Opinion

Telomerase: central regulator of all

of the hallmarks of

1 1,2

Kee Chung Low and Vinay Tergaonkar

1

Division of Cancer Genetics and Therapeutics, Laboratory of NFkB Signaling, Institute of Molecular and Cell Biology (IMCB),

A*STAR (Agency for Science, Technology and Research), Singapore 138673

2

Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore

The hallmarks of cancer described by Hanahan and extends , preventing replicative

Weinberg are properties that cancer cells must possess . Its activity is prominent in highly proliferative

for successful transformation. It is believed that each of cells such as stem cells and germ cells, as well as in 90% of

these hallmarks is independently driven. Although elon- all human [5,7]. Telomerase is composed of protein

gation of telomeres is thought to be the prime function and RNA components; the major being the reverse tran-

of reactivated telomerase reverse transcriptase, this scriptase TERT and its RNA partner TERC (telomerase

activity does not account for all its effects, such as RNA component), which provides a template for TERT;

increasing cell proliferation, resistance to , together they constitute the telomerase reverse transcrip-

and invasion. Recent studies suggest that the telome- tase activity. Besides these two components, other proteins

rase subunit telomerase reverse transcriptase (TERT) are found to be associated with the telomerase enzyme.

has novel molecular functions including transcriptional These include dyskerin (DKC), NOP10 ribonucleoprotein

regulation and metabolic reprogramming. We summa- (NOP10), GAR1 ribonucleoprotein homolog (yeast)

rize these functions and discuss how they could directly (GAR1), NHP2 ribonucleoprotein (NHP2), Reptin, and

regulate the various hallmarks of cancer. Finally, we Pontin [8,9]. Unlike in germ cells or stem cells, TERT is

suggest that therapeutics targeting noncanonical telo- not transcribed in most somatic cells and hence telomerase

merase functions may work better than those that target activity in these cells is low [10]. However, in cancer cells

its role in extension. TERT is transcriptionally reactivated by the oncogenic

transcription factors Myc [11], nuclear factor k-light-

Hallmarks of cancer and telomerase function chain-enhancer of activated B cells (NF-kB) [12], and b-

According to the Hanahan–Weinberg model [1] there are ten catenin [13]. Therefore, oncogene activation leads to telo-

properties, called the hallmarks of cancer, that cancer merase expression, which overcomes replicative senes-

cells must possess for successful oncogenesis: (i) sustaining cence. This newfound ability of cancer cells is entitled

proliferative signaling; (ii) evading growth suppressors; enabling replicative immortality in the Hanahan–Wein-

(iii) inducing ; (iv) resisting cell death; (v) acti- berg model. However, concomitant with telomerase activi-

vating invasion and ; (vi) tumor-promoting in- ty, there are several new properties that cancer cells

flammation; (vii) evading immune destruction; (viii) acquire when TERT is reactivated and these are exempli-

reprogramming energy metabolism; (ix) ; fied by the other nine hallmarks of cancer. This review

and (x) enabling replicative immortality. The hallmark discusses these noncanonical functions of telomerase,

enabling replicative immortality describes the ability to which are largely ascribed to TERT, and suggests how it

grow endlessly and is synonymous with reactivation of may directly regulate the various hallmarks of cancer. This

the telomerase reverse transcriptase. Unlike cancer cells, understanding will be instrumental in designing drugs

most somatic cells divide for a finite number of times that is targeting not just cancers but also, in principle, a whole

directly proportional to the length of their telomeres [2]. host of human ailments that are also characterized by

Telomeres are structures at the ends of chromosomes that telomerase reactivation such as atherosclerosis [14], dys-

are composed of tandem repeats of a TTAGGG sequence. keratosis congenita [15], kidney dysfunction [16], and in-

Telomeric repeats are occupied by proteins collectively flammation [17].

termed the shelterin complex, which maintains telomere

integrity [3,4]. Somatic cells stop division when their telo- First evidence for telomere independent roles of

meres are critically short, which is known as replicative telomerase components

senescence [2] and is characterized by the Hayflick limit, or Some of the earliest evidence for telomere-independent

number of times division will occur before cell division stops roles of telomerase came from mouse studies. Mice have

[5]. Replicative senescence occurs because, during succes- very long telomeres (20–50 kb) compared to humans

sive cell divisions, the ends of chromosomes are lost [6]. (5–10 kb), therefore, there is no need for telomerase upre-

gulation in murine tumors to prevent telomere erosion

Corresponding author: Tergaonkar, V. ([email protected]).

Keywords: signaling; cancer; hTERT; telomere; apoptosis. and replicative senescence. However, it has been found

that TERT is upregulated in breast [18] and skin [19]

0968-0004/$ – see front matter

ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tibs.2013.07.001 cancers in mice, suggesting that TERT could play other

426 Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

roles besides maintaining telomere length. Supporting Xenopus laevis embryos, showing that TERT exerts its

this idea, constitutive expression of TERT has been found noncanonical roles beyond cancer cells.

to increase proliferation of mammary carcinomas [20] and TERT can also regulate NF-kB-dependent transcrip-

epidermal tumors [21] in mice. Additionally, conditional tion [17]. It has been found that TERT binds the p65

induction of TERT in the mouse skin epithelium induces subunit of NF-kB and is recruited to and activates

proliferation of hair follicle cells [22]. Overexpression of NF-kB-regulated promoters such as those of interleukin

TERT in mouse thymocytes (T cells) also leads to T cell (IL)-6, tumor necrosis factor (TNF)a and IL-8, which

lymphomas without affecting telomere length, supporting is critical for inflammation and cancer progression

a telomere-independent role for telomerase in mouse tu- (Figure 1). Interestingly, NF-kB and b-catenin are known

morigenesis across multiple tissue types [23]. activators of TERT expression [12,13]; and because TERT in

Other evidence from human cells suggests that length- turns regulates their transcriptional activities, a feed-

ening of telomeres may not be the only function of TERT forward loop therefore exists, in which the effects of these

that is necessary for transformation. For instance, the major oncogenes are amplified in a cell-autonomous manner

Weinberg laboratory [24] reported that human cells that (Figure 2). We propose that the tumor-promoting effects of

could utilize a TERT-independent, alternative lengthen- TERT might therefore be a consequence of coactivation of

ing of telomeres (ALT) [25] pathway for maintaining NF-kB and b-catenin pathways, which are master regula-

telomere length were not transformed by expression of tors of many oncogenic targets that can directly or indirectly

oncogenic H-Ras [24]. However, coexpression of H-Ras and drive various hallmarks of cancer. The NF-kB and b-catenin

a catalytically active or inactive TERT could successfully pathways can also cross-talk with other oncogenic pathways

transform cells. If maintaining telomere length was the such as Notch [32] and signal transducer and activator of

sole purpose of telomerase, then ALT should have fully transcription (STAT)3 [33] pathways, respectively, there-

substituted for this function. Further supporting a telo- fore, the effects of TERT on NF-kB and b-catenin pathways

mere-independent function of TERT, the Blackburn labo- may potentially have multiple indirect oncogenic effects.

ratory showed that siRNA-mediated loss of TERT, which The TERT-dependent effects described are related to the

leads to loss of telomerase activity, causes very rapid various hallmarks of cancer and we explore them further in

changes in cell proliferation and growth in human cancer the following sections.

cell lines, without any measureable change in telomere

length [26]. These results suggest that the physical pres- Telomerase and the hallmarks of cancer

ence of telomerase components rather than telomerase Regulation of and proliferation

activity also contributes to transformation. Multiple other Expression of TERT in some human and murine cell types

properties of telomerase or its components have been can cause rapid cell proliferation, which occurs without

described. For instance, TERT has been found to associate measurable changes in telomere elongation [22,30,34,35].

with the RNA component of mitochondrial RNA proces- It has been found that human fibroblasts immortalized by

sing endoribonuclease (RMRP) and function as a RNA- overexpression of TERT secrete epiregulin, a

dependent RNA polymerase [27]. DKC, another telome- belonging to the epidermal growth factor (EGF) family.

rase component, has a role in p53 biogenesis and ribosome Importantly, epiregulin neutralization reduces growth of

function [28]. Besides telomerase holoenzyme assembly, these cells [36]. Although it was not directly tested, NF-kB

Reptin and Pontin have functions in remodeling chroma- target genes such as IL-1b and TNF-a have been shown to

tin and DNA damage repair [29]. However, the molecular regulate epiregulin expression, therefore, epiregulin induc-

mechanisms by which these noncanonical activities of tion by TERT might be a consequence of NF-kB activation

telomerase components contribute to oncogenesis are [37].

largely unknown. The various pieces of evidence suggest Activation of EGFR signaling might also be a mecha-

that telomerase components and TERT in particular nism through which TERT stimulates proliferation. Spe-

might have other functions unrelated to their canonical cifically, Smith et al. [34] found that ectopic expression of

role in telomere elongation, which are important in onco- TERT induced proliferation in human mammary epithelial

genesis. We shall explore these functions in the following cells (HMECs) through increased EGFR signaling. Inter-

sections. estingly, there is some evidence that the ability to elongate

telomeres is not required for induction of proliferation by

Telomerase and transcriptional regulation TERT. For instance, telomerase induction can cause pro-

One of the noncanonical roles of TERT is in regulation of liferation of hair follicle cells in mice and this does not

transcription. Artandi and colleagues reported that telo- require TERC/telomerase activity [22]. Supporting and

merase could regulate the transcription of genes involved expanding on these data, Choi et al. [30] more recently

in cell proliferation, , and resistance to apopto- found that a catalytically inactive TERT could promote

sis [30]. Park et al. [31] then uncovered a mechanistic epithelial proliferation in hair follicle and skin cells

link between TERT and transcription, reporting that through the Wnt and Myc signaling pathways. Related

TERT associates with the chromatin remodeler, brahma to these studies, Hrdlicˇkova´ et al. [35] found that alterna-

related gene (BRG)1, and that this complex activates tively spliced variants of TERT that cannot elongate tel-

promoters of Wnt/b-catenin target genes such as Myc omeres, but are able to stimulate Wnt signaling, can

and CyclinD1 (Figure 1). Furthermore, TERT regulation increase human cell proliferation. Overall, these studies

of Wnt signaling was also found to be important in show that TERT can regulate cell growth and proliferation,

the development of the anterior–posterior axis in independent of its role on telomeres.

427

Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9 (A) (B) Ligand Wnt

Receptor (TLRs, TNFR, etc) LRP5/6

Frizzled NEMO NEMO IKK complex

Rap1 Disheveled IKKα IKKβ ELKS

Proteasomal degradaon of lκB IκB β-catenin p50 p65 GSK-3 Axin APC

TERT β-catenin BRG1 TERT TCF

Wnt/β-catenin target genes TERT

p65 p50

NF-κB target genes Nucleus

TiBS

Figure 1. TERT and transcriptional regulation. (A) TERT cooperates with nuclear factor (NF)-kB to regulate its target genes. In canonical NF-kB signaling, a ligand (e.g.,

lipopolysaccharide or TNF-a) binds to its cognate receptor which, though a series of steps, leads to activation of the IKK complex. The IKK complex consists of the IKK1/2

kinases, NEMO the chaperone, and adapters such as ELKS and Rap1. Activated IKK phosphorylates IkB, leading to its degradation by the proteasome, which allows the NF-

kB dimer (p65–p50) to enter the nucleus and activate transcription of its target genes. TERT has been shown to bind p65 subunit of NF-kB in the nucleus and enhance the

transcription of its target genes. (B) TERT cooperates with Wnt to regulate its target genes. In the canonical Wnt signaling, Wnt binds Frizzled and LRP5/6, which activates

Disheveled. Disheveled inhibits a complex containing GSK-3, Axin and APC, allowing b-catenin to be able to enter the nucleus and activate the Wnt target genes. TERT has

been shown to bind BRG1, a chromatin remodeler and enhance the transcription of Wnt target genes. Abbreviations: APC, Adenomatous polyposis coli; BRG1, brahma

related gene 1; ELKS, a protein rich in protein rich in glutamate (E), leucine (L), lysine (K), and serine (S); GSK-3, glycogen synthase kinase 3; IkB, inhibitor of kB; IKK,

inhibitor of NF-kB; LRP, low-density lipoprotein receptor-related protein; NEMO, NF-kB essential modulator; NF-kB, nuclear factor k-light-chain-enhancer of activated B

cells; Rap1, repressor-activator protein 1; TERT, telomerase reverse transcriptase; TNF-a, tumor necrosis factor-a.

Resistance to antigrowth signals TERT might help cancer cells bypass antigrowth signals

Transforming growth factor (TGF)-b is a cytokine that through a general mechanism [34].

inhibits the growth of nonmalignant epithelial cells and TERT might also confer resistance to antigrowth signals

hematopoietic precursor cells, but many malignant cells by causing cells to bypass differentiation, another strategy

become refractory to its antigrowth effects [38]. Ectopic that cancer cells utilize to continue to proliferate [41].

expression of TERT in HMECs and mouse embryonic Oncogenic Myc and Wnt/b-catenin signaling, which TERT

fibroblasts (MEFs) antagonizes TGF-b-dependent growth can positively regulate, can bypass differentiation [42,43].

suppression [39,40]. Furthermore, ectopic expression of Indeed, telomerase-mediated activation of Wnt signaling

TERT in HMECs also downregulates the expression of [31] was shown to suppress differentiation and induce

antigrowth factors such as IL-1 receptor (IL-1R) antagonist proliferation. Furthermore, b-catenin could transcription-

and parathyroid hormone-related peptide, suggesting that ally upregulate Myc expression by binding to b-catenin

428

Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

NF-κB β-catenin

Ponn NHP2 NOP10 Repn GAR1 DKC TERT TERC

GAR1 Telomerase Repn NOP10 holoenzyme DKC Ponn NHP2

TERT TERC

β-catenin

VEGF (1) Proliferaon Telomere NF-κB lengthening (2) Resistance to an-growth Myc signals (3) Invasion and metastasis

Angiogenesis (1) Proliferaon (2) Resistance to apoptosis (1) Reprogramming metabolism (3) Invasion and metastasis (2) Proliferaon (4) Inflammaon and immune survelliance escape

Canonical funcon Noncanonical funcon

TiBS

Figure 2. Feed forward loop of TERT expression. NF-kB and b-catenin regulate TERT expression. Reconstitution of TERT along with other telomerase components

reconstitutes the telomerase reverse transcriptase activity but this complex, or members of this complex, also show other noncanonical activities as depicted. TERT in turn

regulates the expression of NF-kB and b-catenin target genes to maintain itself, forming a feed forward loop. Other oncogenes of these pathways are induced as a

byproduct of this feed forward loop. Key: unbroken red arrow: activation; unbroken blue arrow: complex formation; unbroken black arrow: induction of hallmark of cancer;

broken black arrow: feedback loop to induce TERT. Abbreviations: DKC, dyskerin; GAR1, GAR 1 ribonucleoprotein homolog (yeast); NF-kB, nuclear factor k-light-chain-

enhancer of activated B cells; NHP2, NHP2 ribonucleoprotein; NOP10, NOP10 ribonucleoprotein; TERT, telomerase reverse transcriptase; TERC, telomerase RNA

component; VEGF, vascular endothelial growth factor.

binding sites in the Myc promoter, potentially amplifying show reduced formation of microvessels [47,48]. This effect

TERT-dependent suppression of differentiation [44]. Impor- of TERT, which is independent of its catalytic activity,

tantly, overexpression of an enzymatically inactive TERT could be due to direct activation of VEGF transcription

was shown to inhibit differentiation and induce proliferation [49]. Furthermore, VEGF and acidic/basic fibroblast

of glioma cells through upregulation of EGFR and basic growth factor (a/bFGF) have been shown to activate

fibroblast growth factor (bFGF), supporting the hypothesis the TERT promoter via the phosphoinositide 3-kinase

that this is a telomere-independent effect of TERT [45]. (PI3K/Akt), nitric oxide [50], and extracellular signal-

Taken together, TERT seems to favor tumor growth by regulated kinase (Erk)1/2 pathways [51]. Taken together,

inhibiting antigrowth signals and suppressing differentia- these studies suggest an autocrine feedback loop by which

tion, perhaps independent of its role on telomeres. TERT promotes angiogenesis by inducing the production of

angiogenic factors such as VEGF and a/bFGF, which in

Angiogenesis turn can upregulate the transcription of TERT and rein-

TERT has been reported to modulate angiogenesis as well. force angiogenic factor expression.

TERT knockdown reduces, whereas TERT upregulation

enhances, the formation of microvessels in a mouse model Resistance to apoptosis

of angiogenesis [46]. Compared to controls, xenografted Cancer cells with reactivated TERT gain resistance to

glioblastoma cells transfected with siRNA against TERT death-inducing stimuli through activation of NF-kB,

show reduced production of proangiogenic factors such as which is a master regulator of survival genes [17,52].

vascular endothelial growth factor (VEGF) and bFGF, and Furthermore, TERT can localize to mitochondria [53]

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Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

and inhibit caspase-mediated apoptosis in cancer cells [54] player in reprogramming energy metabolism in cancer

independent of its function in telomere elongation [55]. It cells [77,78]. In fact, it was found that the TERT transcrip-

has also been suggested that mitochondrial TERT tional response is highly similar to that of Wnt and Myc in

can increase intracellular reduced glutathione/oxidized follicular cells [30]. Lastly, it was found that NF-kB can

glutathione ratios, thus increasing the cellular antioxidant upregulate glucose uptake via Glut-3 and increase the rate

buffering capacity and reducing intracellular reactive of aerobic glycolysis [79]. TERT can regulate NF-kB ex-

oxygen species (ROS)-induced apoptosis [56]. These dispa- pression, therefore, this could possibly link TERT to gly-

rate studies suggest that TERT can inhibit apoptosis colysis via the NF-kB pathway.

through various mechanisms, while activating cell prolif-

eration and inhibiting antigrowth signals to have a net Genome instability

progrowth effect. Although TERT enhances the other nine hallmarks of

cancer, which are all protumorigenic, TERT suppresses

Invasion and metastasis the hallmark of genome instability and reduces ,

Liu et al. [57] have shown that TERT overexpression which can be viewed as both a tumor-suppressive effect as

promotes the epithelial–mesenchymal transition (EMT), well as an oncogenic effect. TERT promotes genome

whereas its inhibition decreases it. This is achieved stability through multiple means. First, the canonical

through TERT-mediated Wnt/b-catenin signaling, which function of telomerase is to maintain telomere ends,

then upregulates Snail-1 (snail family zinc finger 1) and without which telomere erosion ensues. This then leads

vimentin; important EMT markers. This study shows that to unstable chromosomes, which increases the rate of

TERT binds to b-catenin directly and localizes to vimentin deletions or amplifications of chromosomal segments

promoters, reiterating its role as a transcriptional coacti- [80]. Second, it has been found that TERT can regulate

vator. Other indirect evidence for TERT involvement in DNA damage signaling through ataxia telangiectasia

invasion and metastasis is through its enhancement of NF- mutated (ATM), breast cancer 1 (BRCA1), and g-H2A

kB signaling. NF-kB has been shown to be critical for EMT histone family, member X (g-H2AX) [81]; thus, knock-

[58] and it also upregulates matrix metalloproteinase down of TERT alters histone modifications and decreases

(MMP)-9, which is important for metastasis [59]. Taken heterochromatin, stifling the DNA damage response and

together these results suggest that TERT could play a role resulting in genome instability. Third, TERT localization

in invasion and metastasis as well. to the mitochondria can protect against DNA damage in

the nucleus by decreasing mitochondrial ROS production,

Inflammation and immune surveillance

Key inflammatory cytokines such as IL-6, IL-8, and TNF-a

have been shown to be direct targets of TERT [17]. Inflam-

mation can upregulate growth factors, survival factors, Resistance to

apopto

promote angiogenesis, and induce EMT [1]. IL-6 has been Angiogenesis sis

shown to be important in promoting various aspects of

ity Inflammaon

cancer progression [60–62] and in the development of che-

moresistant niches [63]. IL-8 and TNF-a have also been

ave immortal

implicated in proliferation, invasion, metastasis, and c VEGF NF-κB

Repli

inhibition of apoptosis in cervical [64] as well as in breast

surve illance

Immune

[65,66] cancers. These and other cytokines thus link inflam- escape

mation to various hallmarks of cancer [67–69]. However,

Telomerase

chronic inflammation can also lead to the formation of an

immunosuppressive environment [70–72]. In fact, IL-6 is a

β

prolif

key cytokine in the chronic inflammatory environment that TGF- β-catenin cell Substained

Genome stability Genome

can cause immunosuppressive effects [73]. Importantly, as eraon

mentioned earlier, these cytokines activate NF-kB, which is

both a positive regulator of TERT and is positively regulated

nals Myc

Invasion and metastasis and Invasion

by TERT, suggesting multiple TERT-dependent mecha-

owth sig owth

r

nisms may reinforce inflammatory signaling [17,74,75]. g

Resistance to and to Resistance

Based on this evidence, TERT might directly and indirectly

metabolism

regulate chronic inflammation and immunosuppression. energy Reprogammed

Reprogramming energy metabolism TiBS

Several genes in the glycolytic pathway were shown to be

Figure 3. Regulation of the hallmarks of cancer by telomerase. TERT regulates the

downregulated by TERT knockdown [76]. There was a various hallmarks of cancer through its enhancement of transcription of VEGF, NF-

kB and b-catenin target genes. b-Catenin also activates Myc. TERT also suppresses

corresponding decrease in glucose consumption and lactate

TGF-b-dependent growth suppression. Telomerase also antagonizes or

production, which are characteristics of the Warburg ef-

suppresses genome instability, in contrast to the other effects, and that this

fect, upon TERT knockdown, showing that TERT can might be through a transcription-independent mechanism (please see text for

details). Arrows not drawn to scale. NF-kB, nuclear factor k-light-chain-enhancer of

directly regulate metabolism in cancer cells. TERT might

activated B cells; TGF-b, transforming growth factor-b; VEGF, vascular endothelial

also regulate energy metabolism via b-catenin, which in

growth factor. ( : Direct activation : Indirect activation. : Inhibition.

turn regulates Myc [44]. Myc is well known to be a major : Two-way activation. : Canonical telomerase activity.)

430

Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

which is one of the telomere-independent functions of reactivation of TERT. For this we would need to delineate

TERT [53]. Thus, telomerase can act as a tumor suppres- which noncanonical activities of telomerase require which

sor by ensuring genome stability and inhibiting muta- components of the telomerase complex besides TERT. Such

tions. a molecular dissection will require precise genetic and

However, maintaining genome stability can be onco- biochemical assays and the understanding that TERT

genic as well. This is because telomere shortening due to reactivation not only induces telomerase activity, but

TERT inhibition leads to apoptosis [82] through a p53- can also regulate other signaling cascades that directly

dependent pathway [83]. Thus, maintaining genome or indirectly regulate all of the hallmarks of cancer.

stability can be viewed as an equilibrium where too little An important strategy to develop effective therapeutics

mutation inhibits tumor progression, whereas too much would be to test the drugability of the interactions between

mutation causes genome catastrophe, leading to cell TERT and its partner transcription and growth factors

cycle arrest or cell death. Figure 3 summarizes the vari- because TERT is not the only crucial regulator controlling

ous potential mechanisms by which the telomerase com- these hallmarks. A putative drug that blocks such an

ponent TERT in particular could regulate the hallmarks interaction would in principle simultaneously target many

of cancer. It is not known if TERT acts alone in these of the hallmarks of cancer. Furthermore, because such a

functions or if the telomerase holoenzyme could partici- drug will only block sustained oncogenic signaling in can-

pate in some of these functions assayed for and ascribed cer cells, and not normal signaling in noncancerous cells,

to TERT. such a drug will be preferable to those that simply block

these master oncogenic regulators that have been unsuc-

Therapeutic targeting of telomerase cessful due to obvious toxicity issues. Numerous chemical

The preceding sections have summarized how telomerase inhibitors of telomere extension have been developed, such

regulates the various hallmarks of cancer; that is, mainly as the telomerase inhibitors MST-312 [84] and BIBR1532,

through TERT acting as a cofactor to regulate NF-kB a non-nucleoside analog [85]: G-quadruplex stabilizers

0

and b-catenin signaling pathways. These findings have such as b-rubromycin [86], PIPER {N,N -bis [2-(1-piperidi-

therapeutic implications for the design of inhibitors that no)ethyl]-3,4,9,10-tetracarboxylic diimide} [87], and

can target human ailments such as cancers, which show TmPyP4 [88]; and general reverse transcriptase inhibitors

(A) Inhibitor Mode of acon Consequences

Convenonal telomerase inhibitors Elongaon of telomeres

Telomere (1) Inhibion of telomerase catalyc acvity shortening (2) Inhibit binding of telomerase to telomere Telomerase

Non-canonical roles

(B) (1) MiRNA (2) Knockdown of tert (3) Non-convenonal telomerase Elongaon of inhibitors telomeres

(1) Reducon in TERT levels (1) Telomere shortening (2) Dissociaon of telomerase components (long term) Telomerase (3) Inhibion of TERT binding to (2) Inhibion of other transcripon factors, DNA hallmarks of cancer (4) Structural changes to complex members (short term)

Non-canonical roles

TiBS

Figure 4. Therapeutics against telomerase. (A) Conventional telomerase inhibitors inhibit the canonical roles of telomerase by negating its catalytic activity or inhibiting its

binding to telomere ends, leading to shortening of telomeres, but do not inhibit its noncanonical roles. (B) Cell proliferation, metastasis, and telomere lengthening might be

better controlled through the use of miRNAs that downregulate TERT concurrently with other oncogenes, nonconventional telomerase inhibitors that affect its

noncanonical roles, and/or drugs that knockdown telomerase levels. These inhibitors downregulate TERT levels, dissociate telomerase complex or inhibit its interaction with transcription factors.

431

Opinion Trends in Biochemical Sciences September 2013, Vol. 38, No. 9

such as AZT (azidothymidine), which acts on the reverse pose a challenge, such inhibitors will inevitably possess

transcriptase activity of TERT [89]. Another way to inhibit fewer toxic side effects compared to current telomerase

telomerase function is to reduce levels of TERT transcripts. inhibitors.

In fact, one such telomerase inhibitor, imetelstat

(GRN163L), demonstrated activity in various cancers Concluding remarks

and has entered Phase I and II clinical trials [90]. It also In this review, we have summarized the existing literature

depleted cancer stem cells in breast and pancreatic cancer that suggests how telomerase in general and its TERT

cell lines without affecting telomere length, suggesting subunit in particular can regulate the various hallmarks

that imetelstat can target noncanonical functions of telo- of cancer. We suggest that co-regulation of NF-kB and

merase [91]. Supporting this strategy, siRNA against b-catenin signaling pathways by TERT could be the key

TERT can induce apoptosis and cause growth inhibition, mechanistic basis