Expression of human proliferation-associated nucleolar antigen p120

The amount of human nucleolar protein p120 is small or nondetectable in most normal resting cells but increases greatly in some actively proliferating cells. The time course was compared for accumulation of mRNAs for p120, c-myc, and histones in phytohemagglutinin-stimulated peripheral blood mononuclear cells. The mRNAs for p120 and c-myc were undetectable in unstimulated peripheral blood mononuclear cells; phytohemagglutinin stimulation increased the p120 mRNA level within 2 h, and it reached a maximum within 24-48 h. The c-myc mRNA level increased within 4 h and had a biphasic maximum at 4 h and 24 h. Indirect immunofluorescence staining and Western blot analysis showed an increase in the level of p120 protein in the nucleolus following the increase in its mRNA level, which suggests that p120 expression is transcriptionally controlled. Both p120 mRNA and c-myc mRNA levels were significantly decreased in 12-O-tetradecanoyl-phorbol-13-acetate-differentiated HL-60 leukemic cells and in confluent normal immortalized human fibroblasts (WSI). These data indicate that p120 is expressed very early in the G1 phase of the cell cycle and suggest that it has an important function in the cell cycle.