Background: Activation of telomerase is essential for the indefinite replication potential of most cancer cells. Inhibition of telomerase is expected to lead to loss of telomere maintenance resulting in cell cycle arrest and/or cancer cell death, making telomerase inhibition an attractive anti-cancer approach. Cancer stem cells (CSCs) are rare cells in tumors implicated in cancer initiation and potentiation, as well as persistence or recurrence after standard treatment. Unlike normal tissue counterparts, all CSCs tested to date have upregulated telomerase activity. Recent identification of several non-canonical roles of telomerase components may expand the functionality of telomerase-inhibiting drugs. Imetelstat, a potent telomerase inhibitor currently in Phase II clinical trials, has been shown to reduce proliferative potential in multiple cancer models. Several recent studies have demonstrated that imetelstat depletes CSCs in various tumor types. The relatively rapid onset of CSC depletion could probably indicate a mechanism independent of telomere shortening. To understand the contributions of non-canonical pathways in the response of cancer cells to imetelstat treatment, we analyzed modulations of key signaling pathways in imetelstat-treated glioblastoma multiforme cancer cell lines (U87-MG and U118) and their CSC and bulk subsets.

Results: Imetelstat inhibited telomerase activity, proliferative capacity and colony-forming potential in the U87 and U118 cell lines. Imetelstat treatment (2 weeks at 3uM) resulted in decreased expression of casein kinase 2 (CK2) subunits alpha and beta, and a reduction in phosphorylation of downstream CK2 substrates such as the DNA repair protein XRCC1. In addition, imetelstat reduced the transcriptional activity of beta-catenin, which is regulated by members of the CK2 family, in U87-MG cells. Cyclin D1, which is regulated by beta-catenin activity, was found to be down regulated by imetelstat treatment. These pathway modulations were not observed in T98G, a cell line resistant to imetelstat-mediated proliferation effects. Reducing CK2-alpha expression had an additive effect on cell growth inhibition in combination with imetelstat in U87-MG cells.

Knock-down of the CK2-alpha, but not CK2-beta subunits, with siRNAs rapidly reduced the numbers of CSCs in U87-MG and U118 cell lines. Concomitant with the down-regulation of CK2-alpha signaling, imetelstat treatment depleted the number of CSCs in U78-MG and U118 cell lines. Preliminary data indicate a more marked reduction of CK2-alpha levels in the CSC subset of U87-MG cells compared to the bulk cells upon short term imetelstat treatment (1 week at 3uM). These in-vitro exposure concentrations of imetelstat are comparable to those attainable intratumorally in a xenograft mouse model.

Conclusions: Our results suggest that CK2-alpha signaling and dysregulation of its downstream targets may play a key role in survival of CSCs as well as bulk tumor cells in U87 and U118 glioblastoma cell lines. These novel insights may help identify drugs that can act synergistically with imetelstat in treating cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A56.