Cancer Stem Cells in Brain Tumors: Identification of Critical Biological Effectors
Human cancer is a leading cause of morbidity and mortality in the developed world. Contrary to the classical model in which tumors are homogeneously composed of malignant cells, accumulating evidence suggests that subpopulations of highly malignant cells play a dominant role in tumor initiation and growth. These cells have the capacity for prolonged self-renewal and they efficiently generate tumors that phenotypically resemble the parental tumor in transplantation assays. Such characteristics are reminiscent of normal stem cells, and these potently tumorigenic cells have therefore been called cancer stem cells (CSCs). Importantly, studies have shown that CSCs are central mediators of therapeutic resistance, tumor angiogenesis, and metastatic or invasive potential. In the case of malignant glioma, poor patient survival and the paucity of effective therapeutic advances have been attributed to inherent CSC growth potential and treatment resistance, respectively. For this reason, there is great interest in elucidating the molecular features of CSCs, with the ultimate hope of developing CSC-directed therapies.
Given the overlap between the highly malignant characteristics exhibited by CSCs and those promoted by the PI3K/AKT pathway, we hypothesized that AKT activity within CSCs could represent a reasonable therapeutic target for CSC-directed therapies. Indeed, a pharmacological inhibitor of AKT preferentially targeted glioma CSCs versus non-CSCs and was associated with increased apoptosis and impaired tumorigenesis. These data suggest that interventions targeting AKT could effectively target glioma CSCs.
Quite distinct from the PI3K/AKT pathway, we hypothesized that the pro-survival and pro-growth features of nitric oxide (NO) might also operate in glioma CSCs. Our experiments found that glioma CSCs produced more NO than non-CSCs, which is attributed to inducible nitric oxide synthase (iNOS) expression and activity within the CSCs. Interference with iNOS activity or expression, as well as selective NO consumption, attenuated CSC growth and tumorigenicity. The mechanism behind iNOS-mediated survival appears to involve, at least in part, suppression of the cell cycle inhibitor CDA1. iNOS inhibition decreased glioma growth in murine xenografts and human expression studies demonstrate an inverse correlation between iNOS expression and patient survival.
To more fully evaluate the biological effects of NO in CSCs, we designed a novel strategy to consume NO within mammalian cells through heterologous expression of E. coli flavohemoglobin (FlavoHb). This enzyme is a highly specific NO dioxygenase which converts NO to inert nitrate several orders of magnitude faster than iNOS synthesizes NO. Expression of FlavoHb in mammalian cells is therefore a novel and functional tool to interrogate the role of NO in cellular stress and signaling.
In summary, this doctoral thesis focuses on several molecular characteristics that define malignant CSCs and describes a novel strategy for studying NO, which is one of the CSC-specific molecular effectors.
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