Gopalakrishnan Kalpana, Venkatesan Shriram, Low Esther Su Hui, Hande M Prakash
Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 117593, Singapore.
Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 117593, Singapore; Tembusu College, National University of Singapore, 138597, Singapore; VIT University, Vellore, 632 014, India; Mangalore University, Mangalore, 574 199, India.
Mutat Res Genet Toxicol Environ Mutagen. 2018 Dec;836(Pt B):103-113. doi: 10.1016/j.mrgentox.2018.03.008. Epub 2018 Mar 28.
The mTOR pathway and the enzyme telomerase are two key players commonly upregulated in cancers. They render survival and proliferative advantage to cancer cells, and are regarded as attractive anticancer targets. Rapamycin, a macrolide antibiotic and mTOR inhibitor, has recently also been implicated in telomerase inhibition and telomere attrition, although the mechanisms remain poorly understood. Using breast cancer cells (MCF-7 and MDA-MB-231) wherein telomerase activity and mTOR pathway are concurrently overexpressed, this study sought to unravel novel mechanisms by which rapamycin may affect these pathways. Short term treatment with an acute dose of rapamycin inhibited the mTOR pathway and telomerase activity and induced G1 arrest. This arrest was independent of cyclin D1 and p21 levels and was not mediated by DNA damage in both cell types. While long term treatment with a clinically relevant dose of rapamycin resulted in compromised population doubling capacity and mTOR pathway inhibition, there was no effect on telomere functionality and telomerase activity as evidenced by our assessments of hTERT protein levels, in vitro telomerase activity, telomere length and telomere FISH analyses. We also found that sustained rapamycin treatment leading to Akt activation may play a role in resistance in the more invasive MDA-MB-231 cells. In summary, rapamycin specifically inhibits the activation of mTOR pathway. Moreover, we show for the first time that while acute short-term treatment with rapamycin induces telomerase inhibition, it does not affect telomerase activity nor does it inflict telomere dysfunction in breast cancer cells upon chronic long-term treatment with a clinically relevant dose. These findings may be useful while designing combinatorial treatment strategies with rapamycin inhibition in the clinic.
mTOR信号通路和端粒酶是癌症中通常上调的两个关键因子。它们赋予癌细胞生存和增殖优势，被视为有吸引力的抗癌靶点。雷帕霉素是一种大环内酯类抗生素和mTOR抑制剂，最近也被认为与端粒酶抑制和端粒损耗有关，但其机制仍知之甚少。本研究利用端粒酶活性和mTOR信号通路同时过度表达的乳腺癌细胞（MCF-7和MDA-MB-231），试图揭示雷帕霉素可能影响这些信号通路的新机制。急性剂量的雷帕霉素短期处理可抑制mTOR信号通路和端粒酶活性，并诱导G1期阻滞。这种阻滞与细胞周期蛋白D1和p21水平无关，且在两种细胞类型中均不是由DNA损伤介导的。虽然临床相关剂量的雷帕霉素长期处理导致群体倍增能力受损和mTOR信号通路抑制，但根据我们对hTERT蛋白水平、体外端粒酶活性、端粒长度和端粒荧光原位杂交分析的评估，对端粒功能和端粒酶活性没有影响。我们还发现，持续的雷帕霉素处理导致Akt激活可能在侵袭性更强的MDA-MB-231细胞的耐药中起作用。总之，雷帕霉素特异性抑制mTOR信号通路的激活。此外，我们首次表明，虽然雷帕霉素急性短期处理可诱导端粒酶抑制，但在临床相关剂量的长期慢性处理后，它不会影响乳腺癌细胞的端粒酶活性，也不会造成端粒功能障碍。这些发现可能有助于在临床上设计与雷帕霉素抑制联合的治疗策略。
