Amplification of the C-MYC Oncogene in Non-small Cell Lung Cancer

Oncogene-induced senescence mediated by c-Myc requires USP10 dependent deubiquitination and stabilization of p14ARF

Oncogene-induced senescence (OIS) is a critical tumor-suppressor mechanism, which prevents hyper-proliferation and transformation of cells. c-Myc promotes OIS through the transcriptional activation of p14ARF followed by p53 activation. Although the oncogene-mediated transcriptional regulation of p14ARF has been well addressed, the post-translational modification of p14ARF regulated by oncogenic stress has yet to be investigated. Here, we found that c-Myc increased p14ARF protein stability by inducing the transcription of ubiquitin-specific protease 10 (USP10). USP10, in turn, mediated the deubiquitination of p14ARF, preventing its proteasome-dependent degradation. USP10-null mouse embryonic fibroblasts and human primary cells depleted of USP10 bypassed c-Myc-induced senescence via the destabilization of p14ARF, and these cells displayed accelerated hyper-proliferation and transformation. Clinically the c-Myc-USP10-p14ARF axis was disrupted in non-small cell lung cancer patients, resulting in significantly worse overall survival. Our studies indicate that USP10 induced by c-Myc has a crucial role in OIS by maintaining the stability of key tumor suppressor p14ARF.

Decoding c-Myc networks of cell cycle and apoptosis regulated genes in a transgenic mouse model of papillary lung adenocarcinomas

The c-Myc gene codes for a basic-helix-loop-helix-leucine zipper transcription factor protein and is reported to be frequently over-expressed in human cancers. Given that c-Myc plays an essential role in neoplastic transformation we wished to define its activity in lung cancer and therefore studied its targeted expression to respiratory epithelium in a transgenic mouse disease model. Using histological well-defined tumors, transcriptome analysis identified novel c-Myc responsive cell cycle and apoptosis genes that were validated as direct c-Myc targets using EMSA, Western blotting, gene reporter and ChIP assays.Through computational analyses c-Myc cooperating transcription factors emerged for repressed and up-regulated genes in cancer samples, namely Klf7, Gata3, Sox18, p53 and Elf5 and Cebpα, respectively. Conversely, at promoters of genes regulated in transgenic but non-carcinomatous lung tissue enriched binding sites for c-Myc, Hbp1, Hif1 were observed. Bioinformatic analysis of tumor transcriptomic data revealed regulatory gene networks and highlighted mortalin and moesin as master regulators while gene reporter and ChIP assays in the H1299 lung cancer cell line as well as cross-examination of published ChIP-sequence data of 7 human and 2 mouse cell lines provided strong evidence for the identified genes to be c-Myc targets. The clinical significance of findings was established by evaluating expression of orthologous proteins in human lung cancer. Taken collectively, a molecular circuit for c-Myc-dependent cellular transformation was identified and the network analysis broadened the perspective for molecularly targeted therapies.

Synergistic induction of the Fas (CD95) ligand promoter by Max and NFkappaB in human non-small lung cancer cells

Fas (CD95/APO-1) ligand is a member of the Tumor Necrosis Factor family and a potent inducer of apoptosis. Fas ligand is expressed in activated T cells and represents a major cytotoxic effector mechanism by which T cells kill their target cells. Activation-induced Fas ligand expression in T cells is under the stringent control of various transcription factors, including nuclear factor kappaB (NFkappaB) and c-Myc/Max. There is accumulating evidence that Fas ligand is also expressed by various non-hematopoietic tumor cells, however, little is known about Fas ligand regulation in tumor cells. In this study, we have analyzed the regulation of the Fas ligand gene promoter induction in two non-small cell lung cancer cell lines, with a major focus on the role of the c-Myc/Max transcription factor. Our results revealed that inhibition of c-Myc/Max did not substantially reduce basal levels of Fas ligand promoter activity, nor did overexpression of c-Myc significantly induce promoter activity. In contrast, we observed that overexpression of Max resulted in a marked increase in basal promoter activity and synergistically enhanced phorbolester- and doxorubicin-induced NFkappaB-mediated Fas ligand promoter activity. These results were confirmed by analyzing endogenous Fas ligand transcription. We conclude that high levels of Max and stress-induced NFkappaB activation may result in elevated expression of Fas ligand in human lung cancer cells and possibly contribute to Fas ligand-associated immune escape mechanisms.

Loss of expression of death-inducing signaling complex (DISC) components in lung cancer cell lines and the influence of MYC amplification

We have previously reported that the key apoptosis related gene caspase 8 (CASP8) is frequently silenced in small cell lung cancer (SCLC) tumors and cell lines usually, but not always, by aberrant promoter methylation. Because CASP8 is a key component of the death-inducing signaling complex (DISC) when specific death receptors (including DR4, DR5, FAS) are activated by their specific ligands (TRAIL/FASL), we examined expression of the components of the DISC complex in lung cancer cell lines. MYC family members are frequently amplified (MYC+ve) in SCLC, and MYC is a potent inducer of apoptosis. We examined 34 SCLC lines (12 of which were MYC+ve) and 22 NSCLC lines. CASP8 gene expression was frequently lost (79%) at message and protein levels in SCLC but not in non-SCLC (NSCLC). MYC amplification was present in 45% of SCLC cell lines, which had lost CASP8 expression, but not in any of the CASP8 positive lines. The frequency of CASP8 loss was significantly higher in MYC+ve SCLC compared to MYC-ve SCLC or in NSCLC. Analyses of other DISC components showed significantly higher rates of loss of expression of CASP10, DR5, FAS and FASL in SCLC compared to NSCLC. The loss of expression of proapoptotic DISC components was significantly higher in MYC+ve SCLC cell lines and these lines were completely resistant to TRAIL. Expression of CASP10 (a caspase closely related to CASP8) was frequently absent at the protein level in both SCLC and NSCLC lines. Expression of c-FLIP (proteolytically inactive homolog of CASP8) was inversely related to expression of CASP8. Our major conclusions are: (a) The death receptor pathway is differently inactivated at multiple levels in lung cancer cell lines; and (b) MYC amplification in SCLC is associated with inactivation of most components of the DISC complex, with resistance to TRAIL and with expression of c-FLIP. These findings may have considerable clinical and therapeutic implications.