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It is now well established that GPR139, a G-protein coupled receptor exclusively expressed in the brain and pituitary, is activated by the essential amino acids L-tryptophan (L-Trp) and L-phenylalanine (L-Phe) via Gαq-coupling. The in vitro affinity and potency values of L-Trp and L-Phe are within the physiological concentration ranges of L-Trp and L-Phe. A recent paper suggests that adrenocorticotropic hormone (ACTH), α and β melanocyte stimulating hormones (α-MSH and β-MSH) and derivatives α-MSH1-9/α-MSH1-10 can also activate GPR139 in vitro. We tested this hypothesis using guanosine 5′-O-(3-[35S]thio)-triphosphate binding (GTPγS), calcium mobilization and [3H]JNJ-63533054 radioligand binding assays. In the GTPγS binding assay, α-MSH, α-MSH1-9/α-MSH1-10, and β-MSH had no effect on [35S]GTPγS incorporation in cell membranes expressing GPR139 up to 30 μM in contrast to the concentration dependent activation produced by L-Trp, JNJ-63533054, and TC-09311 (two small molecule GPR139 agonists). ACTH slightly decreased the basal level of [35S]GTPγS incorporation at 30 μM. In the GPR139 radioligand binding assay, a moderate displacement of [3H]JNJ-63533054 binding by ACTH and β-MSH was observed at 30 μM (40 and 30%, respectively); α-MSH, α-MSH1-9/α-MSH1-10 did not displace any specific binding at 30 μM. In three different host cell lines stably expressing GPR139, α-MSH, and β-MSH did not stimulate calcium mobilization in contrast to L-Trp, JNJ-63533054, and TC-09311. ACTH, α-MSH1-9/α-MSH1-10 only weakly stimulated calcium mobilization at 30 μM ( |
