BACKGROUND

Cancer stem-like cells (CSCs) represent a subset of tumor cells that have the ability to self-renew, a long lifespan and a relatively quiescent phenotype, and show resistance to conventional therapies. Various markers are used to identify CSCs, and have shown that different CSC subtypes may be present within a tumor. One functional property of CSCs is their relative lack of proteasomal activity compared to the tumor bulk.

METHODS

We introduced an unstable fluorescent molecule into FaDu oropharyngeal squamous cell carcinoma cells and analyzed the association of proteasome activity with aldehydehyde dehydrogenase (ALDH) activity as another common CSC marker, and with other stem-cell related properties of glucose metabolism. We also analyzed publicly available gene expression profiling data of ALDH CSCs for alterations in mRNAs associated with proteostasis.

RESULTS

We show that FaDu CSCs identified by low proteasome activity are associated with the population identified by high ALDH activity. Futher characterization shows that these CSCs have a relatively high mitochondrial membrane potential and low levels of glucose transporter, indicating a non-Warburg metabolic phenotype. We also show that proteasome-low FaDu CSCs exhibit decreased rates of protein synthesis. Gene expression profiling of other cancer cell lines reveal common statistically significant differences in proteostasis in ALDH CSCs compared to the bulk of the tumor cells, including reduced levels of Hsp70 and/or Hsp90 in CSCs defined by ALDH, together with reduced levels of UCHL5 mRNA.

CONCLUSIONS

These data provide additional insights into the functional characteristics of proteasome-low/ALDH-high CSCs, indicating a metabolic phenotype of reduced reliance on aerobic glycolysis and a decreased protein synthesis rate. We also identify specific chaperone and ubiquitin ligase activities that can be used to identify CSCs, with corresponding implications for therapeutic strategies that target CSCs through their altered metabolic properties.