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A Novel Role for Telomerase: Regulating Growth of Breast Epithelial Cells
Telomerase is a ribonuclear protein complex that stabilizes chromosome ends by extending telomeric DNA. It is comprised of a catalytic subunit (hTERT) and a structural RNA component (hTR). hTERT is expressed at low levels in somatic cells, and ectopic hTERT expression is sufficient to immortalize primary cells. hTERT-mediated telomere maintenance is well studied and is known to play a critical role in transformation and tumorigenesis. Recent studies have suggested that hTERT can contribute to cancer by a second mechanism that is independent of its role in telomere stabilization. Research from our lab provided the first evidence that hTERT can mediate growth factor-independent proliferation of primary human mammary epithelial cells. It further showed that hTERT modulates the expression of genes that can promote growth and survival of these cells. The goal of this proposal is to elucidate how this novel role of hTERT contributes to the initiation and progression of breast cancer. We will identify hTERT mutants that can carry out only one of the two roles of telomerase. We will use a panel of hTERT mutants to screen for such ?separation of function? mutants. Once identified, we will use these mutants to directly compare the specific contributions that hTERT-dependent telomere maintenance and gene expression make to proliferation, survival, oncogenic transformation and tumorigenesis in mammary epithelial cells. The mutants will also allow us to map the hTERT domain(s) required for transcriptional regulation, and thus determine whether they are distinct from those known to be required for telomere maintenance. We will then determine if the transcriptional regulation function of hTERT is necessary for it to cooperate with specific oncogenes to induce transformation and tumorigenesis in mammary epithelial cells. We will study this cooperation by using soft agar assays to measure changes in anchorage independent growth, and a three-dimensional cell culture model to identify any aberrations in mammary acinar morphogenesis. The results we obtain may directly impact our understanding of telomerase's role in breast cancer. Our studies could also have important therapeutic implications. Reactivation of endogenous telomerase is observed in over ninety percent of breast carcinomas; however, there has been skepticism about developing anti-telomerase drugs based solely on its ability to stabilize chromosome ends. This is because even if a drug successfully inhibited telomerase, tumor cells would still replicate many times before chromosomes shortened critically and crisis occurred. Blocking a second function of telomerase that directly contributes to the growth of cancer cells, could stop tumor growth more rapidly, and thus make anti-telomerase drugs more attractive. Our work could help provide the basis for the rational development of new treatments targeting the novel oncogenic function of hTERT and/or its downstream signaling pathways.
Chromosome ends progressively wear away with each round of cell division, and this eventually causes normal cells to stop growing. This provides a barrier to uncontrolled growth that is a hallmark of cancer. Telomerase is an enzyme which can prolong the lifespan of a cell by restoring the chromosome ends. Not surprisingly, telomerase's lifespan-extending activity is switched on in more than ninety percent of human breast cancers. Recently, a number of studies have raised questions about whether telomerase's role in immortality adequately explains its association with cancer. This led to the discovery of a second cancer-promoting function for telomerase. Research from our laboratory showed for the first time that telomerase could regulate the growth of human breast cells. When telomerase was switched on in these cells, they were no longer dependent on normal stimulatory molecules, called growth factors, in order to multiply. The research further showed that telomerase altered the activity of genes that trigger cell growth and survival. The goal of this proposal is to understand how this novel role of telomerase contributes to the initiation and progression of breast cancer. We have multiple versions (mutants) of telomerase, and in each version a different region of the enzyme is altered (mutated). We will put these mutants into normal breast cells, and determine which mutants can carry out only one of the roles of telomerase. For example, if a mutant can only make breast cells grow without growth factors, but not make them live longer, then we will have identified a mutant that can separate the two roles of telomerase ie. a ?separation of function? mutant. Once we identify such mutants, we will use them in two systems to determine how the two roles of telomerase each contribute to breast cancer. Using the first system, we will determine if the cells acquire the tumor-like property of growing without attaching to a base. In the second system, we will grow cells in a more physiological environment where they can form structures resembling those seen in normal human breasts. We will test if the formation of these structures is altered to resemble early stage breast cancer. The results we obtain may directly impact our understanding of telomerase's role in breast cancer. Our work could also have important implications for treatment. There has been uncertainty about developing anti-telomerase drugs based solely on its ability to stabilize chromosome ends. This is because even if a drug successfully blocked telomerase, tumor cells would still multiply many times before the chromosomes ends wore down and caused cells to stop growing. Blocking a second function of telomerase that directly contributes to the growth of cancer cells, could stop tumor growth more rapidly, and thus make anti-telomerase drugs more attractive. Our studies could help provide the basis for designing new therapies targeting the novel role of telomerase.