Abstract PR02: Myc-Ras cooperation can overwhelm tumor suppressive mechanisms within lung adenocarcinomas
Authors: Wilson Catherine H. , Burkhart Deborah L. , Li Jinyang , Littlewood Trevor D. , Evan Gerard I. 
Published: May 2015

Abstract
The ability of Myc and Ras to cooperate during tumorigenesis was first observed over thirty years ago. Numerous mechanisms have been implicated in their collaboration. For instance, Myc can act to circumvent Ras-induced growth arrest. Alternatively, Ras has been shown to prevent Myc-induced apoptosis by activating the anti-apoptotic kinase Akt. Early experiments in primary rodent cells demonstrated that neither Ras nor Myc alone was sufficient for transformation, whereas when expressed together, transformation occurred. However, these transformation events were rare, suggesting additional co-operating mutations were also required. These and other investigations ultimately led to the concept of ‘intrinsic tumor suppression’, whereby the effects of activation of an individual oncogene is limited by their dual potential to invoke cell cycle arrest or apoptosis, which limit their other oncogenic functions. The dogma suggests that cancers only arise in oncogene expressing cells when anti-apoptotic/cell arrest mutations are acquired and the disruption of these pathways leads to net proliferation.
There is increasing evidence that the activation of intrinsic tumor suppressive mechanisms depend on increased oncogenic flux resulting from changes in the level of oncogene expression. For example, high expression of oncogenic Ras results in premature growth arrest in MEFs, whereas an endogenous level does not. To address this phenomenon with regards to Myc, we previously generated a novel knock-in mouse (R26c-MycER) in which the c-MycERTAM protein was inserted into the relatively ubiquitous, albeit low expressing Rosa26 locus. In contrast to high levels of Myc expressed in classical transgenic models, low-level c-Myc expression did not trigger intrinsic tumor suppression mechanisms in most tissues, suggesting that low levels of oncogenic Myc may be a more efficient initiator of oncogenesis than over expressed Myc, as it might not require cooperating loss of tumor suppressive pathways.
We have now created a R26CAG-c-MycER allele that expresses a 5-10 fold higher constitutive expression of MycERTAM than the R26c-MycER allele. Together with R26c-MycER mice we have been able to generate an allelic series of mice that allow the control, in a spatial and temporal fashion, the activity of de-regulated c-Myc at low, medium and high levels. When we sporadically activated c-MycERTAM within the lung epithelial cells, only very few high-level expressing c-MycERTAM and none of the low-level mice developed tumors within 18 weeks suggesting Myc alone is insufficient for tumorigenesis irrespective of oncogenic level. However, in the presence of an endogenous level of oncogenic K-RasG12D, lung adenomas are evident in mice expressing both low and high levels of c-Myc. Mice expressing the highest levels of c-Myc developed adenocarcinomas with extremely short latency and have a greatly reduced survival when compared to mice with lower levels of c-Myc.
As expected, low level c-Myc did not activate intrinsic tumor suppression via the Arf/p53 pathways within K-Ras driven lesions and these tumors grew faster than lesions with oncogenic K-Ras alone. However, tumors expressing high levels of c-Myc did engage a number of key tumor suppressors and apoptosis increased as a function of c-Myc level, despite the continued growth and extremely short latency of the adenomas/adenocarcinomas in these mice. Thus, when combined with oncogenic K-Ras, high c-Myc cooperation results in rapid tumorigenesis whereby tumors are still able to grow prolifically despite the apparent activation of the surveillance mechanisms. Further analysis is currently underway to determine the specific survival advantage oncogenic K-Ras offers within this system.
This abstract is also presented as Poster B20.
Citation Format: Catherine H. Wilson, Deborah L. Burkhart, Jinyang Li, Trevor D. Littlewood, Gerard I. Evan. Myc-Ras cooperation can overwhelm tumor suppressive mechanisms within lung adenocarcinomas. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr PR02. doi: 10.1158/1557-3125.RASONC14-PR02