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In 1961 Hayflick and Moorhead showed that a somatic cell culture has a limited life span [1]. In 1973 Olovnikov suggested that shortening the chromosomal ends (telomeres) determines the potential number of cell divisions. [2]. Telomeres protect the cellular genome from degradation; they participate in the chromosomal pairing during meiosis and in the gene expression regulation in the telomeres region [3]. In immortal cells that can divide infinitely, this should be the mechanism for compensating the chromosomal shortening. In 1975 Blackburn and Greider discovered the enzyme telomerase that elongates chromosomes [4]. Telomerase is a ribonucleoprotein complex that consists of components that are absolutely required for its activity: the RNA molecule and Telomerase Reverse Transcriptase TERT [5]; also, optionally several telomerase-associated proteins could be included in the telomerase complex. TR is also a template for TERT when telomerase elongates telomeres. Telomerase exists in human cells as dimers and contains two subunits of reverse transcriptase and two RNA molecules [6]. In human telomerase, p23/p90-shaperone, which is responsible for the complex assembling/configuration, binds 14-3-3, which is responsible for nuclear localization, and TP1 with an unknown function. Proteins hGAR1, Dyskerin/NAP57, hNHP2, and C1/C2, which are responsible for the stability, maturation, and localisation of RNA, bind to the hTR; La and hStau, which are supposedly responsible for the binding to telomeres; L22, which acts in processing and nuclear localisation; and hNOP10, A1/UP1, and TP1 with an unknown function [7]; TCAB1, which is responsible for the localisation of hTR in Cajal bodies and binding with telomers [8]. The enzymatic activity of human telomerase in the rabbit reticulocytes lysate is detected by adding hTR and hTERT [9, 10]. Note that telomerase functioning in vivo is not always consistent with the telomerase activity that was measured in vitro. For example, adding the Hemagglutinin epitope to the C-end of hTERT stops telomere alongation but does not suppress telomerase activity [11]. Telomerase activity detected in vitro appears in leucocytes in the G1 phase (Fig. 1) [12]. On the other hand, telomeres are replicated in vivo during the S-phase (Fig. 1) [13, 14]. During most of the cell cycle, TCAB1 helps the hTR accumulation in Cajal bodies [8], and in S-phase it is combined with telomeres in a cell. During the S-phase of the cell cycle, hTERT also moves to telomeres [15, 16]. This means that there is regulation at the level of the spatial localisation of active telomerase (enzyme) and telomere (substrate). A correlation between the telomerase activity and the length of telomeres is not always possible to obtain. For example, there is no dependency between the length of telomeres and telomerase activity in leukaemia [17]. Fig. 1. Cell cycle scheme. The appearance of telomerase activity in vitro happens in the G1-phase, but it works at the S-phase. The two-step hypothesis of cell aging and immortalisation M1/M2 theory describes the activity of telomerase dependency on the number of cell divisions very well (Fig. 2). In embryonic cell lines, telomerase is active and the length of telomeres is constant. In stem cells, the activity of polymerase is lower and it only partially compensates the telomeres shortening. In somatic cells, telomerase in not active. The shortening of telomeres leads to the moment of M1, i.e., the achievement of the Hayflick limit (point A on Fig 1) and the transition of cells to the senescence (aging) condition, which could be rescued by the inactivation or deletion of pRB/p16 or p53. Cells that pass through M1 continue their cell division and achieve the condition of crisis M2 (point B on Fig 2), which leads to massive cellular death. Cells that survive begin their transformation into cancer cells. Cancer cells have the ability for unlimited cell division and support the length of telomeres (usually due to telomerase activity). In the case of the transfection of hTR-expressing somatic cells by the hTERT gene (point C on Fig. 2) before the moment of M2, they, similarly to cancer cells, demonstrate elongation and stabilization of telomeres [7]. Fig. 2. Dependence of telomeres length on the cell cycle number in different types of cells: embryonic cell lines, somatic cells, and hTERT-transfected cells. (A) achieving the Hayflick limit by cells, (B) crisis with the following cell death and transformation ... Telomerase activity, with rare exceptions, does not occur in human somatic cells and tissues. Its activity was shown in reproductive tissues, as well as in intensively renewing tissues, such as some types of blood cells, the intestinal epithelium, and the layer of skin cells [18]; however, the level of telomerase activity in the somatic cells with active polymerase is lower than in cancer cells [19]. |
