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Peripheral T-Cell Lymphomas (PTCL) are rare cancers that lack effective therapies, resulting in poor long-term survival in patients. Cancer stem cells (CSCs) represent a subpopulation within tumors that have been implicated in drug resistance, recurrence, and metastasis, all of which influence the long-term survival of patients. Therefore, to increase patient response, it will be necessary to identify inhibitors that efficiently eliminate CSCs. Initial methods for targeting CSCs have relied on disrupting stem cell proliferation, a process called self-renewal. However, multiple pathways can govern the self-renewal process, potentially providing redundancy to treatments which target only one of these pathways. A better approach would be to block multiple self-renewal pathways simultaneously through inhibition of the molecular chaperone Heat Shock Protein 90 (HSP90) with 17-AAG. As human PTCL is comprised of many tumor subtypes, a transgenic mouse model of T-cell lymphoma was used to examine the efficacy of 17-AAG in treating lymphoma CSCs. Strikingly, lymphoma and Acute Myeloid Leukemia (AML) CSCs were selectively sensitive to 17-AAG treatment, as low
concentrations of 17-AAG induced apoptosis and ablated their ability to form colonies, while having little effect on the differentiated tumor bulk. These effects were found to result from the loss of HIF1a function, which is crucial for the maintenance of both mouse lymphoma and AML CSCs. In addition, the non-CSCs were shown to express higher levels of Heat Shock Factor-1 (HSF1), a transcription factor which regulates chaperone genes such as HSP90 and HSP70. Knockdown of HSF1 resulted in reduced mRNA expression of HSP90α, reduced colony formation, and sensitization of non-CSCs to 17-AAG treatment. This implicates high HSF1 expression as a potential 17-AAG resistance mechanism. Lastly, to broaden the scope of 17-AAG as a CSC-targeting agent, 17-AAG was tested for the ability to sensitize breast CSCs to radiation, a conventional treatment. Interestingly, radioresistance and sensitization to radiation with 17-AAG was not universally observed in CSCs within and between breast cancer lines. Taken together, these results validate 17-AAG mediated HSP90 inhibition as a method for treating lymphoma CSCs, and merits clinical reevaluation as a CSC-sensitizer to conventional cancer therapy. |
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