We have examined the expression of vimentin during the differentiation of mouse myeloid leukemia cells (M1), which were induced to differentiate into macrophages by exposure to conditioned medium (CM) obtained from rat embryo fibroblasts. The synthesis of vimentin, which was examined by two-dimensional gel electrophoresis, increased after 12-24 h of incubation of M1 cells in CM and the elevated level of synthesis continued up to 96 h. A macrophage cell line (Mm1) that was derived from spontaneously differentiated M1 cells constantly synthesized much higher levels of vimentin. The amount of vimentin, which was revealed by immunoblot analysis using an mAb against human vimentin, also increased after differentiation by a factor of 7 when compared on the basis of constant protein and by a factor of 17 on the basis of constant cell numbers. Mm1 cells contained greater than 12- and 45-fold more vimentin compared with undifferentiated M1 cells on the bases of constant protein and constant cell numbers, respectively. Northern blot analysis using vimentin cDNA as a probe revealed increases in vimentin mRNA in the differentiated M1 cells and Mm1 cells. Nuclear run-on assay showed that the expression of vimentin gene during the differentiation of M1 cells was transcriptionally regulated. Observations in indirect immunofluorescence microscopy and EM clearly showed that vimentin bundles were rarely observed in undifferentiated M1 cells, and increased amounts of and large-size vimentin bundles were easily observed in differentiated M1 and Mm1 cells. These results suggest the participation of increased amounts of vimentin filaments in the maldistribution of nuclei in M1 cells during differentiation.

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