Membrane-derived second messenger regulates x-ray-mediated tumor necrosis factor alpha gene induction.

Abstract

Cells adapt to adverse environmental conditions through a wide range of responses that are conserved throughout evolution. Physical agents such as Ionizing radiation are known to initiate a stress response that is triggered by the recognition of DNA damage. We have identified a sgnlIng pathway involving the activation of phospholipase A2 and protein kinase C in human cells that confers x-ray induction of the tumor necrosis factor a gene. Treatment of human cells with ionizing radiation or H202 was associated with the production of arachidonic acid. Inhibition of phospholipase A2 abolished radiation-mediated arachidonate production as well as the subsequent activation of protein kinase C and tumor necrosis factor a gene expression. These findings demonstrate that ionizing radiation-mediated gene expression in human cells is regulated in part by extranuclear signal transduction. One practical application of phospholipase A2 inhibitors is to ameliorate the adverse effects of radiotherapy associated with tumor necrosis factor a production.Signaling pathways activated by DNA (4,5). The complexity ofthis signaling pathway is demonstrated by the number of genes involved in sensing DNA damage and transmitting the signal (5). DNA damage is presumed to be the initiating event in mammalian cell induction of stress response genes following x-ray or UV exposure (6, 7). However, the mechanisms of DNA damage recognition have not been identified in mammalian cells.Signal transduction pathways activated by ionizing radiation include increased phosphotransferase activity of cytoplasmic protein kinases (8-10). Moreover, inhibition of protein kinases blocks radiation-mediated gene induction and effects diverse biological endpoints such as apoptosis (10), radiation survival (11), and induction of the cytokine tumor necrosis factor a (TNF-a) (9). The calcium/phospholipiddependent protein kinase (protein kinase C; PKC) is activated within 15 sec of ionizing radiation exposure and is extinguished by 90 sec in human leukemia HL-60 cells (9). Second messengers that participate in PKC activation following exposure to external stimuli include free fatty acids such as arachidonic acid (12) (16,17). To determine whether arachadonic acid production was associated with radiationmediated signal transduction, we quantified arachidonic acid production in irradiated HL-60 cells and found an increase in arachidonate within 30 min following irradiation.Since phospholipase A2 hydrolyzes phosphatidylcholine to arachidonic acid, we studied the effects of the phospholipase A2 inhibitors mepacrine (18) and bromphenylbromide (BPB) (19). In addition, we studied the effects of dexamethasone and pentoxifylline on radiation-induced fatty acid hydrolysis, as these agents have been shown to inhibit phospholipase A2, reduce the production of cellular mediators of inflammation and tissue injury, and inhibit lipopolysaccharide-induced TNF-a production in monocytes (20, 21). Moreover, glucocorticoids and pentoxifylline are employed clinically to prevent ...

Prevention of late effects of irradiation lung damage by manganese superoxide dismutase gene therapy

“…For example, exposure of epithelial cells to 100-200 cGy irradiation has been demonstrated to induce egr-1, NF-␤, c-jun, c-fos and other transcriptional stimulatory elements. [6][7][8][9][10][11][12][13] These transcriptional regulators activate genes for many cytokine regulators and cellular protective gene products. 9,28 Radioresistance of mammalian cells in culture can be induced by the expression of recombinant oncogenes of several categories 47 or by increasing the availability of specific recombinant growth factors.…”

Dexamethasone-Induced Enhancement of Resistance to Ionizing Radiation and Chemotherapeutic Agents in Human Tumor Cells

“…Several well documented biochemical and molecular mechanisms are known to cause a corticosteroid induced enhancement of cellular resistance to cytotoxic treatment, namely, a dexamethasone induced increase in cellular glutathione content [15], a dexamethasone induced enhancement in expression of the metallothionein gene [19], and a dexamethasoneinducible suppression of radiation induced secretion of tumor necrosis factor alpha (TNFa), a protein enhancing cell inactivation by ionizing radiation [5][6][7]. In addition, we have found some preliminary evidence indicating that there may be a direct activation of DNA repair mechanisms and an inhibition of apoptotic responses following DNA damage (unpublished results).…”

The product of the ataxia-telangiectasia group D complementing gene, ATDC, interacts with a protein kinase C substrate and inhibitor.

“…However, PKC-s has been shown to be induced by ionizing radiation (36) and recent experiments indicate that PKC-e is much less abundant in AT cell lines (37). Hallahan et al (38) have demonstrated that the ionizing radiation-induced signal required for transcriptional induction of the tumor necrosis factor a gene involves the activation of phospholipase A2 at the cell membrane and, subsequently, PKC activity. Interestingly, overexpression of phospholipase A2 has also been shown to complement partially the ionizing radiation sensitivity of AT group D cell lines §.…”