The gene encoding the receptor for macrophage colony-stimulating factor 1 (CSF-1), the c-fins protooncogene, is selectively expressed in immature and mature mononuclear phagocytes and trophoblasts. Exon 1 is expressed only in trophoblasts. Isolation and sequencing of genomic DNA flanking exon 2 of the murine c-ins gene revealed a TATA-less promoter with significant homology to human c-Jins. Reverse transcriptase primer extension analysis using exon 2 primers identified multiple clustered transcription initiation sites. Their position was confirmed by RNase protection. The same primer extension products were detected in equal abundance from macrophage or nonmacrophage sources of RNA. c-Jins mRNA is acutely down-regulated in primary macrophages by CSF-1, bacterial lipopolysaccharide (LPS), and phorbol myristate acetate (PMA). Each of these agents reduced the abundance of c-fns RNA detectable by primer extension using an exon 3 primer without altering the abundance of presumptive short c-Jins transcripts detected with exon 2 primers. Primer extension analysis with an intron 2 primer detected products at greater abundance in nonmacrophages.Templates detected with the intronic primer were induced in macrophages by LPS, PMA, and CSF-1, suggesting that each of the agents caused a shift from full-length c-Jins mRNA production to production of unspliced, truncated transcripts. The c-Jins promoter functioned constitutively in the RAW264 macrophage cell line, the B-cell line MOPC.31C, and several nonhematopoietic cell lines. Macrophage-specific expression and responsiveness to selective repression by LPS and PMA was achieved by the incorporation of intron 2 into the c-Jins promoter-reporter construct. The results suggest that expression of the c-fins gene in macrophages is controlled by sequences in intron 2 that act by regulating transcription elongation.

Cells of the mononuclear phagocyte lineage possess receptors for macrophage colony‐stimulating factor (CSF‐1) encoded by the c‐fms protooncogene and respond to CSF‐1 with increased survival, growth, differentiation, and reversible changes in function. The c‐fms gene is itself a macrophage differentiation marker. In whole mount analyses of mRNA expression in embryos, c‐fms is expressed at very high levels on placental trophoblasts. It is detectable on individual cells in the yolk sac around 8.5 to 9 days postcoitus, appears on isolated cells in the head of the embryo around 9.5 dpc, and appears on numerous cells throughout the embryo by day 10.5. The extent of c‐fms expression is much greater than for other macrophage‐specific genes including lysozyme and a macrophage‐specific protein tyrosine phosphatase. Our studies of the cis‐acting elements of the c‐fms promoter have indicated a key role for collaboration between the macrophage‐specific transcription factor, Pu. 1, which functions in determining the site of transcription initiation, and other members of the Ets transcription factor family. This is emerging as a common pattern in macrophage specific promoters. We have shown that two PU box elements alone can function as a macrophage‐specific promoter. The activity of both the artifical promoter and the c‐fms promoter is activated synergistically by coexpression of Pu. 1 and another Ets factor, c‐Ets‐2. A 3.5kb c‐fms exon 2 promoter (but not the 300bp proximal promoter) is also active in a wide diversity of tumor cell lines. The interesting exception is the melanoma cell line K1735, in which the promoter is completely shut down and expression of c‐fms causes growth arrest and cell death. The activity of the exon 2 promoter in these nonmacrophages is at least as serum responsive as the classic serum‐responsive promoter of the c‐fos gene. It is further inducible in nonmacrophages by coexpression of the c‐fms product. Unlike other CSF‐1/c‐fms‐responsive promoters, the c‐fms promoter is not responsive to activated Ras even when c‐Ets‐2 is coexpressed. In most lines, production of full length c‐fms is prevented by a downstream intronic terminator, but in Lewis lung carcinoma, read‐through does occur, and expression of both c‐fms and other macrophage‐specific genes such as lysozyme and urokinase becomes detectable in conditions of serum deprivation. Mol Reprod Dev 46:46–53, 1997. © 1997 Wiley‐Liss, Inc.

The op/op mouse has a mutation in the macrophage colony-stimulating (CSF-1) gene. The phenotype of gross deficiency in the macrophage and osteoclast lineages corrects significantly with age, suggesting that other factors can substitute for CSF-1. This review examines the evidence that the op/op mouse is completely CSF-1 deficient and considers the possibility that alternative splicing within the CSF-1 gene might bypass the mutation, yielding an incompletely penetrant phenotype.

The gene encoding the receptor for macrophage colony-stimulating factor 1 (CSF-1), the c-fms protooncogene, is selectively expressed in immature and mature mononuclear phagocytes and trophoblasts. Exon 1 is expressed only in trophoblasts. Isolation and sequencing of genomic DNA flanking exon 2 of the murine c-fms gene revealed a TATA-less promoter with significant homology to human c-fms. Reverse transcriptase primer extension analysis using exon 2 primers identified multiple clustered transcription initiation sites. Their position was confirmed by RNase protection. The same primer extension products were detected in equal abundance from macrophage or nonmacrophage sources of RNA. c-fms mRNA is acutely down-regulated in primary macrophages by CSF-1, bacterial lipopolysaccharide (LPS), and phorbol myristate acetate (PMA). Each of these agents reduced the abundance of c-fms RNA detectable by primer extension using an exon 3 primer without altering the abundance of presumptive short c-fms transcripts detected with exon 2 primers. Primer extension analysis with an intron 2 primer detected products at greater abundance in nonmacrophages. Templates detected with the intronic primer were induced in macrophages by LPS, PMA, and CSF-1, suggesting that each of the agents caused a shift from full-length c-fms mRNA production to production of unspliced, truncated transcripts. The c-fms promoter functioned constitutively in the RAW264 macrophage cell line, the B-cell line MOPC.31C, and several nonhematopoietic cell lines. Macrophage-specific expression and responsiveness to selective repression by LPS and PMA was achieved by the incorporation of intron 2 into the c-fms promoter-reporter construct. The results suggest that expression of the c-fms gene in macrophages is controlled by sequences in intron 2 that act by regulating transcription elongation.

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