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Ionizing radiation causes histological changes in normal tissues that resemble those resulting from the inflammatory response. Inflammation is a multistep process requiring expression of adhesion molecules on the surface of endothelial cells which results in leukocyte extravasation. E-selectin is an adhesion molecule that mediates leukocyte "rolling" on the endothelium and is required for the inflammatory response. We quantified E-selectin expression and selectin-dependent adhesion of leukocytes to human endothelial cells after X irradiation to determine whether E-selectin participates in the radiation-mediated inflammation-like response. Immunofluorescence staining of irradiated endothelial cells demonstrated expression of E-selectin on the cell surface similar to that elicited by treatment with interleukin-1 (IL-1). Radiation-mediated expression of E-selectin was dependent on dose and time and occurred at doses as low as 0.5 Gy. Furthermore, the increased adhesion of leukocytes to irradiated endothelial cells was prevented by an E-selectin-blocking antibody. Sialyl Lewis X is one of the molecules on the surface of leukocytes that adheres to E-selectin. The anti-inflammatory agents glycyrrhizin and carminic acid, which are structural analogues of sialyl Lewis X, attenuated adhesion of leukocytes to endothelial cells treated with X rays or IL-1. These data implicate a new class of anti-inflammatory agents in the prevention of adhesions of leukocytes to the irradiated vascular endothelium.

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 ...