Hormonal induction of secretory granules in a pituitary tumor cell line.

GH4C1 cells are a rat pituitary tumor cell strain that secretes PRL and GH but contains almost no secretory granules. Treatment of GH4C1 cells with a combination of estradiol (1 nM), insulin (300 nM), and epidermal growth factor (10 nM) increased the cellular content of PRL by more than 30-fold above control levels but only increased PRL accumulation in the medium 6-fold. To determine whether the increase in intracellular PRL was accompanied by an increase in secretory granules, we compared the numbers of granules in ultrathin sections from untreated GH4C1 cells and from cells treated with the combined hormone regimen and found a nearly 50-fold increase in granule number. Only 75% of the granules stained for PRL by the protein-A gold technique; the other 25% stained for neither PRL nor GH. The occasional granules found in untreated GH4C1 cells stained for PRL. The data demonstrate that the number of granules in GH4C1 cells can be regulated by hormone treatment and that the increase in intracellular PRL is found in storage granules.