Abstract SY33-03: Role of telomerase in normal and neoplastic stem cells (2010)

Abstract We have used hTERT (the catalytic component of telomerase) to immortalize a variety of human cell types (sometimes in combination with Cdk4 to bypass cell culture stress). Cell types immortalized include skin keratinocytes and fibroblasts, muscle satellite cells, breast epithelial and stromal cells, corneal epithelial cells and fibroblasts (keratocytes), and human colonic epithelial cells. In addition, we have immortalized human bronchial epithelial cells (HBEC) and have determined that these cells can terminally differentiate into both central and peripheral lung cell types. These immortalized HBECs have been used to study the molecular pathogenesis of lung cancer by stable “knock down” of TP53 and also by over-expression of C-myc and mutant K-rasV12. When these cells containing multiple genetic alterations are introduced into immunosuppressed mice, the experimentally transformed cells make tumors that represent several distinct histological types. This suggests that these immortalized and transformed HBECs have bronchiolar-alveolar stem-like characteristics that can differentiate into multiple lineages. Stem cells are defined by both their ability to make more stem cells (self renewal) and their ability to produce cells that can differentiate. Experimentally immortalized human bronchial epithelial cells fulfill this definition of normal stem cells by continuous self renewal and by retaining the capability of differentiating into several cell types. Since experimentally transformed cells make lung tumors representing several major lineages, this is also an indication that the HBECs are derived from a multi-potent lung stem cell. Similar to normal stem cells, cancer (initiating) stem cells also have the ability to self-renew as well as undergo differentiation to give rise to phenotypically diverse types of cancer cells. There is mounting evidence that these rare cancer stem cells may be multidrug resistant and responsible for tumor relapse and metastasis. Targeted cancer therapeutic approaches seek to identify pathways that are more tumor specific, resulting in fewer side-effects and that may produce long-term durable responses. Telomerase is a novel cancer therapeutic target since it is activated in the vast majority of human cancers and telomeres of almost all human tumor cells are maintained at short but stable lengths. In addition, telomerase is not expressed or is expressed at levels that do not fully maintain telomeres in normal tissues, and telomeres are generally longer in normal stem cells compared to cancer cells. This potentially provides a therapeutic advantage for targeting telomerase over approaches that affect both normal and cancer cells equally. We have previously reported that telomerase positive cancer cells that are experimentally induced to undergo quiescence, down regulate telomerase. As part of our anti-telomerase therapeutics program, we have addressed the following questions: Are putative cancer stem cell populations quiescent and do they have short or long telomeres? While the molecular characteristics of cancer stem cells are not completely defined and subject to some controversies, we have isolated and examined cells expressing these cancer stem cell putative markers reported for breast, brain, prostate, pancreas, and lung cancer. In each case we have observed that purified cancer stem cell populations are positive for telomerase activity, indicating they are not quiescent. In addition, a telomerase inhibitor currently being tested in clinical trials robustly inhibits the activity of telomerase in these sorted sub-populations of putative cancer stem cells as well as the mass population of cancer cells. Finally, we have observed that cancer stem cells have short telomeres in comparison to normal stem cells. These findings support the idea that there may be a therapeutic window of opportunity to target cancer stem cells by inhibiting telomerase, thus driving telomeres progressively shorter leading to cancer stem cell death, potentially without irreversible damage to normal stem cells. Cancer remains a major cause of death in spite of substantial progress towards understanding the molecular basis of many types of cancers. The discovery of new drugs is a high priority, and telomerase inhibitors have the potential to act by a novel mechanism that will provide new options for cancer therapy. A review of ongoing anti-telomerase clinical trials will be presented. In summary, telomerase inhibitors might not only directly limit or stop the growth of human tumors including cancer stem cells, but might also act in an additive or synergistic fashion with existing therapies to amplify their effectiveness. Citation Format: Jerry W. Shay. Role of telomerase in normal and neoplastic stem cells [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY33-03