Abstract
Ribosomal genes are associated with a subset of acidic proteins called Ag-NOR proteins. The amount of nucleolar Ag-NOR proteins varies, depending on nucleolar activity and/or cell proliferation. To understand the linkage between the amount of Ag-NOR proteins, ribosome biogenesis, and cell proliferation, we investigated the variability of Ag-NOR proteins in rRNA-stimulated cells maintained in G1 and in rRNA-stimulated cells entering the mitotic cycle. Rat hepatocytes were stimulated with cortisol for rRNA synthesis (1, 4, and 8 hr) and the cell cycle was induced by hepatectomy in regenerating hepatocytes (3-21 hr). In non-stimulated hepatocytes, nucleolin and protein B23 were the two major Ag-NOR proteins, corresponding to 70% of total Ag-NOR staining. In hepatocytes stimulated for rRNA synthesis in G1, the amount of Ag-NOR proteins was only slightly increased, whereas in cycle-stimulated cells it was increased 3.04-fold. This is the consequence of a differential increase of the major Ag-NOR proteins that appears earlier and is proportionally more important for nucleolin (3.5-fold) than for protein B23 (twofold) and also for the increase of several minor Ag-NOR proteins. We conclude that, in dividing cells, the mean value of the Ag-NOR proteins measured reflects the percentage of cells in the different phases. This could explain why the amount of Ag-NOR proteins can be used as a marker of cell proliferation.