Tumor cell metabolic heterogeneity is thought to contribute to
tumor recurrence, distant
metastasis and chemo-resistance in
cancer patients, driving poor clinical outcome. To better understand
tumor metabolic heterogeneity, here we used the MCF7
breast cancer line as a model system to metabolically fractionate a
cancer cell population. First, MCF7 cells were stably transfected with an hTERT-promoter construct driving GFP expression, as a
surrogate marker of
telomerase transcriptional activity. To enrich for immortal stem-like
cancer cells, MCF7 cells expressing the highest levels of GFP (top 5%) were then isolated by FACS analysis. Notably, hTERT-GFP(+) MCF7 cells were significantly more efficient at forming mammospheres (i.e., stem cell activity) and showed increased mitochondrial mass and mitochondrial functional activity, all relative to hTERT-GFP(-) cells. Unbiased proteomics analysis of hTERT-GFP(+) MCF7 cells directly demonstrated the over-expression of 33 key
mitochondrial proteins, 17 glycolytic
enzymes, 34 ribosome-related
proteins and 17 EMT markers, consistent with an anabolic
cancer stem-like phenotype. Interestingly, MT-CO2 (
cytochrome c oxidase subunit 2; Complex IV) expression was increased by >20-fold. As MT-CO2 is encoded by mt-
DNA, this finding is indicative of increased mitochondrial biogenesis in hTERT-GFP(+) MCF7 cells. Importantly, most of these candidate
biomarkers were transcriptionally over-expressed in human
breast cancer epithelial cells in vivo. Similar results were obtained using cell size (forward/side scatter) to fractionate MCF7 cells. Larger stem-like cells also showed increased hTERT-GFP levels, as well as increased mitochondrial mass and function. Thus, this simple and rapid approach for the enrichment of immortal anabolic stem-like
cancer cells will allow us and others to develop new prognostic
biomarkers and novel anti-
cancer therapies, by specifically and selectively targeting this metabolic sub-population of aggressive
cancer cells. Based on our proteomics and functional analysis, FDA-approved inhibitors of
protein synthesis and/or mitochondrial biogenesis, may represent novel treatment options for targeting these anabolic stem-like
cancer cells.