Not all those who wander are lost: A study of cancer cells by digital holographic imaging, fluorescence and a combination thereof

Cells are commonly used in research to evaluate toxicity and efficiency of drugs. However, to further increase the usefulness of cells as well as the understandings of effects of different interventions, new methods must constantly be developed and refined. Today, many assays use end-point analysis of large populations of cells, to evaluate the research question. However, there are many cases when this kind of analysis hides important effects or behaviour of individual cells. Therefore, quantitative analysis of individual cells over long time periods is important for the complete understanding of the heterogeneity of cell populations. Dogital holographic imaging is a non-toxic quantitative method that can be used for analysis of individual cells over long periods of time. It is the major analysis method of this thesis.

In cancer, a small population of cells has gained the interest of cancer researchers since the cells resist treatment and have increased capability to migrate and form metastases. Those cells are called cancer stem cells, due to their many similarities to normal stem cells.

The interest in drugs that specifically target cancer stem cells has dramatically increased during the last decade. One of the drugs found to target cancer stem cells in multiple cancers is salinomycin, an ionophore which has been used as an antibiotic for more than 30 years. Almost immediately after addition to the medium of cells, salinomycin is found in the endoplasmatic reticulum resulting in increases the cytosolic Ca2+. This leads to further down-stream effects, which among others includes mesenchymal to epithelial transition.

We have used longitudinal tracking of cells in time-lapses acquired using digital holographic imaging to evaluate cell cycle times and movement of different cancer cell lines as well as normal cell lines. We found that small sub-populations of cells behaved differently than the rest of the individually tracked cells. The existence of these cells could not be distinguished in the population-based data we compared the result to. Further, we also analysed how treatment with salinomycin affected cell cycle time and cell movement.

To further develop our longitudinal assay, we combined digital holographic microscopy with fluorescence microscopy by acquiring images from two systems at the same field of view. We then combined the data from the longitudinal tracking with the expression of cell surface proteins specific for cancer stem cells. We found that salinomycin treatment decreased cell proliferation in cancer stem cells already within 24 hours of treatment, leading to a proportional decrease in this sub-population of the cells.