Progressive increase in telomerase activity from benign melanocytic conditions to malignant melanoma

Skin Cancer Microenvironment: What We Can Learn from Skin Aging?

Aging is a natural intrinsic process associated with the loss of fibrous tissue, a slower cell turnover, and a reduction in immune system competence. In the skin, the continuous exposition of environmental factors superimposes extrinsic damage, mainly due to ultraviolet radiation causing photoaging. Although not usually considered a pathogenic event, photoaging affects cutaneous biology, increasing the risk of skin carcinogenesis. At the cellular level, aging is typified by the rise of senescence cells a condition characterized by reduced or absent capacity to proliferate and aberrant hyper-secretory activity. Senescence has a double-edged sword in cancer biology given that senescence prevents the uncontrolled proliferation of damaged cells and favors their clearance by paracrine secretion. Nevertheless, the cumulative insults and the poor clearance of injured cells in the elderly increase cancer incidence. However, there are not conclusive data proving that aged skin represents a permissive milieu for tumor onset. On the other hand, tumor cells are capable of activating resident fibroblasts onto a pro-tumorigenic phenotype resembling those of senescent fibroblasts suggesting that aged fibroblasts might facilitate cancer progression. This review discusses changes that occur during aging that can prime neoplasm or increase the aggressiveness of melanoma and non-melanoma skin cancer.

How DNA damage and non-canonical nucleotides alter the telomerase catalytic cycle

Telomeres at the ends of linear chromosomes are essential for genome maintenance and sustained cellular proliferation, but shorten with each cell division. Telomerase, a specialized reverse transcriptase with its own integral RNA template, compensates for this by lengthening the telomeric 3' single strand overhang. Mammalian telomerase has the unique ability to processively synthesize multiple GGTTAG repeats, by translocating along its product and reiteratively copying the RNA template, termed repeat addition processivity (RAP). This unusual form of processivity is distinct from the nucleotide addition processivity (NAP) shared by all other DNA polymerases. In this review, we focus on the minimally active human telomerase catalytic core consisting of the telomerase reverse transcriptase (TERT) and the integral RNA (TR), which catalyzes DNA synthesis. We review the mechanisms by which oxidatively damaged nucleotides, and anti-viral and anti-cancer nucleotide drugs affect the telomerase catalytic cycle. Finally, we offer perspective on how we can leverage telomerase's unique properties, and advancements in understanding of telomerase catalytic mechanism, to selectively manipulate telomerase activity with therapeutics, particularly in cancer treatment.

ETS1, nucleolar and non-nucleolar TERT expression in nevus to melanoma progression

TERT (telomerase reverse transcriptase) is the catalytic component of telomerase. TERT shows little expression in normal somatic cells but is commonly re-expressed in cancers, facilitating immortalization. Recently-discovered TERT promoter mutations create binding sites for ETS-family transcription factors to upregulate TERT. ETS1 is reported to be important for TERT upregulation in melanoma. However it is unclear when in melanoma progression TERT and ETS1 proteins are expressed. To elucidate this question, ETS1 and TERT immunohistochemistry were performed on a panel of benign (n=27) and dysplastic nevi (n=34), radial growth phase (n=29), vertical growth phase (n=25) and metastatic melanomas (n=27). Lesions were scored by percentage of positive cells. ETS1 was readily detectable in all lesions, but not in normal melanocytes. TERT was located in either the nucleolus, the nucleoplasm (non-nucleolar) or both. Non-nucleolar TERT increased in prevalence with progression, from 19% of benign nevi to 78% of metastases. It did not however correlate with cell proliferation (Ki-67 immunostaining), nor differ significantly in prevalence between primary melanomas with or without a TERT promoter mutation. These results demonstrate that ETS1 is expressed very early in melanoma progression, and interestingly only non-nucleolar TERT correlates clearly in prevalence with melanoma progression. It can be acquired at various stages and by mechanisms other than promoter mutations.

Melanocytic nevi and melanoma: unraveling a complex relationship

Approximately 33% of melanomas are derived directly from benign, melanocytic nevi. Despite this, the vast majority of melanocytic nevi, which typically form as a result of BRAF^V600E-activating mutations, will never progress to melanoma. Herein, we synthesize basic scientific insights and data from mouse models with common observations from clinical practice to comprehensively review melanocytic nevus biology. In particular, we focus on the mechanisms by which growth arrest is established after BRAF^V600E mutation. Means by which growth arrest can be overcome and how melanocytic nevi relate to melanoma are also considered. Finally, we present a new conceptual paradigm for understanding the growth arrest of melanocytic nevi in vivo termed stable clonal expansion. This review builds upon the canonical hypothesis of oncogene-induced senescence in growth arrest and tumor suppression in melanocytic nevi and melanoma.

Melanoma

Melanoma is a common cancer in the Western world with an increasing incidence. Sun exposure is still considered to be the major risk factor for melanoma. The prognosis of patients with malignant (advanced-stage) melanoma differs widely between countries, but public campaigns advocating early detection have led to significant reductions in mortality rates. As well as sun exposure, distinct genetic alterations have been identified as associated with melanoma. For example, families with melanoma who have germline mutations in CDKN2A are well known, whereas the vast majority of sporadic melanomas have mutations in the mitogen-activated protein kinase cascade, which is the pathway with the highest oncogenic and therapeutic relevance for this disease. BRAF and NRAS mutations are typically found in cutaneous melanomas, whereas KIT mutations are predominantly observed in mucosal and acral melanomas. GNAQ and GNA11 mutations prevail in uveal melanomas. Additionally, the PI3K-AKT-PTEN pathway and the immune checkpoint pathways are important. The finding that programmed cell death protein 1 ligand 1 (PDL1) and PDL2 are expressed by melanoma cells, T cells, B cells and natural killer cells led to the recent development of programmed cell death protein 1 (PD1)-specific antibodies (for example, nivolumab and pembrolizumab). Alongside other new drugs - namely, BRAF inhibitors (vemurafenib and dabrafenib) and MEK inhibitors (trametinib and cobimetinib) - these agents are very promising and have been shown to significantly improve prognosis for patients with advanced-stage metastatic disease. Early signs are apparent that these new treatment modalities are also improving long-term clinical benefit and the quality of life of patients. This Primer summarizes the current understanding of melanoma, from mechanistic insights to clinical progress. For an illustrated summary of this Primer, visit: http://go.nature.com/vX2N9s.

In-situ hybridization-based quantification of hTR: a possible biomarker in malignant melanoma

AIMS

Telomerase is reactivated in most cancers and there is accumulating evidence that this is a driver event in malignant melanoma (MM). Thus, our aim was to evaluate if in-situ hybridization (ISH)-based quantification of telomerase RNA (hTR) could be used to distinguish MM from naevi, and if there was a correlation with the Breslow thickness.

RESULTS AND METHODS

We created a tissue microarray (TMA) from formalin-fixed and paraffin-embedded tissue samples from 17 MM and 23 naevi, performed ISH targeting hTR, and quantified the signals. We found a more than eightfold greater number of hTR signals per nucleus in the MM samples compared to the naevi, and a positive correlation (P = 0.0381) between the number of hTR signals per nucleus and the Breslow thickness.

CONCLUSION

Quantification of hTR ISH signals clearly distinguish MM from naevi (P < 0.0001) and the number of signals per nucleus correlates with the Breslow thickness, suggesting that hTR might be a valuable biomarker in MM. Furthermore, as ISH-based detection requires the presence of both hTR and telomerase reverse transcriptase (hTERT), it might be an indicator of active telomerase and thus have future relevance as a predictive biomarker for anti-telomerase treatment.

The molecular pathology of melanoma: an integrated taxonomy of melanocytic neoplasia

Melanomas comprise multiple biologically distinct categories, which differ in cell of origin, age of onset, clinical and histologic presentation, pattern of metastasis, ethnic distribution, causative role of UV radiation, predisposing germ-line alterations, mutational processes, and patterns of somatic mutations. Neoplasms are initiated by gain-of-function mutations in one of several primary oncogenes, which typically lead to benign melanocytic nevi with characteristic histologic features. The progression of nevi is restrained by multiple tumor-suppressive mechanisms. Secondary genetic alterations override these barriers and promote intermediate or overtly malignant tumors along distinct progression trajectories. The current knowledge about the pathogenesis and clinical, histologic, and genetic features of primary melanocytic neoplasms is reviewed and integrated into a taxonomic framework.

Integrated nanosensors to determine levels and functional activity of human telomerase

Telomerase is a key oncogenic enzyme, and a number of novel telomerase inhibitors are currently under development. Because inhibition can be achieved either at the protein or at the enzymatic activity level, independent measurements of these parameters are important in the development of effective therapeutic agents. In the current study, we have developed a set of functional magnetic nanosensors capable of measuring the concentration of telomerase, as well as its enzymatic activity in parallel. The method is based on a magnetic relaxation switch assay, which can be performed in crude tissue samples and is fast and extremely sensitive. Using this method, we were able to detect different amounts of telomerase protein and activity in various cancer and normal cell lines. Furthermore, we were able to study the effect of phosphorylation on telomerase activity. This system not only could provide a rapid assay for the evaluation of antitelomerase therapies but could also be implemented to the study of other cancer markers.

[Regression in malignant melanoma. Definition, etiopathogenesis, morphology and differential diagnosis]

Malignant melanoma is a neoplasm that more often tends to undergo regression. Clinically, variation in color is perhaps the most important hallmark of primary cutaneous melanoma. The change in color to white, off-white, blue-white and gray-white is a sign of (spontaneous) regression in malignant melanoma. Histopathologically the process starts with a dense lichenoid infiltrate of lymphocytes, and ends with fibrosis and/or melanosis within a thickened papillary dermis. The dense infiltrate of lymphocytes permeates the thin melanoma and destroys the atypical melanocytes in the epidermis and the papillary dermis. A key concern is how to define regression in a reproducible way. Using the following definition, a statistically significant risk of metastases can be demonstrated in thin melanomas (<1.0 mm) with extensive regression (>50%): "fibroplasia with an absence of epidermal and dermal involvement by melanoma cells, but allowing for (lentiginous) single-cell proliferation of atypical melanocytes along the dermo-epidermal junction".

Establishment of a novel melanoma cell line SMYM-PRGP showing cytogenetic and biological characteristics of the radial growth phase of acral melanomas

We established a novel melanoma cell line, SMYM-PRGP, which was non-tumorigenic in vivo, from an acral melanoma in radial growth phase under a low-oxygen environment. SMYM-PRGP was wild-type for known mutation sites in the BRAF and NRAS genes, and showed focal amplification of the human telomerase reverse transcriptase and cyclin D1 genes as well as the fibroblast growth factor-3 and fibroblast growth factor-4 genes. Neither mutation nor copy number loss of the CDKN2A gene was observed. The p16(INK4A) protein was expressed at a level equal to that in normal melanocytes. Among the various melanocyte growth factors added to the culture of SMYM-PRGP cells, endothelin-1 was the strongest growth stimulator, the effect of which was significantly augmented by the addition of calcium chloride. The growth stimulatory effect of endothelin-1 was shown to be mediated via the endothelin B receptor. The protein level of cyclin D1 in SMYM-PRGP cells was approximately 10 times higher than that in normal melanocytes. Although the stimulation with endothelin-1 plus calcium chloride increased cyclin D1 protein levels after 4-6 h, the level of phosphorylated retinoblastoma protein did not increase, suggesting that overexpression of cyclin D1 protein may have little effect on cell cycle progression but rather act as a pro-survival factor. SMYM-PRGP is an excellent tool for investigating the development and progression of acral melanoma.

The role of telomeres and telomerase in the pathology of human cancer and aging

Cellular senescence, the state of permanent growth arrest, is the inevitable fate of replicating normal somatic cells. Postulated to underlie this finite replicative span is the physiology of telomeres, which constitute the ends of chromosomes. The repetitive sequences of these DNA-protein complexes progressively shorten with each mitosis. When the critical length is bridged, telomeres trigger DNA repair and cell cycle checkpoint mechanisms that result in chromosomal fusions, cell cycle arrest, senescence and/or apoptosis. Should senescence be bypassed at such time, continued cell divisions in the face of dysfunctional telomeres and activated DNA repair machinery can result in the genomic instability favourable for oncogenesis. The longevity and malignant progression of the thus transformed cell requires coincident telomerase expression or other means to negate the constitutional telomeric loss. Practically then, telomeres and telomerase may represent plausible prognostic and screening cancer markers. Furthermore, if the argument is extended, with assumptions that telomeric attrition is indeed the basis of cellular senescence and that accumulation of the latter equates to aging at the organismal level, then telomeres may well explain the increased incidence of cancer with human aging.

Telomerase activity in prognostic histopathologic features of melanoma

BACKGROUND

Telomerase activity (TA) is believed to play a role in the regulation of senescence and to limit the number of cell divisions. The deregulation of telomerase appears to contribute to oncogenesis and the formation of immortal cell lines. As a result, it is believed that it could be used as a prognostic marker in melanoma.

METHODS

TA was assayed by the polymerase chain reaction PCR-ELISA-based telomeric repeat amplification protocol (TRAP assay). One hundred and eight samples were distributed in four histological groups: 30 samples from primary cutaneous melanomas, 24 from peritumoural skin sites, 28 from benign melanocytic lesions, and 26 from normal skin sites as a control.

RESULTS

TA was different among the four tested groups (Kruskall-Wallis test p<0.001), and increasing values of TA were observed progressing from normal skin to benign and then to malignant lesions. Among melanoma samples, there was a significant association between TA and ulceration (p=0.025), TA and vascular invasion (p=0.018) and TA and mitotic rate (p=0.029) (Mann-Whitney test). A linear regression analysis showed significant associations between the increase of TA with Breslow thickness (p=0.004) and the presence of satellites (p=0.002).

CONCLUSIONS

We observed that TA had increased from control skin to peritumoural skin, and then to benign melanocytic lesions and finally to melanoma, suggesting tumour progression. TA showed higher values in the presence of some important histopathologic parameters related to poor prognosis in cutaneous melanoma such as ulceration, vascular invasion, satellites, high rates of mitosis, and in thicker tumours.

Modelling cancer in human skin tissue

The capacity to induce neoplasia in human tissue in the laboratory has recently provided a new platform for cancer research. Malignant conversion can be achieved in vivo by expressing genes of interest in human tissue that has been regenerated on immune-deficient mice. Induction of cancer in regenerated human skin recapitulates the three-dimensional architecture, tissue polarity, basement membrane structure, extracellular matrix, oncogene signalling and therapeutic target proteins found in intact human skin in vivo. Human-tissue cancer models therefore provide an opportunity to elucidate fundamental cancer mechanisms, to assess the oncogenic potency of mutations associated with specific human cancers and to develop new cancer therapies.

Early cancers of the skin: clinical, histopathological, and molecular characteristics

Because skin lesions are visible and easily accessible, skin cancers provide us with an excellent in vivo model to study the development of cancers. Cutaneous malignant melanoma and squamous cell carcinoma (SCC) both arise from the epidermis and have an initial progression stage in which proliferation of the neoplastic cells is confined to the epidermis. This stage is called melanoma in situ or SCC in situ. Molecular analyses of melanoma in situ and of solar keratosis, a prototype of early SCC in situ, show that loss of p16(INK4a)/p14(ARF) and dysfunction of p53 play a critical role, respectively. Furthermore, there seems to be potential precursor cells to these in situ lesions, which are not discernible with conventional hematoxylin and eosin-stained sections. The precursor cells have minimal but critical genetic alterations, such as cyclin D1 amplification and p53 mutation, and can be identified using fluorescent in situ hybridization and immunostaining with p53 antibodies, respectively. These precursor cells may be defective in repair response to DNA damage, and would have proliferative or survival advantages over their normal neighboring counterparts in the presence of growth factor stimulation or genotoxic events, such as ultraviolet irradiation. Such precursor clones may be induced at a rather young age, and their number and size increase with accumulating carcinogenic stimuli. If these lesions acquire additional mutations, they could progress to clinically visible lesions of in situ carcinoma. Precise molecular analyses of early stages of skin cancers may have a strong impact on our understanding of in vivo development of cancers in other human organs.

hTERT expression in melanocytic lesions: an immunohistochemical study on paraffin-embedded tissue

BACKGROUND

Telomerase plays a role in the immortalization of cells and carcinogenesis. Previous studies have yielded conflicting results on whether human telomerase RNA (hTER) expression differs in nevi, atypical nevi and melanomas using polymerase chain reaction-based telomeric repeat amplification protocol or in situ hybridization assays. The aim of this study was to evaluate human telomerase reverse transcriptase (hTERT) staining in melanocytic lesions on paraffin-embedded tissues.

METHODS

Paraffin-embedded sections from 12 acquired nevi, seven dysplastic nevi, 11 Spitz nevi, eight primary invasive melanomas, and three metastatic melanomas were studied for staining intensity (0-3+) and percentage of labeled cells with anti-hTERT.

RESULTS

hTERT staining was observed in most cells (>75%), in all but three lesions, and was of greater intensity in the nucleus, especially the nucleolus, compared with the cytoplasm. Spitz nevi tended to have weaker hTERT staining (mean = 1.7) compared with acquired nevi (mean = 2.2), dysplastic nevi (mean = 2.4), primary melanomas (mean = 2.4), or metastatic melanomas (mean = 3).

CONCLUSIONS

Although telomerase activity was weaker in Spitz nevi, there was overlap with other nevi and primary invasive melanomas in our small series. Thus, hTERT expression does not appear to be a reliable adjunct to the histological diagnosis of primary melanocytic lesions.

Telomere-based DNA damage responses: a new approach to melanoma

Melanoma is the most fatal skin cancer, often highly resistant to chemotherapy. Here we show that treatment with an 11-base DNA oligonucleotide homologous to the telomere 3' overhang sequence (T-oligo) induces apoptosis of several established human melanoma cell lines, including the aggressive MM-AN line, whereas normal human melanocytes exposed to the same or higher T-oligo concentrations show only transient cell cycle arrest, implying that malignant cells are more sensitive to T-oligo effects. When MM-AN cells were briefly exposed to T-oligo in culture and injected into the flank or tail vein of SCID mice, eventual tumor volume and number of metastases were reduced 85-95% compared with control mice. Similarly, T-oligos administered intralesionally or systemically selectively inhibited the growth of previously established MM-AN tumor nodules in the flank and peritoneal cavity by 85 to 90% without detectable toxicity. We previously showed that T-oligos act through ATM, p95/Nbs1, E2F1, p16INK4A, p53, and the p53 homologue p73 to modulate downstream effectors and now additionally demonstrate striking down-regulation of the inhibitor of apoptosis protein livin/ML-IAP. We suggest that T-oligo mimics a physiologic DNA damage signal that is frequently masked in malignant cells and thereby activates innate cancer prevention responses. T-oligos may provide a novel therapeutic approach to melanoma.

Molecular genetics of melanocytic neoplasia: practical applications for diagnosis

Pathology is the gold standard for the diagnosis of skin cancer. However, despite morphological criteria that have been constantly refined, there remain areas in which histopathological analysis does not permit an unequivocal diagnosis. Among these, problems most frequently arise with melanocytic lesions. This article reviews recent molecular studies that have revealed differences in the pattern of chromosomal aberrations between naevi and melanoma that could become relevant as adjunctive diagnostic methods in the future.

Classifying melanocytic tumors based on DNA copy number changes

Melanoma and benign melanocytic nevi can overlap significantly in their histopathological presentation and misdiagnoses are common. To determine whether genetic criteria can be of diagnostic help we determined DNA copy number changes in 186 melanocytic tumors (132 melanomas and 54 benign nevi) using comparative genomic hybridization. We found highly significant differences between melanomas and nevi. Whereas 127 (96.2%) of the melanomas had some form of chromosomal aberration, only 7 (13.0%) of the benign nevi cases had aberrations. All seven cases with aberrations were Spitz nevi, in six of which the aberration was an isolated gain involving the entire short arm of chromosome 11. This aberration was not observed in any of the 132 melanomas. We also analyzed the 132 melanomas for genetic differences depending on anatomical site, Clark's histogenetic type, and sun-exposure pattern. We show that melanomas on acral sites have significantly more aberrations involving chromosomes 5p, 11q, 12q, and 15, as well as focused gene amplifications. Melanomas classified as lentigo maligna melanomas or as occurring on severely sun-damaged skin showed markedly more frequent losses of chromosomes 17p and 13q. This study shows a pattern of chromosomal aberration in melanoma that is distinct from melanocytic nevi and should be further evaluated as a diagnostic test for melanocytic lesions that are now ambiguous. In addition, we show marked differences in the genetic make-up of melanomas that depend on anatomical location and sun-exposure pattern indicating that potential therapeutic targets might vary among melanoma types.

Understanding the progression of melanocytic neoplasia using genomic analysis: from fields to cancer

Analysis of DNA copy number changes using comparative genomic hybridization in melanocytic neoplasms has revealed distinct patterns of chromosomal aberrations between benign melanocytic nevi and melanoma. Whereas the vast majority of melanoma expresses chromosomal aberrations, blue nevi, congenital nevi, and most Spitz nevi typically show no aberrations. A subset of Spitz nevi shows an isolated gain of chromosome 11p, an aberration pattern not observed in melanoma. These Spitz nevi frequently harbor mutations in the HRAS gene located on this chromosomal arm. Comparisons among melanoma types showed that melanomas of the palms, soles, and subungual sites can be distinguished by the presence of multiple gene amplifications that arise very early in their progression. About 50% of these amplifications are found at the cyclin D1 locus. By contrast, amplifications are significantly less frequent in other cutaneous melanoma types and if present arise late in progression. The frequent amplifications in melanomas on acral sites allowed the detection of single basal melanocytes with gene amplifications in the histologically normal appearing skin immediately adjacent to a melanoma. These "field cells" represent subtle melanoma in situ and are likely to represent minimal residual disease that can lead to local recurrences if not excised with safety margins. The high frequency of chromosomal aberrations in melanomas and their relative absence in nevi could indicate that melanocytes of melanomas went through telomeric crisis, whereas melanocytes in nevi did not. Such a scenario would suggest that replicative senescence is a tumor-suppressive mechanism in melanocytic neoplasia. It might also explain part of the phenomenon of regression commonly seen in melanoma. Genomic analysis is a powerful tool to obtain insight in the progression of melanocytic neoplasms with potential clinical applications for classification and detection of minimal residual melanoma.

Increased expression of apoptotic markers in melanoma

Extensive labelling for the apoptotic markers calcium channel receptor P2X(7) and caspase-3 and telomerase activity was co-localized at a similar intensity in areas affected by superficial spreading melanoma obtained from 80 patients. Labelling for each of these markers also extended 2 microm from the melanoma into the keratinocyte layer of the adjacent normal epidermis. Conversely, the calcium-regulating receptors P2X(1-3) and P2Y(2) (found in normal but not neoplastic skin) were fully de-expressed within 2 microm of the melanoma but fully expressed beyond that distance. The cell adhesion protein E-cadherin (also only present in normal skin) was progressively de-expressed from a point 2 microm from the melanoma until full de-expression within the lesion. These results show that telomerase-induced proliferation and defensive apoptosis are co-localized and simultaneous processes in melanoma tissue. Melanoma cell proliferation appears to overwhelm the apoptotic defence, perhaps due to the anti-apoptotic effects of telomerase. In addition, keratinocyte regulation of the epidermis and dermis is severely compromised by the loss of E-cadherin and P2X(1-3) and P2Y(2) receptors, resulting in a lesion that is aggressive and malignant.

Malignant transformation of melanocytes to melanoma by constitutive activation of mitogen-activated protein kinase kinase (MAPKK) signaling

Malignant melanoma is the cancer with the most rapid increase in incidence in the United States. Ultraviolet light and deficiency of the p16ink4a gene are known factors that predispose one to the development of malignant melanoma. The signal transduction pathways that underlie the progression of melanoma from their precursors, atypical nevi, are not well understood. We examined activation of the MAP kinase pathway in atypical nevi and melanoma cells and found that this pathway is activated in melanomas. To determine the functional significance of this activation, we introduced constitutively active MAP kinase kinase (MAPKK) into immortalized melanocytes. The introduction of this gene into melanocytes leads to tumorigenesis in nude mice, activation of the angiogenic switch, and increased production of the proangiogenic factor, vascular endothelial growth factor (VEGF), and matrix metalloproteinases (MMPs). Activation of MAP kinase signaling may be an important pathway involved in melanoma transformation. Inhibition of MAP kinase signaling may be useful in the prevention and treatment of melanoma.

Tiptoeing to chromosome tips: facts, promises and perils of today's human telomere biology

The past decade has witnessed an explosion of knowledge concerning the structure and function of chromosome terminal structures-telomeres. Today's telomere research has advanced from a pure descriptive approach of DNA and protein components to an elementary understanding of telomere metabolism, and now to promising applications in medicine. These applications include 'passive' ones, among which the use of analysis of telomeres and telomerase (a cellular reverse transcriptase that synthesizes telomeres) for cancer diagnostics is the best known. The 'active' applications involve targeted downregulation or upregulation of telomere synthesis, either to mortalize immortal cancer cells, or to rejuvenate mortal somatic cells and tissues for cellular transplantations, respectively. This article reviews the basic data on structure and function of human telomeres and telomerase, as well as both passive and active applications of human telomere biology.

The A3 adenosine receptor as a new target for cancer therapy and chemoprotection

Adenosine, a purine nucleoside, acts as a regulatory molecule, by binding to specific G-protein-coupled A(1), A(2A), A(2B), and A(3) cell surface receptors. We have recently demonstrated that adenosine induces a differential effect on tumor and normal cells. While inhibiting in vitro tumor cell growth, it stimulates bone marrow cell proliferation. This dual activity was mediated through the A3 adenosine receptor. This study showed that a synthetic agonist to the A3 adenosine receptor, 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyl-uronamide (Cl-IB-MECA), at nanomolar concentrations, inhibited tumor cell growth through a cytostatic pathway, i.e., induced an increase number of cells in the G0/G1 phase of the cell cycle and decreased the telomeric signal. Interestingly, Cl-IB-MECA stimulates murine bone marrow cell proliferation through the induction of granulocyte-colony-stimulating factor. Oral administration of Cl-IB-MECA to melanoma-bearing mice suppressed the development of melanoma lung metastases (60.8 +/- 6.5% inhibition). In combination with cyclophosphamide, a synergistic anti-tumor effect was achieved (78.5 +/- 9.1% inhibition). Furthermore, Cl-IB-MECA prevented the cyclophosphamide-induced myelotoxic effects by increasing the number of white blood cells and the percentage of neutrophils, demonstrating its efficacy as a chemoprotective agent. We conclude that A3 adenosine receptor agonist, Cl-IB-MECA, exhibits systemic anticancer and chemoprotective effects.

The molecular biology of cancer

The process by which normal cells become progressively transformed to malignancy is now known to require the sequential acquisition of mutations which arise as a consequence of damage to the genome. This damage can be the result of endogenous processes such as errors in replication of DNA, the intrinsic chemical instability of certain DNA bases or from attack by free radicals generated during metabolism. DNA damage can also result from interactions with exogenous agents such as ionizing radiation, UV radiation and chemical carcinogens. Cells have evolved means to repair such damage, but for various reasons errors occur and permanent changes in the genome, mutations, are introduced. Some inactivating mutations occur in genes responsible for maintaining genomic integrity facilitating the acquisition of additional mutations. This review seeks first to identify sources of mutational damage so as to identify the basic causes of human cancer. Through an understanding of cause, prevention may be possible. The evolution of the normal cell to a malignant one involves processes by which genes involved in normal homeostatic mechanisms that control proliferation and cell death suffer mutational damage which results in the activation of genes stimulating proliferation or protection against cell death, the oncogenes, and the inactivation of genes which would normally inhibit proliferation, the tumor suppressor genes. Finally, having overcome normal controls on cell birth and cell death, an aspiring cancer cell faces two new challenges: it must overcome replicative senescence and become immortal and it must obtain adequate supplies of nutrients and oxygen to maintain this high rate of proliferation. This review examines the process of the sequential acquisition of mutations from the prospective of Darwinian evolution. Here, the fittest cell is one that survives to form a new population of genetically distinct cells, the tumor. This review does not attempt to be comprehensive but identifies key genes directly involved in carcinogenesis and demonstrates how mutations in these genes allow cells to circumvent cellular controls. This detailed understanding of the process of carcinogenesis at the molecular level has only been possible because of the advent of modern molecular biology. This new discipline, by precisely identifying the molecular basis of the differences between normal and malignant cells, has created novel opportunities and provided the means to specifically target these modified genes. Whenever possible this review highlights these opportunities and the attempts being made to generate novel, molecular based therapies against cancer. Successful use of these new therapies will rely upon a detailed knowledge of the genetic defects in individual tumors. The review concludes with a discussion of how the use of high throughput molecular arrays will allow the molecular pathologist/therapist to identify these defects and direct specific therapies to specific mutations.