Stimulation of Intracellular Free Calcium in GH3 Cells byγ 3-Melanocyte-Stimulating Hormone. Involvement of a Novel Melanocortin Receptor?

Abstract

Abstract The melanocortin (MC) γ3MSH is a peptide that can be generated from the N-terminal domain of POMC and is believed to signal through the MC3 receptor. We recently showed that it induces a sustained rise in intracellular free calcium levels ([Ca2+]i) in a subpopulation of pituitary cells, particularly in the lactosomatotroph lineage. In the present study we report that γ3MSH and some analogs increase [Ca2+]i in the GH- and PRL-secreting GH3 cell line and evaluate on the basis of pharmacological experiments and gene expression studies which MC receptor may be involved. A dose as low as 1 pm γ3MSH induced an oscillating[ Ca2+]i increase in a significant percentage of GH3 cells. Increasing the dose recruited an increasing number of responding cells; a maximum was reached at 0.1 nm. γ2MSH,α MSH, and NDP-αMSH displayed a similar effect. SHU9119, an MC3 and MC4 receptor antagonist, and an MC5 receptor agonist, did not affect the number of cells showing a [Ca2+]i rise in response to γ3MSH. SHU9119 had also no effect when added alone. MTII, a potent synthetic agonist of the MC3, MC4, and MC5 receptor as well as an N-terminally extended recombinant analog of γ3MSH showed low potency in increasing [Ca2+]i in GH3 cells, but high potency in stimulating cAMP accumulation in HEK 293 cells stably transfected with the MC3 receptor. In contrast, a peptide corresponding to the γ2MSH sequence of POMC-A of Acipenser transmontanus increased [Ca2+]i in GH3 cells, but was about 50 times less potent than γ2- or γ3MSH in stimulating cAMP accumulation in the MC3 receptor expressing HEK 293 cells. By means of RT-PCR performed on a RNA extract from GH3 cells, the messenger RNA of the MC2, MC3, and MC4 receptor was undetectable, but messenger RNA of the MC5 receptor was clearly present. These data suggest that the GH3 cell line does not mediate the effect of γ3MSH through the MC3 receptor. The involvement of the MC5 receptor is unlikely, but cannot definitely be excluded. The findings animate the hypothesis that there exists a second, hitherto unidentified, MC receptor that displays high affinity for γ3MSH.