Cell cycle progression and phenotypic modification of Ki67 antigen-negative G1- and G2-phase cells in phorbol ester-treated Molt-4 human leukemia cells.

To elucidate the relationship between the level of cellular Ki67-reactive antigen and cell proliferation, the effects of 12-O-tetra-decanoylphorbol 13-acetate (TPA) on Ki67 expression, cell cycle progression, and surface phenotypes of human T-lymphoblastic leukemia Molt-4 cells were investigated by multiparameter flow cytometry. The Ki67 antigen is constitutionally expressed in almost all untreated exponentially proliferating Molt-4 cells. Treatment with 10 nM TPA prolonged the duration of the cell cycle time and resulted in a progression arrest of cells in G1- and G2-phases, during which Ki67 expression was decreased to an undetectable level. However, in TPA-treated cultures, the Ki67-positive fraction was invariably smaller than the growth fraction as estimated from continuous 5-bromodeoxyuridine (BrdUrd) labeling curves. This discrepancy could be explained by the finding that some Ki67-negative G1 cells do not enter the resting state but instead remain in the cycling compartment. These results show that Ki67 expression of tumor cells with relatively long G1 duration is downregulated to undetectable levels in late G1-phase and the difference in the level of Ki67 expression between late G1 cells and resting G1 cells is undetectable by conventional immunological methods. Although TPA induced differentiation of Molt-4 cells into mature suppressor T cells, the phenotypic modification was not correlated with cell cycle position and Ki67 reactivity of the cells. These results suggest that growth arrest and phenotypic differentiation of Molt-4 cells are independent effects of TPA.