Modification of the radiosensitivity of human cells to which simian virus 40 T-antigen was transfected

Notch-1 regulates transcription of the epidermal growth factor receptor through p53

The Notch pathway plays a key role in the development and is increasingly recognized for its importance in cancer. We demonstrated previously the overexpression of Notch-1 and its ligands in gliomas and showed that their knockdown inhibits glioma cell proliferation and survival. To elucidate the mechanisms downstream of Notch-1 in glioma cells, we performed microarray profiling of glioma cells transfected with Notch-1 small interfering RNA. Notable among downregulated transcripts was the epidermal growth factor receptor (EGFR), known to be overexpressed or amplified in gliomas and prominent in other cancers as well. Further studies confirmed that Notch-1 inhibition decreased EGFR messenger RNA (mRNA) and EGFR protein in glioma and other cell lines. Transfection with Notch-1 increased EGFR expression. Additionally, we found a significant correlation in levels of EGFR and Notch-1 mRNA in primary high-grade human gliomas. Subsequent experiments showed that p53, an activator of the EGFR promoter, is regulated by Notch-1. Experiments with p53-positive and -null cell lines confirmed that p53 partially mediates the effects of Notch-1 on EGFR expression. These results show for the first time that Notch-1 upregulates EGFR expression and also demonstrate Notch-1 regulation of p53 in gliomas. These observations have significant implications for understanding the mechanisms of Notch in cancer and development.

Selective destruction of glioblastoma cells by interference with the activity or expression of ATF5

Glioblastoma multifome is the most common and most aggressive primary brain tumor with no current curative therapy. We found expression of the bZip transcription factor ATF5 in all 29 human glioblastomas and eight human and rat glioma cell lines assessed. ATF5 is not detectably expressed by mature brain neurons and astrocytes, but is expressed by reactive astrocytes. Interference with ATF5 function or expression in all glioma cell lines tested causes marked apoptotic cell death. In contrast, such manipulations do not affect survival of ATF5-expressing cultured astrocytes or of several other cell types that express this protein. In a proof-of-principle experiment, retroviral delivery of a function-blocking mutant form of ATF5 into a rat glioma model evokes death of the infected tumor cells, but not of infected brain cells outside the tumors. The widespread expression of ATF5 in glioblastomas and the selective effect of interference with ATF5 function/expression on their survival suggest that ATF5 may be an attractive target for therapeutic intervention in such tumors.

The effects of pentoxifylline on the survival of human glioma cells with continuous and intermittent stereotactic radiosurgery irradiation

PURPOSE

In linac-based stereotactic radiosurgery, treatment is delivered intermittently via multiple individual small radiotherapy arcs. The time lapses between the individual arcs permit greater damage repair and increased tumor cell survival in comparison with continuous irradiation. Because pentoxifylline (PTX) has been reported to prevent radiation-induced cell cycle arrest at the G2/M checkpoint, where damage repair is critically linked to cell survival, we hypothesized that PTX would exert a favorable radiosensitization effect by reducing the recovery observed during intermittent stereotactic radiosurgery.

METHODS AND MATERIALS

The human glioma cell line T98G was used to study the effects of continuous vs. intermittent irradiation with or without PTX. Cell cycle patterns were studied using flow cytometry. Clonogenic assays of single cells and spheroid outgrowth assays provided a quantitative measure of PTX-mediated radiosensitization. The PTX effect upon cells in low oxygen conditions was also studied in vitro after enzymatic oxygen scavenging.

RESULTS

Flow Cytometry: T98G cells exposed to both continuous and intermittent irradiation exhibit similar arrest at the G2/M checkpoint. The addition of 2 mM PTX significantly reduced the radiation-induced G2/M block in both irradiation schemes. Clonogenic Assays: The same PTX concentration applied before a continuous dose of 12 Gy, two intermittent doses of 6 Gy, or three intermittent doses of 4 Gy, all given within a 1-h interval, consistently caused radiosensitization. The drug enhancement ratios for PTX were 1.5, 2.7, and 6.0 for the continuous and two different intermittent dose schedules, respectively. Adding PTX after irradiation yielded lower enhancement ratios than pre-irradiation application. A similar pattern was observed after total doses of 4, 6, 9, or 12 Gy, as well. In low oxygen conditions, PTX was seen to have the same effects as in normoxic conditions. Spheroid Outgrowth Assays: The in vitro PTX effects were replicated in the spheroid outgrowth assays.

CONCLUSION

In human glioma cells, PTX abrogates the radiation-induced G2/M block observed after either continuous radiation exposure or intermittent exposures modeling clinical linac-based radiosurgery. The PTX-mediated reduction of the G2/M block translates into radiosensitization, most notably during intermittent exposures, and is presumably a consequence of diminished DNA damage repair at the G2/M checkpoint, though other contributing effects cannot be ruled out. The radiosensitization effect of PTX is sustained under low oxygen conditions. These results support consideration of the clinical evaluation of PTX to enhance the efficacy of linac-based radiosurgery involving intermittent irradiation through multiple arcs.

Radiosensitization by overexpression of the nonphosphorylation form of IkappaB-alpha in human glioma cells

To assess the role of NF-kappaB in cellular radiosensitivity, we constructed mutated IkappaB expression plasmids for SY-IkappaB (with mutations at residues of 32, 36 and 42) expression in human malignant glioma cells (radiosensitive MO54 and radioresistant T98 cells), giving respective cell types referred to as MO54-SY4 and T98-SY14. Both of the clones expressing SY-IkappaB became radiosensitive, compared with the parental MO54 and T98 cells. A treatment with herbimycin A or genistein did not change the radiosensitivity of cells expressing SY-IkappaB, but made both the MO54 and T98 parental cells more sensitive to ionizing radiation. A treatment with TNF-alpha induced DNA fragmentation and apoptosis in cells expressing SY-IkappaB, but not in MO54 and T98 cells. The survival after X-ray exposure of the parental MO54 cells was slightly increased by a TNF-alpha treatment, but that of the parental T98 cells did not change. The change in sensitivity to ultra-violet (UV) radiation and adriamycin in MO54-SY4 cells was very similar to that for X-ray sensitivity, but no change was observed in T98-SY14 cells. Significant sublethal damage repair was observed in T98 cells, whereas MO54 cells showed little repair activity. The expression of p53 was enhanced in the parental MO54 cells, while the p53 levels in the MO54-SY4, and in the parent and clonal T98 cells, did not change. Our data suggest that the serine and tyrosine phosphorylation of IkappaB-alpha may play a role in determining the radiosensitivity of malignant glioma cells.

Effects of protein kinase inhibitors on radiation-induced WAF1 accumulation in human cultured melanoma cells

To examine whether protein kinase C (PKC) and A (PKA) contribute to WAF1 induction by ionizing radiation (IR) in cultured human melanomas, the effect of PK inhibitors 1-(5'-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride (H7), bisindolylmaleimide (GF) and N-[2(p-dromocinnamylamino)ethyl]-5-isoquinolinesulphonamide (H89) on IR-induced WAF1 accumulation was analysed by Western blot analysis. Gamma-ray-induced accumulation of WAF1 showed a peak at 6 Gy in all the cell lines. After gamma-ray irradiation of 6 Gy, a peak of WAF1 accumulation was observed at 6 h in SK-Mel-26, G361 and HM6KO cells, and at 3 h in MeWo cells. In MeWo and SK-Mel-26 cells, the X-ray-induced WAF1 accumulation was decreased by PK inhibitors, GF (PKC inhibitor) or H89 (PKA inhibitor); this did not occur in G361 and HM6KO. In all the cell lines, accumulation of WAF1 induced by X-ray irradiation was suppressed by H7 (PKC and PKA inhibitor). In addition, polymerase chain reaction-single strand conformational polymorphism analysis detected no aberrations in the p53 gene of the four cell lines used. These results suggest that IR-induced WAF1 expression involves PKC and/or PKA activity depending on cell type.

Characterization of the response of human bone marrow endothelial cells to in vitro irradiation

Endothelial cell dysfunction is a classic consequence of radiation damage. Bone marrow endothelial cells (BMEC) are a critical component of the stroma in the regulation of haemopoiesis. In animal models, radiation-induced injury of BMEC has been described and a role for BMEC in haemopoietic regeneration after irradiation has been suggested. However, functions of BMEC involved in the haemopoietic regeneration have not been assessed. Therefore we studied the functional response of human BMEC to irradiation using the transformed human BMEC line (TrHBMEC) irradiated with 2. 5 or 10Gy. Our results showed a time- and a dose-dependent increase in damage to irradiated TrHBMEC measured by a decreased number of adherent cells which correlated with increased apoptosis and augmented release of soluble ICAM-1 and von Willebrand factor. 2 Gy irradiated TrHBMEC expressed more ICAM-1 on their surface than non-irradiated cells, whereas no change in VCAM-1, E-selectin and PECAM-1 expression was observed. An increased production of G-CSF, GM-CSF, IL-8, IL-6, IL-1alpha, IL-11, MIP-1alpha and SCF and no production of LIF, TNF-alpha, TPO and IL-3 by 2 Gy irradiated TrHBMEC was observed. The haemopoietic supportive function of TrHBMEC was not altered after a 2 Gy exposure. These results suggest that although radiation induces endothelial cell damage, irradiated cells still support the proliferation and the differentiation of CD34+ haemopoietic cells.

Increased radiosensitivity of p16 gene-deleted human glioma cells after transfection with wild-type p16 gene

The A1235 and T98 cell lines derived from human gliomas have homozygous deletions in their p16 genes and are radiosensitive and radioresistant, respectively, with respect to other established glioma cell lines. These differences in radiosensitivity may be due to variations to some extent among cell lines, rather than genetically defined resistance or sensitivity. We examined the effect on radiation sensitivity of introducing a wild-type p16 gene into both p16-deficient glioma cell lines. The plasmid pOPMTS containing human wild-type p16 cDNA and a neomycin resistance gene, or the control plasmid pOPRSV1, were transfected into these cells. Clones from both cell lines, which expressed wild-type p16 mRNA constitutively after transfection with pOPMTS, were more radiosensitive than the parental cells and clones obtained after transfection with the negative control plasmid.