Telomerase is a good target for new anticancer drug development because it is present in over 85% of human tumours. However, despite chronic therapy is a condition for anti-telomerase approach, the effects of long-term treatment with telomerase inhibitors remain not well understood. In this work, it was evaluated the effects of long-term treatment of human MDA-MB-231 breast cancer cells with the telomerase inhibitor MST-312. Cells were treated for 72 hours or 140 days, and it was accessed their viability, proliferation rate, morphology, telomeric DNA content, and resistance mechanism. The drug had a clear short-term effect, including chemosensitizing cells for docetaxel and irinotecan, but the chronic exposition led to selection of long telomeres clones, changing characteristics of original cell line. This effect was confirmed in a clonal culture with homogenous karyotype. MRP-1 expression and alternative lengthening of telomeres (ALT) were discarded as additional mechanisms of resistance. This data suggest that, considering the intra-tumour heterogeneity (ITH), what is already a big challenge for treatment of cancer, chronic exposition to telomerase inhibitors can promote tumour adaptations with potential clinical repercussion, drawing attention to ongoing clinical trials and pointing important considerations most times neglected on studies about use of these inhibitors on cancer therapy.Significance of the study: Antitumour action of telomerase inhibitors is well known, but it depends on a long-term exposition because cells will undergo telomere erosion only after many duplication cycles. Recently, the frustrating results of clinical trials with these inhibitors aroused the interest of the scientific community to understand the mechanisms of resistance to anti-telomerase therapy. In this study, we conducted an 18-week experiment to show that telomerase inhibition can lead to cell adaptations and selection of long-telomeres clones, leading to acquisition of resistance. However, we also showed that this inhibitor can sensitize cells to the chemotherapeutic drugs docetaxel and irinotecan.
As células normais tem potencial proliferativo regulado, e na maioria dos tipos celulares, obedecem a um número determinado de ciclos. Para que uma célula possa proliferar indefinidamente e de forma autônoma e ser considerada uma célula de câncer, ela deve passar pelo processo de imortalização. Os telômeros estão intimamente relacionados com o processo de imortalização, pois encurtam a cada ciclo celular, limitando a capacidade proliferativa. A telomerase é uma enzima que restaura a integridade dos telômeros, e está presente em 85% dos tipos de câncer, o que a torna um potencial alvo terapêutico. O câncer de mama é o segundo tipo de câncer mais incidente em todo o mundo, e entre as mulheres é o primeiro em mortalidade. Dada a sua importância epidemiológica, foram utilizadas neste estudo linhagens de câncer de mama que expressam telomerase, MCF-7 e MDA-MB-231, submetidas a tratamentos de curto e longo prazo com o inibidor de telomerase MST-312. Este foi citotóxico para ambas as linhagens em tratamentos de curto prazo, com morte celular acentuada nas doses de 5 µM em MCF-7 e 10 µM em MDA-MB-231, efeito este que independe da crise telomérica. Após 18 semanas de tratamento em dose subtóxica do MST-312, as linhagens foram novamente submetidas aos ensaios de citotoxicidade. A linhagem de MCF-7 apresentou maior sensibilidade ao MST-312 após o tratamento de longo prazo, com sinais de senescência celular acentuadas nos grupos tratados com o inibidor e cariótipos característicos de crise telomérica. Enquanto que a linhagem MDA-MB-231 demonstrou sinais de aquisição de resistência à droga, não demonstrando senescência celular, e maior viabilidade celular no grupo tratado com MST-312 por 18 semanas, quando comparado com o grupo controle. Demonstrando que células que expressam telomerase respondem de maneira diferente ao mesmo inibidor.
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