Abstract 346: p53 mutation in Rb deficient neuroendocrine lung and thyroid cells results in high grade neuroendocrine carcinomas promoted by mTOR pathway activation .
Authors: Akeno Nagako , Miller Ashley , Levinsky Ncik , Wikenheiser-Brokamp Kathryn A. 
Published: August 2014

Abstract
Small cell lung carcinoma (SCLC) and medullary thyroid carcinoma (MTC) are aggressive, rapidly metastatic neuroendocrine carcinomas that portend a dismal prognosis. Rb loss is detected in ≥ 90% of SCLC and p53 is mutated in ∼80% of cases providing evidence that Rb and p53 suppress SCLC. Extensive genetic evidence in mouse models also supports a critical role for Rb in suppressing MTC: however, the mechanisms promoting tumorigenesis in Rb and p53 deficient cells and their relation to therapeutic response remain unclear. In this study, a conditional SCLC and MTC mouse model was generated by targeting Rb and p53 ablation or mutant protein expression to lung and thyroid epithelial cells. Rb loss alone resulted in slowly progressing MTC and lung neuroendocrine hyperplasia. While p53 alteration alone was not sufficient for tumor initiation, p53 loss or mutation in Rb deficient cells led to MTC and SCLC with metastases, phenotypically mimicking human disease. Mice expressing p53 R172H (R175H in human), but not R270H (R273H in human), mutant protein had a gain-of-function phenotype as evidenced by decreased survival of Rb/p53R172H as compared to Rb/p53 ablated mice. No dominant negative effect was detected by comparing Rb/p53+/mutant to Rb/p53+/− mice. Mechanistic studies revealed p53 dependent mammalian target of rapamycin (mTOR) pathway activation in MTC without coincident AKT or MAPK pathway activation. Similar mTOR pathway activation was detected in Rb/p53 deficient and mutant lungs. Rapamycin dramatically inhibited mTOR pathway signaling in Rb/p53 deficient or mutant thyroid tumors and lungs in vivo as well as in MTC and SCLC tumor cells in culture. Interestingly, mTORC1 inhibition was not associated with feedback AKT or MAPK pathway activation in vitro or in vivo as previously reported for breast and prostate cancers. Rapamycin inhibited MTC and SCLC tumor cell growth demonstrating a functional role for mTOR activation in tumor progression. These studies identify distinct, cooperative Rb and p53 tumor suppressive functions. The oncogenic stimulus provided by Rb loss induces aberrant cell growth and leads to a p53 tumor suppressive response that inhibits tumor progression and mTOR dependent cell growth demonstrating that mTOR pathway inhibition is a critical p53 tumor suppressive function. Notably, rapamycin did not result in feedback MAPK or AKT pathway activation providing evidence this is not a general mechanism limiting therapeutic response to mTORC1 inhibition. Together these studies provide mechanistic links between genetic alterations and aberrant signaling pathways critical in carcinogenesis. In addition to providing mechanistic insights, this genetically engineered mouse model provides a robust preclinical platform to explore novel therapeutic strategies and define mechanisms underlying treatment response and resistance.
Citation Format: Nagako Akeno, Ashley Miller, Ncik Levinsky, Kathryn A. Wikenheiser-Brokamp. p53 mutation in Rb deficient neuroendocrine lung and thyroid cells results in high grade neuroendocrine carcinomas promoted by mTOR pathway activation . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 346. doi:10.1158/1538-7445.AM2013-346