The effect of potent iron chelators on the regulation of p53: examination of the expression, localization and DNA-binding activity of p53 and the transactivation of WAF1

Cell Permeable Iron Chelators as Potential Cancer Chemotherapeutic Agents

Iron is an essential micronutrient for the growth and function of all cells. It is, therefore, an attractive target for chemotherapeutic compounds. Numerous studies in vitro and in vivo provide evidence that iron chelators may be effective antitumor agents. Lipophilic iron chelators that are readily cell permeable and can bind intracellular iron stores may selectively kill cancer cells without damaging normal cells. In this review we discuss the role of iron in cellular processes and how these processes differ between normal and neoplastic cells. We also review the effects on normal and cancer cell growth of several lipophilic iron chelators.

Antitumor activity and mechanism of action of the iron chelator, Dp44mT, against leukemic cells

Iron chelators have been reported to induce apoptosis and cell cycle arrest in cancer cells. Recent studies suggest broad and selective antitumor activity of the new iron chelator, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT; Whitnall et al., Proc Natl Acad Sci USA 2006;103:14901-14906). However, little is known concerning its effects on hematological malignancies. Using acute leukemia cells, the effect of Dp44mT on apoptosis, cell cycle, caspase-3 activation, and mitochondrial trans-membrane potential has been examined by flow cytometry. Dp44mT acted to induce a G(1)/S arrest in NB4 promyelocytic leukemia cells at low concentrations (0.5-2.5 microM), being far more effective than the clinically used chelator, desferrioxamine (DFO). Moreover, Dp44mT induced apoptosis of NB4 cells in a dose- and time-dependent manner with markedly less effect on nonproliferating cells. The apoptosis-inducing activity of Dp44mT was significantly more effective than DFO. Furthermore, this study also showed that Dp44mT had broad activity, inducing apoptosis in several types of acute leukemia and also multiple myeloma cell lines. Additional studies examining the cytotoxic mechanisms of Dp44mT showed that a reduction in the mitochondrial trans-membrane potential and caspase-3 activation could be involved in the mechanism of apoptosis. Our results suggest that Dp44mT possesses potential as an effective cytotoxic agent for the chemotherapeutic treatment of acute leukemia.

Pharmacological targeting of the integrated protein kinase B, phosphatase and tensin homolog deleted on chromosome 10, and transforming growth factor-beta pathways in prostate cancer

Prostate cancer is a highly heterogenous disease in which a patient-tailored care program is much desired. Central to this goal is the development of novel targeted pharmacological interventions. To develop these treatment strategies, an understanding of the integration of cellular pathways involved in both tumorigenesis and tumor suppression is crucial. Of further interest are the events elicited by drug treatments that exploit the underlying molecular pathology in cancer. This review briefly describes the evidence that suggests integration of three established pathways: the tumorigenic phosphoinositide 3-kinase/protein kinase B (AKT) pathway, the tumor suppressive phosphatase and tensin homolog deleted on chromosome 10 pathway, and the tumor suppressive transforming growth factor-beta pathway. More importantly, we discuss novel pharmaceutical agents that target key points of integration in these three pathways. These new therapeutic strategies include the use of agents that target iron to inhibit proliferation via multiple mechanisms and suppression of AKT by cytosolic phospholipase A(2)-alpha inhibitors.

Chemical substructures that enrich for biological activity

MOTIVATION

Certain chemical substructures are present in many drugs. This has led to the claim of 'privileged' substructures which are predisposed to bioactivity. Because bias in screening library construction could explain this phenomenon, the existence of privilege has been controversial.

RESULTS

Using diverse phenotypic assays, we defined bioactivity for multiple compound libraries. Many substructures were associated with bioactivity even after accounting for substructure prevalence in the library, thus validating the privileged substructure concept. Determinations of privilege were confirmed in independent assays and libraries. Our analysis also revealed 'underprivileged' substructures and 'conditional privilege'-rules relating combinations of substructure to bioactivity. Most previously reported substructures have been flat aromatic ring systems. Although we validated such substructures, we also identified three-dimensional privileged substructures. Most privileged substructures display a wide variety of substituents suggesting an entropic mechanism of privilege. Compounds containing privileged substructures had a doubled rate of bioactivity, suggesting practical consequences for pharmaceutical discovery.

Cancer cell iron metabolism and the development of potent iron chelators as anti-tumour agents

Cancer contributes to 50% of deaths worldwide and new anti-tumour therapeutics with novel mechanisms of actions are essential to develop. Metabolic inhibitors represent an important class of anti-tumour agents and for many years, agents targeting the nutrient folate were developed for the treatment of cancer. This is because of the critical need of this factor for DNA synthesis. Similarly to folate, Fe is an essential cellular nutrient that is critical for DNA synthesis. However, in contrast to folate, there has been limited effort applied to specifically design and develop Fe chelators for the treatment of cancer. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) group of ligands that demonstrate marked and selective anti-tumour activity in vitro and also in vivo against a wide spectrum of tumours. Indeed, administration of these compounds to mice did not induce whole body Fe-depletion or disturbances in haematological or biochemical indices due to the very low doses required. The mechanism of action of these ligands includes alterations in expression of molecules involved in cell cycle control and metastasis suppression, as well as the generation of redox-active Fe complexes. This review examines the alterations in Fe metabolism in tumour cells and the systematic development of novel aroylhydrazone and thiosemicarbazone Fe chelators for cancer treatment.

Transferrin receptor 2 is frequently expressed in human cancer cell lines

Different proteins ensure the fine control of iron metabolism at the level of various tissues. Among these proteins, it was discovered a second transferrin receptor (TfR2), that seems to play a key role in the regulation of iron homeostasis. Its mutations are responsible for type 3 hemochromatosis (Type 3 HH). Although TfR2 expression in normal tissues was restricted at the level of liver and intestine, we observed that TfR2 was frequently expressed in tumor cell lines. Particularly frequent was its expression in ovarian cancer, colon cancer and glioblastoma cell lines; less frequent was its expression in leukemic and melanoma cell lines. Interestingly, in these tumor cell lines, TfR2 expression was inversely related to that of receptor 1 for transferrin (TfR1). Experiments of in vitro iron loading or iron deprivation provided evidence that TfR2 is modulated in cancer cell lines according to cellular iron levels following two different mechanisms: (i) in some cells, iron loading caused a downmodulation of total TfR2 levels; (ii) in other cell types, iron loading caused a downmodulation of membrane-bound TfR2, without affecting the levels of total cellular TfR2 content. Iron deprivation caused in both conditions an opposite effect compared to iron loading. These observations suggest that TfR2 expression may be altered in human cancers and warrant further studies in primary tumors. Furthermore, our studies indicate that, at least in tumor cells, TfR2 expression is modulated by iron through different biochemical mechanisms, whose molecular basis remains to be determined.

Iron chelation and regulation of the cell cycle: 2 mechanisms of posttranscriptional regulation of the universal cyclin-dependent kinase inhibitor p21CIP1/WAF1 by iron depletion

Iron (Fe) plays a critical role in proliferation, and Fe deficiency results in G(1)/S arrest and apoptosis. However, the precise role of Fe in cell-cycle control remains unclear. We observed that Fe depletion increased the mRNA of the universal cyclin-dependent kinase inhibitor, p21(CIP1/WAF1), while its protein level was not elevated. This observation is unique to the G(1)/S arrest seen after Fe deprivation, as increased p21(CIP1/WAF1) mRNA and protein are usually found when arrest is induced by other stimuli. In this study, we examined the posttranscriptional regulation of p21(CIP1/WAF1) after Fe depletion and demonstrated that its down-regulation was due to 2 mechanisms: (1) inhibited translocation of p21(CIP1/WAF1) mRNA from the nucleus to cytosolic translational machinery; and (2) induction of ubiquitin-independent proteasomal degradation. Iron chelation significantly (P < .01) decreased p21(CIP1/WAF1) protein half-life from 61 (+/- 4 minutes; n = 3) to 28 (+/- 9 minutes, n = 3). Proteasomal inhibitors rescued the chelator-mediated decrease in p21(CIP1/WAF1) protein, while lysosomotropic agents were not effective. In Fe-replete cells, p21(CIP1/WAF1) was degraded in an ubiquitin-dependent manner, while after Fe depletion, ubiquitin-independent proteasomal degradation occurred. These results are important for considering the mechanism of Fe depletion-mediated cell-cycle arrest and apoptosis and the efficacy of chelators as antitumor agents.

Iron chelation regulates cyclin D1 expression via the proteasome: a link to iron deficiency-mediated growth suppression

Iron (Fe) plays an important role in proliferation, and Fe deficiency results in G(1)/S arrest. Despite this, the precise role of Fe in cell-cycle control remains unclear. Cyclin D1 plays a critical function in G(1) progression by interacting with cyclin-dependent kinases. Previously, we examined the effect of Fe depletion on the expression of cell-cycle control molecules and identified a marked decrease in cyclin D1 protein, although the mechanism involved was unknown. In this study, we showed that cyclin D1 was regulated posttranscriptionally by Fe depletion. Iron chelation of cells in culture using desferrioxamine (DFO) or 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311) decreased cyclin D1 protein levels after 14 hours and was rescued by the addition of Fe. Cyclin D1 half-life in control cells was 80 +/- 15 minutes (n = 5), while in chelator-treated cells it was significantly (P < .008) decreased to 38 +/- 3 minutes (n = 5). Proteasomal inhibitors rescued the Fe chelator-mediated decrease in cyclin D1 protein, suggesting the role of the proteasome. In Fe-replete cells, cyclin D1 was degraded in an ubiquitin-dependent manner, while Fe depletion induced a ubiquitin-independent pathway. This is the first report linking Fe depletion-mediated growth suppression at G(1)/S to a mechanism inducing cyclin D1 proteolysis.

Iron regulation and the cell cycle: identification of an iron-responsive element in the 3'-untranslated region of human cell division cycle 14A mRNA by a refined microarray-based screening strategy

Iron regulatory proteins (IRPs) 1 and 2 post-transcriptionally control mammalian iron homeostasis by binding to iron-responsive elements (IREs), conserved RNA stem-loop structures located in the 5'- or 3'-untranslated regions of genes involved in iron metabolism (e.g. FTH1, FTL, and TFRC). To identify novel IRE-containing mRNAs, we integrated biochemical, biocomputational, and microarray-based experimental approaches. IRP/IRE messenger ribonucleoproteins were immunoselected, and their mRNA composition was analyzed using an IronChip microarray enriched for genes predicted computationally to contain IRE-like motifs. Among different candidates, this report focuses on a novel IRE located in the 3'-untranslated region of the cell division cycle 14A mRNA. We show that this IRE motif efficiently binds both IRP1 and IRP2. Differential splicing of cell division cycle 14A produces IRE- and non-IRE-containing mRNA isoforms. Interestingly, only the expression of the IRE-containing mRNA isoforms is selectively increased by cellular iron deficiency. This work describes a new experimental strategy to explore the IRE/IRP regulatory network and uncovers a previously unrecognized regulatory link between iron metabolism and the cell cycle.

Using High-Throughput Screening Data To Discriminate Compounds with Single-Target Effects from Those with Side Effects

The most desirable compound leads from high-throughput assays are those with novel biological activities resulting from their action on a single biological target. Valuable resources can be wasted on compound leads with significant 'side effects' on additional biological targets; therefore, technical refinements to identify compounds that primarily have effects resulting from a single target are needed. This study explores the use of multiple assays of a chemical library and a statistic based on entropy to identify lead compound classes that have patterns of assay activity resulting primarily from small molecule action on a single target. This statistic, called the coincidence score, discriminates with 88% accuracy compound classes known to act primarily on a single target from compound classes with significant side effects on nonhomologous targets. Furthermore, a significant number of the compound classes predicted to have primarily single-target effects contain known bioactive compounds. We also show that a compound's known biological target or mechanism of action can often be suggested by its pattern of activities in multiple assays.

Maternal iron deficiency identifies critical windows for growth and cardiovascular development in the rat postimplantation embryo

Imbalances in nutrition during pregnancy can lead to long-, as well as short-term consequences, a phenomenon known as fetal programming. However, there is little information about when the fetus is most sensitive to its environment during gestation. We hypothesize that different fetal systems are most vulnerable to nutritional stress during periods of maximal growth and differentiation. We used iron (Fe) deficiency, which causes hypertension in the offspring, to test this hypothesis. We examined development between embryonic day (E) 10.5 and 12.5, when cardiovascular development is maximal, using whole embryo culture. Female rats were fed Fe-deficient or control diet for 4 wk before mating and up to E10.5. The embryos were cultured for 48 h in 95% rat serum collected from males fed either a control or Fe-deficient diet. Growth was impaired and heart size increased in embryos taken from Fe-deficient mothers and cultured in deficient serum compared with control embryos cultured in control serum. To test whether restoring normal Fe levels could reverse these effects, we cultured embryos from control and deficient dams in either control or deficient medium. The yolk sac circulation of embryos from dams fed either diet cultured in deficient medium was less developed, with a thinner and less branched network than that in all embryos cultured in control serum. The heart was enlarged in embryos of deficient dams cultured in deficient serum compared with the heart size of those cultured in control serum. Culturing embryos in control serum reversed these changes. We conclude, therefore, that this period of cardiovascular organogenesis is one of the sensitive windows during which optimal Fe status is critical for normal development.

The Evolution of Iron Chelators for the Treatment of Iron Overload Disease and Cancer

The evolution of iron chelators from a range of primordial siderophores and aromatic heterocyclic ligands has lead to the formation of a new generation of potent and efficient iron chelators. For example, various siderophore analogs and synthetic ligands, including ICL670A [4-[3,5-bis-(hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid], 4'-hydroxydesazadesferrithiocin, and Triapine, have been developed from predecessors and illustrate potent iron-mobilizing or antineoplastic activities. This review focuses on the evolution of iron chelators from initial lead compounds through to the development of novel chelating agents, many of which show great potential to be clinically applied in the treatment of iron overload disease and cancer.

Hypoxia-inducible factor-1-mediated activation of stanniocalcin-1 in human cancer cells

Stanniocalcin-1 (STC1) is an endocrine hormone originally discovered in the corpuscles of Stannius, endocrine glands on kidneys of bony fishes, and also has been identified in mammals. The mammalian STC1 gene is widely expressed in various tissues and appears to be involved in diverse biological processes. There is growing evidence to suggest that altered patterns of gene expression have a role in human cancer development. Recently STC1 has been identified as a stimulator of mitochondrial respiration and has been hypothesized to be functionally related to the Warburg effect, of which hypoxia-inducible factor (HIF)-1 plays a key role in reprogramming tumor metabolism. This prompted us to examine the involvement of HIF-1 in the regulation of STC1 expression in tumor hypoxia. Our data reveal that hypoxia can stimulate STC1 gene expression in various human cancer cell lines, including those derived from colon carcinomas, nasopharyngeal cancer (CNE-2, HONE-1, HK-1), and ovarian cancer (CaOV3, OVCAR3, SKOV3). By far, the greatest response was observed in CNE-2 cells. In further studies on CNE-2 cells, desferrioxamine, cobalt chloride, and O(2) depletion all increased HIF-1alpha protein and STC1 mRNA levels. Desferrioxamine treatment, when coupled with Fe replenishment, abolished these effects. RNA interference studies further confirmed that endogenous HIF-1alpha was a key factor in hypoxia-induced STC1 expression. The ability of vascular endothelial growth factor to stimulate STC1 expression in CNE-2 cells was comparatively low. Collectively, the present findings provide the first evidence of HIF-1 regulation of STC1 expression in human cancer cells. The studies have implications as to the role of STC1 in hypoxia induced adaptive responses in tumor cells.

Iron chelators with high antiproliferative activity up-regulate the expression of a growth inhibitory and metastasis suppressor gene: a link between iron metabolism and proliferation

Iron (Fe) is critical for proliferation, but its precise role in cell cycle progression remains unclear. In this study, we examined the mechanisms involved by assessing the effects of Fe chelators on the expression of molecules that play key roles in this process. In initial studies, gene arrays were used to assess gene expression after incubating cells with 2 Fe chelators, namely, desferrioxamine (DFO) and 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311), or the DNA-damaging agent, actinomycin D. From the genes assessed, only the N-myc downstream-regulated gene 1 (Ndrg1) was specifically up-regulated by Fe chelation. Although the function of Ndrg1 is unclear, previous studies showed it markedly slows tumor growth and acts as a potent metastasis suppressor. Incubation of cells with chelators markedly increased Ndrg1 mRNA and protein expression, but this was not found with their Fe complexes or when the Fe-binding site had been inactivated. Increased Ndrg1 expression following Fe chelation was related to the permeability and antiproliferative activity of chelators and could be reversed by Fe repletion. Moreover, Ndrg1 up-regulation after chelation occurred at the transcriptional level and was mediated by hypoxia inducible factor-1α (HIF-1α)-dependent and -independent mechanisms. Our investigation suggests Ndrg1 is a novel link between Fe metabolism and the control of proliferation.

Novel di-2-pyridyl-derived iron chelators with marked and selective antitumor activity: in vitro and in vivo assessment

Aroylhydrazone and thiosemicarbazone iron (Fe) chelators have potent antitumor activity. The aim of the current study was to examine the antitumor effects and mechanisms of action of a novel series of Fe chelators, the di-2-pyridyl thiosemicarbazones. Of 7 new chelators synthesized, 4 showed pronounced antiproliferative effects. The most active chelator was Dp44mT, which had marked and selective antitumor activity-for example, an IC(50) of 0.03 microM in neuroepithelioma cells compared with more than 25 microM in mortal fibroblasts. Indeed, this antiproliferative activity was the greatest yet observed for an Fe chelator. Efficacy was greater than it was for the cytotoxic ligand 311 and comparable to that of the antitumor agent doxorubicin. Strikingly, Dp44mT significantly (P <.01) decreased tumor weight in mice to 47% of the weight in the control after only 5 days, whereas there was no marked change in animal weight or hematologic indices. Terminal deoxyribonucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) staining demonstrated apoptosis in tumors taken from mice treated with Dp44mT. This chelator caused a marked increase of caspase-3 activity in murine Madison-109 (M109) cells. Caspase activation was at least partially mediated by the release of mitochondrial holo-cytochrome c (h-cytc) after incubation with Dp44mT. In conclusion, Dp44mT is a novel, highly effective antitumor agent in vitro and in vivo that induces apoptosis.