Isocitrate dehydrogenase 1 (IDH1) is mutated in various types of human cancer to IDH1R132H, a structural alteration that leads to catalysis of α-ketoglutarate to the oncometabolite D-2-hydroxyglutarate. In this study, we present evidence that small molecule inhibitors of IDH1R132H that are being developed for cancer therapy may pose risks with co-administration of radiotherapy. Cancer cells heterozygous for the IDH1R132H mutation exhibited less IDH-mediated production of NADPH, such that after exposure to ionizing radiation (IR) there were higher levels of reactive oxygen species (ROS), DNA double-strand breaks and cell death compared to IDH1 wild-type cells. These effects were reversed by the IDH1R132H inhibitor AGI-5198. Exposure of IDH1 wild-type cells to D-2-hydroxyglutarate was sufficient to reduce IDH-mediated NADPH production and increase IR sensitivity. Mechanistic investigations revealed that the radiosensitivity of heterozygous cells was independent of the well-described DNA hypermethylation phenotype in IDH1-mutated cancers. Thus, our results argue that altered oxidative stress responses are a plausible mechanism to understand the radiosensitivity of IDH1-mutated cancer cells. Further, they offer an explanation for the relatively longer survival of patients with IDH1-mutated tumors, and they imply that administration of IDH1R132H inhibitors in these patients may limit irradiation efficacy in this setting.

The serine/threonine kinase LKB1 is a well-characterized tumor suppressor that governs diverse cellular processes including growth, polarity and metabolism. Somatic inactivating mutations in LKB1 are observed in about 15-30% of non-small cell lung cancers (NSCLC). LKB1 inactivation confers lung adenocarcinomas (ADC) with malignant features that remain refractory to therapeutic intervention. YAP activation has been linked to LKB1 deficiency, but the role of YAP in lung ADC formation and progression is uncertain. In this study, we showed that ectopic expression of YAP in Type II alveolar epithelial cells led to hyperplasia in mouse lungs. YAP overexpression in the KrasG12D lung cancer mouse model accelerated lung ADC progression. Conversely, YAP deletion dramatically delayed the progression of lung ADC in LKB1-deficient KrasG12D mice. Mechanistic studies identified the anti-apoptotic oncoprotein Survivin as the downstream mediator of YAP responsible for promoting malignant progression of LKB1-deficient lung ADC. Collectively, our findings identify YAP as an important contributor to lung cancer progression, rationalizing YAP inhibition in the context of LKB1-deficiency as a therapeutic strategy to treat lung ADC.

The Fas receptor ligand FasL regulates immune cell levels by inducing apoptosis of Fas receptor-positive cells. Here we studied the impact of host FasL on tumor development in mice. Genetically targeting FasL in naive mice increased myeloid cell populations, but in marked contrast it reduced the levels of myeloid-derived suppressor cells (MDSC) in mice bearing LLC lung tumors. Analysis of the MDSC subset distribution revealed that FasL deficiency skewed cell populations toward the M-MDSC subset, which displays a highly immunosuppressive activity. Furthermore, tumor-bearing mice that were FasL-deficient displayed an enhanced proportion of tumor associated macrophages and T regulatory cells. Overall, the immunosuppressive environment produced by FasL targeting correlated with reduced survival of tumor-bearing mice. These results disclose a new role of FasL in modulating immunosuppressive cells.

Platelet-activating factor receptor (PAFR), a G-protein–coupled receptor, has been implicated in tumorigenesis, but its contributions to metastatic progression have not been investigated. Here, we show that PAFR is overexpressed in non–small cell lung cancer (NSCLC) as well as in breast, colorectal, and gastric carcinomas. Expression of PAFR correlates closely with clinical stages, survival time, and distant metastasis. In human NSCLC cells, activation of the PAF/PAFR signaling axis accentuated malignant character, including by stimulating epithelial–mesenchymal transition (EMT). In contrast, silencing PAFR in aggressive NSCLC cells inhibited these effects. Mechanistic investigations showed that PAFR stimulated EMT by activating STAT3 via upregulation of G-protein–dependent SRC or JAK2 kinase activity. Notably, STAT3 transcriptionally elevated PAFR expression. Thus, activation of PAFR in NSCLC cells initiated a forward feedback loop responsible for mediating the aggressive malignant character of NSCLC cells in vitro and in vivo. Reinforcing this reciprocal activation loop, PAF/PAFR signaling also upregulated IL6 expression and thereby STAT3 activation. Overall, our results elucidated an important role for PAFR dysregulation in the pathogenicity of NSCLC and unraveled a forward feedback loop between PAFR and STAT3 that acts to drive the malignant progression of NSCLC. Cancer Res; 75(19); 1–13. ©2015 AACR.

Restoration of wild-type p53 tumor suppressor function has emerged as an attractive anticancer strategy. Therapeutics targeting the two p53 negative regulators, MDM2 and MDM4, have been developed, but most agents selectively target the ability of only one of these molecules to interact with p53, leaving the other free to operate. Therefore, we developed a method that targets the activity of MDM2 and MDM4 simultaneously based on recent studies indicating that formation of MDM2/MDM4 heterodimer complexes are required for efficient inactivation of p53 function. Using computational and mutagenesis analyses of the heterodimer binding interface, we identified a peptide that mimics the MDM4 C-terminus, competes with endogenous MDM4 for MDM2 binding, and activates p53 function. This peptide induces p53-dependent apoptosis in vitro and reduces tumor growth in vivo. Interestingly, interfering with the MDM2/MDM4 heterodimer specifically activates a p53-dependent oxidative stress response. Consistently, distinct subcellular pools of MDM2/MDM4 complexes were differentially sensitive to the peptide; nuclear MDM2/MDM4 complexes were particularly highly susceptible to the peptide-displacement activity. Taken together, these data identify the MDM2/MDM4 interaction interface as a valuable molecular target for therapeutic reactivation of p53 oncosuppressive function.