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

A senescence restriction point acting on chromatin integrates oncogenic signals

We identify a senescence restriction point (SeRP) as a critical event for cells to commit to senescence. The SeRP integrates the intensity and duration of oncogenic stress, keeps a memory of previous stresses, and combines oncogenic signals acting on different pathways by modulating chromatin accessibility. Chromatin regions opened upon commitment to senescence are enriched in nucleolar-associated domains, which are gene-poor regions enriched in repeated sequences. Once committed to senescence, cells no longer depend on the initial stress signal and exhibit a characteristic transcriptome regulated by a transcription factor network that includes ETV4, RUNX1, OCT1, and MAFB. Consistent with a tumor suppressor role for this network, the levels of ETV4 and RUNX1 are very high in benign lesions of the pancreas but decrease dramatically in pancreatic ductal adenocarcinomas. The discovery of senescence commitment and its chromatin-linked regulation suggests potential strategies for reinstating tumor suppression in human cancers.

Disruption of the MYC Superenhancer Complex by Dual Targeting of FLT3 and LSD1 in Acute Myeloid Leukemia

Mutations in Fms-like tyrosine kinase 3 (FLT3) are common drivers in acute myeloid leukemia (AML) yet FLT3 inhibitors only provide modest clinical benefit. Prior work has shown that inhibitors of lysine-specific demethylase 1 (LSD1) enhance kinase inhibitor activity in AML. Here we show that combined LSD1 and FLT3 inhibition induces synergistic cell death in FLT3-mutant AML. Multi-omic profiling revealed that the drug combination disrupts STAT5, LSD1, and GFI1 binding at the MYC blood superenhancer, suppressing superenhancer accessibility as well as MYC expression and activity. The drug combination simultaneously results in the accumulation of repressive H3K9me1 methylation, an LSD1 substrate, at MYC target genes. We validated these findings in 72 primary AML samples with the nearly every sample demonstrating synergistic responses to the drug combination. Collectively, these studies reveal how epigenetic therapies augment the activity of kinase inhibitors in FLT3-ITD (internal tandem duplication) AML.

IMPLICATIONS

This work establishes the synergistic efficacy of combined FLT3 and LSD1 inhibition in FLT3-ITD AML by disrupting STAT5 and GFI1 binding at the MYC blood-specific superenhancer complex.

CTCF controls three-dimensional enhancer network underlying the inflammatory response of bone marrow-derived dendritic cells

Dendritic cells are antigen-presenting cells orchestrating innate and adaptive immunity. The crucial role of transcription factors and histone modifications in the transcriptional regulation of dendritic cells has been extensively studied. However, it is not been well understood whether and how three-dimensional chromatin folding controls gene expression in dendritic cells. Here we demonstrate that activation of bone marrow-derived dendritic cells induces extensive reprogramming of chromatin looping as well as enhancer activity, both of which are implicated in the dynamic changes in gene expression. Interestingly, depletion of CTCF attenuates GM-CSF-mediated JAK2/STAT5 signaling, resulting in defective NF-κB activation. Moreover, CTCF is necessary for establishing NF-κB-dependent chromatin interactions and maximal expression of pro-inflammatory cytokines, which prime Th1 and Th17 cell differentiation. Collectively, our study provides mechanistic insights into how three-dimensional enhancer networks control gene expression during bone marrow-derived dendritic cells activation, and offers an integrative view of the complex activities of CTCF in the inflammatory response of bone marrow-derived dendritic cells.

Changes of Gene Expression Patterns of Muscle Pathophysiology-Related Transcription Factors During Denervated Muscle Atrophy

Peripheral nerve injury is common, and can lead to skeletal muscle atrophy and dysfunction. However, the underlying molecular mechanisms are not fully understood. The transcription factors have been proved to play a key role in denervated muscle atrophy. In order to systematically analyze transcription factors and obtain more comprehensive information of the molecular regulatory mechanisms in denervated muscle atrophy, a new transcriptome survey focused on transcription factors are warranted. In the current study, we used microarray to identify and analyze differentially expressed genes encoding transcription factors in denervated muscle atrophy in a rat model of sciatic nerve dissection. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses were used to explore the biological functions of differentially expressed transcription factors and their target genes related to skeletal muscle pathophysiology. We found that the differentially expressed transcription factors were mainly involved in the immune response. Based on correlation analysis and the expression trends of transcription factors, 18 differentially expressed transcription factors were identified. Stat3, Myod1, Runx1, Atf3, Junb, Runx2, Myf6, Stat5a, Tead4, Klf5, Myog, Mef2a, and Hes6 were upregulated. Ppargc1a, Nr4a1, Lhx2, Ppara, and Rxrg were downregulated. Functional network mapping revealed that these transcription factors are mainly involved in inflammation, development, aging, proteolysis, differentiation, regeneration, autophagy, oxidative stress, atrophy, and ubiquitination. These findings may help understand the regulatory mechanisms of denervated muscle atrophy and provide potential targets for future therapeutic interventions for muscle atrophy following peripheral nerve injury.

Inhibition of the PLK1-Coupled Cell Cycle Machinery Overcomes Resistance to Oxaliplatin in Colorectal Cancer

Dysregulation of the cell cycle machinery leads to genomic instability and is a hallmark of cancer associated with chemoresistance and poor prognosis in colorectal cancer (CRC). Identifying and targeting aberrant cell cycle machinery is expected to improve current therapies for CRC patients. Here,upregulated polo-like kinase 1 (PLK1) signaling, accompanied by deregulation of cell cycle-related pathways in CRC is identified. It is shown that aberrant PLK1 signaling correlates with recurrence and poor prognosis in CRC patients. Genetic and pharmacological blockade of PLK1 significantly increases the sensitivity to oxaliplatin in vitro and in vivo. Mechanistically, transcriptomic profiling analysis reveals that cell cycle-related pathways are activated by oxaliplatin treatment but suppressed by a PLK1 inhibitor. Cell division cycle 7 (CDC7) is further identified as a critical downstream effector of PLK1 signaling, which is transactivated via the PLK1-MYC axis. Increased CDC7 expression is also found to be positively correlated with aberrant PLK1 signaling in CRC and is associated with poor prognosis. Moreover, a CDC7 inhibitor synergistically enhances the anti-tumor effect of oxaliplatin in CRC models, demonstrating the potential utility of targeting the PLK1-MYC-CDC7 axis in the treatment of oxaliplatin-based chemotherapy.

Methods to Study Myc-Regulated Cellular Senescence: An Update

Cellular senescence plays a role in several physiological processes including aging, embryonic development, tissue remodeling, and wound healing and is considered one of the main barriers against tumor development. Studies of normal and tumor cells both in culture and in vivo suggest that MYC plays an important role in regulating senescence, thereby contributing to tumor development. We have previously described different common methods to measure senescence in cell cultures and in tissues. Unfortunately, there is no unique marker that unambiguously defines a senescent state, and it is therefore necessary to combine measurements of several different markers in order to assure the correct identification of senescent cells. Here we describe protocols for simultaneous detection of multiple senescence markers in situ, a quantitative fluorogenic method to measure senescence-associated β-galactosidase activity (SA-β-gal), and a new method to detect senescent cells based on the Sudan Black B (SBB) analogue GL13, which is applicable to formalin-fixed paraffin-embedded tissues. The application of these methods in various systems will hopefully shed further light on the role of MYC in regulation of senescence, and how that impacts normal physiological processes as well as diseases and in particular cancer development.

Pharmacological inactivation of CDK2 inhibits MYC/BCL-XL-driven leukemia in vivo through induction of cellular senescence

Deregulated expression of the MYC oncogene is a frequent event during tumorigenesis and generally correlates with aggressive disease and poor prognosis. While MYC is a potent inducer of apoptosis, it often suppresses cellular senescence, which together with apoptosis is an important barrier against tumor development. For this latter function, MYC is dependent on cyclin-dependent kinase 2 (CDK2). Here, we utilized a MYC/BCL-X_L-driven mouse model of acute myeloblastic leukemia (AML) to investigate whether pharmacological inhibition of CDK2 can inhibit MYC-driven tumorigenesis through induction of senescence. Purified mouse hematopoietic stem cells transduced with MYC and BCL-X_L were transplanted into lethally irradiated mice, leading to the development of massive leukemia and subsequent death 15–17 days after transplantation. Upon disease onset, mice were treated with the selective CDK2 inhibitor CVT2584 or vehicle either by daily intraperitoneal injections or continuous delivery via mini-pumps. CVT2584 treatment delayed disease onset and moderately but significantly improved survival of mice. Flow cytometry revealed a significant decrease in tumor load in the spleen, liver and bone marrow of CVT2584-treated compared to vehicle-treated mice. This was correlated with induced senescence evidenced by reduced cell proliferation, increased senescence-associated β-galactosidase activity and heterochromatin foci, expression of p19^ARF and p21^CIP1, and reduced phosphorylation (activation) of pRb, while very few apoptotic cells were observed. In addition, phosphorylation of MYC at Ser-62 was decreased. In summary, inhibition of CDK2 delayed MYC/BCL-X_L-driven AML linked to senescence induction. Our results suggest that CDK2 is a promising target for pro-senescence cancer therapy, in particular for MYC-driven tumors, including leukemia.

STAT3 and STAT5 Activation in Solid Cancers

The Signal Transducer and Activator of Transcription (STAT)3 and 5 proteins are activated by many cytokine receptors to regulate specific gene expression and mitochondrial functions. Their role in cancer is largely context-dependent as they can both act as oncogenes and tumor suppressors. We review here the role of STAT3/5 activation in solid cancers and summarize their association with survival in cancer patients. The molecular mechanisms that underpin the oncogenic activity of STAT3/5 signaling include the regulation of genes that control cell cycle and cell death. However, recent advances also highlight the critical role of STAT3/5 target genes mediating inflammation and stemness. In addition, STAT3 mitochondrial functions are required for transformation. On the other hand, several tumor suppressor pathways act on or are activated by STAT3/5 signaling, including tyrosine phosphatases, the sumo ligase Protein Inhibitor of Activated STAT3 (PIAS3), the E3 ubiquitin ligase TATA Element Modulatory Factor/Androgen Receptor-Coactivator of 160 kDa (TMF/ARA160), the miRNAs miR-124 and miR-1181, the Protein of alternative reading frame 19 (p19ARF)/p53 pathway and the Suppressor of Cytokine Signaling 1 and 3 (SOCS1/3) proteins. Cancer mutations and epigenetic alterations may alter the balance between pro-oncogenic and tumor suppressor activities associated with STAT3/5 signaling, explaining their context-dependent association with tumor progression both in human cancers and animal models.

mTOR as a central regulator of lifespan and aging

The mammalian/mechanistic target of rapamycin (mTOR) is a key component of cellular metabolism that integrates nutrient sensing with cellular processes that fuel cell growth and proliferation. Although the involvement of the mTOR pathway in regulating life span and aging has been studied extensively in the last decade, the underpinning mechanisms remain elusive. In this review, we highlight the emerging insights that link mTOR to various processes related to aging, such as nutrient sensing, maintenance of proteostasis, autophagy, mitochondrial dysfunction, cellular senescence, and decline in stem cell function.

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.

Phosphorylated STAT5 regulates p53 expression via BRCA1/BARD1-NPM1 and MDM2

Signal transducer and activator of transcription 5 (STAT5) and nucleophosmin (NPM1) are critical regulators of multiple biological and pathological processes. Although a reciprocal regulatory relationship was established between STAT5A and a NPM–ALK fusion protein in T-cell lymphoma, no direct connection between STAT5 and wild-type NPM1 has been documented. Here we demonstrate a mutually regulatory relationship between STAT5 and NPM1. Induction of STAT5 phosphorylation at Y694 (P-STAT5) diminished NPM1 expression, whereas inhibition of STAT5 phosphorylation enhanced NPM1 expression. Conversely, NPM1 not only negatively regulated STAT5 phosphorylation but also preserved unphosphorylated STAT5 level. Mechanistically, we show that NPM1 downregulation by P-STAT5 is mediated by impairing the BRCA1-BARD1 ubiquitin ligase, which controls the stability of NPM1. In turn, decreased NPM1 levels led to suppression of p53 expression, resulting in enhanced cell survival. This study reveals a new STAT5 signaling pathway regulating p53 expression via NPM1 and uncovers new therapeutic targets for anticancer treatment in tumors driven by STAT5 signaling.

STAT5A is regulated by DNA damage via the tumor suppressor p53

Here we report that the STAT5A transcription factor is a direct p53 transcriptional target gene. STAT5A is well expressed in p53 wild type cells but not in p53-null cells. Inhibition of p53 reduces STAT5A expression. DNA damaging agents such as doxorubicin also induced STAT5A expression in a p53 dependent manner. Two p53 binding sites were mapped in the STAT5A gene and named PBS1 and PBS2; these sites were sufficient to confer p53 responsiveness in a luciferase reporter gene. Chromatin immunoprecipitation experiments revealed that PBS2 has constitutive p53 bound to it, while p53 binding to PBS1 required DNA damage. In normal human breast lobules, weak p53 staining correlated with regions of intense STAT5A staining. Interestingly, in a cohort of triple negative breast tumor tissues there was little correlation between regions of p53 and STAT5A staining, likely reflecting a high frequency of p53 mutations that stabilize the protein in these tumors. We thus reveal an unexpected connection between cytokine signaling and p53.

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.

PLA2R1: expression and function in cancer

The phospholipase A2 receptor 1 (PLA2R1 or PLA2R) was isolated twenty years ago for its ability to bind several secretory phospholipase A2 proteins (sPLA2). Since its identification, it has attracted only a limited interest, mainly in the sPLA2 biology field, as it is viewed uniquely as a regulator of sPLA2 activities. Recent discoveries outline novel important functions of this gene in cancer biology. Indeed, PLA2R1 gain or loss of function experiments in vitro and in vivo shows that this receptor promotes several tumor suppressive responses including senescence, apoptosis and inhibition of transformation. Supporting a tumor suppressive role of PLA2R1, its expression decreases in numerous cancers, and known oncogenes such as HIF2α and c-MYC repress its expression. PLA2R1 promoter methylation, a classical way to repress tumor suppressive gene expression in cancer cells, is observed in leukemia, in kidney and in breast cancer cells. Mechanistically, PLA2R1 activates the kinase JAK2 and orients its activity towards a tumor suppressive one. PLA2R1 also promotes accumulation of reactive oxygen species which induce cell death and senescence. This review compiles recent data demonstrating an unexpected tumor suppressive role of PLA2R1 and outlines the future work needed to improve our knowledge of the functions of this gene in cancer.

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.

Cystathionase mediates senescence evasion in melanocytes and melanoma cells

The development of malignant melanoma is a highly complex process, which is still poorly understood. A majority of human melanomas are found to express a few oncogenic proteins, such as mutant RAS and BRAF variants. However, these oncogenes are also found in nevi, and it is now a well-accepted fact that their expression alone leads to senescence. This renders the understanding of senescence escape mechanisms an important point to understand tumor development. Here, we approached the question of senescence evasion by expressing the transcription factor v-myc myelocytomatosis viral oncogene homolog (c-MYC), which is known to act synergistically with many oncogenes, in melanocytes. We observed that MYC drives the evasion of reactive-oxygen stress-induced melanocyte senescence, caused by activated receptor tyrosine kinase signaling. Conversely, MIZ1, the growth suppressing interaction partner of MYC, is involved in mediating melanocyte senescence. Both, MYC overexpression and Miz1 knockdown led to a strong reduction of endogenous reactive-oxygen species (ROS), DNA damage and senescence. We identified the cystathionase (CTH) gene product as mediator of the ROS-related MYC and MIZ1 effects. Blocking CTH enzymatic activity in MYC-overexpressing and Miz1 knockdown cells increased intracellular stress and senescence. Importantly, pharmacological inhibition of CTH in human melanoma cells also reconstituted senescence in the majority of cell lines, and CTH knockdown reduced tumorigenic effects such as proliferation, H2O2 resistance and soft agar growth. Thus, we identified CTH as new MYC target gene with an important function in senescence evasion.

PAK-dependent STAT5 serine phosphorylation is required for BCR-ABL-induced leukemogenesis

The transcription factor STAT5 (signal transducer and activator of transcription 5) is frequently activated in hematological malignancies and represents an essential signaling node downstream of the BCR-ABL oncogene. STAT5 can be phosphorylated at three positions, on a tyrosine and on the two serines S725 and S779. We have investigated the importance of STAT5 serine phosphorylation for BCR-ABL-induced leukemogenesis. In cultured bone marrow cells, expression of a STAT5 mutant lacking the S725 and S779 phosphorylation sites (STAT5(SASA)) prohibits transformation and induces apoptosis. Accordingly, STAT5(SASA) BCR-ABL(+) cells display a strongly reduced leukemic potential in vivo, predominantly caused by loss of S779 phosphorylation that prevents the nuclear translocation of STAT5. Three distinct lines of evidence indicate that S779 is phosphorylated by group I p21-activated kinase (PAK). We show further that PAK-dependent serine phosphorylation of STAT5 is unaffected by BCR-ABL tyrosine kinase inhibitor treatment. Interfering with STAT5 phosphorylation could thus be a novel therapeutic approach to target BCR-ABL-induced malignancies.

c-Myc is essential to prevent endothelial pro-inflammatory senescent phenotype

The proto-oncogene c-Myc is vital for vascular development and promotes tumor angiogenesis, but the mechanisms by which it controls blood vessel growth remain unclear. In the present work we investigated the effects of c-Myc knockdown in endothelial cell functions essential for angiogenesis to define its role in the vasculature. We provide the first evidence that reduction in c-Myc expression in endothelial cells leads to a pro-inflammatory senescent phenotype, features typically observed during vascular aging and pathologies associated with endothelial dysfunction. c-Myc knockdown in human umbilical vein endothelial cells using lentivirus expressing specific anti-c-Myc shRNA reduced proliferation and tube formation. These functional defects were associated with morphological changes, increase in senescence-associated-β-galactosidase activity, upregulation of cell cycle inhibitors and accumulation of c-Myc-deficient cells in G1-phase, indicating that c-Myc knockdown in endothelial cells induces senescence. Gene expression analysis of c-Myc-deficient endothelial cells showed that senescent phenotype was accompanied by significant upregulation of growth factors, adhesion molecules, extracellular-matrix components and remodeling proteins, and a cluster of pro-inflammatory mediators, which include Angptl4, Cxcl12, Mdk, Tgfb2 and Tnfsf15. At the peak of expression of these cytokines, transcription factors known to be involved in growth control (E2f1, Id1 and Myb) were downregulated, while those involved in inflammatory responses (RelB, Stat1, Stat2 and Stat4) were upregulated. Our results demonstrate a novel role for c-Myc in the prevention of vascular pro-inflammatory phenotype, supporting an important physiological function as a central regulator of inflammation and endothelial dysfunction.

IL-6-STAT3 signaling and premature senescence

Cytokines play several roles in developing and/or reinforcing premature cellular senescence of young cells. One such cytokine, interleukin-6 (IL-6), regulates senescence in some systems in addition to its known functions of immune regulation and promotion of tumorigenesis. In this review, we describe recent advances in studies on the roles of IL-6 and its downstream signal transducer and activator of transcription 3 (STAT3) in regulating premature cellular senescence. IL-6/sIL-6Rα stimulation forms a senescence-inducing circuit involving the STAT3-insulin-like growth factor-binding protein 5 (IGFBP5) as a key axis triggering and reinforcing component in human fibroblasts. We describe how cytokines regulate the process of senescence by activating STAT3 in one system and anti-senescence or tumorigenesis in other systems. The roles of other STAT members in premature senescence also will be discussed to show the multiple mechanisms leading to cytokine-induced senescence.

Preliminary study on XAGE-1b gene and its mechanism for promoting tumor cell growth

The XAGE-1b gene has been identified in numerous malignancies in the human body. However, little is known regarding its mechanism for promoting tumorigenesis in adenoid cystic carcinoma. The aim of this study was to explore the correlation between tumor cell growth and the XAGE-1b gene. The constructed PCMV-Myc plasmid vector containing the XAGE-1b gene and transfected adenoid cystic carcinoma (ACC)-2 cells was applied to study cell cycle alterations and anti-apoptotic effects. These were assessed by flow cytometry with PI staining and the measurement of cell content at its Sub-G₁ phase, respectively. The fluorescence intensity representing the regulation of XAGE-1b on the transcription factors located downstream of the signaling pathway using the Mercury pathway profiling system was also detected. XAGE-1b over expression promoted cell growth by shortening G₀-G₁ and prolonging the G₂-M phase. Additionally, XAGE-1b overexpression enhanced the anti-apoptotic effects induced by tumor necrosis factor-α (TNF-α) and serum deprivation in ACC-2 cells. The results of the present study suggested that XAGE-1b gene is crucial in the tumorigenesis of ACC, and its mechanism should be further investigated.

Methods to study MYC-regulated cellular senescence

Studies in primary and tumor cells suggest that MYC plays an important role in regulating cellular senescence, thereby impacting on tumor development. Here we describe different common methods to measure senescence in cell cultures and in tissues. These include measurement of senescence-associated β-galactosidase activity (SA-β-gal), senescence-associated heterochromatin foci (SAHFs), proliferative arrest, morphological changes, and expression and activity of proteins involved in the senescence process, such as p53 and Rb pathway proteins and secretory proteins. It is important to note that there is no unique marker that unambiguously defines a senescent state, and it is therefore necessary to combine measurements of several different markers that together determine whether cells are senescent or not. Measurement of senescence is an important aspect of studies of MYC biology and will improve our understanding of MYC function and regulation both in preclinical and clinical settings. This may form the basis for new concepts of pro-senescence therapy to combat MYC in cancer.

Hepatic growth hormone and glucocorticoid receptor signaling in body growth, steatosis and metabolic liver cancer development

Growth hormone (GH) and glucocorticoids (GCs) are involved in the control of processes that are essential for the maintenance of vital body functions including energy supply and growth control. GH and GCs have been well characterized to regulate systemic energy homeostasis, particular during certain conditions of physical stress. However, dysfunctional signaling in both pathways is linked to various metabolic disorders associated with aberrant carbohydrate and lipid metabolism. In liver, GH-dependent activation of the transcription factor signal transducer and activator of transcription (STAT) 5 controls a variety of physiologic functions within hepatocytes. Similarly, GCs, through activation of the glucocorticoid receptor (GR), influence many important liver functions such as gluconeogenesis. Studies in hepatic Stat5 or GR knockout mice have revealed that they similarly control liver function on their target gene level and indeed, the GR functions often as a cofactor of STAT5 for GH-induced genes. Gene sets, which require physical STAT5-GR interaction, include those controlling body growth and maturation. More recently, it has become evident that impairment of GH-STAT5 signaling in different experimental models correlates with metabolic liver disease, ranging from hepatic steatosis to hepatocellular carcinoma (HCC). While GH-activated STAT5 has a protective role in chronic liver disease, experimental disruption of GC-GR signaling rather seems to ameliorate metabolic disorders under metabolic challenge. In this review, we focus on the current knowledge about hepatic GH-STAT5 and GC-GR signaling in body growth, metabolism, and protection from fatty liver disease and HCC development.

Interleukin 6 signaling regulates promyelocytic leukemia protein gene expression in human normal and cancer cells

Tumor suppressor PML is induced under viral and genotoxic stresses by interferons and JAK-STAT signaling. However, the mechanism responsible for its cell type-specific regulation under non-stimulated conditions is poorly understood. To analyze the variation of PML expression, we utilized three human cell types, BJ fibroblasts and HeLa and U2OS cell lines, each with a distinct PML expression pattern. Analysis of JAK-STAT signaling in the three cell lines revealed differences in levels of activated STAT3 but not STAT1 correlating with PML mRNA and protein levels. RNAi-mediated knockdown of STAT3 decreased PML expression; both STAT3 level/activity and PML expression relied on IL6 secreted into culture media. We mapped the IL6-responsive sequence to an ISRE(-595/-628) element of the PML promoter. The PI3K/Akt/NFκB branch of IL6 signaling showed also cell-type dependence, being highest in BJ, intermediate in HeLa, and lowest in U2OS cells and correlated with IL6 secretion. RNAi-mediated knockdown of NEMO (NF-κ-B essential modulator), a key component of NFκB activation, suppressed NFκB targets LMP2 and IRF1 together with STAT3 and PML. Combined knockdown of STAT3 and NEMO did not further promote PML suppression, and it can be bypassed by exogenous IL6, indicating the NF-κB pathway acts upstream of JAK-STAT3 through induction of IL6. Our results indicate that the cell type-specific activity of IL6 signaling pathways governs PML expression under unperturbed growth conditions. As IL6 is induced in response to various viral and genotoxic stresses, this cytokine may regulate autocrine/paracrine induction of PML under these pathophysiological states as part of tissue adaptation to local stress.

Oncogene- and tumor suppressor gene-mediated suppression of cellular senescence

Data accumulating during the last two decades suggest that tumorigenesis is held in check by two major intrinsic failsafe mechanisms; apoptosis and cellular senescence. While apoptosis is a programmed cell death process, cellular senescence, which is the focus of this article, is defined as irreversible cell cycle arrest. This process is triggered either by telomere erosion or by acute stress signals including oncogenic stress induced by overactive oncogenes or underactive tumor suppressor genes. The outcome of this is often replication overload and oxidative stress resulting in DNA damage. Oncogenic stress induces at least three intrinsic pathways, p16/pRb-, Arf/p53/p21- and the DNA damage response (DDR)-pathways, that induce premature senescence if the stress exceeds a threshold level. Oncogene-induced senescence (OIS) is frequently observed in premalignant lesions both in animal tumor models and in human patients but is essentially absent in advanced cancers, suggesting that malignant tumor cells have found ways to bypass or escape senescence. This review focuses on cell-autonomous mechanism by which certain oncogenes, tumor suppressor genes and components of the DDR/DNA-repair machinery suppress senescence - mechanisms that are exploited by tumor cells to evade senescence and continue to multiply. In this way, tumor cells become addicted to the continuous activity of senescence suppressor proteins. However, some senescence pathways, although under suppression, may remain intact and can be re-established if senescence suppressor proteins are inactivated or if senescence inducers are reactivated. This can hopefully form the basis for a "pro-senescence therapy" strategy to combat cancer in the future.

Troglitazone suppresses c-Myc levels in human prostate cancer cells via a PPARγ-independent mechanism

Troglitazone is a ligand for the peroxisome proliferator activated receptor gamma (PPARγ) that decreases growth of human prostate cancer cells in vitro and in vivo. However, the mechanism by which troglitazone reduces prostate cancer cell growth is not fully understood. To understand the signaling pathways involved in troglitazone-induced decreases in prostate cancer growth, we examined the effect of troglitazone on androgen-independent C4-2 human prostate cancer cells. Initial experiments revealed troglitazone inhibited C4-2 cell proliferation by arresting cells in the G(0)/G(1) phase of the cell cycle and inducing apoptosis. Since the proto-oncogene product c-Myc regulates both apoptosis and cell cycle progression, we next examined whether troglitazone altered expression of c-Myc. Troglitazone decreased c-Myc protein levels as well as expression of downstream targets of c-Myc in a dose-dependent manner. In C4-2 cells, troglitazone-induced decreases in c-Myc protein involve proteasome-mediated degradation of c-Myc protein as well as reductions in c-Myc mRNA levels. It appears that troglitazone stimulates degradation of c-Myc by increasing c-Myc phosphorylation, for the level of phosphorylated c-Myc was elevated in prostate cancer cells exposed to troglitazone. While troglitazone dramatically decreased the amount of c-Myc within C4-2 cells, the PPARγ ligands ciglitazone, rosiglitazone and pioglitazone did not reduce c-Myc protein levels. Furthermore the down-regulation of c-Myc by troglitazone was not blocked by the PPARγ antagonist GW9662 and siRNA-mediated decreases in PPARγ protein. Thus, our data suggest that troglitazone reduces c-Myc protein independently of PPARγ.

Angiogenic properties of aged adipose derived mesenchymal stem cells after hypoxic conditioning

Background

Mesenchymal stem cells derived from adipose tissue (ADSC) are multipotent stem cells, originated from the vascular-stromal compartment of fat tissue. ADSC are used as an alternative cell source for many different cell therapies, however in ischemic cardiovascular diseases the therapeutic benefit was modest. One of the reasons could be the use of autologous aged ADSC, which recently were found to have impaired functions. We therefore analysed the effects of age on age markers and angiogenic properties of ADSC. Hypoxic conditioning was investigated as a form of angiogenic stimulation.

Methods

ADSC were harvested from young (1-3 month), adult (12 month) and aged (18-24 month) mice and cultured under normoxic (20%) and hypoxic (1%) conditions for 48 h. Differences in proliferation, apoptosis and telomere length were assessed in addition to angiogenic properties of ADSC.

Results

Proliferation potential and telomere length were decreased in aged ADSC compared to young ADSC. Frequency of apoptotic cells was higher in aged ADSC. Gene expression of pro-angiogenic factors including vascular endothelial growth factor (VEGF), placental growth factor (PlGF) and hepatic growth factor (HGF) were down-regulated with age, which could be restored by hypoxia. Transforming growth factor (TGF-β) increased in the old ADSC but was reduced by hypoxia.

Expression of anti-angiogenic factors including thrombospondin-1 (TBS1) and plasminogen activator inhibitor-1 (PAI-1) did increase in old ADSC, but could be reduced by hypoxic stimulation. Endostatin (ENDS) was the highest in aged ADSC and was also down-regulated by hypoxia. We noted higher gene expression of proteases system factors like urokinase-type plasminogen activator receptor (uPAR), matrix metalloproteinases (MMP2 and MMP9) and PAI-1 in aged ADSC compared to young ADSC, but they decreased in old ADSC. Tube formation on matrigel was higher in the presence of conditioned medium from young ADSC in comparison to aged ADSC.

Conclusions

ADSC isolated from older animals show changes, including impaired proliferation and angiogenic stimulation. Angiogenic gene expression can be partially be improved by hypoxic preconditioning, however the effect is age-dependent. This supports the hypothesis that autologous ADSC from aged subjects might have an impaired therapeutic potential.

Role of the growth hormone-IGF-1 axis in cancer

A substantial body of evidence supports a role for the growth hormone (GH)-IGF-1 axis in cancer incidence and progression. This includes epidemiological evidence relating elevated plasma IGF-1 to cancer incidence as well as a lack of cancers in GH/IGF-1 deficiency. Rodent models lacking GH or its receptor are strikingly resistant to the induction of a wide range of cancers, and treatment with the GH antagonist pegvisomant slows tumor progression. While GH receptor expression is elevated in many cancers, autocrine GH is present in several types, and overexpression of autocrine GH can induce cell transformation. While the mechanism of autocrine action is not clear, it does involve both STAT5 and STAT3 activation, and probably nuclear translocation of the GH receptor. Development of a more potent GH receptor antagonist or secretion inhibitor is warranted for cancer therapy.

SOCS1, a novel interaction partner of p53 controlling oncogene-induced senescence

Members of the signal transducers and activators of transcription (STATs) family of proteins, which connect cytokine signaling to activation of transcription, are frequently activated in human cancers. Suppressors of cytokine signaling (SOCS) are transcriptional targets of activated STAT proteins that negatively control STAT signaling. SOCS1 expression is silenced in multiple human cancers suggesting a tumor suppressor role for this protein. However, SOCS1 not only regulates STAT signaling but can also localize to the nucleus and directly interact with the p53 tumor suppressor through its central SH2 domain. Furthermore, SOCS1 contributes to p53 activation and phosphorylation on serine 15 by forming a ternary complex with ATM or ATR. Through this mechanism SOCS1 regulates the process of oncogene-induced senescence, which is a very important tumor suppressor response. A mutant SOCS1 lacking the SOCS box cannot interact with ATM/ATR, stimulate p53 or induce the senescence phenotype, suggesting that the SOCS box recruits DNA damage activated kinases to its interaction partners bound to its SH2 domain. Proteomic analysis of SOCS1 interaction partners revealed other potential targets of SOCS1 in the DNA damage response. These newly discovered functions of SOCS1 help to explain the increased susceptibility of Socs1 null mice to develop cancer as well as their propensity to develop autoimmune diseases. Consistently, we found that mice lacking SOCS1 displayed defects in the regulation of p53 target genes including Mdm2, Pmp22, PUMA and Gadd45a. The involvement of SOCS1 in p53 activation and the DNA damage response defines a novel tumor suppressor pathway and intervention point for future cancer therapeutics.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

Regulation of cell cycle genes and induction of senescence by overexpression of OTX2 in medulloblastoma cell lines

The transcription factor orthodenticle homeobox 2 (OTX2) has been implicated in the pathogenesis of medulloblastoma, as it is often highly expressed and sometimes amplified in these tumors. Little is known of the downstream pathways regulated by OTX2. We therefore generated MED8A and DAOY medulloblastoma cell lines with doxycycline-inducible OTX2 expression. In both cell lines, OTX2 inhibited proliferation and induced a senescence-like phenotype with senescence-associated β-galactosidase activity. Expression profiles of time series after OTX2 induction in MED8A showed early upregulation of cell cycle genes related to the G(2)-M phase, such as AURKA, CDC25C, and CCNG2. Paradoxically, G(1)-S phase genes such as MYC, CDK4, CDK6, CCND1, and CCND2 were strongly downregulated, in line with the observed G(1) arrest. ChIP-on-chip analyses of OTX2 binding to promoter regions in MED8A and DAOY showed a strong enrichment for binding to the G(2)-M genes, suggesting a direct activation. Their mRNA expression correlated with OTX2 expression in primary tumors, underscoring the in vivo relevance of this regulation. OTX2 induction activated the P53 pathway in MED8A, but not in DAOY, which carries a mutated P53 gene. In DAOY cells, senescence-associated secretory factors, such as interleukin-6 and insulin-like growth factor binding protein 7, were strongly upregulated after OTX2 induction. We hypothesize that the imbalance in cell cycle stimulation by OTX2 leads to cellular senescence either by activating the P53 pathway or through the induction of secretory factors. Our data indicate that OTX2 directly induces a series of cell cycle genes but requires cooperating genes for an oncogenic acceleration of the cell cycle.

SOCS1 links cytokine signaling to p53 and senescence

SOCS1 is lost in many human tumors, but its tumor suppression activities are not well understood. We report that SOCS1 is required for transcriptional activity, DNA binding, and serine 15 phosphorylation of p53 in the context of STAT5 signaling. In agreement, inactivation of SOCS1 disabled p53-dependent senescence in response to oncogenic STAT5A and radiation-induced apoptosis in T cells. In addition, SOCS1 was sufficient to induce p53-dependent senescence in fibroblasts. The mechanism of activation of p53 by SOCS1 involved a direct interaction between the SH2 domain of SOCS1 and the N-terminal transactivation domain of p53, while the C-terminal domain of SOCS1 containing the SOCS Box mediated interaction with the DNA damage-regulated kinases ATM/ATR. Also, SOCS1 colocalized with ATM at DNA damage foci induced by oncogenic STAT5A. Collectively, these results add another component to the p53 and DNA damage networks and reveal a mechanism by which SOCS1 functions as a tumor suppressor.

Mitochondrial dysfunction contributes to oncogene-induced senescence

The expression of oncogenic ras in normal human cells quickly induces an aberrant proliferation response that later is curtailed by a cell cycle arrest known as cellular senescence. Here, we show that cells expressing oncogenic ras display an increase in the mitochondrial mass, the mitochondrial DNA, and the mitochondrial production of reactive oxygen species (ROS) prior to the senescent cell cycle arrest. By the time the cells entered senescence, dysfunctional mitochondria accumulated around the nucleus. The mitochondrial dysfunction was accompanied by oxidative DNA damage, a drop in ATP levels, and the activation of AMPK. The increase in mitochondrial mass and ROS in response to oncogenic ras depended on intact p53 and Rb tumor suppression pathways. In addition, direct interference with mitochondrial functions by inhibiting the expression of the Rieske iron sulfur protein of complex III or the use of pharmacological inhibitors of the electron transport chain and oxidative phosphorylation was sufficient to trigger senescence. Taking these results together, this work suggests that mitochondrial dysfunction is an effector pathway of oncogene-induced senescence.

Granulocyte macrophage--colony-stimulating factor-dependent proliferation is impaired in macrophages from senescence-accelerated mice

A senescence-accelerated (SAMP8) mouse model was used to determine the effect of aging on the immune system. We produced in vitro bone marrow-derived macrophages from SAMP8 mice and compared them against senescence-resistant, long-lived mice (SAMR1). Although macrophages from both strains of mice proliferated in a similar manner in response to monocyte-colony-stimulating factor (M-CSF), SAMP8 macrophages showed an impaired response to granulocyte macrophage-colony-stimulating factor (GM-CSF). Similar levels of external regulated kinases (ERK)1/2 and signaling transducer and activator of transcription 5 (STAT5) phosphorylation were observed in macrophages from both strains of mice. The lack of proliferation was not caused by the induction of apoptosis. Differentiation of bone marrow cells into dendritic cells was similar in both strains of mice, as was the induction of major histocompatibility complex (MHC) class II molecules by interferon-gamma (IFN-gamma). Finally, we determined the density of Langerhans cells in vivo in the skin of the two mouse strains, but no differences were found.