Telomerase and human tumorigenesis

Amplification of the telomerase reverse transcriptase (hTERT) gene in cervical carcinomas

The expression of telomerase reverse transcriptase (hTERT), the catalytic component of the telomerase complex, is required for activation of telomerase during immortalization and transformation of human cells. However, the biochemical and genetic mechanisms governing hTERT expression remain to be elucidated. In the present study, we examined hTERT amplification as a potential genetic event contributing to telomerase activation in cervical carcinomas. An amplification of the hTERT gene was found in 1/4 cervical cancer cell lines and 21/88 primary tumor samples derived from the patients with cervical carcinomas. An increase in the hTERT copy number was significantly correlated with higher levels of hTERT protein expression. Moreover, the hTERT alterations with the enhanced hTERT expression were exclusively observed in those tumors with high-risk human papillomavirus infection. Taken together, the hTERT gene amplification, directly or indirectly targeted by human papillomavirus, may be one of the driving forces responsible for upregulation of hTERT expression and activation of telomerase in cervical cancers.

Does p53 affect organismal aging?

The p53 protein plays a critical role in the prevention of cancer. It responds to a variety of cellular stresses to induce either apoptosis, a transient cell cycle arrest, or a terminal cell cycle arrest called senescence. Senescence in cultured cells is associated with augmented p53 activity and abrogation of p53 activity may delay in vitro senescence. Increasing evidence suggests that p53 may also influence aspects of organismal aging. Several mutant mouse models that display alterations in longevity and aging-related phenotypes have defects in genes that alter p53 signaling. Recently, my laboratory has developed and characterized a p53 mutant mouse line that appears to have an enhanced p53 response. These p53 mutants exhibit increased cancer resistance, yet have a shortened longevity and display a number of early aging-associated phenotypes, suggesting a role for p53 in the aging process. The nature of the aging phenotypes observed in this p53 mutant line is consistent with a model in which aging is driven in part by a gradual depletion of stem cell functional capacity.

Hepatitis C virus core protein is necessary for the maintenance of immortalized human hepatocytes

Hepatitis C virus (HCV) core protein has many intriguing properties and plays an important role in cell growth regulation. We have recently shown that the HCV core protein from genotype 1a promotes primary human hepatocytes to an immortalized phenotype. Here, we investigated whether the presence of core protein is necessary for maintenance of the immortalized hepatocytes and investigated its consequences on cellular gene expression. The introduction of an antisense orientation of the core gene into immortalized hepatocytes led to the onset of cell death. Further analysis suggested that cell death occurred through apoptosis associated with the activation of tumor suppressor pathways. Antisense core gene expression in immortalized hepatocytes increased p53 expression at both the mRNA and the protein levels. A decreased telomere length and reduced c-myc protein expression were also observed in hepatocytes when the antisense core gene was introduced. Results from these studies suggested that modulation of cell cycle regulatory genes by repression of core protein expression is responsible for reversion of the immortalized phenotype of the hepatocytes. Thus, targeted inhibition may contribute to the development of new therapeutic modalities for prevention of HCV core protein function.

Involvement of the telomeric protein Pin2/TRF1 in the regulation of the mitotic spindle

Pin2/TRF1 was independently identified as a telomeric DNA-binding protein (TRF1) that regulates telomere length, and as a protein (Pin2) that can bind the mitotic kinase NIMA and suppress its lethal phenotype. We have previously demonstrated that Pin2/TRF1 levels are cell cycle-regulated and its overexpression induces mitotic arrest and then apoptosis. This Pin2/TRF1 activity can be potentiated by microtubule-disrupting agents, but suppressed by phosphorylation of Pin2/TRF1 by ATM; this negative regulation is critical in mediating for many, but not all, ATM-dependent phenotypes. Interestingly, Pin2/TRF1 specifically localizes to mitotic spindles in mitotic cells and affects the microtubule polymerization in vitro. These results suggest a role of Pin2/TRF1 in mitosis. However, nothing is known about whether Pin2/TRF1 affects the spindle function in mitotic progression. Here we characterized a new Pin2/TRF1-interacting protein, EB1, that was originally identified in our yeast two-hybrid screen. Pin2/TRF1 bound EB1 both in vitro and in vivo and they also co-localize at the mitotic spindle in cells. Furthermore, EB1 inhibits the ability of Pin2/TRF1 to promote microtubule polymerization in vitro. Given that EB1 is a microtubule plus end-binding protein, these results further confirm a specific interaction between Pin2/TRF1 and the mitotic spindle. More importantly, we have shown that inhibition of Pin2/TRF1 in ataxia-telangiectasia cells is able to fully restore their mitotic spindle defect in response to microtubule disruption, demonstrating for the first time a functional involvement of Pin2/TRF1 in mitotic spindle regulation.

SV40 infection induces telomerase activity in human mesothelial cells

Mesotheliomas are malignant tumors of the pleural and peritoneal membranes which are often associated with asbestos exposure and with Simian virus 40 (SV40) infection. Telomerase activity is repressed in somatic cells and tissues but is activated in immortal and malignant cells. We evaluated telomerase activity in seven primary malignant mesothelioma biopsies and matched lung specimens and 20 mesothelioma cell lines and eight corresponding primary tumor cultures. All the tumor biopsies, and nearly all primary cell mesothelioma cultures and cell lines were telomerase positive. The findings in cell lines paralleled those observed in primary cultures in cases where paired samples were available. Next, we found that SV40, a DNA tumor virus present in approximately 50% of mesothelioma biopsies in the USA, induced telomerase activity in primary human mesothelial cells, but not in primary fibroblasts. Telomerase activity became detectable as early as 72 h following wild-type (strain 776) SV40 infection, and a clear DNA ladder was detectable 1 week after infection. The amount of telomerase activity increased during passage in cell culture and appeared to parallel increases in the cellular amounts of the SV40 large T-antigen. Thus, SV40 infection leads to telomerase activity before the infected mesothelial cells become transformed and immortalized. SV40 infection of human fibroblasts did not cause detectable telomerase activity. We also determined that the SV40 small t-antigen (tag) plays an important role in inducing telomerase activity because this activity was undetectable or minimal in mesothelial cells infected and/or transformed by SV40 tag mutants. Asbestos alone did not induce telomerase activity, and asbestos did not influence telomerase activity in mesothelial cells infected with SV40. Induction of telomerase activity by SV40 may be related to the very high rate of mesothelial cell immortalization that is characteristically associated with SV40 infection of mesothelial cells.

The complex structure and dynamic evolution of human subtelomeres

Subtelomeres are extraordinarily dynamic and variable regions near the ends of chromosomes. They are defined by their unusual structure: patchworks of blocks that are duplicated near the ends of multiple chromosomes. Duplications among subtelomeres have spawned small gene families, making inter-individual variation in subtelomeres a potential source of phenotypic diversity. The ectopic recombination that occurs between subtelomeres might also have a role in reconstituting telomeres in the absence of telomerase. However, the propensity for subtelomeres to interchange is a double-edged sword, as extensive subtelomeric homology can mediate deleterious rearrangements of the ends of chromosomes to cause human disease.

Telomerase in the human organism

The intent of this review is to describe what is known and unknown about telomerase in somatic cells of the human organism. First, we consider the telomerase enzyme. Human telomerase ribonucleoproteins undergo at least three stages of cellular biogenesis: accumulation, catalytic activation and recruitment to the telomere. Next, we describe the patterns of telomerase regulation in the human soma. Telomerase activation in some cell types appears to offset proliferation-dependent telomere shortening, delaying but not defeating the inherent mitotic clock. Finally, we elaborate the connection between telomerase misregulation and human disease, in the contexts of inappropriate telomerase activation and telomerase deficiency. We discuss how our current perspectives on telomerase function could be applied to improving human health.

The Pin2/TRF1-interacting protein PinX1 is a potent telomerase inhibitor

Telomerase activity is critical for normal and transformed human cells to escape from crisis and is implicated in oncogenesis. Here we describe a novel Pin2/TRF1 binding protein, PinX1 that inhibits telomerase activity and affects tumorigenicity. PinX1 and its small TID domain bind the telomerase catalytic subunit hTERT and potently inhibit its activity. Overexpression of PinX1 or its TID domain inhibits telomerase activity, shortens telomeres, and induces crisis, whereas depletion of endogenous PinX1 increases telomerase activity and elongates telomeres. Depletion of PinX1 also increases tumorigenicity in nude mice, consistent with its chromosome localization at 8p23, a region with frequent loss of heterozygosity in a number of human cancers. Thus, PinX1 is a potent telomerase inhibitor and a putative tumor suppressor.