Inhibition of telomerase induces alternative lengthening of telomeres during human esophageal carcinogenesis.

Immortalization is an important step toward the malignant transformation of human cells and is critically dependent upon telomere maintenance. Two mechanisms are known to maintain human telomeres. The process of telomere maintenance is either mediated through activation of the enzyme telomerase or through an alternative mechanism of telomere lengthening called alternative lengthening of telomeres (ALT). Whereas 85% of all human tumors show reactivation of telomerase, the remaining 15% are able to maintain telomeres via ALT. Telomerase inhibitors are already investigated in clinical trials, although the regulation as well as potential coexistence and redundancy of both telomere maintenance mechanisms during distinct steps of carcinogenesis are poorly understood. Herein, we demonstrate that telomerase activity and ALT alternate in a cell cycle dependent fashion in human esophageal epithelial cells, and can coexist in a genetically defined model of oral-esophageal squamous carcinogenesis. Moreover, we show that immortalized premalignant cells as well as cancer cells are able to switch from telomerase activation to ALT upon inhibition of telomerase. This indicates that cancer cells treated with telomerase inhibitors can use alternative and adaptive ways to maintain their telomeres and thereby escape telomere-based therapeutic strategies.

Telomerase gene mutations are associated with cirrhosis formation.

UNLABELLED: Telomere shortening impairs liver regeneration in mice and is associated with cirrhosis formation in humans with chronic liver disease. In humans, telomerase mutations have been associated with familial diseases leading to bone marrow failure or lung fibrosis. It is currently unknown whether telomerase mutations associate with cirrhosis induced by chronic liver disease. The telomerase RNA component (TERC) and the telomerase reverse transcriptase (TERT) were sequenced in 1,121 individuals (521 patients with cirrhosis induced by chronic liver disease and 600 noncirrhosis controls). Telomere length was analyzed in patients carrying telomerase gene mutations. Functional defects of telomerase gene mutations were investigated in primary human fibroblasts and patient-derived lymphocytes. An increased incidence of telomerase mutations was detected in cirrhosis patients (allele frequency 0.017) compared to noncirrhosis controls (0.003, P value 0.0007; relative risk [RR] 1.859; 95% confidence interval [CI] 1.552-2.227). Cirrhosis patients with TERT mutations showed shortened telomeres in white blood cells compared to control patients. Cirrhosis-associated telomerase mutations led to reduced telomerase activity and defects in maintaining telomere length and the replicative potential of primary cells in culture. CONCLUSION: This study provides the first experimental evidence that telomerase gene mutations are present in patients developing cirrhosis as a consequence of chronic liver disease. These data support the concept that telomere shortening can represent a causal factor impairing liver regeneration and accelerating cirrhosis formation in response to chronic liver disease.

EGFR overexpression induces activation of telomerase via PI3K/AKT-mediated phosphorylation and transcriptional regulation through Hif1-alpha in a cellular model of oral-esophageal carcinogenesis.

Telomerase plays an important role during immortalization and malignant transformation as crucial steps in the development of human cancer. In a cellular model of oral-esophageal carcinogenesis, recapitulating the human disease, immortalization occurred independent of the activation of telomerase but through the recombination-based alternative lengthening of telomeres (ALT). In this stepwise model, additional overexpression of EGFR led to in vitro transformation and activation of telomerase with homogeneous telomere elongation in already immortalized oral squamous epithelial cells (OKF6-D1_dnp53). More interestingly, EGFR overexpression activated the PI3K/AKT pathway. This strongly suggested a role for telomerase in tumor progression in addition to just elongating telomeres and inferring an immortalized state. Therefore, we sought to identify the regulatory mechanisms involved in this activation of telomerase and in vitro transformation induced by EGFR. In the present study we demonstrate that telomerase expression and activity are induced through both direct phosphorylation of hTERT by phospho-AKT as well as PI3K-dependent transcriptional regulation involving Hif1-alpha as a key transcription factor. Furthermore, EGFR overexpression enhanced cell cycle progression and proliferation via phosphorylation and translocation of p21. Whereas immortalization was induced by ALT, in vitro transformation was associated with telomerase activation, supporting an additional role for telomerase in tumor progression besides elongating telomeres.