Cell Cycle-related Expression of p120 Nucleolar Antigen in Normal Human Lymphocytes and in Cells of HL-60 and MOLT-4 Leukemic Lines: Effects of Methotrexate, Camptothecin, and Teniposide¹ Free

Expression of the proliferation-associated nucleolar antigen p120 was studied by flow cytometry in human quiescent and phytohemagglutininstimulated lymphocytes, as well as in human lymphocytic (MOLT-4) and promyelocytic (HL-60) cell lines. Bivariate analysis of p120 and DNA content made it possible to correlate p120 expression with cell position in the cycle. Proliferating lymphocytes and MOLT-4 and HL-60 cells had a similar pattern of p120 expression. Populations of G₁ cells, in all three cell types, were very heterogeneous with respect to p120, and a threshold in G₁ was observed. The cells with a p120 level below the threshold value did not enter S phase. An increase in p120 was observed during progression through S phase, and the antigen was maximally expressed in G₂ cells. The p120/DNA content ratio, however, was highest in late G₁ cells (G_1B) and was declining during S and G₂. The data thus suggest that p120 may be degraded during mitosis and that the postmitotic cells inherit little, if any, of this protein; the antigen then accumulates predominantly during G₁, and must reach a threshold level to enable the cells to enter S phase. Antigen p120 could not be detected in noncycling lymphocytes nor in HL-60 cells induced to myeloid differentiation by growth in the presence of dimethyl sulfoxide. Treatment of MOLT-4 cells with pharmacological concentrations of methotrexate, camptothecin, or teniposide induced cell arrest in S or G₂; expression of p120 in the arrested cells was unchanged from that of untreated MOLT-4 controls at the same phase of the cycle. The level of p120 was minimal in MOLT-4 or HL-60 cells arrested in M phase by vinblastine, but vinblastine had no effect on p120 fluorescence of interphase cells. Camptothecin or teniposide induced apoptosis selectively in S phase of HL-60 cells; apoptotic cells from camptothecin-treated cultures, however, despite the marked nucleolysis, still expressed p120. The data on the drug-treated cells indicate that the p120 level in tumors of patients may be used as a marker of tumor/malignancy even in clinical samples obtained during treatment.