Role of human telomerase reverse transcriptase and telomeric-repeat binding factor proteins 1 and 2 in human hematopoietic cells

Telomerase, an enzyme that adds hexameric repeats of 5'-TTAGGG-3', termed t elomeres, to the ends of chromosomal DNA, has been implicated in cellular i mmortalization and cellular senescence. Recently several relevant genes hav e been cloned, including those encoding three major components of human tel omerase: human telomerase RNA component (hTR), human telomerase reverse tra nscriptase (hTERT), and telomerase-associated protein-1 (TEP1). Also import ant are genes encoding human telomeric-repeat binding factor proteins (TRF) 1 and 2. We compared 10 human malignant hematopoietic cell lines, 19 sampl es from patients with acute leukemia and normal granulocytes and monocytes to study telomerase activity and expression of these various genes using a reverse transcription-polymerase chain reaction (RT-PCR). In all 10 maligna nt cell lines with telomerase activity, hTR, hTERT mRNA, and TEP1 mRNA were expressed, while in normal monocytes and granulocytes without telomerase a ctivity, expression of hTR, but not hTERT mRNA was detected. TEP1 mRNA was expressed in normal monocytes, but not granulocytes. Expression of TRF1 and TRF2 mRNAs was greater in the normal cells than in human malignant hematop oietic cell lines and in 16 samples of patients with acute leukemia. When d ifferentiation of the malignant hematopoietic cell line HL-60 was induced u sing tumor-necrosis-factor 471 and all-trans retinoic acid (ATRA), telomera se activity decreased gradually during differentiation. Of the three telome rase components, only hTERT mRNA expression showed changes paralleling telo merase activity, becoming undetectable with differentiation. In contrast, i nitially low expression of TRF1 and TRF2 mRNAs increased during differentia tion. Not only hTERT, but also TRF1 and TRF2 are important regulators of te lomerase activity that represent potential targets for gene therapy against cancer.