DNA ligase IV and artemis act cooperatively to suppress homologous recombination in human cells: implications for DNA double-strand break repair.

Nonhomologous end-joining (NHEJ) and homologous recombination (HR) are two major pathways for repairing DNA double-strand breaks (DSBs); however, their respective roles in human somatic cells remain to be elucidated. Here we show using a series of human gene-knockout cell lines that NHEJ repairs nearly all of the topoisomerase II- and low-dose radiation-induced DNA damage, while it negatively affects survival of cells harbouring replication-associated DSBs. Intriguingly, we find that loss of DNA ligase IV, a critical NHEJ ligase, and Artemis, an NHEJ factor with endonuclease activity, independently contribute to increased resistance to replication-associated DSBs. We also show that loss of Artemis alleviates hypersensitivity of DNA ligase IV-null cells to low-dose radiation- and topoisomerase II-induced DSBs. Finally, we demonstrate that Artemis-null human cells display increased gene-targeting efficiencies, particularly in the absence of DNA ligase IV. Collectively, these data suggest that DNA ligase IV and Artemis act cooperatively to promote NHEJ, thereby suppressing HR. Our results point to the possibility that HR can only operate on accidental DSBs when NHEJ is missing or abortive, and Artemis may be involved in pathway switching from incomplete NHEJ to HR.

Degradation of oxidative stress-induced denatured albumin in rat liver endothelial cells.

We previously identified conformationally denatured albumin (D2 and D3 albumin) in rats with endotoxicosis (Bito R, Shikano T, and Kawabata H. Biochim Biophys Acta 1646: 100-111, 2003). In the present study, we attempted first to confirm whether the denatured albumins generally increase in conditions of oxidative stress and second to characterize the degradative process of the denatured albumin using primary cultured rat liver endothelial cells. We used five models of oxidative stress, including endotoxicosis, ischemic heart disease, diabetes, acute inflammation, and aging, and found that serum concentrations of D3 albumin correlate with the serum levels of thiobarbituric acid-reactive substance (R = 0.87), whereas the concentrations of D2 albumin are 0.52. Ligand blot analysis showed that the D3 albumin binds to gp18 and gp30, which are known endothelial scavenger receptors for chemically denatured albumin. Primary cultured rat liver endothelial cells degraded the FITC-D3 albumin, and the degradation rate decreased to approximately 60% of control levels in response to anti-gp18 and anti-gp30 antibodies, respectively. An equimolar mixture of these antibodies produced an additive inhibitory effect on both uptake and degradation, resulting in levels approximately 20% those of the control. Furthermore, filipin and digitonin, inhibitors of the caveolae-related endocytic pathway, reduced the FITC-D3 albumin uptake and degradation to <20%. Laser-scanning confocal microscopic observation supported these data regarding the uptake and degradation of D3 albumin. These results indicate that conformationally denatured D3 albumin occurs generally under oxidative stress and is degraded primarily via gp18- and gp30-mediated and caveolae-related endocytosis in liver endothelial cells.