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

Expression of the proliferation-associated nucleolar antigen pi 20 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. Divariate analysis of pl20 and DNA content made it possible to correlate p120 expression with cell position in the cycle. Proliferating lymphocytes and MOLT-4 and HL60 cells had a similar pattern of pi 20 expression. Populations of Gt cells, in all three cell types, were very heterogenous with respect to p120, and a threshold in (., was observed. The cells with a pi20 level below the threshold value did not enter S phase. An increase in pi 20 was observed during progression through S phase, and the antigen was maximally expressed in (.. cells. The pi 20/1)\.A content ratio, however, was highest in late Ci cells (<HB)and was declining during S and (,¡.The data thus suggest that p120 may be degraded during mitosis and that the post mitot k-cells inherit little, if any, of this protein; the antigen then accumulates predominantly during (•,, 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 (Ã-?; expression of pi 20 in the arrested cells was unchanged from that of untreated MOLT-4 controls at the same phase of the cycle. The level of pi 20 was minimal in MOLT4 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 pi 20 level in tumors of patients may be used as a marker of tumor/malignancy even in clinical samples obtained during treatment.