Irradiation as preparative regimen for hepatocyte transplantation causes prolonged cell cycle block

Purpose: Hepatocyte transplantation following liver irradiation (IR) and partial hepatectomy (PH) leads to extensive liver repopulation. We investigated the changes in the liver induced by IR explaining the loss of reproductive integrity in endogenous hepatocytes. Materials and methods: Right lobules of rat liver underwent external beam IR (25 Gy). A second group was subjected to additional 33% PH of the untreated left liver lobule. Liver specimens and controls were analyzed for DNA damage, apoptosis, proliferation and cell cycle related genes (1 hour to up to 12 weeks). Results: Double strand breaks (phosphorylated histone H2AX) induced by IR rapidly declined within hours and were no longer detectable after 4 days. No significant apoptosis was noted and steady mRNA levels (B-cell lymphoma 2-associated X protein (BAX), caspase 3 and 9) were in line with the lack of DNA fragmentation. However, gene expression of p53 and p21 in irradiated liver tissue increased. Transcripts of cyclin D1, proliferating cell nuclear antigen (PCNA), and cyclin B augmented progressively, whereas cyclin E was only affected moderately. Following PH, irradiated livers displayed persistently high protein levels of p21 and cyclin D1. However, cell divisions were infrequent, as reflected by low PCNA levels up to four weeks. Conclusion: IR leads to a major arrest in the G1/S phase and to a lesser extent in the G2/M transition of the cell cycle, resulting in reduced regenerative response following PH. The persistent block of at least four weeks may promote preferential proliferation of transplanted hepatocytes in this milieu.