Telomerase Components in Cell Signaling
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Telomerase Components in Cell Signaling
The extended growth potential of cancer cells is critically dependent upon the maintenance of functional telomeres, which are specialized chromosomal DNA-protein structures in the terminal regions of eukaryotic chromosomes (Ref.1). In order to divide, a normal cell has to replicate the entire DNA in its chromosomes. But normal cells have difficulty in copying the last few bases on the telomere. As a result, the telomere shortens with each round of DNA replication and cell division and as a cell ages, the telomere keeps shortening until it reaches a finite length. At that point cells stop dividing and this halt in growth is triggered by a gene p53 that is activated in response to DNA damage. A telomere that becomes too short no longer protects the chromosome from DNA damage and when the damage takes place, p53 responds by stopping cell replication and forcing it into senescence. Thus, telomeres protect the genetic material carried by eukaryotic chromosomes by a mechanism in which the linear chromosomal DNA can be replicated completely, without the loss of terminal DNA nucleotides from the 5 end of each strand of the DNA. This is necessary to prevent progressive loss of terminal DNA sequences in successive cycles of chromosomal replication (Ref.1).

Telomeric structural proteins fall into two general groups: those that bind telomeric DNA directly, and those that interact, directly or indirectly, with the telomeric DNA-binding proteins. Some telomeric DNA-binding proteins bind single-stranded telomeric DNA and others bind duplex telomeric DNA. The ribonucleoprotein enzyme telomerase binds the protruding single-stranded end of the G-rich telomeric DNA strand in order to extend it and make-up for the loss of terminal sequences resulting from normal semi-conservative DNA replication. Telomerase synthesizes its species-specific telomeric repeat sequence by elongating a DNA primer. It has two essential components, the RNA component hTERC (human Telomerase RNA Component), and a catalytic subunit hTERT (human Telomerase Reverse Transcriptase). TERC, the RNA subunit act in concert to elongate telomeres by reading from the RNA template sequence carried by the RNA subunit and synthesizing a complementary DNA strand (Ref.3). The mechanism of telomerase synthesis involves telomerase first recognizing the 3 overhanging telomeric sequence that exists at the chromosome ends. The telomerase RNA template sequence base pairs with the terminal TTAGGG repeat to initiate elongation of the 3 DNA end. The RNA template has only 11 bases that match the TTAGGG repeat sequence, such that only one repeats of the sequence can be added in a single elongation. Synthesis terminates with the circularly permuted sequence GGTTAG. Telomerase can continue to synthesize telomeric repeats on the same DNA strand by unwinding the DNA from the DNA-RNA hybrid, holding the DNA end while the RNA slides down 6 bases to allow proper alignment and base pairing. Coordination between C-strand and G-strand synthesis is required for proper telomere length maintenance (Ref.4).

The expression of catalytic subunit, TERT, is regulated by several GFs (Growth Factors) like KRas, BCL2 (B-Cell Lymphomal Leukemia-2) and c-Myc and inhibiting factors like p53 and Rb (Retinoblastoma susceptibility protein) that promote apoptosis or block cell division. Other post-translational signaling events acting directly on hTERT or on other proteins involved in the complex play a role in regulation of telomerase activity. The phosphorylation status of hTERT is also involved in modulation of the catalytic activity of telomerase: both PP2A (Protein Phosphatase-2A) and the c-Abl tyrosine kinase act as negative regulators of telomerase function, whereas PKC (Protein Kinase-C) and the Akt/PKB (Protein Kinase-B) pathway which are activated by GFs like EGF (Epidermal Growth Factor) and IGF1 (Insulin-Like Growth Factor-1) upregulate the telomerase activity (Ref.5). Additional molecules that regulate the activity of hTERC-hTERT and the maintenance of telomere structure include TRF1 (Telomeric Repeat binding Factor-1), TRF2, TANK (TRF1-interacting, Ankyrin-related ADP-ribose polymerase, also known as Tankyrase), TIN2 (TRF1-Interacting Nuclear Factor-2), RAP-1 (Repressor Activator Protein-1)/TERF2IP (Telomeric Repeat Binding Factor-2 Interacting Protein) and POT1 (Protection Of Telomeres-1). These proteins interact with the telomere and regulate the opening and closing of the free telomere end and access to the telomere by other protein complexes including the telomerase components. The telomerase ribonucleoprotein component, TEP1 inhibits the release of active telomerase by not releasing hTERT or by TRF1 which binds the end repeats and prevents access to the chromosome ends (Ref.2). A variety of proteins and ribonucleoproteins like DKC1 (Dyskeratosis Congenita-1), p23 and HSP90 (Heat Shock Protein-90), assist in telomerase assembly and facilitate interactions between telomerase and the telomere. Other proteins such as MRE11 (Meiotic Recombination-11), NBS1 (Nijmegen Breakage Syndrome-1), Ku70, Ku80, and ATM (Ataxia Telangiectasia Mutated) function in the detection of short telomeres and trigger DNA-damage response pathways or the repair of telomere sequences (Ref.5).

Telomeres are required to attract the telomerase replication machinery to the chromosomal terminus, as well as to regulate its action there. In addition, telomeres are essential for stabilizing eukaryotic chromosomes in a variety of ways. Telomeres shield the chromosomal termini from recognition by the DNA damage response system of the cell. They cap chromosome ends, preventing them from being degraded or fused together. Such fused chromosomes mis-segregate in mitosis or meiosis. Telomere integrity is essential for chromosome numerical and positional stability, and telomere shortening facilitates the evolution of cancer cells by promoting chromosome end-to-end fusions and the development of aneuploidy (Ref.6).