In This Issue pubs.acs.org/crt
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SPECIAL FEATURES Since 1925, 4-(p-nitrobenzyl)pyridine (NBP) has been used to detect and evaluate the activity of alkylating agents. Frequently used as a model compound for the highly reactive N-7 position of guanine, NBP has proven useful to predict the reactivity of potential mutagens. Now, Gómez-Bombarelli et al. (DOI 10.1021/tx300065v) assess the validity of this model and review its contributions to our understanding of DNA adduct formation. As a repository for most metals, the skeleton both protects the body from toxic elements and serves as a target for that toxicity. Yet, despite its importance in this regard, little is known about the dynamic processes that regulate bone metal metabolism. This is particularly true in the case of the actinides and lanthanides, the focus of a review by Vidaud et al. (DOI 10.1021/tx300064m), who enlighten us on what we know about skeleton-related toxicity of these “f-elements”.
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CARBON NANOTUBES TARGET THE CELL CYCLE
no DNA damage was detected in MWCNT-treated cells, and HCT-116 cells lacking p53 expression responded to MWCNT exposure with the same increase in p21 as was observed in p53expressing HCT-116 cells. Previous studies from the same lab had shown that singlewalled CNTs inhibit signaling via bone morphogenic proteins (BMPs), leading Zhang et al. to consider this pathway as an explanation for their findings. The promoter region of p21 contains an E-box motif, which is regulated by the helix−loop− helix (HLH) family of transcription factors. The Id proteins are HLH proteins lacking a DNA binding domain that block transcriptional activation at E-box promoter regions. Stimulation of BMP signaling pathways leads to increased Id protein activity, and reduced expression of p21 (see figure). Zhang et al. had previously found that MWCNT-dependent suppression of BMP signaling leads to an decrease in the level of Id proteins. They used cells transfected with a luciferase gene driven by the p21 promoter to show that the E-box motifs are required to observe the effect of MWCNTs on p21 expression. They concluded that, through the effects of MWCNTs on BMP signaling, Id-mediated suppression of p21 expression is removed, and p21 levels rise, resulting in cell cycle arrest.
Of the over 1300 nanomaterial-containing consumer products now on the market, approximately 30 are based on carbon nanotubes (CNTs). As this number is likely to grow rapidly, the toxicity of these nanoparticles is a major concern. Prior studies have shown that CNTs can induce inflammation, granuloma formation, and a fibrotic response in cells associated with the respiratory tract. Furthermore, their ability to induce malignant transformation and aneuploidy suggests that they may induce loss of cell cycle control. These observations led Zhang et al. (DOI 10.1021/tx300059m) to explore the effects of carboxylated multiwalled CNTs (MWCNTs) on cell cycle regulation. Exposure to MWCNTs led to arrest at the G1/S phase of the cell cycle in three different cell lines. The MWCNTs were not cytotoxic at the concentrations tested, and the cells resumed cycling after 24 h when the particles were washed away. Immunoblot analysis revealed no change in the levels of several cell cycle regulatory cyclins and cyclin-dependent kinases (CDKs); however, an increase in the level of the CDK inhibitor p21 was observed. Since p21 blocks the phosphorylation of retinoblastoma protein (Rb) by complexes of cyclins and CDKs, a decrease in the level of phosphoryated Rb was also observed. The finding that mouse embryo fibroblasts (MEFs) from p21 knockout mice failed to undergo cell cycle arrest in response to MWCNTs confirmed that increased expression of p21 was required for the response. MWCNTs had no effect on the degradation rate of p21 in cells treated with the protein synthesis inhibitor cycloheximide, indicating that the nanoparticles’ effect on p21 protein expression must be due to increased synthesis. The expression of p21 is frequently regulated by the DNA damage response protein p53. However, © 2012 American Chemical Society
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TOXIC DANGER FROM THE SEA
Palytoxin is one of the most potent nonpeptide marine toxins currently known. This very large, complex molecule disrupts the activity of the Na+/K+-ATPase, which is required to maintain the plasma membrane potential in all cells. Thus, the discovery that Ostreopsis species of dinoflagellates produce Published: June 18, 2012 1157
dx.doi.org/10.1021/tx300188k | Chem. Res. Toxicol. 2012, 25, 1157−1158
Chemical Research in Toxicology
In This Issue
palytoxin-like compounds is of great concern. This is particularly true since blooms of these organisms in the Mediterranean Sea during the past decade have caused intoxication of numerous individuals who were exposed to sea aerosols. The toxins produced by Ostreopsis confronta (cf.) ovata, the species of dinoflagellate isolated from Mediterranean waters, include a putative palytoxin (pPLTX) and palytoxin analogues that have been designated ovatoxins (OVTX-a, -b, -c, -c, and -e). Thus far, OVTX-a has been the major component of toxins isolated from O. cf. ovata strains. Now, Ciminiello et al. (DOI 10.1021/tx300085e) announce a new ovatoxin, OVTX-f, produced by an O. cf. ovata isolate (CBA2-122) obtained from a bloom in the mid-Adriatic Sea near Portonovo (Ancona, Italy). LC-MS analysis of extracts of the CBA2-122 strain revealed peaks for all five of the known ovatoxins plus pPLTX. Despite the complexity of the molecules, and the presence of multiple species of differing charges, high resolution collision-induced dissociation MSn analyses allowed the identification of individual compounds. The results indicated two sets of ions present in the chromatographic region corresponding to the retention time of OVTX-a. These two sets of ions corresponded to OVTX-a and a previously uncharacterized compound with a highly similar spectrum to that of OVTX-a. Using combined assignments of the most abundant singly-, doubly-, and triply charged ions of the new OVTX-f gave an elemental formula of C 131 H227 N 3 O 52 , as compared to C129H223N3O52 for OVTX-a. Thus, the two compounds differ by only a C2H4 group. Further MSn analysis localized the structural difference to the region between carbons-95 and -102, which has not previously been a recognized site of variability between palytoxin analogues. OVTX-f accounted for 50% of the total toxin extract from strain CBA2-122, followed by OVTX-a (23%) and OVTX-b (17%). The remaining ovatoxins were minor components. Exposure of primary human macrophages to the toxic extracts induced an increase in the levels of mRNAs encoding inflammation-related proteins via the nuclear factor-κB and p38 mitogen activated protein kinase signaling pathways. The new toxin profile of the O. cf. ovata isolate indicates that the range of toxic species produced by O. cf. ovata may be more complex than previously realized. The increasing presence of these microorganisms in the Mediterranean Sea and other temperate waters poses a serious potential health hazard to individuals living along the coast, as well a significant ecotoxicologic threat to the entire marine food chain.
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