ToxSci Advance Access published online on June 18, 2009
Toxicological Sciences, doi:10.1093/toxsci/kfp136
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Long-term exposure to zidovudine delays cell cycle progression, induces apoptosis, and decreases telomerase activity in human hepatocytes
Division of Biochemical Toxicology, National Center for Toxicological Research, Jefferson, Arkansas 72079, U.S.A
1 To whom correspondence should be addressed. Tel: 870-543-7612. FAX: 870-543-7136. E-mail: jia-long.fang{at}fda.hhs.gov.
Received May 12, 2009; revision received June 12, 2009; accepted June 12, 2009
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Abstract |
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Zidovudine (3’-azido-3’-deoxythymidine; AZT), which is currently used in the treatment of acquired immunodeficiency syndrome, has been shown to have anticancer properties. In the present study, we examined the mechanisms contributing to increased sensitivity of cancer cells to the growth-inhibitory effects of AZT. This was accomplished by incubating a hepatoma cell line (HepG2) and a normal liver cell line (THLE2) with AZT in continuous culture for up to 4 weeks and evaluating the number of viable and necrotic cells, the induction of apoptosis, cell cycle alterations, and telomerase activity. In HepG2 cells, AZT (2 - 100 µM) caused significant dose-dependent decreases in the number of viable cells at exposures > 24 h. During a 1-week recover period, there was only a slight increase in the number of viable cells treated with AZT. The decrease in viable cells was associated with an induction of apoptosis, a decrease in telomerase activity, and S and G2/M phase arrest of the cell cycle. During the recovery period, the extent of apoptosis and telomerase activity returned to control levels, while the disruption of cell cycle progression persisted. Western blot analysis indicated that AZT caused a decrease in checkpoint kinase 1 (Chk1) and kinase 2 (Chk2) and an increase in phosphorylated Chk1 (Ser345) and Chk2 (Thr68). Similar effects, to lesser extent, were observed in THLE2 cells given much higher concentrations of AZT (50 - 2,500 µM). These data show that HepG2 cells are much more sensitive than THLE2 cells to AZT. They also indicate that a combination of a delay of cell cycle progression, an induction of apoptosis, and a decrease in telomerase activity is contributing to the decrease in the number of viable cells from AZT treatment, and that checkpoint enzymes Chk1 and Chk2 may play an important role in the delay of cell cycle progression.
Key Words: AZT; Cell cycle; Apoptosis; Telomerase activity; Checkpoint kinases.
The views presented in this article do not necessarily reflect those of the U.S. Food and Drug Administration