Administration of rat acute-phase protein α2-macroglobulin before total-body irradiation initiates cytoprotective mechanisms in the liver

Previously, we showed that administration of the acute-phase protein alpha(2)-macroglobulin (alpha M-2) to rats before total-body irradiation with 6.7 Gy (LD50/30) of X-rays provides the same level of radioprotection as amifostine. Here, we compare the cytoprotective effects of alpha M-2 and amifostine on rat liver. The potential of the liver to replenish cells destroyed by ionizing radiation was assessed by immunoblot analysis with antibody to proliferating cell nuclear antigen (PCNA). After irradiation, in unprotected rats PCNA decreased 6-fold from the basal level. In rats pretreated with either alpha M-2 or amifostine, PCNA was increased throughout a 4 week follow-up period, indicating that hepatocyte proliferation was unaffected. Since PCNA is an important component of the repair machinery, its increased expression was accompanied by significantly lower DNA damage in alpha M-2- and amifostine-treated rats. At 2 weeks after irradiation, the Comet assay revealed a 15-fold increase in DNA damage in unprotected rats, while in alpha M-2- and amifostine-treated rats we observed 3- and 4-fold rise in damage, respectively. The improved protection to DNA damage was supported by elevated activity of the antioxidant systems. Compared to untreated rats, pretreatments with alpha M-2 and amifostine led to similar increases in levels of the inflammatory cytokine IL-6 and the redox-sensitive transcription factor NF kappa B, promoting upregulation of MnSOD, the major component of the cell's antioxidant axis, and subsequent increases in Mn/CuZnSOD and catalase enzymatic activities. The results show that alpha M-2 induces protein factors whose interplay underlies radioprotection and support the idea that alpha M-2 is the central effector of natural radioprotection in the rat.