DIFFERENTIAL UTILIZATION OF RAS SIGNALING PATHWAYS BY MACROPHAGE-COLONY-STIMULATING FACTOR (CSF) AND GRANULOCYTE-MACROPHAGE CSF RECEPTORS DURING MACROPHAGE DIFFERENTIATION

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and macropha ge colony-stimulating factor (M-CSF) independently stimulate the proli feration and differentiation of macrophages from bone marrow progenito r cells. Although the GM-CSP and M-CSF receptors are unrelated, both c ouple to Ras-dependent signal transduction pathways, suggesting that t hese pathways might account for common actions of GM-CSF and M-CSF on the expression of macrophage-specific genes. To test this hypothesis, we have investigated the mechanisms by which GM-CSF and M-CSF regulate the expression of the macrophage scavenger receptor A (SR-A) gene. We demonstrate that induction of the SR-A gene by M-CSF is dependent on AP-1 and cooperating Ets domain transcription factors that bind to sit es in an M-CSF-dependent enhancer located 4.1 to 4.5 kb upstream of th e transcriptional start site. In contrast, regulation by GM-CSF requir es a separate enhancer located 4.5 to 4.8 kb upstream of the transcrip tional start site that confers both immediate-early and sustained tran scriptional responses. Results of a combination of DNA binding experim ents and functional assays suggest that immediate transcriptional resp onses are mediated by DNA binding proteins that are constitutively bou nd to the GM-CSF enhancer and are activated by Ras. At 12 to 24 h afte r GM-CSF treatment, the GM-CSF enhancer becomes further occupied by ad ditional DNA binding proteins that may contribute to sustained transcr iptional responses. In concert, these studies indicate that GM-CSF and M-CSF differentially utilize Ras-dependent signal transduction pathwa ys to regulate scavenger receptor gene expression, consistent with the distinct functional properties of M-CSF- and GM-CSF-derived macrophag es.