AMOUNT VARIABILITY OF TOTAL AND INDIVIDUAL AG-NOR PROTEINS IN CELLS STIMULATED TO PROLIFERATE

Ribosomal genes are associated with a subset of acidic proteins called Ag-NOR proteins, The amount of nucleolar Ag-NOR proteins varies, depe nding on nucleolar activity and/or cell proliferation, To understand t he linkage between the amount of Ag-NOR proteins, ribosome biogenesis, and cell proliferation, we investigated the variability of Ag-NOR pro teins in rRNA-stimulated cells maintained in G(1) and in rRNA-stimulat ed cells entering the mitotic cycle, Rat hepatocytes were stimulated w ith cortisol for rRNA synthesis (1, 4, and 8 hr) and the cell cycle wa s 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 hep atocytes stimulated for rRNA synthesis in G(1), the amount of Ag-NOR p roteins 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 pro portionally more important for nucleolin (3.5-fold) than for protein B 23 (twofold) and also for the increase of several minor Ag-NOR protein s, We conclude that, in dividing cells, the mean value of the Ag-NOR p roteins measured reflects the percentage of cells in the different pha ses, This could explain why the amount of Ag-NOR proteins can be used as a marker of cell proliferation.