Effect of Long-Term HT on Telomere Length mere erosion in this specific condition. Another important factor contributing to telomere short- ening is the oxidative stress. Telomeres are highly sensitive to oxidative stress-mediated damage in vitro. In fact, the telomere shortening rate in human fibroblasts among different cultures varies by more than 1 - 2 orders of magnitude depending on the oxidative stress.20,21 The preferential accu- mulation of oxidative damage causes single- stranded breaks that result in telomere erosion by mechanisms that are independent of the end replication problem.21-23 Antioxidants aid in the attenuation of telomere shortening.3,24 Considering that estrogen and its derivatives are known to have antioxidant properties via diverse mecha- nisms,15 the antioxidant capacity of long- term HT might modify the influence of ROS on the telo- mere shortening rate. However, we failed to find any significant dif- ferences in the serum total antioxidant status (TAS) level between the HT and the non-HT group. Also, there was no significant correlation between the TAS level and the relative T/S ratio. This result was irrelevant because estrogen is believed to influence telomere attrition primarily by its antioxidant effects. This discrepancy raised the possibility that serum levels of the total antioxidant capacity in one point in time might not necessarily indicate the oxidative damages of DNA in vivo. DNA damage is not only caused by oxidative stress but is itself regarded as the most sensitive biological marker for oxidative stress. Despite the status of high sensitivity, there are studies which demonstrate that there is no asso- ciation between the percentage of DNA damage and serum levels of antioxidant capacity.25,26 Mendoza-Nunez26 proposed that low serum levels of total antioxidants are not always indicative of oxidative stress. Moreover, it would be compli- cated to measure the full spectrum of estrogen's antioxidant actions by conventional methods. This is because of the diverse mechanisms through which estrogen exerts an antioxidant effect,15 e.g., scavenging free radicals, inhibiting free radical formation and stimulating free radical enzymes. In addition to its antioxidant effect, estrogen has been shown to stimulate telomerase in a vitro study.27 This mode of action is thought to be mediated by posttranscriptional modification through stimulation of the phosphoinositol 3- Kinase/Akt pathway.28,29 Estrogen can also in- fluence telomerase activation by stimulating NO, which has been proven to activate telomerase in the endothelial cell line in recent vitro studies.30,31 Because serum estrogen levels in the HT group were significantly higher than in the non-HT group, we could hypothesize that telomerase acti- vation aided in the alleviation of telomere attrition in the HT group. Further studies will be needed to confirm this hypothesis. The traditional method for measuring telomere length in human DNA is to determine a mean TRF length by Southern blot analysis. However, this method is time consuming, requires a large amount of DNA, and shows variability depending upon the particular restriction enzyme used. However, measurement of the relative telomere length ratio by quantitative PCR is simple and allows for the rapid processing of large numbers of samples. In previous research, relative T/S ratios determined using quantitative PCR were found to correlate well with mean TRF lengths.11,19 Also, agarose gel electrophoresis demonstrated that the approximate size of major PCR products was 78 bp with new primer set, as was expected. In this study, postmenopausal women having had HT were more prone to take vitamins and to exercise regularly, so they may have practiced better health habits than women in the non-HT group. Serum levels of fasting blood glucose and HDL-cholesterol were significantly different between the two groups. These factors might be a source of bias, but estrogen use still showed a significant correlation with the relative telomere length ratio in linear regression models after adjusting for these factors. There are some limitations to this study. First, the retrospective nature of our study design does not permit us to confirm the effect of estrogen replacement on changes in telomere attrition. Prospective studies are needed to evaluate the role that estrogen therapy may have on the alle- viation of telomere attrition. Also, because telo- mere length has been shown to vary considerably among individuals in previous studies, the small sample size in this study may not offset an enrol- ling bias. In conclusion, telomere lengths were longer in Yonsei Med J Vol. 46, No. 4, 2005