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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.

Specific changes in gene expression during cancer initiation should enable discovery of biomarkers for risk assessment, early detection and targets for chemoprevention. It has been previously demonstrated that altered mRNA and proteome signatures of morphologically normal cells bearing a single inherited "hit" in a tumor suppressor gene parallel many changes observed in the corresponding sporadic cancer. Here, we report on the global gene expression profile of morphologically normal, cultured primary breast epithelial and stromal cells from Li-Fraumeni syndrome (LFS) TP53 mutation carriers. Our analyses identified multiple changes in gene expression in both morphologically normal breast epithelial and stromal cells associated with TP53 haploinsufficiency, as well as interlocking pathways. Notably, a dysregulated p53 signaling pathway was readily detectable. Pharmacological intervention with the p53 rescue compounds CP-31398 and PRIMA-1 provided further evidence in support of the central role of p53 in affecting these changes in LFS cells and treatment for this cancer. Because loss of signaling mediated by TP53 is associated with the development and survival of many human tumors, identification of gene expression profiles in morphologically normal cells that carry "one-hit" p53 mutations may reveal novel biomarkers, enabling the discovery of potential targets for chemoprevention of sporadic tumors as well.

HER2 amplification in breast cancer is associated with a more aggressive disease, greater likelihood of recurrence, and decreased survival compared to women with HER2-negative breast cancer. Trastuzumab is a monoclonal antibody that inhibits HER2 activity, making this compound an important therapeutic option for patients with HER2-positive breast cancer. However, resistance to trastuzumab develops rapidly in a large number of breast cancer patients. The objective of this study was to determine whether GRN163L, a telomerase template antagonist currently in clinical trials for cancer treatment, can augment the effects of trastuzumab in breast cancer cells with HER2 amplification. GRN163L was effective in inhibiting telomerase activity and shortening telomeres in HER2-positive breast cancer cells. We show that GRN163L acts synergistically with trastuzumab in inhibiting HER2-positive breast cancer cell growth. More importantly, we show that GRN163L can restore the sensitivity of therapeutic-resistant breast cancer cells to trastuzumab. These findings implicate that telomerase template antagonists have potential use in the treatment of cancers that have developed resistance to traditional cancer therapy.

Telomeres are repetitive (TTAGGG)(n) DNA sequences found at the end of chromosomes that protect the ends from recombination, end to end fusions, and recognition as damaged DNA. Telomerase activity can be detected in 85% to 90% of human tumors, which stabilizes telomeres to prevent apoptosis or cellular senescence. Previous reports showed the efficacy of the novel telomerase template antagonist, GRN163L, as a potential anticancer agent. The objective of the present study was to elucidate the molecular effects of GRN163L in MDA-MB-231 breast cancer cells and to determine whether GRN163L could be used in mechanism-based combination therapy for breast cancer. We observed that GRN163L reduced MDA-MB-231 growth rates without a significant effect on breast cancer cell viability within the first 14 days in vitro. In addition, GRN163L altered cell morphology, actin filament organization, and focal adhesion formation in MDA-MB-231 cells. Importantly, the cellular response to GRN163L significantly augmented the effects of the microtubule stabilizer paclitaxel in MDA-MB-231 breast cancer cell growth in vitro and in vivo compared with paclitaxel alone or a mismatch control oligonucleotide plus paclitaxel. Furthermore, in vitro MDA-MB-231 invasive potential was significantly inhibited with GRN163L and paclitaxel. These data support a rationale for potentially combining GRN163L with paclitaxel for the treatment of breast cancer in the clinical setting.

Purpose: Telomerase is expressed in 80-90% of tumor cells, but is absent in most somatic cells. The absence of telomerase activity results in progressive telomere shortening, leading to cellular senescence or death through deoxyribonucleic acid (DNA) damage signals. In addition, a role for telomerase in DNA damage repair has also been suggested. A specific telomerase inhibitor, GRN163L that is complementary to the template region of the telomerase ribonucleic acid component (hTR). We hypothesized that exposure to GRN163L, either through immediate inhibition of telomerase activity or through eventual telomere shortening and dysfunction, may enhance radiation sensitivity. Our goal was to test whether the treatment with GRN163L enhances sensitivity to irradiation (IR) in MDA-MB-231 breast cancer cells., Methods and Materials: The MDA-MB-231 breast cancer cells were treated with or without GRN163L for 2-42 days. Inhibition of telomerase activity and shortening of telomeres were confirmed. Cells were then irradiated and clonogenic assays were performed to show cell survival differences. In vivo studies using MDA-MB-231 xenografts were performed to corroborate the in vitro results., Results: We show that cells with shortened telomeres due to GRN163L enhance the effect on IR reducing survival by an additional 30% (p < 0.01). These results are confirmed in vivo, with a significant decrease in tumor growth in mice exposed to GRN163L., Conclusions: We found that GRN163L is a promising adjuvant treatment in combination with radiation therapy that may improve the therapeutic index by enhancing the radiation sensitivity. These studies prompt further investigation as to whether this combination can be applied to other cancers and the clinic.

Two isoforms of proliferating cell nuclear antigen (PCNA) have been observed in breast cancer cells. Commercially available antibodies to PCNA recognize both isoforms and, therefore, cannot differentiate between the PCNA isoforms in malignant and nonmalignant breast epithelial cells and tissues. We have developed a unique antibody that specifically detects a PCNA isoform (caPCNA) associated with breast cancer epithelial cells grown in culture and breast-tumor tissues. Immunostaining studies using this antibody suggest that the caPCNA isoform may be useful as a marker of breast cancer and that the caPCNA-specific antibody could potentially serve as a highly effective detector of malignancy. We also report here that the caPCNA isoform functions in breast cancer-cell DNA replication and interacts with DNA polymerase delta. Our studies indicate that the caPCNA isoform may be a previously uncharacterized detector of breast cancer.

Purpose: Maintenance of telomeres by telomerase is critical for the continuing proliferation of most advanced cancer cells. Telomerase activity has been detected in the vast majority of cancer cells but not most normal cells, making the enzyme an attractive target for anticancer therapy. The aim of this study was to address the breast cancer translational potential of the novel telomerase inhibitor, GRN163L., Experimental Design: In the present study, we investigated the effects of GRN163L treatment on a panel of breast cancer cells representing different tumor subtypes with varying genetic backgrounds, including ER+, ER-, HER2+, BRCA1 mutant breast tumor cells as well as doxorubicin-resistant cancer cells. To investigate the in vivo effects of GRN163L, we employed a breast cancer xenograft and metastasis model that simulates a clinical situation in which a patient arrives with a primary tumor that may be then treated or surgically removed., Results: GRN163L effectively inhibited telomerase activity in a dose-dependent fashion in all breast cancer cell lines resulting in progressive telomere shortening. A mismatch control oligonucleotide showed no effect on telomerase activity and GRN163L did not significantly affect telomere shortening in normal human mammary epithelial cells or in endothelial cells. Breast cancer cells that exhibited telomerase inhibition also exhibited significant reduction in colony formation and tumorigenicity. Furthermore, GRN163L suppressed tumor growth and lung metastases (P = 0.017) of MDA-MB-231 cells in vivo after 4 weeks of treatment., Conclusions: These results show in vivo effectiveness of GRN163L in breast cancer and support its promising clinical potential for breast cancer treatment.

Recent microarray studies have identified distinct subtypes of breast tumors that arise from different cell types and that show statistically significant differences in patient outcome. To gain insight into these differences, we identified in vitro and in vivo changes in gene expression induced by chemotherapeutics. We treated two cell lines derived from basal epithelium (immortalized human mammary epithelial cells) and two lines derived from luminal epithelium (MCF-7 and ZR-75-1) with chemotherapeutics used in the treatment of breast cancer and assayed for changes in gene expression using DNA microarrays. Treatment doses for doxorubicin and 5-fluorouracil were selected to cause comparable cytotoxicity across all four cell lines. The dominant expression response in each of the cell lines was a general stress response; however, distinct expression patterns were observed. Both cell types induced DNA damage-response genes such as p21(waf1), but the response in the luminal cells showed higher fold changes and included more p53-regulated genes. Luminal cell lines repressed a large number of cell cycle-regulated genes and other genes involved in cellular proliferation, whereas the basal cell lines did not. Instead, the basal cell lines repressed genes that were involved in differentiation. These in vitro responses were compared with expression responses in breast tumors sampled before and after treatment with doxorubicin or 5-fluorouracil/mitomycin C. The in vivo data corroborated the cell-type-specific responses to chemotherapeutics observed in vitro, including the induction of p21(waf1). Similarities between in vivo and in vitro responses help to identify important response mechanisms to chemotherapeutics.

Human telomerase is a unique reverse transcriptase that is expressed in multiple cancers, but not in the vast majority of normal cells. The enzyme is responsible for telomere protection and maintenance, and supports the proliferative immortality of cancer cells. Thus, it has been proposed that the specific inhibition of telomerase activity in tumors might have significant and beneficial therapeutic effects. To this goal we have designed, synthesized, and evaluated several oligonucleotide N3'-->P5' phosphoramidates as telomerase inhibitors. These oligonucleotides are complementary to the template region of the RNA domain of telomerase (hTR). The prepared compounds were evaluated in HME50-5E breast epithelial cells, where their effects on telomerase activity were determined using a cell-based telomerase (TRAP) assay at 24 as well as 72 h after exposure to compounds. The oligo-N3'-->P5' phosphoramidate inhibited telomerase activity in cells in the presence of the cellular up-take enhancer (FuGENE6) in a dose- and sequence-dependent manner, with IC(50) values of approximately 1 nM. Inhibition of telomerase activity by this compound without the lipid carrier was not efficient. However, the isosequential oligonucleotide N3'-->P5' thio-phosphoramidate was able to inhibit telomerase activity with or without lipid carriers at nM, or low-microM concentrations, respectively. This inhibition of telomerase activity in HME50-5E cells by the oligonucleotide thio-phosphoramidates was also sequence specific. Long-term treatment of the cells with 0.5 microM of FuGENE6 formulated 13-mer thio-phosphoramidates, fully complementary to hTR, resulted in gradual telomere shortening, followed by cellular senescence and apoptosis, as would be predicted for a telomerase inhibitor. The mismatched control compound had no effect on cell proliferation. The results suggest that the oligonucleotide N3'-->P5' phosphoramidates, and particularly thio-phosphoramidates, might be further developed as selective anti-telomerase reagents.