Glucocorticoids Down-Regulate Somatostatin Receptors on Pituitary Cells in Culture*

GH4C1 cells, a clonal strain of rat pituitary tumor cells which synthesize and secrete PRL and GH, have specific functional receptors for the two hypothalamic peptides TRH and somatostatin (SRIF). Chronic treatment of GH4C1 cells with cortisol has previously been shown to increase the number of TRH receptors per GH4C1 cell. The results reported here demonstrate that cortisol also causes a 20–40% decrease in the specific binding of [125I-Tyr1]SRIF. Treatment of GH4C1 cells with 5 × 10-6 M cortisol caused a time-dependent decrease in [125I-Tyr1]SRIF-specific binding. A maximal effect was achieved after 8 h of treatment, and the effect persisted for at least 48 h. Receptor affinity was the same in control and cortisol-pretreated cells (Kd = 7.8 × 10-10 M), but the number of SRIF-binding sites decreased from 21,900 to 15,20 receptors/cell. The half-maximal decrease in [125I-Tyr1]SRIF-specific binding occurred with 3 × 10-8 M cortisol. Pretreatment of cells with 5 × 10-7 M dexamethasone(1,4-pregnadien-9-fluoro-16α-methyl-llβ,17α,21-triol-3,20-dione) for 43 h caused the same maximal decrease in [125I-Tyr1] SRIF-specific binding as 5 × 10-6 M cortisol; however, dexamethasone was 10 times more potent than cortisol (ED50 = 3 × 10-9 M). Pretreatment of GH4C1 cells for 43 h with 5 × 10-6 M 17β-estradiol, 17α-methyltestosterone, testosterone, or progesterone had no effect on [125I-Tyr1]SRIF-specific binding, indicating that this effect was glucocorticoid specific. Glucocorticoids also caused a decrease in [125I-Tyr1]SRIF-specific binding to AtT20/Dl6 cells, a clonal strain of mouse pituitary tumor cells which respond biologically to SRIF with decreased ACTH release. Treatment of GH4C1 cells with both dexamethasone and cycloheximide caused the same decrease in [125I-Tyr1]SRIF-specific binding as did either agent alone. In contrast, the downmodulation of SRIF receptors by chronic TRH treatment was previously shown to be additive with cycloheximide. However, maximal concentrations of TRH and cortisol added together had no greater effect on SRIF receptors than TRH alone. Therefore, even though the effects of TRH and cortisol are differentially sensitive to cycloheximide, the pathways by which they downregulate SRIF receptors probably overlap at a common ratelimiting step. In conclusion, the receptors for TRH and SRIF are modulated in opposite directions on GH4C1 cells by chronic glucocorticoid treatment. Since these two hypothalamic peptides have opposing effects on hormone release, such receptor modulation may play an important role in regulating pituitary responsiveness to these hypothalamic regulatory factors.