AMOUNT OF THE 2 MAJOR AG-NOR PROTEINS, NUCLEOLIN, AND PROTEIN B23 IS CELL-CYCLE DEPENDENT

To know the biological basis allowing the use of Ag-NOR protein expres sion as proliferation marker in human malignancies, the relationship b etween cell cycle and amount of Ag-NOR protein was analysed. The quant ification of the two major Ag-NOR proteins, nucleolin and protein B23, was performed in exponentially growing, serum-deprived, and cell-cycl e stimulated cells. Expression of nucleolin was low in serum-deprived cells and increased mostly in S phase during cell-cycle stimulation. C onversely, expression of protein B23 was slightly repressed in serum-d eprived cells, and increased progressively until G(2) phase during cel l-cycle stimulation. The accumulation of nucleolin and protein B23 in G(2) compared to G(1) was demonstrated using sorted phase-specific cel ls. In G(0), cells sorted according to their very low RNA content, and the amount of Ag-NOR proteins was half of that found in G(1) cells, n ucleolin being only weakly detectable. Therefore, the expression of nu cleolin increased between G(0)-G(1) and G(1)-S phases. These data supp ort the hypothesis that quantification of Ag-NOR proteins is an estima tion of the percentage of cells in each cell cycle phase because their amount is high in S-G(2) and low in G(1) phases. (C) Wiley-Liss, Inc.