Regulation of urokinase plasminogen activator gene transcription in the RAW264 murine macrophage cell line by macrophage colony-stimulating factor (CSF-1) is dependent upon the level of cell-surface receptor

Macrophage colony-stimulating factor (CSF-1) binds to a receptor (CSF-1R) e ncoded by the c-fms proto-oncogene and activates transcription of the uroki nase plasminogen activator (uPA) gene in murine bone-marrow-derived macroph ages. This article demonstrates that the murine macrophage cell line RAW264 responds to CSF-1 with inducible phosphorylation of cytoplasmic proteins o n tyrosine residues but fails to induce transcription of uPA. The defect wa s correlated with a selective failure to maintain CSF-1Rs on the cell surfa ce, whereas all RAW264 cells contained abundant CSF-1Rs within the presumpt ive Golgi/endoplasmic reticulum compartment. Transfection with a CSF-1R exp ression plasmid permitted CSF-1-dependent activation of the signalling path way targeting an Ets/AP1 (activator protein 1)element in the uPA promoter t hat has been shown previously to be a target of oncogenic ras and protein k inase C pathways. Mutation of the expressed CSF-1R at either Y807 or Y559, sites of receptor tyrosine phosphorylation implicated in signal transductio n, reduced but did not abolish uPA promoter activation by CSF-1. Activation by mutant CSF1R plasmids was additive; there was no evidence of mutual com plementation. The results indicate that maintenance of elevated uPA transcr iption by CSF-1 requires new receptors emerging continuously on the cell su rface. Parallel, partly redundant, signalling pathways arising from phospho rylated tyrosines on the CSF-1R activate multiple cis-acting elements on th e complex uPA promoter.