Honghong Chen

The mTOR inhibitor rapamycin suppresses DNA double-strand break repair.

Abstract mTOR (mammalian target of rapamycin) signaling plays a key role in the development of many tumor types. Therefore, mTOR is an attractive target for cancer therapeutics. Although mTOR inhibitors are thought to have radiosensitization activity, the molecular bases remain largely unknown. Here we show that treating MCF7 breast cancer cells with rapamycin (an mTOR inhibitor) results in significant suppression of homologous recombination (HR) and nonhomologous end joining (NHEJ), two major mechanisms required for repairing ionizing radiation-induced DNA DSBs. We observed that rapamycin impaired recruitment of BRCA1 and Rad51 to DNA repair foci, both essential for HR. Moreover, consistent with the suppressive role of rapamycin on both HR and NHEJ, persistent radiation-induced DSBs were detected in cells pretreated with rapamycin. Furthermore, the frequency of chromosome and chromatid breaks was increased in cells treated with rapamycin before and after irradiation. Thus our results show that radiosensitization by mTOR inhibitors occurs via disruption of the major two DNA DSB repair pathways.

Silibinin Inhibits Wnt/β-catenin Signaling by Suppressing Wnt Co-receptor LRP6 Expression in Human Prostate and Breast Cancer Cells

Silibinin is a natural compound isolated from milk thistle seed extracts, and has traditionally been used as a hepatoprotectant. A number of studies have also established the cancer therapeutic and chemopreventive role of silibinin in both in vitro and in vivo models. The low density lipoprotein receptor-related protein-6 (LRP6) is an essential Wnt co-receptor for the Wnt/β-catenin pathway and represents a promising target for cancer prevention and therapy. In the present study, we found that silibinin was able to repress endogenous LRP6 expression and block Wnt3A-induced LRP6 phosphorylation and Wnt/β-catenin signaling activation in HEK293 cells. Importantly, silibinin was also able to suppress endogenous LRP6 expression and phosphorylation and block Wnt/β-catenin signaling in prostate cancer PC-3 and DU-145 cells and breast cancer MDA-MB-231 and T-47D cells. Mechanistically, silibinin inhibited LRP6 promoter activity and decreased LRP6 mRNA levels in prostate and breast cancer cells. Finally, we demonstrated that silibinin displayed anticancer activity with IC(50) values comparable to those shown to suppress LRP6 expression and Wnt/β-catenin signaling activities in prostate and breast cancer cells. Our data indicate that silibinin is a novel small molecule Wnt/β-catenin signaling inhibitor by suppressing Wnt co-receptor LRP6 expression at the transcription level, and that the anti-cancer activity of silibinin is associated with its inhibitory effect on Wnt/LRP6 signaling.

Efficacy of a novel chelator BPCBG for removing uranium and protecting against uranium-induced renal cell damage in rats and HK-2 cells.

Chelation therapy is a known effective method to increase the excretion of U(VI) from the body. Until now, no any uranium chelator has been approved for emergency medical use worldwide. The present study aimed to evaluate the efficacy of new ligand BPCBG containing two catechol groups and two aminocarboxylic acid groups in decorporation of U(VI) and protection against acute U(VI) nephrotoxicity in rats, and further explored the detoxification mechanism of BPCBG for U(VI)-induced nephrotoxicity in HK-2 cells with comparison to DTPA-CaNa₃. Chelating agents were administered at various times before or after injections of U(VI) in rats. The U(VI) levels in urine, kidneys and femurs were measured 24 h after U(VI) injections. Histopathological changes in the kidney and serum urea and creatinine and urine protein were examined. After treatment of U(VI)-exposed HK-2 cells with chelating agent, the intracellular U(VI) contents, formation of micronuclei, lactate dehydrogenase (LDH) activity and production of reactive oxygen species (ROS) were assessed. It was found that prompt, advanced or delayed injections of BPCBG effectively increased 24 h-urinary U(VI) excretion and decreased the levels of U(VI) in kidney and bone. Meanwhile, BPCBG injection obviously reduced the severity of the U(VI)-induced histological alterations in the kidney, which was in parallel with the amelioration noted in serum indicators, urea and creatinine, and urine protein of U(VI) nephrotoxicity. In U(VI)-exposed HK-2 cells, immediate and delayed treatment with BPCBG significantly decreased the formation of micronuclei and LDH release by inhibiting the cellular U(VI) intake, promoting the intracellular U(VI) release and inhibiting the production of intracellular ROS. Our data suggest that BPCBG is a novel bi-functional U(VI) decorporation agent with a better efficacy than DTPA-CaNa₃.

Comparison of cellular damage response to low-dose-rate 125I seed irradiation and high-dose-rate gamma irradiation in human lung cancer cells

PURPOSE To investigate the difference of cellular response between low-dose-rate (LDR) 125I seed irradiation and high-dose-rate (HDR) γ-irradiation in human lung cancer cells. METHODS AND MATERIALS A549 and NCI-H446 cells with or without wortmannin (WM) treatment were exposed to 125I seeds and γ-rays, respectively. Cell survival, micronuclei (MN) formation, and the expressions of Ku70/Ku80 proteins were measured. RESULTS There was a strong negative correlation between survival and MN formation for both irradiations, and the MN inductions of NCI-H446 were about twofolds of those of A549, and the survival of NCI-H446 was lower than that of A549, indicating the radiosensitivity of NCI-H446 cells was greater than that of A549 cells. Interestingly, at 4-Gy radiation, NCI-H446 cells were more sensitive to LDR irradiation than HDR irradiation. WM treatment enhanced the radiosensitivity of A549 cells evenly to (125I seed and γ-irradiation, but this treatment led NCI-H446 cells to be more sensitive to LDR 125I. Further results revealed that the expression of phosphorylated Ku80 protein was enhanced in irradiated A549, but in contrast, it was markedly decreased in NCI-H446 cells after 4-Gy LDR 125I irradiation as that compared with γ-irradiated and nonirradiated cells. CONCLUSION NCI-H446 cells were more sensitive to LDR 125I irradiation than HDR irradiation, and this sensitivity could be further enhanced by WM treatment. But no obvious differences of cellular response to both irradiations were observed in A549. Ku as molecular markers together with cell proliferation rate can be used to predict the radiosensitivity of tumor cells to LDR 125I seed irradiation.

Combination effects of chronic cadmium exposure and gamma-irradiation on the genotoxicity and cytotoxicity of peripheral blood lymphocytes and bone marrow cells in rats.

This work investigated the effects of chronic cadmium (Cd) exposure combined with γ-ray irradiation on the cytotoxicity and genotoxicity of peripheral blood cells and bone marrow cells in rats. Results showed that when the rats were exposed to low dose (LD) Cd of 0.1mg CdCl₂/(kgd) for 8 and 12 weeks, the Cd concentration in blood reached to 135-140 μg/L and no toxic effects on peripheral blood lymphocytes, white blood cells (WBC) and granulocyte-monocyte (GM) progenitor cells were observed except polychromatic erythrocytes (PCE) of bone marrow. Moreover, this chronic LD Cd exposure significantly decreased irradiation-induced micronucleus (MN) formation and hypoxanthine-guanine phosphoribosyl transferase (hprt) mutation in lymphocytes and PCE, while the combination of LD Cd exposure and irradiation induced the additive metallothionein (MT) protein expression in bone marrow cells. When the rats were exposed to a high dose (HD) Cd of 0.5mg CdCl/₂(kgd) for 8 and 12 weeks, the blood Cd level approached to 458-613 μg/L and an inflammatory response was induced, meanwhile, MN formation and hprt mutation were markedly increased, and the ratio of PCE/NCE (normochromatic erythrocyte) was significantly decreased. Furthermore, when the rats were exposed to HD Cd plus 2 Gy irradiation, additive toxic effects on MN formation, hprt mutation, PCE damage and GM progenitor cell proliferation were observed, while this combination treatment resulted in an obvious reduction of MT protein compared to HD Cd group. In conclusion, chronic exposure to LD Cd induced the adaptive response to irradiation in the genotoxicity of peripheral blood lymphocytes and PCE of bone marrow by the up-regulation of Cd-induced MT protein, but the combination of HD Cd exposure and irradiation generated the additive effects on the cytotoxicity and genotoxicity in peripheral blood lymphocytes and bone marrow cells.

Cadmium-Induced Adaptive Response in Cells of Chinese Hamster Ovary Cell Lines with Varying DNA Repair Capacity

Abstract Pan, Y., Yuan, D., Zhang, J., Xu, P., Chen, H. and Shao, C. Cadmium-Induced Adaptive Response in Cells of Chinese Hamster Ovary Cell Lines with Varying DNA Repair Capacity. Radiat. Res. 171, 446–453 (2009). The combined exposure to environmental toxicants such as heavy metals and radiation is an important research area in health protection. The aim of this study was to investigate the role of DNA repair and the phosphatidylinositol 3-kinase (PI3K) family in the cadmium-induced adaptive response to toxicants or radiation. Using cells of three Chinese hamster ovary (CHO) cell lines with different capacities to repair DNA damage, we found that pre-exposure to cadmium at a nonlethal concentration could induce an adaptive response to a subsequent challenge with cadmium or radiation in all the cell lines. The magnitude of the adaptive response in adapted cells was dependent on several factors, including DNA repair capacity, the priming dose of cadmium, and the challenging dose of cadmium or radiation. When the cells were challenged with 50 μM CdCl2, the adaptive response was less evident in XRCC1-defective EM-C11 cells than in cells of the other two cell lines. Moreover, treatment of cells with wortmannin or KU-55933 eliminated the adaptive response in all the cell lines. Our data suggest that the triggering of a cadmium-induced adaptive response was independent of DNA repair capacity. Single-strand break (SSB) repair or base excision repair (BER) rather than double-strand break (DSB) repair was mainly involved in the adaptive response. This response of cells to a further challenge with cadmium or radiation may be mediated through the ataxia telangiectasia mutated (ATM) pathway.

Effect of gap junctional intercellular communication on radiation responses in neoplastic human cells

Abstract Shao, C., Furusawa, Y., Matsumoto, Y., Pan, Y., Xu, P. and Chen, H. Effect of Gap Junctional Intercellular Communication on Radiation Responses in Neoplastic Human Cells. Radiat. Res. 167, 283–288 (2007). Gap junctional intercellular communication (GJIC) is an important function of metazoan cells and is believed to have beneficial effects in anti-tumor therapy. In this study, we found that, when neoplastic human salivary gland (HSG) cells were irradiated with a 100 keV/μm carbon-ion beam, micronuclei, G2/M-phase arrest, and cell killing were induced and that their induction increased with dose. Treatment of confluent HSG cells with 8-Br-cAMP increased GJIC between cells. After release from this treatment, the cell cycle progress and the formation of binucleated cells were still similar to those of untreated cells. However, radiation-induced cellular damage, including micronucleus (MN) formation and G2/M-phase arrest of that cAMP-treated population, was less than that of the untreated population and that the surviving fraction was slightly enhanced by cAMP treatment, suggesting that increased GJIC protects HSG cells from lethal radiation damage. Moreover, when confluent HSG cells were treated with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), a scavenger of nitric oxide (NO) free radical, MN induction and cell killing in the irradiated population were increased. Our results indicate that NO may be involved in GJIC-mediated radioprotection of HSG cells, which may have implications for radiotherapy.

The role of nucleophosmin/B23 in radiation-induced chromosomal instability in human lymphoblastoid cells of different p53 genotypes

Purpose: To investigate the role of nucleophosmin (NPM/B23) in radiation-induced chromosomal instability and apoptosis in human lymphoblastoid cells with different protein 53 (p53) status. Materials and methods: Wild type (wt) p53 TK6 and mutant type (mt) p53 WTK1 with or without short hairpin RNA (shRNA)-mediated silencing of NPM, TK6 with or without short interfering RNA (siRNA)-mediated silencing of p53 (p53i and NEGi) were irradiated with 4 Gy gamma-rays. Six to 48 h after irradiation, the index of apoptosis, chromosome aberration, cell cycle distribution and the levels of total NPM and phosphorylated-threonine 199 (pThr199) NPM proteins were measured. Cells in some dishes were treated with 10 μM Olomoucine (OLO) for 3 h before irradiation and remained in the medium after irradiation. Results: The rates of radiation-induced apoptosis in TK6 and TK6/NEGi were about 2-fold of those in WTK1 and TK6/p53i, while the frequencies of polyploidy in TK6 and TK6/NEGi were obviously lower than those in WTK1 and TK6/p53i. Moreover, after irradiation, pThr199 NPM levels increased significantly in WTK1 and TK6/p53i, and slightly increased in TK6 and TK6/NEGi, indicating that the increased level of pThr199 NPM was related to p53 status. When Thr199 hyperphosphorylation of NPM was inhibited by OLO or when NPM was knocked down, we found that radiation-induced apoptosis was more pronounced and polyploidy formation was reduced as compared with negative control while the magnitude of these changes in TK6 was obviously higher than that in WTK1, indicating that NPM has an antagonistic interaction with wt p53. Conclusions: NPM/B23 plays an important role in protecting cells from radiation-induced apoptosis and increasing polyploidy formation via either a p53 or non-p53 pathway.