Inhibition of human telomerase in immortal human cells leads to progressive telomere shortening and cell death

Abstract

The correlation between telomerase activity and human tumors has led to the hypothesis that tumor growth requires reactivation of telomerase and that telomerase inhibitors represent a class of chemotherapeutic agents. Herein, we examine the effects of inhibition of telomerase inside human cells. Peptide nucleic acid and 2'-O-MeRNA oligomers inhibit telomerase, leading to progressive telomere shortening and causing immortal human breast epithelial cells to undergo apoptosis with increasing frequency until no cells remain. Telomere shortening is reversible: if inhibitor addition is terminated, telomeres regain their initial lengths. Our results validate telomerase as a target for the discovery of anticancer drugs and supply general insights into the properties that successful agents will require regardless of chemical type. Chemically similar oligonucleotides are in clinical trials and have well characterized pharmacokinetics, making the inhibitors we describe practical lead compounds for testing for an antitelomerase chemotherapeutic strategy.

Figure 1

(a) Inhibition of telomerase activity by 2′-O-MeRNA oligomers delivered into HME50-5E cells by using FuGENE6 lipid, measured 3 days after transfection. Similar results were observed on introduction of 2′-O-MeRNA oligomers into HME50-hTERT cells and on introduction of 2′-O-MeRNA or PNA oligomers into DU145 cells. (b) Inhibition of telomerase activity in HME50-5E and DU145 cells by 2′-O-MeRNA and PNA oligomers as detected by the TRAP assay. HME50-5E and DU145 cells were collected 1 and 3 days after transfection with 2′-O-MeRNA or PNA oligomers. Telomerase activity was quantitated as described (38).

Figure 2

Effects on cell growth after long-term transfection with match or mismatch 2′-O-MeRNA. (a) HME50-5E cells transfected with 2′-O-MeRNA oligomers. (b) DU145 cells transfected with 2′-O-MeRNA oligomers. (c) HME50-hTERT cells transfected with 2′-O-MeRNA oligomers.

Figure 3

Increase of apoptosis of HME50-5E cells after repeated transfection with fully complementary 2′-O-MeRNA. Levels of apoptosis were measured by staining with 4′6-diamidino-2-phenylindole followed by microscopy. Background apoptosis of cells that were untreated or that were treated with 2′-O-MeRNA containing mismatched bases was 2–3%. Levels of apoptosis were confirmed by flow cytometry with the ApoAlert Annexin V Apoptosis kit (CLONTECH).

Figure 4

Measurement of telomere restriction fragment length (TRF) in HME50-5E and DU145 cells treated with 2′-O-MeRNA and PNA oligomers. (a) HME50-5E cells that had been treated with 2′-O-MeRNA oligomers for 60 days, with the results of independent experiments shown in triplicate. (b) DU145 cells treated with 2′-O-MeRNA and PNA oligomers for 76 days, with the results of independent experiments shown in triplicate. In parts a and b, the signal intensity in the lanes showing the outcome of treatment with fully complementary oligomer is weak, because telomeres have eroded and few telomeric repeats remain to hybridize with radiolabeled probe. Equivalent amounts of chromosomal DNA were loaded in each lane. TRF lengths are expressed as kilobase pairs.

Figure 5

The TRF length of DU145 as a function of inhibitor addition. TRF lengths were measured after 76 days of addition of fully complementary or mismatch-containing 2′-O-MeRNA or PNA oligomers. TRF lengths were then measured again 3 or 5 weeks after terminating oligomer addition. kB, kilobase.