Effects of thiols on topoisomerase-II alpha activity and cell cycle progression

Curcumin-induced mitotic arrest is characterized by spindle abnormalities, defects in chromosomal congression and DNA damage

The chemopreventive agent curcumin has anti-proliferative effects in many tumour types, but characterization of cell cycle arrest, particularly with physiologically relevant concentrations, is still incomplete. Following oral ingestion, the highest concentrations of curcumin are achievable in the gut. Although it has been established that curcumin induces arrest at the G₂/M stage of the cell cycle in colorectal cancer lines, it is not clear whether arrest occurs at the G₂/M transition or in mitosis. To elucidate the precise stage of arrest, we performed a direct comparison of the levels of curcumin-induced G₂/M boundary and mitotic arrest in eight colorectal cancer lines (Caco-2, DLD-1, HCA-7, HCT116p53+/+, HCT116p53^–/^–, HCT116p21^–/^–, HT-29 and SW480). Flow cytometry confirmed that these lines underwent G₂/M arrest following treatment for 12h with clinically relevant concentrations of curcumin (5–10 μM). In all eight lines, the majority of this arrest occurred at the G₂/M transition, with a proportion of cells arresting in mitosis. Examination of the mitotic index using fluorescence microscopy showed that the HCT116 and Caco-2 lines exhibited the highest levels of curcumin-induced mitotic arrest. Image analysis revealed impaired mitotic progression in all lines, exemplified by mitotic spindle abnormalities and defects in chromosomal congression. Pre-treatment with inhibitors of the DNA damage signalling pathway abrogated curcumin-induced mitotic arrest, but had little effect at the G₂/M boundary. Moreover, pH2A.X staining seen in mitotic, but not interphase, cells suggests that this aberrant mitosis results in DNA damage.

Chiral toxicology: it's the same thing...only different

Chiral substances possess a unique architecture such that, despite sharing identical molecular formulas, atom-to-atom linkages, and bonding distances, they cannot be superimposed. Thus, in the environment of living systems, where specific structure-activity relationships may be required for effect (e.g., enzymes, receptors, transporters, and DNA), the physiochemical and biochemical properties of racemic mixtures and individual stereoisomers can differ significantly. In drug development, enantiomeric selection to maximize clinical effects or mitigate drug toxicity has yielded both success and failure. Further complicating genetic polymorphisms in drug disposition, stereoselective metabolism of chiral compounds can additionally influence pharmacokinetics, pharmacodynamics, and toxicity. Optically pure pharmaceuticals may undergo racemization in vivo, negating single enantiomer benefits or inducing unexpected effects. Appropriate chiral antidotes must be selected for therapeutic benefit and to minimize adverse events. Enantiomers may possess different carcinogenicity and teratogenicity. Environmental toxicology provides several examples in which compound bioaccumulation, persistence, and toxicity show chiral dependence. In forensic toxicology, chiral analysis has been applied to illicit drug preparations and biological specimens, with the potential to assist in determination of cause of death and aid in the correct interpretation of substance abuse and "doping" screens. Adrenergic agonists and antagonist, nonsteroidal anti-inflammatory agents, SSRIs, opioids, warfarin, valproate, thalidomide, retinoic acid, N-acetylcysteine, carnitine, penicillamine, leucovorin, glucarpidase, pesticides, polychlorinated biphenyls, phenylethylamines, and additional compounds will be discussed to illustrate important concepts in "chiral toxicology."

WR-1065, the active metabolite of amifostine, mitigates radiation-induced delayed genomic instability

Compounds that can protect cells from the effects of radiation are important for clinical use, in the event of an accidental or terrorist-generated radiation event, and for astronauts traveling in space. One of the major concerns regarding the use of radio-protective agents is that they may protect cells initially, but predispose surviving cells to increased genomic instability later. In this study we used WR-1065, the active metabolite of amifostine, to determine how protection from direct effects of high- and low-LET radiation exposure influences genomic stability. When added 30 min before irradiation and in high concentrations, WR-1065 protected cells from immediate radiation-induced effects as well as from delayed genomic instability. Lower, nontoxic concentrations of WR-1065 did not protect cells from death; however, it was effective in significantly decreasing delayed genomic instability in the progeny of irradiated cells. The observed increase in manganese superoxide dismutase protein levels and activity may provide an explanation for this effect. These results confirm that WR-1065 is protective against both low- and high-LET radiation-induced genomic instability in surviving cells.

Global gene expression analysis in time series following N-acetyl L-cysteine induced epithelial differentiation of human normal and cancer cells in vitro

Background

Cancer prevention trials using different types of antioxidant supplements have been carried out at several occasions and one of the investigated compounds has been the antioxidant N-acetyl-L-cysteine (NAC). Studies at the cellular level have previously demonstrated that a single supplementation of NAC induces a ten-fold more rapid differentiation in normal primary human keratinocytes as well as a reversion of a colon carcinoma cell line from neoplastic proliferation to apical-basolateral differentiation [1]. The investigated cells showed an early change in the organization of the cytoskeleton, several newly established adherens junctions with E-cadherin/β-catenin complexes and increased focal adhesions, all features characterizing the differentiation process.

Methods

In order to investigate the molecular mechanisms underlying the proliferation arrest and accelerated differentiation induced by NAC treatment of NHEK and Caco-2 cells in vitro, we performed global gene expression analysis of NAC treated cells in a time series (1, 12 and 24 hours post NAC treatment) using the Affymetrix GeneChip™ Human Genome U95Av2 chip, which contains approximately 12,000 previously characterized sequences. The treated samples were compared to the corresponding untreated culture at the same time point.

Results

Microarray data analysis revealed an increasing number of differentially expressed transcripts over time upon NAC treatment. The early response (1 hour) was transient, while a constitutive trend was commonly found among genes differentially regulated at later time points (12 and 24 hours). Connections to the induction of differentiation and inhibition of growth were identified for a majority of up- and down-regulated genes. All of the observed transcriptional changes, except for seven genes, were unique to either cell line. Only one gene, ID-1, was mutually regulated at 1 hour post treatment and might represent a common mediator of early NAC action.

The detection of several genes that previously have been identified as stimulated or repressed during the differentiation of NHEK and Caco-2 provided validation of results. In addition, real-time kinetic PCR analysis of selected genes also verified the differential regulation as identified by the microarray platform.

Conclusion

NAC induces a limited and transient early response followed by a more consistent and extensively different expression at later time points in both the normal and cancer cell lines investigated. The responses are largely related to inhibition of proliferation and stimulation of differentiation in both cell types but are almost completely lineage specific. ID-1 is indicated as an early mediator of NAC action.

Interaction of amifostine and ionizing radiation on transcriptional patterns of apoptotic genes expressed in human microvascular endothelial cells (HMEC)

PURPOSE

Amifostine is a prodrug that requires dephosphorylation by alkaline phosphatase to become activated. This process occurs rapidly within the bloodstream after its i.v. administration to patients undergoing cancer treatment with selected radiation and chemotherapies. Vascular endothelial cells will, therefore, represent a normal cell system that is among the first to experience the radioprotective effects of this agent. Amifostine's active free thiol WR-1065 was investigated to determine its effect on radiation-induced changes in transcriptional patterns and subsequent apoptosis in human microvascular endothelial cells (HMEC) growing in vitro.

METHODS AND MATERIALS

Human microvascular endothelial cells were grown to confluency and then exposed to WR-1065 at a concentration of 4 mM for 30 min, radiation doses that ranged from 0 to 6 Gy, and WR-1065 at a concentration of 4 mM for 30 min before exposure to ionizing radiation. Cell survival was assessed by clonogenic assay, cell cycle phase was analyzed by flow cytometry, apoptosis was also assessed by flow cytometry in which Anexin V staining and sub-G1 fraction analysis were applied, and gene expression was analyzed by the Clontech Atlas Human cDNA array to identify synergistic and antagonistic effects as a function of amifostine and radiation exposure conditions with a focus on apoptotic-related factors.

RESULTS

Exposure of HMEC to 4 mM WR-1065 30 min before irradiation resulted in a protection enhancement factor of 2.0; that is, D(O-IRR) of 1.25 Gy and D(O-IRR+WR) of 2.56 Gy. Expression profiling revealed 29 genes that were synergistically activated by the combined action of WR-1065 and ionizing radiation, and an additional 12 genes were synergistically or additively suppressed. In particular, a subset of apoptosis-related genes that included caspases 2, 4, and 9 and different members of the bcl family, along with apoptosis-related receptors, were identified as being significantly affected by the combined treatment of WR-1065 and radiation exposure. In addition, a number of cell cycle-related genes that express cyclins A, G1, G2, and D3 and DNA damage/check point proteins ATM, DNA-PK and RAD23B were also found to be significantly affected. Functional assays of apoptosis were also performed that demonstrated the ability of WR-1065 to protect against radiation-induced apoptosis.

CONCLUSIONS

WR-1065, the active thiol form of amifostine, is an effective radioprotector of HMEC as determined by use of clonogenic and apoptotic assays for cell survival. Expression profiling successfully defined the transcriptional response of HMEC to both WR-1065 and ionizing radiation exposure, either alone or in combination, and demonstrated both synergistic and antagonistic effects on the expression of different cellular genes, along with corresponding functional responses. The radioprotective effects of amifostine are not limited to its well-characterized physiochemical properties, which include free-radical scavenging, auto-oxidation leading to intracellular hypoxia, and chemical repair by hydrogen atom donation, but include its ability to modulate the complex transcriptional regulation of genes that are involved in apoptosis, cell cycle, and DNA repair.

Antioxidant effects of stereoisomers of N-acetylcysteine (NAC), L-NAC and D-NAC, on angiotensin II-stimulated MAP kinase activation and vascular smooth muscle cell proliferation

We examined the effects of the stereoisomers of N-acetylcysteine (NAC), L-NAC and D-NAC, on cellular glutathione (GSH) concentration and whether NAC-regulated cellular GSH levels are directly associated with angiotensin II (Ang II)-induced intracellular signaling events in vascular smooth muscle cells (VSMC). Both L-NAC and D-NAC similarly increased intracellular GSH concentration. We found that L-NAC and D-NAC both inhibited Ang II-induced c-Jun N-terminal kinase and p38 mitogen-activated protein kinase activation and [(3)H]-thymidine incorporation in VSMC. Our present study indicates the comparable effects of NAC stereoisomers in regulating intracellular GSH and the redox-dependent intracellular signaling mechanisms in VSMC.

Inhibition of chronic ulcerative colitis-associated colorectal adenocarcinoma development in a murine model by N-acetylcysteine

Long-term ulcerative colitis (UC) patients are at increased risk for developing colorectal cancer. In order to develop strategies for preventing UC-associated carcinogenesis, we studied the effect of the antioxidant N-acetylcysteine (NAC) on UC-associated cancer development in a mouse model. Female C57BL/6J mice were subjected to long-term administration of dextran sulfate sodium (DSS) in the drinking fluid and 2-fold iron-enriched AIN76A diet, with or without NAC. In the DSS-plus-2-fold iron positive control group, gross tumor incidence was 88.5% (23/26 mice) after 12 DSS cycles (1 DSS cycle = 7 day DSS treatment period followed by 10 day recovery period). The tumor multiplicity was 2.1 +/- 0.2 tumors/tumor-bearing mouse, and the tumor volume was 0.054 +/- 0.019 cm3. With 0.2% NAC administration, tumor incidence was significantly reduced (68%, 17/25 mice; P < 0.05), as was the tumor multiplicity (1.5 +/- 0.1 tumors/tumor-bearing mouse; P < 0.05). The tumor volume was lower (0.014 +/- 0.004 cm3), but not significantly decreased. The proliferation index was significantly decreased in non-cancerous epithelia (48.5 +/- 6.0% vs 32.0 +/- 3.7%; P < 0.05), but not in tumor cells. NAC significantly induced apoptosis in both non-cancerous epithelia and colorectal adenocarcinoma. The number of cells immunostained-positive for nitrotyrosine was markedly decreased in the non-cancerous mucosa of NAC-treated mice (102.4 +/-16.6 positive cells/mm2 mucosa vs 53.6 +/- 14.9 cells/mm2; P < 0.05). In addition, the number of inducible nitric oxide synthase (iNOS)-positive inflammatory cells in the non-cancerous mucosa of the distal colon was markedly decreased by NAC. This study indicates that the antioxidant NAC has the potential to serve as a preventive agent for UC-associated colorectal cancer, possibly via inhibition of cellular proliferation and nitrosative stress-caused cellular damage.

Mechanisms of N-acetylcysteine in the prevention of DNA damage and cancer, with special reference to smoking-related end-points

Although smoking cessation is the primary goal for the control of cancer and other smoking-related diseases, chemoprevention provides a complementary approach applicable to high risk individuals such as current smokers and ex-smokers. The thiol N-acetylcysteine (NAC) works per se in the extracellular environment, and is a precursor of intracellular cysteine and glutathione (GSH). Almost 40 years of experience in the prophylaxis and therapy of a variety of clinical conditions, mostly involving GSH depletion and alterations of the redox status, have established the safety of this drug, even at very high doses and for long-term treatments. A number of studies performed since 1984 have indicated that NAC has the potential to prevent cancer and other mutation-related diseases. N-Acetylcysteine has an impressive array of mechanisms and protective effects towards DNA damage and carcinogenesis, which are related to its nucleophilicity, antioxidant activity, modulation of metabolism, effects in mitochondria, decrease of the biologically effective dose of carcinogens, modulation of DNA repair, inhibition of genotoxicity and cell transformation, modulation of gene expression and signal transduction pathways, regulation of cell survival and apoptosis, anti-inflammatory activity, anti-angiogenetic activity, immunological effects, inhibition of progression to malignancy, influence on cell cycle progression, inhibition of pre-neoplastic and neoplastic lesions, inhibition of invasion and metastasis, and protection towards adverse effects of other chemopreventive agents or chemotherapeutical agents. These mechanisms are herein reviewed and commented on with special reference to smoking-related end-points, as evaluated in in vitro test systems, experimental animals and clinical trials. It is important that all protective effects of NAC were observed under a range of conditions produced by a variety of treatments or imbalances of homeostasis. However, our recent data show that, at least in mouse lung, under physiological conditions NAC does not alter per se the expression of multiple genes detected by cDNA array technology. On the whole, there is overwhelming evidence that NAC has the ability to modulate a variety of DNA damage- and cancer-related end-points.

Activation of NFkappaB and MnSOD gene expression by free radical scavengers in human microvascular endothelial cells

The effect of nonprotein thiol (NPT) free radical scavengers WR-1065 (SH) and WR-33278 (SS), the active thiol and disulfide metabolites of amifostine, N-acetylcysteine (NAC; both L- and D- isomers), mesna, captopril, and dithiothreitol (DTT) on NFkappaB activation in human microvascular endothelial cells (HMEC) was investigated and contrasted to TNFalpha. The use of each of these NPTs at millimolar concentrations independent of oxidative damage-inducing agents resulted in a marked activation of NFkappaB, with the maximum effect observed between 30 min and 1 h after treatment. Only the SH and SS forms of amifostine, however, were effective in activating NFkappaB when administered at micromolar levels. Using a supershift assay, SH and SS equally affected the p50-p65 heterodimer, but not homodimers or heterodimers containing p52 or c-Rel subunits of NFkappaB. Neither catalase nor pyruvate when added to the culture medium to minimize hydrogen peroxide production had an effect on NFkappaB activation by SH. Thus, while oxidative damage is known to activate NFkappaB, the intracellular redox environment may also be affected by the addition of free radical scavenging agents such as NPT, and these in turn are capable of activating the redox sensitive transcription factor NFkappaB. There does not appear to be a significant role, if any, for the production of H(2)O(2) as an intermediate step in the activation of NFkappaB by either the SH or the SS form of amifostine. Rather, the underlying mechanism of action, especially for the SS form, may be related to the close structural and functional similarities of these agents to polyamines, which have been reported to be capable of activating NFkappaB. In contrast to TNFalpha, exposure of cells to either 40 microM or 4 mM of SH for 30 min did not induce intercellular adhesion molecule-1 (ICAM-1) gene expression, but did increase manganese superoxide dismutase (MnSOD) gene expression. MnSOD expression rose by 2-fold and remained elevated from 4 to 22 h following SH exposure.

In-vivo glutathione elevation protects against hydroxyl free radical-induced protein oxidation in rat brain

Glutathione deficiency has been associated with a number of neurodegenerative diseases including Lou Gehrig's disease, Parkinson's disease, and HIV. A crucial role for glutathione is as a free radical scavenger. Alzheimer's disease (AD) brain is characterized by oxidative stress, manifested by protein oxidation, lipid oxidation, oxidized glutathione, and decreased activity of glutathione S-transferase, among others. Reasoning that elevated levels of endogenous glutathione would offer protection against free radical-induced oxidative stress, rodents were given in vivo injections of N-acetylcysteine (NAC), a known precursor of glutathione, to study the vulnerability of isolated synaptosomal membranes treated with Fe2+/H2O2, a known hydroxyl free radical producer. Protein carbonyls, a marker of protein oxidation, were measured. NAC significantly increased endogenous glutathione levels in cortical synaptosome cytosol (P < 0.01). As reported previously, protein carbonyl levels of the Fe2+/H2O2-treated synaptosomes were significantly higher compared to that of non-treated controls (P < 0.01), consistent with increased oxidative stress. In contrast, protein carbonyl levels in Fe2+/H2O2-treated synaptosomes isolated from NAC-injected animals were not significantly different from saline-injected non-treated controls, demonstrating protection against hydroxyl radical induced oxidative stress. These results are consistent with the notion that methods to increase endogenous glutathione levels in neurodegenerative diseases associated with oxidative stress, including AD, may be promising.