Cancer cell dynamics in presence of telomerase inhibitors: analysis of in vitro data

Telomere Length: A Cardiovascular Biomarker and a Novel Therapeutic Target

Coronary artery disease (CAD) is a multifactorial disease with a high prevalence, particularly in developing countries. Currently, the investigation of telomeres as a potential tool for the early detection of the atherosclerotic disease seems to be a promising method. Telomeres are repetitive DNA sequences located at the extremities of chromosomes that maintain genetic stability. Telomere length (TL) has been associated with several human disorders and diseases while its attrition rate varies significantly in the population. The rate of TL shortening ranges between 20 and 50 bp and is affected by factors such as the end-replication phenomenon, oxidative stress, and other DNA-damaging agents. In this review, we delve not only into the pathophysiology of TL shortening but also into its association with cardiovascular disease and the progression of atherosclerosis. We also provide current and future treatment options based on TL and telomerase function, trying to highlight the importance of these cutting-edge developments and their clinical relevance.

Role of Telomeres Shortening in Atherogenesis: An Overview

It is known that the shortening of the telomeres leads to cell senescence, accompanied by acquiring of pro-inflammatory phenotype. The expression of telomerase can elongate telomeres and resist the onset of senescence. The initiation of atherosclerosis is believed to be associated with local senescence of the endothelial cells of the arteries in places with either low or multidirectional oscillatory wall shear stress. The process of regeneration of the artery surface that has begun does not lead to success for several reasons. Atherosclerotic plaques are formed, which, when developed, lead to fatal consequences, which are the leading causes of death in the modern world. The pronounced age dependence of the manifestations of atherosclerosis pushes scientists to try to link the development of atherosclerosis with telomere length. The study of the role of telomere shortening in atherosclerosis is mainly limited to measuring the telomeres of blood cells, and only in rare cases (surgery or post-mortem examination) are the telomeres of local cells available for measurement. The review discusses the basic issues of cellular aging and the interpretation of telomere measurement data in atherosclerosis, as well as the prospects for the prevention and possible treatment of atherosclerosis.

Topochemical Models for Prediction of Telomerase Inhibitory Activity of Flavonoids

In the present study, the relationship between the topochemical indices and telomerase inhibitory activity of flavonoids has been investigated. Three topochemical indices, Wiener's topochemical index (a distance-based topochemical descriptor), molecular connectivity topochemical index (an adjacency-based topochemical descriptor) and superadjacency topochemical index (an adjacency cum distance-based topochemical descriptor) were used for the present investigation. The values of the Wiener's topochemical index, molecular connectivity topochemical index and superadjacency topochemical index for each of the 30 analogues comprising the data set were computed using an in-house computer program. Resultant data was analysed and suitable models were developed after identification of the active ranges. Subsequently, a biological activity was assigned to each analogue involved in the data set using these models, which was then compared with the reported telomerase inhibitory activity. Statistical significance of proposed models was investigated using intercorrelation analysis. Accuracy of prediction using proposed models was found to vary from 80% to 83%.

Stabilization of telomeres in nonlinear models of proliferating cell lines

We analyse an age-structured model of telomere loss in a proliferating cell population. The cell population is divided into telomere classes, which shorten each round of division. The model consists of a nonlinear system of partial differential equations for the telomere classes. We prove that if the highest telomere class is exempted from mortality, then all the classes stabilize to a nontrivial equilibrium dependent on the initial state of cells in the highest telomere class.

Targeting telomerase

Given the constitutive expression of telomerase in the majority of human tumors, telomerase inhibition is an attractive, broad-spectrum therapeutic target for cancer treatment. Therapeutic strategies for inhibiting telomerase activity have included both targeting components of telomerase (the protein component, TERT, or the RNA component, TERC) or by directly targeting telomere DNA structures. Recently a combination telomerase inhibition therapy has been studied also. The TERT promoter has been used to selectively express cytotoxic gene(s) in cancer cells and a TERT vaccine for immunization against telomerase has been tested. The 10% to 15% of immortalized cancer cells that do not express telomerase use a recombination-based mechanism for maintaining telomere structures that has been called the alternative lengthening of telomeres (ALT). In view of the increasing study of telomerase inhibitors as anticancer treatments, it will be crucial to determine whether inhibition of telomerase will select for cancer cells that activate ALT mechanisms of telomere maintenance.

Trisubstituted Acridines as G-quadruplex Telomere Targeting Agents. Effects of Extensions of the 3,6- and 9-Side Chains on Quadruplex Binding, Telomerase Activity, and Cell Proliferation

The synthesis is reported of a group of 3,6,9-trisubstituted acridine compounds as telomeric quadruplex-stabilizing ligands with systematic variations at the 3-, 6-, and 9-positions. A new microwave-assisted methodology has been developed for trisubstituted acridine synthesis. Structure-activity relationships are reported using surface plasmon resonance and a fluorescence melting assay to examine quadruplex binding, together with a telomerase inhibition assay. These reveal relationships between G-quadruplex stabilization and telomerase inhibition and optimal 3,6- and 9-substituent side-chain lengths for maximal activity. Qualitative molecular modeling using molecular dynamics simulations has been undertaken on four quadruplex-DNA complexes. Long-term exposure of MCF7 cancer cells to a subset of the most active compounds, at doses lower than the IC(50) values, showed that one compound produced a marked decrease in population growth, accompanied by senescence, which is consistent with telomere targeting by this agent.

Induction of prolonged survival of CD4+ T lymphocytes by intermittent IL-2 therapy in HIV-infected patients

HIV infection leads to decreases in the number of CD4 T lymphocytes and an increased risk for opportunistic infections and neoplasms. The administration of intermittent cycles of IL-2 to HIV-infected patients can lead to profound increases (often greater than 100%) in CD4 cell number and percentage. Using in vivo labeling with 2H-glucose and BrdU, we have been able to demonstrate that, although therapy with IL-2 leads to high levels of proliferation of CD4 as well as CD8 lymphocytes, it is a remarkable preferential increase in survival of CD4 cells (with half-lives that can exceed 3 years) that is critical to the sustained expansion of these cells. This increased survival was time-dependent: the median half-life, as determined by semiempirical modeling, of labeled CD4 cells in 6 patients increased from 1.7 weeks following an early IL-2 cycle to 28.7 weeks following a later cycle, while CD8 cells showed no change in the median half-life. Examination of lymphocyte subsets demonstrated that phenotypically naive (CD27+CD45RO-) as well as central memory (CD27+CD45RO+) CD4 cells were preferentially expanded, suggesting that IL-2 can help maintain cells important for host defense against new antigens as well as for long-term memory to opportunistic pathogens.

A kinetic model of telomere shortening in infants and adults

We have previously demonstrated that telomeres shorten more rapidly in peripheral mononuclear cells (PBMC) of infants than in adults (Zeichner et al., Blood 93 (1999) 2824). Here we describe a mathematical model that allows quantification of telomere dynamics both in infants and in adults. In this model the dependence of the telomere dynamics on age is accounted by assuming proportionality between the body growth, as approximated by the Gompertz equation, and the increase in the number of PBMCs. The model also assumes the existence of two subpopulations of PBMC with significantly different rates of division. This assumption is based on the results from a previous analysis of in vitro data for telomere dynamics in presence of telomerase inhibitors and our recent data obtained by measurements of BrdU incorporation in T lymphocytes in humans (Kovacs et al., J. Exp. Med. 194 (2001) 1731). The average telomere length of PBMC was calculated as the average length of these two subpopulations. The model fitted our experimental data well and allowed to derive a characteristic time of conversion of the rapidly proliferating cells to slowly proliferating cells on the order of 20 days. The half-life of the slowly proliferating cells was estimated to be about 6 months, which is in good agreement with data obtained by independent methodologies. Comparison of the one-population and two-subpopulations models demonstrated that one population model cannot explain the observed parameters of the terminal restriction fragment (TRF) dynamics while two-subpopulations model does. These results suggest that the rapid telomere shortening in infants is largely determined by the faster PBMC turnover compared to adults. This may have major implications for elucidation of the HIV pathogenesis in infants. One can speculate that the more rapid course of the HIV disease in infants is due to the existence of rapidly dividing cells, which are susceptible to HIV infection. In addition, these results could have implications for understanding of mechanisms of aging.