Single-cell analysis of the relationship among transferrin receptors, proliferation, and cell cycle phase in K562 cells

Multiparameter single-cell analysis by flow cytometry was used to distinguish between size-related changes in K562 cell transferrin receptor (TfR) expression and changes in membrane receptor density throughout the cell cycle and over time in culture. Light-scatter pulse-width time-of-flight, a direct and readily calibrated measure of cell diameter, was used to calculate receptor density as the average number of receptors per unit cell surface area. Cell surface TfRs were unimodally distributed over the cell population and were present throughout the cell cycle. The number of receptors increased as cells progressed through the cell cycle, but cell cycle phase was also correlated with cell volume. However, when size heterogeneity was factored out by reanalysis of listmode data, there was a clear cell-cycle effect: among cells of the same size, both the number of receptors per cell and the receptor density increased from G1 to S to G2/M. TfR expression was also followed over time in culture after dilution into fresh medium. A decrease in growth rate after four days was preceded by one to two days by a decrease in both number of TfRs per cell and mean receptor density, indicating that decreased TfR expression represented true "down-regulation" and not just decreased cell size or an increase in the proportion of smaller G1 cells. This type of analysis is generally applicable for resolving the effects of cell size heterogeneity and cell cycle on membrane protein distribution and for other studies of ligand-receptor interaction.