S. Kawanishi, S. Oikawa

In humans, shortening of telomere which contains highly conserved repeats of a characteristic hexameric sequence (5'-TTAGGG-3') is believed to be associated with cell senescence. Recently, Zglinicki et al. reported an increase of the rate of telomere shortening by oxidative stress in human fibroblasts [1]. However, the mechanism for the increase of telomere shortening rate by oxidative stress remains to be clarified. We investigated whether UVA, which contributes to photoaging, accelerates the telomere shortening in human cultured cells. The terminal restriction fragment (TRF) from WI-38 fibroblasts irradiated with UVA (365-nm light) decreased with increasing the irradiation dose. Furthermore, UVA irradiation dose-dependently increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in both WI-38 fibroblasts and HL-60 cells [2]. In order to clarify the mechanism of the acceleration of telomere shortening, we investigated site-specific DNA damage induced by UVA irradiation in the presence of endogenous photosensitizers using 32P 5' end-labeled DNA fragments containing telomeric oligonucleotide (TTAGGG)4. UVA irradiation with riboflavin induced 8-oxodG formation in the DNA fragments containing telomeric sequence, and Fpg protein treatment led to chain cleavages at the central guanine of 5'-GGG-3' in telomere sequence. The amount of 8-oxodG formation in DNA fragment containing telomere sequence (5'-CGC(TTAGGG)7CGC-3') was approximately 5 times more than that in DNA fragment containing non-telomere sequence (5'-CGC(TGTGAG)7CGC-3'). Furthermore, H2O2 plus Cu(II) caused DNA damage, including 8-oxodG formation, specifically at the GGG sequence in the telomere sequence (5'-TTAGGG-3') [3]. Human 8-oxodG-DNA glycosylase introduces a chain break in a double-stranded oligonucleotide specifically at an 8-oxodG residue. Therefore, the formation of 8-oxodG at the GGG triplet in telemore sequence induced by oxidative stress could participate in accerelation of telomere shortening. Finally, it is concluded that sequence-specific oxidative damage to DNA may play important roles in aging.

[1] T. von Zglinicki, G. Saretzki, W. Docke, C. Lotze, Mild hyperoxia shortens telomeres and inhibits proliferation of fibroblasts: a model for senescence? Exp. Cell Res. 220 (1995) 186-193.
[2] S. Oikawa, S. Tada-Oikawa, S. Kawanishi. Site-specific DNA damage at the GGG sequence by UVA involves acceleration of telomere shortening. Biochemistry. 40, 4763-4768 (2001).
[3] S. Oikawa, S. Kawanishi, Site-specific DNA damage at GGG sequence by oxidative stress may accelerate telomere shortening, FEBS Lett. 453 (1999) 365-368.

Keywords (Optional): 
oxidative stress
DNA damage