Current Drug Targets - Volume 12, Issue 9, 2011

New therapeutic strategies that focus on disease-relevant molecular events and interfere with specific signaling pathways have raised considerable expectations for the treatment of many human diseases. Forkhead box O (FOXO) proteins are emerging as transcriptional integrators of pathways that regulate a variety of cellular processes and have been considered as potential therapeutic targets for a broad range of human health conditions, including obesity, diabetes, hypertriglyceridemia, aging, infertility, muscle atrophy, inflammation, immune diseases and cancer [1] (see reviews in this issue). The four members of the mammalian Foxo family of proteins_Foxo1, Foxo3a, Foxo4 and Foxo6_belong to class O of the forkhead/winged helix transcription factors (Fox) and function as transcriptional regulators in the cell nucleus [2]. Foxo proteins influence the transcription of an ever-growing list of target genes through direct binding to their consensus DNA sequence or via protein-protein interactions with other transcription factors and coactivators [3]. Foxo factors are ancient, evolutionarily conserved targets of insulin-like signaling and have evolved to respond to multiple intracellular and extracellular stimuli with engagement of adaptive gene expression programs. The activity of Foxo factors is regulated by a sophisticated signaling network that integrates metabolic, mitogenic and stress signals resulting in a specific pattern of posttranscriptional modifications. Phosphorylation, acetylation, methylation, glycosylation and ubiquitination modulate FoxO function and control nuclearcytoplasmic shuttling, DNA binding and protein-protein interactions. Foxo proteins have been recently established as bona fide tumor suppressors and correspondingly the abrogation of Foxo function is a key feature of many tumor cells. Contrary to other tumor suppressors like p53 or PTEN whose functions are abrogated via genetic or epigenetic changes, inactivation of FOXOs occurs mostly due to posttranscriptional up-regulation of their inhibitory inputs. That offers a wide range of possibilities for restoring FOXO activity e.g. with small molecule inhibitors targeting up-regulated FOXO repressors [4-6]. However, as Foxo factors regulate a broad variety of cellular functions some of which are seemingly opposing such as apoptosis and resistance against oxidative stress, their therapeutic activation or inhibition may lead to undesirable clinical outcome. Therapeutic interference with FOXO functions might have both beneficial effects in one disease setting while having deleterious effects in another. A number of potential therapeutic limitations could arise particularly from chronic modulation of FOXO function. Selective inhibition of FOXO1 activity in the liver might ameliorate metabolic abnormalities associated with obesity and diabetes, while at the same time promote hepatic fibrosis (see Kim et al., this issue [7]). Therapies aimed at reactivating FOXO activity could provide exciting opportunities for innovative treatments for cancer patients [8]. Accordingly, the cytostatic and cytotoxic effects of a diverse spectrum of anti-cancer drugs are mediated through the activation of FOXO factors. Paradoxically, FOXO proteins also contribute to drug resistance by driving the expression of genes important for drug efflux as well as DNA repair and cell survival pathways in drug resistant cancers (see Wilson et al., this issue). In addition, while FOXO proteins exert many if not all the properties attributed to tumor suppressors they may promote the maintenance of the very few tumor-initiating cells that regenerate the disease [9, 10] (see Ghaffari et al., this issue). Based on our current knowledge on the regulation and functions of FOXO proteins the success of their exploitation in the clinic critically relies on the capability to engage specific FOXO-dependent transcriptional programs. The development of therapeutic agents, drug combinations and treatment regimes that modulate specific subsets of FOXO target genes at the right place at the right time constitute a major challenge for basic and drug discovery research. In this special issue of CDT leading experts in the field discuss controversies and advances in our understanding of FOXO biology and its biomedical applications. Huarui Lu and Haojie Huang focus on the therapeutic potential of the FOXO family member FOXO1 for the treatment of diseases such as cancer, diabetes and muscular atrophy. Although Foxo1 null mice exhibit embryonic lethal phenotypes with marked evidence of vascular defects precluding the analysis of the role of this isoform in adult tissues several genetically modified mouse models have been developed that shed light on the role of FOXO1 in human diseases. Dae Hyun Kim, Ting Zhang, Steven Ringquist and H. Henry Dong discuss the hypothesis that selective inhibition of FOXO1 in the liver would ameliorate hypertriglyceridemia, a common lipid disorder associated with obesity and type 2 diabetis and a hallmark of metabolic syndrome. Miranda S. C. Wilson, Jan J. Brosens, Helma D. C. Schwenen and Eric W.-F. Lam review evidence indicating that targeting the FOXO-FOXM1 axis could be a viable strategy for treatment of cancer and for overcoming drug resistance. Saghi Ghaffari, Safak Yalcin, Maite Rielland and Xin Zhang describe the critical relationship between the FOXO driven detoxification of reactive oxygen species (ROS) and the regulation of quiescence in stem cell populations including tumorinitiating cells and explore the therapeutic implications.....

The forkhead box O (FoxO) transcription factors are known to be involved in many physiological and pathological processes including apoptosis, cell cycle arrest, stress resistance, glucose metabolism, cellular differentiation and development, and tumor suppression. The environmental cues, such as growth factors, nutrients, oxidative stress and irradiation, can either positively or negatively modulate FoxO proteins'; activities, thereby ensuring distinctive transcription programs in the cell. The potent activities of FoxOs are tightly controlled by multiple mechanisms, which include posttranslational modification such as phosphorylation, acetylation, methylation and ubiquitination, subcellular localization, and direct protein-protein interaction. Mounting evidence suggests that the human FOXO1 protein, a founding member of the FoxO family is likely involved in carcinogenesis, diabetes and other human diseases. Here we give an overview of most recent findings regarding the regulation and function of FoxO1, its potential role in human diseases and useful animal models for functional studies on FoxO1. Prospective ways in which the discoveries from the basic research of FoxO1 can be utilized for drug targeting and development of novel therapeutics for human diseases are also discussed.

Hypertriglyceridemia is characterized by increased production and decreased clearance of triglyceride-rich lipoproteins including very low-density lipoprotein (VLDL) and chylomicron. Due to its proatherogenic profile, hypertriglyceridemia contributes to the development of atherosclerosis and coronary artery disease. While the pathophysiology of hypertriglyceridemia remains poorly understood, its close association with obesity and type 2 diabetes implicates insulin resistance in the pathogenesis of hypertriglyceridemia. However, the molecular basis linking insulin resistance to hypertriglyceridemia remains elusive. Preclinical studies show that FoxO1 plays a pivotal role in controlling insulin-dependent regulation of microsomal triglyceride transfer protein (MTP) and apolipoprotein C-III (ApoC-III), two key components that catalyze the rate-limiting steps in the production and clearance of triglyceride-rich lipoproteins. Under physiological conditions, FoxO1 activity is inhibited by insulin. In insulin resistant states, FoxO1 becomes deregulated, contributing to unbridled FoxO1 activity in the liver. This effect contributes to hepatic overproduction of VLDL and impaired catabolism of triglyceride-rich particles, accounting for the pathogenesis of hypertriglyceridemia. These data spur the hypothesis that selective inhibition of FoxO1 activity in the liver would improve triglyceride metabolism and ameliorate hypertriglyceridemia. In this article, we review the role of FoxO1 in insulin action and lipid metabolism, and evaluate the therapeutic potential of targeting FoxO1 for treating hypertriglyceridemia in insulin resistant subjects with obesity and type 2 diabetes.

FOXO transcription factors, functioning downstream of the PI3K-PTEN-AKT (PKB) signalling cascade, are essential for cell proliferation, differentiation, DNA damage repair and apoptosis. Recent research indicates that the related transcription factor FOXM1 is a direct target of repression by FOXO proteins. Inactivation of FOXO or overexpression of FOXM1 is associated with tumorigenesis and cancer progression. In addition, the cytostatic and cytotoxic effects of a diverse spectrum of anti-cancer drugs, such as paclitaxel, doxorubicin, lapatinib, gefitinib, imatinib and cisplatin, are mediated through the activation of FOXO3a and/or the inhibition of its target FOXM1. Paradoxically, FOXO proteins also contribute to drug resistance by driving the expression of genes important for drug efflux as well as DNA repair and cell survival pathways in drug resistant cancers. Given its pivotal roles in drug sensitivity as well as resistance, targeting the FOXO-FOXM1 axis could be a viable strategy for treatment of cancer and for overcoming drug resistance. Studying the expression profiles of the components of the FOXO-FOXM1 axis and their cofactors in cancer patients might also help to predict and monitor their clinical response to chemotherapy. A better understanding of the mechanism by which FOXO and FOXM1 are regulated, as well as their roles in drug sensitivity and resistance, may render these proteins crucial prognostic markers and therapeutic targets for breast cancer and other malignancies.

Forkhead FoxO transcription factors exert critical biological functions in response to genotoxic stress. In mammals four FoxOs proteins are known. FoxOs induce cell cycle arrest, repair damaged DNA, or initiate apoptosis by modulating genes that control these processes. In particular, FoxO proteins are critical regulators of oxidative stress by modulating the expression of several anti-oxidant enzyme genes. This function of FoxO is essential for the regulation of stem and progenitor cell pool in the hematopoietic system and possibly in cellular systems. Overall functions of FoxOs are consistent with their role as tumor suppressors as has been shown in animal models. As such, FoxOs are suppressed in various cancer cells. However, recent reports strongly suggest that FoxOs are critical for the maintenance of leukemic stem cells. The diverse functions of FoxOs are orchestrated by tight regulations of expression and activity of its family members. Here we discuss the recent progress in understanding the function of FoxOs specifically in normal and cancer stem cells and what is known about the regulation of these proteins in various cell types and tissues including in the physiological setting of primary cells in vivo. These studies underscore the importance of regulation of FoxO proteins and whether these factors play distinct or redundant functions. Understanding how FoxOs are modulated is critical for devising novel therapies based on targeted restoration/or inhibition of FoxO function in cancer and in other diseased cells in which FoxOs have a key function.

Forkhead O transcription factors (FOXO) are critical for the regulation of cell cycle arrest, cell death, and DNA damage repair. Inactivation of FOXO proteins may be associated with tumorigenesis, including breast cancer, prostate cancer, glioblastoma, rhabdomyosarcoma, and leukemia. Accumulated evidence shows that activation of oncogenic pathways such as phosphoinositide-3-kinase/AKT/IKK or RAS/mitogen-activated protein kinase suppresses FOXO transcriptional activity through the phosphorylation of FOXOs at different sites that ultimately leads to nuclear exclusion and degradation of FOXOs. In addition, posttranslational modifications of FOXOs such as acetylation, methylation and ubiquitination also contribute to modulating FOXO3a functions. Several anti-cancer drugs like paclitaxel, imatinib, and doxorubicin activate FOXO3a by counteracting those oncogenic pathways which restrain FOXOs functions. In this review, we will illustrate the regulation of FOXOs and reveal potential therapeutics that target FOXOs for cancer treatment.

All fundamental reproductive events in the human ovary and uterus, including ovulation, implantation and menstruation, are dependent upon profound tissue remodelling, characterized by cyclical waves of cell proliferation, differentiation, recruitment of inflammatory cells, apoptosis, tissue breakdown and regeneration. Although the rise and fall in ovarian hormones, estradiol and progesterone, orchestrate these reproductive events, FOXO transcription factors, an evolutionary conserved subfamily of forkhead transcription factors, have emerged major downstream effector molecules, capable of integrating hormonal cues with a variety of stress, growth factor and cytokine signal transduction pathways. The ability of FOXOs to regulate seemingly opposing cellular responses, ranging from cell cycle arrest and oxidative stress responses to differentiation and apoptosis, renders these transcription factors indispensable for cyclic tissue remodelling in the reproductive tract. Aberrant expression or perturbed activity of FOXO transcription factors are increasingly linked to prevalent reproductive disorders, such as endometriosis, endometrial cancer, primary ovarian insufficiency and pregnancy failure, which in turn highlights their potential as therapeutic targets.

The Forkhead Box O (Foxo) proteins represent an evolutionarily conserved family of transcription factors that play an important role in regulating processes including metabolism, longevity, and cell death/survival. How is it that a single transcription factor can initiate such divergent cellular responses? We will review the evidence that specific patterns of post-translational modifications play a key role in directing Foxo into various transcriptional readouts. This regulation appears to take on a two tiered regulatory model; with a group of well defined post-translational modifications regulating nuclear localization and transcriptional activity while a second set of modifications regulate the transcriptional specificity of Foxo

The promotion of cellular survival, dedifferentiation, and uncontrolled proliferation via the suppression of apoptotic effectors is a fundamental characteristic of tumor cells. As substrates that are negatively regulated by oncogenic signaling cascades driven by AKT, SGK (serum- and glucocorticoid-inducible kinase), IkB kinase (IKK), ERK, and cyclin-dependent kinases (CDK), forkhead box-class O (FOXO) transcription factors have emerged as bona fide tumor suppressors. These transcription factors indeed regulate a variety of cellular responses and themselves are regulated by reversible phosphorylation, acetylation, ubiquitination and miRNAs. This review will discuss our current understanding of mechanisms for FOXO regulation and the potential implications for therapeutically restoring FOXO transcriptional activity.

FOXO transcription factors control proliferation, apoptosis, differentiation and metabolic processes. Loss of FOXO function has been identified in several human cancers, and results in increased cellular survival and a predisposition to neoplasia, especially in epithelial cancer. FOXO factors are therefore bona fide tumor suppressors, and their potential use as therapeutic targets in cancer has been a matter of debate. Importantly, FOXO factors can also positively regulate cell survival through the activation of several detoxification genes, complicating its putative therapeutic potential. Targeting of FOXO factors has also been proposed for the treatment of metabolic dysfunctions such as diabetes mellitus, immunological disorders and neurodegeneration, as well as for the prevention of aging by maintaining the hematopoyetic stem cells niche. But again, data has accumulated that cautions against the potential use of the FOXO activators in these settings. Therefore, greater understanding of the regulation of FOXO target specificity is still needed to boost its use as a therapeutic target. The four members of the FOXO family (FOXO1, FOXO3A, FOXO4 and FOXO6) have distinct but overlapping cellular functions, although they seem to bind a common set of DNA sites. This fact together with the observation that FOXOs are only partially dependent on their DNA binding activity to regulate their target genes highlights the fact that the interaction of the FOXOs with other transcription factors is crucial for the FOXO-mediated transcriptional programs. In this review, we provide an overview of recent progress in the understanding of the modulation of FOXO activity and target specificity by transcription factors and coactivators.

Until recently, nitrite has been considered a stable and inert metabolite of nitric oxide (•NO) metabolism. This view is now changing as it has been shown that nitrite can be reduced back to •NO and thus one may consider a reversible interaction regarding ˙NO:nitrite couple. Not only physiological regulatory actions have been assigned to nitrite but also may represent, in addition to nitrate, the largest ˙NO reservoir in the body. This notion has obvious importance when considering that ˙NO is a ubiquitous regulator of cell functions, ranging from neuromodulation to the regulation of vascular tone. Particularly in the stomach, following ingestion of nitrate and food or beverages-containing polyphenols, a rich chemistry occurs in which ˙NO, ˙NO-derived species and nitroso or nitrated derivatives may be formed. Most of these molecules may play an important role in vivo. For instance, it has been shown that polyphenol-catalyzed nitrite reduction to ˙NO may induce local vasodilation and that ethanol (from wine) reacts with ˙NO-derived species yielding nitroso derivatives endowed with ˙NO-donating properties. Thus, this review reveals new pathways for the biological effects of dietary nitrite encompassing its interaction with dietary components (polyphenols, red wine, lipids), yielding products with impact on human physiology and pathology, namely cardiovascular, urinary and gastrointestinal systems. Novel therapeutic strategies are therefore expected to follow the elucidation of the mechanisms of nitrite biology.

The advent of the biological era has seen many improvements in the management of inflammatory bowel disease (IBD). These agents, however, are not a ubiquitous panacea as they are neither universally available nor are they universally efficacious in the short or long-term. There is, therefore, still a need for other therapies and it is important to remember about the medications that have been effective in the past. The use of azathioprine and 6-mercoptopurine has been the mainstay of long-term therapy for many IBD patients for many years. Their role as steroid sparing agents and in the maintenance of remission is well recognized, and with the advent of metabolite testing their use has been refined. Methotrexate is a second line immunomodulator with less impressive data but still with observed benefits in Crohn's disease (CD) and two newer immunosuppressive agents, mycophenylate mofetil and tacrolimus have sparked some interest as they appear to be efficacious in some patients. As IBD is a chronic incurable condition that primarily presents in young patients, the treating clinician's goal is to induce and maintain long-term remission. So when one agent is ineffective, or unavailable, other agents need to be considered. This review aims to provide clinicians with practical and up to date knowledge about the use of the immunomodulators in the management of IBD, which is vital in order to offer the best management for their patients.