KRAS-driven lung cancers frequently inactivate TP53 and/or STK11/LKB1, defining tumor subclasses with emerging clinical relevance. Specifically, KRAS-LKB1 (KL) mutant lung cancers are particularly aggressive, lack PD-L1, and respond poorly to immune checkpoint blockade (ICB). The mechanistic basis for this impaired immunogenicity, despite the overall high mutational load of KRAS mutant lung cancers, remains obscure. Here we report that LKB1 loss results in marked silencing of STING expression and insensitivity to cytoplasmic double strand DNA (dsDNA) sensing. This effect is mediated at least in part by hyperactivation of DNMT1 and EZH2 activity related to elevated S-adenylmethionine (SAM) levels, and reinforced by DNMT1 upregulation. Ectopic expression of STING in KL cells engages IRF3 and STAT1 signaling downstream of TBK1 and impairs cellular fitness, due to the pathologic accumulation of cytoplasmic mitochondrial dsDNA associated with mitochondrial dysfunction. Thus, silencing of STING avoids these negative consequences of LKB1 inactivation, while facilitating immune escape.

Histone methylation is implicated in a number of biological and pathological processes, including cancer development. In this study, we investigated the molecular mechanism for the recruitment of Polycomb repressive complex-2 (PRC2) and its accessory component, JARID2, to chromatin, which regulates methylation of lysine 27 of histone H3 (H3K27), during epithelial-mesenchymal transition (EMT) of cancer cells. The expression of MEG3 long noncoding RNA (lncRNA), which could interact with JARID2, was clearly increased during transforming growth factor-β (TGF-β)-induced EMT of human lung cancer cell lines. Knockdown of MEG3 inhibited TGF-β-mediated changes in cell morphology and cell motility characteristic of EMT and counteracted TGF-β-dependent changes in the expression of EMT-related genes such as CDH1, ZEB family, and the microRNA-200 family. Overexpression of MEG3 influenced the expression of these genes and enhanced the effects of TGF-β in their expressions. Chromatin immunoprecipitation (ChIP) revealed that MEG3 regulated the recruitment of JARID2 and EZH2 and histone H3 methylation on the regulatory regions of CDH1 and microRNA-200 family genes for transcriptional repression. RNA immunoprecipitation and chromatin isolation by RNA purification assays indicated that MEG3 could associate with JARID2 and the regulatory regions of target genes to recruit the complex. This study demonstrated a crucial role of MEG3 lncRNA in the epigenetic regulation of the EMT process in lung cancer cells.

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Histone methylation plays a crucial role in various biological and pathological processes including cancer development. In this study, we discovered that JARID2, an interacting component of Polycomb repressive complex-2 (PRC2) that catalyzes methylation of lysine 27 of histone H3 (H3K27), was involved in Transforming Growth Factor-beta (TGF-ß)-induced epithelial-mesenchymal transition (EMT) of A549 lung cancer cell line and HT29 colon cancer cell line. The expression of JARID2 was increased during TGF-ß-induced EMT of these cell lines and knockdown of JARID2 inhibited TGF-ß-induced morphological conversion of the cells associated with EMT. JARID2 knockdown itself had no effect in the expression of EMT-related genes but antagonized TGF-ß-dependent expression changes of EMT-related genes such as CDH1, ZEB family and microRNA-200 family. Chromatin immunoprecipitation assays showed that JARID2 was implicated in TGF-ß-induced transcriptional repression of CDH1 and microRNA-200 family genes through the regulation of histone H3 methylation and EZH2 occupancies on their regulatory regions. Our study demonstrated a novel role of JARID2 protein, which may control PRC2 recruitment and histone methylation during TGF-ß-induced EMT of lung and colon cancer cell lines.