Retinoblastoma protein promotes oxidative phosphorylation through upregulation of glycolytic genes in oncogene‐induced senescent cells

Metabolism is closely linked with cellular state and biological processes, but the mechanisms controlling metabolic properties in different contexts remain unclear. Cellular senescence is an irreversible growth arrest induced by various stresses, which exhibits active secretory and metabolic phenotypes. Here, we show that retinoblastoma protein (RB) plays a critical role in promoting the metabolic flow by activating both glycolysis and mitochondrial oxidative phosphorylation (OXPHOS) in cells that have undergone oncogene‐induced senescence (OIS). A combination of real‐time metabolic monitoring, and metabolome and gene expression analyses showed that OIS‐induced fibroblasts developed an accelerated metabolic flow. The loss of RB downregulated a series of glycolytic genes and simultaneously reduced metabolites produced from the glycolytic pathway, indicating that RB upregulates glycolytic genes in OIS cells. Importantly, both mitochondrial OXPHOS and glycolytic activities were abolished in RB‐depleted or downstream glycolytic enzyme‐depleted OIS cells, suggesting that RB‐mediated glycolytic activation induces a metabolic flux into the OXPHOS pathway. Collectively, our findings reveal that RB essentially functions in metabolic remodeling and the maintenance of the active energy production in OIS cells.     

Bcl-xL and E1B-19K proteins inhibit p53-induced irreversible growth arrest and senescence by preventing reactive oxygen species-dependent p38 activation

In this study, we describe novel functions of the anti-apoptotic Bcl-2 family proteins. Bcl-x(L) and E1B-19K were found to inhibit p53-induced irreversible growth arrest and senescence, but not to inhibit transient growth arrest, implying that Bcl-x(L) and E1B-19K are specifically involved in senescence without participating in growth arrest. We provide several lines of evidences showing that the functions of Bcl-x(L) and E1B-19K to prevent generation of reactive oxygen species (ROS) are important to inhibit senescence induction. First, we found that that ROS are increased during p53-induced senescence. Moreover, Bcl-x(L) and E1B-19K inhibit this p53-induced ROS generation. Second, antioxidants prevent the induction of senescence and ROS by p53, but not the persistence of the senescence phenotype. Third, the anti-senescence functions of Bcl-x(L) and E1B-19K were suppressed by adding exogenous ROS. These results suggest that Bcl-x(L) and E1B-19K inhibit senescence induction by preventing ROS generation. Furthermore, p38 kinase was found to be activated during p53-induced senescence, but not in cells expressing Bcl-x(L) or E1B-19K, or in cells treated with anti-oxidants. Consistently, a chemical inhibitor of p38 kinase, SB203580, was found to inhibit p53-induced senescence, but only when treated before the cellular commitment to senescence, implying that p38 kinase is necessary for senescence induction. Therefore, Bcl-x(L) and E1B-19K inhibit p53-induced senescence by preventing ROS generation, which in turn leads to the activation of p38 kinase. These results also suggest that the oncogenic potential of Bcl-2 is due to its ability to inhibit senescence as well as apoptosis.     

Activities of glucose metabolic enzymes in human preantral follicles: in vitro modulation by follicle-stimulating hormone, luteinizing hormone, epidermal growth factor, insulin-like growth factor I, and transforming growth factor beta1

Modulation of glucose metabolic capacity of human preantral follicles in vitro by gonadotropins and intraovarian growth factors was evaluated by monitoring the activities of phosphofructokinase (PFK) and pyruvate kinase (PK), two regulatory enzymes of the glycolytic pathway, and malate dehydrogenase (MDH), a key mitochondrial enzyme of the Krebs cycle. Preantral follicles in classes 1 and 2 from premenopausal women were cultured separately in vitro in the absence or presence of FSH, LH, epidermal growth factor (EGF), insulin-like growth factor (IGF-I), or transforming growth factor beta1 (TGFbeta1) for 24 h. Mitochondrial fraction was separated from the cytosolic fraction, and both fractions were used for enzyme assays. FSH and LH significantly stimulated PFK and PK activities in class 1 and 2 follicles; however, a 170-fold increase in MDH activity was noted for class 2 follicles that were exposed to FSH. Although both EGF and TGFbeta1 stimulated glycolytic and Krebs cycle enzymes for class 1 preantral follicles, TGFbeta1 consistently stimulated the activities of both glycolytic enzymes more than that of EGF. IGF-I induced PK and MDH activities in class 1 follicles but negatively influenced PFK activity for class 1 follicles. In general, only gonadotropins consistently stimulated both glycolytic and Krebs cycle enzyme activities several-fold in class 2 follicles. These results suggest that gonadotropins and ovarian growth factors differentially influence follicular energy-producing capacity from glucose. Moreover, gonadotropins may either directly influence glucose metabolism in class 2 preantral follicles or do so indirectly through factors other than the well-known intraovarian growth factors. Because growth factors modulate granulosa cell mitosis and functionality, their role on energy production may be related to specific cellular activities.     

ECRG1, a novel candidate of tumor suppressor gene in the esophageal carcinoma, triggers a senescent program in NIH3T3 cells

Esophageal cancer-related gene 1 (ECRG1) is a novel tumor-suppressor gene candidate identified from the human esophagus. Previous studies showed that ECRG1 overexpression could inhibit cell growth and induce G1 cell cycle arrest and p15(INK4b) expression by interacting with Miz-1 (Myc-interacting zinc finger protein). Such evidence suggests the alterations in ECRG1 may play an important role in tumorigenesis. To further study the biological function of the ECRG1 gene, we transfected ECRG1 into NIH3T3 cells. Expression of ECRG1 in these cells caused senescence-like changes characterized in terms of altered cell morphology, cell cycle arrest at the G1/S phase, and significantly impaired cell proliferation (P < 0.01). Moreover, NIH3T3 cells transfected with ECRG1 stained positive for SA-beta-gal staining (pH 6.0), which is a specific marker of cellular senescence. We also studied changes in telomerase activity and the related senescence genes, such as p21 and p16. The results indicated that when ECRG1 induced a senescence-like state, telomerase activity was markedly decreased (P < 0.05), and expression of p21 was distinctly increased, whereas no changes were detected in p16 and telomerase-component RNA levels. These findings suggest that ECRG1 may be of importance in murine cell senescence, promoting senescence by regulating expression of p21.     

Mitochondrial effectors of cellular senescence: beyond the free radical theory of aging

Cellular senescence is a process that results from a variety of stresses, leading to a state of irreversible growth arrest. Senescent cells accumulate during aging and have been implicated in promoting a variety of age‐related diseases. Mitochondrial stress is an effective inducer of cellular senescence, but the mechanisms by which mitochondria regulate permanent cell growth arrest are largely unexplored. Here, we review some of the mitochondrial signaling pathways that participate in establishing cellular senescence. We discuss the role of mitochondrial reactive oxygen species (ROS), mitochondrial dynamics (fission and fusion), the electron transport chain (ETC), bioenergetic balance, redox state, metabolic signature, and calcium homeostasis in controlling cellular growth arrest. We emphasize that multiple mitochondrial signaling pathways, besides mitochondrial ROS, can induce cellular senescence. Together, these pathways provide a broader perspective for studying the contribution of mitochondrial stress to aging, linking mitochondrial dysfunction and aging through the process of cellular senescence.     

Regulation by survivin of cancer cell death induced by F14512, a polyamine-containing inhibitor of DNA topoisomerase II

F14512, an epipodophyllotoxin derivative equipped with a spermine moiety, is selectively taken up by the polyamine transport system over-active in tumor cells. F14512 was identified as a selective anticancer agent with a broad spectrum of antitumor activities and is currently undergoing phase I clinical trial in onco-hematology. However, the mechanism by which F14512 exerts its selective effects on neoplastic cells remains poorly understood. In this study, using mainly P388 leukemia cells, we showed that activation of the DNA damage response by F14512 did not induce immediate apoptosis but resulted in an early growth arrest. F14512-induced G2 arrest was accompanied by the appearance of a senescence-like phenotype (characterized by an increased β-galactosidase staining) with up-regulation of the cyclin-dependent kinase inhibitor p16, and cyclin D1. The early senescence-based cell cycle block was characterized by a marked increase of the level of the IAP protein survivin, but not cIAP2, in P388 cells as well as in three other leukemia and melanoma cell types. The Thr(34)-phosphorylated form of survivin was observed within 4 h after F14512 exposure. Inhibition of survivin by siRNA resulted in a switch from senescence-like growth arrest to apoptosis. Compared with the parental drug etoposide, F14512-induced DNA damage signaling pathway resulted in greater senescence like-growth arrest and delayed apoptosis. Collectively, our data show that senescence arrest and subsequent apoptosis are powerful mechanisms mediating the chemotherapeutic effects of F14512 and identify survivin as the molecular determinant responsible for a qualitative shift in cell fate from senescence to apoptosis upon treatment with F14512.     

Oncogene-induced senescence results in marked metabolic and bioenergetic alterations

Oncogene-induced senescence (OIS) is characterized by permanent growth arrest and the acquisition of a secretory, pro-inflammatory state. Increasingly, OIS is viewed as an important barrier to tumorgenesis. Surprisingly, relatively little is known about the metabolic changes that accompany and therefore may contribute to OIS. Here, we have performed a metabolomic and bioenergetic analysis of Ras-induced senescence. Profiling approximately 300 different intracellular metabolites reveals that cells that have undergone OIS develop a unique metabolic signature that differs markedly from cells undergoing replicative senescence. A number of lipid metabolites appear uniquely increased in OIS cells, including a marked increase in the level of certain intracellular long chain fatty acids. Functional studies reveal that this alteration in the metabolome reflects substantial changes in overall lipid metabolism. In particular, Ras-induced senescent cells manifest a decline in lipid synthesis and a significant increase in fatty acid oxidation. Increased fatty acid oxidation results in an unexpectedly high rate of basal oxygen consumption in cells that have undergone OIS. Pharmacological or genetic inhibition of carnitine palmitoyltransferase 1, the rate-limiting step in mitochondrial fatty acid oxidation, restores a presenescent metabolic rate and, surprisingly, selectively inhibits the secretory, pro-inflammatory state that accompanies OIS. Thus, Ras-induced senescent cells demonstrate profound alterations in their metabolic and bioenergetic profiles, particularly with regards to the levels, synthesis and oxidation of free fatty acids. Furthermore, the inflammatory phenotype that accompanies OIS appears to be related to these underlying changes in cellular metabolism.Key words: oncogene-induced senescence, metabolomics, Ras, fatty acid oxidation     

Coming up for air: HIF-1 and mitochondrial oxygen consumption

Hypoxic cells induce glycolytic enzymes; this HIF-1-mediated metabolic adaptation increases glucose flux to pyruvate and produces glycolytic ATP. Two papers in this issue of Cell Metabolism (Kim et al., 2006; Papandreou et al., 2006) demonstrate that HIF-1 also influences mitochondrial function, suppressing both the TCA cycle and respiration by inducing pyruvate dehydrogenase kinase 1 (PDK1). PDK1 regulation in hypoxic cells promotes cell survival.     

Oncogene-induced senescence results in marked metabolic and bioenergetic alterations

Oncogene-induced senescence (OIS) is characterized by permanent growth arrest and the acquisition of a secretory, pro-inflammatory state. Increasingly, OIS is viewed as an important barrier to tumorgenesis. Surprisingly, relatively little is known about the metabolic changes that accompany and therefore may contribute to OIS. Here, we have performed a metabolomic and bioenergetic analysis of Ras-induced senescence. Profiling approximately 300 different intracellular metabolites reveals that cells that have undergone OIS develop a unique metabolic signature that differs markedly from cells undergoing replicative senescence. A number of lipid metabolites appear uniquely increased in OIS cells, including a marked increase in the level of certain intracellular long chain fatty acids. Functional studies reveal that this alteration in the metabolome reflects substantial changes in overall lipid metabolism. In particular, Ras-induced senescent cells manifest a decline in lipid synthesis and a significant increase in fatty acid oxidation. Increased fatty acid oxidation results in an unexpectedly high rate of basal oxygen consumption in cells that have undergone OIS. Pharmacological or genetic inhibition of carnitine palmitoyltransferase 1, the rate-limiting step in mitochondrial fatty acid oxidation, restores a pre-senescent metabolic rate and, surprisingly, selectively inhibits the secretory, pro-inflammatory state that accompanies OIS. Thus, Ras-induced senescent cells demonstrate profound alterations in their metabolic and bioenergetic profiles, particularly with regards to the levels, synthesis and oxidation of free fatty acids. Furthermore, the inflammatory phenotype that accompanies OIS appears to be related to these underlying changes in cellular metabolism.     

DOT1L modulates the senescence-associated secretory phenotype through epigenetic regulation of IL1A

Oncogene-induced senescence (OIS) is a stable cell cycle arrest that occurs in normal cells upon oncogene activation. Cells undergoing OIS express a wide variety of secreted factors that affect the senescent microenvironment termed the senescence-associated secretory phenotype (SASP), which is beneficial or detrimental in a context-dependent manner. OIS cells are also characterized by marked epigenetic changes. We globally assessed histone modifications of OIS cells and discovered an increase in the active histone marks H3K79me2/3. The H3K79 methyltransferase disruptor of telomeric silencing 1-like (DOT1L) was necessary and sufficient for increased H3K79me2/3 occupancy at the IL1A gene locus, but not other SASP genes, and was downstream of STING. Modulating DOT1L expression did not affect the cell cycle arrest. Together, our studies establish DOT1L as an epigenetic regulator of the SASP, whose expression is uncoupled from the senescence-associated cell cycle arrest, providing a potential strategy to inhibit the negative side effects of senescence while maintaining the beneficial inhibition of proliferation.     

miR-181a mediates metabolic shift in colon cancer cells via the PTEN/AKT pathway

Cancer cell metabolism is often characterized by a shift from an oxidative to a glycolytic bioenergetics pathway, a phenomenon known as the warburg effect. Whether the deregulation of miRNAs contributes to the warburg effect remains largely unknown. Here we show that miR-181a expression is increased and thus induces a metabolic shift in colon cancer cells. miR-181a performs this function by inhibiting the expression of PTEN, leading to an increase of phosphorylated AKT which triggers metabolic shift. The increase of lactate production induced by miR-181a results in the rapid growth of cancer cells. These results identify miR-181a as a molecular switch involved in the orchestration of the warburg effect in colon cancer cells via the PTEN/AKT pathway.     

Cell senescence: hypertrophic arrest beyond the restriction point

Withdrawal of mitogens (growth factors) arrests normal cells in G0 (quiescence). All other stresses and factors arrest cell cycle beyond the restriction point in G1 and G2 (non-G0 arrest), in the presence of mitogenic stimulation. Strong mitogenic stimuli by themselves cause non-G0 arrest. Unlike G0, arrest beyond restriction point is characterized by both high levels of cyclins and CDK inhibitors, activated mitogenic pathways with a secondary GF resistance, and continuous mass growth (cell hypertrophy). Prolonged hypertrophic arrest culminates in cell senescence. This review discusses that quiescence and senescence are two opposite, mutually exclusive conditions and that cell senescence can be reversed and prevented.     

Oncogenes induce senescence with incomplete growth arrest and suppress the DNA damage response in immortalized cells

Activation of the Her2 (ErbB2) oncogene is implicated in the development of breast, ovary and other cancers. Here, we show that expression of NeuT, a mutant-activated rodent isoform of Her2, in immortalized breast epithelial cells, while promoting senescence-associated morphological changes, up-regulation of senescence-associated β-galactosidase activity, and accumulation of the cyclin-dependent kinase inhibitor p21, failed to trigger the major senescence end-point, i.e. permanent growth arrest. Similar senescence-associated phenotype with incomplete growth arrest, which we dubbed senescence with incomplete growth arrest (SWING), could also be triggered by the expression of the Ras oncogene. SWING phenotype was stable, and persisted in tumor xenografts established from NeuT-transduced cells. Furthermore, a significant population of cells in SWING state was found in tumors in the MMTV/NeuT transgenic mouse model. SWING cells showed downregulation of histone H2AX, critical for repair of double-stranded DNA breaks, and impaired activation of Chk1 kinase. Overall, SWING cells were characterized by increased DNA instability and hypersensitivity to genotoxic stresses. We propose that the SWING state could be a stage in the process of cancer development.     

Effects of metal ions on simultaneous production of glucose oxidase and catalase by Aspergillus niger

The effects of various metal ions on the simultaneous production of glucose oxidase and catalase by Aspergillus niger were investigated. Calcium carbonate induced synthesis of both enzymes. The induction of calcium carbonate was accompanied by a metabolic shift from the glycolytic pathway (EMP, Embden-Meyerhof-Parnas) to direct oxidation of glucose by glucose oxidase. The time course of the biosynthesis of both enzymes is reported. The logistic model was in good agreement with the experimental growth results. The production of both enzymes was growth-associated. Finally, a model of growth and product formation was also proposed.     

Cellular senescence and chromatin structure

Cellular senescence is characterized by stable cell cycle arrest that is triggered by various forms of stress stimuli. Senescent cells show a series of morphological and physiological alterations including a flat and enlarged morphology, an increase in acidic β-galactosidase activity, chromatin condensation, and changes in gene expression pattern. These features are not observed in proliferating cells or quiescent cells in vitro. Using these senescence markers, cellular senescence has been shown to occur in benign or premalignant lesions but not in malignant lesions and to act as a tumor-suppressing mechanism in vivo. The onset and maintenance of the senescent state are regulated by two tumor suppressor proteins, p53 and Rb, which mediate senescence signals through p38 mitogen-activated protein kinase and cyclin-dependent kinase inhibitors. Alterations of chromatin structure are believed to contribute to the irreversible nature of the senescent state. Senescent cells form characteristic heterochromatin structure called senescence-associated heterochromatic foci (SAHFs), which may repress the expression of proliferation-promoting genes, such as E2F target genes. Recent studies have provided molecular insights into the structure and the mechanism of SAHF formation. In this paper, we review the role of cellular senescence in tumor suppression in vivo and the molecular mechanism of stable growth arrest in senescent cells, focusing on the special form of heterochromatin, SAHFs.