An Autocrine Activity Capable of Substituting for Serum in Cell Cultures

Abstract

Provided that high cell densities (above 10⁶/ml) are maintained, a factor-dependent murine hemopoietic progenitor cell line (FDC-PI) will proliferate in serum-free medium. The conditioned medium (CM) from high-density FDC-PI cells permits the serum-free survival of FDC-PI cells even at low density, indicating the existence of a diffusible autocrine factor. The requirement of FDC-PI for a colony-stimulating factor (either IL-3 or GM-CSF) is not abrogated by culturing the cells at high cell density or in the conditioned medium. Furthermore, the CM from FDC-PI enhances the mitogenic stimulation of normal human skin fibroblasts (HSF) by epidermal growth factor (EGF): i.e., the lag period before entry into the cell cycle is shortened by up to 6 hr. The fibroblasts themselves secrete an activity into serum-free medium that appears to be required during mitogenic stimulation by EGF. The HSF-CM also allows FDC-PI cells to survive and proliferate serum-free at low cell densities. Low concentrations of fetal calf serum or human plasma (0.2-2%) have the same effect as FDC-PI-CM and HSF-CM. We have tested many of the known growth factors, and none of them mimicked the autocrine serum replacing activity (ASRA). The activity in human plasma elutes from a gel-filtration column with an apparent molecular weight of 60,000. It appears as if cultured normal cells and cell lines produce molecules capable of complementing the growth factors required for the survival and proliferation of a range of cells in serum-free cultures.