Ionizing radiation modulates vascular endothelial growth factor (VEGF) expression through multiple mitogen activated protein kinase dependent pathways

We investigated the role of radiation-induced mitogen activated protein kin ase (MAPK) pathway activity in the regulation of proliferation, cell surviv al and vascular endothelial growth factor (VEGF) production in primary astr ocytes and in To and RT2 glioblastoma cells derived from Fisher 344 rats, I n these cells, ionizing radiation (2 Gy) caused activation of the MAPK path way which was blocked by specific inhibitor drugs, Blunting of radiation-in duced MAPK activity weakly enhanced radiation-induced apoptosis 24 h after exposure in RT2 cells, Furthermore, blunting of MAPK activation weakly enha nced the ability of radiation to reduce RT2 cell growth in clonogenic growt h assays, These findings argue that inhibition of MAPK signaling reduces pr oliferation and enhances cell killing by ionizing radiation in transformed astrocytes, Proliferation and survival of cancer cells has been linked in v ivo to enhanced expression of angiogenic growth factors, Recently we demons trated that the gene product of a novel rodent radiation-responsive gene, p rogression elevated gene 3 (PEG-3), could enhance vascular endothelial grow th factor (VEGF) promoter activity in rodent fibroblasts, leading to increa sed VEGF protein levels and tumorigenic behavior in vivo. Thus PEG-3 and VE GF expression could be expected to directly correlate with the oncogenic po tential of transformed cells, RT2 cells expressed more PEG-3 and VEGF prote in than To cells, and were more tumorigenic in vivo than To cells, Radiatio n activated the PEG-3 promoter via MAPK signaling and ectopic over-expressi on of PEG-3 enhanced both basal MARK activity and basal VEGF promoter activ ity, Basal MAPK activity partially correlated with basal VEGF promoter acti vity and VEGF protein levels in primary astrocytes, T9 and RT2 cells, Radia tion increased the activity of the VEGF promoter and VEGF protein levels in primary astrocytes, T9 and RT2 cells which were dependent upon MAPK functi on, Furthermore, inhibition of AP-1 transcription factor signaling by domin ant negative c-Jun (TAM67) also significantly reduced basal, and to a lesse r extent radiation-induced, VEGF promoter function in RT2 cells, Collective ly, our data demonstrate that radiation-induced MAPK signaling can both pro tect cells: from radiation-induced cell death as well as enhance protein le vels of pro-angiogenic factors such as VEGF, Enhanced VEGF expression in RT 2 cells may be mediated via MAPK and JNK pathway signaling which converges upon the AP-1 transcription factor complex.