Regulation of the synthesis of S-100 protein in rat glial cells.

The influence of serum on the synthesis of the nervous system specific S-100 protein has been investigated in the rat glial cell clone C61a. In sparse cells, where S-100 synthesis is low, removal of serum leads to a 2- to 3-fold increase of the rate of S-100 synthesis relative to total soluble cell protein. Measurements of S-100 content by radioimmunoassay showed that sparse cells are accumulating S-100 much more rapidly in the absence of serum. Our findings suggest that serum components inhibit the expression of S-100 synthesis in sparse cells by a concentration-dependent mechanism. This effect does not correlate directly with the reduced cellular growth rate in low serum or serum-free medium. Lipid-free medium, which has little growth-promoting activity, maintains the capacity to lower the rate of S-100 synthesis. A partially purified extract of platelets can substitute for serum in repressing the synthesis of the S-100 protein.