The telomerase inhibitor imetelstat alone, and in combination with trastuzumab, decreases the cancer stem cell population and self-renewal of HER2+ breast cancer cells
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Cancer stem cells (CSCs) are thought to be responsible for tumor progression, metastasis, and recurrence. HER2 overexpression is associated with increased CSCs, which may explain the aggressive phenotype and increased likelihood of recurrence for HER2+ breast cancers. Telomerase is reactivated in tumor cells, including CSCs, but has limited activity in normal tissues, providing potential for telomerase inhibition in anti-cancer therapy. The purpose of this study was to investigate the effects of a telomerase antagonistic oligonucleotide, imetelstat (GRN163L), on CSC and non-CSC populations of HER2+ breast cancer cell lines. The effects of imetelstat on CSC populations of HER2+ breast cancer cells were measured by ALDH activity and CD44/24 expression by flow cytometry as well as mammosphere assays for functionality. Combination studies in vitro and in vivo were utilized to test for synergism between imetelstat and trastuzumab. Imetelstat inhibited telomerase activity in both subpopulations. Moreover, imetelstat alone and in combination with trastuzumab reduced the CSC fraction and inhibited CSC functional ability, as shown by decreased mammosphere counts and invasive potential. Tumor growth rate was slower in combination-treated mice compared to either drug alone. Additionally, there was a trend toward decreased CSC marker expression in imetelstat-treated xenograft cells compared to vehicle control. Furthermore, the observed decrease in CSC marker expression occurred prior to and after telomere shortening, suggesting that imetelstat acts on the CSC subpopulation in telomere length-dependent and -independent mechanisms. Our study suggests addition of imetelstat to trastuzumab may enhance the effects of HER2 inhibition therapy, especially in the CSC population.
KeywordsTelomerase Breast cancer HER2+ Cancer stem cells Trastuzumab
We would like to thank the members of the Herbert Lab for helpful discussion preparing this manuscript. We thank Geron Corporation for generously providing the imetelstat and sense oligonucleotides, Dr. Francisco Esteva for kindly sharing the SKBR3 and SKBR3-R cell lines, Dr. Harikrishna Nakshatri for kindly sharing the TMD-231 cell line, Dr. Harlan Shannon for help with the combination studies, Dr. George Sandusky for help with histology, Dr. Hiromi Tanaka for help with TeloTAGGG analysis, the Indiana University Simon Cancer Center (IUSCC) flow cytometry core facility for their services and expertise, and the IUSCC infusion pharmacy for generously providing the trastuzumab. This investigator was supported, in part, by the National Institutes of Health, National Research Service Award Number T32 HL007910- Basic Science Studies on Gene Therapy of Blood Diseases. This work was also supported in part by a Grant from Susan G. Komen for the Cure®, an IUSCC Cancer Biology Training Program Predoctoral Fellowship, a grant from the Mary Kay Ash Charitable Foundation, and the Indiana Genomics Initiative (INGEN; supported in part by the Lilly Endowment, Inc). We are also grateful for the philanthropic support made to the Herbert laboratory through IUSCC in memory of Carol Herbert.
Conflict of interest
Imetelstat and sense oligonucleotides were generously provided by the Geron Corporation. The authors declare they have no other conflicts of interest.
The authors declare that the experiments performed comply with the current laws.
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