had nearly absent PTPRT mRNA and had high levels of pSTAT3Tyr705, while H520 had little or no pSTAT3Tyr705 protein expression. Silencing of PTPRT using siRNA in H520, where PTPRT was not endogenously silenced by promoter hypermethylation, resulted in the upregulation of the STAT3 target genes such as cyclin D1 and Bcl-XL mRNA and protein expression as well as increased pSTAT3Tyr705 level. Analyses of the methylation status in the PTPRT promoter region in primary lung tumor samples revealed methylation in one of the 3 tumor samples. This correlated with increased STAT3 target genes (cyclin D1 and Bcl-XL) mRNA expression. In lung cancer cell lines, PTPRT promoter methylation is associated with sensitivity to STAT3 inhibition. Our data suggests that silencing of PTPRT by promoter hypermethylation is a frequent mechanism of STAT3 hyperactivation, and targeting STAT3 may be an effective molecular targeted approach in the treatment of this subset of lung cancer. Citation Format: Malabika Sen, Audrey Kindsfather, James G. Herman. Epigenetic inactivation of PTPRT and sensitivity to STAT3 inhibition in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3365. doi:10.1158/1538-7445.AM2017-3365