The effect of inducing agents on the numbers of interphase fibrillar centers in the U937 promonocytic cell line.

The mean numbers of interphase fibrillar centers have been determined in triplicate experiments, using the argyrophil (AgNOR) method. Promonocytic U937 cells were incubated with each of three inducing agents, namely, interleukin-6, granulocyte-macrophage colony-stimulating factor, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. The cells were examined at 24, 48, and 72 h during the induction periods and their doubling time and mean AgNOR score were calculated. After 72 h, the cells were maintained in culture for a further 24 h in the absence of an inducing agent and these parameters were determined again. It was found that whereas the unstimulated U937 cells had a mean value in the region of 50 AgNORs per nucleus, this diminished to about 20 after a 72 h incubation, but rose to 30 or more when the inducing agents had been withdrawn for 24 h. These observation confirm the results of previous studies using melanoma and HL60 cell lines: however, it has now been demonstrated that a variety of agents can modulate the numbers of fibrillar centers in a very similar way in a single cell line. Furthermore, we have shown that the "undifferentiated" U937 cell AgNOR score recovers when the agents no longer act upon the cells; this implies that fibrillar center numbers are intimately related to differentiation state in cell lines, as in the case in, for example, tumor specimens.