Amount of the two major Ag‐NOR proteins, nucleolin, and protein B23 is cell‐cycle dependent

To know the biological basis allowing the use of Ag-NOR protein expression as proliferation marker in human malignancies, the relationship between cell cycle and amount of Ag-NOR protein was analyzed. The quantification of the two major Ag-NOR proteins, nucleolin and protein B23, was performed in exponentially growing, serum-deprived, and cell-cycle stimulated cells. Expression of nucleolin was low in serum-deprived cells and increased mostly in S phase during cell-cycle stimulation. Conversely, expression of protein B23 was slightly repressed in serum-deprived cells, and increased progressively until G2 phase during cell-cycle stimulation. The accumulation of nucleolin and protein B23 in G2 compared to G1 was demonstrated using sorted phase-specific cells. In G0, cells sorted according to their very low RNA content, and the amount of Ag-NOR proteins was half of that found in G1 cells, nucleolin being only weakly detectable. Therefore, the expression of nucleolin increased between G0–G1 and G1–S phases. These data support the hypothesis that quantification of Ag-NOR proteins is an estimation of the percentage of cells in each cell cycle phase because their amount is high in S-G2 and low in G1 phases. Cytometry 28:147–156, 1997. © 1997 Wiley-Liss Inc.