Telomerase Inhibitors as Anticancer Therapy Pp. 567-575
Masaharu Akiyama, Teru Hideshima, Nikhil C. Munshi, and Kenneth C. Anderson
Targeting Human Telomerase in Cancer Therapy Pp. 577-587
Sylvain
Huard and Chantal Autexier
The ‘Other’ Telomerase Inhibitors: Non-G-Quadruplex Interactive Agent, Non-Antisense, Non-Reverse Transcriptase Telomerase Inhibitors Pp. 589-603
L.A.
Beltz
Targeting Human Telomerase by Antisense Oligonucleotides and Ribozymes Pp. 605-612
M.
Folini, M. Pennati, and N. Zaffaroni
How to Inhibit Telomerase Activity for Cancer Therapy Pp. 613-626
Satoru
Kyo and Masaki Inoue
Unusual DNA Conformations: Implications for Telomeres Pp. 627-644
Martin
Mills, Laurent Lacroix, Paola B. Arimondo, Jean-Louis Leroy, Jean-Christophe
François, Horst Klump and Jean-Louis Mergny
[Back to top] Telomerase Inhibitors as Anticancer Therapy
Masaharu Akiyama, Teru
Hideshima, Nikhil C. Munshi, and
Kenneth C. Anderson
Telomerase
inhibitors have been touted as a novel cancer specific therapy, as most tumor cells
have high expression of telomerase, whereas most normal somatic cells express
low or undetectable levels of telomerase. Continued proliferation of tumor
cells requires activation of telomerase to maintain chromosomal stability and
extend life span, because telomerase elongates telomere length and rewinds the
cellular mitotic clock. Conversely, shortening of telomeres by inhibition of
telomerase activity induces growth arrest (senescence) and apoptosis in tumor
cells. Moreover, it has been reported that inhibition of telomerase increases
the susceptibility of tumor cells to apoptosis induced by anticancer agents.
Thus, telomerase inhibitors could be used as an adjuvant with conventional
therapy. However, there are also several potential limitations of telomerase
inhibition as a therapeutic strategy. For example, there is a lag phase between
telomerase inhibition and telomere shortening, with growth arrest and cell
death. In this review, we will discuss the basic biology of telomeres and
telomerase as a platform for the development of treatments based upon
inhibition of telomerase activity.
[Back to top] Targeting Human Telomerase in Cancer Therapy
Sylvain Huard and
Chantal Autexier
Telomerase
is a specialized RNA template-containing reverse transcriptase that mediates
telomere repeat synthesis at chromosome ends. The maintenance of telomere
length and integrity is essential for cell survival. Telomerase is active in
most immortal and tumor cells, whereas the majority of normal human cells
demonstrate no detectable activity and undergo telomere shortening. The
identification of a possible role for telomerase in cellular aging and cancer
has led to numerous studies designed to characterize this ribonucleoprotein
enzyme. Inhibiting telomerase activity in immortal human cells reduces cellular
proliferative capacity and can lead to cell death. Identifying mechanisms to
specifically inhibit telomerase activity in malignant cells could thus be of
great therapeutic value in the treatment of cancer. In this review, we
summarize the current understanding of the mechanism of action of human
telomerase. The biochemical characterization of telomerase is necessary for the
design and evaluation of antitelomerase therapies. Different strategies are
currently under investigation to design inhibitors that target the reverse
transcriptase and RNA components of the telomerase complex. Recent advances in
the design of these inhibitors and their properties are discussed.
[Back to top] The ‘Other’ Telomerase Inhibitors: Non-G-Quadruplex Interactive Agent, Non-Antisense, Non-Reverse Transcriptase Telomerase Inhibitors
L.A. Beltz
Human
telomeres are several kilobases of repeated (TTAGGG)n sequences at the ends of chromosomes,
a short fragment of which is lost with each cell division. This shortening
serves as a "mitotic clock" which limits the number of divisions that
a normal somatic cell can undergo. Cells undergoing continuous division need
some method of bypassing this clock. One such method is the expression of
telomerase. This ribonucleoprotein is an enzyme that rebuilds the lost portion
of the telomeres. Between 80- 95% of tumors are telomerase-positive, including
ovarian carcinoma, hepatocellular carcinoma, neuroblastoma, leukemia/lymphoma,
and cancers of the breast, prostate, lung, kidneys and bladder, as well as many
immortalized cell lines . While absent in most normal tissues, this enzyme is
expressed at higher levels in germline tissues, bone marrow, and lymphocytes.
Due
to the expression of telomerase in most tumor cells and its absence in most
normal tissues, telomerase inhibitors are being investigated as possible
anticancer agents. This review focuses on non-reverse transcriptase inhibitor,
non-oligonucleotide and non-G-quartet interactive agent telomerase inhibitors.
These agents include: differentiating agents, kinases and phosphatases, cell
cycle and apoptosis regulating agents, immunotherapeutic agents, antibiotics,
steroids, bisindole derivatives, and a variety of other compounds. These agents
hold much promise for the future treatment of malignancies.
[Back to top] Targeting Human Telomerase by Antisense Oligonucleotides and Ribozymes
M. Folini, M. Pennati,
and N. Zaffaroni
Human
telomerase is a ribonucleoprotein enzyme complex that enables cells to maintain
telomere length, allowing indefinite replicative capacity. The notion that
telomerase is reactivated in 80-90% of human cancers has led to the proposal of
telomerase as a promising therapeutic target for novel anticancer
interventions. Due to its inherent accessibility to nucleic acids, telomerase
appears an ideal target for strategies based on the use of antisense
oligonucleotides and ribozymes that target its RNA template. In this review a
summary of the different antisense- and ribozyme-based approaches used thus far
to inhibit telomerase activity in human cancer cells is provided. All these
strategies significantly inhibited the enzyme’s catalytic activity in in vitro
and in vivo tumor models. However, while in some studies tumor cell growth
arrest was observed as a consequence of telomere shortening after prolonged
telomerase inhibition, other studies have shown that antisense- and
ribozyme-based treatments targeting telomerase induced rapid loss (i.e. within
a few days) of tumor cell viability with concomitant apoptosis. In the latter
case it is unlikely that cell death was related to telomere erosion since the
cells would not have undergone enough divisions to significantly shorten their
telomeres. A possible explanation is that telomerase inhibitors may induce
apoptosis in cancer cells directly by interfering with the capping function of
the enzyme. Overall, the available results indicate antisense oligonucleotides
and ribozymes as good tools to inhibit telomerase and suggest that abrogation
of telomerase activity may affect tumor cell proliferation also through
pathways that are not dependent on telomere erosion.
[Back to top] How to Inhibit Telomerase Activity for Cancer Therapy
Satoru Kyo and Masaki
Inoue
Telomerase
is a target for anticancer research because telomerase activity is closely
correlated with malignancy. Inhibition of telomerase activity should increase
telomere shortening, which destabilizes chromosomes, thus leading to cellular
senescence and death. Extensive investigations have addressed the molecular
mechanisms of telomerase activation in cancers. Based on results from these
studies, various attempts have been made to inhibit telomerase activity using
molecular techniques in cancer cells. Antisense oligonucleotides directed to
human telomerase RNA, the dominant negative form of human telomerase reverse
transcriptase (hTERT), hammerhead ribozymes that cut hTR and agents that
interact with quadruplex DNA represent potential telomerase inhibitors. This
review includes a summary of recent attempts to inhibit telomerase activity in
cancer cells and a discussion of how these tools can be applied to cancer
therapy, especially in combination with established anti-cancer agents.
[Back to top] Unusual DNA Conformations: Implications for Telomeres
Martin
Mills, Laurent Lacroix, Paola B. Arimondo, Jean-Louis Leroy, Jean-Christophe
François, Horst Klump and Jean-Louis Mergny
DNA
is prone to structural polymorphism: its three-dimensional structure can differ
markedly from the classical double helix. Nucleic acid structures composed of more
than two strands have also been observed. The guanine-rich sequence of both the
telomere and centromere can form a quadruplex based on G-quartets while the
complementary cytosine-rich strand can fold into an intercalated tetramer
called the i-motif. The G-quartet is a gold mine for structural biologists and
the telomere has become a target for anti-cancer drug design since it was
observed that deregulation of telomerase favors proliferation of certain
tumors. Other DNA sequences may adopt unusual confor-mations.
Polypurine-polypyrimidine sequences capable of forming a triple-stranded
structure called H-DNA are found abundantly in the eukaryotic genome and may
play a significant role in DNA metabolism, transcription and replication.
Triplex-forming oligonucleotides are currently being developed as
"anti-gene" agents. Unusual DNA structures may therefore be
implicated in fundamental processes such as gene expression and
represent
unique targets for both structural-specific and sequence-specific agents. In this
review, we present work characterizing some of these unusual conformations in
terms of structure, stability and formation kinetics and discuss their
biological implications.