Telomerase: putting an end to DNA

The effects of oxidative stress on telomerase activity and other stress-related proteins in lens epithelial cells

Telomerase is a ribonucleoprotein complex responsible for maintaining the ends of chromosomes and for repair of DNA strand breaks. While telomerase activity is generally found in cells that have unlimited proliferative potential such as neoplastic cells, germline cells and some stem cells, lens epithelial cells (though not highly proliferative) have telomerase activity. Our previous studies indicated that lens epithelial cells express high levels of telomerase despite their low proliferative potential, thus we hypothesized that telomerase expression protects lens epithelial cells from oxidative stress. We also determined levels of the stress proteins gadd45 and p16 and the stress and proliferation-related protein, proliferating cell nuclear antigen (PCNA). In acute studies, lenses were exposed to TBHP for 0-120 min. In recovery studies, lenses were exposed to TBHP for 1 hr, then allowed to recover for up to 18 hr. In acute studies, telomerase activity was increased, p16 initially decreased then normalized, PCNA levels did not change significantly even in the overnight recovery groups, and gadd45 was decreased in some TBHP exposed groups. In recovery studies, telomerase activity was increased in all groups, gadd45 decreased then became elevated, and p16 levels were decreased at later recovery times. PCNA levels remained constant during the studies, indicating that there was no change in proliferation. These studies showed that elevated telomerase activity did not correlate with increased proliferation in lens epithelial cells; instead, increased telomerase activity was associated with increased levels of the stress-related protein gadd45 only in the later recovery times. These findings support the hypothesis that telomerase plays a protective rather than a proliferative role in lens epithelial cells.

Telomerase activity in lens epithelial cells of normal and cataractous lenses

Telomerase is a ribonucleoprotein responsible for maintaining telomere length, preventing chromosomal degradation and recombination, and repairing DNA strand breaks. These activities are believed to be important in preventing cell senescence. Telomerase activity is normally found in germinal, neoplastic and stem cells, but not any ocular tissue studied to date. The epithelium of the crystalline lens is comprised of a population of cells with diverse mitotic potential including the germinative epithelium which contains cells with the potential for unlimited replicative capacity, equatorial cells which terminally differentiate into lens fibers, and the central epithelium which are considered to be quiescent and nonreplicative under normal circumstances. We speculated that the germinative region of lens epithelial cells might have telomerase activity, and that dysregulation of its activity might be associated with cataractogenesis. We investigated these hypotheses in lens capsule specimens from normal and cataractous dogs and from cultures of canine lens epithelial cells using standard assays for telomerase activity and telomere length. Telomerase activity was found in normal canine lens epithelial cells in the central, germinative and equatorial regions of the anterior lens capsule at equivalent levels. Similar findings were made in feline and murine lens epithelial cells, indicating that the presence of telomerase activity in the lens was not species specific. Lens fiber cells, corneal epithelium and endothelium and nonpigmented ciliary epithelium were telomerase negative. Telomerase activity and telomere lengths were significantly greater in lens epithelia from cataractous lenses when compared with normal lenses. Since telomerase activity is associated with an immortal phenotype, the presence of telomerase activity in the lens epithelial cells may function to prevent conversion to senescence. It was, therefore, difficult to explain why these cells cannot be passaged more than four times in culture. We found that telomerase activity and telomere lengths gradually decreased with increased passages until telomerase activity was no longer present at passage two. Consistent with these findings, there were no senescent cells present on the lens capsule when the lens was initially dissected for culture, but an increasing number of cells were senescent with each passage, correlating well with the loss of telomerase activity. Telomerase activity is likely important in the germinative epithelium to maintain its proliferative potential and prevent cell senescence. Telomerase may function in the quiescent, central lens to maintain telomeres damaged by oxidative stress and ultraviolet light exposure, thereby preventing accelerated loss of these elements which triggers cell senescence. It remains to be determined if the increase in telomerase activity in lens epithelial cells from cataractous lenses is a primary dysregulation that may have a role in the development of the cataract, or is secondary to cataract formation.

Telomere-related components are coordinately synthesized during human T-lymphocyte activation

Since telomerase activity is present in most malignant cells, but absent in most normal cells, its induction in normal cells warrants scrutiny. Therefore we have analyzed the inducibility of telomere-related components in normal lymphocytes during their activation. Telomerase activity increased over 400-fold, telomerase reverse transcriptase (hTERT) mRNA 52 x , telomerase RNA 32 x , TTAGGG repeat binding factor 1 mRNA 19 x , TTAGGG repeat binding factor 2 mRNA 20 x , and telomerase-associated protein mRNA 17 x . The peak value for each was reached at about 72 h. However hTERT rose fastest and synchronously with telomerase activity. Thus in normal human lymphocytes (1) the syntheses of all cloned telomerase-related components are coordinately regulated and (2) hTERT may have a priming role.

Decrease in telomerase activity in U-87MG human glioblastomas after treatment with an antagonist of growth hormone-releasing hormone

Antagonists of growth hormone-releasing hormone (GH-RH) inhibit the growth of various tumors through mechanisms that involve the suppression of the insulin-like growth factor I and/or insulin-like growth factor II levels or secretion. In the present study, we tested the hypothesis that the tumor inhibition is associated with a decrease in telomerase activity because telomerase is considered obligatory for continued tumor growth. Nude mice bearing xenografts of U-87MG human glioblastomas were treated with GH-RH antagonist MZ-5-156. Telomerase activity was assessed by the telomerase repeat amplification protocol. Treatment with MZ-5-156 reduced levels of telomerase activity as compared with controls. When U-87 glioblastomas, H-69 small cell lung carcinomas, H-23 non-small cell lung carcinomas, and MDA-MB-468 breast carcinoma cells were cultured in vitro, addition of 3 microM MZ-5-156 also inhibited telomerase activity. Reverse transcription-PCR analysis revealed that in U-87MG glioblastomas, the expression of the hTRT gene encoding for the telomerase catalytic subunit was significantly decreased by MZ-5-156, whereas the levels of mRNA for hTR and TP1, which encode for the telomerase RNA and telomerase-associated protein, respectively, were unaffected. The repression of the telomerase activity was not accompanied by a significant decrease of mRNA level for the c-myc protooncogene that regulates telomerase. Our findings suggest that tumor inhibition induced by the GH-RH antagonists in U-87MG glioblastomas is associated with the down-regulation of the hTRT gene, resulting in a decrease in telomerase activity. Further studies are needed to establish whether GH-RH antagonists produce telomerase inhibition in other tumors.