Study on Mechanisms of Telomerase Regulation in Arsenic Trioxide Inducing Apoptosis in Myelodysplasia Cell Line.

The telomerase activity can be down regulated by arsenic trioxide (As2O3), which is regarded as an apoptotic induction agent, is confirmed in many kinds of tumor cells. To investigate the mechanisms of telomerase regulation and to explore the correlation of As2O3 inducing apoptosis and telomerase regulation in MUTZ-1 cells, which are established as a high-risk myelodysplasia Cell line that derived from a MDS patient (FAB subtype refractory anemia with excess of blasts), a quantitative assessment of the telomerase activity by TRAP-ELISA and detection of the expression levels of hTERT, TRF1 (TTAGGG repeat binding factor 1), TRF2 (TTAGGG repeat binding factor 2), bcl-2, bax mRNA were performed, together with the assessment of the apoptosis by means of translocation of phosphatidylserine (PS) through flow cytometry assay. The results indicated that a typical apoptotic cell group distribution of DNA content was represented in the MUTZ-1 cells after being exposed to As2O3 at the range of concentration from 1μmol/L to 8μmol/L in a dose-dependent manner (r=0.736, P<0.001) and time-dependent manner (r=0.674, p<0.05), and the telomerase activity was down-regulated in a time-dependent manner (r=−0.976,P=0.024), and the expression level of hTERT mRNA in MUTZ-1 cells was represented in a dose-dependent manner (r=−0.892,P=0.042) and time-dependent manner (r=−1.000,P=0.04), after the cells were treated by As2O3 at the dosage as above. It was showed that a significant correlation between the decreased telomerase activity and the increased percentage of apoptotic cells in the treated cells (r=0.938,P=0.018), and there was a strong relationship between the telomerase activity and the mRNA expression of hTERT gene (r=0.783,P=0.022). However, As2O3 has no obvious effect on the expression level of TRF1 mRNA and TRF2 mRNA, which were regarded as two telomere-binding proteins. Further findings indicated that the inhibition of telomerase activity in MUTZ-1 cells was accompanied with down-regulated mRNA expression of bcl-2 gene (densitometry readings: 0.255±0.017 vs 0.466±0.069, P<0.05) and decreased ration of bcl-2/bax (densitometry reading ratios: 0.890±0.083 vs 0.546±0.014, P<0.05) at the dosage of 4μmol/L for 24 hours. These observations suggest that the apoptosis induced by As2O3 on MUTZ- 1 cells might be mediated through the inhibition of telomerase activity regulated by expression of hTERT gene, which implies that may be one of the mechanisms of As2O3 inducing apoptosis in MUTZ-1 cells.

Control of Human Telomere Length by TRF1 and TRF2

ABSTRACT Telomere length in human cells is controlled by a homeostasis mechanism that involves telomerase and the negative regulator of telomere length, TRF1 (TTAGGG repeat binding factor 1). Here we report that TRF2, a TRF1-related protein previously implicated in protection of chromosome ends, is a second negative regulator of telomere length. Overexpression of TRF2 results in the progressive shortening of telomere length, similar to the phenotype observed with TRF1. However, while induction of TRF1 could be maintained over more than 300 population doublings and resulted in stable, short telomeres, the expression of exogenous TRF2 was extinguished and the telomeres eventually regained their original length. Consistent with their role in measuring telomere length, indirect immunofluorescence indicated that both TRF1 and TRF2 bind to duplex telomeric DNA in vivo and are more abundant on telomeres with long TTAGGG repeat tracts. Neither TRF1 nor TRF2 affected the expression level of telomerase. Furthermore, the presence of TRF1 or TRF2 on a short linear telomerase substrate did not inhibit the enzymatic activity of telomerase in vitro. These findings are consistent with the recently proposed t loop model of telomere length homeostasis in which telomerase-dependent telomere elongation is blocked by sequestration of the 3′ telomere terminus in TRF1- and TRF2-induced telomeric loops.

An essential yeast gene encoding a TTAGGG repeat-binding protein

A yeast gene encoding a DNA-binding protein that recognizes the telomeric repeat sequence TTAGGG found in multicellular eukaryotes was identified by screening a lambda gt11 expression library with a radiolabeled TTAGGG multimer. This gene, which we refer to as TBF1 (TTAGGG repeat-binding factor 1), encodes a polypeptide with a predicted molecular mass of 63 kDa. The TBF1 protein, produced in vitro by transcription and translation of the cloned gene, binds to (TTAGGG)n probes and to a yeast telomeric junction sequence that contains two copies of the sequence TTAGGG separated by 5 bp. TBF1 appears to be identical to a previously described yeast TTAGGG-repeat binding activity called TBF alpha. TBF1 produced in vitro yields protein-DNA complexes with (TTAGGG)n probes that have mobilities on native polyacrylamide gels identical to those produced by partially purified TBF alpha from yeast cells. Furthermore, when extracts are prepared from a strain containing a TBF1 gene with an antigen tag, we find that the antigen copurifies with the predominant (TTAGGG)n-binding activity in the extracts. The DNA sequence of TBF1 was determined. The predicted protein sequence suggests that TBF1 may contain a nucleotide-binding domain, but no significant similarities to any other known proteins were identified, nor was an obvious DNA-binding motif apparent. Diploid cells heterozygous for a tbf1::URA3 insertion mutation are viable but upon sporulation give rise to tetrads with only two viable spores, both of which are Ura-, indicating that the TBF1 gene is essential for growth. Possible functions of TBF1 (TFB alpha) are discussed in light of these new results.

An essential yeast gene encoding a TTAGGG repeat-binding protein.

A yeast gene encoding a DNA-binding protein that recognizes the telomeric repeat sequence TTAGGG found in multicellular eukaryotes was identified by screening a lambda gt11 expression library with a radiolabeled TTAGGG multimer. This gene, which we refer to as TBF1 (TTAGGG repeat-binding factor 1), encodes a polypeptide with a predicted molecular mass of 63 kDa. The TBF1 protein, produced in vitro by transcription and translation of the cloned gene, binds to (TTAGGG)n probes and to a yeast telomeric junction sequence that contains two copies of the sequence TTAGGG separated by 5 bp. TBF1 appears to be identical to a previously described yeast TTAGGG-repeat binding activity called TBF alpha. TBF1 produced in vitro yields protein-DNA complexes with (TTAGGG)n probes that have mobilities on native polyacrylamide gels identical to those produced by partially purified TBF alpha from yeast cells. Furthermore, when extracts are prepared from a strain containing a TBF1 gene with an antigen tag, we find that the antigen copurifies with the predominant (TTAGGG)n-binding activity in the extracts. The DNA sequence of TBF1 was determined. The predicted protein sequence suggests that TBF1 may contain a nucleotide-binding domain, but no significant similarities to any other known proteins were identified, nor was an obvious DNA-binding motif apparent. Diploid cells heterozygous for a tbf1::URA3 insertion mutation are viable but upon sporulation give rise to tetrads with only two viable spores, both of which are Ura-, indicating that the TBF1 gene is essential for growth. Possible functions of TBF1 (TFB alpha) are discussed in light of these new results.

A mammalian factor that binds telomeric TTAGGG repeats in vitro

We have identified a DNA-binding activity with specificity for the TTAGGG repeat arrays found at mammalian telomeres. This factor, called TTAGGG repeat factor (TRF), is present in nuclear extracts of human, mouse, and monkey cells. TRF from HeLa cells was characterized in detail by electrophoretic mobility shift assays. It binds double-stranded TTAGGG repeats in linear and circular DNAs. Single-stranded repeats are not recognized. The optimal site for TRF appears to contain more than six contiguous TTAGGG repeats. Tandem arrays of TAGGG, TTTAGGG, TTTTAGGG, TTGGGG, and TTAGGC repeats do not bind TRF well, indicating that TRF preferentially recognizes the telomeric repeat sequence present at mammalian chromosome ends. The apparent molecular mass of this factor, based on recovery of TRF from sodium dodecyl sulfate-polyacrylamide gels, is approximately 50 kDa. We suggest that TRF binds along the length of mammalian telomeres.

A mammalian factor that binds telomeric TTAGGG repeats in vitro.

We have identified a DNA-binding activity with specificity for the TTAGGG repeat arrays found at mammalian telomeres. This factor, called TTAGGG repeat factor (TRF), is present in nuclear extracts of human, mouse, and monkey cells. TRF from HeLa cells was characterized in detail by electrophoretic mobility shift assays. It binds double-stranded TTAGGG repeats in linear and circular DNAs. Single-stranded repeats are not recognized. The optimal site for TRF appears to contain more than six contiguous TTAGGG repeats. Tandem arrays of TAGGG, TTTAGGG, TTTTAGGG, TTGGGG, and TTAGGC repeats do not bind TRF well, indicating that TRF preferentially recognizes the telomeric repeat sequence present at mammalian chromosome ends. The apparent molecular mass of this factor, based on recovery of TRF from sodium dodecyl sulfate-polyacrylamide gels, is approximately 50 kDa. We suggest that TRF binds along the length of mammalian telomeres.