Telomerase activity and its regulation in malignant hematopoietic cells
Sammanfattning: Telomerase, a multicomponent ribonucleoprotein, synthesizes TTAGGG telomeric repeats essential for the stability and integrity of chromosomes. Most normal human somatic cells exhibit undetectable telomerase activity. Activation of telomerase is widely observed in both primary malignant tissues and immortal cell lines and has been suggested as a critical step during turnorigenesis. The aim of this thesis was to define clinical and biological implications of telomerase activity in acute myelogenous leukemia (AML) and the regulatory pathways of telomerase expression in malignant hematopoietic cells. Ninety-five leukemic cell samples from 66 patients with AML were analyzed for telomerase activity and for the expression of its subunits telomerase reverse transcriptase (hTERT), telomerase RNA (hTER) and telomerase associated protein (hTEPI). Eighty out of 95 AML samples exhibited elevated telomerase activity, which was in accordance with the upregulation of hTERT expression. Higher levels of enzymatic activity was significantly associated with progressive disease, CD34 expression, and abnormal karyotypes. This data suggests that activation of telomerase may play a role in the development and progression of AML. We found that terminal differentiation led to the repression of telomerase activity in the HL60 promyelocytic leukemic cell line when treated with differentiation-inducing agents. Downregulation of hTERT rnRNA expression at the transcriptional level preceded the reduction of telomerase activity in differentiated HL60 cells. During this process, ongoing protein synthesis was required for the inhibition of hTERT expression. These findings indicate that tumor cells may maintain an intact repression pathway that can be triggered to signal a shut-off of hTERT and telomerase expression, which provides a base for the manipulation of telomerase in human tumors. We demonstrated that either the overexpression of wild type (wt) tumor suppressor p53 in the BL41 Burkitt lymphoma cell line or the activation of endogenous wt p53 in the breast carcinoma MCF-7 cell line transcriptionally downregulated hTERT rnRNA expression. The activation of the hTERT proximal promoter in Drosophila Schneider SL2 cells completely depended on the ectopic expression of the transcription factor Sp1. Sp1-mediated transactivation of the promoter could be abrogated by wt p53. wt p53 repressed Sp1-binding to the hTERT promoter by forming a p53-Sp1 complex. Thus, p53-mediated repression of hTERT/telomerase may reflect yet another anti-tumor mechanism of p53. Interferon-[alpha] (IFN-[alpha]) has acquired use in the treatment of various human malignancies. However, the underlying mechanisms for its action remain unclear. We found that IFN-[alpha] treatment induces a rapid downregulation of hTERT expression followed by a decline in telomerase activity in hematopoietic cell lines Daudi, P3HR1, U266 and H9 and in leukemic cells from patients with acute leukemia. The suppression of hTERT/telomerase mediated by IFN-[alpha] was apparently independent of cell growth arrest and alterations in myc expression. This suppression could not be blocked by inhibition of protein synthesis. The anti-telomerase effect of IFN-[alpha] may represent one of the mechanisms contributing to its action against human tumors.
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