Oncogene-induced Cellular Senescence Research Articles

Abstract PO2-01-08: Differential HOXB13 gene expression and promoter methylation analysis in breast cancer

Abstract Background: The Breast Cancer Index (BCI) is a gene expression-based signature comprising two functional biomarker panels: the Molecular Grade Index (MGI) and BCI (H/I), which is a ratio of the HOXB13 and IL17BR gene expression. Several studies have shown that BCI (H/I) is a predictive biomarker for extended adjuvant endocrine therapy benefit in hormone receptor-positive (HR+) early-stage breast cancer. Molecular mechanisms underlying endocrine responsiveness remain to be fully explored. The objective of this analysis is to evaluate the correlation between gene expression and methylation in breast cancer patients displaying different HOXB13 mRNA expression levels from The Cancer Genome Atlas (TCGA) project. Methods: Data from methylation microarrays and mRNA sequencing were examined in combination with clinical metadata from 1,095 TCGA breast cancer patients. After removing outliers, HR+ samples were ordered based on their HOXB13 expression levels and the top 10% of samples (HOXB13-high; n=95) and bottom 10% (HOXB13-low; n=95) were selected for further analysis. Differentially expressed genes (DEGs) were determined by comparing all protein-coding genes between the HOXB13-high and HOXB13-low sets, based on an absolute log-fold change above 2 and a Benjamini-Hochberg adjusted p-value threshold of 0.01. Among the subset of samples with matching methylation data (n=60 for the HOXB13-high group and n=48 for the HOXB13-low group), hypo-methylated and hyper-methylated differentially methylated probes (DMPs) were identified based on at least 10% change of β (i.e., average promoter methylation) and a Benjamini-Hochberg adjusted p-value threshold of 0.01. Motif and transcription factor (TF) enrichment analyses were identified at a minimum incidence of 10 and a lower boundary of 1.5 for the 95% confidence interval of the odds ratio. Results: Our analysis identified a total of 613 DEGs between the HOXB13-high and HOXB13-low groups. Gene ontology analysis of DEGs revealed a statistically significant enrichment of genes associated with PTK6 regulated cell cycle, oncogene-induced cellular senescence, and regulation of T cell activation. Analysis of protein-protein interactions of differentially expressed genes revealed differences in key biological processes including changes in cell cycle regulation, p53 pathway, and IL-17 signaling. A total of 5,295 hyper-methylated and 580 hypo-methylated DMPs were discovered when comparing HOXB13-high and HOXB13-low samples. Changes in global methylation patterns enabled classification of breast cancer samples with high or low HOXB13 expression with an accuracy of 0.90. Reconstruction of gene regulatory networks from DNA methylation and transcriptome profiles elucidated significant pairs of DMPs and differentially expressed genes. Specifically, a total of 197 differentially expressed genes were attributed to hyper-methylated DMPs, including genes involved in response to estrogen, activation of HOX genes during differentiation, and drug-mediated inhibition of CDK4/6 activity. Additionally, 8 differentially expressed genes were attributed to hypo-methylated DMPs and were associated with proteasomal protein catabolic processes. Lastly, motif analysis of transcriptional factors for the hyper-methylated DMPs revealed enrichment of binding motifs attributed to FOXA1, ESR1, XBR1, and FOXP1. Conclusion: A comprehensive and comparative analysis of mRNA expression levels and methylation patterns between TCGA breast cancer samples with high and low expression of HOXB13 revealed key signaling pathways and biological processes that may provide insights on the molecular mechanism of HOXB13 in the regulation of response to endocrine therapy in HR+ breast cancer. Citation Format: Natalia Siuliukina, Yi Zhang, Kai Treuner, Mark Pegram. Differential HOXB13 gene expression and promoter methylation analysis in breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-01-08.

C-Jun N-terminal kinase signaling in cellular senescence.

Cellular senescence leads to decreased tissue regeneration and inflammation and is associated with diabetes, neurodegenerative diseases, and tumorigenesis. However, the mechanisms of cellular senescence are not fully understood. Emerging evidence has indicated that c-Jun N-terminal kinase (JNK) signaling is involved in the regulation of cellular senescence. JNK can downregulate hypoxia inducible factor-1α to accelerate hypoxia-induced neuronal cell senescence. The activation of JNK inhibits mTOR activity and triggers autophagy, which promotes cellular senescence. JNK can upregulate the expression of p53 and Bcl-2 and accelerates cancer cell senescence; however, this signaling also mediates the expression of amphiregulin and PD-LI to achieve cancer cell immune evasion and prevents their senescence. The activation of JNK further triggers forkhead box O expression and its target gene Jafrac1 to extend the lifespan of Drosophila. JNK can also upregulate the expression of DNA repair protein poly ADP-ribose polymerase 1 and heat shock protein to delay cellular senescence. This review discusses recent advances in understanding the function of JNK signaling in cellular senescence and includes a comprehensive analysis of the molecular mechanisms underlying JNK-mediated senescence evasion and oncogene-induced cellular senescence. We also summarize the research progress in anti-aging agents that target JNK signaling. This study will contribute to a better understanding of the molecular targets of cellular senescence and provides insights into anti-aging, which may be used to develop drugs for the treatment of aging-related diseases.

Investigating EWS‐FLI1 Induced Replication Stress in Ewing Sarcoma Pathogenesis

Ewing sarcoma (EWS) is the second most common malignant bone cancer affecting young children and adolescents. Although most patients achieve long‐lasting remission, patients with metastatic or recurrent disease have very poor outcomes and chemo‐resistance is a significant problem. The majority of Ewing sarcomas are driven by a chromosomal translocation resulting in the EWS‐FLI1 fusion gene. EWS‐FLI1 is an oncogenic transcription factor that plays roles in transcriptional regulation, DNA damage response, cell cycle checkpoint control, and alternative splicing. However, the exact molecular mechanisms behind these functions are not understood fully. Although EWS‐FLI1 is specific to EWS cells and required for tumorigenesis, successfully targeting EWS‐FLI1 has proved challenging. Therefore, other therapeutic targets must be identified to improve outcomes for patients with metastatic or recurrent disease.Using a quantitative PCR array, we identified USP1, a deubiquitinase that functions in both the DNA damage and replication stress responses, as being overexpressed in EWS cell lines compared to its cell of origin, the human mesenchymal stem cell. Importantly, we found USP1 to be essential for EWS cell line proliferation, as growth of these cell lines is arrested with knockdown of USP1. USP1 downregulation has been linked to oncogene‐induced cellular senescence dependent on functional p53. However, because p53 is mutated in EWS, the growth arrest we observe following USP1 downregulation must occur via a p53‐independent mechanism. Therefore, we hypothesize that USP1 overexpression functions to bypass cellular senescence in a p53‐independent manner in response to EWS‐FLI1 induced replication stress, resulting in tumorigenesis. Our studies aim to test this hypothesis in EWS and explore the potential to target USP1 alone or in combination with the current chemotherapy regimen for treating EWS.Support or Funding InformationThis work was supported by Pediatric Cancer pilot funding UNMC and UNMC startup funds to Gargi Ghosal and UNMC Graduate Assistantship Fellowship to Halle Halbkat

Krüppel-Like Factor 6 Is Required for Oxidative and Oncogene-Induced Cellular Senescence.

Krüppel-like factor 6 (KLF6) is a transcription factor involved in the regulation of several cellular processes. Regarding its role in tumorigenesis, KLF6 is considered a tumor suppressor. Numerous reports demonstrate its frequent genomic loss or down-regulation, implying a functional inactivation in a broad range of human cancers. Previous work from our laboratory showed that the down-regulation of KLF6 expression in normal fibroblasts leads to cellular transformation, while its ectopic expression interferes with the oncogenic transformation triggered by activated Ras through a cell cycle arrest. We hypothesize that the growth suppressor activity of KLF6 may involve the induction of cellular senescence thereby helping to prevent the proliferation of cells at risk of neoplastic transformation. Here, we explored the association of KLF6 up-regulation in two different cellular senescence scenarios. We found that KLF6 silencing bypasses both oxidative and oncogene-induced senescence. In this context, KLF6 expression per se was capable to trigger cellular senescence in both normal and tumoral contexts. As such, the findings presented in this report provide insights into a potential mechanism by which KLF6 may play a suppressing role of uncontrolled or damaged cell proliferation.

Oncogene-induced senescence: a double edged sword in cancer.

Oncogene-induced cellular senescence (OIS) is a complex program that is triggered in response to aberrant activation of oncogenic signaling. Initially, OIS was thought to be a barrier to malignant transformation because of its suppression on cell proliferation. Later studies showed that senescence induced by oncogenes can also promote the initiation and development of cancer. The opposing effects of OIS occur through different combinations of downstream effectors as well as the interplay of senescent cells and the microenvironment, such as senescence-associated inflammation. Here, we review the common features and molecular mechanisms underlying OIS and the interaction between senescent cells and the microenvironment. We propose that targeting senescent cells may have a beneficial therapeutic effect during the treatment of cancer.

The human T-cell leukemia virus type-1 p30II protein activates p53 and induces the TIGAR and suppresses oncogene-induced oxidative stress during viral carcinogenesis

In normal cells, aberrant oncogene expression leads to the accumulation of cytotoxic metabolites, including reactive oxygen species (ROS), which can cause oxidative DNA-damage and apoptosis as an intrinsic barrier against neoplastic disease. The c-Myc oncoprotein is overexpressed in many lymphoid cancers due to c-myc gene amplification and/or 8q24 chromosomal translocations. Intriguingly, p53 is a downstream target of c-Myc and hematological malignancies, such as adult T-cell leukemia/lymphoma (ATL), frequently contain wildtype p53 and c-Myc overexpression. We therefore hypothesized that p53-regulated pro-survival signals may thwart the cell's metabolic anticancer defenses to support oncogene-activation in lymphoid cancers. Here we show that the Tp53-induced glycolysis and apoptosis regulator (TIGAR) promotes c-myc oncogene-activation by the human T-cell leukemia virus type-1 (HTLV-1) latency-maintenance factor p30II, associated with c-Myc deregulation in ATL clinical isolates. TIGAR prevents the intracellular accumulation of c-Myc-induced ROS and inhibits oncogene-induced cellular senescence in ATL, acute lymphoblastic leukemia, and multiple myeloma cells with elevated c-Myc expression. Our results allude to a pivotal role for p53-regulated antioxidant signals as mediators of c-Myc oncogenic functions in viral and non-viral lymphoid tumors.

Caveolin-1 promotes the tumor suppressor properties of oncogene-induced cellular senescence

Oncogene-induced senescence (OIS) is considered a powerful tumor suppressor mechanism. Caveolin-1 acts as a scaffolding protein to functionally regulate signaling molecules. We demonstrate that a lack of caveolin-1 expression inhibits oncogenic K-Ras (K-RasG12V)-induced premature senescence in mouse embryonic fibroblasts and normal human bronchial epithelial cells. Oncogenic K-Ras induces senescence by limiting the detoxification function of MTH1. We found that K-RasG12V promotes the interaction of caveolin-1 with MTH1, which results in inhibition of MTH1 activity. Lung cancer cells expressing oncogenic K-Ras have bypassed the senescence barrier. Interestingly, overexpression of caveolin-1 restores cellular senescence in both A549 and H460 lung cancer cells and inhibits their transformed phenotype. In support of these findings, our in vivo data demonstrate that overexpression of oncogenic K-Ras (K-RasG12D) induces cellular senescence in the lung of wildtype but not caveolin-1-null mice. A lack of K-RasG12D-induced premature senescence in caveolin-1-null mice results in the formation of more abundant lung tumors. Consistent with these data, caveolin-1-null mice overexpressing K-RasG12D display accelerated mortality. Finally, our animal data were supported by human sample analysis in which we show that caveolin-1 expression is dramatically down-regulated in lung adenocarcinomas from lung cancer patients, both at the mRNA and protein levels, and that low caveolin-1 expression is associated with poor survival. Together, our data suggest that lung cancer cells escape oncogene-induced premature senescence through down-regulation of caveolin-1 expression to progress from premalignant lesions to cancer.

Abstract 5483: Investigating the role of the retinoblastoma protein in induction and maintenance of oncogene-induced cellular senescence in tumor suppression

Abstract Oncogene-induced cellular senescence is a tumor suppressor response resulting in irreversible cell cycle exit, and is dependent on the p53 and RB pathways. Loss of p53 has recently been shown to allow re-entry of senescent cells into the cell cycle. The role of the RB1 protein in maintenance of the senescent state is less well studied. We investigated whether RB1 is necessary for the induction as well as the maintenance of oncogene-induced senescence, and we also evaluated whether RB1 activation is sufficient to induce and/or maintain senesce in the absence of a functional p53 pathway. Inactivation of RB1 prior to onset of senescence led to failure of senescence induction, with impaired cell cycle exit and failure to induce senescence markers. On the other hand, loss of RB1 after cells had already entered oncogene-induced senescence was able to reverse some features of the senescence phenotype, but did not result in successful cell division and accumulation. Interestingly, in the absence of p53, constitutively active RB1 was sufficient for cells to undergo oncogene-induced senescence, and effectively prevented colony formation. Finally, while p53 inactivation was sufficient for senescent cells to re-enter the cell cycle, constitutive activation of RB1 prevented cell cycle re-entry, even in the absence of p53. Our findings show that RB1 is necessary for both the induction and maintenance of oncogene-induced senescence, and that active RB is sufficient for senescence induction as well as its maintenance, both in the presence and absence of p53. These results have direct implications on targeting RB activation as a senescence-inducing approach in both p53 wild type and p53-defective premalignant and malignant tumors. Note: This abstract was not presented at the meeting. Citation Format: Mohamad Harajly, Hasan Zalzali, Farah Ghamloush, Raya H. Saab. Investigating the role of the retinoblastoma protein in induction and maintenance of oncogene-induced cellular senescence in tumor suppression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5483. doi:10.1158/1538-7445.AM2017-5483

Induction and Detection of Oncogene-Induced Cellular Senescence in Drosophila.

Cellular senescence is induced by various cellular stresses, including activation of the Ras oncogene. In Drosophila imaginal epithelia, clones of cells expressing oncogenic Ras (RasV12) show several markers of cellular senescence, such as elevation of SA-β-gal activity, upregulation of the Cdk inhibitor Dacapo (Dap), and heterochromatinization. However, these cells do not undergo cell cycle arrest or exhibit a DNA damage response (DDR), cellular hypertrophy, or a senescence-associated secretory phenotype (SASP), other essential markers of cellular senescence. However, we found that inducing mitochondrial dysfunction within RasV12-expressing cells caused all above-mentioned aspects of cellular senescence. This provided the first evidence that cellular senescence occurs in invertebrates and is intriguing because mitochondrial dysfunction is frequently observed in human cancers. Here, we describe the procedures for the induction and detection of cellular senescence in Drosophila epithelia.

Derepression of hTERT gene expression promotes escape from oncogene-induced cellular senescence

Oncogene-induced senescence (OIS) is a critical tumor-suppressing mechanism that restrains cancer progression at premalignant stages, in part by causing telomere dysfunction. Currently it is unknown whether this proliferative arrest presents a stable and therefore irreversible barrier to cancer progression. Here we demonstrate that cells frequently escape OIS induced by oncogenic H-Ras and B-Raf, after a prolonged period in the senescence arrested state. Cells that had escaped senescence displayed high oncogene expression levels, retained functional DNA damage responses, and acquired chromatin changes that promoted c-Myc-dependent expression of the human telomerase reverse transcriptase gene (hTERT). Telomerase was able to resolve existing telomeric DNA damage response foci and suppressed formation of new ones that were generated as a consequence of DNA replication stress and oncogenic signals. Inhibition of MAP kinase signaling, suppressing c-Myc expression, or inhibiting telomerase activity, caused telomere dysfunction and proliferative defects in cells that had escaped senescence, whereas ectopic expression of hTERT facilitated OIS escape. In human early neoplastic skin and breast tissue, hTERT expression was detected in cells that displayed features of senescence, suggesting that reactivation of telomerase expression in senescent cells is an early event during cancer progression in humans. Together, our data demonstrate that cells arrested in OIS retain the potential to escape senescence by mechanisms that involve derepression of hTERT expression.

The HTLV-1 latency-maintenance factor p30II activates p53-dependent metabolic effectors to promote oncogene-activation and aberrant lymphoproliferation

The human T-cell leukemia retrovirus type-1 (HTLV-1) transforms CD4+ T-lymphocytes and causes adult T-cell leukemia/lymphoma (ATLL), an aggressive hematological malignancy that is refractive to most anticancer treatments. The highly-conserved pX sequence of HTLV-1 encodes three latency-maintenance factors: p30II, p13II and Hbz, which suppress proviral gene expression and help HTLV-1-infected cells evade detection by host immune responses as a prelude to neoplastic disease. We have previously demonstrated that p30II enhances the transcriptional and oncogenic functions of the c-Myc oncoprotein and induces aberrant lymphoproliferation through molecular interactions with the TIP60 acetyltransferase. Our recent studies have demonstrated the cooperation between p30II and c-Myc is dependent upon the induction of p53-dependent pro-survival signals. The p30II protein activates p53 and induces expression of p53-dependent metabolic effectors, including the Tp53-induced glycolysis and apoptosis regulator (TIGAR). Acute and lymphoma-stage ATLL clinical isolates frequently over express c-Myc and contain wildtype p53. We therefore hypothesize the induction of p53-dependent pro-survival signals by p30II could promote c-myc oncogene-activation during retroviral carcinogenesis by preventing c-Myc-induced cytotoxicity and apoptosis. Indeed, we have found that lentiviral-p30II induces mitochondrial expression of TIGAR and prevents the intracellular accumulation of c-Myc-induced reactive oxygen species (ROS) and inhibits oncogene-induced cellular senescence. Knockdown of TIGAR expression, using a small-interfering RNA (siRNA-tigar), inhibited the ability of p30II to suppress c-Myc-induced cytotoxicity. A panel of primary ATLL patient tumor lymphocytes exhibited over expression of TIGAR which correlated with c-Myc deregulation, compared to donor hu-PBMCs. Furthermore, we have shown that siRNA-knockdown of TIGAR expression sensitizes HTLV-1-transformed SLB1 lymphoma cells to oncogene-induced ROS and causes cellular senescence, suggesting siRNA-inhibition of TIGAR may be a plausible approach to sensitize ATLL tumor lymphocytes to anticancer chemotherapy drugs that induce oxidative damage. Our studies reveal a paradoxical role for p53-dependent metabolic effectors in promoting oncogene-activation by HTLV-1 p30II, which could promote proviral replication and leukemic disease progression.

Registered report: senescence surveillance of pre-malignant hepatocytes limits liver cancer development.

The Kang et al. (2011), published in Nature in 2011. The experiments that will be replicated are those reported in Figures 3B, 3C, 3E, and 4A. In these experiments, Kang et al. (2011) demonstrate the phenomenon of oncogene-induced cellular senescence and immune-mediated clearance of senescent cells after intrahepatic injection of NRAS (Figures 2I, 3B, 3C, and 3E). Additionally, Kang et al. (2011) show the specific necessity of CD4+ T cells for immunoclearance of senescent cells (Figure 4A). The Reproducibility Project: Cancer Biology is a collaboration between the eLife.

CDK2 transcriptional repression is an essential effector in p53-dependent cellular senescence-implications for therapeutic intervention.

Cellular senescence, a form of cell-cycle arrest, is a tumor-suppressor mechanism triggered by multiple tumor-promoting insults, including oncogenic stress and DNA damage. The role of cyclin-dependent kinase 2 (CDK2) regulation has been evaluated in models of replicative senescence, but little is known regarding its role in other senescence settings. Using in vitro and in vivo models of DNA damage-and oncogene-induced cellular senescence, it was determined that activation of the tumor-suppressor protein p53 (TP53) resulted in repression of the CDK2 transcript that was dependent on intact RB. Ectopic CDK2 expression was sufficient to bypass p53-dependent senescence, and CDK2-specific inhibition, either pharmacologically (CVT313) or by use of a dominant-negative CDK2, was sufficient to induce early senescence. Pharmacologic inhibition of CDK2 in an in vivo model of pineal tumor decreased proliferation and promoted early senescence, and it also decreased tumor penetrance and prolonged time to tumor formation in animals lacking p53. In conclusion, for both oncogene- and DNA damage-induced cellular senescence, CDK2 transcript and protein are decreased in a p53- and RB-dependent manner, and this repression is necessary for cell-cycle exit during senescence. These data show that CDK2 inhibition may be useful for cancer prevention in premalignant hyperproliferative lesions, as well as established tumors.

Myc and Ras oncogenes engage different energy metabolism programs and evoke distinct patterns of oxidative and DNA replication stress

Both Myc and Ras oncogenes impact cellular metabolism, deregulate redox homeostasis and trigger DNA replication stress (RS) that compromises genomic integrity. However, how are such oncogene-induced effects evoked and temporally related, to what extent are these kinetic parameters shared by Myc and Ras, and how are these cellular changes linked with oncogene-induced cellular senescence in different cell context(s) remain poorly understood. Here, we addressed the above-mentioned open questions by multifaceted comparative analyses of human cellular models with inducible expression of c-Myc and H-RasV12 (Ras), two commonly deregulated oncoproteins operating in a functionally connected signaling network. Our study of DNA replication parameters using the DNA fiber approach and time-course assessment of perturbations in glycolytic flux, oxygen consumption and production of reactive oxygen species (ROS) revealed the following results. First, overabundance of nuclear Myc triggered RS promptly, already after one day of Myc induction, causing slow replication fork progression and fork asymmetry, even before any metabolic changes occurred. In contrast, Ras overexpression initially induced a burst of cell proliferation and increased the speed of replication fork progression. However, after several days of induction Ras caused bioenergetic metabolic changes that correlated with slower DNA replication fork progression and the ensuing cell cycle arrest, gradually leading to senescence. Second, the observed oncogene-induced RS and metabolic alterations were cell-type/context dependent, as shown by comparative analyses of normal human BJ fibroblasts versus U2-OS sarcoma cells. Third, the energy metabolic reprogramming triggered by Ras was more robust compared to impact of Myc. Fourth, the detected oncogene-induced oxidative stress was due to ROS (superoxide) of non-mitochondrial origin and mitochondrial OXPHOS was reduced (Crabtree effect). Overall, our study provides novel insights into oncogene-evoked metabolic reprogramming, replication and oxidative stress, with implications for mechanisms of tumorigenesis and potential targeting of oncogene addiction.

Abstract 3138: IL-6/Stat3 signaling is an indispensable modulator of oncogene-induced cellular senescence

Abstract IL-6/Stat3 signaling plays a role as a proto-oncogene in various tumors including prostate cancer, where IL-6 has been explored as therapeutic target. Deficiency of Pten, one of the most commonly mutated genes in cancer, directly triggers a senescence program critically depending on the p19Arf-p53 fail-safe pathway. We identified a novel and unexpected tumor suppressive role of IL-6/Stat3 signaling in a Pten-deficient murine prostate cancer model. Our data provide evidence that activation of IL-6/Stat3 signaling axis is implicated in oncogene-induced senescence (OIS). Strikingly, genetic ablation of IL-6 strongly promoted prostate cancer initiation, progression and metastasis in Pten-deficient mice. Mechanistically, IL6/Stat3 signaling regulates p19Arf-p53-induced senescence by modulating expression of the ubiquitin ligase MDM2 in pre-neoplastic cells. Critically, in prostate cancer patients low expression of Stat3 or p14Arf, the human homologue of p19Arf, correlates with poor survival. Therefore, the IL-6/Stat3 signaling axis may represent a novel molecular target for senescence-based cancer therapies. Citation Format: Jan Pencik, Michaela Schlederer, Melanie Hassler, Wolfgang Gruber, Fritz Aberger, Richard Kennedy, Stephen Walker, Stephan Rose-John, Valeria Poli, Robert Eferl, Harald Esterbauer, Osman Aksoy, Merima Herac, Peter Mazal, Andrea Haitel, Martin Susani, Richard Moriggl, Zoran Culig, Lukas Kenner. IL-6/Stat3 signaling is an indispensable modulator of oncogene-induced cellular senescence. [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 3138. doi:10.1158/1538-7445.AM2014-3138

Abstract 2260: c-Myc induced senescence is dependent on transactivation of actin depolymerizing factor (ADF)/cofilin to mediate potent bystander effect

Abstract Oncogene-induced senescence may prevent unlimited proliferation and lead to cell death. This phenomenon is likely to be associated with up-regulation of p53 and p16INK4 tumor suppressor. Morphological transformation is also an important feature of senescence that exhibits enlarged cell volume and flat morphology. In this study, we found that c-Myc was co-upregulated with cofilin-1, an actin-binding protein essential for actin dynamics and morphological maintenance in replicative senescence. Additionally, oxidative stress induced senescent phenotypes also accompanied by up-regulation of both c-myc and cofilin-1. Knockdown of cofilin-1 was sufficient to minimize the cell shapes and senescence associated β-galactosidase marker that was induced by over-expressed c-myc or oxidative stress. Over-expression of cofilin-1 was also sufficient to induce senescence in the absence of p53 and p16INK4, but was unable to influence c-Myc level. On the other hand, over-expression of c-Myc could induce cofilin-1 expression. We next found that c-myc was able to induce the transactivation of cofilin-1 gene according to results of quantitative polymerase chain reaction (qPCR) and luciferase reporter gene assay. Furthermore, the results of ChIP assay provided evidence that c-myc can bind to two proximal E-box elements nearby the transcriptional initiation on cofilin-1 gene promoter. Although c-myc promoted cellular senescence in vitro, the conditional medium from c-myc over-expressing A549 cells accelerated migration and proliferation in separate cultured cells without c-myc over-expression. Knockdown of cofilin-1 also alleviated this bystander effect caused by over-expression of c-myc. Therefore, c-myc oncogene may require cofilin-1 for cell senescence that influences the growth of other tumor cells. Taken together, current data provide evidence that c-myc/cofilin-1 signaling pathway is a novel mechanism on promoting oncogene-induced cellular senescence and tumor progression. Citation Format: Shih-Ting Lin, Cheng-Han Tsai, Yi-Jang Lee, Chia-Chien Lo, Yen-Ting Chou. c-Myc induced senescence is dependent on transactivation of actin depolymerizing factor (ADF)/cofilin to mediate potent bystander effect. [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 2260. doi:10.1158/1538-7445.AM2014-2260

The Nuclear Receptor Peroxisome Proliferator-activated Receptor-β/δ (PPARβ/δ) Promotes Oncogene-induced Cellular Senescence through Repression of Endoplasmic Reticulum Stress

Endoplasmic reticulum (ER) stress and ER stress-associated unfolded protein response (UPR) can promote cancer cell survival, but it remains unclear whether they can influence oncogene-induced senescence. The present study examined the role of ER stress in senescence using oncogene-dependent models. Increased ER stress attenuated senescence in part by up-regulating phosphorylated protein kinase B (p-AKT) and decreasing phosphorylated extracellular signal-regulated kinase (p-ERK). A positive feed forward loop between p-AKT, ER stress, and UPR was discovered whereby a transient increase of ER stress caused reduced senescence and promotion of tumorigenesis. Decreased ER stress was further correlated with increased senescence in both mouse and human tumors. Interestingly, H-RAS-expressing Pparβ/δ null cells and tumors having increased cell proliferation exhibited enhanced ER stress, decreased cellular senescence, and/or enhanced tumorigenicity. Collectively, these results demonstrate a new role for ER stress and UPR that attenuates H-RAS-induced senescence and suggest that PPARβ/δ can repress this oncogene-induced ER stress to promote senescence in accordance with its role as a tumor modifier that suppresses carcinogenesis.

Oncogene-induced reactive oxygen species fuel hyperproliferation and DNA damage response activation

Oncogene-induced reactive oxygen species (ROS) have been proposed to be signaling molecules that mediate proliferative cues. However, ROS may also cause DNA damage and proliferative arrest. How these apparently opposite roles can be reconciled, especially in the context of oncogene-induced cellular senescence, which is associated both with aberrant mitogenic signaling and DNA damage response (DDR)-mediated arrest, is unclear. Here, we show that ROS are indeed mitogenic signaling molecules that fuel oncogene-driven aberrant cell proliferation. However, by their very same ability to mediate cell hyperproliferation, ROS eventually cause DDR activation. We also show that oncogenic Ras-induced ROS are produced in a Rac1 and NADPH oxidase (Nox4)-dependent manner. In addition, we show that Ras-induced ROS can be detected and modulated in a living transparent animal: the zebrafish. Finally, in cancer we show that Nox4 is increased in both human tumors and a mouse model of pancreatic cancer and specific Nox4 small-molecule inhibitors act synergistically with existing chemotherapic agents.

Translation‐dependent mechanisms lead to PML upregulation and mediate oncogenic K‐RAS‐induced cellular senescence

Expression of oncogenic K-RAS in primary cells elicits oncogene-induced cellular senescence (OIS), a form of growth arrest that potently opposes tumourigenesis. This effect has been largely attributed to transcriptional mechanisms that depend on the p53 tumour suppressor protein. The PML tumour suppressor was initially identified as a component of the PML-RARα oncoprotein of acute promyelocytic leukaemia (APL). PML, a critical OIS mediator, is upregulated by oncogenic K-RAS in vivo and in vitro. We demonstrate here that oncogenic K-RAS induces PML protein upregulation by activating the RAS/MEK1/mTOR/eIF4E pathway even in the absence of p53. Under these circumstances, PML mRNA is selectively associated to polysomes. Importantly, we find that the PML 5′ untranslated mRNA region plays a key role in mediating PML protein upregulation and that its presence is essential for an efficient OIS response. These findings demonstrate that upregulation of PML translation plays a central role in oncogenic K-RAS-induced OIS. Thus, selective translation initiation plays a critical role in tumour suppression with important therapeutic implications for the treatment of solid tumours and APL.

Abstract LB-128: The N-terminus of the ASPP2 tumor suppressor binds Ras and enhances Ras/Raf/MEK activation to promote oncogene-induced cellular senescence

Abstract The ASPP2 tumor suppressor is a pro-apoptotic member of a family of p53 binding proteins. It functions in part by selectively enhancing p53 transcriptional activation of pro-apoptotic target genes. Although the ASPP2 C-terminal domain binds the p53 core domain, little is known about the physical and functional interactions of other proteins to the highly structured ASPP2 N-terminus. Moreover, mounting evidence from our lab and others demonstrate that ASPP2 also mediates non-apoptotic p53-independent functions. However, the molecular mechanisms remain poorly understood. Previous reports have found that ASPP2 can localize to the cytoplasm and have identified a potential Ras-association domain in the ASPP2 N-terminus. Therefore, we wondered if novel ASPP2 functions could be mediated by ASPP2 N-terminal interaction with cytoplasmic Ras. Using co-immunoprecipitation and immunofluorescence microscopy, we identified ASPP2 in a complex with active Ras on the plasma membrane. Moreover, we also found that this interaction required the ASPP2 N-terminus since BBP, a naturally occurring alternatively spliced ASPP2 isoform lacking the N-terminus, showed markedly reduced ability to bind Ras. To determine if ASPP2 could modulate Ras function, we ectopically expressed ASPP2 in cells and found enhancement of Ras activation and subsequent stimulation of ERK phosphorylation. Conversely, ASPP2 knockdown by siRNA drastically reduced ERK phosphorylation and activation by EGF. Although it is oncogenic in tumor cells, constitutive Ras activation in normal cells leads to premature senescence in a Raf-MEK-ERK activation dependent manner. To explore if ASPP2 enhanced senescence, we knocked down ASPP2 expression with siRNA and found that senescence induced by RasV12 lentiviral infection in normal human fibroblasts was significantly attenuated. This was similar to the attenuation of senescence achieved by the MEK inhibitor PD98059. Together, our results demonstrate a novel function of the ASPP2 N-terminus in promoting cellular senescence via Ras-interaction and stimulation of Raf-MEK-ERK activity. These findings suggest a novel mechanism by which ASPP2-induced senescence may mediate its tumor suppression ability, and opens new avenues for investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-128. doi:10.1158/1538-7445.AM2011-LB-128