title: Targeting tumor-initiating cells: eliminating anabolic cancer stem cells with inhibitors of protein synthesis or by mimicking caloric restriction.
abstract: We have used an unbiased proteomic profiling strategy to identify new potential therapeutic targets in tumor-initiating cells (TICs), a.k.a., cancer stem cells (CSCs). Towards this end, the proteomes of mammospheres from two breast cancer cell lines were directly compared to attached monolayer cells. This allowed us to identify proteins that were highly over-expressed in CSCs and/or progenitor cells. We focused on ribosomal proteins and protein folding chaperones, since they were markedly over-expressed in mammospheres. Overall, we identified >80 molecules specifically associated with protein synthesis that were commonly upregulated in mammospheres. Most of these proteins were also transcriptionally upregulated in human breast cancer cells in vivo, providing evidence for their potential clinical relevance. As such, increased mRNA translation could provide a novel mechanism for enhancing the proliferative clonal expansion of TICs. The proteomic findings were functionally validated using known inhibitors of protein synthesis, via three independent approaches. For example, puromycin (which mimics the structure of tRNAs and competitively inhibits protein synthesis) preferentially targeted CSCs in both mammospheres and monolayer cultures, and was ~10-fold more potent for eradicating TICs, than "bulk" cancer cells. In addition, rapamycin, which inhibits mTOR and hence protein synthesis, was very effective at reducing mammosphere formation, at nanomolar concentrations. Finally, mammosphere formation was also markedly inhibited by methionine restriction, which mimics the positive effects of caloric restriction in cultured cells. Remarkably, mammosphere formation was >18-fold more sensitive to methionine restriction and replacement, as directly compared to monolayer cell proliferation. Methionine is absolutely required for protein synthesis, since every protein sequence starts with a methionine residue. Thus, the proliferation and survival of CSCs is very sensitive to the inhibition of protein synthesis, using multiple independent approaches. Our findings have important clinical implications, since they may also explain the positive therapeutic effects of PI3-kinase inhibitors and AKT inhibitors, as they ultimately converge on mTOR signaling and would block protein synthesis. We conclude that inhibition of mRNA translation by pharmacological or protein/methionine restriction may be effective strategies for eliminating TICs. Our data also indicate a novel mechanism by which caloric/protein restriction may reduce tumor growth, by targeting protein synthesis in anabolic tumor-initiating cancer cells.
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Rebecca
authors: R
authors: Lamb
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Hannah
authors: H
authors: Harrison
authors: The Cancer Research UK Manchester Institute, University of Manchester, UK.
authors: Duncan L
authors: DL
authors: Smith
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.
authors: Paul A
authors: PA
authors: Townsend
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.
authors: Thomas
authors: T
authors: Jackson
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Bela
authors: B
authors: Ozsvari
authors: The Kimmel Cancer Center, Philadelphia, PA, USA.
authors: Ubaldo E
authors: UE
authors: Martinez-Outschoorn
authors: The Kimmel Cancer Center, Philadelphia, PA, USA.
authors: Richard G
authors: RG
authors: Pestell
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Anthony
authors: A
authors: Howell
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Michael P
authors: MP
authors: Lisanti
authors: The Manchester Centre for Cellular Metabolism (MCCM), Institute of Cancer Sciences, University of Manchester, UK.|The Breakthrough Breast Cancer Research Unit, Institute of Cancer Sciences, University of Manchester, UK.
authors: Federica
authors: F
authors: Sotgia
mesh_terms: D032701:Biomimetics; D031204:Caloric Restriction; D049109:Cell Proliferation / Q000187:drug effects; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000494:pharmacology; D014411:Neoplastic Stem Cells / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D014176:Protein Biosynthesis / Q000187:drug effects*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology*; D011691:Puromycin / Q000494:pharmacology; D020123:Sirolimus / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D007166:Immunosuppressive Agents; D011500:Protein Synthesis Inhibitors; D011691:Puromycin; D020123:Sirolimus
keywords: methionine restriction; protein synthesis; puromycin; rapamycin; tumor initiating cells