Synthesis, Post-translational Processing, and Autocrine Transforming Activity of a Carboxylterminal Truncated Form of Colony Stimulating Factor-1

The mononuclear phagocyte colony stimulating factor encoded by a 1.6 kilobase pair human cDNA is synthesized as a homodimeric transmembrane glycoprotein that is released from the plasma membrane by proteolysis. Premature termination of the CSF-1 coding sequence upstream of its carboxylterminal transmembrane-spanning segment and expression of the truncated CSF-1 cDNA in either bovine papilloma virus or retrovirus vectors led to the synthesis of a soluble, biologically active growth factor that was rapidly secreted from cells. Like the full-length CSF-1 precursor, the truncated polypeptide was rapidly assembled through disulfide bonds immediately after synthesis and acquired asparagine-linked oligosaccharide chains that underwent progressive post-translational modifications during intracellular transport. Soluble CSF-1 encoded by the truncated cDNA stimulated the formation of bone marrow-derived mouse macrophage colonies in semisolid medium and induced transformation of mouse NIH-3T3 cells when coexpressed with the human c-fms proto-oncogene product (CSF-1 receptor). Compared to results obtained with the full-length CSF-1 cDNA, the efficiency of transformation obtained with the truncated CSF-1 gene was reduced, in spite of the fact that transfected cultures produced similar levels of the extracellular growth factor. The results indicate that CSF-1 amino acid residues 1-158 (together with the aminoterminal signal peptide at residues -32 to -1) are sufficient for biological activity and that CSF-1 cDNAs encoding either membrane-bound or soluble precursors are active in autocrine transformation.