Melatonin regulates catecholamine biosynthesis by modulating bone morphogenetic protein and glucocorticoid actions.
Here we investigated roles of the pituitary bone morphogenetic protein (BMP) system in modulating GH production regulated by a somatostatin analog, octreotide (OCT) and a dopamine agonist, bromocriptine (BRC) in rat pituitary somatolactotrope tumor GH3 cells. The GH3 cells were found to express BMP ligands, including BMP-4 and BMP-6; BMP type-1 and type-2 receptors (except the type-1 receptor, activin receptor-like kinase (ALK)-6); and Smad signaling molecules. Forskolin stimulated GH production in accordance with cAMP synthesis. BRC, but not OCT, suppressed forskolin-induced cAMP synthesis by GH3 cells. Individual treatment with OCT and BRC reduced forskolin-induced GH secretion. A low concentration (0.1 microM) of OCT in combination with BRC (1-100 microM) exhibited additive effects on reducing GH and cAMP production induced by forskolin. However, a high concentration (10 microM) of OCT in combination with BRC failed to suppress GH and cAMP production. BMP-4 specifically enhanced GH secretion and cAMP production induced by forskolin in GH3 cells. BRC, but not OCT, inhibited BMP-4-induced activation of Smad1,5,8 phosphorylation and Id-1 transcription and decreased ALK-3 expression. Of note, in the presence of a high concentration of OCT, the BRC effects suppressing BMP-4-Smad1,5,8 signaling were significantly impaired. In the presence of BMP-4, a high concentration of OCT also attenuated the BRC effects suppressing forskolin-induced GH and cAMP production. Collectively, a high concentration of OCT interferes with BRC effects by reducing cAMP production and suppressing BMP-4 signaling in GH3 cells. These findings may explain the mechanism of resistance of GH reduction to a combination therapy with OCT and BRC for GH-producing pituitary adenomas.