Beta-carotene: a cancer chemopreventive agent or a co-carcinogen?

Evidence from both epidemiological and experimental observations have fueled the belief that the high consumption of fruits and vegetables rich in carotenoids may help prevent cancer and heart disease in humans. Because of its well-documented antioxidant and antigenotoxic properties, the carotenoid beta-carotene (betaCT) gained most of the attention in the early 1980s and became one of the most extensively studied cancer chemopreventive agents in population-based trials supported by the National Cancer Institute. However, the results of three randomized lung cancer chemoprevention trials on betaCT supplementation unexpectedly contradicted the large body of epidemiological evidence relating to the potential benefits of dietary carotenoids. Not only did betaCT show no benefit, it was associated with significant increases in lung cancer incidence, cardiovascular diseases, and total mortality. These findings aroused widespread scientific debate that is still ongoing. It also raised the suspicion that betaCT may even possess co-carcinogenic properties. In this review, we summarize the current data on the co-carcinogenic properties of betaCT that is attributed to its role in the induction of carcinogen metabolizing enzymes and the over-generation of oxidative stress. The data presented provide convincing evidence of the harmful properties of this compound if given alone to smokers, or to individuals exposed to environmental carcinogens, as a micronutrient supplement. This has now been directly verified in a medium-term cancer transformation bioassay. In the context of public health policies, while the benefits of a diet rich in a variety of fruits and vegetables should continue to be emphasized, the data presented here point to the need for consideration of the possible detrimental effects of certain isolated dietary supplements, before mass cancer chemoprevention clinical trials are conducted on human subjects. This is especially important for genetically predisposed individuals who are environmentally or occupationally exposed to mutagens and carcinogens, such as those found in tobacco smoke and in industrial settings.     

How can we improve antibody-based cancer therapy?

Monoclonal antibodies (mAbs) as a class of novel oncology therapeutics are demonstrating clinical efficacy as measured by tumor response (shrinkage in tumor size), and prolongations in progression-free survival (PFS) and overall survival (OS). However, clinical benefits are often limited to when antibodies are used in combination with chemotherapy or radiation modalities, with tumor responses only seen in a fraction of patients, and improvements in PFS and OS are incremental.¹ The potential of mAbs and mAb constructs has yet to be fully exploited for maximal clinical benefit. New approaches to further improve the effectiveness of these mAb therapies include (1) selection of patients who may derive the most benefit based on the molecular characteristics of their tumors; (2) improvements in biodistribution to maximize delivery of mAbs to susceptible tumor cells; and (3) optimization of antibody immune effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC).Key words: monoclonal antibodies, solid tumors, cancer, pharmacogenomics, biodistribution, bioengineeringMonoclonal antibodies (mAbs) as a class of novel oncology therapeutics are demonstrating clinical efficacy as measured by tumor response (shrinkage in tumor size), and prolongations in progression-free survival (PFS) and overall survival (OS). However, clinical benefits are often limited to when antibodies are used in combination with chemotherapy or radiation modalities, with tumor responses only seen in a fraction of patients, and improvements in PFS and OS are incremental.¹ The potential of mAbs and mAb constructs has yet to be fully exploited for maximal clinical benefit. New approaches to further improve the effectiveness of these mAb therapies include (1) selection of patients who may derive the most benefit based on the molecular characteristics of their tumors; (2) improvements in biodistribution to maximize delivery of mAbs to susceptible tumor cells; and (3) optimization of antibody immune effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC) (Fig. 1).Open in a separate windowFigure 1Efficacy of monoclonal antibodies may be improved by selecting responding patient subpopulations, improving biodistribution and delivery of antibody to the tumor and maximizing antibody-mediated immune responses through application of protein and glyco-engineering.     

Histamine: an undercover agent in multiple rare diseases?

Histamine is a biogenic amine performing pleiotropic effects in humans, involving tasks within the immune and neuroendocrine systems, neurotransmission, gastric secretion, cell life and death, and development. It is the product of the histidine decarboxylase activity, and its effects are mainly mediated through four different G-protein coupled receptors. Thus, histamine-related effects are the results of highly interconnected and tissue-specific signalling networks. Consequently, alterations in histamine-related factors could be an important part in the cause of multiple rare/orphan diseases. Bearing this hypothesis in mind, more than 25 rare diseases related to histamine physiopathology have been identified using a computationally assisted text mining approach. These newly integrated data will provide insight to elucidate the molecular causes of these rare diseases. The data can also help in devising new intervention strategies for personalized medicine for multiple rare diseases.     

Tumor-induced lymphangiogenesis: a target for cancer therapy?

Recent advances in understanding the biology of lymphangiogenesis, the new growth of lymphatic vessels, have cast new light on the molecular basis of metastasis to regional lymph nodes. The receptor tyrosine kinase VEGFR-3 is virtually exclusively expressed on lymphatic but not blood endothelium in the adult, and activation of VEGFR-3 by its ligands VEGF-C and VEGF-D is sufficient to induce lymphangiogenesis. Correlative studies with human tumors and functional studies using animal tumor models show that increased levels of VEGF-C or VEGF-D in tumors lead to enhanced numbers of lymphatic vessels in the vicinity of tumors, which in turn promotes metastasis to regional lymph nodes by providing a greater number of entry sites into the lymphatic system for invading tumor cells. These findings have prompted studies to investigate whether inhibitors of VEGFR-3 activation might represent novel therapeutic agents for the suppression of metastasis. However, a number of points regarding the therapeutic potential of anti-lymphangiogenic treatments in the context of cancer remain to be addressed. The spectrum and relative importance of molecules that induce lymphangiogenesis and the regulation of their expression during tumor progression, the reversibility of tumor-induced lymphangiogenesis, and possible side-effects of anti-lymphangiogenesis-based therapies all need to be investigated. Most importantly, the extent to which lymph node metastases contribute to the formation of metastases in other organs remains to be elucidated. These aspects are the focus of this review, and their investigation should serve as a roadmap to possible translational application.     

Urocortin: a beneficial or detrimental agent to endothelium?

As a member of the corticotropin-releasing factor (CRF) family, urocortin (UCN) has been demonstrated to show diverse effects on cardiovascular system. It is commonly considered as a protective agent on vascular function. However, a growing body of evidence suggests that some effects of UCN may lead to endothelial dysfunction (ED) which may be the cause or consequence of vascular disease and a hallmark of known cardiovascular risk factors. The present review is then focused on recent evidence that reveals the beneficial and detrimental effects of UCN on endothelium.     

Downregulation of CK2 induces apoptosis in cancer cells – A potential approach to cancer therapy

We have previously documented that naked antisense CK2α ODN can potently induce apoptosis in cancer cells in culture and in mouse xenograft human prostate cancer. The effects of the antisense CK2α are related to downregulation of CK2α message and rapid loss of the CK2 from the nuclear compartment. Here we demonstrate that downregulation of CK2 elicited by diverse methods leads to inhibition of cell growth and induction of apoptosis. The various approaches to downregulation of CK2 employed were transfection with kinase-inactive plasmid, use of CK2α siRNA, use of inhibitors of CK2 activity, and use of antisense CK2α ODN packaged in sub-50 nm nanocapsules made from tenascin. In all cases, the downregulation of CK2 is associated with loss in cell survival. We have also described preliminary observations on an approach to targeting CK2 in cancer cells. For this, sub-50 nm tenascin-based nanocapsules bearing the antisense CK2α ODN were employed to test that the antisense is delivered to the cancer cells in vivo. The results provide the first preliminary evidence that such an approach may be feasible for targeting CK2 in cancer cells. Together, our results suggest that CK2 is potentially a highly plausible target for cancer therapy.     

Parkin: a multipurpose neuroprotective agent?

An autosomal recessive juvenile-onset form of Parkinson's disease (AR-JP) is caused by loss-of-function mutations of the parkin gene, which encodes a ubiquitin-protein ligase. Three recent reports demonstrate that parkin can protect neurons from diverse cellular insults, including alpha-synuclein toxicity, proteasomal dysfunction, Pael-R accumulation, and kainate-induced excitotoxicity. These findings suggest a central role for parkin in maintaining dopaminergic neuronal integrity and strengthen the link between AR-JP and the more common sporadic form of Parkinson's disease.     

Histamine in cardiac sympathetic ganglia: a novel neurotransmitter?

Although ganglia in the heart are well known to be cholinergic, many other neurotransmitters and neuropeptides also influence (and are produced in) cardiac neurons, including adrenergic and purinergic compounds. Recently, histamine was suggested as a possible neurotransmitter in cardiac tissue. Although histamine does elicit many effects in the heart, does it stand up to rigorous scrutiny and fulfill certain criteria that are used to define neurotransmitters?     

Proteomics: a new way to improve human African trypanosomiasis diagnosis?

African trypanosomiases, including the human disease referred to as ‘sleeping sickness’ and the animal diseases such as nagana, surra and dourine, are neglected vector-borne diseases that after years of research still need improved diagnosis and chemotherapy. Advances in proteomics offer new tools to define biomarkers, whose expression may reflect host–parasite interactions occurring during the infection. In this review, the authors first describe the current diagnostic tools used to detect a trypanosome infection during field surveys, and then discuss their interests, limits and further evolutions. The authors also report on the contribution of molecular diagnostics, and the recent advances and developments that make it suitable for fieldwork. The authors then explore the recent uses of proteomics technology to define host and parasite biomarkers that allow detection of the infection, the power and constraints of the technology. The authors conclude by discussing the urgent need to use the biomarkers discovered in order to develop tools to improve trypanosomiasis control in the near future.     

Targeted cancer therapy: What if the driver is just a messenger?

“Shoot the driver” is the paradigm of targeted cancer therapy. However, resistance to targeted inhibitors of signaling pathways is a major problem. In part, the redundancy of signaling networks can bypass targeted inhibitors and thereby reduce their biological effect. In this case, the driver turns out to be one of several potential messengers and is easily replaced. Cocktails of multiple targeted inhibitors are an obvious solution. This is limited, however, by the lack of potent inhibitors and may also produce increased toxicity. Therefore, we explored the direct blockade of a key biological activity downstream from multiple converging oncogenic signals. Specifically, several oncogenic signaling pathways, including AKT, MAPK and PIM kinase signals, converge on the activation of cap-dependent translation. In cancer cells, aberrant activation of cap-dependent translation favors the increased expression of short-lived oncoproteins like c-MYC, MCL1, CYCLIND1 and the PIM kinases. Intriguingly, cancer cells are especially sensitive to even temporary reductions in these proteins. We will discuss our findings concerning translational inhibitor therapy in cancer.Key words: targeted therapy, cancer, lymphoma, translation, eIF4ETargeted cancer therapies are designed to block selected pathways or molecules that are required for tumor cell survival. The most successful examples are inhibitors of the BCR-ABL fusion protein that drive chronic myeloid leukemia (CML). By shutting down the activity of a single molecule driving the growth of CML cells, imatinib and its successors dasatinib and nilotinib can produce complete and sustained remissions as single agents (reviewed in ref. ¹). However, it has proven difficult to translate this success to other cancers. In metastatic melanoma, for example, the BRAF inhibitor vemurafenib produces high response rates in patients whose tumors bear the BRAF-V600E activating mutation (reviewed in ref. ²). Excitement generated by these results is well-deserved, as they have opened a new treatment paradigm in a disease with few available options and a grim prognosis. Unfortunately, however, resistance to the drug generally is seen within a few months. Median progression-free survival resulting from vemurafenib for treatment-naïve metastatic melanoma patients with V600E was about six months,³ compared with imatinib in CML, which kept 93% of patients progression-free at five years.⁴ The success of TKIs in CML is still the exception and not the rule in targeted cancer therapeutics.Resistance to targeted inhibitors is an emerging problem with multiple causes and potential solutions. While mechanisms of resistance to BRAF inhibition in melanoma remain to be elucidated, they are well-described for some targets in other cancers. Drugs against the epidermal growth factor receptor (EGFR) kinase, for example, are bypassed both by activation of downstream mediators, most prominently KRAS, and by signaling through parallel pathways like the MET oncogene (reviewed in ref. ⁵). Resistance mechanisms are varied and complex in some ways, but most boil down to the same idea: evolution has provided multiple routes to the crucial endpoints that allow sustained growth and proliferation of cancer cells. Cocktails of multiple inhibitors have been proposed to prevent or thwart resistance (reviewed in ref. ⁶), and this approach appears highly promising based on some recent preclinical studies. In prostate cancer, for example, combined blockade of androgen receptor signaling and the PI3K/AKT/mTOR pathway showed potent synergy in model systems.⁷ Unfortunately, similar approaches against many other cancers are currently limited by the availability of potent, selective inhibitors and a need to identity which combinations will be effective. Moreover, combined toxicities from simultaneous use of multiple inhibitors will pose limits on the number and intensity of drugs that can be used.An alternative and potentially complementary approach is to directly target the downstream biological processes that are activated by signaling pathways and that cancer cells rely on. Experiences with inhibitors of EGFR, BRAF, and other signaling molecules suggest that most tumors can reliably activate parallel or downstream messengers to thwart efficacy. So inhibiting a signaling intermediate (i.e., a messenger) allows resistance if the biological effects can be achieved via an alternate route. Ultimately, tumor cells don''t depend on the messenger, often a kinase, and, instead, require a downstream biological function. This opens the possibility that targeting the critical effect directly may be an effective cancer therapy and could overcome the problem of redundant messengers.Several inherent properties of signaling pathways are relevant to targeted therapies, their side effects and mechanisms of resistance.⁸ For example, signaling pathways frequently converge on key activities. As explained above, this redundancy can produce resistance to targeted agents. Often redundant pathways are induced via feedback mechanisms, which provide robust signals and can also bypass selective inhibitors. Signals also diverge, however, and seemingly parallel pathways therefore also produce pleiotropic and non-overlapping effects. It is possible not all activities are required by cancer cells. Hence, an upstream block may produce toxicities, in part, by blocking activities that are not strictly required by tumor cells as much as by normal tissues. Finally, unlike metabolic pathways, where a limiting substrate is passed down, signaling cascades amplify signals toward a key activity, and the initial signal or message is both energetically “cheap” and infinitely recurrent. Accordingly, it is conceivable that direct block of the downstream effects provide an alternative or complementary approach to targeting upstream signaling molecules.Multiple oncogenic signals, including the PI3K, MAPK/ERK and PIM kinase pathways converge on the activation of cap-dependent translation, the process by which most capped mRNAs are translated into proteins (reviewed in ref. ⁹). Signaling pathways control the availability of the cap-binding protein eIF4E that is the limiting component of the multimeric translation complex eIF4F, which also includes scaffolds (eIF4G) and RNA helicase activities (eIF4A).^10–12 The complex ultimately mediates loading of mRNAs onto ribosomes. Availability of the eIF4E factor is especially important for mRNAs with long and structured 5′ UTRs. These include, in particular, short-lived cell cycle regulators and oncoproteins. Hence, regulation of eIF4E via upstream signals provides an immediate level of expression control that directly controls levels of proteins, including c-MYC, cyclin D1, BCL2, MCL1 and PIM1.^13–17 Cancer cells require continuous expression of these proteins. For example, even brief loss of MYC expression produces widespread cell death in several cancers but only produces reversible cell cycle arrest in normal tissues.¹⁸ Hence, the increased requirement for the continuous translation of oncoproteins in cancer cells may provide a therapeutic window for inhibitors of capdependent translation.We recently investigated the therapeutic potential of directly blocking capdependent translation in non-Hodgkin lymphoma (NHL).^15,16,19 Rapalogs, inhibitors of mTORC1, the upstream activator of cap-dependent translation, have been extensively studied in NHL in clinical trials (reviewed in ref. ²⁰). However, their activity has, overall, been modest, and our results implicate mTORC1-independent activation of translation by PIM kinases as one mechanism of rapalog resistance in NHLs.¹⁶ The PIM family kinases are active upon expression and do not require activating modifications. They have been known for some time to be able to promote phosphorylation of 4E-BP1 in a manner resistant to rapamycin.^21,22 We now report expression of PIM1 and/or PIM2 in more than 60% of diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL) and more than 75% of mantle cell and small lymphocytic lymphomas. Our study found PIM expression (either PIM1 or PIM2) was associated with worse time to event and overall survival in FL, while another recent report points to PIM2 as a driver of aggressive disease in activated B-cell type DLBCL.²³ In addition, two recent studies of chromosomal translocations mediated by activation-induced deaminase (AID) identified PIM1 as a frequent target.^24,25 In sum, expression of PIM kinases is common in NHLs, may be associated with a more aggressive clinical course and exemplifies how the redundancy of messaging molecules can bypass the clinical activity of selective signaling inhibitors.In experimental systems, we found that direct blockade of cap-dependent translation was highly effective against lymphomas with redundant PI3K/AKT and PIM signals.¹⁶ Briefly, using both a constitutively active mutant 4E-BP1 allele that blocks eIF4E activity and a small molecule inhibitor of the eIF4A helicase, silvestrol, we were able to completely restore rapalog sensitivity in lymphomas engineered to express PIM2 kinase activity. Mechanistically, we found that silvestrol dramatically reduced the translation of critical oncoproteins, including c-MYC, cyclin D1 and MCL1. Interestingly, silvestrol also blocked the translation of both PIM kinases themselves. Moreover, consistent with prior reports, silvestrol treatment at an effective dose was well-tolerated in animals, and we observed no frank toxicity.¹⁹ Hence, blocking cap-dependent translation disrupts upstream signaling molecules, the PIM kinases and also key oncoproteins commonly considered “undruggable” oncoproteins, including c-MYC.Silvestrol worked dramatically better than inhibition of the upstream kinases. Briefly, we tested the SuperGen Inc. PIM kinase inhibitors SGI-1776 and SGI-1773 side by side with silvestrol in a panel of these PIM-expressing human NHL cell lines. Notably, SGI-1776 is the only PIM inhibitor that has entered clinical trials, although these had to be discontinued due to cardiac toxicity of the compound (SuperGen press release, 2010). In any case, silvestrol showed in vitro potency at IC₅₀ of less than 10 nM in all cases, and the PIM kinase inhibitors were 100 to 1,000 times less active. These results highlight some problems associated with the “inhibitor cocktail” approach and indicate a complementary strategy that includes direct blockade of a key biological activity, in this instance, cap-dependent translation of oncoproteins.Silvestrol is not the only means to block cap-dependent translation, and others are reviewed in reference ⁹. These include antisense oligonucleotides against eIF4E and peptide inhibitors of eIF4F complex formation, though neither has entered clinical trials. Silvestrol is the most well-studied of several compounds that emerged from library screens for ability to disrupt the function of the eIF4F subunit eIF4A, an RNA helicase required for its ability to promote mRNA translation. A plant-derived flavagline, silvestrol has shown activity against a variety of tumor types and can be given to mice at high enough concentrations for antitumor activity without major toxicity. The drug shows activity as a single agent against human breast and prostate cancer cell lines in xenograft experiments in nude mice.²⁶ This produced mild transient impairment of hepatic synthetic function but no toxicities producing morbidity or mortality. In genetically defined murine tumor models, silvestrol showed potent synergy with chemotherapy when used against tumors bearing translational activation due to loss of Pten or overexpression of eIf4e.¹⁹ Originally isolated from Aglaia silvestris, silvestrol has a complex structure that has proved difficult to chemically synthesize in quantity. For this reason, the parent compound is not an ideal clinical drug candidate. Efforts are underway by Drs. Pelletier (McGill) and Porco (Boston University) to develop analogs with more efficient synthesis profiles and that retain its biochemical properties. In sum, cap-dependent translation is a promising drug target alternate to mTORC1 and upstream kinase inhibitors.     

Tp53 gene therapy: a key to modulating resistance to anticancer therapies?

Abnormalities in the p53 tumor suppressor have been identified in over 60% of human cancers. The status of p53 within tumor cells has been proposed to be one of the major determinants of the response to anticancer therapies. In this review we examine the relationship between functional p53 and sensitivity, or resistance, to chemotherapy and radiotherapy. We also discuss the potential of current gene-therapy approaches to restore functional p53 to tumors as a means of modulating the effects of radiation and chemotherapy.