Regulation of DNA synthesis in mouse macrophages. II. Studies on mechanisms of action of the macrophage growth factor

When mouse macrophages are incubated with medium conditioned by mouse fibroblasts, they are induced to synthesize DNA and divide. This phenomenon is triggered by a macrophage growing factor (MGF) released by the fibroblasts. The presence of a serum cofactor is essential to the activity of the MGF; this cofactor can be removed by dialysis and seems unrelated to the “growth-promoting substances” normally present in serum. The kinetics of DNA synthesis in macrophages stimulated by conditioned medium (CM) is characterized by a lag phase of 24–48 h and a peak synthesis at 4–5 days, followed by a rapid decrease. This decrease is caused by depletion of the MGF from the CM by the growing macrophages.