Growth of C6 glioma cells in serum-containing medium decreases beta-adrenergic receptor number

Rat C6 glioma cells were grown in 5% fetal bovine serum-containing medium and under serum-free, defined conditions. In order to ask whether cells grown in serum-free medium are phenotypically identical to cells grown in serum, we examined effects of cell growth under both conditions on the beta-adrenergic receptor, a cell surface marker that activates adenylate cyclase and thereby regulates these cells. beta-Adrenergic receptors were qualitatively similar in cells grown in serum-containing and serum-free media, but the number of receptors was 30-50% less in cells grown with serum. This effect required several days to occur or to be reversed. The decreased number of receptors appeared to be caused by an inhibitory effect of serum on receptor number and not by a stimulatory action of the constituents of the serum-free medium. Growth medium with 5% fetal bovine serum maximally inhibited beta-adrenergic receptor numbers with 1% serum causing a half-maximal inhibition. The ability of serum to inhibit the expression of beta-adrenergic receptors could be blocked by dialyzing but not by boiling fetal bovine serum. beta-Adrenergic receptor-stimulated cyclic AMP accumulation was also decreased by growth in medium containing serum. These studies demonstrate that compared to growth of cells in serum-free medium, growth in serum-containing medium can inhibit expression of cell surface beta-adrenergic receptors. These results imply that the presence of serum in medium in which cells are grown alters properties in the plasma membrane and may alter hormonal responses in such cells.