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Prolactin and insulin are targeted to the regulated pathway in GH4C1 cells, but their storage is differentially regulated.

PRL storage in GH4C1 cells, rat pituitary tumor cells, can be induced by treatment with a combination of estradiol, epidermal growth factor, and bovine insulin. This increase in storage is characterized by a preferential increase in intracellular PRL compared with secreted PRL and a 50-fold increase in the number of secretory granules. Treatment with the combined hormones stimulates PRL synthesis approximately 6-fold, but this effect is not sufficient to increase PRL storage, because epidermal growth factor alone increases PRL synthesis to the same extent without affecting storage. The cDNA for human proinsulin down-stream of the RSV-LTR promoter was transfected into GH4C1 cells to determine whether storage of another protein known to be targeted to the regulated pathway would also increase with hormone treatment. Proinsulin and PRL release were stimulated over the same time course and to the same peak height, compared to basal release, by both KCl and TRH, indicating that proinsulin is targeted to the regulated pathway in GH4C1 cells. There was little intracellular processing of proinsulin to insulin. Proinsulin synthesis increased 3.9-fold with the combined treatment, assessed by accumulation of proinsulin immunoreactivity in the medium and increases at the mRNA level. Treatment with the combined hormones did not cause the preferential increase in intracellular proinsulin that occurred with PRL; the increase in intracellular proinsulin could be accounted for primarily by effects on synthesis. These results suggest that storage of the two hormones can be differentially regulated.