Oncogenes as Novel Targets for Cancer Therapy (Part I)

ATAD2 in cancer: a pharmacologically challenging but tractable target

INTRODUCTION

ATAD2 protein is an emerging oncogene that has strongly been linked to the etiology of multiple advanced human cancers. Therapeutically, despite the fact that genetic suppression/knockdown studies have validated it as a compelling drug target for future therapeutic development, recent druggability assessment data suggest that direct targeting of ATAD2's bromodomain (BRD) may be a very challenging task. ATAD2's BRD has been predicted as a 'difficult to drug' or 'least druggable' target due to the concern that its binding pocket, and the areas around it, seem to be unfeasible for ligand binding. Areas covered: In this review, after shedding light on the multifaceted roles of ATAD2 in normal physiology as well as in cancer-etiology, we discuss technical challenges rendered by ATAD2's BRD active site and the recent drug discovery efforts to find small molecule inhibitors against it. Expert opinion: The identification of a novel low-nanomolar semi-permeable chemical probe against ATAD2's BRD by recent drug discovery campaign has demonstrated it to be a pharmacologically tractable target. Nevertheless, the development of high quality bioavailable inhibitors against ATAD2 is still a pending task. Moreover, ATAD2 may also potentially be utilized as a promising target for future development of RNAi-based therapy to treat cancers.

Immunomicelles for advancing personalized therapy

Personalized medicine, which ultimately seeks to afford tailored therapeutic regimens for individual patients, is quickly emerging as a new paradigm in the diagnosis and treatment of diseases. The idea of casting aside generic treatments in favor of patient-centric therapies has become feasible owing to advances in nanotechnology and drug delivery coupled with an enhanced knowledge of genomics and an understanding of disease at the molecular level. This review highlights polymeric immunomicelles as a class of nanocarriers that have the potential to combine diagnosis, targeted drug therapy, as well as imaging and monitoring of therapeutic response, to render a personalized approach to the management of disease. Smart multi-functional immunomicelles, as the next generation of nanocarriers, are poised for facilitating personalized cancer treatment. This review provides an assessment of immunomicelles as tools for advancing personalized therapy of diseases, with cancer being the major focus.

The renin-angiotensin system in the breast and breast cancer

Much evidence now suggests that angiotensin II has roles in normal functions of the breast that may be altered or attenuated in cancer. Both angiotensin type 1 (AT1) and type 2 (AT2) receptors are present particularly in the secretory epithelium. Additionally, all the elements of a tissue renin-angiotensin system, angiotensinogen, prorenin and angiotensin-converting enzyme (ACE), are also present and distributed in different cell types in a manner suggesting a close relationship with sites of angiotensin II activity. These findings are consistent with the concept that stromal elements and myoepithelium are instrumental in maintaining normal epithelial structure and function. In disease, this system becomes disrupted, particularly in invasive carcinoma. Both AT1 and AT2 receptors are present in tumours and may be up-regulated in some. Experimentally, angiotensin II, acting via the AT1 receptor, increases tumour cell proliferation and angiogenesis, both these are inhibited by blocking its production or function. Epidemiological evidence on the effect of expression levels of ACE or the distribution of ACE or AT1 receptor variants in many types of cancer gives indirect support to these concepts. It is possible that there is a case for the therapeutic use of high doses of ACE inhibitors and AT1 receptor blockers in breast cancer, as there may be for AT2 receptor agonists, though this awaits full investigation. Attention is drawn to the possibility of blocking specific AT1-mediated intracellular signalling pathways, for example by AT1-directed antibodies, which exploit the possibility that the extracellular N-terminus of the AT1 receptor may have previously unsuspected signalling roles.

CREPT accelerates tumorigenesis by regulating the transcription of cell-cycle-related genes

Tumorigenesis is caused by an uncontrolled cell cycle and the altered expression of many genes. Here, we report a gene CREPT that is preferentially expressed in diverse human tumors. Overexpression of CREPT accelerates tumor growth, whereas depletion of CREPT demonstrates a reversed effect. CREPT regulates cyclin D1 expression by binding to its promoter, enhancing its transcription both in vivo and in vitro, and interacting with RNA polymerase II (RNAPII). Interestingly, CREPT promotes the formation of a chromatin loop and prevents RNAPII from reading through the 3' end termination site of the gene. Our findings reveal a mechanism where CREPT increases cyclin D1 transcription during tumorigenesis, through enhancing the recruitment of RNAPII to the promoter region, possibly, as well as chromatin looping.

Use of recombinant cell-permeable small peptides to modulate glucocorticoid sensitivity of acute lymphoblastic leukemia cells

Glucocorticoid (GC) hormones induce apoptosis in T-cell and pre-B-cell acute lymphoblastic leukemia (ALL) cells. Steroid-mediated apoptosis requires a threshold level of the glucocorticoid receptor (GR) protein, and increasing the intracellular GR levels in ALL cells would augment their hormone sensitivity. A protein transduction domain (PTD) approach was used to accomplish this. We produced an HIV Tat PTD domain fusion protein (Tat-GR(554-777)) that potentially competes for the degradation of GR protein by the ubiquitin-proteasome system and should thus increase its intracellular levels by "stabilizing" the GR. We also designed a fusion peptide for the c-Myb DNA binding domain, Tat-c-Myb DBD, since the biological function of this peptide as a dominant negative inhibitor of the c-Myb protein was already known. Purified, bacterially expressed Tat-c-Myb DBD and Tat-GR(554-777) exhibited highly efficient transduction into cultured ALL cell lines including 697 (pre-B-ALL) and CEM-C7 (T-ALL) cells. As expected, the transduced Tat-c-Myb DBD peptide inhibited steroid-mediated stimulation of a GR promoter-luciferase reporter gene. Significantly, transduced Tat-GR(554-777) effectively increased intracellular GR levels in the GC-resistant T-ALL cell line, CEM-C1, and in the pre-B-ALL 697 cell line. Furthermore, transduction of Tat-GR(554-777) rendered GC-resistant CEM-C1 cells sensitive to steroid killing and further sensitized 697 cells to steroid. The use of Tat-fusion peptide transduction may eventually lead to innovative therapeutic modalities to improve the clinical response of patients suffering from T-cell and pre-B-cell acute lymphoblastic leukemia by increasing steroid responsiveness and perhaps converting steroid-resistant leukemia to a hormone-responsive phenotype.

Pathway sensitivity analysis for detecting pro-proliferation activities of oncogenes and tumor suppressors of epidermal growth factor receptor-extracellular signal-regulated protein kinase pathway at altered protein levels

BACKGROUND

Mathematic models and sensitivity analyses of biologic pathways have been used for exploring the dynamics and for detecting the key components of signaling pathways.

METHODS

The authors previously developed a mathematic model of the epidermal growth factor receptor-extracellular signal-regulated protein kinase (EGFR-ERK) pathway using ordinary differential equations from existing EGFR-ERK pathway models. By using prolonged ERK activation as an indicator that may lead to cell proliferation under certain circumstances, in the current study, a pathway sensitivity analysis was performed to test its capability of detecting pro-proliferative activities through altered protein levels to examine the effects on ERK activation.

RESULTS

The analysis revealed that 12 of 20 oncoproteins and 4 of 5 tumor suppressors were detected, consistent with reported experimental works. Because pathway dynamics depend on many factors, some of which were not included in the current models, failure to detect all known oncogenes and tumor suppressors can be because of the failure to include relevant crosstalk to other pathway components.

CONCLUSIONS

Overall, the current results indicated that pathway sensitivity analysis is a useful approach for detecting and distinguishing pro-proliferation activities of oncoproteins and suppressed proliferative activities of tumor suppressors at altered protein levels at least in the EGFR-ERK model.

Recent advances in validating MDM2 as a cancer target

The MDM2 oncogene is overexpressed in various human cancers. Its expression correlates with the phenotypes of high-grade, late-stage, and more resistant tumors. The auto-regulatory loop between MDM2 and the tumor suppressor p53 has long been considered the epitome of a rational target for cancer therapy. As such, many novel agents have been generated to interfere with the interaction of the two proteins, which results in the activation of p53. Among these agents are several small molecule inhibitors synthesized based upon the crystal structures of the MDM2-p53 complex. With use of high-throughput screening, several specific and effective agents for inhibition of the protein-protein interaction were discovered. Recent investigations, however, have demonstrated that many proteins regulate the MDM2-p53 interaction, and that MDM2 may have p53-independent oncogenic functions. In order for novel MDM2 inhibitors to be translated to the clinic, it is necessary to obtain a better understanding of the regulation of MDM2 and of the MDM2-p53 interaction. In particular, the implications of various interactions between certain regulator(s) and MDM2/p53 under different circumstances need to be elucidated to determine which pathway(s) represent the best targets for therapy. Targeting both MDM2 itself and regulators of MDM2 and the MDM2-p53 interaction, or use of MDM2 inhibitors in combination with conventional treatments, may improve prospects for tumor eradication.

Extracellular activity of cyclic AMP-dependent protein kinase as a biomarker for human cancer detection: distribution characteristics in a normal population and cancer patients

The overexpression of cyclic AMP (cAMP)-dependent protein kinase (PKA) has been reported in patients with cancer, and PKA inhibitors have been tested in clinical trials as a novel cancer therapy. The present study was designed to characterize the population distribution of extracellular activity of cAMP-dependent protein kinase (ECPKA) and its potential value as a biomarker for cancer detection and monitoring of cancer therapy. The population distribution of ECPKA activity was determined in serum samples from a Chinese population consisting of a total of 603 subjects (374 normal healthy volunteers and 229 cancer patients). The serum ECPKA was determined by a validated sensitive radioassay, and its diagnostic values (including positive and negative predictive values) were analyzed. The majority of normal subjects (>70%) have undetectable or very low levels of serum ECPKA. In contrast, the majority of cancer patients (>85%) have high levels of ECPKA. The mean ECPKA activity in the sera of cancer patients was 10.98 units/mL, 5-fold higher than that of the healthy controls (2.15 units/mL; P < 0.001). In both normal subjects and cancer patients, gender and age had no significant influence on the serum ECPKA. Among factors considered, logistic analysis revealed that the disease (cancer) is the only factor contributing to the elevation of ECPKA activity in cancer patients. In conclusion, ECPKA may function as a cancer marker for various human cancers and can be used in cancer detection and for monitoring response to therapy with other screening or diagnostic techniques.

Vascular endothelial growth factor (VEGF) signaling in tumor progression

Vascular endothelial cells are ordinarily quiescent in adult humans and divide less than once per decade. When tumors reach a size of about 0.2-2.0mm in diameter, they become hypoxic and limited in size in the absence of angiogenesis. There are about 30 endogenous pro-angiogenic factors and about 30 endogenous anti-angiogenic factors. In order to increase in size, tumors undergo an angiogenic switch where the action of pro-angiogenic factors predominates, resulting in angiogenesis and tumor progression. One mechanism for driving angiogenesis results from the increased production of vascular endothelial growth factor (VEGF) following up-regulation of the hypoxia-inducible transcription factor. The human VEGF family consists of VEGF (VEGF-A), VEGF-B, VEGF-C, VEGF-D, and placental growth factor (PlGF). The VEGF family of receptors consists of three protein-tyrosine kinases and two non-protein kinase receptors (neuropilin-1 and -2). Owing to the importance of angiogenesis in tumor progression, inhibition of VEGF signaling represents an attractive cancer treatment.

Discovery and development of sorafenib: a multikinase inhibitor for treating cancer

Since the molecular revolution of the 1980s, knowledge of the aetiology of cancer has increased considerably, which has led to the discovery and development of targeted therapies tailored to inhibit cancer-specific pathways. The introduction and refinement of rapid, high-throughput screening technologies over the past decade has greatly facilitated this targeted discovery and development process. Here, we describe the discovery and continuing development of sorafenib (previously known as BAY 43-9006), the first oral multikinase inhibitor that targets Raf and affects tumour signalling and the tumour vasculature. The discovery cycle of sorafenib (Nexavar; Bayer Pharmaceuticals) - from initial screening for a lead compound to FDA approval for the treatment of advanced renal cell carcinoma in December 2005 - was completed in just 11 years, with approval being received approximately 5 years after the initiation of the first Phase I trial.

Mechanistic Investigation and Implications of Photodynamic Therapy Induction of Vascular Endothelial Growth Factor in Prostate Cancer

Photodynamic therapy (PDT) is now an approved therapeutic modality, and induction of vascular endothelial growth factor (VEGF) following subcurative PDT is of concern as VEGF may provide a survival stimulus to tumors. The processes that limit the efficacy of PDT warrant investigation so that mechanism-based interventions may be developed. This study investigates VEGF increase following subcurative PDT using the photosensitizer benzoporphyrin derivative (BPD) both in an in vitro and in an orthotopic model of prostate cancer using the human prostate cancer cell line LNCaP. The two subcurative doses used, 0.25 and 0.5 J/cm(2), mimicked subcurative PDT and elicited a 1.6- and 2.1-fold increase, respectively, in secreted VEGF 24 hours following PDT. Intracellular VEGF protein measurement and VEGF mRNA showed a 1.4- and 1.6-fold increase only at 0.5 J/cm(2). In vivo subcurative PDT showed an increase in VEGF by both immunohistochemistry and ELISA. In vitro analysis showed no activation of hypoxia-inducible factor-1alpha (HIF-1alpha) or cyclooxygenase-2 (COX-2) following subcurative PDT; furthermore, small interfering RNA inhibition of HIF-1alpha and COX-2 inhibitor treatment had no effect on PDT induction of VEGF. PDT in the presence of phosphatidylinositol 3-kinase/AKT inhibitor or mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase inhibitor still induced VEGF. However, subcurative PDT increased phosphorylated p38 and stress-activated protein kinase/c-Jun NH(2)-terminal kinase. The p38 MAPK inhibitor abolished PDT induction of VEGF. The results establish the importance of VEGF in subcurative BPD-PDT of prostate cancer and suggest possible molecular pathways for its induction. These findings should provide the basis for the development of molecular-based interventions for enhancing PDT and merit further studies.

Double minutes, cytogenetic equivalents of gene amplification, in human neoplasia - a review

Double minutes are tiny spherical chromatin bodies of a few mega-base pairs of size which are found occasionally in hematopoietic neoplasia and more or less often in human solid tumors. They have been associated with worse prognosis and poor outcome of the malignancies where present. With the beginning era of molecular cytogenetics they could be defined as cytogenetic equivalents of amplified DNA sequences. The identification of involved chromosomal segments and their molecular nature led to the development of molecular genetic techniques for a rapid and reliable detection of prognostically important oncogene amplifications in human tumors and,as a consequence, to gene-targeted therapy.

Oncogenes as novel targets for cancer therapy (part II): Intermediate signaling molecules

This is the second part of a four-part review on potential therapeutic targeting of oncogenes. The previous part introduced the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes, which we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part I included a discussion of growth factors and tyrosine kinases. This portion of the review covers intermediate signaling molecules and the various strategies used to inhibit their expression or decrease their activities.

Protein kinases as targets for antimalarial intervention: Kinomics, structure-based design, transmission-blockade, and targeting host cell enzymes

The surge of interest in protein kinases as targets for chemotherapeutic intervention in a number of diseases such as cancer and neurodegenerative disorders has stimulated research aimed at determining whether enzymes of this class might also be considered as targets in the context of diseases caused by parasitic protists. Here, we present an overview of recent developments in this field, concentrating (i) on the benefits gained from the availability of genomic databases for a number of parasitic protozoa, (ii) on the emerging field of structure-aided design of inhibitors targeting protein kinases of parasitic protists, (iii) on the concept known as transmission-blockade, whereby kinases implicated in the development of the parasite in their arthropod vector might be targeted to interfere with disease transmission, and (iv) on the possibility of controlling parasitic diseases through the inhibition of host cell protein kinases that are required for the establishment of infection by the parasites.

Oncogenes as Novel Targets for Cancer Therapy (Part IV)

This is the final part of a four-part serial review on oncogenes and their potential use as targets for cancer therapy. Previous sections discussed various categories of oncogenes (growth factors, tyrosine kinases, intermediate signaling molecules, and transcription factors) and the advances made in various strategies being used to alter their actions. This part describes four oncogenes, MDM2, BCL2, XIAP, and Survivin, that are involved in regulation of the cell cycle and apoptosis.