Tk Mutants Research Articles

Mutagenicity and genotoxicity evaluation of 15 nitrosamine drug substance-related impurities in human TK6 cells

Nitrosamine drug substance-related impurities (NDSRIs) are a sub-category of N-nitrosamine drug impurities that share structural similarity to the corresponding active pharmaceutical ingredient. The mutagenicity of NDSRIs is poorly understood. We previously tested a series of NDSRIs using the Enhanced Ames Test (EAT). In this follow-up study, we further examined the genotoxicity and mutagenicity of 15 of these NDSRIs in human TK6 cells. Seven EAT-positive NDSRIs, including N-nitroso-nortriptyline, N-nitroso-fluoxetine, N-nitroso-desmethyl-diphenhydramine, N-nitroso-duloxetine, N-nitroso-lorcaserin, N-nitroso-varenicline, and N-nitroso-sertraline, induced concentration-dependent increases in micronuclei after bioactivation with hamster liver S9. These NDSRIs were also mutagenic in the TK and HPRT gene mutation assays, consistent with their positive EAT results. In the presence of hamster liver S9, the eight EAT-negative NDSRIs were negative in the micronucleus assay and negative for mutation induction. Using TK6 cells endogenously expressing a single human cytochrome P450 (CYP), we found that CYP2C19, CYP2B6, CYP2A6, and CYP3A4 are key enzymes activating the genotoxicity and mutagenicity of these NDSRIs. Overall, the hamster S9-mediated TK6 cell mutagenicity results agreed with those observed in the EAT, indicating consistency in the mutagenic responses produced by NDSRIs across different testing systems. These data support the use of EAT for hazard identification and safety assessment of NDSRIs.

Next-generation EGFR tyrosine kinase inhibitors to overcome C797S mutation in non-small cell lung cancer (2019-2024).

Lung cancer is a leading cause of cancer-related deaths worldwide. Non-small cell lung cancer (NSCLC) accounts for the major portion (80-85%) of all lung cancer cases. Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) are commonly used as the targeted therapy for EGFR-mutated NSCLC. The FDA has approved first-, second- and third-generation EGFR-TKIs as therapeutics options. Osimertinib, the third-generation irreversible EGFR-TKI, has been approved for the treatment of NSCLC patients with the EGFRT790M mutation. However, due to the EGFRC797S mutation in the kinase domain of EGFR, resistance to osimertinib is observed and that limits the long-term effectiveness of the drug. The C797S mutation is one of the major causes of drug resistance against the third-generation EGFR TKIs. The C797S mutations including EGFR double mutations (19Del/C797S or L858R/C797S) and or EGFR triple mutations (19Del/T790M/C797S or L858R/T790M/C797S) cause major resistance to the third-generation EGFR-TKIs. Therefore, the discovery and development of fourth-generation EGFR-TKIs to target triple mutant EGFR with C797S mutation is a challenging topic in medicinal chemistry research. In this review, we discuss the discovery of novel fourth-generation EGFR TKIs, medicinal chemistry approaches and the strategies to overcome the C797S mutations. In vitro activities of EGFR-TKIs (2019-2024) against mutant EGFR TK, anti-proliferative activities, structural modifications, binding modes of the inhibitors and in vivo efficacies in animal models are discussed here.

FTO regulates the DNA damage response via effects on cell-cycle progression.

The fat mass and obesity-associated protein FTO is an "eraser" of N6-methyladenosine, the most abundant mRNA modification. FTO plays important roles in tumorigenesis. However, its activities have not been fully elucidated and its possible involvement in DNA damage - the early driving event in tumorigenesis - remains poorly characterized. Here, we have investigated the role of FTO in the DNA damage response (DDR) and its underlying mechanisms. We demonstrate that FTO responds to various DNA damage stimuli. FTO is overexpressed in mice following exposure to the promutagens aristolochic acid I and benzo[a]pyrene. Knockout of the FTO gene in TK6 cells, via CRISPR/Cas9, increased genotoxicity induced by DNA damage stimuli (micronucleus and TK mutation assays). Cisplatin- and diepoxybutane-induced micronucleus frequencies and methyl methanesulfonate- and azathioprine-induced TK mutant frequencies were also higher in FTO KO cells. We investigated the potential roles of FTO in DDR. RNA sequencing and enrichment analysis revealed that FTO deletion disrupted the p38 MAPK pathway and inhibited the activation of nucleotide excision repair and cell-cycle-related pathways following cisplatin (DNA intrastrand cross-links) treatment. These effects were confirmed by western blotting and qRT-PCR. FTO deletion impaired cell-cycle arrest at the G2/M phase following cisplatin and diepoxybutane treatment (flow cytometry analysis). Our findings demonstrated that FTO is involved in several aspects of DDR, acting, at least in part, by impairing cell cycle progression.

How Bulky of Base Adducts can be Responsible for Clastogenicity rather than Mutagenicity?

Two types of TK mutants are induced by genotoxic factors; normally growing (NG) TK mutants due to point mutations of targeted TK locus, and slowly growing (SG) mutants due to gross structural changes involving the growth-regulating gene outside targeted TK locus. In this study, human lymphoblstoid WTK1 cells were used to consider how bulky n- alkylated bases can induce SG mutants. For this purpose, n-alkyl methanesulfonates (AMS) having an n-alkyl group with 3-7 carbons [n-propy methanesulfonate (PMS), n-butyl methanesulfonate (BMS), n-pentyl methanesulfonate (PeMS), n- hexyl methanesulfonate (HexMS), and heptyl methanesulfonate(HepMS)] were synthesized. n-alkyl methanesulfonates having n-alkyl groups with 1-7 carbons induced NG mutants, but n-alkyl methanesulfonates having n-alkyl groups with ≥4 carbons but not with ≤3 carbons induced SG mutants. n-Alkyl methanesulfonates having n-Alkyl groups with ≥4 carbons have been shown to induce bulky adducts that cause disturbances to the helical DNA structure and are removed by nucleotide excision repair. It could be considered that n-alkyl groups with ≥4 carbons causing disturbances to the helical DNA structure induce SG mutants to result in clastogenicity rather than mutagenicity.

PB1871: PROGNOSTIC SIGNIFICANCE OF THE NOTCH1 GENE MUTATION AND OTHER LABORATORY FACTORS IN CHRONIC LYMPHOCYTIC LEUKEMIA

Background: An important stage in the management of patients with CLL is the prediction of the course of the disease in order to assess the need for an earlier onset of therapy and the use of alternative therapy on an individual basis, depending on the detectable prognostic markers. Aims: To determine the interrelationship of clinical-laboratory (as well as molecular-genetic) prognostic markers with various courses of CLL. Methods: We have examined 107 patients diagnosed with CLL in SI «RRC RM&HE» from 2017 to 2020. This cohort was divided into 2 groups: 1st group (49 patients) without signs of progression of CLL, and the 2nd group (58 patients) with clinical signs of disease progression after 3 years of observation. The median age in the 1st group was 61 years, in the 2nd - 62 years. The material for the study was venous blood. To determine the immunophenotype of tumor cells we used the flow cytofluorimeter FacsCanto II. Detection of β2-mg, TK and TPA was performed using the chemiluminescent analyzer. To determine the mutations of the NOTCH1 gene (34th exon), SSCP-PCR was used, followed by direct sequencing of DNA samples with conformational polymorphism. We used nonparametric statistics methods calculated in the Statistica 10.0. Statistically significant differences were considered for p<0.05. Results: Patients in group 1 differed from patients in group 2 in levels of β2-mg-1.8 and 3.5 μg/L (p<0.001) and LDH-220 and 326 U/L (p<0.002). TK levels (p<0.001) and TPA (p<0.011) were also significantly higher in group 2 (7.7 and 49.8 IU/L) compared with patients in group 1 (4.2 and 34.3 IU/L). According to immunophenotyping data, statistically significant differences were obtained on the Zap70 marker: in the 1st group the median was 10.1%, in the 2nd group - 21.6%. As a result of molecular genetic analysis, mutations in the NOTCH1 gene were identified in 14 patients. It was found that patients without progression at the time of inclusion in the study in the presence of mutations in the NOTCH1 gene had a statistically significantly shorter progression-free survival (mean 33.4 months (95% CI [22.0-44.8]) compared with patients without mutations (59.2 months (95% CI [54.1–64.3]) (p=0.018), which indicates the association of NOTCH1 mutations with a poor prognosis for disease progression in patients with CLL. Summary/Conclusion: It was found that markers such as LDH, β2-mg, TK, TPA, ZAP70 and NOTCH1 gene mutation have certain prognostic significance, which was confirmed by comparing their levels in patients with and without CLL progression after 3 years of observation. We can use NOTCH1 gene mutation as an additional factor in predicting the course of CLL. Knowledge of reliable prognostic markers may allow to initiate in-time treatment or to adjust therapy for patients with high-risk CLL with a probable increase in their life span.

Improving the Safety of Mesenchymal Stem Cell-Based Ex Vivo Therapy Using Herpes Simplex Virus Thymidine Kinase.

Human mesenchymal stem cells (MSCs) are multipotent stem cells that have been intensively studied as therapeutic tools for a variety of disorders. To enhance the efficacy of MSCs, therapeutic genes are introduced using retroviral and lentiviral vectors. However, serious adverse events (SAEs) such as tumorigenesis can be induced by insertional mutagenesis. We generated lentiviral vectors encoding the wild-type herpes simplex virus thymidine kinase (HSV-TK) gene and a gene containing a point mutation that results in an alanine to histidine substitution at residue 168 (TK(A168H)) and transduced expression in MSCs (MSC-TK and MSC-TK(A168H)). Transduction of lentiviral vectors encoding the TK(A168H) mutant did not alter the proliferation capacity, mesodermal differentiation potential, or surface antigenicity of MSCs. The MSC-TK(A168H) cells were genetically stable, as shown by karyotyping. MSC-TK(A168H) responded to ganciclovir (GCV) with an half maximal inhibitory concentration (IC50) value 10-fold less than that of MSC-TK. Because MSC-TK(A168H) cells were found to be non-tumorigenic, a U87-TK(A168H) subcutaneous tumor was used as a SAE-like condition and we evaluated the effect of valganciclovir (vGCV), an oral prodrug for GCV. U87-TK(A168H) tumors were more efficiently ablated by 200 mg/kg vGCV than U87-TK tumors. These results indicate that MSC-TK(A168H) cells appear to be pre-clinically safe for therapeutic use. We propose that genetic modification with HSV-TK(A168H) makes allogeneic MSC-based ex vivo therapy safer by eliminating transplanted cells during SAEs such as uncontrolled cell proliferation.

Is Genotoxicity of Peroxisome Proliferator-Activated Receptor Agonists Due to Oxidative Stress Via Agonistic Pathways?

In order to investigate the relationship between genotoxicity and peroxisome proliferator-activated receptor (PPAR) agonistic effects, we conducted two kinds of comet assays (cellular and acellular), a micronucleus (MN) test, and a TK mutation assay with and without PPAR antagonists using human lymphoblastoid cells. PPARα agonist clofibrate (CLF) and PPARγ agonists indomethacin (IND) and pioglitazone (PGZ) showed positive responses in the cellular comet assay, TK mutation assay, and detection of intracellular reactive oxygen species (ROS), but not in the acellular comet assay and MN test. PPARα antagonist (GW6471) suppressed the induction of ROS, DNA damage, and TK mutation by CLF. PPARγ antagonist (BADGE) suppressed the induction of ROS, DNA damage, and TK mutation by IND and PGZ. Therefore, CLF and two PPARγ agonists (PGZ and IND) show genotoxicity by oxidative stress via PPARα and PPARγ agonistic pathways, respectively. Considering that some unrepaired DNA lesions induced by them persist to form gene mutations but not chromosome aberrations, there is a possibility that their genotoxic potential is due to mutagenic but not clastogenic potential by the production of ROS via agonistic pathway

ZEBRAFISH AS MODELS OF TYROSINE KINASE‐DRIVEN CANCER

The protein kinase gene family encodes proteins involved in several essential functions, with cell growth and migration being essential outcomes. Mutations in kinases underlie a broad range of diseases, primarily inflammation and cancer. Tyrosine kinase inhibitors (TKIs) in particular, are successfully used as a targeted therapy for cancer and inflammation‐driven disorders. Current unmet needs include addressing the adverse effects of TKIs, the emergence of resistance, and the necessity of newer TKIs. Experiments using accessible animal models such as zebrafish can help address these needs. Zebrafish studies can help in efficient characterization of toxicological and pharmacological effects of inhibition using known or potential TKIs, via the combination of phenotypic and molecular analysis. The recent application of CRISPR‐Cas9 further enables the development of genetically engineered cancer models in zebrafish. Following our previous work showing conservation of zebrafish tyrosine kinases compared to humans, we are now studying the effects of chemical or genetic tyrosine kinase modulation in zebrafish.To identify the phenotypic endpoints of tyrosine kinase inhibition in zebrafish, we selected a few drugs approved by the USFDA for treating cancer and inflammation‐related disorders in humans. We evaluated phenotypic effects including adverse effects to identify NOELs and detectable and consistent phenotypic endpoints of such compounds in zebrafish. These effects provide a basis to evaluate novel TKI compounds. Further, tyrosine kinase genes involved in inflammation and cancer were selected for targeted genetic modification in zebrafish using CRISPR Cas9. Our initial data demonstrate the development of MET (HGFR) homozygous mutant zebrafish and the phenotypes associated with this mutant. Detailed characterization of this mutant including molecular analysis is under way. The characteristics of this mutant provide the basis to identify MET‐specific inhibitors. Development of zebrafish models carrying oncogenic TK mutants, and detailed characterization of the effects of TKIs in zebrafish are ongoing.In summary, our work is focussed on identifying TKI treatment‐related phenotypes in zebrafish, and generation of TK‐crispant zebrafish as cancer models.Support or Funding InformationDepartment of Biotechnology and the Indian Council of Medical Research, Govt. of India

Arylidenes of Quinolin-2-one scaffold as Erlotinib analogues with activities against leukemia through inhibition of EGFR TK/ STAT-3 pathways.

A new series of 6-substiuted-4-(2-(4-substituted-benzylidene)hydrazinyl)quinolin-2(1H)-one derivatives have been designed and synthesized. The structure of the synthesized compounds was proved by 1H NMR, 13C NMR, 2D NMR, mass and elemental analyses. The target compounds were evaluated for their in vitro cytotoxic activity against 60 cancer cell lines according to NCI protocol. Consequently, the most active compounds were further examined against the most sensitive leukemia RPMI-8226 and on healthy cell lines. 6-Chloro-derivative was the most active one; with IC50=15.72±1.21 and 46.05±2.36μM against RPMI-8226 and normal cell lines, respectively. Also, it showed a remarkable inhibitory activity compared to gefitinib on the EGFR TK mutant, wild and on H-RAS in addition to STAT-3 with IC50=695.49±21.8, 263.15±15.13, 10.61±0.27 and 1.753±0.81nM, respectively. Cell cycle analysis of RPMI-8226 cells treated with the 6-chloro-derivative showed cell cycle arrest at G2/M phase (supported by Caspases-3,8, BAX and Bcl-2 studies) with a significant pro-apoptotic activity as indicated by annexin V-FITC staining. Moreover, the docking studies ROCS analysis and Tanimoto scores supported the results. The study illustrated the effect of several factors on compounds activity.

Detection of Loss of Heterozygosity in Tk-Deficient Mutants from L5178Y Tk+/--3.7.2C Mouse Lymphoma Cells.

The mouse lymphoma assay (MLA), a forward mutation assay using the Tk+/--3.7.2C clone of the L5178Y mouse lymphoma cell line and the Thymidine kinase (Tk) gene, has been widely used as an in vitro genetic toxicity assay for more than four decades. The MLA can evaluate the ability of mutagens to induce a wide range of genetic events including both gene mutations and chromosomal mutations and has been recommended as one component of several genotoxicity test batteries. Tk-deficient mutants often exhibit chromosomal abnormalities involving the distal end of chromosome 11 where the Tk gene is located, in mice, and the type of chromosome alteration can be analyzed using a loss of heterozygosity (LOH) approach. LOH has been considered an important event in human tumorigenesis and can result from any of the following several mechanisms: large deletions, mitotic recombination, and chromosome loss. In this chapter, the authors describe the procedures for the detection of LOH in the Tk mutants from the MLA, and apply LOH analysis for understanding the types of genetic damage that is induced by individual chemicals.

Whole genome sequencing analysis of small and large colony mutants from the mouse lymphoma assay.

The Thymidine kinase (Tk) gene forward mutation assay, known as the mouse lymphoma assay (MLA), has been widely used for evaluating the genotoxicity of chemical agents. A striking morphological feature of Tk mutant colonies is the bimodal distribution of their sizes, with cells from the large colonies growing at a normal rate and cells from the small colonies growing at a slower rate than normal. To understand the molecular distinction for the different growth rates, we performed whole genome sequencing (WGS) analysis of the large and small colony mutants generated from the MLA. Three large colony and three small colony mutants generated from cells treated with 4-nitroquinoline 1-oxide (4-NQO) or the vehicle control were selected for analysis. The WGS data were analyzed for loss of heterozygosity (LOH) and chromosome copy number along chromosome 11, where the Tk gene is located. Although there were LOH alterations in both large and small colony mutants, copy number changes near Tk locus were found only in small colony mutants produced bythe vehicle control and 4-NQO treatments. The chromosome copy number in the regions near the Tk locus increased from two to three or four in the spontaneous small colony mutants and decreased from two to one in the 4-NQO-induced small colony mutants. These results suggest that chromosome damage was repaired differently in the large and small colony mutants, resulting in significant chromosome alterations in the small colony mutants, but not in the large colony mutants. Thus, chromosome alterations near the Tk locus may play a major role in the inhibition of cell growth in the Tk small colony mutants.

Safety and immunogenicity of an attenuated Chinese pseudorabies variant by dual deletion of TK&gE genes

BackgroundSince the outbreak of a new emerging virulent pseudorabies virus mutant in Chinese pig herds, intensive research has been focused on the construction of novel gene deletion vaccine based on the variant virulent viruses. An ideal vaccine candidate is expected to have a balanced safety and immunogenicity.ResultsFrom the infectious clone of PRV AH02LA strain, a TK deletion mutant was generated through two-step Red mutagenesis. After homologous recombination with a transfer vector, a TK&gE dual deficient mutant PRV (PRVΔTK&gE-AH02) was generated, and its structure verified by PCR, RFLP and sequencing. Growth kinetics test showed that PRVΔTK&gE-AH02 reached a titer of 107.5 TCID50 /mL on ST cells.The PRVΔTK&gE-AH02 at a dose of 106.0 TCID50 /animal was not virulent in mice or 1-day-old piglets with maternal PRV antibodies. No clinical signs or virus shedding were detected in 28~ 35-day-old piglets without maternal PRV antibodies after nasal or intramuscular administration with a dose of 106.0 TCID50, although it caused one death of four 1-day-old piglets without maternal PRV antibodies. In the efficiency test of PRVΔTK&gE-AH02, all four 28~ 35-day-old piglets without PRV antibody in the challenge control showed typical clinical symptoms and virus shedding, and two died at 4~ 5 days post challenge. All piglets in 105.0, 104.0 and 103.0 TCID50/dose PRVΔTK&gE-AH02 groups provided complete protection against challenge at only 7 days post intramuscular vaccination. More importantly, PRVΔTK&gE-AH02 stopped virus shedding in these piglets. In contrast, all four piglets in PRV Bartha K61 vaccine group developed high body temperature (≥40.5 °C) and viral shedding, despite they had mild or even no clinical symptoms.ConclusionsThe constructed TK&gE dual deletion mutant PRVΔTK&gE-AH02 can reach high titers on ST cells. The live vaccine of PRVΔTK&gE-AH02 is highly safe, and can not only provide clinical protection but also stops virus shedding. This study suggests that PRVΔTK&gE-AH02 might work as a promising vaccine candidate to combat the PRV variant emerging in Chinese herds since 2011.

A review of the genotoxic potential of 1,4-naphthoquinone

1,4-Naphthoquinone (1,4-NQ; CAS RN 130-15-4), a derivative of naphthalene, is a commonly used pre-cursor in industrial processes. Since the early 1980’s 1,4-NQ has been tested in a number of genotoxicity assays, both in vitro and in vivo. There is strong evidence that 1,4-NQ does not induce gene mutations in bacteria or mammalian cells in vitro with predominantly negative Ames tests and negative Hprt and tk mutation studies. However, there is clear evidence of a clastogenic response in vitro from positive micronucleus, sister chromatid exchange and chromosome aberration assays. 1,4-NQ-treated mice and hamsters were, however, negative for micronucleus or chromosomal aberration induction in GLP-compliant studies with clear evidence of target tissue exposure, suggesting an in vitro only effect. Evidence indicates that the mechanism of in vitro clastogenicity is predominantly via ROS generation, and since in vitro mammalian cell tests systems have poor anti-oxidant defence mechanisms, they are particularly sensitive to oxidative DNA damage. On the other hand, healthy mammalian tissues have more efficient anti-oxidant defence mechanisms, and therefore it is not surprising that 1,4-NQ is not genotoxic in vivo.

Abstract CT149: MP0250, a VEGF- and HGF-blocking multi-DARPin drug candidate, in combination with tyrosine-kinase-inhibitors targeting EGFR-mutated NSCLC: Preclinical rationale and phase Ib/II study outline

Abstract Background: Relapse of patients with EGFR-mutated NSCLC treated with TKI regimens is most often due to TK mutation but also in a significant number of cases due to upregulation of bypass pathways, the most common being the HGF/cMET signaling pathway. Osimertinib, a third generation TKI, is currently approved for T790M-mutated NSCLC but options for treatment following relapse are unsatisfactory. Hence, there is a high medical need for therapies overcoming EGFR-TKI resistance, including inhibition of bypass mechanisms. In this respect, we are investigating combination of HGF/cMET pathway inhibition with EGFR-TKI therapy. MP0250 is a multi-DARPin drug candidate, specifically neutralizing HGF and VEGF, with the potential to block HGF-mediated bypass of EGFR-TKI inhibition in addition to exerting EGFR-independent antitumor activity via dual HGF and VEGF inhibition. We have shown that HGF expression is upregulated in most human tumors including in 4 of 5 NSCLC samples. In contrast, cMET was not highly overexpressed. MP0250 showed single-agent activity in 3 HGF-autocrine NSCLC PDX-models and an additive/synergistic effect in combination with erlotinib in an erlotinib-resistant model. These preclinical data support the reported observation that the HGF/cMET signaling pathway can confer resistance to EGFR-TKI therapy and that sensitivity to EGFR-TKIs can be restored by blocking HGF. Consequently, we have initiated a phase 1b/2 study for MP0250 in combination with osimertinib in the treatment of EGFR-mutated NSCLC patients. Trial Design: This is a phase Ib/II, single-arm, open-label, multi-center study of MP0250 in combination with osimertinib in patients with EGFR-mutated non-squamous non-small cell lung cancer (NSCLC) pretreated with osimertinib.The primary objective is to estimate the antitumor efficacy, as measured by Objective Response Rate (ORR) after 8 cycles of treatment with MP0250 i.v. every 3 weeks in combination with osimertinib orally once daily, when administered to patients with EGFR-mutated, advanced, non-squamous NSCLC after tumor progression on osimertinib and on or after the most recent therapy. Secondary and exploratory objectives are to determine safety, tolerability, progression-free survival, pharmacokinetics, immunogenicity and biomarkers. An initial dose escalation safety lead-in part, Part A, is intended to establish a safe recommended dose (RD) of MP0250 in combination with osimertinib. The cohort at RD from Part A will be expanded in order to confirm the tolerability and to further estimate the efficacy of MP0250 in combination with osimertinib. A total of 40 patients will be enrolled, 12 in part A and an additional 28 in part B at RD. Further details will be shown in the poster. Citation Format: Joachim Kiemle-Kallee, Ulrike Fiedler, Keith M. Dawson, Jutta Haunschild, Sandra Dietschy, Michael T. Stumpp, Frank Hermann, Andreas Harstrick. MP0250, a VEGF- and HGF-blocking multi-DARPin drug candidate, in combination with tyrosine-kinase-inhibitors targeting EGFR-mutated NSCLC: Preclinical rationale and phase Ib/II study outline [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT149.

A review of the genotoxicity of trimethylolpropane triacrylate (TMPTA)

Trimethylolpropane triacrylate (TMPTA) is a trifunctional acrylate monomer which polymerizes rapidly when exposed to sources of free radicals. It is widely used as a reactive diluent and polymer building block in the formulation of overprint varnishes, inks and a variety of wood, plastic and metal coatings. TMPTA has been tested in a range of in vitro and in vivo genotoxicity tests. There is no clear evidence of induction of gene mutations by TMPTA in bacteria or mammalian cells in vitro, but there is evidence of clastogenicity from induction of small colony tk mutants in the mouse lymphoma assay, and also induction of micronuclei and chromosomal aberrations. However, TMPTA was negative in bone marrow or blood micronucleus tests in vivo following oral or repeated dermal application, and did not induce comets in bone marrow or liver of mice following intravenous administration, which would have achieved plasma (and therefore tissue) concentrations estimated to exceed those inducing clastogenic effects in vitro. It is concluded that TMPTA is not genotoxic in vivo.

Comparative Genotoxicity of TEMPO and 3 of Its Derivatives in Mouse Lymphoma Cells.

TEMPO (2, 2, 6, 6-tetramethylphiperidine-1-oxyl) and its derivatives are stable free radical nitroxides widely used in the field of chemistry, biology, and pharmacology. TEMPO was previously found to be mutagenic and to induce micronuclei in mammalian cells. In this study, we investigated and quantified the genotoxicity of 4 structurally similar nitroxides, TEMPO and 3 of its derivatives (4-hydroxy-TEMPO, 4-oxo-TEMPO, and 4-methoxy-TEMPO), using the mouse lymphoma assay (MLA) and Comet assay in L5178Y Tk+/- cells. The results showed that all tested nitroxides were cytotoxic and mutagenic in the MLA, both in the presence and absence of S9, with metabolic activation significantly enhancing the cytotoxicity and/or mutagenicity. In addition, the 4 nitroxides caused DNA-strand breakage. The mutagenicity and DNA damaging dose-responses of the test articles were compared using the PROAST benchmark dose software package. The potency ranking of the 4 nitroxides for mutagenicity was different from the ranking of the DNA damaging effects. The mode of action analysis by a multi-endpoint DNA damage pathway assay classified all 4 nitroxides as clastogens. In addition, the majority of the induced Tk mutants showed loss of heterozygosity at the Tk and D11Mit42 loci (ie, chromosome damage <31 Mbp). These results suggest that TEMPO and its 3 derivatives are cytotoxic and mutagenic in mouse lymphoma cells through a mechanism that involves strand breakage and large alterations to DNA. The potency rankings indicate that the different TEMPO derivatives vary in their mutagenic and DNA damaging potential.

MiR-182 promotes cancer invasion by linking RET oncogene activated NF-\u03baB to loss of the HES1/Notch1 regulatory circuit

BackgroundDominant-activating mutations in the RET proto-oncogene, a receptor tyrosine kinase, are responsible for the development of medullary thyroid carcinoma (MTC) and causative for multiple endocrine neoplasia (MEN) type 2A and 2B. These tumors are highly aggressive with a high propensity for early metastasis and chemoresistance. This attribute makes this neoplasia an excellent model for probing mechanisms underlying cancer progression.MethodsThe expression level of miR-182 was measured in MTC tumor specimens and in TT cells by real-time RT-PCR. TT cells and modified NThy-ori 3.1 that stably express RETM918T were used to investigate RET-dependent regulation of miR-182. Identification and validation of miR-182 targets and pathways was accomplished with luciferase assays, qRT-PCR, Western blotting and immunofluorescence. In vitro, overexpression and knockdown experiments were carried out to examine the impact of miR-182 and HES1 on invasion and migration.ResultsWe found that miR-182 expression is significantly upregulated in MTC patient samples and tumor-derived cell lines harboring mutated RET. Inhibition of RET oncogenic signaling through a dominant-negative RET∆TK mutant in TT cells reduces miR-182, whereas overexpression of RETM918T in NThy-ori 3.1 cells increases miR-182 levels. We further show that overexpression of this miRNA in NThy.miR-182 cells promotes the invasive and migratory properties without affecting cell proliferation. MiR-182 is upregulated after RET induced NF-κB translocation into the nucleus via binding of NF-κB to the miR-182 promoter. Database analysis revealed that HES1, a repressor of the Notch pathway, is a target of miR-182, whose upregulation correlates with loss of HES1 transcription in MTC tissue samples and mutant RET cell lines. Moreover, we demonstrated that the 3′UTR of the HES1 mRNA bearing the targeting sequence for miR-182 clearly reduced luciferase reporter activity in cells expressing miR-182. Decreased expression of HES1 promotes migration by upregulating Notch1 inhibitor Deltex1 and consequent repression of Notch1.ConclusionWe demonstrate a novel mechanism for MTC aggressiveness in which mutated RET/NF-κB-driven expression of miR-182 impedes HES1 activation in a negative feedback loop. This observation might open new possibilities to treat RET oncogene associated metastatic cancer.

Homologous Recombination and Translesion DNA Synthesis Play Critical Roles on Tolerating DNA Damage Caused by Trace Levels of Hexavalent Chromium.

Contamination of potentially carcinogenic hexavalent chromium (Cr(VI)) in the drinking water is a major public health concern worldwide. However, little information is available regarding the biological effects of a nanomoler amount of Cr(VI). Here, we investigated the genotoxic effects of Cr(VI) at nanomoler levels and their repair pathways. We found that DNA damage response analyzed based on differential toxicity of isogenic cells deficient in various DNA repair proteins is observed after a three-day incubation with K2CrO4 in REV1-deficient DT40 cells at 19.2 μg/L or higher as well as in TK6 cells deficient in polymerase delta subunit 3 (POLD3) at 9.8 μg/L or higher. The genotoxicity of Cr(VI) decreased ~3000 times when the incubation time was reduced from three days to ten minutes. TK mutation rate also significantly decreased from 6 day to 1 day exposure to Cr(VI). The DNA damage response analysis suggest that DNA repair pathways, including the homologous recombination and REV1- and POLD3-mediated error-prone translesion synthesis pathways, are critical for the cells to tolerate to DNA damage caused by trace amount of Cr(VI).

Size- and coating-dependent cytotoxicity and genotoxicity of silver nanoparticles evaluated using in vitro standard assays

The physicochemical characteristics of silver nanoparticles (AgNPs) may greatly alter their toxicological potential. To explore the effects of size and coating on the cytotoxicity and genotoxicity of AgNPs, six different types of AgNPs, having three different sizes and two different coatings, were investigated using the Ames test, mouse lymphoma assay (MLA) and in vitro micronucleus assay. The genotoxicities of silver acetate and silver nitrate were evaluated to compare the genotoxicity of nanosilver to that of ionic silver. The Ames test produced inconclusive results for all types of the silver materials due to the high toxicity of silver to the test bacteria and the lack of entry of the nanoparticles into the cells. Treatment of L5718Y cells with AgNPs and ionic silver resulted in concentration-dependent cytotoxicity, mutagenicity in the Tk gene and the induction of micronuclei from exposure to nearly every type of the silver materials. Treatment of TK6 cells with these silver materials also resulted in concentration-dependent cytotoxicity and significantly increased micronucleus frequency. With both the MLA and micronucleus assays, the smaller the AgNPs, the greater the cytotoxicity and genotoxicity. The coatings had less effect on the relative genotoxicity of AgNPs than the particle size. Loss of heterozygosity analysis of the induced Tk mutants indicated that the types of mutations induced by AgNPs were different from those of ionic silver. These results suggest that AgNPs induce cytotoxicity and genotoxicity in a size- and coating-dependent manner. Furthermore, while the MLA and in vitro micronucleus assay (in both types of cells) are useful to quantitatively measure the genotoxic potencies of AgNPs, the Ames test cannot.

Sensitivity of human cells expressing low-fidelity or weak-catalytic-activity variants of DNA polymerase ζ to genotoxic stresses

Translesion DNA polymerases (TLS pols) play critical roles in defense mechanisms against genotoxic agents. The defects or mutations of TLS pols are predicted to result in hypersensitivity of cells to environmental mutagens. In this study, human cells expressing DNA polymerase ζ (Pol ζ) variants with low fidelity or weak catalytic activity have been established with Nalm-6-MSH+ cells and their sensitivity to mutagenicity and cytotoxicity of benzo[a]pyrene diol epoxide (BPDE) and ultraviolet-C light (UV-C) was examined. The low-fidelity mutants were engineered by knocking-in DNA sequences that direct changes of leucine 2618 to either phenylalanine (L2618F) or methionine (L2618M) of Pol ζ. The weak-catalytic-activity mutants were generated by knocking-in DNA sequences that direct changes of either tyrosine 2779 to phenylalanine (Y2779F) or aspartate 2781 to asparagine (D2781N). In addition, a +1 frameshift mutation, i.e., CCC to CCCC, was introduced in the coding region of the TK1 gene to measure the mutant frequencies. Doubling time and spontaneous TK mutant frequencies of the established cell lines were similar to those of the wild-type cells. The low-fidelity mutants displayed, however, higher sensitivity to the mutagenicity of BPDE and UV-C than the wild-type cells although their cytotoxic sensitivity was not changed. In contrast, the weak-catalytic-activity mutants were more sensitive to the cytotoxicity of BPDE and UV-C than the wild-type cells, and displayed much higher sensitivity to the clastogenicity of BPDE than the wild-type cells in an in vitro micronucleus assay. These results indicate that human Pol ζ is involved in TLS across DNA lesions induced by BPDE and UV-C and also that the TLS plays important roles in induction of mutations, clastogenicity and in cellular survival of the damaged human cells. Similarities and differences in in vivo roles of yeast and human Pol ζ in genome integrity are discussed.