Ionizing Radiation Up-Regulates Telomerase Activity in Cancer Cell Lines; Mechanisms and Possible Clinical Relevance.

Background: Activation of telomerase has been recently recognized as one of the pivotal events in malignant transformation. Up-regulation of this enzyme is probably necessary for transformation and, more importantly, for perpetuation of the malignant clone. Down regulation of telomerase activity may compromise the viability of cancer cells. Therefore, telomerase is considered to be a central target for future anti cancer therapies. Indeed, others and we have shown that telomere shortening by inhibition of telomerase sensitizes malignant cells to several cytotoxic drugs. Very little is known about the effect of irradiation, an important anticancer modality, on telomerase activity. The available data is sketchy and controversial. In this study we characterize the effect of irradiation on telomerase activity in malignant cells and its molecular mechanism.

Methods: Several cancer cell lines were exposed to a range of radiation doses. Telomerase activity was assessed at various time points. All levels of telomerase regulation were evaluated: transcriptional, alternative splicing, posttranslational phosphorylation and dephosphorylation and intracellular compartmentalization of the enzyme. Subsequently, the involvement of several signal transduction pathways triggered by irradiation and relevant to telomerase regulation was evaluated by point inhibition of numerous signaling compounds.

Results: In all cell lines, irradiation caused a fourfold increase in telomerase activity with a maximal effect at 24 hours at a dose of 400 Rad. This elevation was mediated by posttranslational modification of the enzyme as phospho-AKT; the enzyme that phosphorylates telomerase was increased after irradiation. Transcriptional activity and intracellular compartmentalization of telomerase were not affected. PI3K/AKT pathway mediates the irradiation-induced upregulation of telomerase activity, as wortmanin decreased that effect. RAS oncogene was the upstream effector of PI3K, since its inhibition by FTS decreased irradiation dependent telomerase activity. PTEN was not involved in regulating that effect. In addition, several membrane receptors known to be activated by radiation including Erb1, Erb2 andIGF1R were not involved in the signaling cascade leading to the up regulation of telomerase.

Conclusions: Radiation causes posttranslational upregulation of telomerase in cancer cells lines. Telomerase activation probably enables non-specific “chromosomal healing” in certain low irradiation dosages. This activation was mediated via the PI3K/AKT signaling pathway activated by Ras. The membrane receptors classically activated by irradiation were not involved in this process. These finding have potential clinical implication, since the upregulation of telomerase by irradiation in cancer cells may interfere with future cytotoxic treatment and necessitate concomitant inhibition of the enzyme.