Telomerase is an
RNA-dependent polymerase that synthesizes telomeric
DNA (TTAGGG)n repeats. The overall goal of our work was to establish human
cancer models that can be used to design clinical trials with
telomerase inhibitors. The objectives of this study were (1) to set up a human
breast cancer system that allows evaluation of the effects of
telomerase inhibitors in cultured cells using a non-amplified
telomerase assay and (2) to test this system using two drugs (
cisplatin and
TMPyP4) that affect the
telomerase expression in
breast cancer cells in culture. We first compared the
telomerase activity in a variety of human
breast cancer cell lines to that of other tumour types using a new biotinylated-primer extension assay. Our method, based on a non-amplified primer extension assay shows the direct incorporation of 32P-labelled
nucleotides induced by
telomerase on human telomeric primers. The 32P-dGTP labelled
telomerase-extended 5'-biotinylated (TTAGGG)3 primer can subsequently be separated using
streptavidin-coated magnetic beads. As compared to other non-amplified method, we showed that this procedure improved the characterization and the quantification of the banding pattern resulting from
telomerase extension by reducing the radioactive background. Using this method, we observed that
telomerase activity varies markedly in a panel of 39 human
cancer cell lines. For example, MCF7
breast cancer cells in culture showed intermediate
telomerase activity corresponding to 33.8+/-3.4% of that of the HeLa cells (reference cell line). Similarly, the telomere length varied with each cell line (average: 6.24+/-6.16). No correlation between the level of
telomerase and telomere length was observed, suggesting that a high processivity is not required to maintain telomeres and that, in some cell lines, another mechanism of telomere elongation can maintain telomere length. From this study, we selected MCF7 and MX1 models that showed reproducible
telomerase activity and a relatively limited telomere length for the testing of potential telomere-
telomerase interacting agents. Using
cisplatin and a new
porphyrin-derived compound
TMPyP4, we showed that our model was able to detect a down-regulation of the
telomerase activity in MCF7 cells in culture and in a human MX1 tumour xenografts. Based on these results, a
breast cancer model for evaluating
telomerase and telomere interactive agents is proposed.