mRNA levels for human nucleolar protein P120 in tumor and nontumor cells

A monoclonal antibody to a human tumor nucleolar 120 kD protein was developed by Freeman et al. (Cancer Res. 48: 1244-1251, 1988). Its complementary DNA (cDNA) has been isolated and sequenced (Fonagy et al., submitted). To determine the relative messenger RNA (mRNA) level for protein p120, cellular mRNA was extracted, slot-blotted onto nitrocellulose filters, and hybridized to radioactive p120 cDNA fragments. Human tumor cells contained 15-60 times more p120 mRNA than human term placenta. The rat Novikoff hepatoma ascites cell mRNA hybridized to the p120 cDNA probes, but the p120 monoclonal antibody did not react with the Novikoff hepatoma proteins. Novikoff hepatoma mRNA contained 8 times as much p120 mRNA as normal rat liver. As a control, a cDNA was used for protein B23, an abundant nucleolar protein; there were 3.5, 29, and 14 times more B23 mRNA than p120 mRNA in normal rat liver, Novikoff hepatoma ascites cells, and HeLa cells, respectively. Whereas the increased levels of the mRNA and protein B23 reflect increased activity of the nucleolus for any increment of nucleolar function, the increased levels of p120 mRNA and the p120 protein reflect the activity of the G1 phase of the cell cycle. The elevated level of p120 mRNA in tumors may reflect the heightened G1 cascade in transformed cells.