The Response of Malignant B Lymphocytes to Ionizing Radiation: Cell Cycle Arrest, Apoptosis and Protection against the Cytotoxic Effects of the Mitotic Inhibitor Nocodazole

The inhibition of microtubule dynamics instability alters lipid homeostasis in TM4 Sertoli cells

Sertoli cells (SC) structurally support and transport nutrients to germ cells during spermatogenesis facilitated by an active cytoskeleton. Chemical perturbation of SC microtubule (MT) dynamics instability leads to premature germ cell exfoliation demonstrating that this process is essential for male fertility, yet the effects of MT damaging drugs on SC lipid metabolism have been less explored. The aim of this study was to advance our understanding of how adequate SC MT dynamicity is needed to finely tune lipid homeostasis. To elucidate the role of MT dynamics instability on the latter, we suppressed MT dynamicity by long-term exposures to 10 nM of nocodazole (NCZ) on TM4-SC cultures. Inhibition of MT dynamics instability affected the distribution of [³H] arachidonate on TM4-SC. Triacylglycerols (TAG) exhibited a higher proportion of the [³H] label, with significantly lower percentages in the mitochondrial phospholipid cardiolipin, and notably, also in phosphatidylethanolamine. A noteworthy and progressive accumulation of lipid droplets during the period of exposure to NCZ was accompanied by increased TAG levels but not cholesterol levels in TM4-SC. NCZ-exposed cells reduced their mitochondrial membrane potential and increased ROS production without triggering apoptosis, had a compromised autophagic flux, and lost their transferrin expression. Although SC morphology was preserved, the NCZ-exposed cells displayed alteration of the normal organization of microfilaments (f-actin) and intermediate filaments (vimentin). Our findings suggest that a preserved MT dynamicity is essential in the maintenance of lipid and fatty acids homeostasis in SC, and thus highlights a novel target in these cells for drugs that impair MT dynamicity.

Karonudib has potent anti-tumor effects in preclinical models of B-cell lymphoma

Chemo-immunotherapy has improved survival in B-cell lymphoma patients, but refractory/relapsed diseases still represent a major challenge, urging for development of new therapeutics. Karonudib (TH1579) was developed to inhibit MTH1, an enzyme preventing oxidized dNTP-incorporation in DNA. MTH1 is highly upregulated in tumor biopsies from patients with diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma, hence confirming a rationale for targeting MTH1. Here, we tested the efficacy of karonudib in vitro and in preclinical B-cell lymphoma models. Using a range of B-cell lymphoma cell lines, karonudib strongly reduced viability at concentrations well tolerated by activated normal B cells. In B-cell lymphoma cells, karonudib increased incorporation of 8-oxo-dGTP into DNA, and prominently induced prometaphase arrest and apoptosis due to failure in spindle assembly. MTH1 knockout cell lines were less sensitive to karonudib-induced apoptosis, but were displaying cell cycle arrest phenotype similar to the wild type cells, indicating a dual inhibitory role of the drug. Karonudib was highly potent as single agent in two different lymphoma xenograft models, including an ABC DLBCL patient derived xenograft, leading to prolonged survival and fully controlled tumor growth. Together, our preclinical findings provide a rationale for further clinical testing of karonudib in B-cell lymphoma.

Cell Cycle Analysis and Relevance for Single-Cell Gating in Mass Cytometry

Cell cycle analysis by mass cytometry (MC) is hampered by the poor resolution of the Iridium-labeled DNA intercalator compared to DNA-specific fluorescent dyes. We report here a minimum cell cycle panel for MC consisting of Ir-intercalator (DNA content), IdU (S phase), anti-pS28HistoneH3 (mitosis), anti-CDT1 (G₁ phase) and anti-Geminin (non-G₁ phases). Cell cycle distributions obtained by MC were not significantly different from fluorescence flow cytometry results (r² = 0.98, P < 0.001). Further subdivision of the G₁ and G₂ phases could be done with anti-pS780RB1 (late G₁ ) and anti-PLK1 (late G₂ ), respectively. A disadvantage of MC is that aggregates of cells cannot easily be removed while retaining all single cells. We have developed an analysis pipeline including unsupervised clustering by FlowSOM and subsequent single-cell gating. When performed on cells stained with the cell cycle panel, this analysis pipeline successfully identified debris, dead/apoptotic cells, nonsingle-cell populations and the major cell cycle phases. The presented cell cycle panel and analysis pipeline thus achieves true single-cell analysis at the same time as any additional channels in the panel are open for phenotyping and cell cycle-resolved expression or modification analysis. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC. on behalf of International Society for Advancement of Cytometry.

Effects of low-dose irradiation on human osteoblasts and periodontal ligament cells

OBJECTIVE

To investigate the effects of dental x-ray on proliferation and mineralization in human primary osteoblasts as well as on proliferation and apoptotic potential in human periodontal ligament (PDL) cells.

DESIGN

Primary osteoblasts and PDL cells were irradiated with various doses of periapical radiography by repeated exposures and further incubated for 1, 3 or 7 days. Cell proliferation was assayed by BrdU incorporation. The effect of dental x-ray on mineralization in osteoblasts either before or after x-ray exposures was determined by Alizarin red staining. Both mRNA and protein expressions of BCL-2, an anti-apoptotic gene, and BAX, a pro-apoptotic gene, in PDL cells were analyzed by RT-qPCR and immunoblotting analysis, respectively.

RESULTS

Neither the proliferative nor the mineralization ability of irradiated osteoblasts was different from that of non-irradiated osteoblasts at any doses or time points. By contrast, there was a significant decrease in the proliferation of PDL cells on day 3 after repeated exposures to dental x-ray for 20 times (P < 0.05), whereas the ratio of BCL-2 to BAX mRNA and protein expressions in these irradiated PDL cells was significantly increased (P < 0.05).

CONCLUSIONS

Upon multiple exposures to dental x-ray used in intraoral radiography up to 20 times, there is no effect on the proliferation or the mineralization of osteoblasts, whereas the proliferative and apoptotic potentials of PDL cells are transiently decreased.

Combined inhibition of Wee1 and Chk1 gives synergistic DNA damage in S-phase due to distinct regulation of CDK activity and CDC45 loading

Recent studies have shown synergistic cytotoxic effects of simultaneous Chk1- and Wee1-inhibition. However, the mechanisms behind this synergy are not known. Here, we present a flow cytometry-based screen for compounds that cause increased DNA damage in S-phase when combined with the Wee1-inhibitor MK1775. Strikingly, the Chk1-inhibitors AZD7762 and LY2603618 were among the top candidate hits of 1664 tested compounds, suggesting that the synergistic cytotoxic effects are due to increased S-phase DNA damage. Combined Wee1- and Chk1-inhibition caused a strong synergy in induction of S-phase DNA damage and reduction of clonogenic survival. To address the underlying mechanisms, we developed a novel assay measuring CDK-dependent phosphorylations in single S-phase cells. Surprisingly, while Wee1-inhibition alone induced less DNA damage compared to Chk1-inhibition, Wee1-inhibition caused a bigger increase in S-phase CDK-activity. However, the loading of replication initiation factor CDC45 was more increased after Chk1- than Wee1-inhibition and further increased by the combined treatment, and thus correlated well with DNA damage. Therefore, when Wee1 alone is inhibited, Chk1 suppresses CDC45 loading and thereby limits the extent of unscheduled replication initiation and subsequent S-phase DNA damage, despite very high CDK-activity. These results can explain why combined treatment with Wee1- and Chk1-inhibitors gives synergistic anti-cancer effects.

Replication-induced DNA damage after PARP inhibition causes G2 delay, and cell line-dependent apoptosis, necrosis and multinucleation

PARP inhibitors have been approved for treatment of tumors with mutations in or loss of BRCA1/2. The molecular mechanisms and particularly the cellular phenotypes resulting in synthetic lethality are not well understood and varying clinical responses have been observed. We have investigated the dose- and time-dependency of cell growth, cell death and cell cycle traverse of 4 malignant lymphocyte cell lines treated with the PARP inhibitor Olaparib. PARP inhibition induced a severe growth inhibition in this cell line panel and increased the levels of phosphorylated H2AX-associated DNA damage in S phase. Repair of the remaining replication related damage caused a G₂ phase delay before entry into mitosis. The G₂ delay, and the growth inhibition, was more pronounced in the absence of functional ATM. Further, Olaparib treated Reh and Granta-519 cells died by apoptosis, while U698 and JVM-2 cells proceeded through mitosis with aberrant chromosomes, skipped cytokinesis, and eventually died by necrosis. The TP53-deficient U698 cells went through several rounds of DNA replication and mitosis without cytokinesis, ending up as multinucleated cells with DNA contents of up to 16c before dying. In summary, we report here for the first time cell cycle-resolved DNA damage induction, and cell line-dependent differences in the mode of cell death caused by PARP inhibition.

New distinct compartments in the G2 phase of the cell cycle defined by the levels of γH2AX

Induction of DNA double strand breaks leads to phosphorylation and focus-formation of H2AX. However, foci of phosphorylated H2AX (γH2AX) appear during DNA replication also in the absence of exogenously applied injury. We measured the amount and the number of foci of γH2AX in different phases of the cell cycle by flow cytometry, sorting and microscopy in 4 malignant B-lymphocyte cell lines. There were no detectable γH2AX and no γH2AX-foci in G₁ cells in exponentially growing cells and cells treated with PARP inhibitor (PARPi) for 24 h to create damage and reduce DNA repair. The amount of γH2AX increased immediately upon S phase entry, and about 10 and 30 γH2AX foci were found in mid-S phase control and PARPi-treated cells, respectively. The γH2AX-labeled damage caused by DNA replication was not fully repaired before entry into G₂. Intriguingly, G₂ cells populated a continuous distribution of γH2AX levels, from cells with a high content of γH2AX and the same number of foci as S phase cells (termed “G₂H” compartment), to cells that there were almost negative and had about 2 foci (termed “G₂L” compartment). EdU-labeling of S phase cells revealed that G₂H was directly populated from S phase, while G₂L was populated from G₂H, but in control cells also directly from S phase. The length of G₂H in particular increased after PARPi treatment, compatible with longer DNA-repair times. Our results show that cells repair replication-induced damage in G₂H, and enter mitosis after a 2–3 h delay in G₂L.

Inhibition of mTOR sensitizes breast cancer stem cells to radiation-induced repression of self-renewal through the regulation of MnSOD and Akt

The sensitization of breast cancer stem cells (BrCSCs) to the inhibitive effects of radiotherapy through adjuvant therapy which targets oncogenic pathways represents a prospective strategy for improving the effect of radiation in patients with triple-negative breast cancer (TNBC). Mammalian target of rapamycin (mTOR) activation is one of the most frequent events in human malignancies, and is critical for sustaining the self-renewing ability of cancer stem cells (CSCs); inhibition by rapamycin is an effective and promising strategy in anticancer treatments. In the present study, we found that mTOR activity was closely related to the self-renewal ability of BrCSCs, and in triple negative MDA-MB-453 and MDA-MB-468 cells, rapamycin repression of mTOR phosphorylation decreased the number of mammospheres and helped to sensitize the resistant CSCs to low-dose radiation therapy. By inhibiting mTOR and mitochondrial manganese superoxide dismutase (MnSOD), we confirmed that rapamycin functioned through the mTOR/MnSOD/reactive oxygen species (ROS) signaling pathway, and the existence of Akt governed the rapamycin-induced asymmetric division (AD) of stem cells in cases of radiation-treated breast cancer. The synergic effects of rapamycin and low-dose radiation induced the AD of stem cells, which then resulted in a decrease in the number of mammospheres, and both were mediated by MnSOD. Governed by Akt, the consequent inhibition of ROS formation and oxidative stress preserved the AD mode of stem cells, which is critical for an improved radiotherapy response in clinical treatment, as the tumor group is thus easier to eliminate with radiation therapy. We posit that an in-depth understanding of the interaction of radiation with CSCs has enormous potential and will make radiation even better and more effective.

T0070907, a PPAR γ inhibitor, induced G2/M arrest enhances the effect of radiation in human cervical cancer cells through mitotic catastrophe

Overexpression of peroxisome proliferator activator receptor γ (PPARγ) has been implicated in many types of cancer including cervical cancer. Radiation therapy remains the main nonsurgical modality for the treatment of cervical cancer. The present study reports the impact of pharmacological inhibition of PPARγ in enhancing the radiosensitization of cervical cancer cells in vitro. Three cervical cancer cell lines (HeLa, SiHa, and Me180) were treated with a PPARγ inhibitor, T0070907, and/or radiation. The changes in protein, cell cycle, DNA content, apoptosis, and cell survival were analyzed. The PPARγ is differentially expressed in cervical cancer cells with maximum expression in ME180 cells. T0070907 has significantly decreased the tubulin levels in a time-dependent manner in ME180 cells. The decrease in the tubulin levels after T0070907 in ME180 and SiHa cells was associated with significant increase in the cells at the G2/M phase. The changes in the tubulin and G2/M phase were not evident in HeLa cells. T0070907 reduced the protein levels of PPARγ; however, PPARγ silencing had no effect on the α-tubulin level in ME180 cells suggesting the PPARγ-dependent and -independent actions of T0070907. To ascertain the impact of synergistic effect of T0070907 and radiation, HeLa and ME180 cells were pretreated with T0070907 and subjected to radiation (4 Gy). Annexin V-fluorescein isothiocyanate analysis revealed increased apoptosis in cells treated with radiation and T0070907 when compared to control and individual treatment. In addition, T0070907 pretreatment enhanced radiation-induced tetraploidization reinforcing the additive effect of T0070907. Confocal analysis of tubulin confirmed the onset of mitotic catastrophe in cells treated with T0070907 and radiation. These results strongly suggest the radiosensitizing effects of T0070907 through G2/M arrest and mitotic catastrophe.

Molecular hydrogen protects human lymphocyte AHH-1 cells against 12C6+ heavy ion radiation

PURPOSE

To investigate the potential protective role of molecular hydrogen (H(2)) against (12)C(6+) heavy ion radiation, which is a major hazard for space travel and has been also widely used in heavy ion radiotherapy.

MATERIALS AND METHODS

H(2) was dissolved in Roswell Park Memorial Institute (RPMI) 1640 medium under high pressure (0.4 Mpa) to a saturated level by using an apparatus produced by our department. A 2-[6-(4'-hydroxy) phenoxy-3H-xanthen-3-on-9-yl] benzoate (HPF) probe and a 2',7'-Dichlorodihydrofluorescein diacetate (H2DCFH-DA) fluorescent dye were used to measure the intracellular reactive oxygen species (ROS) level. Cell apoptosis were determined by double-staining with Annexin V-fluorescein isothiocyanate (Annexin V-FITC) and propidium iodide (PI) as well as a Hoechst 33342 staining method alternatively. Subsequently, cell cycle analysis was performed using a PI staining method and the expression of apoptotic protein was examined by Western blot.

RESULTS

In this study, we demonstrated H(2) reduced ROS level in Human lymphocyte AHH-1 cells as well as in the radiolysis of water. Our data also showed H(2) attenuated (12)C(6+) radiation- induced cell apoptosis and also alleviated radiation-induced G2/M cell cycle arrest. Heavy ion radiation-induced Caspase 3 activation was also inhibited by H(2) treatment.

CONCLUSION

In conclusion, these data showed that H(2) attenuated (12)C(6+) radiation-induced cell apoptosis through reducing the ROS level and modulating apoptotic molecules, thus indicating the potential of H(2) as a safe and effective radioprotectant.

Standard fluorescent imaging of live cells is highly genotoxic

Fluorescence microscopy is commonly used for imaging live mammalian cells. Here, we describe studies aimed at revealing the potential genotoxic effects of standard fluorescence microscopy. To assess DNA damage, a high throughput platform for single cell gel electrophoresis is used (e.g., the CometChip). Light emitted by three standard filters was studied: (a) violet light [340-380 nm], used to excite DAPI and other blue fluorophores, (b) blue light [460-500 nm] commonly used to image green fluorescent protein (GFP) and Calcein AM, and (c) green light [528-553 nm], useful for imaging red fluorophores. Results show that exposure of samples to light during imaging is indeed genotoxic even when the selected wavelengths are outside the range known to induce significant damage levels. Shorter excitation wavelengths and longer irradiation times lead to higher levels of DNA damage. We have also measured DNA damage in cells expressing enhanced GFP or stained with Calcein AM, a widely used green fluorophore. Data show that Calcein AM leads to a synergistic increase in the levels of DNA damage and that even cells that are not being directly imaged sustain significant DNA damage from exposure to indirect light. The nature of light-induced DNA damage during imaging was assessed using the Fpg glycosylase, an enzyme that enables quantification of oxidative DNA damage. Oxidative damage was evident in cells exposed to violet light. Furthermore, the Fpg glycosylase revealed the presence of oxidative DNA damage in blue-light exposed cells for which DNA damage was not detected using standard analysis conditions. Taken together, the results of these studies call attention to the potential confounding effects of DNA damage induced by standard imaging conditions, and identify wavelength, exposure time, and fluorophore as parameters that can be modulated to reduce light-induced DNA damage.

In vitro cytotoxicity of low-dose-rate radioimmunotherapy by the alpha-emitting radioimmunoconjugate Thorium-227-DOTA-rituximab

PURPOSE

To determine whether the low-dose-rate alpha-particle-emitting radioimmunoconjugate (227)Th-1,4,7,10-p-isothiocyanato-benzyl-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-rituximab can be used to inactivate lymphoma cells growing as single cells and small colonies.

METHODS AND MATERIALS

CD20-positive lymphoma cell lines were treated with (227)Th-DOTA-rituximab for 1-5 weeks. To simulate the in vivo situation with continuous but decreasing supply of radioimmunoconjugates from the blood pool, the cells were not washed after incubation with (227)Th-DOTA-rituximab, but half of the medium was replaced with fresh medium, and cell concentration and cell-bound activity were determined every other day after start of incubation. A microdosimetric model was established to estimate the average number of hits in the nucleus for different localizations of activity.

RESULTS

There was a specific targeted effect on cell growth of the (227)Th-DOTA-rituximab treatment. Although the cells were not washed after incubation with (227)Th-DOTA-rituximab, the average contribution of activity in the medium to the mean dose was only 6%, whereas the average contribution from activity on the cells' own surface was 78%. The mean dose rates after incubation with 800 Bq/mL (227)Th-DOTA-rituximab varied from 0.01 to 0.03 cGy/min. The average delay in growing from 10(5) to 10(7) cells/mL was 15 days when the cells were treated with a mean absorbed radiation dose of 2 Gy alpha-particle radiation from (227)Th-DOTA-rituximab, whereas it was 11 days when the cells were irradiated with 6 Gy of X-radiation. The relative biologic effect of the treatment was estimated to be 2.9-3.4.

CONCLUSIONS

The low-dose-rate radioimmunoconjugate (227)Th-DOTA-rituximab is suitable for inactivation of single lymphoma cells and small colonies of lymphoma cells.

Effects of tetramethoxystilbene on hormone-resistant breast cancer cells: biological and biochemical mechanisms of action

Secondary resistance to hormonal therapy for breast cancer commonly develops after an initial response to tamoxifen or aromatase inhibitors. Agents to abrogate these adaptive changes would substantially enhance the long-term benefits of hormonal therapy. Our studies with a stilbene derivative called TMS (2,3',4,5'-tetramethoxystilbene) identified unexpected effects with potential utility for treatment of breast tumors secondarily resistant to hormonal therapy. TMS was originally developed as an inhibitor of cytochrome P450 1B1 to block the conversion of estradiol to 4-OH-estradiol. While studying this agent in three models of hormone resistance, we detected direct antitumor effects not related to its role as an inhibitor of catecholestrogens. During examination of the mechanisms involved, we showed that treatment with 3 micromol/L TMS for 24 h inhibited tubulin polymerization and microtubule formation, caused a cell cycle block at the G2-M phase, and induced apoptosis. TMS also inhibited activated focal adhesion kinase (FAK), Akt, and mammalian target of rapamycin (mTOR) and stimulated c-jun-NH2-kinase and p38 mitogen-activated protein kinase activity. With respect to antitumor effects, TMS at a concentrations of 0.2 to 0.3 micromol/L inhibited the growth of long-term tamoxifen-treated MCF-7 cells by 80% and fulvestrant-treated MCF-7 cells by 70%. In vivo studies, involving 8 weeks of treatment with TMS via a 30-mg s.c. implant, reduced tumor volume of tamoxifen-resistant MCF-7 breast cancer xenografts by 53%. Our data suggest that TMS is a promising therapeutic agent because of its unique ability to block several pathways involved in the development of hormone resistance.

Targeted cancer therapy with a novel low-dose rate alpha-emitting radioimmunoconjugate

Alpha-emitting radionuclides are highly cytotoxic and are of considerable interest in the treatment of cancer. A particularly interesting approach is in radioimmunotherapy. However, alpha-emitting antibody conjugates have been difficult to exploit clinically due to the short half-life of the radionuclides, low production capability, or limited source materials. We have developed a novel technology based on the low-dose rate alpha-particle-emitting nuclide (227)Th, exemplified here using the monoclonal antibody rituximab. In vitro, this radioimmunoconjugate killed lymphoma cells at Becquerel per milliliter (Bq/mL) levels. A single injection of (227)Th-rituximab induced complete tumor regression in up to 60% of nude mice bearing macroscopic (32-256 mm(3)) human B-lymphoma xenografts at Becquerel per gram (Bq/g) levels without apparent toxicity. Therapy with (227)Th-rituximab was significantly more effective than the control radioimmunoconjugate (227)Th-trastuzumab and the standard beta-emitting radioimmunoconjugate for CD20(+) lymphoma(90)Y-tiuxetan-ibritumomab. Thorium-227 based constructs may provide a novel approach for targeted therapy against a wide variety of cancers.

99mTc-HMPAO labelling inhibits cell motility and cell proliferation and induces apoptosis of NC-NC cells

(99m)Tc-hexamethyl-propylenamine-oxime ((99m)Tc-HMPAO)-labelled leukocytes have been used in standard diagnostic procedures for the detection of infection and inflammation. Although some investigators have already pointed out that labelling of leukocytes with (99m)Tc-HMPAO has detrimental effects on the cells, still very little is known regarding the effects of ionizing radiation on lymphocyte function. The effects of (99m)Tc-HMPAO-labelling on lymphocyte adhesion, proliferation, mitotic index, migration and apoptosis were evaluated. The lymphoblastoid cell line NC-NC was used as the lymphocyte population. (99m)Tc-HMPAO-labelling decreased cell adhesion, proliferation, mitotic index and motility, whereas it induced apoptosis and cell-cycle arrest. The rate of decrease in cell proliferation was up to 70% (P<0.001) by day 4 after labelling. (99m)Tc-HMPAO-labelling led a 35% decrease (P<0.001) in adhesion ability of the cells on fibronectin at 16h. Using the Boyden chamber motility assay, it was shown that both spontaneous and monocyte chemotactic protein (MCP-1)-induced lymphocyte motility were strongly reduced by (99m)Tc-HMPAO-labelling. The decrease in motility was approximately five-fold (P<0.05). In addition, a 12-fold increase (P<0.05) was observed in apoptosis of the (99m)Tc-HMPAO-treated cells compared with control cells. Besides, it was shown that cell-cycle arrest was induced starting from the 3rd day after treatment with (99m)Tc-HMPAO. Our observations indicate that (99m)Tc-HMPAO-labelling has damaging effects on lymphocyte function including cell adhesion, proliferation, mitotic index, motility and cell cycle under in vitro conditions.

Point-source irradiation with eccentric rotation causes inhomogeneous dose distribution

Radioactive point sources are regularly used for irradiating cell culture and other biological materials. Eccentric rotation is often used to minimize dose disparities that arise from irradiating samples that span a distance from the point source. Rotation provides a great improvement in dose homogeneity compared to inert irradiation yet still presents an obvious shortcoming for exposures in which the sample completes only partial rotation or fractional rotation. In such cases, certain areas of the sample have a closer average distance to the radiation source than other areas within the same sample. This obstacle can be partially overcome by adjusting rotation speed so the sample traverses a full rotation (or multiple thereof) throughout the total irradiation time. Here we investigate the effects of irradiation with eccentric rotation on dose homogeneity. We show that due to the inverse square law that governs dose, even exposures with full rotation result in inhomogeneous dose distributions. This dose inhomogeneity can be substantial, especially for large samples and small source- sample distances. We observed a 33% difference in survival across 100-mm dishes and a 400% difference for 150-mm dishes. The dose inhomogeneity inherent to eccentric rotation increases the actual average dose delivered across the sample compared to that delivered at sample center. We offer a table of correction factors that account for this dose increase and correct the dose delivered at center to the actual average dose delivered across the entire sample.

Ionizing radiation enhances the expression of the nonsteroidal anti-inflammatory drug-activated gene (NAG1) by increasing the expression of TP53 in human colon cancer cells

The induction of apoptosis in cells of human colon cancer cell lines after gamma irradiation was investigated to determine whether apoptosis was mediated by TP53 and the subsequent expression of its downstream target, the NSAID-activated gene (NAG1). HCT116 (TP53(+/+)), HCT15 (TP53 mutant) and TP53 null HCT116 (TP53(-/-)) cells were irradiated with gamma rays, and apoptosis was measured at various times after irradiation. In HCT116 TP53(+/+) cells, apoptosis was increased after irradiation; the increase was dependent on the time after treatment and the dose of gamma rays. However, in HCT15 TP53 mutant cells and HCT116 TP53(-/-) cells, there were no remarkable changes in apoptosis. The expression of TP53 protein in HCT116 cells was increased after irradiation and was followed by an increase in the expression of NAG1 protein. In contrast, the expression of NAG1 protein in TP53 mutant cells and TP53(-/-) cells was not increased by the radiation treatment, suggesting that NAG1 was required for apoptosis. The expression of NAG1 increased apoptosis in HCT116 cells, but radiation treatment did not further increase apoptosis. The transfection of a NAG1 siRNA into HCT116 cells suppressed radiation-induced apoptosis and inhibited the induction of NAG1 protein without altering the expression of TP53. a NAG1 luciferase promoter construct that included both of the TP53 binding sites, was activated by radiation in dose-dependent manner, while the promoters lacking one or both of the TP53 binding sites in the NAG1 promoter activity either was less responsive or did not respond. The findings reported here indicate that gamma radiation activates the TP53 tumor suppressor, which then increases the expression of NAG1. NAG1 mediates the induction of apoptosis in human colorectal cells.

Cross-talk between DNA damage and cell survival checkpoints during G2and mitosis: pharmacologic implications

In this study, we wanted to clarify the role of survivin-mediated survival signaling during G2 and M in tumor cells treated with DNA-damaging agents. As a cellular model, we selected MOLT-4 human T-cell lymphoblastic leukemia cells that overexpress survivin and nonfunctional p53. Treatment with melphalan, a classic DNA-damaging agent, led to the induction of the DNA damage checkpoint and growth arrest in the G2 phase of the cell cycle. Checkpoint abrogation by caffeine was accompanied by mitotic entry and rapid apoptotic cell death, whereas cells remaining in G2 remained viable during the same time interval. Unexpectedly, when the spindle checkpoint was activated following G2 abrogation, two different effects could be observed. If the microtubules of the melphalan-treated cells were destabilized by nocodazole, cells became arrested in prometaphase with low survivin levels and entered apoptosis. In contrast, if the microtubules of the melphalan-treated cells were stabilized by taxol, cells were still arrested in prometaphase, but apoptotic execution was inhibited. This effect is, most likely, directly mediated by survivin itself given its well-established antiapoptotic functions. In conclusion, depending on the way the spindle checkpoint was activated in cells with damaged DNA, cells could be either protected by survivin or die during mitosis. We suggest that the efficacy of DNA damage checkpoint abrogators used in combination with DNA-damaging agents may critically depend on whether DNA damage is able to invoke spindle checkpoint response and to activate survivin-associated survival signaling during mitosis.

Response of malignant B lymphocytes to ionizing radiation: gene expression and genotype

The human malignant B-lymphocyte cell lines Reh and U698 show arrest in G2 phase after ionizing radiation (IR), but only Reh cells arrest in G1 phase and die by apoptosis. We have used cDNA microarrays to measure changes in gene expression at 2, 4 and 6 hr after irradiation of Reh and U698 cells with 0.5 and 4 Gy in order to begin exploring the molecular mechanisms underlying the phenotypic changes. We also investigated whether gene expression changes could be caused by possible aberrations of genes, as measured by comparative genomic hybridization. Reh cells showed upregulation of CDKN1A that likely mediated the G1 arrest. In contrast, U698 cells have impaired function of TP53 protein and no activation of CDKN1A, suppressing the arrest in G1. The G2 arrest in both cell lines was likely due to repression of PLK1 and/or CCNF. IR-induced apoptosis in Reh cells was probably mediated by TP53 and CDKN1A, whereas a high expression level of MCL1, caused by gene amplification, and activation of the NFKB pathway may have suppressed the apoptotic response in U698 cells. Genes suggested to be involved in apoptosis were activated long before this phenotype was detectable and showed the same temporal expression profiles as genes involved in cell cycle arrest. Our results suggest that differences in functionality and/or copy number of several genes involved in IR-regulated pathways contributed to the phenotypic differences between Reh and U698 cells after IR, and that multiple molecular factors control the radiation response of malignant B lymphocytes.