Responses of rat and chick chondrocytes and rate hepatoma cells to plasma fractions with insulin-like and growth-promoting activities.

Plasma protein Cohn fraction IV-1 was extracted and fractionated according to the method of Van Wyk et al.[1], with the omission of the Sephadex G-75 step. The fractionated polypeptides were investigated for insulin-like activity (lipid synthesis and lactate production) and growth-stimulatory activity (DNA synthesis) in three bioassay systems: rat hepatoma cells, and rat and chick chondrocyte suspensions. Binding of each fraction to insulin receptors in isolated plasma membranes was also determined. Three peptide subfractions, with molecular weights ranging from 400 to 20 000, stimulated lipid synthesis, lactate production and DNA synthesis in all three bioassay systems. There were distinct species- and cell-type-specific variations in the quantitative patterns produced by each subfraction. In contrast, authentic insulin (0.005-0.1 ng/ml) had no effect on DNA and lipid synthesis anad lactate production in hepatoma cells and chondrocytes. One of the three active subfractions and two relatively inactive fractions displaced insulin from its receptors. These observations indicate that acid-ethanol extracts of plasma contain a variety of peptide factors with insulin-like and growth-promoting effects whose expression in different cell types is modulated by inherent properties of each cell type which are determined genetically presumably. Surface receptors for authentic insulin do not appear to play an essential role in the insulin-like activities of the isolated plasma fractions.