Flow cytometric quantification of cyclin E in human cell lines and hematopoietic malignancies

Cyclins are important in the regulatory system controlling cell cycle progression and overexpression or aberrant expression of G1 cyclins, as cyclin E, may result in an inadequate G1-S transition. In this study, the expression of cyclin E was analyzed in 12 human cell lines and 18 hematological malignancies by flow cytometry, and protein levels were quantified using calibrating microbeads. The majority of the tested cell lines demonstrated a cell cycle-specific expression of cyclin E, even though some cell lines expressed lower and more constant cyclin E levels throughout the cell cycle. No correlation between the fraction of cyclin E-positive cells and cyclin E content could be observed in the cell lines. Similar cyclin E levels were observed in the tumor samples as for the cell lines, although the fractions of positive cells were low. The flow cytometric quantification of cyclin E was compared with densitometric quantification of cyclin E Western blots, and a significant correlation between the two methods was observed (rs = 0.56, P = 0.03). The variation of cyclin E levels between cell lines was significantly higher than the variation for repetitive analyses of the same cell line, suggesting that the expression of cyclin E could reliably be quantified using flow cytometry. Because of the potential to analyze specific subgroups such as cyclin E-positive cells, this method may facilitate the detection of cyclin E-overexpressing tumor cells.