Effect of Differentiation on the Expression of a Nucleolar Antigen with a Molecular Weight of 145,000 in HE-60 Cells

M_r145,000 nucleolar protein antigen (pl45) is associated with growing cells (R. L. Ochs el al., J. Cell Biol., 101: 211a, 1985) and has been found in a broad range of human cancers (J. W. Freeman et al., Cancer Res., 46: 3593–3598, 1986). In this study the presence of nucleolar antigen p145 was examined in the human promyelocytic tumor cell line HL-60 which was induced to differentiate by retinoic acid. Differentiation was monitored by morphological changes, [3H]thymidine accumulation, the ability of cells to reduce nitroblue tetrazolium, and cell number. The monoclonal antibody to nucleolar antigen p145 produced bright immunofluorescence in all cycling interphase HL-60 cells; during mitosis only diffuse staining was detected. Nucleolar antigen p145 in HL-60 cells was undetectable after 132 h of treatment with retinoic acid. The absence of nucleolar antigen p145 was associated with an 81% decline in thymidine accumulation and apparent inactivation of ribosomal and nonribosomal DNA transcription as observed by electron microscopy. The loss in expression of the antigen also correlated with increased nitroblue tetrazolium-positive cells, appearance of morphologically distinct myeloid cells, and termination of cell proliferation. These data indicate that the expression of nucleolar antigen p145 occurred in cycling HL-60 cells but not in terminally differentiated noncycling HL-60 cells.