A usable model of "decathlon winner" cancer cells in triple-negative breast cancer: survival of resistant cancer cells in quiescence.

We previously described a strategy for selecting highly adaptable rare triple-negative breast cancer (TNBC) cells based on their ability to survive a severe and prolonged metabolic challenge, e.g., a lack of glutamine. We hypothesized that metabolically adaptable (MA) cancer cells selected from the SUM149 cell line in this manner have the capacity to survive a variety of challenges that postulated "decathlon winner" cancer cells must survive to succeed in metastasis. These MA cells were resistant to glutaminase inhibitor CB-839, as predicted from their ability to proliferate without exogenous glutamine. They were also resistant to hypoxia, surviving treatment with hypoxia inducer cobalt chloride. Investigating the nature of intrinsic resistance in SUM149-MA cells, we found that 1-2 mM metformin completely inhibited the emergence of MA colonies in SUM149 cells in glutamine-free medium. These highly resistant MA cells grew into colonies upon removal of metformin, indicating that they survived in quiescence for several weeks under metformin treatment. This approach of selecting resistant cells worked equally well with additional TNBC cell lines, specifically inflammatory breast cancer cell line FC-IBC02 and mouse breast cancer cell line 4T07. In both cases, less than 1% of cells survived metformin treatment and formed colonies in glutamine-free medium. The MA cells selected in this manner were significantly more resistant to the chemotherapeutic drug doxorubicin than the parental cell lines. We conclude that our approach may be useful in developing usable models of cancer cell quiescence and therapy resistance in TNBC.

Highly adaptable triple-negative breast cancer cells as a functional model for testing anticancer agents.

A major obstacle in developing effective therapies against solid tumors stems from an inability to adequately model the rare subpopulation of panresistant cancer cells that may often drive the disease. We describe a strategy for optimally modeling highly abnormal and highly adaptable human triple-negative breast cancer cells, and evaluating therapies for their ability to eradicate such cells. To overcome the shortcomings often associated with cell culture models, we incorporated several features in our model including a selection of highly adaptable cancer cells based on their ability to survive a metabolic challenge. We have previously shown that metabolically adaptable cancer cells efficiently metastasize to multiple organs in nude mice. Here we show that the cancer cells modeled in our system feature an embryo-like gene expression and amplification of the fat mass and obesity associated gene FTO. We also provide evidence of upregulation of ZEB1 and downregulation of GRHL2 indicating increased epithelial to mesenchymal transition in metabolically adaptable cancer cells. Our results obtained with a variety of anticancer agents support the validity of the model of realistic panresistance and suggest that it could be used for developing anticancer agents that would overcome panresistance.

Selection of metastatic breast cancer cells based on adaptability of their metabolic state.

A small subpopulation of highly adaptable breast cancer cells within a vastly heterogeneous population drives cancer metastasis. Here we describe a function-based strategy for selecting rare cancer cells that are highly adaptable and drive malignancy. Although cancer cells are dependent on certain nutrients, e.g., glucose and glutamine, we hypothesized that the adaptable cancer cells that drive malignancy must possess an adaptable metabolic state and that such cells could be identified using a robust selection strategy. As expected, more than 99.99% of cells died upon glutamine withdrawal from the aggressive breast cancer cell line SUM149. The rare cells that survived and proliferated without glutamine were highly adaptable, as judged by additional robust adaptability assays involving prolonged cell culture without glucose or serum. We were successful in isolating rare metabolically plastic glutamine-independent (Gln-ind) variants from several aggressive breast cancer cell lines that we tested. The Gln-ind cells overexpressed cyclooxygenase-2, an indicator of tumor aggressiveness, and they were able to adjust their glutaminase level to suit glutamine availability. The Gln-ind cells were anchorage-independent, resistant to chemotherapeutic drugs doxorubicin and paclitaxel, and resistant to a high concentration of a COX-2 inhibitor celecoxib. The number of cells being able to adapt to non-availability of glutamine increased upon prior selection of cells for resistance to chemotherapy drugs or resistance to celecoxib, further supporting a linkage between cellular adaptability and therapeutic resistance. Gln-ind cells showed indications of oxidative stress, and they produced cadherin11 and vimentin, indicators of mesenchymal phenotype. Gln-ind cells were more tumorigenic and more metastatic in nude mice than the parental cell line as judged by incidence and time of occurrence. As we decreased the number of cancer cells in xenografts, lung metastasis and then primary tumor growth was impaired in mice injected with parental cell line, but not in mice injected with Gln-ind cells.