c- fos expression is neither sufficient nor obligatory for differentiation of monomyelocytes to macrophages

The c- fos gene is rapidly and transiently expressed when human U-937 and HL-60 leukemia cells are induced to differentiate to macrophages. We show that the expression of c- fos is controlled primarily at the transcriptional level. c- fos mRNA is very labile, with a half-life of less than 30 min. Superinduction of c- fos in the presence of cycloheximide occurs primarily because of stabilization of c- fos mRNA. When U-937 cells are serum-stimulated or treated with diacylglycerol, a c-kinase agonist, c- fos is transiently expressed to high levels; however, the cells fail to differentiate to macrophages. Furthermore, HL-60 cell variants resistant to TPA can be induced to differentiate to macrophages in the absence of detectable c- fos expression.