Circumventing senescence is associated with stem cell properties and metformin sensitivity

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.

Ribosomal protein RPL22/eL22 regulates the cell cycle by acting as an inhibitor of the CDK4-cyclin D complex

Senescence is a tumor suppressor program characterized by a stable growth arrest while maintaining cell viability. Senescence-associated ribogenesis defects (SARD) have been shown to regulate senescence through the ability of the ribosomal protein S14 (RPS14 or uS11) to bind and inhibit the cyclin-dependent kinase 4 (CDK4). Here we report another ribosomal protein that binds and inhibits CDK4 in senescent cells: L22 (RPL22 or eL22). Enforcing the expression of RPL22/eL22 is sufficient to induce an RB and p53-dependent cellular senescent phenotype in human fibroblasts. Mechanistically, RPL22/eL22 can interact with and inhibit CDK4-Cyclin D1 to decrease RB phosphorylation both in vitro and in cells. Briefly, we show that ribosome-free RPL22/eL22 causes a cell cycle arrest which could be relevant during situations of nucleolar stress such as cellular senescence or the response to cancer chemotherapy.

The senescence-associated secretory phenotype and its regulation

The senescence-associated secretory phenotype (SASP) defines the ability of senescent cells to express and secrete a variety of extracellular modulators that includes cytokines, chemokines, proteases, growth factors and bioactive lipids. The role of the SASP depends on the context. The SASP reinforces the senescent cell cycle arrest, stimulates the immune-mediated clearance of potentially tumorigenic cells, limits fibrosis and promotes wound healing and tissue regeneration. On the other hand, the SASP can mediate chronic inflammation and stimulate the growth and survival of tumor cells. The regulation of the SASP occurs at multiple levels including chromatin remodelling, activation of specific transcription factors such as C/EBP and NF-κB, control of mRNA translation and intracellular trafficking. Several SASP modulators have already been identified setting the stage for future research on their clinical applications.

SOCS1 regulates senescence and ferroptosis by modulating the expression of p53 target genes

The mechanism by which p53 suppresses tumorigenesis remains poorly understood. In the context of aberrant activation of the JAK/STAT5 pathway, SOCS1 is required for p53 activation and the regulation of cellular senescence. In order to identify p53 target genes acting during the senescence response to oncogenic STAT5A, we characterized the transcriptome of STAT5A-expressing cells after SOCS1 inhibition. We identified a set of SOCS1-dependent p53 target genes that include several secreted proteins and genes regulating oxidative metabolism and ferroptosis. Exogenous SOCS1 was sufficient to regulate the expression of p53 target genes and sensitized cells to ferroptosis. This effect correlated with the ability of SOCS1 to reduce the expression of the cystine transporter SLC7A11 and the levels of glutathione. SOCS1 and SOCS1-dependent p53 target genes were induced during the senescence response to oncogenic STAT5A, RasV12 or the tumor suppressor PML. However, while SOCS1 sensitized cells to ferroptosis neither RasV12 nor STAT5A mimicked the effect. Intriguingly, PML turned cells highly resistant to ferroptosis. The results indicate different susceptibilities to ferroptosis in senescent cells depending on the trigger and suggest the possibility of killing senescent cells by inhibiting pathways that mediate ferroptosis resistance.

Sponges against miR-19 and miR-155 reactivate the p53-Socs1 axis in hematopoietic cancers

Normal cell proliferation is controlled by a balance between signals that promote or halt cell proliferation. Micro RNAs are emerging as key elements in providing fine signal balance in different physiological situations. Here we report that STAT5 signaling induces the miRNAs miR-19 and miR-155, which potentially antagonize the tumor suppressor axis composed by the STAT5 target gene SOCS1 (suppressor of cytokine signaling-1) and its downstream effector p53. MiRNA sponges against miR-19 or miR-155 inhibit the functions of these miRNAs and potentiate the induction of SOCS1 and p53 in mouse leukemia cells and in human myeloma cells. Adding a catalytic RNA motif of the hammerhead type within miRNA sponges against miR-155 leads to decreased miR-155 levels and increased their ability of inhibiting cell growth and cell migration in myeloma cells. The results indicate that antagonizing miRNA activity can reactivate tumor suppressor pathways downstream cytokine stimulation in tumor cells.

Abstract 2154: ERK/MAPK pathway inhibits tumorigenesis and cellular reprogramming of pancreatic cancer cells

Reprogrammation of cancer cells into stem-like state is one the most important mechanisms implicated in tumor initiation, metastasis and resistance to chemotherapies. Acquisition of stem-like cell property is directly correlated with the level of activation of the ERK/MAPK pathway. Our study focuses on the implication of the downregulation of this pathway during the transition of pancreatic benign neoplasms to pancreatic ductal adenocarcinoma. This transformation is directly correlated with the acquisition of stem-like cells properties. Conversely, hyperactivation of the ERK pathway using phosphatase inhibitors abrogates the stem-like cell phenotype.

We analyzed the variation of genes expression by microarrays analysis between cells obtained from pancreatic intraepithelial neoplasias and pancreatic cancer. These analyses implicate the expression of DUSPs and genes related to stem cell biology in tumor progression during pancreatic cancer.

Citation Format: Benjamin le Calvé, Xavier Deschenes-Ximard, Filippos Kottakis, Véronique Bourdeau, Frédéric Lessard, Karine Moineau-Vallée, Emmanuelle Saint-Germain, Julien Fitamant, Rushika Miriam Perera, Nabeel Bardeesy, Gerardo Ferbeyre. ERK/MAPK pathway inhibits tumorigenesis and cellular reprogramming of pancreatic cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2154. doi:10.1158/1538-7445.AM2015-2154

Abstract 3895: Tumor suppressor activity of the ERK/MAPK signaling: inhibition of cell reprogramming by degradation of specific proteins

In this study, we investigated the contribution of the ERK signals to tumor initiation and the differentiation state of cancer cells. Oncogenic forms of RAS are found in up to 30% of all human cancers and are established drivers of tumor initiation and maintenance. However, strong expression of these oncogenes in normal cells induces cellular senescence, a putative tumor-suppressive barrier. RAS activates several signaling pathways, such as the PI3K/AKT pathway, the RAL pathway and the classical RAF/MEK/ERK MAP Kinase pathway. We previously found that RASV12-induced senescence of primary cells is prevented by attenuation of ERK signaling. Mechanistically, strong ERK signaling promotes senescence by inducing selective proteasome-dependent protein degradation. This “Senescence-Associated Protein Degradation” (SAPD) targets proteins required for cell cycle progression, mitochondrial functions, cell migration and cell signaling.

Here we show that in addition to abrogating RAS-induced senescence, a moderate ERK activity allows transformation of primary human cells stably expressing RASV12 and hTERT as well as transformation of RasV12-expressing rodent cells. Furthermore, in a Kras-driven mouse model of multistage pancreatic cancer progression, decreased p-ERK levels correlate with tumor initiation. We found that transformed cells with low p-ERK levels express markers of pluripotency and demonstrate phenotypes of tumor initiating cells, such as formation of free-floating tumor spheres, and show the expression of a gene module associated to stem cells. This depends on moderate p-ERK levels, since increasing the activity of the pathway by the pharmacological and genetic inhibition of the Dual-Specificity Phosphatases 1 and 6 (DUSP1/6) completely abrogates the stem-like cell phenotype. Our results suggest that strong ERK signals could circumvent this phenotype by promoting the degradation of key transcription factors that regulate expression of stem cell-associated genes, and this, even if the activity of the pathway is not sufficient to induce cellular senescence.

Taken together, these results demonstrate a novel anti-tumor effect of strong ERK signaling and suggest that processes attenuating ERK levels and/or activity may contribute to tumor initiation and aggressiveness of oncogenic RAS-driven cancers. Therefore, we propose a model where a moderated level of activated ERK (p-ERK) in RAS-expressing cells promotes transformation and dedifferentiation whereas higher levels limit cancer initiation and maintenance by activating tumor-suppressive mechanisms like senescence and differentiation. Considering the increased resistance of tumor-initiating cells to chemotherapy and their capacity to initiate tumor development, this model suggests a need to target cancer cells with low pERK levels within a tumor.

Citation Format: Xavier Deschênes-Simard, Filippos Kottakis, Frédéric Lessard, Emmanuelle Saint-Germain, Véronique Bourdeau, Nabeel Bardeesy, Gerardo Ferbeyre. Tumor suppressor activity of the ERK/MAPK signaling: inhibition of cell reprogramming by degradation of specific proteins. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3895. doi:10.1158/1538-7445.AM2014-3895

CHES1/FOXN3 regulates cell proliferation by repressing PIM2 and protein biosynthesis

The expression of the forkhead transcription factor checkpoint suppressor 1 (CHES1), also known as FOXN3, is reduced in many types of cancers. We show here that CHES1 decreases protein synthesis and cell proliferation in tumor cell lines but not in normal fibroblasts. Conversely, short hairpin RNA-mediated depletion of CHES1 increases tumor cell proliferation. Growth suppression depends on the CHES1 forkhead DNA-binding domain and correlates with the nuclear localization of CHES1. CHES1 represses the expression of multiple genes, including the kinases PIM2 and DYRK3, which regulate protein biosynthesis, and a number of genes in cilium biogenesis. CHES1 binds directly to the promoter of PIM2, and in cells expressing CHES1 the levels of PIM2 are reduced, as well as the phosphorylation of the PIM2 target 4EBP1. Overexpression of PIM2 or eIF4E partially reverses the antiproliferative effect of CHES1, indicating that PIM2 and protein biosynthesis are important targets of the antiproliferative effect of CHES1. In several human hematopoietic cancers, CHES1 and PIM2 expressions are inversely correlated, suggesting that repression of PIM2 by CHES1 is clinically relevant.