Abstract B01: An allelic series of deregulated c-Myc expression interrogates the oncogenicity of distinct Myc levels in lung adenocarcinoma
Authors: Burkhart Deborah L. , Wilson Catherine H. , Bywater Megan , Kortlever Roderik , Li Jinyang , Arends Mark J. , Littlewood Trevor D. , Evan Gerard I. 
Published: October 2015

A plethora of transgenic mouse models have found that c-Myc induces tumors inefficiently and requires cooperating mutations. Acute activation of high levels of Myc induces cellular proliferation, tempered by intrinsic tumor suppression via apoptosis. We hypothesized that low, near physiological, levels of deregulated Myc that fail to invoke tumor suppression pathways, would more effectively initiate sporadic tumors.

We previously described the Rosa26lslMycER (R26MER) allele in which deregulated c-Myc is expressed at an endogenous-like level following expression of Cre-recombinase. This c-Myc is fused to a modified estrogen receptor, such that Myc function is dependent upon administration of tamoxifen. We have now generated a Rosa26CAGlslMycER (R26(C)MER) allele, in which the addition of the CAG enhancer has increased the amount of MycER driven by this locus 10-fold. Genetic combination of these alleles generates for the first time mice with tightly controlled and defined levels of switchable Myc protein.

Lung cancer is the leading cause of cancer death worldwide and is commonly associated with elevated Myc levels. To determine the role of deregulated Myc expression in lung adenocarcinoma, intranasally administered Adenoviral Cre-recombinase was used to induce sporadic c-MycER expression at different levels in lung epithelial cells, and tamoxifen diet was used to activate c-MycER function. Whereas acute low levels of Myc induced proliferation, high levels of Myc induced both proliferation and apoptosis. Contrary to our hypothesis that Myc-induced apoptosis would delay tumorigenesis, mice with high Myc expression exhibited shorter tumor latency and decreased survival compared to mice with lower levels of expression.

The observation that acute Myc activation stimulates p53 activity suggests that the Arf-p53 pathway would be selected against during tumorigenesis. We observed that the hyperplastic clusters of cells observed following Myc activation in R26(C)MER mice were enhanced in mice lacking functional Arf protein. However at later time points their number gradually decreased and the clusters remained small, suggesting that further mutations are necessary for tumorigenesis.

Of the tumors that develop following long-term Myc activation, 40% carried activating mutations in Kras. We therefore crossed our R26(C)MER mice to the KrasLSL-G12D allele to investigate the mechanism of this oncogene cooperation. Endogenous levels of oncogenic Kras cooperated dramatically with high levels of Myc, inducing rapid growth of lesions in spite of apparent activation of the p53 tumor suppressor pathway and frequent apoptosis.

We then hypothesized that Kras provided increased survival via the Akt pathway to counter Myc-induced apoptosis, as has been previously suggested. Indeed, we found that inhibition of PI3K through treatment with PF-04691502 increased the amount of apoptosis in R26(C)MER/(C)MER;KrasLSL-G12D/+ mice. Importantly, a more dramatic effect was observed when the same lesions were treated with a MEK inhibitor, PD 0325901. Despite having no effect on apoptosis, treatment with PD 0325901 significantly reduced proliferation in these tumors, clearing early lesions within a week of treatment, presumably by allowing Myc-induced apoptosis to overcome the residual proliferation. Consistent with this notion, MEK inhibition of human lung cancer cell lines is more effective at inducing apoptosis in the presence of higher levels of Myc protein. These data suggest that MEK inhibition may have a potent therapeutic role in lung adenocarcinomas that express high levels of Myc.

Citation Format: Deborah L. Burkhart, Catherine H. Wilson, Megan Bywater, Roderik Kortlever, Jinyang Li, Mark J. Arends, Trevor D. Littlewood, Gerard I. Evan. An allelic series of deregulated c-Myc expression interrogates the oncogenicity of distinct Myc levels in lung adenocarcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B01.