Current Molecular Medicine (v.9, #4)

Hypoxia is a common feature of solid tumors, with significant effects on the responses to both chemotherapy and radiotherapy. It has now been well established that hypoxia is associated with a poor prognosis in multiple tumor types, and several mechanisms underlying this association have emerged over the last decade. Hypoxia appears to induce a diverse range of molecular changes in the tumor microenvironment that are manifested at the DNA, RNA and protein level, all of which contribute significantly to the unique, and#x201C;hypoxic phenotypeand#x201D;. At the DNA level, hypoxia is associated with numerous genetic mutations and aberrations, which are in part a result of dynamic changes in the expression of key DNA repair genes. At the RNA level, it is now apparent that hypoxia induces both the transcriptional expression and repression of myriad target genes, and importantly, this regulation occurs via both HIF-dependent and -independent pathways. Finally, at the protein level, it is now clear that hypoxia alters the translation and post-translational stability of numerous proteins involved in diverse regulatory pathways. In this issue of Current Molecular Medicine, we present a series of reviews that address the multifaceted role of hypoxia in tumor progression and its clinical implications. Chan, Koch, and Bristow summarize data regarding the regulation of double-strand break (DSB) repair by hypoxia. In particular, they discuss the effects of both short-term and prolonged hypoxia on the two major DSB repair pathways, homologous recombination (HR) and nonhomologous end-joining (NHEJ). The exciting potential for developing new agents specifically targeting these hypoxia-induced defects in HR and NHEJ is also discussed by the authors. In addition to HIFs, numerous other transcription factors appear to be specifically induced by hypoxia, thus highlighting the complexity of the transcriptome response to hypoxia. Ye and Koumenis discuss one such protein, ATF4, a basic leucine-zipper transcription factor involved in amino acid metabolism, regulation of cellular redox state, and anti-stress responses. Based on analyses of clinical tumor tissue, the authors propose that ATF4 plays a role in the adaptive response to hypoxia in the tumor microenvironment.

Tumor Hypoxia as a Modifier of DNA Strand Break and Cross-Link Repair by Norman Chan, Cameron Koch, Robert Bristow (401-410).
Hypoxia is a common characteristic of many solid tumors and is associated with poor prognosis. Cells with low oxygen levels can have altered sensitivity to radiotherapy and chemotherapy secondary to changes in the incidence of DNA single- and double-strand breaks (DNA-ssb, DNA-dsb), DNA base damage, DNA-DNA cross-links and DNA-protein cross-links. Recent evidence also supports that cells exposed to chronic hypoxia have a decreased capacity of DNA-dsb repair. This review will examine the influence of shortterm and prolonged hypoxia on the two major pathways of DNA-dsb repair: homologous recombination (HR) and non-homologous end-joining (NHEJ). Novel treatment strategies designed to exploit the hypoxic tumor microenvironment are also discussed. Modification of DNA damage sensing and repair due to fluctuating oxygen levels within a dynamic tumor microenvironment may have profound implications for tumor progression and treatment.

Hypoxia/anoxia promotes tumor aggressiveness and negatively impacts tumor response to therapy. Coordinate regulation of HIF-dependent and HIF-independent pathways has been shown to contribute to cellular adaptation to hypoxic stress, and to couple macromolecular synthesis rates to reduced energy availability. An important component of this type of adaptation is the activation of the endoplasmic reticulum kinase PERK by acute or prolonged hypoxia. Activated PERK subsequently induces phosphorylation of the translation initiation factor eIF2and#945; and translational upregulation of the transcription factor ATF4. ATF4 is a basic leucine-zipper (bZip) transcription factor, which regulates amino acid metabolism, cellular redox state, and antistress responses. ATF4 expression can be regulated at transcriptional, translational, and post-translational levels. The functional activation of ATF4 under hypoxia and the overexpression of ATF4 in hypoxic areas of clinical samples of human tumors suggest that ATF4 plays a role in tumor hypoxic adaptation. Here we summarize recent findings regarding the regulation of ATF4 in transformed cells, clinical tumor samples and tumor models, and speculate on its potential role in tumor progression and chemoresistance.

The poorly developed vasculature in solid human tumors is responsible for a profound level of intraand inter-tumor heterogeneity in oxygen concentration. High levels of hypoxia are associated with poor patient prognosis due in part to hypoxia-induced changes in cell metabolism, angiogenesis, invasiveness and resistance to therapy. Over the past decade several distinct oxygen sensing pathways that regulate the cellular response to hypoxia have been defined. These include transcriptional and translational responses initiated by oxygen-dependent stabilization of the HIF-1 transcription factor, activation of the unfolded protein response (UPR) and inhibition of the mTOR (mammalian target of rapamycin) kinase signaling pathway. Variations in the duration and severity of hypoxic stress differentially activate these responses and lead to substantial phenotypic variation amongst otherwise identical tumor cells. Nevertheless, several studies have provided links between HIF-, UPR- and mTOR-mediated signaling and the induction of autophagy. This process facilitates survival during metabolic stress and may also be an important mechanism for the removal of potentially toxic damaged proteins and organelles. We propose that overlapping mechanisms of autophagy regulation by HIF, mTOR and the UPR function to coordinately promote hypoxia tolerance and tumor cell survival.

Hypoxic Tumor Microenvironment and Cancer Cell Differentiation by Yuri Kim, Qun Lin, Peter Glazer, Zhong Yun (425-434).
Hypoxia or oxygen deficiency is a salient feature of solid tumors. Hypoxic tumors are often resistant to conventional cancer therapies, and tumor hypoxia correlates with advanced stages of malignancy. Hypoxic tumors appear to be poorly differentiated. Increasing evidence suggests that hypoxia has the potential to inhibit tumor cell differentiation and thus plays a direct role in the maintenance of cancer stem cells. Studies have also shown that hypoxia blocks differentiation of mesenchymal stem/progenitor cells, a potential source of tumor-associated stromal cells. It is therefore likely that hypoxia may have a profound impact on the evolution of the tumor stromal microenvironment. These observations have led to the emergence of a novel paradigm for a role of hypoxia in facilitating tumor progression. Hypoxia may help create a microenvironment enriched in poorly differentiated tumor cells and undifferentiated stromal cells. Such an undifferentiated hypoxic microenvironment may provide essential cellular interactions and environmental signals for the preferential maintenance of cancer stem cells. This hypothesis suggests that effectively targeting hypoxic cancer stem cells is a key to successful tumor control.

Novel Imaging Provides New Insights into Mechanisms of Oxygen Transport in Tumors by Matthew Hardee, Mark Dewhirst, Nikita Agarwal, Brian Sorg (435-441).
Hypoxia is a common feature of solid tumors, and abnormal tumor oxygen transport is a key factor in the imbalance between tumor oxygen supply and demand. Novel advanced imaging techniques can enable new insights into the complexities of tumor oxygen transport and hypoxia that were not previously known or fully appreciated. In this paper, we document new insights into tumor oxygen transport enabled by spectral imaging of microvascular hemoglobin saturation.

Hypoxia and Radiation Therapy: Past History, Ongoing Research, and Future Promise by Sara Rockwell, Iwona Dobrucki, Eugene Kim, S. Marrison, Van Vu (442-458).
Tumor hypoxia influences the outcome of treatment with radiotherapy, chemotherapy and even surgery, not only for the treatment of large bulky tumors with extensive necrosis, but also in the treatment of very small primary tumors and recurrences, micrometastases, and surgical margins with microscopic tumor involvement. Because hypoxic tumor cells are resistant to radiation and to many anticancer drugs, many approaches to circumventing the therapeutic resistance induced by hypoxia have been examined in laboratory studies and clinical trials. In this review, these approaches and the results of past laboratory and clinical studies are described and the limitations of the past agents and their testing are discussed. We describe the importance of new technologies for measuring hypoxia in human tumors, which allow assessment of pretreatment tumor oxygen levels and changes in hypoxia over the course of prolonged treatment regimens. These offer the possibility of improving the design of clinical trials and the selection of patients who will benefit from hypoxia-directed therapies, as well as the possibility of facilitating the development of better agents and regimens for use in hypoxia-directed therapy. We also discuss how the improved understanding of the abnormal vascular beds in solid tumors and of the effects of hypoxia and related microenvironmental insults, resulting from recent and ongoing research, offers the potential for finding new therapeutic targets, that may lead to the development of new agents and novel therapeutic approaches for selectively targeting cells in the adverse microenvironments within solid tumors.

New Perspectives for Melanoma Immunotherapy: Role of IL-12 by Claudia Cocco, Vito Pistoia, Irma Airoldi (459-469).
Metastatic melanoma is a poor prognosis skin cancer. Since conventional treatments including surgery and chemotherapy often fail, novel therapeutic strategies are needed. In particular, identification of melanoma associated antigen has fostered the progress of both active (vaccines) and adoptive immunotherapy. Some promising results have been obtained, but most melanoma patients are not yet cured possibly because of different immune-escape mechanisms operated by tumor cells. Several studies have addressed the use of interleukin (IL)-12 for melanoma therapy due to its immunoregulatory function and anti-tumor activity mediated by stimulation of T and NK effector cells. Unfortunately, IL-12 has shown considerable toxicity. We [1] have recently demonstrated that IL-12 exerts a direct anti-tumor activity on murine B16 melanoma cells expressing a functional IL-12 receptor (R). In our model low levels of endogenous IL-12 reduced proliferation, increased apoptosis, and defective microvessel formation of tumor cells. This review summarizes imformation about melanoma immunotherapy and highlights a novel mechanism of IL-12-mediated anti-tumor activity based upon the direct effect of the cytokine on IL-12R+ tumor cells. In this view, new therapeutic approaches may be planned including: i) pre-screening of melanoma patients for IL-12Rand#946;2 expression to identify potential responders, ii) administration of small and less frequent doses of IL-12 to avoid toxicity and iii) targeting of IL- 12 to IL-12R+ tumor cells, such as local administration in patients with skin tumors or injection of IL-12 fused to an antibody specific to tumor cells.

Fanconi anemia (FA) is an autosomal recessive disorder characterized by multiple congenital anomalies, progressive bone marrow failure, and high cancer risk. Cells from FA patients exhibit spontaneous chromosomal instability and hypersensitivity to DNA interstrand cross-linking (ICL) agents. Although the precise mechanistic details of the FA/BRCA pathway of ICL-repair are not well understood, progress has been made in the identification of the FA proteins that are required for the pathway. Among the 13 FA complementation groups from which all the FA genes have been cloned, only a few of the FA proteins are predicted to have direct roles in DNA metabolism. One of the more recently identified FA proteins, shown to be responsible for complementation of the FA complementation group J, is the BRCA1 Associated C-terminal Helicase (BACH1, designated FANCJ), originally identified as a protein associated with breast cancer. FANCJ has been proposed to function downstream of FANCD2 monoubiquitination, a critical event in the FA pathway. Evidence supports a role for FANCJ in a homologous recombination (HR) pathway of double strand break (DSB) repair. In this review, we will summarize the current knowledge in terms of FANCJ functions through its enzymatic activities and protein interactions. The molecular roles of FANCJ in DNA repair and the response to replicational stress will be discussed.

Targeting Allergic Airway Diseases by siRNA: An Option for the Future? by Holger Meinicke, Yasemin Darcan, Eckard Hamelmann (483-494).
Allergic diseases emerge as a substantial health problem of the 21st century. Current therapies including combination therapies of corticosteroids and and#946;2-agonists are highly effective, inexpensive and relatively safe. However, these medicaments only relieve symptoms but are not curing disease. So the challenge has to be to develop new therapeutics which are as effective as present medicaments without any side effects and hopefully even heal the disease. It has become clear over the past years that Th2-cells and their cytokines have an outstanding role in the development of airway hyperresponsiveness, airway inflammation, airway remodelling and reversible airway obstruction, the cardinal symptoms of allergic asthma. There is legitimate hope that down regulation of these factors could result in a susceptible damage of pathologic mechanisms. Out of different gene silencing techniques RNAi seems to be the most promising method to achieve long-lasting and effective abrogation of key elements in allergic diseases. In particular, this review will highlight the potential of small interfering RNAs to specifically inhibit the function of transcription factors and tyrosine kinases which are involved in orchestrating an allergic immune response.

The TRAIL to Viral Pathogenesis: The Good, the Bad and the Ugly by Nathan Cummins, Andrew Badley (495-505).
Since the discovery of Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL) in 1995, much has been learned about the protein, its receptors and signaling cascade to induce apoptosis and the regulation of its expression. However, the physiologic role or roles that TRAIL may play in vivo are still being explored. The expression of TRAIL on effector T cells and the ability of TRAIL to induce apoptosis in virally infected cells provided early clues that TRAIL may play an active role in the immune defense against viral infections. However, increasing evidence is emerging that TRAIL may have a dual function in the immune system, both as a means to kill virally infected cells and in the regulation of cytokine production. TRAIL has been implicated in the immune response to viral infections (good), and in the pathogenesis of multiple viral infections (bad). Furthermore, several viruses have evolved mechanisms to manipulate TRAIL signaling to increase viral replication (ugly). It is likely that whether TRAIL ultimately has a proviral or antiviral effect will be dependent on the specific virus and the overall cytokine milieu of the host. Knowledge of the factors that determine whether TRAIL is proviral or antiviral is important because the TRAIL system may become a target for development of novel antiviral therapies.

Schizophrenia is a multifactorial disease characterized by a high heritability. Several candidate genes have been suggested, with the strongest evidences for genes encoding dystrobrevin binding protein 1 (DTNBP1), neuregulin 1 (NRG1), neuregulin 1 receptor (ERBB4) and disrupted in schizophrenia 1 (DISC1), as well as several neurotrophic factors. These genes are involved in neuronal plasticity and play also a role in adult neurogenesis. Therefore, the genetic basis of schizophrenia could involve different factors more or less specifically required for neuroplasticity, including the synapse maturation, potentiation and plasticity as well as neurogenesis. Following the model of Knudson in tumors, we propose a two-hit hypothesis of schizophrenia. In this model of gene-environment interaction, a variant in a gene related to neurogenesis is transmitted to the descent (first hit), and, secondarily, an environmental factor occurs during the development of the central nervous system (second hit). Both of these vulnerability and trigger factors are probably necessary to generate a deficit in neurogenesis and therefore to cause schizophrenia. The literature supporting this gene x environment hypothesis is reviewed, with emphasis on some molecular pathways, raising the possibility to propose more specific molecular medicine.