Marie-France Gaumont-Leclerc

Regulation of E2Fs and senescence by PML nuclear bodies

The tumor suppressor PML (promyelocytic leukemia protein) regulates cellular senescence and terminal differentiation, two processes that implicate a permanent exit from the cell cycle. Here, we show that the mechanism by which PML induces a permanent cell cycle exit and activates p53 and senescence involves a recruitment of E2F transcription factors bound to their promoters and the retinoblastoma (Rb) proteins to PML nuclear bodies enriched in heterochromatin proteins and protein phosphatase 1α. Blocking the functions of the Rb protein family or adding back E2Fs to PML-expressing cells can rescue their defects in E2F-dependent gene expression and cell proliferation, inhibiting the senescent phenotype. In benign prostatic hyperplasia, a neoplastic disease that displays features of senescence, PML was found to be up-regulated and forming nuclear bodies. In contrast, PML bodies were rarely visualized in prostate cancers. The newly defined PML/Rb/E2F pathway may help to distinguish benign tumors from cancers, and suggest E2F target genes as potential targets to induce senescence in human tumors.

Human fibroblasts require the Rb family of tumor suppressors, but not p53, for PML-induced senescence.

Cellular senescence is a permanent cell cycle arrest that can be triggered by a variety of stresses including short telomeres and activated oncogenes. Promyelocytic leukemia protein (PML) is a central component of the senescence response, and is able to trigger the process when overexpressed in human diploid fibroblasts (HDFs). Senescence induced by PML in HDFs is characterized by a modest increase in p53 levels and activity, the accumulation of hypophosphorylated Rb and a reduced expression of E2F-dependent genes. To dissect the p53 and Rb family requirements for PML-induced senescence, we used the oncoproteins E6 and E7 from human papillomavirus type 16. We found that the coexpression of E6 and E7 inhibited the growth arrest and senescence induced by PML. In addition, these viral oncoproteins blocked the formation of PML bodies and excluded both p53 and Rb from PML bodies. Expression of dominant-negative p53 alone failed to block PML-induced senescence and expression of E6 only delayed the process. On the other hand, expression of E7 was sufficient to block PML-induced senescence, while an E7 mutant unable to bind Rb did not. Together, these data indicate that PML-induced senescence engages the Rb tumor-suppressor pathway predominantly.

Tumor suppressor activity of the ERK/MAPK pathway by promoting selective protein degradation

Constitutive activation of growth factor signaling pathways paradoxically triggers a cell cycle arrest known as cellular senescence. In primary cells expressing oncogenic ras, this mechanism effectively prevents cell transformation. Surprisingly, attenuation of ERK/MAP kinase signaling by genetic inactivation of Erk2, RNAi-mediated knockdown of ERK1 or ERK2, or MEK inhibitors prevented the activation of the senescence mechanism, allowing oncogenic ras to transform primary cells. Mechanistically, ERK-mediated senescence involved the proteasome-dependent degradation of proteins required for cell cycle progression, mitochondrial functions, cell migration, RNA metabolism, and cell signaling. This senescence-associated protein degradation (SAPD) was observed not only in cells expressing ectopic ras, but also in cells that senesced due to short telomeres. Individual RNAi-mediated inactivation of SAPD targets was sufficient to restore senescence in cells transformed by oncogenic ras or trigger senescence in normal cells. Conversely, the anti-senescence viral oncoproteins E1A, E6, and E7 prevented SAPD. In human prostate neoplasms, high levels of phosphorylated ERK were found in benign lesions, correlating with other senescence markers and low levels of STAT3, one of the SAPD targets. We thus identified a mechanism that links aberrant activation of growth signaling pathways and short telomeres to protein degradation and cellular senescence.

Myc Down-regulation as a Mechanism to Activate the Rb Pathway in STAT5A-induced Senescence

Senescence is a general antiproliferative program that avoids the expansion of cells bearing oncogenic mutations. We found that constitutively active STAT5A (ca-STAT5A) can induce a p53- and Rb-dependent cellular senescence response. However, ca-STAT5A did not induce p21 and p16INK4a, which are responsible for inhibiting cyclin-dependent protein kinases and engaging the Rb pathway during the senescence response to oncogenic ras. Intriguingly, ca-STAT5A led to a down-regulation of Myc and Myc targets, including CDK4, a negative regulator of Rb. The down-regulation of Myc was in part proteasome-dependent and correlated with its localization to promyelocytic leukemia bodies, which were found to be highly abundant during STAT5-induced senescence. Introduction of CDK4 or Myc bypassed STAT5A-induced senescence in cells in which p53 was also inactivated. These results uncover a novel mechanism to engage the Rb pathway in oncogene-induced senescence and indicate the existence of oncogene-specific pathways that regulate senescence.

Designing small multiple-target artificial RNAs

MicroRNAs (miRNAs) are naturally occurring small RNAs that regulate the expression of several genes. MiRNAs’ targeting rules are based on sequence complementarity between their mature products and targeted genes’ mRNAs. Based on our present understanding of those rules, we developed an algorithm to design artificial miRNAs to target simultaneously a set of predetermined genes. To validate in silico our algorithm, we tested different sets of genes known to be targeted by a single miRNA. The algorithm finds the seed of the corresponding miRNA among the solutions, which also include the seeds of new artificial miRNA sequences potentially capable of targeting these genes as well. We also validated the functionality of some artificial miRNAs designed to target simultaneously members of the E2F family. These artificial miRNAs reproduced the effects of E2Fs inhibition in both normal human fibroblasts and prostate cancer cells where they inhibited cell proliferation and induced cellular senescence. We conclude that the current miRNA targeting rules based on the seed sequence work to design multiple-target artificial miRNAs. This approach may find applications in both research and therapeutics.

Cellular senescence and protein degradation: Breaking down cancer

Autophagy and the ubiquitin–proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Transcriptome analysis and tumor suppressor requirements of STAT5-induced senescence.

Although it is acknowledged that senescent cells accumulate with age, the molecular mechanisms leading to cell senescence as a function of age remain to be identified. In cell culture models, it has been clearly shown that senescence involves the activation of a DNA damage response secondary to short telomeres or oncogene expression. Oncogenes are altered versions of genes coding for proteins that mediate signals from extracellular factors such as cytokines, growth factors, and hormones. In particular, we show here that constitutive activation of the JAK/STAT5 signaling pathway induces senescence in both mouse and human normal cells. The process involves activation of the p53 and Rb tumor suppressor pathways and mitochondrial dysfunction. Gene expression analysis of STAT5‐induced senescence revealed changes in the expression of genes coding for cytokines, proteins in cytokine signaling pathways, and several metabolic enzymes. We discuss a model called senescence‐induced senescence, in which cytokines secreted by senescent cells can propagate the process as a function of age.

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

CHES1/FOXN3 inhibits cell proliferation and protein biosynthesis in tumor cell lines but not in normal fibroblasts. CHES1 directly represses the expression of the gene coding for the protein kinase PIM2, and PIM2 or eIF4E counteracts the antiproliferative effect of CHES1. The levels of CHES1 and PIM2 are inversely correlated in several human cancers.

Abstract 4166: Tumor suppressor activity of the ERK/MAPK pathway by promoting selective protein degradation

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL We report that ERK/MAPK signaling mediates tumor suppression. Oncogenic forms of ras are found in approximately 25% of all human cancers and are generally associated with uncontrolled proliferation of cancer cells. However, expression of oncogenic ras in normal cells is associated with cellular senescence. In primary cells expressing oncogenic ras, ERK knockdown by shRNA prevented the activation of the DNA damage response, p53, p16INK4a/Rb and senescence and cooperated with hTERT in transformation. In human prostate neoplasms, high levels of phosphorylated-ERK were found in benign lesions while in malignant lesions lower levels correlated with a higher Gleason score and earlier disease relapse after treatment. Mechanistically, the antitumor effects of ERK/MAPK signaling included the selective proteasome-dependent degradation of proteins required for cell cycle progression, mitochondrial functions and cell signaling and the inhibition of the phosphoinositide-3-kinase/AKT (PI3K/AKT) signaling pathway. We thus show for the first time that the outcome of ERK/MAPK signaling can be controlled at the level of ERK1/2, the final members of the pathway. A quantitative control at the end of the pathway provides a mechanism for specific modulation of ERK/MAPK signaling since controlling the input may also affect the numerous signaling pathways activated by membrane receptors. The data also indicate that the levels of the ERK kinases control a binary switch between cell proliferation and senescence and explain the maintenance of the senescence phenotype despite the presence of constitutively activated growth factor signaling pathways. Since inhibitors of the ERK pathway are under consideration for clinical use, we caution that they may trigger the escape of dormant senescent cells leading to malignant tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4166. doi:1538-7445.AM2012-4166

Abstract 1061: Context dependent tumor suppressor activity of the ERK pathway explains its inverse correlation with malignancy in prostate neoplasms

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL This study gives considerable insight about the antagonistic functions associated to the ERK1/2 kinases activation during tumorigenesis. The Ras proteins are small GTPases known for their role in growth factor signals transmission from membrane receptors. Oncogenic forms of Ras are found in approximately 25% of all human cancers and are generally associated with uncontrolled cell proliferation. However, expression of oncogenic Ras in normal cells is associated with accumulation of DNA damage, activation of tumor suppressors p53 and Rb and cellular senescence. It is known that Ras activates multiple signaling pathways, such as the PI3K/Akt pathway, the Ral pathway and the classical Raf/Mek/Erk MAP Kinase pathway. Our study demonstrates that Erk1/2 (Extracellular-Regulated Kinases) are crucial for H-RasV12-induced senescence in normal human fibroblasts. We have shown that the expression of different small hairpin RNAs against Erk1 or Erk2 in normal human fibroblasts causes an important bypass of H-RasV12-induced senescence and the acquisition of several characteristics of transformed cells. This finding strongly suggests that the Raf/Mek/Erk pathway can be anti-oncogenic in some contexts by activating the tumor suppressor pathways regulating senescence. We propose a model where a moderated level of activated Erk promotes proliferation and potentially transformation, but a higher level of activated Erk, over a given threshold, activates senescence. Consistent with our model, expression studies in benign prostatic hyperplasia (BPH), a benign tumor of the prostate, reveal that BPH cells display high level of senescence markers and activated Erk in comparison with normal cells and many tumor samples. Similarly, we observed a negative correlation between the p-Erk1/2 levels in the nuclei of cancer cells from different prostate tumors and the Gleason score or the biochemical relapse after chemotherapy. Such correlations suggest that cells with lower p-Erk1/2 levels are in general from more aggressive prostate cancers. To explain the phenotype of transformed cells that bypass senescence in our experiments, an analysis of signaling pathways known to contribute to transformation by Ras was performed. We observed that the Raf/Mek/Erk pathway exerts a negative control on the PI3K/Akt pathway, itself activated by Ras. Thus, cells that bypass H-RasV12-induced senescence by decreasing Raf/Mek/Erk activity show a strong hyperactivation of Akt, which could explain their anarchic proliferation. Our results suggest caution in the clinical use of inhibitors of the ERK pathway alone but also the possibility of designing novel antitumor agents that increase the ERK activity over the threshold required to induce cellular senescence. 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 1061. doi:10.1158/1538-7445.AM2011-1061