Showing papers in "Oncogene in 2003"

Cisplatin: mode of cytotoxic action and molecular basis of resistance

Abstract: Cisplatin is one of the most potent antitumor agents known, displaying clinical activity against a wide variety of solid tumors. Its cytotoxic mode of action is mediated by its interaction with DNA to form DNA adducts, primarily intrastrand crosslink adducts, which activate several signal transduction pathways, including those involving ATR, p53, p73, and MAPK, and culminate in the activation of apoptosis. DNA damage-mediated apoptotic signals, however, can be attenuated, and the resistance that ensues is a major limitation of cisplatin-based chemotherapy. The mechanisms responsible for cisplatin resistance are several, and contribute to the multifactorial nature of the problem. Resistance mechanisms that limit the extent of DNA damage include reduced drug uptake, increased drug inactivation, and increased DNA adduct repair. Origins of these pharmacologic-based mechanisms, however, are at the molecular level. Mechanisms that inhibit propagation of the DNA damage signal to the apoptotic machinery include loss of damage recognition, overexpression of HER-2/neu, activation of the PI3-K/Akt (also known as PI3-K/PKB) pathway, loss of p53 function, overexpression of antiapoptotic bcl-2, and interference in caspase activation. The molecular signature defining the resistant phenotype varies between tumors, and the number of resistance mechanisms activated in response to selection pressures dictates the overall extent of cisplatin resistance.

MALAT-1, a novel noncoding RNA, and thymosin beta4 predict metastasis and survival in early-stage non-small cell lung cancer.

Abstract: Early-stage non-small cell lung cancer (NSCLC) can be cured by surgical resection, but a substantial fraction of patients ultimately dies due to distant metastasis. In this study, we used subtractive hybridization to identify gene expression differences in stage I NSCLC tumors that either did or did not metastasize in the course of disease. Individual clones (n=225) were sequenced and quantitative RT-PCR verified overexpression in metastasizing samples. Several of the identified genes (eIF4A1, thymosin beta4 and a novel transcript named MALAT-1) were demonstrated to be significantly associated with metastasis in NSCLC patients (n=70). The genes' association with metastasis was stage- and histology specific. The Kaplan-Meier analyses identified MALAT-1 and thymosin beta4 as prognostic parameters for patient survival in stage I NSCLC. The novel MALAT-1 transcript is a noncoding RNA of more than 8000 nt expressed from chromosome 11q13. It is highly expressed in lung, pancreas and other healthy organs as well as in NSCLC. MALAT-1 expressed sequences are conserved across several species indicating its potentially important function. Taken together, these data contribute to the identification of early-stage NSCLC patients that are at high risk to develop metastasis. The identification of MALAT-1 emphasizes the potential role of noncoding RNAs in human cancer.

The Bcl-2 family: roles in cell survival and oncogenesis.

Abstract: Apoptosis, the cell-suicide programme executed by caspases, is critical for maintaining tissue homeostasis, and impaired apoptosis is now recognized to be a key step in tumorigenesis. Whether a cell should live or die is largely determined by the Bcl-2 family of anti- and proapoptotic regulators. These proteins respond to cues from various forms of intracellular stress, such as DNA damage or cytokine deprivation, and interact with opposing family members to determine whether or not the caspase proteolytic cascade should be unleashed. This review summarizes current views of how these proteins sense stress, interact with their relatives, perturb organelles such as the mitochondrion and endoplasmic reticulum and govern pathways to caspase activation. It briefly explores how family members influence cell-cycle entry and outlines the evidence for their involvement in tumour development, both as oncoproteins and tumour suppressors. Finally, it discusses the promise of novel anticancer therapeutics that target these vital regulators.

Control of apoptosis by p53.

Abstract: The p53 tumor suppressor acts to integrate multiple stress signals into a series of diverse antiproliferative responses. One of the most important p53 functions is its ability to activate apoptosis, and disruption of this process can promote tumor progression and chemoresistance. p53 apparently promotes apoptosis through transcription-dependent and -independent mechanisms that act in concert to ensure that the cell death program proceeds efficiently. Moreover, the apoptotic activity of p53 is tightly controlled, and is influenced by a series of quantitative and qualitative events that influence the outcome of p53 activation. Interestingly, other p53 family members can also promote apoptosis, either in parallel or in concert with p53. Although incomplete, our current understanding of p53 illustrates how apoptosis can be integrated into a larger tumor suppressor network controlled by different signals, environmental factors, and cell type. Understanding this network in more detail will provide insights into cancer and other diseases, and will identify strategies to improve their therapeutic treatment.

The role of glutathione-S-transferase in anti-cancer drug resistance

Abstract: Glutathione-S-transferases (GSTs) are a family of Phase II detoxification enzymes that catalyse the conjugation of glutathione (GSH) to a wide variety of endogenous and exogenous electrophilic compounds. GSTs are divided into two distinct super-family members: the membrane-bound microsomal and cytosolic family members. Microsomal GSTs are structurally distinct from the cytosolic in that they homo- and heterotrimerize rather than dimerize to form a single active site. Microsomal GSTs play a key role in the endogenous metabolism of leukotrienes and prostaglandins. Human cytosolic GSTs are highly polymorphic and can be divided into six classes: alpha, mu, omega, pi, theta, and zeta. The pi and mu classes of GSTs play a regulatory role in the mitogen-activated protein (MAP) kinase pathway that participates in cellular survival and death signals via protein : protein interactions with c-Jun N-terminal kinase 1 (JNK1) and ASK1 (apoptosis signal-regulating kinase). JNK and ASK1 are activated in response to cellular stress. GSTs have been implicated in the development of resistance toward chemotherapy agents. It is plausible that GSTs serve two distinct roles in the development of drug resistance via direct detoxification as well as acting as an inhibitor of the MAP kinase pathway. The link between GSTs and the MAP kinase pathway provides a rationale as to why in many cases the drugs used to select for resistance are neither subject to conjugation with GSH, nor substrates for GSTs. GSTs have emerged as a promising therapeutic target because specific isozymes are overexpressed in a wide variety of tumors and may play a role in the etiology of other diseases, including neurodegenerative diseases, multiple sclerosis, and asthma. Some of the therapeutic strategies so far employed are described in this review.

A decade of caspases

Abstract: Caspases are a family of cysteine proteases that play important roles in regulating apoptosis. A decade of research has generated a wealth of information on the signal transduction pathways mediated by caspases, the distinct functions of individual caspases and the mechanisms by which caspases mediate apoptosis and a variety of physiological and pathological processes.

PI3K/Akt and apoptosis: size matters.

Abstract: Recent research has examined Akt and Akt-related serine-threonine kinases in signaling cascades that regulate cell survival and are important in the pathogenesis of degenerative diseases and in cancer. We seek to recapitulate the research that has helped to define the current understanding of the role of the Akt pathway under normal and pathologic conditions, also in view of genetic models of Akt function. In particular, we will evaluate the mechanisms of Akt regulation and the role of Akt substrates in Akt-dependent biologic responses in the decisions of cell death and cell survival. Here, we hope to establish the mechanisms of apoptosis suppression by Akt kinase as a framework for a more general understanding of growth factor-dependent regulation of cell survival.

Multidrug resistance mediated by the breast cancer resistance protein BCRP (ABCG2)

Abstract: Observations of functional adenosine triphosphate (ATP)-dependent drug efflux in certain multidrug-resistant cancer cell lines without overexpression of P-glycoprotein or multidrug resistance protein (MRP) family members suggested the existence of another ATP-binding cassette (ABC) transporter capable of causing cancer drug resistance. In one such cell line (MCF-7/AdrVp), the overexpression of a novel member of the G subfamily of ABC transporters was found. The new transporter was termed the breast cancer resistance protein (BCRP), because of its identification in MCF-7 human breast carcinoma cells. BCRP is a 655 amino-acid polypeptide, formally designated as ABCG2. Like all members of the ABC G (white) subfamily, BCRP is a half transporter. Transfection and enforced overexpression of BCRP in drug-sensitive MCF-7 or MDA-MB-231 cells recapitulates the drug-resistance phenotype of MCF-7/AdrVp cells, consistent with current evidence suggesting that functional BCRP is a homodimer. BCRP maps to chromosome 4q22, downstream from a TATA-less promoter. The spectrum of anticancer drugs effluxed by BCRP includes mitoxantrone, camptothecin-derived and indolocarbazole topoisomerase I inhibitors, methotrexate, flavopiridol, and quinazoline ErbB1 inhibitors. Transport of anthracyclines is variable and appears to depend on the presence of a BCRP mutation at codon 482. Potent and specific inhibitors of BCRP are now being developed, opening the door to clinical applications of BCRP inhibition. Owing to tissue localization in the placenta, bile canaliculi, colon, small bowel, and brain microvessel endothelium, BCRP may play a role in protecting the organism from potentially harmful xenobiotics. BCRP expression has also been demonstrated in pluripotential 'side population' stem cells, responsible for the characteristic ability of these cells to exclude Hoechst 33342 dye, and possibly for the maintenance of the stem cell phenotype. Studies are emerging on the role of BCRP expression in drug resistance in clinical cancers. More prospective studies are needed, preferably combining BCRP protein or mRNA quantification with functional assays, in order to determine the contribution of BCRP to drug resistance in human cancers.

P-glycoprotein: from genomics to mechanism

Abstract: Resistance to chemically different natural product anti-cancer drugs (multidrug resistance, or MDR) results from decreased drug accumulation, resulting from expression of one or more ATP-dependent efflux pumps. The first of these to be identified was P-glycoprotein (P-gp), the product of the human MDR1 gene, localized to chromosome 7q21. P-gp is a member of the large ATP-binding cassette (ABC) family of proteins. Although its crystallographic 3-D structure is yet to be determined, sequence analysis and comparison to other ABC family members suggest a structure consisting of two transmembrane (TM) domains, each with six TM segments, and two nucleotide-binding domains. In the epithelial cells of the gastrointestinal tract, liver, and kidney, and capillaries of the brain, testes, and ovaries, P-gp acts as a barrier to the uptake of xenobiotics, and promotes their excretion in the bile and urine. Polymorphisms in the MDR1 gene may affect the pharmacokinetics of many commonly used drugs, including anticancer drugs. Substrate recognition of many different drugs occurs within the TM domains in multiple-overlapping binding sites. We have proposed a model for how ATP energizes transfer of substrates from these binding sites on P-gp to the outside of the cell, which accounts for the apparent stoichiometry of two ATPs hydrolysed per molecule of drug transported. Understanding of the biology, genetics, and biochemistry of P-gp promises to improve the treatment of cancer and explain the pharmacokinetics of many commonly used drugs.

Survivin, versatile modulation of cell division and apoptosis in cancer

Abstract: Survivin is a member of the inhibitor of apoptosis (IAP) gene family that has attracted attention from several viewpoints of basic and translational research. Its cell cycle-regulated expression at mitosis and association with the mitotic apparatus have been of interest to cell biologists studying faithful segregation of sister chromatids and timely separation of daughter cells. Investigators interested in mechanisms of apoptosis have found survivin an evolving challenge: while survivin inhibits apoptosis in vitro and in vivo, this pathway may be more selective as compared to cytoprotection mediated by other IAPs. Finally, basic and translational researchers in cancer biology have converged on survivin as a pivotal cancer gene, not simply for its sharp expression in tumors and not in normal tissues, but also for the potential exploitation of this pathway in cancer diagnosis and therapy. The objective of the present contribution is to line up current evidence and emerging concepts on the multifaceted functions of survivin in cell death and cell division, and how this pathway is being pursued for novel cancer therapeutic strategies.

TRAIL and apoptosis induction by TNF-family death receptors.

Abstract: Tumor necrosis factor-related apoptosis-inducing ligand or Apo 2 ligand (TRAIL/Apo2L) is a member of the tumor necrosis factor (TNF) family of ligands capable of initiating apoptosis through engagement of its death receptors. TRAIL selectively induces apoptosis of a variety of tumor cells and transformed cells, but not most normal cells, and therefore has garnered intense interest as a promising agent for cancer therapy. TRAIL is expressed on different cells of the immune system and plays a role in both T-cell- and natural killer cell-mediated tumor surveillance and suppression of suppressing tumor metastasis. Some mismatch-repair-deficient tumors evade TRAIL-induced apoptosis and acquire TRAIL resistance through different mechanisms. Death receptors, members of the TNF receptor family, signal apoptosis independently of the p53 tumor-suppressor gene. TRAIL treatment in combination with chemo- or radiotherapy enhances TRAIL sensitivity or reverses TRAIL resistance by regulating the downstream effectors. Efforts to identify agents that activate death receptors or block specific effectors may improve therapeutic design. In this review, we summarize recent insights into the apoptosis-signaling pathways stimulated by TRAIL, present our current understanding of the physiological role of this ligand and the potential of its application for cancer therapy and prevention.

Apoptosis and melanoma chemoresistance.

Abstract: Melanoma is the most aggressive form of skin cancer and is notoriously resistant to all current modalities of cancer therapy. A large set of genetic, functional and biochemical studies suggest that melanoma cells become 'bullet proof' against a variety of chemotherapeutic drugs by exploiting their intrinsic resistance to apoptosis and by reprogramming their proliferation and survival pathways during melanoma progression. In recent years, the identification of molecules involved in the regulation and execution of apoptosis, and their alteration in melanoma, have provided new insights into the molecular basis for melanoma chemoresistance. With this knowledge in hand, the challenge is now to devise strategies potent enough to compensate or bypass these cell death defects and improve the actual poor prognosis of patients at late stages of the disease.

Regulation of apoptosis by endoplasmic reticulum pathways

Abstract: Apoptotic programmed cell death pathways are activated by a diverse array of cell extrinsic and intrinsic signals, most of which are ultimately coupled to the activation of effector caspases. In many instances, this involves an obligate propagation through mitochondria, causing egress of critical proapoptotic regulators to the cytosol. Central to the regulation of the mitochondrial checkpoint is a complex three-way interplay between members of the BCL-2 family, which are comprised of an antiapoptotic subgroup including BCL-2 itself, and the proapoptotic BAX,BAK and BH3-domain-only subgroups. Constituents of all three of these BCL-2 classes, however, also converge on the endoplasmic reticulum (ER), an organelle whose critical contributions to apoptosis is only now becoming apparent. In addition to propagating death-inducing stress signals itself, the ER also contributes in a fundamental way to Fas-mediated apoptosis and to p53-dependent pathways resulting from DNA damage and oncogene expression. Mobilization of ER calcium stores can initiate the activation of cytoplasmic death pathways as well as sensitize mitochondria to direct proapoptotic stimuli. Additionally, the existence of BCL-2-regulated initiator procaspase activation complexes at the ER membrane has also been described. Here, we review the potential underlying mechanisms involved in these events and discuss pathways for ER-mitochondrial crosstalk pertinent to a number of cell death stimuli.

Rituximab (monoclonal anti-CD20 antibody): mechanisms of action and resistance.

Abstract: Rituximab, a chimeric monoclonal antibody targeted against the pan-B-cell marker CD20, was the first monoclonal antibody to be approved for therapeutic use. Treatment with rituximab at standard weekly dosing is effective in more than 50% of patients with relapsed or refractory CD20-positive follicular non-Hodgkin's lymphoma, but is not curative. It is less effective in other subtypes of CD20-positive lymphoma and for retreatment, even with CD20 still expressed. Thus, binding of rituximab to CD20 is not sufficient to kill many lymphoma cells, indicating that there are mechanisms of resistance. Mechanisms of cell destruction that have been demonstrated to be activated by rituximab binding to CD20 include direct signaling of apoptosis, complement activation and cell-mediated cytotoxicity. The relative importance of each of these mechanisms in determining clinical response to rituximab treatment remains a matter of conjecture. Thus, the role of various resistance pathways, some documented in experimental systems and others still hypothetical, remains uncertain. Resistance could potentially be mediated by alterations in CD20 expression or signaling, elevated apoptotic threshold, modulation of complement activity or diminished cellular cytotoxicity. As the first of an expanding class of anticancer agents, lessons learned regarding the mechanism of rituximab action and resistance will be of increasing importance.

To be, or not to be: NF-kappaB is the answer--role of Rel/NF-kappaB in the regulation of apoptosis.

Abstract: To be, or not to be: NF- κ B is the answer – role of Rel/NF- κ B in the regulation of apoptosis

Mechanisms of Taxol resistance related to microtubules.

Abstract: Since its approval by the FDA in 1992 for the treatment of ovarian cancer, the use of Taxol has dramatically increased. Although treatment with Taxol has led to improvement in the duration and quality of life for some cancer patients, the majority eventually develop progressive disease after initially responding to Taxol treatment. Drug resistance represents a major obstacle to improving the overall response and survival of cancer patients. This review focuses on mechanisms of Taxol resistance that occur directly at the microtubule, such as mutations, tubulin isotype selection and post-translational modifications, and also at the level of regulatory proteins. A review of tubulin structure, microtubule dynamics, the mechanism of action of Taxol and its binding site on the microtubule are included, so that the reader can evaluate Taxol resistance in context.

BRAF mutations and RET/PTC rearrangements are alternative events in the etiopathogenesis of PTC

Abstract: Rearrangement of RET proto-oncogene is the major event in the etiopathogenesis of papillary thyroid carcinoma (PTC). We report a high prevalence of BRAF(V599E) mutation in sporadic PTC and in PTC-derived cell lines. The BRAF(V599E) mutation was detected in 23 of 50 PTC (46%) and in three of four PTC-derived cell lines. The prevalence of the BRAF(V599E) mutation in PTC is the highest reported to date in human carcinomas, being only exceeded by melanoma. PTC with RET/PTC rearrangement as well as the TPC-1 cell line (the only one harboring RET/PTC rearrangement) did not show the BRAF(V599E) mutation. BRAF(V599E) mutation was not detected in any of 23 nodular goiters, 51 follicular adenomas and 18 follicular carcinomas. A distinct mutation in BRAF (codon K600E) was detected in a follicular adenoma. Activating mutations in RAS genes were detected in 15% of FA, 33% of FTC and 7% of PTC. BRAF(V599E) mutation did not coexist with alterations in any of the RAS genes in any of the tumors. These results suggest that BRAF(V599E) mutation is frequent in the etiopathogenesis of PTC. The BRAF(V599E) mutation appears to be an alternative event to RET/PTC rearrangement rather than to RAS mutations, which are rare in PTC. BRAF(V599E) may represent an alternative pathway to oncogenic MAPK activation in PTCs without RET/PTC activation.

The MRP family of drug efflux pumps.

Abstract: The MRP family is comprised of nine related ABC transporters that are able to transport structurally diverse lipophilic anions and function as drug efflux pumps. Investigations of this family have provided insights not only into cellular resistance mechanisms associated with natural product chemotherapeutic agents, antifolates and nucleotide analogs, but also into factors that influence drug distribution in the body, membrane systems that are involved in the extrusion of reduced folates, cysteinyl leukotrienes and bile acids, and the molecular basis of two hereditary conditions in humans. The review will describe the biochemical properties, drug resistance activities and potential in vivo functions of these unusual pumps.

Axis of evil: molecular mechanisms of cancer metastasis.

Abstract: Although the genetic basis of tumorigenesis may vary greatly between different cancer types, the cellular and molecular steps required for metastasis are similar for all cancer cells. Not surprisingly, the molecular mechanisms that propel invasive growth and metastasis are also found in embryonic development, and to a less perpetual extent, in adult tissue repair processes. It is increasingly apparent that the stromal microenvironment, in which neoplastic cells develop, profoundly influences many steps of cancer progression, including the ability of tumor cells to metastasize. In carcinomas, the influences of the microenvironment are mediated, in large part, by bidirectional interactions (adhesion, survival, proteolysis, migration, immune escape mechanisms lymph-/angiogenesis, and homing on target organs) between epithelial tumor cells and neighboring stromal cells, such as fibroblasts as well as endothelial and immune cells. In this review, we summarize recent advances in understanding the molecular mechanisms that govern this frequently lethal metastatic progression along an axis from primary tumor to regional lymph nodes to distant organ sites. Affected proteins include growth factor signaling molecules, chemokines, cell-cell adhesion molecules (cadherins, integrins) as well as extracellular proteases (matrix metalloproteinases). We then discuss promising new therapeutic approaches targeting the microenvironment. We note, however, that there is still too little knowledge of how the many events are coordinated and integrated by the cancer cell, with conspiratorial help by the stromal component of the host. Before drug development can proceed with a legitimate chance of success, significant gaps in basic knowledge need to be filled.

MAPK pathways in radiation responses.

Abstract: Within the last 15 years, multiple new signal transduction pathways within cells have been discovered. Many of these pathways belong to what is now termed 'the mitogen-activated protein kinase (MAPK) superfamily.' These pathways have been linked to the growth factor-mediated regulation of diverse cellular events such as proliferation, senescence, differentiation and apoptosis. Based on currently available data, exposure of cells to ionizing radiation and a variety of other toxic stresses induces simultaneous compensatory activation of multiple MAPK pathways. These signals play critical roles in controlling cell survival and repopulation effects following irradiation, in a cell-type-dependent manner. Some of the signaling pathways activated following radiation exposure are those normally activated by mitogens, such as the 'classical' MAPK (also known as the ERK) pathway. Other MAPK pathways activated by radiation include those downstream of death receptors and procaspases, and DNA-damage signals, including the JNK and P38 MAPK pathways. The expression and release of autocrine growth factor ligands, such as (transforming growth factor alpha) and TNF-alpha, following irradiation can also enhance the responses of MAPK pathways in cells and, consequently, of bystander cells. Thus, the ability of radiation to activate MAPK signaling pathways may depend on the expression of multiple growth factor receptors, autocrine factors and Ras mutation. Enhanced basal signaling by proto-oncogenes such as K-/H-/N-RAS may provide a radioprotective and growth-promoting signal. In many cell types, this may be via the PI3K pathway; in others, this may occur through nuclear factor-kappa B or multiple MAPK pathways. This review will describe the enzymes within the known MAPK signaling pathways and discuss their activation and roles in cellular radiation responses.

Regulation and mechanisms of mammalian double-strand break repair

Abstract: The double-strand break (DSB) is believed to be one of the most severe types of DNA damage, and if left unrepaired is lethal to the cell. Several different types of repair act on the DSB. The most important in mammalian cells are nonhomologous end-joining (NHEJ) and homologous recombination repair (HRR). NHEJ is the predominant type of DSB repair in mammalian cells, as opposed to lower eucaryotes, but HRR has recently been implicated in critical cell signaling and regulatory functions that are essential for cell viability. Whereas NHEJ repair appears constitutive, HRR is regulated by the cell cycle and inducible signal transduction pathways. More is known about the molecular details of NHEJ than HRR in mammalian cells. This review focuses on the mechanisms and regulation of DSB repair in mammalian cells, the signaling pathways that regulate these processes and the potential crosstalk between NHEJ and HRR, and between repair and other stress-induced pathways with emphasis on the regulatory circuitry associated with the ataxia telangiectasia mutated (ATM) protein.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

Abstract: In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal–thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO•) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO• levels.

Stat3 activation regulates the expression of vascular endothelial growth factor and human pancreatic cancer angiogenesis and metastasis

Abstract: Expression of vascular endothelial growth factor (VEGF), a key angiogenic protein, has been linked with pancreatic cancer progression. However, the molecular basis for VEGF overexpression remains unclear. Immunohistochemical studies have indicated that VEGF overexpression coincides with elevated Stat3 activation in human pancreatic cancer specimens. In our study, more than 80% of the human pancreatic cancer cell lines used exhibited constitutively activated Stat3, with Stat3 activation correlated with the VEGF expression level. Blockade of activated Stat3 via ectopic expression of dominant-negative Stat3 significantly suppressed VEGF expression, angiogenesis, tumor growth, and metastasis in vivo. Furthermore, constitutively activated Stat3 directly activated the VEGF promoter, whereas dominant-negative Stat3 inhibited the VEGF promoter. A putative Stat3-responsive element on the VEGF promoter was identified using a protein-DNA binding assay and confirmed using a promoter mutagenesis assay. These results indicate that Stat3 directly regulates VEGF expression and hence angiogenesis, growth, and metastasis of human pancreatic cancer, suggesting that Stat3 signaling may be targeted for treatment of pancreatic cancer.

Epstein-Barr virus and oncogenesis: from latent genes to tumours.

Abstract: Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus associated with the development of both lymphoid and epithelial tumours. As a common virus infection, EBV appears to have evolved to exploit the process of B cell development to persist as a life-long asymptomatic infection. However, the virus can contribute to oncogenesis as evidenced by its frequent detection in certain tumours, namely Burkitt's lymphoma (BL), post-transplant B cell lymphomas, Hodgkin's disease (HD) and nasopharyngeal carcinoma (NPC), and by its unique ability to efficiently transform resting B cells in vitro into permanently growing lymphoblastoid cell lines (LCLs). These transforming effects are associated with the restricted expression of EBV genes such that only a subset of so-called latent virus proteins are expressed in virus infected tumours and in LCLs. Distinct forms of EBV latency are manifest in the different tumours and these appear to be a vestige of the pattern of latent gene expression used by the virus during the establishment of persistent infection within the B cell pool. This review summarises our current knowledge of EBV latent gene function and how this relates to the role of the virus in the aetiology of different tumours.

Molecular viral oncology of hepatocellular carcinoma.

Abstract: Hepatocellular carcinoma (HCC) is the fifth most common cancer, but the third leading cause of cancer death, in the world, with more than 500,000 fatalities annually. The major etiology of HCC/liver cancer in people is hepatitis B virus (HBV), followed by hepatitis C virus infection (HCV), although nonviral causes also play a role in a minority of cases. Recent molecular studies confirm what was suspected: that HCC tissue from different individuals have many phenotypic differences. However, there are clearly features that unify HCC occurring in a background of viral hepatitis B and C. HCC due to HBV and HCV may be an indirect result of enhanced hepatocyte turnover that occurs in an effort to replace infected cells that have been immunologically attacked. Viral functions may also play a more direct role in mediating oncogenesis. This review considers the molecular and cellular mechanisms involved in primary hepatocellular carcinoma, using a viral perspective.

DNA damage checkpoint control in cells exposed to ionizing radiation.

Abstract: Damage induced in the DNA after exposure of cells to ionizing radiation activates checkpoint pathways that inhibit progression of cells through the G1 and G2 phases and induce a transient delay in the progression through S phase. Checkpoints together with repair and apoptosis are integrated in a circuitry that determines the ultimate response of a cell to DNA damage. Checkpoint activation typically requires sensors and mediators of DNA damage, signal transducers and effectors. Here, we review the current state of knowledge regarding mechanisms of checkpoint activation and proteins involved in the different steps of the process. Emphasis is placed on the role of ATM and ATR, as well on CHK1 and CHK2 kinases in checkpoint response. The roles of downstream effectors, such as P53 and the CDC25 family of proteins, are also described, and connections between repair and checkpoint activation are attempted. The role of checkpoints in genomic stability and the potential of improving the treatment of cancer by DNA damage inducing agents through checkpoint abrogation are also briefly outlined.

P53 and radiation responses.

Abstract: Cells have evolved elaborate mechanisms (checkpoints) to monitor genomic integrity in order to ensure the high-fidelity transmission of genetic information. Cells harboring defects in checkpoint pathways respond to DNA damage improperly, which in turn may enhance the rate of cancer development. Ionizing radiation (IR) primarily leads to double-strand DNA breaks (DSBs), which activate DNA damage checkpoints to initiate signals ultimately leading to a binary decision between cell death and cell survival. TP53 has been recognized as an important checkpoint protein, functioning mainly through transcriptional control of target genes that influence multiple response pathways and leading to the diversity of responses to IR in mammalian cells. We review how the tumor suppressor P53 is involved in the complex response to IR to enforce the cell's fate to live by inducing the growth arrest coupled to DNA damage repair or to die by inducing irreversible growth arrest or apoptosis. Moreover, recent insights have emerged in our understanding of how P53 modulates radiosensitivity in tissues following IR as well as its role in sensitizing cells to chemo- and radiotherapy. The P53 pathway remains an attractive target for exploitation in the war on cancer.

Myc pathways provoking cell suicide and cancer

Abstract: A paradox for the cancer biology field has been the revelation that oncogenes, once thought to simply provide advantages to a cancer cell, actually put it at dire risk of cell suicide. Myc is the quintessential oncogene in this respect, as in normal cells it is required for cell cycle traverse, whereas in cancers it is overexpressed and functions as the angiogenic switch. Nonetheless, Myc overexpression kills normal cells dead in their tracks. Here we review Myc-induced pathways that contribute to the apoptotic response. Molecular analysis of Myc-induced tumors has established that some of these apoptotic pathways are essential checkpoints that guard the cell from cancer, as they are selectively bypassed during tumorigenesis. The precise mechanism(s) by which Myc targets these pathways are largely unresolved, but we propose that they involve crosstalk and feedback regulatory loops between arbiters of cell death.

Radiation and ceramide-induced apoptosis

Abstract: Ceramide is a sphingolipid that acts as a second messenger in ubiquitous, evolutionarily conserved, signaling systems. Emerging data suggest that radiation acts directly on the plasma membrane of several cell types, activating acid sphingomyelinase, which generates ceramide by enzymatic hydrolysis of sphingomyelin. Ceramide then acts as a second messenger in initiating an apoptotic response via the mitochondrial system. Radiation-induced DNA damage can also initiate ceramide generation by activation of mitochondrial ceramide synthase and de novo synthesis of ceramide. In some cells and tissues, BAX is activated downstream of ceramide, regulating commitment to the apoptotic process via release of mitochondrial cytochrome c. Genetic and pharmacologic studies in vivo showed that radiation targets the acid sphingomyelinase apoptotic system of microvascular endothelial cells in the lungs, intestines and brain, as well as in oocytes, to initiate the pathogenesis of tissue damage. Regulated ceramide metabolism may produce metabolites, such as sphingosine 1-phosphate, shown to signal antiapoptosis, thus controlling the intensity of the apoptotic response and constituting a mechanism for radiation sensitivity or resistance. An improved understanding of this signaling system may offer new opportunities for the modulation of radiation effects in the treatment of cancer.

Phosphorylation of Bim-EL by Erk1/2 on serine 69 promotes its degradation via the proteasome pathway and regulates its proapoptotic function.

Abstract: Bim is a proapoptotic member of the Bcl-2 family that shares only the BH3 domain with this family. Three Bim proteins Bim-EL, Bim-L and Bim-S are synthesized from the same transcript. We report here that Bim-EL when phosphorylated by Erk1/2 is rapidly degraded via the proteasome pathway. Using different cellular models we evidence that serine 69 is both necessary and sufficient for Erk1/2-mediated phosphorylation and degradation of Bim-EL. In K562 cells, Phorbol 12-myristate 13-acetate activates Erk1/2 and consequently increases Bim-EL phosphorylation and degradation by the proteasome, resulting in cell survival, while the Bcr-Abl inhibitor imatinib abrogates Bim-EL phosphorylation and degradation and induces caspase activation and apoptosis. We also show that Bim-EL(S69G) promotes apoptosis more efficiently than Bim-EL-WT in K562 cells. Altogether, our findings demonstrate that phosphorylation of Bim-EL by Erk1/2 on serine 69 selectively leads to its proteasomal degradation and therefore represents a new and important mechanism of Bim regulation.