Modulation of Peptide Binding to Specific Receptors on Rat Pituitary Cells by Tumor-promoting Phorbol Esters: Decreased Binding of Thyrotropin-releasing Hormone and Somatostatin as Well as Epidermal Growth Factor¹

Phorbol esters increase prolactin synthesis and release and inhibit growth hormone production by GH₄C₁ cells, a clonal strain of rat pituitary cells. We examined the actions of phorbol esters on the binding of peptides that regulate the function of GH₄C₁ cells and found that phorbol esters decreased the specific binding of epidermal growth factor (EGF), thyrotropinreleasing hormone (TRH), and somatostatin. 12-O-Tetradecanoyl-13-phorbol acetate (TPA) at 100 ng/ml decreased ¹²⁵I-EGF binding to 15% of control within 1 hr, ³H-TRH binding to 30% of control after 4 hr, and ¹²⁵I-Tyr¹-somatostatin binding to 40% of control after 24 hr. The action of TPA on ¹²⁵I-EGF and ³H-TRH binding was temperature dependent, occurring at 37° but not at 4°, and was reversed within 72 hr after removal of TPA. By Scatchard analysis of ¹²⁵I-EGF-binding data, TPA (100 ng/ml) decreased the concentration dependence of ¹²⁵I-EGF binding (7- to 10-fold) when binding was assayed at 37° and decreased both the apparent affinity (2- to 6-fold) and the number of ¹²⁵I-EGF-binding sites (by 25 to 50%) when binding was assayed at 4°. The decrease in ³H-TRH binding induced by TPA (100 ng/ml) was due to a 2- to 5-fold decrease in the apparent affinity of binding at 37°. Thus, unlike findings reported for certain other cell lines, phorbol esters induce an alteration in membrane function in GH₄C₁ cells which decreases the binding to specific receptors for more than one regulatory peptide. Modulation of receptors for regulatory ligands may play a role in the mechanism by which phorbol esters promote neoplastic transformation, but the action on “growth factor” receptors cannot be considered specific in all cell types.