HTERT transcriptase

Beyond the Hayflick limit, cells that have escaped senescence and/or crisis must progress beyond mortality stage 2 (Figure 26.1). Recent studies have shown that human telomerase reverse transcriptase cDNA (hTERT) in combination with viral oncogenes also has the potential to immortalize human airway epithelial cells [15, 78-82], However, even though hTERT alone will cause enhanced growth potential of a cell line, with the exception of one study [81],... 

The telomerase holoenzyme core consists of a catalytic subunit, the reverse transcriptase protein hTERT (1-3), and an RNA template subunit, hTR (4), which are essential for telomerase activity (5). Otherproteins (6,7) and kinases (8-11) are... 

Telomerase activity can also be inhibited by the direct binding of small non-nucleosidic synthetic compounds to the hTERT reverse transcriptase component of telomerase. Schnapp and coworkers have recently reported the first mixed-type noncompetitive (70) catalytic telomerase inhibitor, (2-((E)-3-naphtalen-2-yl-but-2-enoylamino)-benzoic acid) (BIBR1532), which causes telomere shortening and senescence characteristics in various types of cancer cells in vitro and in vivo in mouse xenograft models at nanomolar concentrations (71). 

Last, the hTERT reverse transcriptase inhibitors do not effect their activity by specifically and persistently binding to hTERT rather, they act as competitors for the substrate deoxyribonucleotides used by reverse transcriptases, such as hTERT, to construct DNA chains (or more specifically for hTERT, to construct telomeric DNA extensions). Small nucleoside analogues can act as reverse transcriptase inhibitors, although only some of these compounds, such as 6-thio-2 -deoxyguanosine 5 -triphosphatc (TDG-TP), are selective against the hTERT reverse transcriptase (72). TDG-TP is effective at low micromolar concentrations (72) and stops telomeric DNA extension after incorporation into the DNA (73). 

Kyo S, Takakura M, Taira T, Kanaya T, Itoh H, et al. 2000. Spl cooperates with c-Myc to activate transcription of the human telomerase reverse transcriptase gene (hTERT). Nucleic Acids Res. 28 669-77... 

Dasi F, Lledo S, Garcia-Granero E, RipoU R, Marugan M, Tormo M, et al. Real-time quantification in plasma of human telomerase reverse transcriptase (hTERT) mRNA a simple blood test to monitor disease in cancer patients. Lab Invest 2001 81 767-9. 

As often happens in these days of the human genome project, the human version hTERT) was found shortly thereafter [16, 17]. It was expressed in a variety of transformed cells but not detectable in primary cultures of human somatic cells, already giving a simple answer as to why telomerase activity was deficient in somatic cells. Thus, over a short time span, we went from having no telomerase protein to a whole family of TERTs (Telomerase Reverse Transcriptases). 

Some cells that lack telomerase activity, on the other hand, still have a high level of hTERT transcription. In these cases, regulation at the level of alternative splicing leads to skipping of exons that encode reverse transcriptase function, so any translation product would not give an active enzyme [47]. 

Trott, D.A., Newbold, R.F., and Nabholz, M. (2003). A chromosome 3-encoded repressor of the human telomerase reverse transcriptase (hTERT) gene controls the state ofhTERT chromatin. Gancer Res. 63, 689-695. 

Retroviral-mediated transduction of hTERT (human telomerase reverse transcriptase) in human dermal microvascular endothelial cells (EC) (HDMEC) results in cell lines that form microvascular structures upon subcutaneous implantation in severe combined immunodeficiency mice (SCID) mice. Anti-human type IV collagen basement membrane immii-noreactivity and visualization of enhanced green fluorescent protein (eGFP)-labeled microvessels confirmed the human origin of these vessels. Primary HDMEC-derived vessel density decreased with time, while telomerized HDMEC maintained durable vessels six... 

Figure 10.11 Schematic diagram of (a) formation of small interfering RNA (siRNA) loaded polyethylene glycol (PEG)- carboxymethyl chitosan (CMCS)/CaP hybrid anionic nanoparticles (NPPEG-CMCS/CaP/siRNA). (b) NPPEG-CMCS/CaP/siRNA for systemic delivery of human telomerase reverse transcriptase (hTERT) siRNA in anticancer therapy.

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