Dynamic interconversion of cancer cells: single-cell analysis of population-based equilibrium

Background - Multiple subpopulations of cells with different phenotypes and genotypes coexist within malignant tumours. Different subpopulations seem to maintain themselves in an equilibrium position via stochastic interconversion. Such plasticity plays an important role in cancer progression, such as transition from epithelial states to mesenchymal state during metastasis, dedifferentiation of non-CSCs to CSCs and rapid transition to MDR cells during drug exposure. We attempt to explore this interconversion plasticity at both population and single cell levels to better understand the dynamics of phenotypic transition.
Methods - We isolated CD90+ and CD90- cells from A549 lung adenocarcinoma cell line by flow cytometry and then characterized their phenotypic behaviors and interconversion dynamics at both population and single cells level. Single cells were cultured in 96 well plate for 13 days. Viable cells were then transferred into 6 well plate for further expansion until enough cells were collected for measurement of CD90 expression in flow cytometry.
Results - A549 maintains a stable expression of CD90 (~50%) which is bimodal distributed. The sorted subpopulations eventually achieved a new equilibrium (CD90+ à 60% , CD90 - à 30%) and did not converge during the bulk cell interconversion. Individual single cell clones showed large variation of CD90 expression. The distribution of CD90 expression was significantly different between CD90+ and CD90- single cell clones, while both groups of clones achieved a similar average equilibrium as compared with the bulk cells. We noted two types of clones defined as transition clones and committed clones with differential interconversion capacities. The later one showed no sign of interconversion and was committed to the original CD90+/- expression level. Further experiments showed that the committed CD90+ cells were more mesenchymal with higher migration ability, while the committed CD90- cells were more epithelial and tumorigenic.
Conclusions - Interconversion plasticity varied from cell to cell and the committed cells prohibited the return of sorted cell subpopulations returned back to the original equilibrium. Phenotypic studies of CD90+ and CD90- revealed that the interconversion between them was EMT related.