Molecular quantification of cell cycle-related gene expression at the protein level

Immunofluorescence cytometry of antigen and DNA content provides relative measurements of the cell cycle phase distribution of a specific epitope. Measurement of correlated expression of epitopes on signaling and regulatory proteins will be useful in the study of the complex pathways involved in cell cycle regulation and carcinogenesis. However, to formulate regulatory pathway models, measurements of molecules per cell would be more useful than relative measurements of intensity. Here, we report on a system in which the relationship between molecules and fluorescence is determined for a reference set of cell lines that are then used to directly calculate the number of molecules for unknowns. To demonstrate the process, we calculated the cell cycle phase distribution of SV40 large T antigen (Tag) in the reference cells.

A set of cell line clones expressing different levels of Tag were isolated. Quantitative Western blots of these cells and purified, recombinant Tag were performed. Cells from the same sample were stained and analyzed by flow cytometry for Tag and DNA. The relationship between molecules and fluorescence was established and calculations were performed for the phase distributions of Tag.

The five cell lines had 0.11, 0.27, 1.06, 2.44, and 2.63 x 10(6) molecules of Tag per cell, determined by Western blot. The average coefficient of variation was 10.6%. The relationship of molecules to fluorescence fit a linear equation (r(2) = 0.96) over the range, 0.11 - 2.63 x 10(6) molecules, however, the same equation did not fit the relationship between 0 molecules, defined by isotype staining controls, and the lowest expressing cell line. To calculate the phase distributions of molecules in the lowest cell line, a second linear equation from 0 to 110,000 molecules was used.

This work describes a system where fixed cells expressing various levels of a target antigen quantified by Western blots can be used to standardize flow cytometric measurements of gene expression in absolute terms.