Lateral diffusion of wheat germ agglutinin-labeled glycoconjugates in the membrane of differentiating HL-60 and U-937 cells assessed with fluorescence recovery after photobleaching (FRAP).

The promyelocytic leukemia cell line HL-60 and the histiocytic cell line U-937 were grown in suspension culture. They were induced to differentiate during 5-d cultivation in the presence of dimethylsulfoxide (DMSO; 1.3% w/v) or phorbol-12-myristate-acetate (PMA; 10(-7) M), which yields granulocyte- and macrophage-like cells, respectively. Differentiation was evidenced by increased capacity to recognize and phagocytize IgG- or complement-coated yeast particles. Aliquots taken from the cultures with and without DMSO (or PMA) were spun down directly on glass microscope slides, washed, labeled with fluoresceinated wheat germ agglutinin (WGA), and directly examined at room temperature for the rate of fluorescence recovery after photobleaching (FRAP). It was found that cultivation of the HL-60 and the U-937 cells in the presence of DMSO, which yields granulocyte-like cells, reduced the average value of lateral diffusion coefficient D (X 10(10] from 1.72 +/- 0.13 cm2s-1 to 0.97 +/- 0.13 cm2s-1 and from 1.77 +/- 0.11 cm2s-1 to 0.82 +/- 0.13 cm2s-1, respectively. U-937 cells grown with PMA also showed a reduction of D(X 10(10] to 0.88 +/- 0.10 cm2s-1. There was a larger immobile fraction of fluorescence in the HL-60 cells than in the U-937 cells, viz., 70-80% compared to 10-50%. The total number of binding sites for WGA was not altered, but the surface density changed, since the HL-60 and the U-937 cells became smaller and larger, respectively, when grown in the presence of DMSO. It is concluded that differentiation reduces the average lateral mobility of the WGA-binding membrane component by a factor around 2.