Metabolic regulation by p53

Clair D, Butterfield DA. Lack of p53 affects the expression of several brain mitochondrial proteins: insights from proteomics into important pathways regulated by p53

The tumor suppressor protein p53 has been described "as the guardian of the genome" for its crucial role in regulating the transcription of numerous genes responsible for cells cycle arrest, senescence, or apoptosis in response to various stress signals. Although p53 promotes longevity by decreasing the risk of cancer through activation of apoptosis or cellular senescence, several findings suggest that an increase of its activity may have deleterious effects leading to selected aspects of the aging phenotype and neurodegenerative diseases. There is the link between p53 and oxidative stress, the latter a crucial factor that contributes to neurodegenerative processes like Alzheimer disease (AD). In the present study, using a proteomics approach, we analyzed the impact of lack of p53 on the expression of several brain mitochondrial proteins involved in different pathways, and how lack of p53 may present a target to restore neuronal impairments. Our investigation on isolated brain mitochondria from p53((-/-)) mice also provides a better understanding of the p53-mitochondria relationship and its involvement in the development of many diseases.

Dietary downregulation of mutant p53 levels via glucose restriction: mechanisms and implications for tumor therapy

The majority of human tumors express mutant forms of p53 at high levels, promoting gain of oncogenic functions and correlating with disease progression, resistance to therapy and unfavorable prognosis. p53 mutant accumulation in tumors is attributed to the ability to evade degradation by the proteasome, the only currently recognized machinery for p53 disruption. We report here that glucose restriction (GR) induces p53 mutant deacetylation, routing it for degradation via autophagy. Depletion of p53 leads, in turn, to robust autophagic activation and to cell death, while expression of degradation-defective mutant p53 blocks autophagy and enables survival to GR. Furthermore, we found that a carbohydrate-free dietetic regimen that lowers the fasting glucose levels blunts p53 mutant expression and oncogenic activity relative to a normal diet in several animal model systems. These findings indicate that the stability of mutant forms of p53 is influenced by the levels of glucose and by dietetic habits. They also unravel the existence of an inhibitory loop between autophagy and mutant p53 that can be exploited therapeutically.

Genetically-defined metabolic reprogramming in cancer

Oncogenes and tumor suppressors regulate cell metabolism. Evidence demonstrates that tumorigenic mutations in these genes tend to orchestrate metabolic activity into a platform that promotes cell survival, growth, and proliferation. Recent work has shown that some metabolic enzymes are also mutated in cancer, and that these mutations may influence malignancy directly. Thus, these enzymes seem to function as oncogenes and tumor suppressors, and would appear to be compelling targets for therapeutic intervention. Here, we review several enzymes mutated in cancer - phosphoglycerate dehydrogenase, isocitrate dehydrogenases 1 and 2, succinate dehydrogenase, and fumarate hydratase - and discuss exciting new work that has begun to pull back the curtain on how mutations in these enzymes influence tumorigenesis.

Regulation of lipid metabolism by p53 - fighting two villains with one sword

Both cellular and systemic metabolism of lipids are paramount for homeostasis, and their malfunction leads to devastating pathologies. Recently, exciting findings have linked the p53 tumor suppressor to the regulation of lipid metabolism. Here, we summarize these findings showing a clear role for p53 in enhancing lipid catabolism while inhibiting its anabolism. We also describe the multitude of genes regulated by p53 that participate in or regulate systemic lipid transport. From the compilation of available data a scenario is emerging in which p53 regulates genes involved in lipid metabolism - both in a cancer-preventive effort and, intriguingly, as a means to prevent atherosclerosis. Thus, by regulating lipid metabolism, p53 fights the two major causes of death worldwide - atherosclerosis and cancer.

Chemotherapeutic agents induce the expression and activity of their clearing enzyme CYP3A4 by activating p53

Cytochrome P450 (P450) enzymes are abundantly expressed in the human liver where they hydroxylate organic substrates. In a microarray screen performed in human liver cells, we found a group of eleven P450 genes whose expression was induced by p53 (CYP3A4, CYP3A43, CYP3A5, CYP3A7, CYP4F2, CYP4F3, CYP4F11, CYP4F12, CYP19A1, CYP21A2 and CYP24A1). The mode of regulation of four representative genes (CYP3A4, CYP3A7, CYP4F2 and CYP4F3) was further characterized. The genes were induced in a p53-dependent manner in HepG2 and Huh6 cells (both are cancer-derived human liver cells) and in primary liver cells isolated from human donors. Furthermore, p53 was found to bind to p53-responsive elements in the genes' DNA-regulatory regions and to enhance their transcription in a reporter gene assay. Importantly, when p53 was activated following the administration of either of three different anticancer chemotherapeutic agents (cisplatin, etoposide or doxorubicin), it was able to induce CYP3A genes, which are the main factors in systemic clearance of these agents. Finally, the p53-dependent induction of P450 genes following either Nutlin or chemotherapy treatment led to enhanced P450 enzymatic activity. Thus, in addition to the well-established role of p53 at the tumor site, our data unravels a novel function of hepatic p53 in inducing P450 enzymes and position p53 as a major factor in the hepatic response to xenobiotic and metabolic signals. Importantly, this study reveals a novel pathway for the induction of CYP3As by their substrates through p53, warranting the need for careful consideration when designing systemically administered chemotherapeutic regimens.

Reactive oxygen species regulation of autophagy in cancer: implications for cancer treatment

Reactive oxygen species (ROS) are important in regulating normal cellular processes, but deregulated ROS contribute to the development of various human diseases including cancers. Autophagy is one of the first lines of defense against oxidative stress damage. The autophagy pathway can be induced and upregulated in response to intracellular ROS or extracellular oxidative stress. This leads to selective lysosomal self-digestion of intracellular components to maintain cellular homeostasis. Hence, autophagy is the survival pathway, conferring stress adaptation and promoting viability under oxidative stress. However, increasing evidence has demonstrated that autophagy can also lead to cell death under oxidative stress conditions. In addition, altered autophagic signaling pathways that lead to decreased autophagy are frequently found in many human cancers. This review discusses the advances in understanding of the mechanisms of ROS-induced autophagy and how this process relates to tumorigenesis and cancer therapy.

Metabolic regulation of oxygen and redox homeostasis by p53: lessons from evolutionary biology?

The genetic links between p53 and metabolic processes such as oxidative phosphorylation are being studied with increasing interest given that cellular metabolism seems to play an important role in tumorigenesis. This review focuses on how p53 regulation of various metabolic genes may influence redox homeostasis, as the genome is constantly susceptible to oxidative damage, a consequence of living in an aerobic environment. Because p53-like genetic sequences are also found in life forms that may not necessarily benefit from tumor suppression, an evolutionary introduction is given in an attempt to understand why p53 might regulate a basic cellular activity such as metabolism. The presented epidemiologic and experimental data suggest that one reason may be for the homeostatic regulation of oxygen, the essential substrate for reactive oxygen species generation.

Glucose--a sweet way to die: metabolic switching modulates tumor cell death

TRAIL, a putative anticancer cytokine, induces extrinsic cell death by activating the caspase cascade directly (Type I cells) via the death-inducing signaling complex (DISC) or indirectly (Type II cells) by caspase-8 cleavage of Bid and activation of the mitochondrial cell death pathway. Cancer cells are characterized by their dependence on aerobic glycolysis, which, although inefficient in terms of ATP production, facilitates tumor metabolism. Our studies show that TRAIL-induced cell death is significantly affected by the metabolic status of the cell. Inhibiting glycolysis with 2-deoxyglucose potentiates TRAIL-induced cell death, whereas glucose deprivation can paradoxically inhibit apoptosis. These conflicting responses to glycolysis inhibition are modulated by the balance between the Akt and AMPK pathways and their subsequent downstream regulation of mTORC1. This results in marked changes in protein translation, in which the equilibrium between anti- and pro-apoptotic Bcl-2 family member proteins is decided by their individual degradation rates. This regulates the mitochondrial cell death pathway and alters its sensitivity not only to TRAIL, but to ABT-737, a Bcl-2 inhibitor. Taken together, our studies show that the sensitivity of cancer cells to apoptosis can be modulated by targeting their unique metabolism in order to enhance sensitivity to apoptotic agents.

Phospho-ΔNp63α/SREBF1 protein interactions: bridging cell metabolism and cisplatin chemoresistance

Tumor protein (TP)-p53 family members (TP63, TP63 and TP73) are guardians of the genome and key players in orchestrating the cellular response to cisplatin treatment. Cisplatin-induced phosphorylation of ΔNp63α was shown to have a role in regulating intracellular ΔNp63α protein levels. We previously found that squamous cell carcinoma (SCC) cells exposed to cisplatin displayed the ATM-dependent phosphorylation of ΔNp63α (p-ΔNp63α), which is critical for the transcriptional regulation of specific downstream mRNAs and microRNAs and is likely to underlie the chemoresistance of SCC cells. However, SCC cells expressing non-p-ΔNp63α became more cisplatin-resistant. We also found that p-ΔNp63α forms complexes with a number of proteins involved in cell death response through regulation of cell cycle arrest, apoptosis, autophagy, RNA splicing and chromatin modifications. Here, we showed that p-ΔNp63α induced ARG1, GAPDH, and CPT2 gene transcription in cisplatin-sensitive SCC cells, while non-p-ΔNp63α increased a transcription of CAD, G6PD and FASN genes in cisplatin-resistant SCC cells. We report that the p-ΔNp63α-dependent regulatory mechanisms implicated in the modulation of plethora of pathways, including amino acid, carbohydrate, lipid and nucleotide metabolisms, thereby affect tumor cell response to cisplatin-induced cell death, suggesting that the ATM-dependent ΔNp63α pathway plays a role in the resistance of tumor cells to platinum therapy.

A role for the Warburg effect in preimplantation embryo development: metabolic modification to support rapid cell proliferation

In this essay, we propose that embryos express a metabolic phenotype necessarily different from that of differentiated somatic cells and more like that of rapidly proliferating cancer cells. This metabolic adaptation, known as the Warburg effect, supports rapid cell proliferation. One of the hallmarks of the Warburg effect is that pyruvate is directed away from the tri-carboxylic acid cycle and metabolized to lactate, resulting in a buildup of glycolytic intermediates. Although this is a comparatively inefficient way to generate ATP, this adaptation allows the cell to meet other critical metabolic requirements, including biomass production and redox regulation. Thus, utilization of WE gives proliferating cells a selective growth advantage. This model represents a completely new understanding of embryo metabolism in the context of a broad, interconnected network of metabolic mechanisms that influence viability, versus the current dogma of carbohydrate metabolism via oxidative phosphorylation. A more complete understanding of embryo metabolism is critical to better support embryo viability in vitro, and to avoid forcing embryos to adapt to suboptimal culture conditions at a significant cost to future growth and development.

Death-associated proliferation kinetic in normal and transformed cells

Pyruvate kinase M2 (PKM2) may occur in both a tetrameric and a dimeric form. When the majority of PKM2 molecules are in the highly active tetrameric conformation, glucose is primarily degraded to pyruvate and lactate with the regeneration of energy. A tumor suppressor protein, death-associated protein kinase (DAPK), interacts with PKM2 protein and stabilizes PKM2 in its active tetrameric form in normal proliferating cells. However, DAPK is widely inactivated in cancer cells, leading to the loss of the active conformation of PKM2. This may render PKM2 sensitive to cellular oxidants, switching the enzyme into its inactive dimeric form. Consequently, inhibition of PKM2 after oxidative stress contributes optimal tumor growth and allows cancer cells to withstand oxidative stress.

A novel N-terminal domain may dictate the glucose response of Mondo proteins

Glucose is a fundamental energy source for both prokaryotes and eukaryotes. The balance between glucose utilization and storage is integral for proper energy homeostasis, and defects are associated with several diseases, e.g. type II diabetes. In vertebrates, the transcription factor ChREBP is a major component in glucose metabolism, while its ortholog MondoA is involved in glucose uptake. Both MondoA and ChREBP contain five Mondo conserved regions (MCRI-V) that affect their cellular localization and transactivation ability. While phosphorylation has been shown to affect ChREBP function, the mechanisms controlling glucose response of both ChREBP and MondoA remain elusive. By incorporating sequence analysis techniques, structure predictions, and functional annotations, we synthesized data surrounding Mondo family proteins into a cohesive, accurate, and general model involving the MCRs and two additional domains that determine ChREBP and MondoA glucose response. Paramount, we identified a conserved motif within the transactivation region of Mondo family proteins and propose that this motif interacts with the phosphorylated form of glucose. In addition, we discovered a putative nuclear receptor box in non-vertebrate Mondo and vertebrate ChREBP sequences that reveals a potentially novel interaction with nuclear receptors. These interactions are likely involved in altering ChREBP and MondoA conformation to form an active complex and induce transcription of genes involved in glucose metabolism and lipogenesis.

Chloropicrin induces endoplasmic reticulum stress in human retinal pigment epithelial cells

Chloropicrin is an aliphatic volatile nitrate compound that is mainly used as a pesticide. It has several toxic effects in animals and can cause irritating and other health problems in exposed humans. Since the mode of chloropicrin action is poorly understood, the aim of this study was to investigate molecular responses underlying chloropicrin toxicity. We used human retinal pigment epithelial cells (ARPE-19) as a model cell type because the eyes are one of the main target organs affected by chloropicrin exposure. Transmission electron microscopy images revealed that exposure to a chloropicrin concentration that decreased cell viability by 50%, evoked the formation of numerous electron-lucent, non-autophagy vacuoles in the cytoplasm with dilatation of the endoplasmic reticulum (ER). Lower concentrations led to the appearance of more electron-dense vacuoles, which contained cytoplasmic material and were surrounded by a membrane resembling autophagy vacuoles. According to immunoblotting analyses chloropicrin increased the amount of the ER-stress related proteins, Bip (about 3-fold compared to the controls), IRE1α (2.5-fold) and Gadd 153/Chop (2.5-fold), evidence for accumulation of misfolded proteins in the ER. This property was further confirmed by the increase of reactive oxygen species (ROS) production (2-2.5-fold), induction of heme oxygenase-1 (about 6-fold), and increase in the level of the tumour suppressor protein p53 (2-fold). Thus, the cytotoxicity of chloropicrin in the retinal pigment epithelium is postulated to be associated with oxidative stress and perturbation of the ER functions, which are possibly among the mechanisms involved in oculotoxicity of chloropicrin.

Wild-type tumor repressor protein 53 (Trp53) promotes ovarian cancer cell survival

Loss of Pten in the Kras(G12D);Amhr2-Cre mutant mice leads to the transformation of ovarian surface epithelial (OSE) cells and rapid development of low-grade, invasive serous adenocarcinomas. Tumors occur with 100% penetrance and express elevated levels of wild-type tumor repressor protein 53 (TRP53). To test the functions of TRP53 in the Pten;Kras (Trp53+) mice, we disrupted the Trp53 gene yielding Pten;Kras(Trp53-) mice. By comparing morphology and gene expression profiles in the Trp53+ and Trp53- OSE cells from these mice, we document that wild-type TRP53 acts as a major promoter of OSE cell survival and differentiation: cells lacking Trp53 are transformed yet are less adherent, migratory, and invasive and exhibit a gene expression profile more like normal OSE cells. These results provide a new paradigm: wild-type TRP53 does not preferentially induce apoptotic or senescent related genes in the Pten;Kras(Trp53+) cancer cells but rather increases genes regulating DNA repair, cell cycle progression, and proliferation and decreases putative tumor suppressor genes. However, if TRP53 activity is forced higher by exposure to nutlin-3a (a mouse double minute-2 antagonist), TRP53 suppresses DNA repair genes and induces the expression of genes that control cell cycle arrest and apoptosis. Thus, in the Pten;Kras(Trp53+) mutant mouse OSE cells and likely in human TP53+ low-grade ovarian cancer cells, wild-type TRP53 controls global molecular changes that are dependent on its activation status. These results suggest that activation of TP53 may provide a promising new therapy for managing low-grade ovarian cancer and other cancers in humans in which wild-type TP53 is expressed.

AMP-activated protein kinase (AMPK) controls the aging process via an integrated signaling network

Efficient control of energy metabolic homeostasis, enhanced stress resistance, and qualified cellular housekeeping are the hallmarks of improved healthspan and extended lifespan. AMPK signaling is involved in the regulation of all these characteristics via an integrated signaling network. Many studies with lower organisms have revealed that increased AMPK activity can extend the lifespan. Experiments in mammals have demonstrated that AMPK controls autophagy through mTOR and ULK1 signaling which augment the quality of cellular housekeeping. Moreover, AMPK-induced stimulation of FoxO/DAF-16, Nrf2/SKN-1, and SIRT1 signaling pathways improves cellular stress resistance. Furthermore, inhibition of NF-κB signaling by AMPK suppresses inflammatory responses. Emerging studies indicate that the responsiveness of AMPK signaling clearly declines with aging. The loss of sensitivity of AMPK activation to cellular stress impairs metabolic regulation, increases oxidative stress and reduces autophagic clearance. These age-related changes activate innate immunity defence, triggering a low-grade inflammation and metabolic disorders. We will review in detail the signaling pathways of this integrated network through which AMPK controls energy metabolism, autophagic degradation and stress resistance and ultimately the aging process.

The microtubule cytoskeleton is required for a G2 cell cycle delay in cancer cells lacking stathmin and p53

In several cancer cell lines, depleting the microtubule (MT)-destabilizing protein stathmin/oncoprotein18 leads to a G2 cell cycle delay and apoptosis. These phenotypes are observed only in synergy with low levels of p53, but the pathway(s) activated by stathmin depletion to delay the cell cycle are unknown. We found that stathmin depletion caused greater MT stability in synergy with loss of p53, measured by the levels of acetylated α-tubulin and the rate of centrosomal MT nucleation. Nocodazole or vinblastine-induced MT depolymerization abrogated the stathmin-depletion induced G2 delay, measured by the percentage of cells staining positive for several markers (TPX2, CDK1 with inhibitory phosphorylation), indicating that MTs are required to lengthen G2. Live cell imaging showed that stathmin depletion increased time in G2 without an impact on the duration of mitosis, indicating that the longer interphase duration is not simply a consequence of a previous slowed mitosis. In contrast, stabilization of MTs with paclitaxel (8 nM) slowed mitosis without lengthening the duration of interphase, demonstrating that increased MT stability alone is not sufficient to delay cells in G2.

Crosstalk between p53 and TGF-β Signalling

Wild-type p53 and TGF-β are key tumour suppressors which regulate an array of cellular responses. TGF-β signals in part via the Smad signal transduction pathway. Wild-type p53 and Smads physically interact and coordinately induce transcription of a number of key tumour suppressive genes. Conversely mutant p53 generally subverts tumour suppressive TGF-β responses, diminishing transcriptional activation of key TGF-β target genes. Mutant p53 can also interact with Smads and this enables complex formation with the p53 family member p63 and blocks p63-mediated activation of metastasis suppressing genes to promote tumour progression. p53 and Smad function may also overlap during miRNA biogenesis as they can interact with the same components of the Drosha miRNA processing complex to promote maturation of specific subsets of miRNAs. This paper investigates the crosstalk between p53 and TGF-β signalling and the potential roles this plays in cancer biology.

Molecular Mechanisms of Cytotoxicity and Apoptosis Induced by Inorganic Fluoride

Fluoride (F) is ubiquitous natural substance and widespread industrial pollutant. Although low fluoride concentrations are beneficial for normal tooth and bone development, acute or chronic exposure to high fluoride doses results in adverse health effects. The molecular mechanisms underlying fluoride toxicity are different by nature. Fluoride is able to stimulate G-proteins with subsequent activation of downstream signal transduction pathways such as PKA-, PKC-, PI3-kinase-, Ca2+-, and MAPK-dependent systems. G-protein-independent routes include tyrosine phosphorylation and protein phosphatase inhibition. Along with other toxic effects, fluoride was shown to induce oxidative stress leading to excessive generation of ROS, lipid peroxidation, decrease in the GSH/GSSH ratio, and alterations in activities of antioxidant enzymes, as well as to inhibit glycolysis thus causing the depletion of cellular ATP and disturbances in cellular metabolism. Fluoride triggers the disruption of mitochondria outer membrane and release of cytochrome c into cytosol, what activates caspases-9 and -3 (intrinsic) apoptotic pathway. Extrinsic (death receptor) Fas/FasL-caspase-8 and -3 pathway was also described to be implicated in fluoride-induced apoptosis. Fluoride decreases the ratio of antiapoptotic/proapoptotic Bcl-2 family proteins and upregulates the expression of p53 protein. Finally, fluoride changes the expression profile of apoptosis-related genes and causes endoplasmic reticulum stress leading to inhibition of protein synthesis.

p53, a novel regulator of lipid metabolism pathways

BACKGROUND & AIMS

In this study we aimed at characterizing the regulation of hepatic metabolic pathways by the p53 transcription factor.

METHODS

Analysis of gene expression following alteration of p53 status in several human- and mouse-derived cells using microarray analysis, quantitative real-time PCR, chromatin immunoprecipitation, and reporter gene assays. A functional assay was performed to determine lipid transfer activity.

RESULTS

We identified a novel role for the p53 protein in regulating lipid and lipoprotein metabolism, a process not yet conceived as related to p53, which is known mainly for its tumor suppressive functions. We revealed a group of 341 genes whose expression was induced by p53 in the liver-derived cell line HepG2. Twenty of these genes encode proteins involved in many aspects of lipid homeostasis. The mode of regulation of three representative genes (Pltp, Abca12, and Cel) was further characterized. In addition to HepG2, the genes were induced following activation of p53 in human primary hepatic cells isolated from liver donors. p53-dependent regulation of these genes was evident in other cell types namely Hep3B cells, mouse hepatocytes, and fibroblasts. Furthermore, p53 was found to bind to the genes' promoters in designated p53 responsive elements and thereby increase transcription. Importantly, p53 augmented the activity of secreted PLTP, which plays a major role in lipoprotein biology and atherosclerosis pathology.

CONCLUSIONS

These findings expose another facet of p53 functions unrelated to tumor suppression and render it a novel regulator of hepatic lipid metabolism and consequently of systemic lipid homeostasis and atherosclerosis development.

Increased dosage of tumor suppressors limits the tumorigenicity of iPS cells without affecting their pluripotency

Embryonic stem (ES) cells and induced pluripotent stem (iPS) cells represent a promising therapeutic tool for many diseases, including aged tissues and organs at high risk of failure. However, the intrinsic self-renewal and pluripotency of ES and iPS cells make them tumorigenic, and hence, the risk of tumor development hinders their clinical application. Here, we present a novel approach to limit their tumorigenicity and increase their safety through increased copy number of tumor suppressors. iPS containing an extra copy of the p53 or Ink4a/ARF locus show normal pluripotency, as determined by in vitro and in vivo differentiation assays. Yet, while retaining full pluripotency, they also possess an improved engagement of the p53 pathway during teratocarcinoma formation, which leads to a reduced tumorigenic potential in various in vitro and in vivo assays. Furthermore, they show an improved response to anticancer drugs, which could aid in their elimination in case tumors arise with no adverse effects on cell function or aging. Our system provides a model for studying tumor suppressor pathways during reprogramming, differentiation, and cell therapy applications. This offers an improved understanding of the pathways involved in tumor growth from engrafted pluripotent stem cells, which could facilitate the use of ES and iPS cells in regenerative medicine.

p53: exercise capacity and metabolism

PURPOSE OF REVIEW

There is an inverse relationship between cancer incidence and cardiorespiratory fitness in large population studies. Mechanistic insights into these observations may strengthen the rationale for encouraging exercise fitness in the clinics for cancer prevention and may promote the development of new preventive strategies.

RECENT FINDINGS

Studying the multifaceted activities of p53, a critical tumor suppressor gene, has revealed various cellular pathways necessary for adapting to environmental stresses. Genetic connections are being made between p53 and an increasing number of metabolic activities such as oxidative phosphorylation, glycolysis and fatty acid oxidation. In-vivo mouse models show that p53 plays an important role in determining both basal aerobic exercise capacity and its improvement by training.

SUMMARY

The genetic pathways by which p53 regulates metabolism and exercise may help explain significant epidemiologic observations connecting cardiorespiratory fitness and cancer. Further understanding of these molecular pathways through human translational studies may promote the development of new cancer preventive strategies.

SIRT1, metabolism and cancer

PURPOSE OF REVIEW

SIRT1 impacts upon diverse cellular processes via its roles in the determination of chromatin structure, chromatin remodelling and gene expression. This review covers the recent discoveries linking SIRT1 with the regulation of mammalian metabolism and considers ways in which abnormal metabolism in disease may, in turn, impact upon SIRT1 because of SIRT1's functional dependency upon NAD.

RECENT FINDINGS

Diverse signalling pathways are integrated to regulate energy metabolism and homeostasis. Such pathways involve intracellular networks and mitochondria, and also intercellular signalling within and between tissues to co-ordinate adaptive metabolic responses within the organism as a whole. Here, we outline the recent studies exploring the regulatory links between SIRT1 and mitochondrial biogenesis, cellular redox and associated metabolic pathways, and angiogenesis/Notch signalling. These links are effected by the SIRT1-mediated deacetylation of transcriptional regulators and enzymes with key roles in metabolism.

SUMMARY

SIRT1 activity is directly coupled with homeostasis and metabolism. SIRT1 is also a metabolic sensor. It follows that disease-related metabolic abnormalities are likely to impinge upon SIRT1 functioning. Disease-related functions of SIRT1, in their turn, offer potential targets for the development of novel SIRT1-based therapies. In cancer, for example, the survival function of SIRT1 may reflect abnormal cancer metabolism and identifies SIRT1 as a target for anticancer therapy.

Does expression of the retrogene UTP14c in the ovary pre-dispose women to ovarian cancer?

It has been previously shown that the spermatogenesis associated retrogene, UTP14c, is expressed in over 50% of normal human ovaries and 80% of ovarian cancers. UTP14c is located on chromosome 13 as an intronless copy of the X-linked housekeeping gene, UTP14a. Like all spermatogenesis associated retrogenes, UTP14c is expressed in the testis and is essential for sperm production. It has no known role in the female and is not normally expressed in any cells or organs outside of the gonads. By comparison the protein encoded by UTP14a is found in all cell types and has a dual function. It is primarily involved in the biosynthesis of 18S ribosomal RNA in the nucleolus where it is a component of the U3 small nucleolar RNA associated protein complex. In addition, it down regulates TP53 in both the nucleus and cytoplasm by targeting it for proteolytic degradation. By analogy, we propose that the UTP14c peptide also targets TP53 for degradation. This in turn may prevent cells expressing UTP14c from entering apoptosis. The loss of TP53 in ovarian cells can also result in the down regulation of microRNA-145 (miR-145) expression. The loss of miR-145 can result in the activation of factors that promote oncogenesis and cellular pluripotency which in turn could lead to the development of ovarian cancer. We hypothesize that women, whose ovaries express UTP14c, are predisposed to ovarian cancer due to the disruption of protective signals that normally trigger TP53-mediated apoptosis and the dysregulation of genes that promote oncogenesis, such as c-Myc, that occurs when miR-145 synthesis is disrupted.

A p53-inducible microRNA-34a downregulates Ras signaling by targeting IMPDH

p53 is a well-known transcription factor that controls cell cycle arrest and cell death in response to a wide range of stresses. Moreover, p53 regulates glucose metabolism and its mutation results in the metabolic switch to the Warburg effect found in cancer cells. Nucleotide biosynthesis is also critical for cell proliferation and the cell division cycle. Nonetheless, little is known about whether p53 regulates nucleotide biosynthesis. Here we demonstrated that p53-inducible microRNA-34a (miR-34a) repressed inosine 5'-monophosphate dehydrogenase (IMPDH), a rate-limiting enzyme of de novo GTP biosynthesis. Treatment with anti-miR-34a inhibitor relieved the expression of IMPDH upon DNA damage. Ultimately, miR-34a-mediated inhibition of IMPDH resulted in repressed activation of the GTP-dependent Ras signaling pathway. In summary, we suggest that p53 has a novel function in regulating purine biosynthesis, aided by miR-34a-dependent IMPDH repression.

Role of p53 within the regulatory network controlling muscle mitochondrial biogenesis

The tumor suppressor protein p53 is recognized to contribute significantly to the regulation of mitochondrial content. Mice without p53 have reduced endurance capacity and muscle performance. However, the function of p53 in muscle remains to be fully established. Understanding how p53 coordinates mitochondrial homeostasis will facilitate a better comprehension of how exercise could constitute as a therapy for cancer treatment.

MicroRNA-211 expression promotes colorectal cancer cell growth in vitro and in vivo by targeting tumor suppressor CHD5

Background

Chromodomain-helicase-DNA-binding protein 5 (CHD5) is a newly identified tumor suppressor that is frequently downregulated in a variety of human cancers. Our previous work revealed that the low expression of CHD5 in colorectal cancer is correlated with CHD5 promoter CpG island hypermethylation. In this study, we investigated the effect of microRNA-211 (miR-211)-regulated CHD5 expression on colorectal tumorigenesis.

Methodology/Principal Findings

miR-211 was predicted to target CHD5 by TargetScan software analysis. A stably expressing exogenous miR-211 colorectal cancer cell line (HCT-116^miR-211) was generated using lentiviral transduction and used as a model for in vitro and in vivo studies. The expression level of miR-211 in HCT-116^miR-211 cells was upregulated by 16-fold compared to vector control cells (HCT-116^vector). Exogenous miR-211 directly binds to the 3′-untranslated region (3′-UTR) of CHD5 mRNA, resulting in a 50% decrease in CHD5 protein level in HCT-116^miR-211 cells. The levels of cell proliferation, tumor growth, and cell migration of HCT-116^miR-211 cells were significantly higher than HCT-116^vector cells under both in vitro and in vivo conditions, as determined using the methods of MTT, colony formation, flow cytometry, scratch assay, and tumor xenografts, respectively. In addition, we found that enforced expression of miR-211 in HCT-116 cells was able to alter p53 pathway-associated regulatory proteins, such as MDM2, Bcl-2, Bcl-xL, and Bax.

Conclusion/Significance

Our results demonstrate that CHD5 is a direct target of miR-211 regulation. Enforced expression of miR-211 promotes tumor cell growth at least in part by downregulating the expression level of the CHD5 tumor suppressor. Our results provide a better understanding of the association of between miR-211-regulated CHD5 expression and CHD5 function in colorectal tumorigenesis.

Mitochondria and cancer: a growing role in apoptosis, cancer cell metabolism and dedifferentiation

At the beginning of the twentieth century, Otto Warburg demonstrated that cancer cells have a peculiar metabolism. These cells preferentially utilise glycolysis for energetic and anabolic purposes, producing large quantities of lactic acid. He defined this unusual metabolism "aerobic glycolysis". At the same time, Warburg hypothesised that a disruption of mitochondrial activities played a precise pathogenic role in cancer. Because of this so-called "Warburg effect", mitochondrial physiology and cellular respiration in particular have been overlooked in pathophysiological studies of cancer. Over time, however, many studies have shown that mitochondria play a fundamental role in cell death by apoptosis or necrosis. Moreover, metabolic enzymes of the Krebs cycle have also recently been recognised as oncosuppressors. Recently, a series of studies were undertaken to re-evaluate the role of oxidative mitochondrial metabolism in cancer cell growth and progression. Some of these data indicate that modulation of mitochondrial respiration may induce an arrest of cancer cell proliferation and differentiation (pseudodifferentiation) and/or or death, suggesting that iatrogenic manipulation of some mitochondrial activities may induce anticancer effects. Moreover, studying the role of mitochondria in cancer cell dedifferentiation/differentiation processes may allow further insight into the pathophysiology and therapy of so-called cancer stem cells.

Intermittent hypoxia activates temporally coordinated transcriptional programs in visceral adipose tissue

Obstructive sleep apnea (OSA) is a prevalent disorder characterized by intermittent hypoxia (IH) during sleep. OSA is strongly associated with obesity and dysregulation of metabolism-yet the molecular pathways linking the effects of IH on adipocyte biology remain unknown. We hypothesized that exposure to IH would activate distinct, time-dependent transcriptional programs in visceral adipose tissue of mice. We exposed 36 mice to IH or normoxia for up to 13 days. We transcriptionally profiled visceral fat tissue harvested from the animals and performed functional enrichment and network analysis on differentially expressed genes. We identified over 3,000 genes with significant expression patterns during the time course of IH exposure. The most enriched pathways mapped to metabolic processes, mitochondrion, and oxidative stress responses. We confirmed the pathophysiological relevance of these findings by demonstrating that mice exposed to chronic IH developed dyslipidemia and underwent significant lipid and protein oxidation within their visceral adipose depots. We applied gene-gene interaction network analysis to identify critical controllers of IH-induced transcriptional programs in adipocytes-these network hubs represent putative targets to modulate the effects of chronic IH on adipose tissue. Our approach to integrate computational methods with gene expression profiling of visceral fat tissue during IH exposure shows promise in helping unravel the mechanistic links between OSA and adipocyte biology.

p53, Stem Cells, and Reprogramming: Tumor Suppression beyond Guarding the Genome

p53 is well recognized as a potent tumor suppressor. In its classic role, p53 responds to genotoxic insults by inducing cell cycle exit or programmed cell death to limit the propagation of cells with corrupted genomes. p53 is also implicated in a variety of other cellular processes in which its involvement is less well understood including self-renewal, differentiation, and reprogramming. These activities represent an emerging area of intense interest for cancer biologists, as they provide potential mechanistic links between p53 loss and the stem cell-like cellular plasticity that has been suggested to contribute to tumor cell heterogeneity and to drive tumor progression. Despite accumulating evidence linking p53 loss to stem-like phenotypes in cancer, it is not yet understood how p53 contributes to acquisition of "stemness" at the molecular level. Whether and how stem-like cells confer survival advantages to propagate the tumor also remain to be resolved. Furthermore, although it seems reasonable that the combination of p53 deficiency and the stem-like state could contribute to the genesis of cancers that are refractory to treatment, direct linkages and mechanistic underpinnings remain under investigation. Here, we discuss recent findings supporting the connection between p53 loss and the emergence of tumor cells bearing functional and molecular similarities to stem cells. We address several potential molecular and cellular mechanisms that may contribute to this link, and we discuss implications of these findings for the way we think about cancer progression.

An indirect role for ASPP1 in limiting p53-dependent p21 expression and cellular senescence

In addition to acting as a transcriptional cofactor for p53, ASPP1 has been shown to function in the cytoplasm to regulate the nuclear localization and activity of YAP/TAZ. We show here that the ability of ASPP1 to activate YAP results in the decreased expression of LATS2, which lowers the ability of p53 to induce p21, cell-cycle arrest and senescence. ASPP1 expression peaks in S-phase, and down-regulation of ASPP1 leads to a reduction in DNA synthesis and enhanced senescence in response to drugs that impede DNA replication. These activities of cytoplasmic ASPP1 in opposing p53-mediated p21 expression are in contrast to the role of nuclear ASPP1 in cooperating with p53 to induce the expression of apoptotic target genes, and may help to dampen p53 activity in normal cells.

The FSHD atrophic myotube phenotype is caused by DUX4 expression

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is linked to deletions in 4q35 within the D4Z4 repeat array in which we identified the double homeobox 4 (DUX4) gene. We found stable DUX4 mRNAs only derived from the most distal D4Z4 unit and unexpectedly extended to the flanking pLAM region that provided an intron and a polyadenylation signal. DUX4 encodes a transcription factor expressed in FSHD but not control primary myoblasts or muscle biopsies. The DUX4 protein initiates a large transcription deregulation cascade leading to muscle atrophy and oxidative stress, which are FSHD key features.

METHODOLOGY/PRINCIPAL FINDINGS

We now show that transfection of myoblasts with a DUX4 expression vector leads to atrophic myotube formation associated with the induction of E3 ubiquitin ligases (MuRF1 and Atrogin1/MAFbx) typical of muscle atrophy. DUX4 induces expression of downstream targets deregulated in FSHD such as mu-crystallin and TP53. We developed specific siRNAs and antisense oligonucleotides (AOs) targeting the DUX4 mRNA. Addition of these antisense agents to primary FSHD myoblast cultures suppressed DUX4 protein expression and affected expression of the above-mentioned markers.

CONCLUSIONS/SIGNIFICANCE

These results constitute a proof of concept for the development of therapeutic approaches for FSHD targeting DUX4 expression.