Epithelial-mesenchymal transition (EMT) is a highly conserved process in which polarized, immobile epithelial cells lose tight junctions, associated adherence, and become migratory mesenchymal cells. Several transcription factors, including the Snail/Slug family, Twist, δ EF1/ZEB1, SIP1/ZEB2 and E12/E47 respond to microenvironmental stimuli and function as molecular switches for the EMT program. Snail is a zinc-finger transcriptional repressor controlling EMT during embryogenesis and tumor progression. Through its N-terminal SNAG domain, Snail interacts with several corepressors and epigenetic remodeling complexes to repress specific target genes, such as the E-cadherin gene (CDH1). An integrated and complex signaling network, including the RTKs, TGF-β, Notch, Wnt, TNF-α, and BMPs pathways, activates Snail, thereby inducing EMT. Snail expression correlates with the tumor grade, nodal metastasis of many types of tumor and predicts a poor outcome in patients with metastatic cancer. Emerging evidences indicate that Snail causes a metabolic reprogramming, bestows tumor cells with cancer stem cell-like traits, and additionally, promotes drug resistance, tumor recurrence and metastasis. Despite many new and exciting developments, several challenges remain to be addressed in order to understand more thoroughly the role of Snail in metastasis. Additional investigations are required to disclose the contribution of microenvironmental factors on tumor progression. This information will lead to a comprehensive understanding of Snail in cancer and will provide us with novel approaches for preventing and treating metastatic cancers.
Targeting the ubiquitin-proteasome pathway has emerged as a rational approach in the treatment of human cancer. Based on positive preclinical and clinical studies, bortezomib was subsequently approved for the clinical use as a front-line treatment for newly diagnosed multiple myeloma patients and for the treatment of relapsed/refractory multiple myeloma and mantle cell lymphoma, for which this drug has become the staple of treatment. The approval of bortezomib by the US Food and Drug Administration (FDA) represented a significant milestone as the first proteasome inhibitor to be implemented in the treatment of malignant disease. Bortezomib has shown a positive clinical benefit either alone or as a part of combination therapy to induce chemo-/radio-sensitization or overcome drug resistance. One of the major mechanisms of bortezomib associated with its anticancer activity is through upregulation of NOXA, which is a proapoptotic protein, and NOXA may interact with the anti-apoptotic proteins of Bcl-2 subfamily Bcl-XL and Bcl-2, and result in apoptotic cell death in malignant cells. Another important mechanism of bortezomib is through suppression of the NF-κB signaling pathway resulting in the down-regulation of its anti-apoptotic target genes. Although the majority of success achieved with bortezomib has been in hematological malignancies, its effect toward solid tumors has been less than encouraging. Additionally, the widespread clinical use of bortezomib continues to be hampered by the appearance of dose-limiting toxicities, drug-resistance and interference by some natural compounds. These findings could help guide physicians in refining the clinical use of bortezomib, and encourage basic scientists to generate next generation proteasome inhibitors that broaden the spectrum of efficacy and produce a more durable clinical response in cancer patients. Other desirable applications for the use of proteasome inhibitors include the development of inhibitors against specific E3 ligases, which act at an early step in the ubiquitin-proteasome pathway, and the discovery of less toxic and novel proteasome inhibitors from natural products and traditional medicines, which may provide more viable drug candidates for cancer chemoprevention and the treatment of cancer patients in the future.
One of the major causes of failure in cancer chemotherapy is multidrug resistance (MDR), where cancer cells simultaneously become resistant to different anticancer drugs. Over-expression of membrane efflux pumps like Pglycoprotein (P-gp) that recognizes different chemotherapeutic agents and transports them out of the cell, plays a major role in MDR. The shortcoming of P-gp inhibitors in clinic has been attributed to their non-specific action on P-gp and/or non-selective distribution to non-target organs that leads to intolerable side effects by the P-gp inhibitor at doses required for P-gp inhibition upon systemic administration. Another major issue is the reduced elimination of P-gp substrates (e.g. anticancer drugs) and intolerable toxicities by anticancer drugs when co-administered with P-gp inhibitors. To overcome these shortcomings, new generation of P-gp inhibitors with improved specificity for P-gp have been developed. More recently, attention has been paid to the use of drug delivery systems primarily to restrict P-gp inhibition to tumor and reduce the non-selective inhibition of P-gp in non-target organs. This review will provide an overview and update on the status of P-gp inhibition approaches and the role of drug delivery systems in overcoming P-gp mediated MDR.
Luteolin, 3,4,5,7-tetrahydroxyflavone, is a common flavonoid that exists in many types of plants including fruits, vegetables, and medicinal herbs. Plants rich in luteolin have been used in Chinese traditional medicine for treating various diseases such as hypertension, inflammatory disorders, and cancer. Having multiple biological effects such as anti-inflammation, anti-allergy and anticancer, luteolin functions as either an antioxidant or a pro-oxidant biochemically. The biological effects of luteolin could be functionally related to each other. For instance, the anti-inflammatory activity may be linked to its anticancer property. Luteolins anticancer property is associated with the induction of apoptosis, and inhibition of cell proliferation, metastasis and angiogenesis. Furthermore, luteolin sensitizes cancer cells to therapeuticinduced cytotoxicity through suppressing cell survival pathways such as phosphatidylinositol 3-kinase (PI3K)/Akt, nuclear factor kappa B (NF-κB), and X-linked inhibitor of apoptosis protein (XIAP), and stimulating apoptosis pathways including those that induce the tumor suppressor p53. These observations suggest that luteolin could be an anticancer agent for various cancers. Furthermore, recent epidemiological studies have attributed a cancer prevention property to luteolin. In this review, we summarize the progress of recent research on luteolin, with a particular focus on its anticancer role and molecular mechanisms underlying this property of luteolin.
PTEN/PI3K/AKT constitutes an important pathway regulating the signaling of multiple biological processes such as apoptosis, metabolism, cell proliferation and cell growth. PTEN is a dual protein/lipid phosphatase which main substrate is the phosphatidyl-inositol,3,4,5 triphosphate (PIP3), the product of PI3K. Increase in PIP3 recruits AKT to the membrane where it is activated by other kinases also dependent on PIP3. Many components of this pathway have been described as causal forces in cancer. PTEN activity is lost by mutations, deletions or promoter methylation silencing at high frequency in many primary and metastatic human cancers. Germ line mutations of PTEN are found in several familial cancer predisposition syndromes. Activating mutations which have been reported for PI3K and AKT, in tumours are able to confer tumourigenic properties in several cellular systems. Additionally, the binding of PI3K to oncogenic ras is essential for the transforming properties of ras. In summary, the data strongly support the view of the PTEN/PI3K/AKT pathway as an important target for drug discovery.
Over the past ten years, proteasome inhibition has emerged as an effective therapeutic strategy for treating multiple myeloma (MM) and some lymphomas. In 2003, Bortezomib (BTZ) became the first proteasome inhibitor approved by the U.S. Food and Drug Administration (FDA). BTZ-based therapies have become a staple for the treatment of MM at all stages of the disease. The survival rate of MM patients has improved significantly since clinical introduction of BTZ and other immunomodulatory drugs. However, BTZ has several limitations. Not all patients respond to BTZbased therapies and relapse occurs in many patients who initially responded. Solid tumors, in particular, are often resistant to BTZ. Furthermore, BTZ can induce dose-limiting peripheral neuropathy (PN). The second generation proteasome inhibitor Carfizomib (CFZ; U.S. FDA approved in August 2012) induces responses in a minority of MM patients relapsed from or refractory to BTZ. There is less PN compared to BTZ. Four other second-generation proteasome inhibitors (Ixazomib, Delanzomib, Oprozomib and Marizomib) with different pharmacologic properties and broader anticancer activities, have also shown some clinical activity in bortezomib-resistant cancers. While the mechanism of resistance to bortezomib in human cancers still remains to be fully understood, targeting the immunoproteasome, ubiquitin E3 ligases, the 19S proteasome and deubiquitinases in pre-clinical studies represents possible directions for future generation inhibitors of ubiquitin-proteasome system in the treatment of MM and other cancers.
Studies on erythropoietin regulation led to discovery of hypoxia-inducible factor 1 (HIF-1), a transcription factor which is central component of oxygen sensing mechanism in mammalian cells. The number of HIF-1 and hypoxiaregulated target genes has grown exponentially and includes genes that encode proteins with roles in erythropoiesis, angiogenesis, glycolytic pathway, glucose transport, metastasis, and cell survival. Thus, HIF-1 claimed the role of the master that orchestrates cellular responses to oxygen deprivation. In addition, HIF-1 is also activated or influenced through oxygen-independent mechanisms via growth factors, deregulated oncogenes, and/or tumor suppressors. Whereas HIF prolyl hydroxylases (PHDs) regulate HIF-1 (and subsequently identified HIF-2) during hypoxia, the PI3K, AKT and MAPK pathways mediate primarily non-hypoxic HIF regulation. Here we will focus primarily on pathways that lead to HIF activation via PI3K/AKT, and mTOR/p70S6K1. In addition, recent studies have revealed novel factors and mechanisms that regulate oxygen-independent HIF-1α and HIF-2α degradation. HIFs play important roles in many processes in health and disease. Consequently, HIFs and pathways (PI3K/AKT and mTOR/p70S6K1) that lead to normoxic HIF activation are considered potential therapeutic targets in these pathologies.
AKT/PKB (Protein Kinase B) are central proteins mediating signals from receptor tyrosine kinases and phosphatidylinositol 3-kinase. AKT kinases are involved in a number of important cellular processes including cell proliferation and survival, cell size in response to nutrient availability, tumor invasion/metastasis, and angiogenesis. Various components of the AKT signaling pathway are encoded by tumor suppressor genes and oncogenes whose loss or activation, respectively, plays an important role in tumorigenesis. The growing body of evidence connecting deregulated AKT signaling with sporadic human cancers and inherited cancer predisposition syndromes is discussed. We also highlight new findings regarding the involvement of activating mutations of AKT1, AKT2, and AKT3 in somatic overgrowth disorders: Proteus syndrome, hypoglycemia with hypertrophy, and hemimegalencephaly, respectively. In addition, we review recent literature documenting the various ways the AKT signaling pathway is activated in human cancers and consequences for molecularly targeted therapies.
Glioblastoma (glioblastoma multiforme; GBM; WHO Grade IV) accounts for the majority of primary malignant brain tumors in adults. Amplification and mutation of the epidermal growth factor receptor (EGFR) gene represent signature genetic abnormalities encountered in GBM. A range of potential therapies that target EGFR or its mutant constitutively active form, ΔEGFR, including tyrosine kinase inhibitors (TKIs), monoclonal antibodies, vaccines, and RNA-based agents, are currently in development or in clinical trials for the treatment of GBM. Data from experimental studies evaluating these therapies have been very promising; however, their efficacy in the clinic has so far been limited by both upfront and acquired drug resistance. This review discusses the current status of anti-EGFR agents and the recurrent problem of resistance to these agents that strongly indicates that a multiple target approach will provide a more favorable future for these types of targeted therapies in GBM.
Patients with inflammatory bowel diseases, such as ulcerative colitis and Crohns disease, are at increased risk of developing colon cancer, confirming that chronic inflammation predisposes to development of tumors. Moreover, it appears that colon cancers that do not develop as a complication of inflammatory bowel disease are also driven by inflammation, because it has been shown that regular use of nonsteroidal anti-inflammatory drugs (NSAIDs) lowers the mortality from sporadic colon cancer and results in regression of adenomas in familial adenomatous polyposis (FAP) patients, who inherit a mutation in the Apc gene. Colorectal cancer therefore represents a paradigm for the link between inflammation and cancer. Inflammation is driven by soluble factors, cytokines and chemokines, which can be produced by tumor cells themselves or, more often, by the cells recruited to the tumor microenvironment. Inflammatory cytokines and chemokines promote growth of tumor cells, perturb their differentiation, and support the survival of cancer cells. Tumor cells become addicted to inflammatory stroma, suggesting that the tumor microenvironment represents an attractive target for preventive and therapeutic strategies. Proinflammatory cytokines, such as TNFα, IL-6 and IL-1β, or transcription factors that are required for signaling by these cytokines, including NF-κB and STATs, are indeed emerging as potential targets for anticancer therapy. TNFα antagonists are in phase I/II clinical trials and have been shown to be well tolerated in patients with solid tumors, and IL-1β antagonists that ameliorate several inflammatory disorders characterized by excessive IL-1β production, will likely follow. Therefore, development of drugs that normalize the tumor microenvironment or interrupt the crosstalk between tumor and the tumor microenvironment is an important approach to the management of cancer.
Epigenetic modifications determine phenotypic characteristics in a reversible, stable and genotype-independent manner. Epigenetic modifications mainly encompass CpG island methylation and histone modifications, both being important in the pathogenesis of malignancies. The reversibility of epigenetic phenomenon provides a suitable therapeutic option that is reactivation of epigenetically silenced tumor-suppressor genes. Inhibition of DNA methyltransferase, histone deacetylase and Aurora B kinase, individually or collectively, could feasibly prevent or reverse the impact of epigenetic silencing. MicroRNAs [miRNAs] are an important layer of epigenetic controlling of gene expression, and serve as diagnostic and prognostic biomarkers as well as treatment targets for several types of cancer. miRNAs are involved inepigenetically silencing or activation of genes, tumor suppressor genes and oncogenes, and their modulation opens new horizons for designing novel cancer therapeutic agents.
The proteasome has emerged as an important clinically relevant target for the treatment of hematologic malignancies. Since the Food and Drug Administration approved the first-in-class proteasome inhibitor bortezomib (Velcade® ) for the treatment of relapsed/refractory multiple myeloma (MM) and mantle cell lymphoma, it has become clear that new inhibitors are needed that have a better therapeutic ratio, can overcome inherent and acquired bortezomib resistance and exhibit broader anti-cancer activities. Marizomib (NPI-0052; salinosporamide A) is a structurally and pharmacologically unique β-lactone-γ-lactam proteasome inhibitor that may fulfill these unmet needs. The potent and sustained inhibition of all three proteolytic activities of the proteasome by marizomib has inspired extensive preclinical evaluation in a variety of hematologic and solid tumor models, where it is efficacious as a single agent and in combination with biologics, chemotherapeutics and targeted therapeutic agents. Specifically, marizomib has been evaluated in models for multiple myeloma, mantle cell lymphoma, Waldenstroms macroglobulinemia, chronic and acute lymphocytic leukemia, as well as glioma, colorectal and pancreatic cancer models, and has exhibited synergistic activities in tumor models in combination with bortezomib, the immunomodulatory agent lenalidomide (Revlimid® ), and various histone deacetylase inhibitors. These and other studies provided the framework for ongoing clinical trials in patients with MM, lymphomas, leukemias and solid tumors, including those who have failed bortezomib treatment, as well as in patients with diagnoses where other proteasome inhibitors have not demonstrated significant efficacy. This review captures the remarkable translational studies and contributions from many collaborators that have advanced marizomib from seabed to bench to bedside.
Overcoming intrinsic and acquired chemoresistance is the major challenge in treating ovarian cancer patients. Initially nearly 75% of ovarian cancer patients respond favourably to chemotherapy, but subsequently the majority gain acquired resistance resulting in recurrence, cancer dissemination and death. This review summarizes recent advances in our understanding of the cellular origin and the molecular mechanisms defining the basis of cancer initiation and malignant transformation with respect to epithelial-mesenchymal transition (EMT) of ovarian cancer cells. We discuss the critical role of EMT frequently encountered in different phases of ovarian cancer progression and its involvement in regulating cancer growth, survival, migration, invasion and drug resistance. Using a model ovarian cancer cell line we highlight the relationship between EMT and the 'migrating cancer stem (MCS) cell-like phenotype' in response to drug treatment, and relate how these processes can impact on chemoresistance and ultimately recurrence. We propose the molecular targeting of distinct 'EMT transformed cancer stem-like cells' and suggest ways that may improve the efficacy of current chemotherapeutic regimens much needed for the management of this disease.
The urokinase plasminogen activator (uPA) system (uPAS) consists of the uPA, its cognate receptor (uPAR) and two specific inhibitors, the plasminogen activator inhibitor 1 (PAI-1) and 2 (PAI-2). The uPA converts the proenzyme plasminogen in the serine protease plasmin, involved in a number of physiopathological processes requiring basement membrane (BM) and/or extracellular matrix (ECM) remodelling, including tumor progression and metastasis. Data accumulated over the past years have made increasingly clear that the uPAS has a multifunctional task in the neoplastic evolution, affecting tumor angiogenesis, malignant cell proliferation, adhesion and migration, intravasation and growth at the metastatic site. In agreement with their role in cancer progression and metastasis, an increased expression of uPA, uPAR, and PAI-1 has been documented in several malignant tumors, and a positive correlation between the levels of one or more uPAS members and a poor prognosis has been frequently reported. This is particularly evident in breast cancer, for which uPA has been demonstrated to be the most potent independent prognostic factor described to date. The involvement of the uPAS in cancer progression identifies its components as suitable targets for anti-cancer therapy. Several therapeutical approaches aimed at inhibiting the uPA/uPAR functions have been shown to possess anti-tumor effects in xenograft models, including selective inhibitors of uPA activity, antagonist peptides, monoclonal antibodies able to prevent uPA binding to uPAR and gene therapy techniques silencing uPA/uPAR expression. All these strategies, however, although promising, need definitive confirmation in humans as, up to now, only few uPA inhibitors entered clinical trial.
Biological processes that drive cell growth are exciting targets for cancer therapy. The fibroblast growth factor (FGF) signaling network plays a ubiquitous role in normal cell growth, survival, differentiation, and angiogenesis, but has also been implicated in tumor development. Elucidation of the roles and relationships within the diverse FGF family and of their links to tumor growth and progression will be critical in designing new drug therapies to target FGF receptor (FGFR) pathways. Recent studies have shown that FGF can act synergistically with vascular endothelial growth factor (VEGF) to amplify tumor angiogenesis, highlighting that targeting of both the FGF and VEGF pathways may be more efficient in suppressing tumor growth and angiogenesis than targeting either factor alone. In addition, through inducing tumor cell survival, FGF has the potential to overcome chemotherapy resistance highlighting that chemotherapy may be more effective when used in combination with FGF inhibitor therapy. Furthermore, FGFRs have variable activity in promoting angiogenesis, with the FGFR-1 subgroup being associated with tumor progression and the FGFR-2 subgroup being associated with either early tumor development or decreased tumor progression. This review highlights the growing knowledge of FGFs in tumor cell growth and survival, including an overview of FGF intracellular signaling pathways, the role of FGFs in angiogenesis, patterns of FGF and FGFR expression in various tumor types, and the role of FGFs in tumor progression.
A large number of novel therapeutics is currently undergoing clinical evaluation for the treatment of prostate cancer, and small molecule signal transduction inhibitors are a promising class of agents. These inhibitors have recently become a standard therapy in renal cell carcinoma and offer significant promise in prostate cancer. Through an understanding of the key pathways involved in prostate cancer progression, a rational drug design can be aimed at the molecules critical to cellular signaling. This may enable administration of selective therapies based on the expression of molecular targets, more appropriately individualizing treatment for prostate cancer patients. One pathway with a prominent role in prostate cancer is the PI3K/Akt/mTOR pathway. Current estimates suggest that PI3K/Akt/mTOR signaling is upregulated in 30-50% of prostate cancers, often through loss of PTEN. Molecular changes in the PI3K/Akt/mTOR signaling pathway have been demonstrated to differentiate benign from malignant prostatic epithelium and are associated with increasing tumor stage, grade, and risk of biochemical recurrence. Multiple inhibitors of this pathway have been developed and are being assessed in the laboratory and in clinical trials, with much attention focusing on mTOR inhibition. Current clinical trials in prostate cancer are assessing efficacy of mTOR inhibitors in combination with multiple targeted or traditional chemotherapies, including bevacizumab, gefitinib, and docetaxel. Completion of these trials will provide substantial information regarding the importance of this pathway in prostate cancer and the clinical implications of its targeted inhibition. In this article we review the data surrounding PI3K/Akt/mTOR inhibition in prostate cancer and their clinical implications.
DNA methylation is an epigenetic modification involved in gene expression regulation. In cancer, the DNA methylation pattern becomes aberrant, causing an array of tumor suppressor genes to undergo promoter hypermethylation and become transcriptionally silent. Reexpression of methylation silenced tumor suppressor genes by inhibiting the DNA methyltransferases (DNMT1, DNMT3A, and DNMT3B) has emerged as an effective strategy against cancer. The expression of DNA methyltransferase 1 (DNMT1) being high in S-phase of cell cycle makes it a specific target for methylation inhibition in rapidly dividing cells as in cancer. This review discusses nucleoside analogues (azacytidine, decitabine, zebularine, SGI-110, CP-4200), non-nucleoside ihibitors both synthetic (hydralazine, RG108, procaine, procainamide, IM25, disulfiram) and natural compounds (curcumin, genistein, EGCG, resveratrol, equol, parthenolide) which act through different mechanisms to inhibit DNMTs. The issues of bioavailability, toxicity, side effects, hypomethylation resistance and combinatorial therapies have also been highlighted.
Mammalian target of rapamycin (mTOR) is a key protein kinase controlling signal transduction from various growth factors and upstream proteins to the level of mRNA translation and ribosome biogenesis, with pivotal regulatory effects on cell cycle progression, cellular proliferation and growth, autophagy and angiogenesis. The mTOR pathway, and its upstream regulators in the PI3K/PTEN/AKT cascade, are altered in a variety of experimental and human malignancies. This has led to the prediction that mTOR inhibitors may be used as anticancer agents. With the recent approval of two mTOR-targeted drugs (temsirolimus and everolimus) for the treatment of renal cell carcinoma and mantle cell lymphoma, this paradigm has been effectively translated into the clinical setting. In this review, we discuss mTOR biology and regulation, the mode of action of mTOR inhibitors as anti-cancer agents, and current clinical evidence supporting the use of rapamycin-like mTOR inhibitors in cancer treatment.
Overcoming intrinsic and acquired drug resistance is a major challenge in treating cancer. Poor responses to drug treatment can result in metastasis, cancer dissemination and death. Recently, the epithelial-mesenchymal transition (EMT) has been found to play a critical role in cancer drug resistance, but the nature of this intrinsic link remains unclear. This review summarizes recent advances in the understanding of drug resistance and focuses especially on the association between EMT and drug resistance. We discuss the roles of EMT in regulating drug resistance across different types of cancer, focusing simultaneously on the molecular mechanisms and potential pathways involved in the regulation of drug resistance by EMT. In addition, we discuss potential therapeutic strategies to target EMT to reverse drug resistance.
As of 21st century, cancer is arguably the most complex and challenging disease known to mankind and an inevitable public health concern of this millennium. Nanotechnology, suitably amalgamated with cancer research, has ushered an era of highly personalized and safer medicines which can improve cancer diagnosis and therapy. A wide variety of nanomedicines are currently under investigation, including polymeric/non-polymeric nanoparticles, dendrimers, quantum dots, carbon nanotubes, lipid- and micelle-based nanoparticles. The bases of these nanomedicines in reducing toxicity associated with cancer therapy are their ability to carry a large payload and multivalent-ligand targeting. This imparts specificity for targeting the tissues as well as bypass resistance mechanisms. The major hurdles on these future medicines are potential toxicity of nanoparticles, which imposes the need of extensive regulatory evaluation before nanomedicines could be utilized as cancer therapeutics. This review highlights nanopharmaceuticals that have been investigated in oncology for various applications (diagnosis, therapeutic delivery and theranostics). It also discusses the effects of nano-sized materials on tissues/organ functions, the possibility of overcoming multi-drug resistance by using nanomedicines and their current clinical status.