Gamma-MSH peptides in the pituitary: effects, target cells, and receptors.
The melanocortin (MC) gamma3-MSH is believed to signal through the MC3 receptor. We showed that it induces a sustained increase in intracellular free calcium levels ([Ca(2+)](i)) in a subpopulation of pituitary cells. Most of the cells responding to gamma3-MSH express more than one pituitary hormone mRNA. The effect of gamma3-MSH is blocked by SHU9119, a MC3R and MC4R antagonist, in only 50% of the responsive cells, suggesting that in half of these cells the mediating receptor is not the MC3R. Low picomolar doses of gamma3-MSH increase [Ca(2+)](i) in the growth hormone (GH)- and prolactin (PRL)-secreting GH3 cell line. gamma2-MSH and alpha-MSH display a similar effect. SHU9119 does not affect the gamma3-MSH-induced [Ca(2+)](i) response. MTII, a potent synthetic agonist of the MC3R, MC4R, and MC5R, also shows no or low potency in increasing [Ca(2+)](i). By means of RT-PCR, the mRNA of the MC2R, MC3R, and MC4R receptors is undetectable. Experiments testing gamma2-MSH analogues with single alanine replacements show that, unlike the classic MCRs, the His(5)-Phe(6)-Arg(7)-Trp(8) sequence in gamma2-MSH is not a core sequence for activating the gamma-MSH receptor in GH3 cells, whereas Met(3) is essential. Low nanomolar doses of gamma-MSH increase intracellular cAMP levels. Blockade of protein kinase A abolishes the [Ca(2+)](i) responses to gamma3-MSH. gamma2-MSH increases binding of [S(35)]GTPgammaS to membrane preparations of GH3 cells. The pharmacological characteristics of gamma-MSH peptides and analogues on [Ca(2+)](i) and the signal-transduction pathways present strong evidence for the expression of a hitherto uncharacterized gamma-MSH receptor in GH3 cells, belonging to the G protein-coupled receptor family.
Denef
C
C
Laboratory of Cell Pharmacology, University of Leuven Medical School, B-3000 Leuven, Belgium. Carl.Denef@med.kuleuven.ac.be
Lu
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J
Swinnen
E
E
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D010455:Peptides / Q000235:genetics / Q000378:metabolism*; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D011388:Prolactin / Q000235:genetics / Q000378:metabolism; D018043:Receptors, Corticotropin / Q000378:metabolism; D044101:Receptors, Melanocortin; D016415:Sequence Alignment; D015398:Signal Transduction / Q000502:physiology; D019825:gamma-MSH / Q000235:genetics / Q000378:metabolism*
D010455:Peptides; D018043:Receptors, Corticotropin; D044101:Receptors, Melanocortin; D019825:gamma-MSH; D011388:Prolactin; D002118:Calcium