Investigations of telomeres and telomerase following ionizing radiation exposure

Abstract

Telomeres are critical structures located at the termini of eukaryotic chromosomes that regulate the replicative lifespan of human cells. Telomeres shorten with cell division, a process that eventually leads to telomere based growth arrest and cellular senescence. Telomere length is maintained through the activity of the reverse transcriptase, telomerase that functions in embryonic and adult stem cells to elongate telomeres and prolong replicative lifespan. Telomerase is repressed in the vast majority of human somatic cells, and its reactivation is a critical early step in carcinogenesis. Thus, telomerase and telomere maintenance are critical factors in the processes of carcinogenesis, tumor maintenance, and tumor recurrence following interventional therapy. Ionizing radiation (IR) has long been acknowledged as both a potent carcinogen and an effective agent in the treatment of cancer. To investigate the role telomeres and telomerase play in the cellular response to IR exposure, we tracked telomerase activity and telomere length in a panel of cancer and immortalized non-cancer cell lines following both acute and low dose rate (LDR) exposures to γ-rays. We observed elevations of telomerase activity in cancer, but not non-cancer, cell lines following acute exposures to IR. Further, telomere length was significantly reduced in both cancer and non-cancer cells post-acute IR exposure. Taken together, these studies suggest telomerase activity is playing a role in accelerated tumor repopulation following radiation therapy and that the associated telomere loss may be contributing to genomic instability. As IR induced enrichment of cancer stem cells (CSC) in established cancer cell lines was recently suggested to play a role in accelerated tumor repopulation following radiation therapy, we investigated a potential role for telomerase in the IR induced enrichment of CSCs. Consistent with previous reports, we detected a significant enrichment of putative breast CSCs in MCF-7 mammary carcinoma cells at 5 days post exposure, and demonstrate significant enrichment of putative CSCs in the non-tumorigenic MCF-10a, WTK1, and LCL15044 cell lines. Further small molecule inhibition of telomerase activity was able to effectively block CSC enrichment in both MCF-7 and MCF-10a cells. Together, these results suggest that telomerase inhibition is a significant player in the IR induced enrichment of putative CSCs in both cancer and non-cancer mammary epithelial cells. Further, this process seems to be driven by non-canonical roles of telomerase.