Abstract
Although the somatomedin polypeptides stimulate the growth of cells in vitro, the magnitude of this effect is less than that of whole serum. We have investigated the possibility that serum may contain a factor(s) which would enhance the mitogenic response of cells in tissue culture to purified MSA (Multiplication Stimulating Activity), a somatomedin-like polypeptide. MSA alone is unable to achieve the full stimulating effect of serum on cultured chick embryo fibroblasts (CEF). However, Cohn Fraction VI prepared from human serum greatly enhances the ability of MSA to stimulate DNA synthesis and cell multiplication in CEF. This activity is not accounted for by the three major components of fraction VI-albumin, alpha, acid glycoprotein, and alpha2 acid glycoprotein. The factor is heat stable and has an apparent molecular weight of 1000-1500. Brief exposure of CEF to the factor prior to the addition of MSA is sufficient for enhancement to occur. In addition, the factor also enhances the stimulation of (3H) thymidine incorporation in CEF by insulin. This factor may modulate the cellular response to diverse insulin-like peptides.
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