Characterization of macrophage- and granulocyte-inducing proteins for normal and leukemic myeloid cells produced by the Krebs ascites tumor.

Medium from serum-free cultures of Krebs ascites tumor cells contains two macrophage and granulocyte inducing (MGI) activities that can act on the myeloid precursors of these hematopoietic cells. One activity, MGI-1, induced the formation of macrophage and granulocyte colonies from normal myeloid precursors. The second activity, MGI-2, induced macrophage and granulocyte differentiation in myeloid leukemic cells that no longer required MGI-1 for colony formation. The medium contained one species of MGI-1 and two species of MGI-2. One species of MGI-2, MGI-2A, copurified through five stages of purification with MGI-1, but separated from the other MGI-2 species, MGI-2B, at an early stage in purification. MGI-1, MGI-2A and MGI-2B were purified 1490, 1140 and 678-fold, respectively. When bands with biological activity gel from non-denaturing polyacrylamide gels were run on SDS-polyacrylamide gel electrophoresis, MGI-1 and MGI-2A activities were associated with similar Mr and each activity showed two bands, one of 23 000 and the other 25 000. MGI-2B activity showed one band with a Mr of 45 000. Secretion did not appear to involve glycosylation, none of the species bound to concanavalin A, soybean agglutinin, or wheat germ agglutinin agarose columns and they did not appear to contain carbohydrates. The assays for MGI-1 and MGI-2 activities were not affected by adding protease inhibitors. But MGI-2 was more readily destroyed by treatment with proteases and was more labile at high temperature and low pH than MGI-1. It is suggested that the level of cellular proteases may play a role in regulating the relative amounts of MGI-1 and MGI-2 that are present in vivo.