Nucleolin and fibrillarin expression in stimulated lymphocytes and differentiating HL-60 cells. A flow cytometric assay

Nucleolin and fibrillarin are two histone‐like major proteins in the nucleolus that were found to be overexpressed in proliferating cells. Using specific antibodies to either nucleolin or fibrillarin flow cytometric, measurements were carried out to demonstrate quantitative changes of these proteins during lymphocyte mitogenic activation and differentiation of HL‐60 promyelocytic leukaemia cells. The expression of nucleolin increased during lymphocyte stimulation and decreased slowly but constantly in the course of differentiation of HL‐60 cells. Expression of fibrillarin reached a maximum in the first cell cycle and then dropped to a basic level in stimulated lymphocytes. Compared to nucleolin, the level of fibrillarin decreased more rapidly and more extensively in differentiating HL‐60 cells. The data support other observations that nucleolin is a stabile structural protein at the ribosomal genes while fibrillarin may have a more specific functional role in nucleologenesis and ribosome production.