Effects of radiation and/or tumor necrosis factor alpha on cell damage in a healthy liver: a role for PECAM-1

The liver is considered to be radiosensitive; however, the mechanisms of radiation-induced liver damage are poorly understood. Platelet endothelial cell adhesion molecule 1 (PECAM-1/CD31) is an adhesion molecule and expressed mainly in blood cells and endothelial cells. Its expression is decreased during inflammatory processes. Tumor necrosis factor (TNF)-alpha, which is induced by radiation, is known to downregulate PECAM-1.

The aim of the current study was to investigate if combined treatment with TNF-alpha and irradiation would enhance liver damage through regulation of the PECAM-1 signaling pathway. This was studied in-vivo in mouse models of single-dose selective liver irradiation w/wo TNF-alpha administered intraperitoneally shortly before irradiation. The mice were sacrificed at different time points, serum and tissues were collected for further analyses. RNA- and protein analyses were performed by RT-PCR and Western blotting, respectively.

Both irradiation and TNF-alpha administration alone induced elevated aspartate aminotransferase (AST)-levels (hepatic damage) in serum, compared to sham-irradiated mice (control). This hepatic damage was further enhanced in mice that received combined treatment with irradiation and TNF-alpha. In parallel to hepatic damage, a time-dependent decrease in the expression level of hepatic PECAM-1 was found in mice that received each single irradiation or TNF-alpha treatment. The administration of irradiation together with TNF-alpha showed additional decline in the expression of PECAM-1. In contrast, increased expression of hepatic lipocalin-2 (LCN-2), an acute phase protein, was detected at mRNA and protein levels after irradiation or TNF-alpha treatment. The level of LCN-2 was further increased in mice that received combined treatment with TNF-alpha and irradiation, compared to irradiation or TNF-alpha alone. This induction seems to be mediated by the activation of the signal transducer and activator of transcription (STAT)-3 signaling pathway.

In order to study the role of PECAM-1 in hepatic damage, the liver of both wild-type (WT) and PECAM-1 knock-out (KO)-mice were selectively irradiated (25 Gy). PECAM-1 KO mice showed higher liver damage in parallel to increased LCN-2 expression compared to WT-mice at RNA and protein levels. By means of Western blotting, an increased level of cell death-related proteins (SOD-1, BAX) was observed after irradiation in both WT- and PECAM-1 KO mice. However, the level of Cyt-C was reduced only in PECAM-1 KO mice after irradiation. Our study shows a synergistic effect of radiation and TNF-alpha on hepatic cell-damage, probably through regulation of PECAM-1. Our results may help to develop protective strategies to reduce radiation-induced defects in normal liver tissue, as well as strategies, which may increase the effects of radiation on tumor tissue.