Human serum spreading factor: relationship to somatomedin B

Human serum spreading factor (SF) is a blood glycoprotein that promotes the attachment and spreading of a variety of cell types in serum-free culture, as well as affecting migration, proliferation and differentiation of some cell types under appropriate conditions. The amino acids occupying 17 of the first 23 positions from the NH2-terminus of SF were determined, and this sequence was found to be identical to that reported for somatomedin B (SmB) by Fryklund and Sievertsson (FEBS Letters 87:55). Immunoassay of SF in plasma from subjects with conditions related to altered GH levels indicated that serum SF levels were not GH-responsive to any marked degree. No effect of purified SF was observed in a cell growth assay used previously to detect mitogenic activity in SmB preparations. These results support the conclusion that SF acts as a substratum molecule to elicit its biological effects in cell culture, and does not act in a manner similar to peptide growth factors. These data also are consistent with the conclusion of Heldin et al. (Science 213:1122) that the mitogenic activity of SmB preparations is derived from a contaminating factor, and is not due to SmB.