RAS/MAPK signaling functions in oxidative stress,DNA damage response and cancer progression

Mitogen-activated protein kinase (MAPK) signaling pathways organize a great constitution network that regulates several physiological processes, like cell growth, differentiation, and apoptotic cell death. Due to the crucial importance of this signaling pathway, dysregulation of the MAPK signaling cascades is involved in the pathogenesis of various human cancer types. Oxidative stress and DNA damage are two important factors which in common lead to carcinogenesis through dysregulation of this signaling pathway. Reactive oxygen species (ROS) are a common subproduct of oxidative energy metabolism and are considered to be a significant physiological modulator of several intracellular signaling pathways including the MAPK pathway. Studies demonstrated that the MAP kinases extracellular signal-regulated kinase (ERK) 1/2 and p38 were activated in response to oxidative stress. In addition, DNA damage is a partly common circumstance in cell life and may result in mutation, cancer, and even cell death. Recently, accumulating evidence illustrated that the MEK/ERK pathway is associated with the suitable performance of cellular DNA damage response (DDR), the main pathway of tumor suppression. During DDR, the MEK/ERK pathway is regularly activated, which contributes to the appropriate activation of DDR checkpoints to inhibit cell division. Therefore, the aim of this review is to comprehensively discuss the critical function of MAPK signaling in oxidative stress, DNA damage, and cancer progression.     

Regulation of the mTOR complex 1 pathway by nutrients, growth factors, and stress

The large serine/threonine protein kinase mTOR regulates cellular and organismal homeostasis by coordinating anabolic and catabolic processes with nutrient, energy, and oxygen availability and growth factor signaling. Cells and organisms experience a wide variety of insults that perturb the homeostatic systems governed by mTOR and therefore require appropriate stress responses to allow cells to continue to function. Stress can manifest from an excess or lack of upstream signals or as a result of genetic perturbations in upstream effectors of the pathway. mTOR nucleates two large protein complexes that are important nodes in the pathways that help buffer cells from stresses, and are implicated in the progression of stress-associated phenotypes and diseases, such as aging, tumorigenesis, and diabetes. This review focuses on the key components of the mTOR complex 1 pathway and on how various stresses impinge upon them.     

Akt-dependent and -independent mechanisms of mTOR regulation in cancer

The protein kinase mTOR (mammalian target of rapamycin) is a critical regulator of cellular metabolism, growth, and proliferation. These processes contribute to tumor formation, and many cancers are characterized by aberrant activation of mTOR. Although activating mutations in mTOR itself have not been identified, deregulation of upstream components that regulate mTOR are prevalent in cancer. The prototypic mechanism of mTOR regulation in cells is through activation of the PI3K/Akt pathway, but mTOR receives input from multiple signaling pathways. This review will discuss Akt-dependent and -independent mechanisms of mTOR regulation in response to mitogenic signals, as well as its regulation in response to energy and nutrient-sensing pathways. Preclinical and clinical studies have demonstrated that tumors bearing genetic alterations that activate mTOR are sensitive to pharmacologic inhibition of mTOR. Elucidation of novel pathways that regulate mTOR may help identify predictive factors for sensitivity to mTOR inhibitors, and could provide new therapeutic targets for inhibiting the mTOR pathway in cancer. This review will also highlight pharmacologic approaches that inhibit mTOR via activation of the AMP-activated protein kinase (AMPK), an important inhibitor of the mTOR pathway and an emerging target in cancer.     

Long noncoding RNAs in the mTOR signaling network: biomarkers and therapeutic targets

As an evolutionarily conserved serine/threonine kinase of the phosphoinositide 3-kinase (PI3K) related kinase family, the mechanistic/mammalian target of rapamycin (mTOR) plays vital roles in the PI3K/AKT/mTOR pathway, participating in different cellular processes including cell survival, metabolism and proliferation. Aberrant activity of this signaling pathway may lead to oncogenesis. Over the last two decades, great progress has been made in the understanding of mTOR activation and how its response is counteracted for maintaining tissue homeostasis. Besides regulatory proteins and microRNAs, long noncoding RNA (lncRNA) is another emerging critical layer of the intricate modulatory architecture for the control of the mTOR signaling circuit. Also, the production of numerous lncRNAs is induced by mTOR treatment. These findings offer new perspectives for designing novel diagnostic and therapeutic strategies. In this review, we summarize the interactions between the mTOR signaling pathway and lncRNAs in the development and progression of various types of tumors, focusing on the mechanisms of these interactions, and also discuss the potential use of lncRNAs as biomarkers and therapeutic targets for malignancies.     

AMPK/mTOR信号通路的研究进展

mTOR是细胞生长和增殖的中枢调控因子。mTOR形成2个不同的复合物mTORC1和mTORC2。mTORC1受多种信号调节,如生长因子、氨基酸和细胞能量,同时,mTORC1调节许多重要的细胞过程,包括翻译、转录和自噬。AMPK作为一种关键的生理能量传感器,是细胞和有机体能量平衡的主要调节因子,协调多种代谢途径,平衡能量的供应和需求,最终调节细胞和器官的生长。能量代谢平衡调控是由多个与之相关的信号通路所介导,其中AMPK/mTOR信号通路在细胞内共同构成一个合成代谢和分解代谢过程的开关。此外,AMPK/mTOR信号通路还是一个自噬的重要调控途径。本文着重于目前对AMPK和mTOR信号传导之间关系的了解,讨论了AMPK/mTOR在细胞和有机体能量稳态中的作用。     

Prolyl hydroxylase domain enzymes and their role in cell signaling and cancer metabolism

The prolyl hydroxylase domain (PHD) enzymes regulate the stability of the hypoxia-inducible factor (HIF) in response to oxygen availability. During oxygen limitation, the inhibition of PHD permits the stabilization of HIF, allowing the cellular adaptation to hypoxia. This adaptation is especially important for solid tumors, which are often exposed to a hypoxic environment. However, and despite their original role as the oxygen sensors of the cell, PHD are currently known to display HIF-independent and hydroxylase-independent functions in the control of different cellular pathways, including mTOR pathway, NF-kB pathway, apoptosis and cellular metabolism. In this review, we summarize the recent advances in the regulation and functions of PHD in cancer signaling and cell metabolism.     

mTOR in programmed cell death and its therapeutic implications

Mechanistic target of rapamycin (mTOR), a highly conserved serine/threonine kinase, is involved in cellular metabolism, protein synthesis, and cell death. Programmed cell death (PCD) assists in eliminating aging, damaged, or neoplastic cells, and is indispensable for sustaining normal growth, fighting pathogenic microorganisms, and maintaining body homeostasis. mTOR has crucial functions in the intricate signaling pathway network of multiple forms of PCD. mTOR can inhibit autophagy, which is part of PCD regulation. Cell survival is affected by mTOR through autophagy to control reactive oxygen species production and the degradation of pertinent proteins. Additionally, mTOR can regulate PCD in an autophagy-independent manner by affecting the expression levels of related genes and phosphorylating proteins. Therefore, mTOR acts through both autophagy-dependent and -independent pathways to regulate PCD. It is conceivable that mTOR exerts bidirectional regulation of PCD, such as ferroptosis, according to the complexity of signaling pathway networks, but the underlying mechanisms have not been fully explained. This review summarizes the recent advances in understanding mTOR-mediated regulatory mechanisms in PCD. Rigorous investigations into PCD-related signaling pathways have provided prospective therapeutic targets that may be clinically beneficial for treating various diseases.     

DNA damage response and repair in ovarian cancer: Potential targets for therapeutic strategies

Ovarian cancer is among the most lethal gynecologic malignancies with a poor survival prognosis. The current therapeutic strategies involve surgery and chemotherapy. Research is now focused on novel agents especially those targeting DNA damage response (DDR) pathways. Understanding the DDR process in ovarian cancer necessitates having a detailed knowledge on a series of signaling mediators at the cellular and molecular levels. The complexity of the DDR process in ovarian cancer and how this process works in metastatic conditions is comprehensively reviewed. For evaluating the efficacy of therapeutic agents targeting DNA damage in ovarian cancer, we will discuss the components of this system including DDR sensors, DDR transducers, DDR mediators, and DDR effectors. The constituent pathways include DNA repair machinery, cell cycle checkpoints, and apoptotic pathways. We also will assess the potential of active mediators involved in the DDR process such as therapeutic and prognostic candidates that may facilitate future studies.     

Targeting the mTOR signaling network in cancer

The mammalian target of rapamycin (mTOR) is an unconventional protein kinase that is centrally involved in the control of cancer cell metabolism, growth and proliferation. The mTOR pathway has attracted broad scientific and clinical interest, particularly in light of the ongoing clinical cancer trials with mTOR inhibitors. The mixed clinical results to date reflect the complexity of both cancer as a disease target, and the mTOR signaling network, which contains two functionally distinct mTOR complexes, parallel regulatory pathways, and feedback loops that contribute to the variable cellular responses to the current inhibitors. In this review, we discuss the regulatory pathways that govern mTOR activity, and highlight clinical results obtained with the first generation of mTOR inhibitors to reach the oncology clinics.     

Targeting the mTOR regulatory network in hepatocellular carcinoma: Are we making headway?

The mechanistic target of rapamycin (mTOR) pathway coordinates organismal growth and homeostasis in response to growth factors, nutrients, and cellular energy stage. The pathway regulates several major cellular processes and is implicated in various pathological conditions, including hepatocellular carcinoma (HCC). This review summarizes recent advances of the mTOR pathway, highlights the potential of the mTOR pathway as a therapeutic target, and explores clinical trials targeting the mTOR pathway in HCC. Although the review focuses on the mTOR pathway involved in HCC, more comprehensive discussions (eg, developing a rational design for future trials targeting the mTOR pathway) are also applicable to other tumors.     

mTORC1 signaling in hepatic lipid metabolism

The mechanistic target of rapamycin (mTOR) signaling pathway regulates many metabolic and physiological processes in different organs or tissues. Dysregulation of mTOR signaling has been implicated in many human diseases including obesity, diabetes, cancer, fatty liver diseases, and neuronal disorders. Here we review recent progress in understanding how mTORC1 (mTOR complex 1) signaling regulates lipid metabolism in the liver.     

mTOR-Dependent Cell Survival Mechanisms

The mechanistic target of rapamycin (mTOR) kinase is a conserved regulator of cell growth, proliferation, and survival. In cells, mTOR is the catalytic subunit of two complexes called mTORC1 and mTORC2, which have distinct upstream regulatory signals and downstream substrates. mTORC1 directly senses cellular nutrient availability while indirectly sensing circulating nutrients through growth factor signaling pathways. Cellular stresses that restrict growth also impinge on mTORC1 activity. mTORC2 is less well understood and appears only to sense growth factors. As an integrator of diverse growth regulatory signals, mTOR evolved to be a central signaling hub for controlling cellular metabolism and energy homoeostasis, and defects in mTOR signaling are important in the pathologies of cancer, diabetes, and aging. Here we discuss mechanisms by which each mTOR complex might regulate cell survival in response to metabolic and other stresses.The mechanistic target of rapamycin (mTOR) is a highly conserved serine/threonine protein kinase belonging to the phosphatidylinositol kinase-related kinase (PIKK) family and in mammalian cells is a central regulator of cell growth, proliferation, and survival (for review, see Sengupta et al. 2010; Zoncu et al. 2010). As its name implies, mTOR is the target of the naturally occurring compound rapamycin, which in association with the FK506-binding protein (FKBP12) is an allosteric inhibitor of mTOR. Although rapamycin is now known to only partially inhibit mTOR activity, derivatives of the drug have important clinical applications in oncology, in preventing restenosis after angioplasty, and as an immunosuppressant following organ transplants.     

MKK7 and ARF

Sensing, integrating, and processing of stressogenic signals must be followed by accurate differential response(s) for a cell to survive and avoid malignant transformation. The DNA damage response (DDR) pathway is vital in this process, as it deals with genotoxic/oncogenic insults, having p53 as a nodal effector that performs most of the above tasks. Accumulating data reveal that other pathways are also involved in the same or similar processes, conveying also to p53. Emerging questions are if, how, and when these additional pathways communicate with the DDR axis. Two such stress response pathways, involving the MKK7 stress-activated protein kinase (SAPK) and ARF, have been shown to be interlocked with the ATM/ATR-regulated DDR axis in a highly ordered manner. This creates a new landscape in the DDR orchestrated response to genotoxic/oncogenic insults that is currently discussed.     

Sestrin2 integrates Akt and mTOR signaling to protect cells against energetic stress-induced death

The phosphoinositide-3 kinase/Akt (PI3K/Akt) pathway has a central role in cancer cell metabolism and proliferation. More importantly, it is one of the cardinal pro-survival pathways mediating resistance to apoptosis. The role of Akt in response to an energetic stress is presently unclear. Here, we show that Sestrin2 (Sesn2), also known as Hi95, a p53 target gene that protects cells against oxidative and genotoxic stresses, participates in the protective role of Akt in response to an energetic stress induced by 2-deoxyglucose (2-DG). Sesn2 is upregulated in response to an energetic stress such as 2-DG and metformin, and mediates the inhibition of mammalian target of rapamycin (mTOR), the major cellular regulator of energy metabolism. The increase of Sesn2 is independent of p53 but requires the anti-apoptotic pathway, PI3K/Akt. Inhibition of Akt, as well as loss of Sesn2, sensitizes cells to 2-DG-induced apoptosis. In addition, the rescue of Sesn2 partially reverses the pro-apoptotic effects of 2-DG. In conclusion, we identify Sesn2 as a new energetic stress sensor, which appears to be protective against energetic stress-induced apoptosis that integrates the pro-survival function of Akt and the negative regulation of mTOR.     

Beyond ATM: the protein kinase landscape of the DNA damage response

The DNA of all organisms is constantly subjected to damaging agents, both exogenous and endogenous. One extremely harmful lesion is the double-strand break (DSB), which activates a massive signaling network - the DNA damage response (DDR). The chief activator of the DSB response is the ATM protein kinase, which phosphorylates numerous key players in its various branches. Recent phosphoproteomic screens have extended the scope of damage-induced phosphorylations beyond the direct ATM substrates. We review the evidence for the involvement of numerous other protein kinases in the DDR, obtained from documentation of specific pathways as well as high-throughput screens. The emerging picture of the protein phosphorylation landscape in the DDR broadens the current view on the role of this protein modification in the maintenance of genomic stability. Extensive cross-talk between many of these protein kinases forms an interlaced signaling network that spans numerous cellular processes. Versatile protein kinases in this network affect pathways that are different from those they have been identified with to date. The DDR appears to be one of the most extensive signaling responses to cellular stimuli.     

Elevated cyclin G2 expression intersects with DNA damage checkpoint signaling and is required for a potent G2/M checkpoint arrest response to doxorubicin

To maintain genomic integrity DNA damage response (DDR), signaling pathways have evolved that restrict cellular replication and allow time for DNA repair. CCNG2 encodes an unconventional cyclin homolog, cyclin G2 (CycG2), linked to growth inhibition. Its expression is repressed by mitogens but up-regulated during cell cycle arrest responses to anti-proliferative signals. Here we investigate the potential link between elevated CycG2 expression and DDR signaling pathways. Expanding our previous finding that CycG2 overexpression induces a p53-dependent G(1)/S phase cell cycle arrest in HCT116 cells, we now demonstrate that this arrest response also requires the DDR checkpoint protein kinase Chk2. In accord with this finding we establish that ectopic CycG2 expression increases phosphorylation of Chk2 on threonine 68. We show that DNA double strand break-inducing chemotherapeutics stimulate CycG2 expression and correlate its up-regulation with checkpoint-induced cell cycle arrest and phospho-modification of proteins in the ataxia telangiectasia mutated (ATM) and ATM and Rad3-related (ATR) signaling pathways. Using pharmacological inhibitors and ATM-deficient cell lines, we delineate the DDR kinase pathway promoting CycG2 up-regulation in response to doxorubicin. Importantly, RNAi-mediated blunting of CycG2 attenuates doxorubicin-induced cell cycle checkpoint responses in multiple cell lines. Employing stable clones, we test the effect that CycG2 depletion has on DDR proteins and signals that enforce cell cycle checkpoint arrest. Our results suggest that CycG2 contributes to DNA damage-induced G(2)/M checkpoint by enforcing checkpoint inhibition of CycB1-Cdc2 complexes.     

DNA Damage Sensing by the ATM and ATR Kinases

In eukaryotic cells, maintenance of genomic stability relies on the coordinated action of a network of cellular processes, including DNA replication, DNA repair, cell-cycle progression, and others. The DNA damage response (DDR) signaling pathway orchestrated by the ATM and ATR kinases is the central regulator of this network in response to DNA damage. Both ATM and ATR are activated by DNA damage and DNA replication stress, but their DNA-damage specificities are distinct and their functions are not redundant. Furthermore, ATM and ATR often work together to signal DNA damage and regulate downstream processes. Here, we will discuss the recent findings and current models of how ATM and ATR sense DNA damage, how they are activated by DNA damage, and how they function in concert to regulate the DDR.During their lifespan, cells are inevitably challenged by extrinsic and intrinsic stresses that threaten the integrity of their genomes. To survive these adverse conditions and pass on intact genetic information to subsequent generations, cells have evolved a highly organized and coordinated effort to ameliorate genotoxic stress called the DNA damage response (DDR). This response underlies the organismal ability to sense and signal problems in its DNA, to arrest cell-cycle progression (cell-cycle checkpoints) and activate appropriate DNA repair mechanisms, or to eliminate cells with unrepairable genomes. The importance of the DDR network for the development and well being of humans is illustrated by the large variety of diseases and cancer-predisposition syndromes that have been linked to mutations of DDR genes (Ciccia and Elledge 2010).In eukaryotic cells, the cellular response to DNA damage is regulated and coordinated by the DDR signaling pathway. Like classic signal transduction pathways, the DDR uses signal sensors, transducers, and effectors (Fig. 1) (Zhou and Elledge 2000). In contrast to the signal transduction pathways that are activated by ligands of receptor kinases, the DDR signaling pathway is activated by aberrant DNA structures induced by DNA damage or DNA replication stress. The sensors of this pathway are the proteins that directly recognize these aberrant DNA structures and activate the most upstream DDR kinases. The DDR signaling pathway consists of a protein kinase cascade as well as mediator proteins that facilitate the phosphorylation events within the DDR network. The effectors of the DDR signaling pathway are substrates of the DDR kinases that participate in a wide spectrum of cellular processes important for genomic stability, such as DNA replication, DNA repair, and cell-cycle control. Although unique in the way through which it is activated, the DDR is a bona fide signal transduction pathway that is primarily driven by protein phosphorylation.Open in a separate windowFigure 1.The framework of the DDR signaling pathway. Like other signal transduction pathways, the DDR signaling pathway consists of signal sensors, transducers, and effectors. The sensors of this pathway are proteins that recognize DNA structures induced by DNA damage and DNA replication stress. The transducers of this pathway are kinases, including ATM, ATR, and their downstream kinases. The effectors of this pathway are substrates of ATM, ATR, and their downstream kinases. These effectors of ATM and ATR are involved in a broad spectrum of cellular processes that are important for maintenance of genomic stability of organisms.In mammalian cells, the ATM (ataxia-telangiectasia mutated), ATR (ATM- and Rad3-Related), and DNA-PKcs (DNA-dependent protein kinase) kinases are the most upstream DDR kinases. These large serine/threonine kinases are members of the phosphatidylinositol-3-kinase-like kinase family (PIKKs) (Lempiainen and Halazonetis 2009; Lovejoy and Cortez 2009). In response to DNA damage, hundreds of proteins are phosphorylated at Ser/Thr-Glu motifs and additional sites in an ATM- or ATR-dependent manner, whereas DNA-PKcs appears to regulate a smaller number of targets and play a role primarily in nonhomologous end joining (NHEJ) (Matsuoka et al. 2007; Smolka et al. 2007; Stokes et al. 2007; Bensimon et al. 2010; Beli et al. 2012). ATM and ATR also activate a second wave of phosphorylation through their activation of Chk1, Chk2, and MK2 protein kinases (Matsuoka et al. 1998; Liu et al. 2000; Reinhardt et al. 2007). Although the functional significance of many of these phosphorylation events remains to be understood, the potential ATM/ATR-regulated substrates are involved in a wide variety of cellular processes that may contribute to the DDR. In the picture emerging from these genome-scale studies, ATM and ATR are the master transducers of DNA signals, and they orchestrate a large network of cellular processes to maintain genomic integrity. In vivo and in vitro studies also suggest that the DNA-damage specificities and functions of ATM and ATR are distinct. Whereas ATM is primarily activated by double-stranded DNA breaks (DSBs), ATR responds to a broad spectrum of DNA damage, including DSBs and a variety of DNA lesions that interfere with replication. In this review, we will discuss how ATM and ATR are activated by DNA damage, how they are distinct from each other, and how they function in concert to regulate the DDR.