Telomerase activity, cell proliferation, and cancer

Telomerases

Telomerases are RNA-dependent polymerases that catalyse the synthesis of the telomeric DNA at the tips of eukaryotic chromosomes. The recent identification of the catalytic subunit of telomerases from several different species suggests that the core of the telomerase is conserved. The proposed sequence and structural homology between the telomerase catalytic subunit and reverse transcriptases, together with a wealth of genetic and biochemical information, has led to significant advances in our understanding of the mechanism by which telomerases synthesise telomeric DNA.

Three molecular determinants of malignant conversion and their potential as therapeutic targets

The past decade has been marked by an explosion of knowledge regarding the dysregulation of cancer at the molecular level. It has become apparent that oncogenes, tumor suppressor genes, and other ancillary molecules interact in complex pathways that govern cellular homeostasis. We review three molecular events that have been implicated in tumorigenesis and define pathways ripe for the development of new therapeutic approaches: 1) activation of telomerase, 2) dysregulation of the patched/sonic hedgehog pathway, and 3) mutation of the INK4 alpha-ARF locus.

Cellular senescence and cancer

The proliferative lifespan of normal mammalian cells is limited by intrinsic controls, which desensitize the cell-cycle machinery to extrinsic stimulation after a given number of cell divisions. One underlying clock driving this process of 'replicative senescence' is the progressive erosion of chromosome telomeres, which occurs with each round of DNA replication. This appears to trigger growth inhibition via activation of the tumour suppressor gene (TSG) product, p53, and the consequent up-regulation of the cell-cycle inhibitor p21WAF1. Other inhibitory pathways are also activated (possibly by additional clocks), including the TSG p16INK4a and the less well-defined complementation group genes. Loss of one pathway can be compensated, after a limited extension of lifespan, by further up-regulation of the others, so that to escape mortality a developing tumour must overcome multiple 'proliferative lifespan barriers' (PLBs) by successive genetic events, each conferring a new wave of clonal expansion. This provides one explanation for the existence of multiple genetic abnormalities in human cancers; furthermore, the diversity in the nature and timing of these PLBs between different cell types may explain the variation in the spectrum of abnormalities observed between the corresponding cancers. Even if all senescence pathways are inactivated, immortalization can only be achieved if erosion of telomeres is halted, before their end-protecting function is lost. This usually requires either activation of telomerase during tumour development, if the cell of origin is telomerase-negative, or up-regulation if the normal cell already has some activity, but not enough to prevent erosion. In either case, cancers often maintain near-critical telomere lengths; hence pharmacological inhibition of telomerase remains an attractive approach to the selective killing of tumour cells.

Telomerase expression in chickens: constitutive activity in somatic tissues and down-regulation in culture

Although human and rodent telomeres have been studied extensively, very little is known about telomere dynamics in other vertebrates. Moreover, our current dependence on mice as a model for human tumorigenesis and aging poses a problem because human and mouse telomere biology is very different. To explore whether chickens might provide a more useful model, we have examined telomerase activity and telomere length in chicken tissues as well as in primary cell cultures. Although chicken telomeres resemble human telomeres in that they are 8-20 kb in length, the distribution of telomerase activity in chickens resembles what is found in mice. Active enzyme is present in germline tissue as well as in a wide range of somatic tissues. Because chicken cells exhibit extremely low rates of spontaneous immortalization, this finding indicates that constitutive telomerase expression does not necessarily lead to an increased immortalization frequency. Finally, we found that telomerase activity is greatly down-regulated when primary cultures are established from chicken embryos. Although this down-regulation explains the telomere loss and replicative senescence that we observed in fibroblast cultures, it raises questions concerning how relevant studies of senescence in primary cell cultures are to aging in whole animals.

Telomerase activity in the differentiation of benign and malignant adrenal tumors

BACKGROUND

Telomerase is an RNA-dependent DNA polymerase that extends the ends of chromosomes by synthesizing the 6 oligonucleotide repeat TTAGGG and thus serves as a marker for cellular immortality. Although absent in most adult somatic tissues, telomerase activity is present in stem cells and is reactivated in nearly all primary human malignancies. In this study we sought to determine whether tumors of the adrenal glands contain telomerase activity and whether telomerase activity can be used to differentiate benign and malignant tumors of the adrenal glands.

METHODS

Tissue was obtained from 23 specimens at adrenalectomy. Adjacent normal adrenal tissue was obtained for control. All specimens were rapidly frozen and stored at -80 degrees C until assay. Telomerase activity was determined by the telomeric repeat amplification protocol (TRAP).

RESULTS

Telomerase activity was present in 5 of 23 (22%) of the adrenal tumors. All 3 malignant tumors were strongly TRAP positive. There was a single cortical adenoma that had very weak telomerase activity. The single TRAP-positive tumor of the adrenal medulla was a ganglioneuroma.

CONCLUSIONS

Benign adrenal tumors infrequently contain telomerase activity, whereas telomerase reactivation appears to be common in malignant tumors of the adrenal glands. These data suggest that determination of telomerase activity may offer a novel way to facilitate the differentiation of benign and malignant adrenal tumors.

Telomerase is not an epidermal stem cell marker and is downregulated by calcium

The ribonucleoprotein complex telomerase, which was found to be active in germ line, immortal, and tumor cells, and in cells from continuously renewing normal tissues such as epidermis or bone marrow, is thought to be correlated with an indefinite life span. Therefore, it has been postulated that in the normal tissues, telomerase activity may be restricted to stem cells, the possible precursors of tumor cells. Here, we demonstrate that a 56% enriched population of epidermal stem cells exhibited less telomerase activity than the more actively proliferating transit amplifying cells, which are destined to differentiate after a finite number of cell divisions. Thus telomerase is not a stem cell marker. In human epidermis we found a heterogeneous expression of the telomerase RNA component (hTR) within the basal layer, with clusters of hTR-positive cells showing variable activities. Histone-3 expressing S-phase basal cells were distributed evenly, illustrating that hTR upregulation may not strictly be correlated with proliferation. We further show for human epidermal cells that differentiation-dependent downregulation of telomerase correlates with Ca++-induced cell differentiation and that increasing the amount of Ca++ but not Mg++ or Zn++ reduced telomerase activity in a dose-dependent manner in a cell-free system (differentiation-independent). Furthermore, addition of ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid completely reversed this Ca++-induced inhibition. These data indicate that Ca++ is not only an important regulator of epidermal differentiation but also a key regulator of telomerase.

Increased telomerase activity in intrahepatic cholangiocellular carcinomas induced by N-nitrosobis(2-oxopropyl)amine in hamsters

Telomerase activities in intrahepatic cholangiocarcinomas induced by N-nitrosobis(2-oxopropyl)amine (BOP) in female hamsters were determined using a telomeric repeat amplification protocol (TRAP) assay followed by densitometric quantification. Each determination was repeated to confirm the results and telomerase activity was also detected by gel electrophoresis. An increase was evident in all of 10 cholangiocarcinomas examined, with levels ranging from 2.48 to 4.40 times the normal liver value by densitometric quantification. This finding of a consistent increase suggests that telomerase activation is involved in the development of intrahepatic cholangiocarcinomas and immortalization of cancer cells.

Developmentally programmed assembly of higher order telomerase complexes with distinct biochemical and structural properties

In Euplotes crassus, telomerase is responsible for telomere maintenance during vegetative growth and de novo telomere synthesis during macronuclear development. Here we show that telomerase in the vegetative stage of the life cycle exists as a 280-kD complex that can add telomeric repeats only onto telomeric DNA primers. Following the initiation of macronuclear development, telomerase assembles into larger complexes of 550 kD, 1600 kD, and 5 MD. In the 1600-kDa and 5-MDa complexes, telomerase is more processive than in the two smaller complexes and can add telomeres de novo onto nontelomeric 3' ends. Assembly of higher order telomerase complexes is accompanied by an extended region of RNase V1 and RNase T1 protection in the telomerase RNA subunit that is not observed with telomerase from vegetatively growing cells. The protected residues encompass a highly conserved region previously proposed to serve as a platform for formation of higher order structures. These findings provide the first direct demonstration of developmentally regulated higher order telomerase complexes with unique biochemical and structural properties.

Telomerase activity in 'immortal' fish

Eukaryotic chromosome termini consist of telomeres, short sequence repeats. According to the telomere hypothesis, DNA replication leads to telomere shortening, resulting in a cellular mitotic clock. Telomerase resets it by telomere synthesis. In mammals with a limited growth phase, telomerase activity in somatic tissues is restricted to stem cell derivatives with high proliferation potential. But other animals, like some fish, grow throughout their life with little senescence. All somatic cells require a high proliferation capacity and telomerase should be active in all cells, irrespective of fish age. Indeed, we detected high telomerase activities in all analyzed organs of rainbow trout (Oncorhynchus mykiss).

Telomerase activity in oral lichen planus

The purpose of this study was to determine the expression of telomerase in refractory oral lichen planus. Using a polymerase chain reaction-based telomerase activity assay, we investigated telomerase activity in 20 oral lichen planus specimens (erosive 9, atrophic 11). Telomerase activity was detected in 14 cases (erosive 7, atrophic 7). Furthermore, 13 cases of lichen planus with mild dysplasia proved telomerase-positive in eight specimens and six of seven cases devoid of dysplasia were also positive in the telomerase assay. The data indicate that, in general, telomerase activity might be frequently detectable in OLP. The data also suggest that telomerase activity might not be particularly associated with the premalignant phenotype in OLP.

Myc activates telomerase

Telomere maintenance has been proposed as an essential prerequisite to human tumor development. The telomerase enzyme is itself a marker for tumor cells, but the genetic alterations that activate the enzyme during neoplastic transformation have remained a mystery. Here, we show that Myc induces telomerase in both normal human mammary epithelial cells (HMECs) and normal human diploid fibroblasts. Myc increases expression of hEST2 (hTRT/TP2), the limiting subunit of telomerase, and both Myc and hEST2 can extend the life span of HMECs. The ability of Myc to activate telomerase may contribute to its ability to promote tumor formation.

Expression of TERT in early premalignant lesions and a subset of cells in normal tissues

Activation of telomerase, the enzyme that synthesizes the telomere ends of linear chromosomes, has been implicated in human cell immortalization and cancer cell pathogenesis. Enzyme activity is undetectable in most normal cells and tissues, but present in immortal cells and cancer tissues. While expression of TERC, the RNA component of telomerase, is widespread, the restricted expression pattern of TERT, the telomerase catalytic subunit gene, is correlated with telomerase activity, and its ectopic expression in telomerase-negative cells is sufficient to reconstitute telomerase activity and extend cellular lifespan. We have used in situ hybridization to study TERT expression at the single-cell level in normal tissues and in various stages of tumour progression. In normal tissues, including some that are known to be telomerase-negative, TERT mRNA was present in specific subsets of cells thought to have long-term proliferative capacity. This included mitotically inactive breast lobular epithelium in addition to some actively regenerating cells such as the stratum basale of the skin. TERT expression appeared early during tumorigenesis in vivo, beginning with early pre-invasive changes in human breast and colon tissues and increasing gradually during progression, both in the amount of TERT mRNA present within individual cells and in the number of expressing cells within a neoplastic lesion. The physiological expression of TERT within normal epithelial cells that retain proliferative potential and its presence at the earliest stages of tumorigenesis have implications for the regulation of telomerase expression and for the identification of cells that may be targets for malignant transformation.

The rate of telomere sequence loss in human leukocytes varies with age

A gradual loss of telomeric repeat sequences with aging previously has been noted in normal adult tissues, and this process has been implicated in cell senescence. No data exist that address the rate of telomere shortening in normal human cells within families or early in life. To address these questions, we measured telomere lengths in peripheral blood leukocytes (PBLs) from 75 members of 12 families and in a group of unrelated healthy children who were 5-48 months old. Here we report the surprising observation that rates of telomere attrition vary markedly at different ages. Telomeric repeats are lost rapidly (at a rate of >1 kilobase per year) from the PBLs of young children, followed by an apparent plateau between age 4 and young adulthood, and by gradual attrition later in life. These data suggest that the loss of telomeric repeats in hematopoietic cells is a dynamic process that is differentially regulated in young children and adults. Our results have implications for current models of how telomeric sequences are lost in normal somatic cells and suggest that PBLs are an excellent tissue to investigate how this process is controlled.