Date Published: January 06, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Xavier Deschênes‐Simard, Maxime Parisotto, Marie‐Camille Rowell, Benjamin Le Calvé, Sebastian Igelmann, Karine Moineau‐Vallée, Emmanuelle Saint‐Germain, Paloma Kalegari, Véronique Bourdeau, Filippos Kottakis, Nabeel Bardeesy, Gerardo Ferbeyre.
Most cancers arise in old individuals, which also accumulate senescent cells. Cellular senescence can be experimentally induced by expression of oncogenes or telomere shortening during serial passage in culture. In vivo, precursor lesions of several cancer types accumulate senescent cells, which are thought to represent a barrier to malignant progression and a response to the aberrant activation of growth signaling pathways by oncogenes (oncogene toxicity). Here, we sought to define gene expression changes associated with cells that bypass senescence induced by oncogenic RAS. In the context of pancreatic ductal adenocarcinoma (PDAC), oncogenic KRAS induces benign pancreatic intraepithelial neoplasias (PanINs), which exhibit features of oncogene‐induced senescence. We found that the bypass of senescence in PanINs leads to malignant PDAC cells characterized by gene signatures of epithelial‐mesenchymal transition, stem cells, and mitochondria. Stem cell properties were similarly acquired in PanIN cells treated with LPS, and in primary fibroblasts and mammary epithelial cells that bypassed Ras‐induced senescence after reduction of ERK signaling. Intriguingly, maintenance of cells that circumvented senescence and acquired stem cell properties was blocked by metformin, an inhibitor of complex I of the electron transport chain or depletion of STAT3, a protein required for mitochondrial functions and stemness. Thus, our studies link bypass of senescence in premalignant lesions to loss of differentiation, acquisition of stemness features, and increased reliance on mitochondrial functions.
A variety of oncogenes induce DNA damage, protein degradation, and mitochondrial dysfunction ultimately triggering a stable cell cycle arrest known as oncogene‐induced senescence (OIS). OIS is considered a powerful anti‐cancer response mediated by bona fide tumor suppressors such as p53, RB, and p16INK4a (Salama, Sadaie, Hoare, & Narita, 2014). In mice, expression of oncogenes in a variety of tissues leads to premalignant lesions with the characteristics of OIS that often progress to form malignant tumors (Collado & Serrano, 2010). Although inactivation of tumor suppressors such as p53 accelerates the formation of tumors in these mouse models (Collado & Serrano, 2010), the mechanism of senescence bypass in tumors that spontaneously arise from premalignant lesions remains unclear. In particular, there is an extensive reprogramming of the cancer genome resulting in loss of genetic programs of cell differentiation and gain of gene expression programs of embryonic stem cells (ESCs) (Ben‐Porath et al., 2008). While many comparisons have been made between tumor cells and their normal counterparts, there is much to learn by comparing malignant cells to their precursor premalignant lesions being both populations expressing the same driving oncogene.
We show here that bypass of OIS was associated with the acquisition of a gene expression program characteristic of stem cells, EMT, and mitochondrial genes. This gene expression transition was observed after comparing premalignant pancreatic intraepithelial neoplasias (PanINs) with pancreatic ductal adenocarcinoma cells that spontaneously arose from PanIN cells. Also, these results were reproduced in other models of OIS in primary human and mouse cells. We found that PanIN cells that progress into PDAC show enrichment for genes regulated by known drivers of CSCs such as NF‐κB (Sun et al., 2013), STAT3 (Yang et al., 2010), and MYC (Sancho et al., 2015). These cells also acquire properties of tumor‐initiating cells including the ability to form tumor spheres in cell culture and ectopic tumors in vivo by extravasation. The protumorigenic pathways activated in cells that bypass senescence can be also stimulated by activation of PRRs that play an important role in the origin of pancreatic cancer (Zambirinis et al., 2013). Finally, the subpopulation of tumor‐initiating cells in PDAC cells show increased mitochondrial content and are sensitized to metformin. Taken together, these observations suggest that OIS acts as an initial barrier for the proliferation of cells with oncogenic mutations but seems to prime cells to activate a stemness gene expression program that can contribute to progression into malignant tumors.