Mol Biol Rep (2011) 38:3339–3349
DOI 10.1007/s11033-010-0439-x
Human telomerase activity regulation
Aneta Wojtyla • Marta Gladych • Blazej Rubis
Received: 16 July 2010 / Accepted: 8 November 2010 / Published online: 18 November 2010
Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Telomerase has been recognized as a relevant
factor distinguishing cancer cells from normal cells. Thus,
it has become a very promising target for anticancer therapy. The cell proliferative potential can be limited by
replication end problem, due to telomeres shortening,
which is overcome in cancer cells by telomerase activity or
by alternative telomeres lengthening (ALT) mechanism.
However, this multisubunit enzymatic complex can be
regulated at various levels, including expression control but
also other factors contributing to the enzyme phosphorylation status, assembling or complex subunits transport.
Thus, we show that the telomerase expression targeting
cannot be the only possibility to shorten telomeres and
induce cell apoptosis. It is important especially since the
transcription expression is not always correlated with the
enzyme activity which might result in transcription modulation failure or a possibility for the gene therapy to be
overcome. This review summarizes the current state of
knowledge of numerous telomerase regulation mechanisms
that take place after telomerase subunits coding genes
transcription. Thus we show the possible mechanisms
of telomerase activity regulation which might become
attractive anticancer therapy targets.
Keywords Telomerase Telomeres Cancer
Telomerase activity regulation
A. Wojtyla M. Gladych B. Rubis (&)
Department of Clinical Chemistry and Molecular Diagnostics,
Poznan University of Medical Sciences,
Przybyszewskiego 49 St, 60-355 Poznan, Poland
e-mail: blazejr@ump.edu.pl
Introduction
Telomeres, the guanine-rich repeated sequences located at
the ends of chromosomes, function as a biological clock
limiting the cell proliferation ability with every next cell
division (Hayflick limit). However, most cancer cells
reveal a telomere length maintenance mechanism (TMM)
which is responsible for telomeres rebuilt that accompanies
cellular proliferation [1]. Majority of cancer cells demonstrate the chromosome ends renewal mechanism involving
telomerase, that utilizes its integral RNA molecule as a
template for reverse transcription of new telomeric DNA.
Other cell lines use a non-telomerase mechanism, known as
Alternative Lengthening of Telomeres (ALT) which
involves the use of a DNA template [2]. The telomere
lengthening is either undetectable or has low level of
activity in normal somatic cells while it is rather common
in the vast majority of cancers [3, 4]. Thus it provides
important target for detection and treatment of cancer cells.
Generally, the length of telomeres varies between 2 and
15 kb in germline cells [5, 6], whereas in human ALT cell
lines, telomere length varies between 2 and over 50 kb
[7, 8]. The studies across many tumor types [9] have shown
that the majority (*85%) of tumors express telomerase
and hence are able to maintain a stable and homogenous
telomere length and so avoid replicative senescence. The
remaining *15% of tumors either do not maintain telomere length or they activate ALT [7]. For example almost
100% of adenocarcinomas express telomerase [10] whereas
the ALT mechanism among the sarcomas is much more
common i.e. *47% in osteosarcomas and *34% in
astrocytomas [11], *25% in liposarcomas [12, 13]. The
reasons for the tissue difference in the utilization of telomerase or ALT is not understood but it has been proposed
that telomerase (TERT expression) may be more heavily
123
3340
repressed in tissues of mesenchymal origin [14, 15]. It is
known that ALT positive primary tumors can give rise to
telomerase-positive secondary tumors and vice versa [14].
Furthermore, there is evidence that both TMMs can be
active in the same cell line and similarly that a small
proportion of tumors express telomerase and markers for
the ALT mechanism [12, 13]. There is the interest in
targeting novel anticancer therapies at telomerase but
tumors that utilize the ALT mechanism will not respond
to such therapies. Thus quadruplex stabilizers appear to
be the most promising strategies against ALT revealing
cancers.
It was demonstrated in many studies that telomeres
renewal is a multifactorial process in mammalian cells,
involving telomerase gene expression, post-translational
protein–protein interactions and protein phosphorylation
(see Table 1). Numerous proto-oncogenes and tumor suppressor genes are also engaged in this mechanism and the
complexity of telomerase control mechanisms is studied in
the context of tumor development as well as ageing. Due to
significant role of telomerase in those processes it is of
great interest to identify the enzyme regulators. Additionally, since numerous studies reveal a correlation between
short telomere length and increased mortality, the telomerase expression/activity appears to be one of the most
crucial factors to study in order to improve the anticancer
therapy and prevention. Thus, the enzyme seems to be the
most promising target for therapy. However, because both,
telomerase expression and activity, are not always correlated, targeting complex activity seems to be of great
interest.
Mol Biol Rep (2011) 38:3339–3349
Table 1 Human telomerase regulation factors
Factor
Up(:)/down(;)
regulation
References
Phosphorylation
Akt
:
[60]
Estrogena
:
[120]
Insulin-like growth factor I (IGFI)a
:
[65, 122]
Interleukin 6 (IL-6)a
:
[66]
Interleukin 2 (IL-2)a
:
[125]
PKC (izoforms a, ß, d, e, f)
Ionizing radiationa
:
:
[66, 68, 70]
[125]
Ultraviolet irradiationa
:
[126]
dimethyl sulfoxide (DMSO)a
:
[126]
Abl
;
[55]
PP2A
;
[57]
Imatinib mesylate (Gleevec)a
;
[117]
PTENa
;
[118]
Gambogic acida
;
[62]
Retinoic acida
;
[119]
Mistletoe lectina
;
[121]
;
[67]
;
[124]
14-3-3 signaling proteins
:
[74]
NF-jBp65
Shp-2
:
:
[75]
[76]
IP6 (inositol hexaphosphate)
Oxygena
a
Transport
Nucleolin
:
[81]
H2Oa2
;
[77]
Ran (GTPase)a
;
[77]
hPinX1
;
[80]
hnRNPA1
:
[98]
TCAB1
:
[99]
POT1
:
[104]
TPP1
:
[104]
TRF1, TRF2
;
[109]
Ku
;
[117]
hRap1a
;
[113]
Complex assembly
Telomerase splicing variants
Alternative mRNA splicing is a common mechanism that
controls gene expression in higher eukaryotes and this
process is known to be tissue-, development- and
sex-specific [16]. The same phenomenon concerns TERT
expression that can be alternatively spliced and different
variants of that gene can be observed e.g. in testis and
colonic crypt, suggesting complex regulation of this gene
in the development [17–19].
TERT
As already reported, TERT splice variants may be
expressed in normal, pre-crisis, and alternative lengthening
of telomeres cells (ALT) that lack detectable telomerase
activity [17–19]. Thus, transcriptional control of TERT is
supposed to play the crucial role in the complex regulation
of telomerase activity, however alternative splicing
123
Arrows indicate activation (:) or inhibition (;) of telomerase activity
a
Indirect influence on telomerase activity
variants are also suggested to play an important role in
telomerase regulation. Ten different splice variants of
TERT have been identified so far [17, 18, 20, 21]. The most
relevant and widely studied variants involve splicing at two
main sites: the a splice site and the b site [18, 19]. The a
site deletion of 36 bp from exon 6 within reverse transcriptase (RT) motif A does not influence translation process [17, 22]. The other deletions from exons 7 and 8
(b site, 182 bp) and one insertion (38 bp) cause premature
Mol Biol Rep (2011) 38:3339–3349
translation terminations upstream of the essential RT
motifs effectively deleting the remaining three reverse
transcriptase motifs [23, 24]. Splicing at either site can
occur independently or in combination to produce different full-length variants at different proportions within
various cancer cell lines [25]. To date, only the a-/b?
variant has been shown to exhibit any regulatory function,
acting as a dominant-negative inhibitor of telomerase
activity when overexpressed in either normal or tumor
cells [25, 26]. These variants do not have catalytic
activity since the RT motifs are required for this function
[23, 24].
Another deletion variant of the TERT transcript was
identified in hepatocellular carcinoma cell lines [20]. The
deleted transcript was characterized by an in-frame deletion of 189 bp, spanning nucleotides 2710 to 2898, corresponding to the complete loss of exon 11 (gammadeletion). Interestingly, in gastric and hepatocellular carcinoma cell lines, the gamma deletion variant and its
combination variants, a- and c-, b- and c-, a-, b- and
c-deletion variants were frequently detected, while they
were not detected in colorectal carcinoma cell lines [27].
The other three insertions occur downstream of the RT
motifs and produce truncations of the protein. Since some
C-terminal modifications have been shown to interfere with
the ability of telomerase to maintain telomeres [28, 29],
these insertion variants may not produce biologically
functional proteins [25]. In the studies performed in osteosarcoma [30] it was concluded that in cell lines exhibiting
full-length TERT mRNA/protein without any splice variants telomerase activity was higher than in cell lines
expressing splice variants. It was also demonstrated that
alternative splicing might be involved in controlling the
telomerase activity in osteosarcoma cell lines, thereby
contributing to the telomere maintenance mechanism.
However, it is still unclear whether the ratio of full length
to spliced TERT is important in determining telomerase
activity [31]. In some studies it was shown that the absolute
expression of TERT was well correlated with telomerase
activity [32–35], while in other no correlation with either
relative or absolute amounts of variant transcripts was
revealed [36]. Thus, the regulatory function of various
TERT transcripts is supposed to be cell type–specific,
however, the many different methods used to quantify
TERT mRNA in multiple studies [30, 36–46] made it
difficult to standardize these findings [47]. It was also
shown that the alternative splicing could be regulated by
changes in the subcellular re-localization of splicing factors
[48]. Accordingly, it was suggested that TERT could be
regulated by TGF-b1 through alternative splicing [38].
Alternatively, TGF-b1 might induce rapid degradation of
the TERT transcripts whereas c-Myc preferentially stabilizes the b variant.
3341
Splicing variants of POT1
It was shown that human POT1 protein (protection of
telomeres, hPOT1) known to bind specifically to the G-rich
telomere strand could act as a telomerase-dependent,
positive regulator of telomere length [49]. In addition to
full-length POT1 protein (variant v1), the human POT1
gene encodes four other variants due to alternative RNA
splicing (variants v2, v3, v4, and v5), whose functions are
poorly understood [50]. Importantly, a COOH-terminally
truncated variant (v5), which consists of the NH2-terminal
oligonucleotide-binding (OB) folds and the central region
of unknown function, was found to protect telomeres and
prevent cellular senescence as efficiently as v1. However,
detailed mechanistic and functional differences between v1
and v5 were found [50] but their contribution to whole
enzyme activity remains to be elucidated.
Posttranslational regulation
Transcriptional regulation is a crucial stage affecting telomerase activity, however, it has been well documented,
that the regulation of the holoenzyme takes place during
posttranslational mechanism and plays a pivotal role in
modulating telomerase activity as well [51]. Posttranslational regulation of telomerase activity can occur via
reversible phosphorylation of TERT catalytic subunit at
specific serine/threonine or tyrosine residues [51]. Due to
multiple kinase and phosphatase activators and inhibitors
the telomerase phosphorylation status may affect its
structure, localization and enzyme activity [51]. Numerous
non-specific phosphorylation sites within TERT protein are
postulated but only a few of them appear to be the key
residues, and their phosphorylation influences telomerase
activity (both, activation and inhibition) [52].
Telomerase repression
c-Abl
Specific phosphorylation site at TERT is present at prolinerich region (308-PSTSRPPRP-316) [51]. It was revealed
that the contribution of c-Abl tyrosine kinase to TERT
phosphorylation at specific tyrosine residue led to
decreased telomerase activity. It was shown that overexpression of c-Abl inhibited cell growth by causing cell
cycle arrest [53]. Because of c-Abl’s role in stress response
to DNA damage, exposure of cells to ionizing radiation led
to a significant increase in TERT phosphorylation by c-Abl
[54]. It was also demonstrated that c-Abl phosphorylated
TERT leading to inhibition of telomerase activity and
123
3342
decrease in telomere length [54] suggesting a direct association between c-Abl and TERT. A crosstalk between BcrAbl tyrosine kinase, protein kinase C and telomerase was
also suggested as a potential reason for resistance to Glivec
in chronic myelogenous leukemia [55].
PP2A
Protein phosphatase 2A (PP2A), which is engaged in the
negative control of cell growth and division, reveals
inhibitory function on telomerase activity in human breast
cancer, PMC42 cells. As reported, PP2A remarkably
abolished telomerase activity in nucleus while this effect
was not observed when the other main cellular protein
phospatases 1 and 2B were applied [56]. When active,
PP2A dephosporylates TERT protein on Ser and/or Thr
residue [57]. Another study showed that PP2A caused
dephosphorylation of Akt kinase on Ser and/or Thr residue
and thus abolished its activatory effect on TERT. However,
it is still unclear if PP2A might directly dephosphorylate
TERT without protein–protein interaction [58]. The effect
of PP2A on TERT is blocked by its inhibitor, okadaic acid.
Thus, endogenous protein kinase(s) might again phosphorylate telomerase catalytic subunit and reactivate the
protein. It was then concluded that TERT phosphorylation
and dephosphorylation was crucial to telomerase activity
regulation in human breast cancer cells [56]. Another
studies revealed that PP2A was a direct target for simian
virus 40, since this phosphatase is inhibited by viral
oncogenic protein small-t antigen binding and consequently, the cell proliferation is stimulated. These data
suggested the mechanism of telomerase activation and
tumor genesis by oncogenic viruses [59]. Thus, pharmacological stimulation of PP2A dephosphorylation of telomerase in cancer may be of potential therapeutic
significance.
Telomerase activation
PKB
The Akt kinase (known also as protein kinase B) shows a
specificity to serine/threonine residues and enhances
human telomerase activity through TERT phosphorylation.
This explains Akt kinase’s role in protecting cell from
apoptosis and augmenting the cell proliferation capacity. It
was shown in melanoma cells, that Akt kinase carried out
this modification on serine residue at position 824 of TERT
protein [60]. Phosphorylation at two different sites is necessary to activate Akt kinase (Ser473 and Thr308) [61].
However, some studies showed that Akt phosphorylation at
serine 473 residue influenced subsequent phosphorylation
123
Mol Biol Rep (2011) 38:3339–3349
of the TERT subunit. The authors also showed that Akt
submitted to dephosphorylation at Ser473 by gambogic
acid (GA, a natural antitumor compound) caused a
decrease in TERT phosphorylation through Akt and subsequently decreased telomerase activity. Thus, Akt was
suggested to be a limiting factor for human telomerase
activity, especially since it plays a crucial role in human
telomerase activity regulation through TERT phosphorylation at PI3K/Akt/mTOR pathway-dependent posttranscriptional level [62].
In physiological conditions down-regulation of telomerase activity takes place during differentiation of CD8? T
cells since it is crucial to maintain the replicative capacity
of memory T cells. However, CD8? T cells may lose their
ability to phosphorylate Akt kinase during gradual differentiation. Telomerase down-regulation in highly differentiated CD8?CD28-CD27- T cells leads to inevitable
replicative end stage after their activation [63]. It was
shown that TERT was associated with both, Akt and the
heat shock protein HSP90 in human embryonic kidney and
endothelial cells. This association is necessary for telomerase activity by Ser473 phosphorylation of its catalytic
subunit while HSP90 prevents Akt kinase dephosphorylation (inactivation) by protein phosphatase 2A (PP2A) and
consequently decrease of telomerase activity [59]. Another
study demonstrated that novobiocin (competitive inhibitor
of HSP90) [64] inhibited formation of the Akt and Hsp90
complex and resulted in dephosphorylation and inactivation of Akt [58]. Additionally, it was shown that the proliferative and survival factors for human multiple myeloma
(MM) cells, i.e. interleukin 6 (IL-6) and insulin-like growth
factor 1 (IGF-1) up-regulated telomerase activity without
alteration of human telomerase reverse transcriptase
(TERT) protein expression [65]. As reported, that increase
of telomerase activity caused by these cytokines was
mediated by phosphatidylinositol 30 -kinase (PI3k)/Akt/
nuclear factor jB (NFjB) signaling. Thus telomerase
activity was shown to be related not only to transcriptional
regulation of TERT by NFjB but also to posttranscriptional regulation because of phosphorylation of TERT by
Akt kinase. These studies therefore demonstrated that telomerase activity was associated with cell growth, survival,
and drug resistance in MM cells.
PKC
First studies of PKC (protein kinase C) in breast cancer
cells showed that PKCa phosphorylated both, TERT and
human telomerase associated protein 1 (hTEP1) [66]. It
was shown that inositol hexaphosphate (IP6) repressed
telomerase activity via deactivation of Akt and PKCa
(Ser657) in prostate cancer cells [67]. Consequently, the
lack of TERT phosphorylation makes impossible to bind its
Mol Biol Rep (2011) 38:3339–3349
nuclear translocator and telomerase catalytic subunit is
forced to go back from nucleus to cytoplasm. Thus, it was
proposed that IP6 may also decrease the level of proteins
involved in telomerase transport to the nucleus [67]. Other
studies performed in human nasopharyngeal carcinoma
(NPC) cells showed that telomerase activity is controlled
by PKCf isoform that phosphorylates TERT [68]. This
kinase was postulated to be critical for phosphorylation of
TERT (thus telomerase activation) during T cell activation
as well. However, the participation of other PKC isoforms
has not been excluded. The same author reported that in T
lymphocytes the PKC activity was essential not only for
post-transcriptional control of telomerase activity but also
for induction of its expression through PKC-dependent
signal pathway and induction of c-Myc expression [69].
The research on head and neck cancer cells revealed that
expression of PKC isoenzymes a, b, d, e and f and TERT
phosphorylation was also correlated with higher telomerase
activity in tumor cells [70].
Transport
When posttranslational modification are brought to an end,
two main subunits of telomerase (TERT and TR) are
delivered to the place of their action, nucleus.
TERT nuclear transport
In nonactivated CD4? T cells telomerase activity is detected
only in cytoplasm. However, in activated T cells, the TERT
protein is present in both, cytoplasm and nucleus. Thus, total
cellular amount of TERT remains constant—before and after
activation, while transport of TERT from cytoplasm into
nucleus takes place during T cells activation. Consequently,
higher telomerase activity is observed in activated T cells
which leads to telomere elongation in nucleus [71]. It is not
clear yet whether TERT is transported into nucleus first,
followed by telomerase complex assembly or if the whole
telomerase complex is transferred into nucleus. Based on
certain research, a hypothesis that TERT is transported into
nucleus where telomerase complex is assembled seems to be
more likely. It is supported by the fact that the vast majority
of TR (human telomerase RNA) [72] as well as p23 and
Hsp90 (telomerase complex components) are present in
nucleus [73]. Several factors involved in TERT translocation
into the nucleus has been identified. It was demonstrated that
TERT binds to the 14-3-3 signaling proteins. This association is crucial for localization of TERT in the nucleus while it
does not affect telomerase activity. The 14-3-3 proteins act as
repressors of TERT after binding to a receptor for the nuclear
export machinery (CRM1/exportin1) [74]. Another studies
showed that TERT protein interacts directly with nuclear
3343
factor NFjB p65 in multiple myeloma cells which suggested
that this factor modulated nuclear translocation of telomerase and played a crucial role in its regulation. Tumor
necrosis factor a (TNF a) activates NFjB p65 that causes an
increase in translocation of TERT bound to NFjB p65 from
the cytoplasm to the nucleus. Moreover, this factor binds
only phosphorylated TERT [75], thus other researchers
concluded that Shp-2 (SH2-containing protein tyrosine
phosphatase) might participate in the nuclear transport of
TERT as well and suggested that Shp-2 is a negative regulator of Src mediated export. They also showed that in
nucleus the Shp-2 was associated with TERT, but the complex dissociated just before TERT export. It was revealed
that Shp-2 activity was significant for retaining TERT in the
nucleus [76]. It was also shown that H2O2 treatment induced
translocation of TERT from the nucleus into the cytoplasm.
ROS-induced phosphorylation of tyrosine 707 within TERT
is crucial for this nuclear export and Src kinase family is
responsible for this modification. Nuclear export of TERT is
a specific mechanism because it is connected with mitogenactivated protein kinase1/2 (MAPK1/2) import to the
nucleus. When the TERT molecule is phosphorylated by Src
kinase it binds with Ran (GTPase) that enables nuclear
export of TERT via CRM1-related mechanism [77]. After
TERT translocation from cytoplasm to nucleus the subsequent shuttling to the nucleolus (in normal cells) takes
place [78]. However, in cancer cells, TERT protein is distributed mainly in nucleoplasm, where substrates for telomerase are present [79]. As reported, one of the nucleolar
protein hPinX1 bound to TERT and therefore inhibited telomerase enzymatic activity [80]. Recent studies revealed
that hPinX1 also increased the TERT transport from nucleoplasm to nucleolus. It was proposed that these two functions
of hPinx1 protein are independent from each other [79]. The
other authors suggested that telomerase retaining in the
nucleolus might prevent from interaction with its substrates
present in nucleoplasm [78]. Despite binding of TERT with
hPinX1, it can associate with TR and form a telomerase
holoenzyme. This complex is stored in nucleolus where
its function is kept dormant till cell division signal [78].
Nucleolin, a nucleolus protein, forms a complex with telomerase and facilitates the export of telomerase from nucleoli
to the nucleoplasm. This process may involve masking of a
nucleolar retention signal of TERT and/or nucleolin. It is
supposed that nucleolin maintains telomerase in the nucleoplasm and therefore makes it ready for the delivery to the
telomeres [81].
TR nuclear transport
As shown, translocation of TR and TERT is regulated and
multiple nuclear structures participate in transport and
123
3344
biogenesis of telomerase [82]. Throughout most of the cell
cycle TR is present in Cajal bodies that act as its transmitters to telomeres [83]. These subnuclear structures are
general sites of RNP assembly and RNA modification [84].
In contrary to TR, TERT is located in distinct nucleoplasmic foci and thus, two main subunits of telomerase are
separated during almost the whole cell cycle. In early S
phase TERT is translocated to nucleoli. At the same time
Cajal bodies containing TR accumulate at the periphery of
nucleoli. Interestingly, TR accumulates at the pole of Cajal
body that precedes localization to telomeres in mid-S phase
when Cajal bodies deliver telomerase to individual telomeres. Furthermore, it was revealed that the same kinases
and phosphatases that act during S-phase may modify telomerase subunits [82]. However, the mechanisms involved
in targeting and accumulation of TR is not fully understood. To date, within telomerase RNA molecule the CAB
box and H/ACA motif has been identified to influence the
TR translocation to Cajal bodies and nucleoli [85, 86].
Analogically, one of TERT domain is known to mediate
nucleolar translocation [87, 88].
Telomerase complex assembly
Human telomerase assembly occurs by complex mechanism consisted of few steps, depending on energy and
involving first of all stabilization of TR and its subsequent
association with TERT protein [52, 89]. Only TERT protein and TR are necessary to gain telomerase activity in
vitro. However, in vivo telomerase complex is composed
of additional multiple proteins (see Fig. 1), that facilitate
the enzyme to act [90]. Similarly to the transport of telomerase subunit TERT to the nucleus, assembly of the telomerase enzyme complex may be regulated during cell
cycle. Telomerase assembly could take place during S
phase and it is disassembled probably during M phase.
Prevention of premature binding of the essential telomerase
subunits (TERT and TR) is possible due to different sites of
their compartmentalization and keeping them away from
their substrates (telomeres) [82]. Thus, two telomerase
assembling sites are possible during S phase i.e. at the
telomere ends (similarly to yeast) [91] or in Cajal bodies
[82]. It has been suggested that survival of motor neuron
(SMN) complex, a RNP assembly factor present in Cajal
bodies, takes part in telomerase biogenesis. It was demonstrated that TR is associated with GAR1, a protein which
interacts with SMN complex [92]. However, it requires
further analysis to establish where does the telomerase
assembly occur. Recent studies showed that the localization of TR in Cajal bodies and near telomeres depends on
TERT. This suggests that TR assembles a complex with
TERT and then both proteins are transported to telomeres.
123
Mol Biol Rep (2011) 38:3339–3349
p23
other
subunits
Hsp 90
TR
dyskerin
TERT
NHP2
TEP1
NOP10
GAR1
Fig. 1 Human telomerase complex. hTR, human telomerase RNA;
hTERT, human Telomerase Reverse Transcriptase; Hsp90, heat shock
protein 90; P23, the Hsp90-associated protein; TEP1, telomeraseassociated protein 1; dyskerin, NHP2, NOP10, GAR1, members of
the H/ACA snoRNPs (small nucleolar ribonucleoproteins)
Alternatively, TERT is supposed to indirectly influence the
trafficking of TR or a transient interaction of the two
components that contribute to TR localization [93].
Telomerase RNA goes pseudourdylation by w synthase
dyskerin. This RNA modification is essential for assembly
and stability of TR. Then, three additional H/ACA proteins
bind: Nop10, Nhp2 and Gar1. Nop10 associates with Cbf5
(dyskerin homolog in yeast) which makes a core in TR
complex. Association of other subunits (La, Staufen, L22
and hnRNP C1/C2, TEP1, p23 and Hsp90) enables stabilization and final structure formation [52, 73, 94]. However,
the roles of these proteins in telomerase action is still
unclear and further investigation is required.
ATPases and DNA helicases pontin and reptin reveal an
essential role in telomerase assembly. The amount of
TERT bound to pontin and reptin peaks in S phase [95].
When the two pivotal subunits of telomerase enzyme are
stabilized and bound with auxiliary proteins the TERT and
TR dimerization occurs. Two regions of TR are necessary
for its binding with TERT: the template region (nucleotides
44-186) and a putative double hairpin element in the 50 stem
of the H/ACA domain, where TR stabilizing H/ACA proteins bind (nucleotides 243-326) [96].
Single-molecule fluorescence two-color coincidence
detection technique (TCCD) made possible to show that
active human telomerase comprises of TERT and TR in a
1:1 stoichiometry ratio [97]. When telomerase assemblage
is finished, hnRNP A1 association with TR is required for
holoenzyme access to telomeres. hnRNP A1 can bind
simultaneously with TR and telomeric DNA and therefore
acts as a potential link between telomerase enzyme and
telomeres [98]. Recently, a novel protein TCAB1 (telomerase and Cajal body protein1) was isolated. This protein
is a component of active telomerase and interacts with
Mol Biol Rep (2011) 38:3339–3349
dyskerin. It is required for telomerase association both,
with Cajal bodies (which deliver telomerase to chromosome ends) and with telomeres. Thus TCAB1 protein
facilitates to elongate telomeres by telomerase [99].
Telomeric proteins
Beyond multiple levels of telomerase activity regulation
presented in this review, an additional one exists. Interaction of telomerase with numerous telomere binding proteins (TBP) that may influence telomerase enzyme activity.
It is supposed that binding some of them to telomeres and
therefore making it impossible for telomerase to access the
chromosome ends is an indirect way of telomerase activity
regulation [51, 99].
A number of telomere binding proteins in human were
identified [100, 101] and shown to play crucial role in
telomere protection. They allow to distinguish telomeres
from damaged DNAs by forming telomere structure with D
and T loops and therefore prevent them from degradation
and fusion. Shelterin, a very dynamic structure, is implicated in the generation of t-loops, and it controls the synthesis of telomeric DNA by telomerase. All six shelterin
subunits (TRF1, TRF2, Rap1, TIN2, TPP1, POT1) can be
found in a single complex in fractionated nuclear extracts
[102, 103]. While shelterin complex seems to be responsible for negative telomerase regulation, the POT1–TPP1
proteins may activate telomerase processivity in certain
conditions. Apparently, these two opposite functions of
POT1–TPP1 complex seem to be impossible. However, it
was proposed that POT1–TPP1 switches from inhibiting
telomerase access to the telomere, as a component of
shelterin, to serving as a processivity factor for telomerase
during telomere extension. Three-state model of telomere
length regulation was worked out [104]. At first state POT1
is directly bound at the 30 end of telomere and associates
with TPP1. This POT1–TPP1 position prevents binding of
telomerase to chromosome ends [100]. TPP1–POT1 association enhanced POT1 affinity for telomeric ssDNA and
TPP1 associates with the telomerase, providing a physical
link between telomerase and the shelterin complex [105].
According to this model, these proteins are removed from
their binding sites by an unidentified mechanism. Posttranslational modification or disruption of the shelterin
might be involved in that process. At third state, released
POT1–TPP1 complex may serve as an activator of telomerase during telomere extension. When elongated, telomere reaches a certain threshold, the newly synthesized
repeats bind shelterin complexes and the 3’ end of the
overhang is re-bound by POT1–TPP1. This causes telomerase inhibition and return to the first state of the complex
[105]. POT1 binds single stranded DNA at telomere end,
3345
but the number of RNA r(UUAGGGUUNG) sequences
matching the POT1-binding site were identified [106].
Despite the possibility to bind with these RNAs, POT1
strongly binds only with telomeric DNA. Providing binding experiments with mixed DNA–RNA oligonucleotides
and a high resolution crystal structure it was shown that a
single ribouridine (rU4) instead of a deoxythymidine (dT4)
in a telomeric sequence d(GGTTAGGGTTAG) is the primary determinant of RNA discrimination by hPOT1 [107].
However, other authors suppose that some of the POT1–
TPP1 complexes are not associated with single stranded
DNA [108].
The TTAGGG repeat binding factor 1 and 2 (TRF1 and
TRF2, respectively) are the main proteins responsible for
negative feedback control in mammals. They are bound to
double stranded DNA at T-loop, which is a ‘‘closed’’ state
of telomere in which the telomerase enzyme cannot access
and therefore extends the telomere terminus. TRF1 and
TRF2 act as a negative regulators of telomeres length
because they are involved in T-loop formation [109, 110].
TRF1 and TRF2 were shown to act in cis to repress telomere elongation. TRF1 was reported to repress the telomerase action on telomeres while, on the contrary, TRF2
appears to activate a telomeric degradation without showing any influence on telomerase [111]. Another proteins
with the negative-feedback regulation of telomere length
have been identified in human cells. The proteins acting on
TRF1 are: Tankyrase 1 and 2 (TANK 1 and 2), TIN2,
PINX1, three TRF1-interacting factors but also hRAP1
which interacts with TRF2 [111, 112]. PINX1 can inhibit
telomerase by forming a stable complexes with catalytic
subunit of telomerase and TRF1 molecule. It binds with
TERT by its telomerase inhibitory domain (TID) placed at
C terminal 74 aa [80]. The human repressor activator
protein 1 (hRap1) was identified as a protein that specifically interacts with TRF2 and negatively regulates telomere length in vivo. It was shown that in addition to TRF2,
the hRap1 forms a complex with a multiple DNA repair
proteins: Rad50, Mre11, PARP1 (poly(ADP-ribose) polymerase), and Ku86/Ku70 [113]. One of these Ku proteins
has been proposed to be a direct regulator of human telomerase. It is a heterodimer of Ku70 and Ku 80 (or Ku 86)
subunits and it is involved in DNA repair pathway. It was
shown that Ku protein associated with human telomerase
both, in vivo and in vitro [114]. It was reported that Ku
associated with TERT and this interaction might regulate
the access of telomerase to telomeres. It is possible that the
association of telomerase with Ku might trap telomerase at
the double-stranded region of telomeric DNA and this
leads to decrease of the telomerase ability to access the
exposed 30 overhang [114]. On the contrary, other authors
showed that human Ku70/80 associated with TR both, in
vitro and in vivo. It was shown in TERT deficient cell lines
123
3346
not requiring the presence of telomerase catalytic subunit,
which suggested that Ku interacted directly with TR (with
a region of 47 nucleotides in the 30 end of TR precisely).
Thus, Ku may promote telomere elongation either by
recruiting TR to chromosome ends or by stabilizing
TR/TERT complexes once they form at the ends [115].
Therefore, it has been proposed that hRap1 associated with
TRF2 may indirectly regulate telomerase by recruitment or
regulation of Ku protein [113]. Recently, a novel protein
(MOV10 helicase) binding to G-strand of both single- and
double-stranded telomeric DNA was identified. MOV10
undergoes expression in human testis and ovary and seems
to be necessary to maintain telomerase activity in those
tissues. It is proven that MOV10 associates with TERT and
telomere and therefore probably takes part in the progression of telomere lengthening [116].
Summary and conclusions
Intensive studies of telomerase functioning in human cells
gave new perspectives on the mechanism of senescence,
stem cells and cancer therapy. The studies show that
numerous enzymes are required for telomerase functioning
that facilitate new approaches for inhibiting telomerase in
treating cancer. Probably there are still numerous unrevealed proteins that contribute to regulation of such a
dynamic complex. In conclusion, TERT expression is
regulated at both, the transcriptional and post-transcriptional levels, and the alternative splicing of TERT is also
involved in the control of telomerase activity. However,
contradictive reports concern the correlation of telomere
length with telomerase activity or TERT expression in
different cells which might confirm the tissue-specificity of
the regulatory mechanism. Since telomerase plays a very
important role in telomeres maintaining and thus it is
responsible for unlimited survival of cancer cell but also
for stem cells resources it seems very important to study
the enzyme in the context of anticancer therapy but also
tissue regeneration and aging.
One of the most promising strategies against telomerase
is RNA interference and antisense nucleotide which is
already in the second and even third clinic phase study
however, multiple side effects were observed which stopped the enthusiasm about this method generally. Thus, the
study of telomerase activity regulation at the level of
enzymatic complex activity seems to give an alternative.
Anyway, it is still supposed to use both, activity and
expression regulation methods, as adjuvant therapies similarly to G-quadruplex stabilization. Understanding of telomerase activity may then bring a new insight into many
serious clinical problems that we have to face in aging
society.
123
Mol Biol Rep (2011) 38:3339–3349
Acknowledgment The present review was supported by N N401
223 534 research grant.
Conflict of interest The authors declare that there are no conflicts
of interest, financial or otherwise.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
References
1. Henson JD, Reddel RR (2010) Assaying and investigating
alternative lengthening of telomeres activity in human cells and
cancers. FEBS Lett [Epub ahead of print]
2. Bryan TM, Reddel RR (1997) Telomere dynamics and telomerase activity in in vitro immortalised human cells. Eur J Cancer
33:767–773
3. Wu KJ, Grandori C, Amacker M et al (1999) Direct activation of
TERT transcription by c-MYC. Nat Genet 21:220–224
4. Ryan KM, Birnie GD (1997) Cell-cycle progression is not
essential for c-Myc to block differentiation. Oncogene
14:2835–2843
5. Allshire RC, Dempster M, Hastie ND (1989) Human telomeres
contain at least three types of G-rich repeat distributed nonrandomly. Nucleic Acids Res 17:4611–4627
6. de Lange T, Shiue L, Myers RM, Cox DR, Naylor SL, Killery
AM, Varmus HE (1990) Structure and variability of human
chromosome ends. Mol Cell Biol 10:518–527
7. Bryan TM, Englezou A, Gupta J, Bacchetti S, Reddel RR (1995)
Telomere elongation in immortal human cells without detectable
telomerase activity. EMBO J 14:4240–4248
8. Park KH, Rha SY, Kim CH et al (1998) Telomerase activity and
telomere lengths in various cell lines: changes of telomerase
activity can be another method for chemosensitivity evaluation.
Int J Oncol 13:489–495
9. Kim NW, Piatyszek MA, Prowse KR et al (1994) Specific
association of human telomerase activity with immortal cells
and cancer. Science 266:2011–2015
10. Kammori M, Kanauchi H, Nakamura K, Kawahara M, Weber
TK, Mafune K, Kaminishi M, Takubo K (2002) Demonstration
of human telomerase reverse transcriptase in human colorectal
carcinomas by in situ hybridization. Int J Oncol 20:15–21
11. Henson JD, Hannay JA, McCarthy SW et al (2005) A robust
assay for alternative lengthening of telomeres in tumors shows
the significance of alternative lengthening of telomeres in sarcomas and astrocytomas. Clin Cancer Res 11:217–225
12. Johnson JE, Varkonyi RJ, Schwalm J, Cragle R, Klein-Szanto A,
Patchefsky A, Cukierman E, von Mehren M, Broccoli D (2005)
Multiple mechanisms of telomere maintenance exist in liposarcomas. Clin Cancer Res 11:5347–5355
13. Costa A, Daidone MG, Daprai L et al (2006) Telomere
maintenance mechanisms in liposarcomas: association with
histologic subtypes and disease progression. Cancer Res
66:8918–8924
14. Henson JD, Neumann AA, Yeager TR, Reddel RR (2002)
Alternative lengthening of telomeres in mammalian cells.
Oncogene 21:598–610
15. Serakinci N, Hoare SF, Kassem M, Atkinson SP, Keith WN
(2006) Telomerase promoter reprogramming and interaction
with general transcription factors in the human mesenchymal
stem cell. Regen Med 1:125–131
Mol Biol Rep (2011) 38:3339–3349
16. Adams AK, Holm C (1996) Specific DNA replication mutations
affect telomere length in Saccharomyces cerevisiae. Mol Cell
Biol 16:4614–4620
17. Kilian A, Bowtell DD, Abud HE et al (1997) Isolation of a
candidate human telomerase catalytic subunit gene, which
reveals complex splicing patterns in different cell types. Hum
Mol Genet 6:2011–2019
18. Ulaner GA, Hu JF, Vu TH et al (1998) Telomerase activity in
human development is regulated by human telomerase reverse
transcriptase (hTERT) transcription and by alternate splicing of
hTERT transcripts. Cancer Res 58:4168–4172
19. Ulaner GA, Hu JF, Vu TH et al (2000) Regulation of telomerase
by alternate splicing of human telomerase reverse transcriptase
(hTERT) in normal and neoplastic ovary, endometrium and
myometrium. Int J Cancer 85:330–335
20. Hisatomi H, Ohyashiki K, Ohyashiki JH et al (2003) Expression
profile of a gamma-deletion variant of the human telomerase
reverse transcriptase gene. Neoplasia 5:193–197
21. Saebøe-Larssen S, Fossberg E, Gaudernack G (2006) Characterization of novel alternative splicing sites in human telomerase
reverse transcriptase (hTERT): analysis of expression and
mutual correlation in mRNA isoforms from normal and tumour
tissues. BMC Mol Biol 7:26
22. Wick M, Zubov D, Hagen G (1999) Genomic organisation and
promoter characterization of the gene encoding the human telomerase reverse transcriptase (hTERT). Gene 232:97–106
23. Nakamura TM, Morin GB, Chapman KB et al (1997) Telomerase catalytic subunit homologs from fission yeast and human.
Science 277:911–912
24. Lingner J, Hughes TR, Shevchenko A et al (1997) Reverse
transcriptase motifs in the catalytic subunit of telomerase. Science 276:561–567
25. Yi X, White DM, Aisner DL et al (2000) An alternate splicing
variant of the human telomerase catalytic subunit inhibits telomerase activity. Neoplasia 2:433–440
26. Colgin LM, Wilkinson C, Englezou A et al (2000) The
hTERTalpha splice variant is a dominant negative inhibitor of
telomerase activity. Neoplasia 2:426–432
27. Nagao K, Katsumata K, Aizawa Y et al (2004) Differential
alternative splicing expressions of telomerase reverse transcriptase in gastrointestinal cell lines. Oncol Rep 11:127–131
28. Counter CM, Hahn WC, Wei W et al (1998) Dissociation among
in vitro telomerase activity, telomere maintenance, and cellular
immortalization. Proc Natl Acad Sci USA 95:14723–14728
29. Ouellette MM, Aisner DL, Savre-Train I et al (1999) Telomerase activity does not always imply telomere maintenance.
Biochem Biophys Res Commun 254:795–803
30. Fujiwara-Akita H, Maesawa C, Honda T et al (2005) Expression
of human telomerase reverse transcriptase splice variants is well
correlated with low telomerase activity in osteosarcoma cell
lines. Int J Oncol 26:1009–1016
31. Villa R, Porta CD, Folini M et al (2001) Possible regulation of
telomerase activity by transcription and alternative splicing of
telomerase reverse transcriptase in human melanoma. J Invest
Dermatol 116:867–873
32. Kotoula V, Hytiroglou P, Pyrpasopoulou A et al (2002)
Expression of human telomerase reverse transcriptase in
regenerative and precancerous lesions of cirrhotic livers. Liver
22:57–69
33. Shimojima M, Komine F, Hisatomi H et al (2004) Detection of
telomerase activity, telomerase RNA component, and telomerase reverse transcriptase in human hepatocellular carcinoma.
Hepatol Res 29:31–38
34. Sun PM, Wei LH, Luo MY et al (2007) The telomerase activity
and expression of hTERT gene can serve as indicators in the
3347
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
anti-cancer treatment of human ovarian cancer. Eur J Obstet
Gynecol Reprod Biol 130:249–257
Satra M, Tsougos I, Papanikolaou V et al (2006) Correlation
between radiation-induced telomerase activity and human telomerase reverse transcriptase mRNA expression in HeLa cells.
Int J Radiat Biol 82:401–409
Barclay JY, Morris A, Nwokolo CU (2005) Telomerase, hTERT
and splice variants in Barrett’s oesophagus and oesophageal
adenocarcinoma. Eur J Gastroenterol Hepatol 17:221–227
Krams M, Claviez A, Heidorn K et al (2001) Regulation of
telomerase activity by alternate splicing of human telomerase
reverse transcriptase mRNA in a subset of neuroblastomas. Am
J Pathol 159:1925–1932
Cerezo A, Kalthoff H, Schuermann M et al (2002) Dual regulation of telomerase activity through c-Myc-dependent inhibition and alternative splicing of hTERT. J Cell Sci 115:
1305–1312
Fajkus J, Borsky M, Kunická Z et al (2003) Changes in telomerase activity, expression and splicing in response to differentiation of normal and carcinoma colon cells. Anticancer Res
23:1605–1612
Fujiwara M, Kamma H, Wu W et al (2004) Expression and
alternative splicing pattern of human telomerase reverse transcriptase in human lung cancer cells. Int J Oncol 24:925–930
Liu D, O’Connor MS, Qin J, Songyang Z (2004) Telosome, a
mammalian telomere-associated complex formed by multiple
telomeric proteins. J Biol Chem 279:51338–51342
Zaffaroni N, Villa R, Pastorino U et al (2005) Lack of telomerase activity in lung carcinoids is dependent on human telomerase reverse transcriptase transcription and alternative splicing
and is associated with long telomeres. Clin Cancer Res 11:2832–
2839
Brandt S, Heller H, Schuster KD, Grote J (2005) The tamoxifeninduced suppression of telomerase activity in the human hepatoblastoma cell line HepG2: a result of post-translational regulation. J Cancer Res Clin Oncol 131:120–128
Rha SY, Jeung HC, Yang WI et al (2006) Alteration of hTERT
full-length variant expression level showed different gene
expression profiles and genomic copy number changes in breast
cancer. Oncol Rep 15:749–755
Mavrogiannou E, Strati A, Stathopoulou A et al (2007) Realtime RT-PCR quantification of human telomerase reverse
transcriptase splice variants in tumor cell lines and non-small
cell lung cancer. Clin Chem 53:53–61
Wang Y, Kowalski J, Tsai HL et al (2008) Differentiating
alternative splice variant patterns of human telomerase reverse
transcriptase in thyroid neoplasms. Thyroid 18:1055–1063
Lincz LF, Mudge LM, Scorgie FE et al (2008) Quantification of
hTERT splice variants in melanoma by SYBR green real-time
polymerase chain reaction indicates a negative regulatory role
for the beta deletion variant. Neoplasia 10:1131–1137
van Oordt W, Diaz-Meco MT, Lozano J et al (2000) The
MKK(3/6)-p38-signaling cascade alters the subcellular distribution of hnRNP A1 and modulates alternative splicing regulation. J Cell Biol 149:307–316
Colgin LM, Baran K, Baumann P et al (2003) Human POT1
facilitates telomere elongation by telomerase. Curr Biol
13:942–946
Yang Q, Zhang R, Horikawa I et al (2007) Functional diversity
of human protection of telomeres 1 isoforms in telomere protection and cellular senescence. Cancer Res 67:11677–11686
Cong YS, Wright WE, Shay JW (2002) Human telomerase and
its regulation. Microbiol Mol Biol Rev 66:407–425
Aisner DL, Wright WE, Shay JW (2002) Telomerase regulation:
not just flipping the switch. Curr Opin Genet Dev 12:80–85
123
3348
53. Sawyers CL, McLaughlin J, Goga A, Havlik M, Witte O (1994)
The nuclear tyrosine kinase c-Abl negatively regulates cell
growth. Cell 77:121–131
54. Kharbanda S, Kumar V, Dhar S et al (2000) Regulation of the
hTERT telomerase catalytic subunit by the c-Abl tyrosine
kinase. Curr Biol 10:568–575
55. Bakalova R, Ohba H, Zhelev Z, Ishikawa M, Shinohara Y, Baba
Y (2003) Cross-talk between Bcr-Abl tyrosine kinase, protein
kinase C and telomerase-a potential reason for resistance to
Glivec in chronic myelogenous leukaemia. Biochem Pharmacol
66:1879–1884
56. Li H, Zhao LL, Funder JW, Liu JP (1997) Protein phosphatase
2A inhibits nuclear telomerase activity in human breast cancer
cells. J Biol Chem 272:16729–16732
57. Avci CB, Sahin F, Gunduz C et al (2007) Protein phosphatase
2A (PP2A) has a potential role in CAPE-induced apoptosis of
CCRF-CEM cells via effecting human telomerase reverse
transcriptase activity. Hematology 12:519–525
58. Haendeler J, Hoffmann J, Rahman S et al (2003) Regulation of
telomerase activity and anti-apoptotic function by protein–protein interaction and phosphorylation. FEBS Lett 536:180–186
59. Janssens V, Goris J, Van Hoof C (2005) PP2A: the expected
tumor suppressor. Curr Opin Genet Dev 15:34–41
60. Kang SS, Kwon T, Kwon DY, Do SI (1999) Akt protein kinase
enhances human telomerase activity through phosphorylation of
telomerase reverse transcriptase subunit. J Biol Chem
274:13085–13090
61. Jacinto E, Facchinetti V, Liu D et al (2006) SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity. Cell 127:125–137
62. Zhao Q, Yang Y, Yu J et al (2008) Posttranscriptional regulation
of the telomerase hTERT by gambogic acid in human gastric
carcinoma 823 cells. Cancer Lett 262:223–231
63. Plunkett FJ, Franzese O, Finney HM et al (2007) The loss of
telomerase activity in highly differentiated CD8?CD28-CD27T cells is associated with decreased Akt (Ser473) phosphorylation. J Immunol 178:7710–7719
64. Marcu MG, Schulte TW, Neckers L (2000) Novobiocin and
related coumarins and depletion of heat shock protein 90dependent signaling proteins. J NatlCancer Inst 92:242–248
65. Akiyama M, Hideshima T, Hayashi T et al (2002) Cytokines
modulate telomerase activity in a human multiple myeloma cell
line. Cancer Res 62:3876–3882
66. Li H, Zhao L, Yang Z et al (1998) Telomerase is controlled by
protein kinase Ca in human breast cancer cells. J Biol Chem
273:33436–33442
67. Jagadeesh S, Banerjee PP (2006) Inositol hexaphosphate
represses telomerase activity and translocates TERT from the
nucleus in mouse and human prostate cancer cells via the
deactivation of Akt and PKCa. Biochem Biophys Res Commun
349:1361–1367
68. Yu CC, Lo SC, Wang TC (2001) Telomerase is regulated by
protein kinase C-f in human nasopharyngeal cancer cells. Biochem J 355:459–464
69. Sheng WY, Chien YL, Wang TC (2003) The dual role of protein
kinase C in the regulation of telomerase activity in human
lymphocytes. FEBS Lett 540:91–95
70. Chang JT, Lu YC, Chen YJ et al (2006) hTERT phosphorylation
by PKC is essential for telomerase holoprotein integrity and
enzyme activity in head neck cancer cells. Br J Cancer
94:870–878
71. Liu K, Hodes RJ, Weng NP (2001) Telomerase activation in
human T lymphocytes does not require increase in telomerase
reverse transcriptase (hTERT) protein but is associated with
hTERT phosphorylation and nuclear translocation. J Immunol
166:4826–4830
123
Mol Biol Rep (2011) 38:3339–3349
72. Hiyama E, Hiyama K, Yokoyama T, Shay JW (2001) Immunohistochemical detection of telomerase (hTERT) protein in
human cancer tissues and a subset of cells in normal tissues.
Neoplasia 3:17–26
73. Forsythe HL, Jarvis JL, Turner JW et al (2001) Stable association of Hsp90 and p23, but not Hsp70, with active human telomerase. J Biol Chem 276:15571–15574
74. Seimiya H, Sawada H, Muramatsu Y et al (2000) Involvement
of 14-3-3 proteins in nuclear localization of telomerase. EMBO
J 19:2652–2661
75. Akiyama M, Hideshima T, Hayashi T et al (2003) Nuclear
factor-jB p65 mediates tumor necrosis factor a-induced nuclear
translocation of telomerase reverse transcriptase protein. Cancer
Res 63:18–21
76. Jakob S, Schroeder P, Lukosz M et al (2008) Nuclear protein
tyrosine phosphatase Shp-2 is one important negative regulator
of nuclear export of telomerase reverse transcriptase. J Biol
Chem 283:33155–33161
77. Haendeler J, Hoffmann J, Brandes RP et al (2003) Hydrogen
peroxide triggers nuclear export of telomerase reverse transcriptase via Src kinase family-dependent phosphorylation of
tyrosine 707. Mol Cell Biol 23:4598–4610
78. Wong JM, Kusdra L, Collins K (2002) Subnuclear shuttling of
human telomerase induced by transformation and DNA damage.
Nat Cell Biol 4:731–736
79. Lin J, Jin R, Zhang B, et al (2007) Characterization of a novel
effect hPinX1 on hTERT nucleolar localization. Biochem Biophys Res Commun 353:946–952
80. Zhou XZ, Lu KP (2001) The Pin2/TRF1-interacting protein
PinX1 is a potent telomerase inhibitor. Cell 107:347–359
81. Khurts S, Masutomi K, Delgermaa L, et al (2004) Nucleolin
interacts with telomerase. J Biol Chem 279:51508–51515
82. Tomlinson RL, Ziegler TD, Supakorndej T et al (2006) Cell
cycle-regulated trafficking of human telomerase to telomeres.
Mol Biol Cell 17:955–965
83. Jády BE, Richard P, Bertrand E, Kiss T (2006) Cell cycledependent recruitment of telomerase RNA and Cajal bodies to
human telomeres. Mol Biol Cell 17:944–954
84. Cioce M, Lamond AI (2005) Cajal bodies: a long history of
discovery. Annu Rev Cell Dev Biol 21:105–131
85. Lukowiak AA, Narayanan A, Li ZH et al (2001) The snoRNA
domain of vertebrate telomerase RNA functions to localize the
RNA within the nucleus. RNA 7:1833–1844
86. Jády BE, Bertrand E, Kiss T (2004) Human telomerase RNA
and box H/ACA scaRNAs share a common Cajal body-specific
localization signal. J Cell Biol 164:647–652
87. Etheridge KT, Banik SS, Armbruster BN, et al (2002) The
nucleolar localization domain of the catalytic subunit of human
telomerase. J Biol Chem 277:24764–24770
88. Yang Y, Chen Y, Zhang C et al (2002) Nucleolar localization of
hTERT protein is associated with telomerase function. Exp Cell
Res 277:201–209
89. Collins K (2008) Physiological assembly and activity of human
telomerase complexes. Mech Ageing Dev 129:91–98
90. McEachern MJ, Krauskopf A, Blackburn EH (2000) Telomeres
and their control. Annu Rev Genet 34:331–358
91. Taggart AK, Teng SC, Zakian VA (2002) Est1p as a cell cycleregulated activator of telomere-bound telomerase. Science
297:1023–1026
92. Bachand F, Boisvert FM, Côté J et al (2002) The product of the
survival of motor neuron (SMN) gene is a human telomeraseassociated protein. Mol Biol Cell 13:3192–3202
93. Tomlinson RL, Abreu EB, Ziegler T et al (2008) Telomerase
reverse transcriptase is required for the localization of telomerase RNA to Cajal bodies and telomeres in human cancer cells.
Mol Biol Cell 19:3793–3800
Mol Biol Rep (2011) 38:3339–3349
94. Hamma T, Ferré-D’Amaré AR (2010) The box H/ACA ribonucleoprotein complex: interplay of the RNA and protein
structures in post-transcriptional RNA modification. J Biol
Chem 285:805–809
95. Venteicher AS, Meng Z, Mason PJ et al (2008) Identification of
ATPases pontin and reptin as telomerase components essential
for holoenzyme assembly. Cell 132:945–957
96. Collins K, Mitchell JR (2002) Telomerase in the human
organism. Oncogene 21:564–579
97. Alves D, Li H, Codrington R et al (2008) Single-molecule
analysis of human telomerase monomer. Nat Chem Biol
4:287–289
98. Fiset S, Chabot B (2001) hnRNP A1 may interact simultaneously with telomeric DNA and the human telomerase RNA in
vitro. Nucleic Acids Res 29:2268–2275
99. Venteicher AS, Artandi SE (2009) TCAB1: driving telomerase
to Cajal bodies. Cell Cycle 8:1329–1331
100. de Lange T (2005) Shelterin: the protein complex that shapes
and safeguards human telomeres. Genes Dev 19:2100–2110
101. Blasco MA (2007) The epigenetic regulation of mammalian
telomeres. Nat Rev Genet 8:299–309
102. Liu WJ, Zhang YW, Zhang ZX, Ding J (2004) Alternative
splicing of human telomerase reverse transcriptase may not be
involved in telomerase regulation during all-trans-retinoic acidinduced HL-60 cell differentiation. J Pharmacol Sci 96:106–114
103. Ye JZ, Donigian JR, Van Overbeek M et al (2004) TIN2 binds
TRF1 and TRF2 simultaneously and stabilizes the TRF2 complex on telomeres. J Biol Chem 279:47264–47271
104. Wang F, Podell ER, Zaug AJ et al (2007) The POT1–TPP1
telomere complex is a telomerase processivity factor. Nature
445:506–510
105. Xin H, Liu D, Wan M et al (2007) TPP1 is a homologue of
ciliate TEBP-ß and interacts with POT1 to recruit telomerase.
Nature 445:559–562
106. Luke B, Lingner J (2009) TERRA: telomeric repeat-containing
RNA. EMBO J 28:2503–2510
107. Nandakumar J, Podell ER, Cech TR (2010) How telomeric
protein POT1 avoids RNA to achieve specificity for singlestranded DNA. Proc Natl Acad Sci USA 107:651–656
108. Takai KK, Hooper S, Blackwood S et al (2010) In vivo stoichiometry of shelterin components. J Biol Chem 285:1457–1467
109. Smogorzewska A, Van Steensel B, Bianchi A et al (2000)
Control of human telomere length by TRF1 and TRF2. Mol Cell
Biol 20:1659–1668
110. Shore D, Bianchi A (2009) Telomere length regulation: coupling
DNA end processing to feedback regulation of telomerase.
EMBO J 28:2309–2322
111. Ancelin K, Brunori M, Bauwens S et al (2002) Targeting assay
to study the cis functions of human telomeric proteins: evidence
for inhibition of telomerase by TRF1and for activation of telomere degradation by TRF2. Mol Cell Biol 22:3474–3487
3349
112. Smogorzewska A, de Lange T (2004) Regulation of telomerase
by telomeric proteins. Annu Rev Biochem 73:177–208
113. O’Connor MS, Safari A, Liu D et al (2004) The human Rap1
protein complex and modulation of telomere length. J Biol
Chem 279:28585–28591
114. Chai W, Ford LP, Lenertz L et al (2002) Human Ku70/80
associates physically with telomerase through interaction with
hTERT. J Biol Chem 277:47242–47247
115. Ting NS, Yu Y, Pohorelic B et al (2005) Human Ku70/80
interacts directly with hTR, the RNA component of human
telomerase. Nucleic Acids Res 33:2090–2098
116. Nakano M, Kakiuchi Y, Shimada Y et al (2009) MOV10 as a
novel telomerase-associated protein. Biochem Biophys Res
Commun 388:328–332
117. Uziel O, Fenig E, Nordenberg J et al (2005) Imatinib mesylate
(Gleevec) downregulates telomerase activity and inhibits proliferation in telomerase-expressing cell lines. Br J Cancer
92:1881–1891
118. Zhou C, Bae-Jump VL, Whang YE et al (2006) The PTEN
tumor suppressor inhibits telomerase activity in endometrial
cancer cells by decreasing hTERT mRNA levels. Gynecol
Oncol 101:305–310
119. Kunisada M, Budiyanto A, Bito T et al (2005) Retinoic acid
suppresses telomerase activity in HSC-1 human cutaneous
squamous cell carcinoma. Br J Dermatol 152:435–443
120. Kimura A, Ohmichi M, Kawagoe J et al (2004) Induction of
hTERT expression and phosphorylation by estrogen via Akt cascade in human ovarian cancer cell lines. Oncogene 23:4505–4515
121. Choi SH, Lyu SY, Park WB (2004) Mistletoe lectin induces
apoptosis and telomerase inhibition in human A253 cancer cells
through dephosphorylation of Akt. Arch Pharm Res 27:68–76
122. Wetterau LA, Francis MJ, Ma L, Cohen P (2003) Insulin-like
growth factor I stimulates telomerase activity in prostate cancer
cells. J Clin Endocrinol Metab 88:3354–3359
123. Kawauchi K, Ihjima K, Yamada O (2005) IL-2 increases human
telomerase reverse transcriptase activity transcriptionally and
posttranslationally through phosphatidylinositol 30 -kinase/Akt,
heat shock protein 90, and mammalian target of rapamycin in
transformed NK cells. J Immunol 174:5261–5269
124. Minamino T, Mitsialis SA, Kourembanas S (2001) Hypoxia
extends the life span of vascular smooth muscle cells through
telomerase activation. Mol Cell Biol 21:3336–3342
125. Ram R, Uziel O, Eldan O et al (2009) Ionizing radiation upregulates telomerase activity in cancer cell lines by post-translational mechanism via ras/phosphatidylinositol 3-kinase/Akt
pathway. Clin Cancer Res 15:914–923
126. Alfonso-De Matte MY, Cheng JQ, Kruk PA (2001) Ultraviolet
irradiation- and dimethyl sulfoxide-induced telomerase activity
in ovarian epithelial cell lines. Exp Cell Res 267:13–27
123