Telomerase inhibition using azidothymidine in the HT-29 colon cancer cell line

Background: We investigated the effects of telomerase inhibition by using the reverse transcriptase inhibitor azidothymidine (AZT) in the human colorectal cancer cell line HT-29 in the presence and absence of 5-fluorouracil (5-FU). Methods: HT-29 cells were cultured in the presence of AZT. Telomerase activity was measured by using the telomerase repeat amplification protocol. Telomere length was determined by Southern analysis. The colorimetric microtiter assay was performed to determine the cytotoxic effects of AZT, alone and in combination with 5-FU. Results: The presence of 3'-azido-3'-deoxythymidine triphosphate, (AZT-TP) effectively inhibited telomerase extracted from HT-29 cells. HT-29 cells cultured with 125 muM of AZT underwent fewer total population doublings over 91 days. Southern analysis revealed that telomere attrition occurred within this period. Exposure to 125 muM of AZT resulted in slightly reduced viability (10%) of HT-29 cells. However, the presence of AZT increased 5-FU cytotoxicity, suggesting that the effects of these two drugs are synergistic. Conclusions: The data are consistent with telomerase inhibition having growth-inhibitory effects in addition to those predicted to accompany loss of telomere function. Further studies using specific small-molecule inhibitors will confirm whether the growth-inhibitory and 5-FU-sensitivity effects seen here are a direct result of telomerase inhibition.