Dissociation of Cell Division Stimulating Capacity for Balb/c-3T3 from the Insulin-like Activity in Human Serum

Abstract

TEMIN et al.^1,2 have shown that insulin and serum proteins with insulin-like activity, isolated from either human or calf serum stimulate cell division in diploid populations of chick cells, while virus-transformed cells grow in the absence of these factors. An established line of rat liver cells, with a unique ability to grow in the absence of serum, releases macromolecular factors into the medium that have insulin-like activity and induce non-transformed cells to divide^3,4; this suggests again that the factors with insulin-like activity are required to initiate cell division. Crystalline insulin stimulates DNA synthesis in BHK cells at limiting serum concentrations⁵, and is a requirement for growth in Agarose of some non-transformed variants of these cells⁶. Both insulin and whole serum have a pleotropic effect on non-transformed 3T3 cells⁷; they induce RNA and protein synthesis, and prevent protein degradation. These factors are not needed for the maintenance of the pleotropic effect in virus-transformed cells⁷. In addition, insulin and serum⁸ both cause a rapid decrease in the concentration of intra-cellular cyclic AMP in 3T3 cells, the decline of which may be a prerequisite for cell division^9,10. We have isolated a serum fraction containing the factors that stimulate DNA synthesis and have insulin-like activity, and have found that heating separates the two activities.