Dual intracellular pathways in gonadotropin releasing hormone (GNRH) induced desensitization of luteinizing hormone (LH) secretion.
The mechanisms of GnRH-induced desensitization of LH secretion are poorly understood. Protein kinase C (PKC) and protein kinase A (PKA) desensitize some receptors of the 7-membrane type, and the GnRH receptor has consensus phosphorylation sites for PKC in the first and third intracellular loops, and a site for PKA in the first intracellular loop. In the first set of experiments we determined whether synthetic peptides representing the three intracellular loops of the receptor could be phosphorylated in vitro by purified PKC and PKA. As compared with a model substrate peptide for PKC, the third intracellular loop was phosphorylated 74% and the first intracellular loop 21%; PKA-phosphorylated the first intracellular loop peptide 17% as well as a model peptide substrate. In the second set of experiments, we used phorbol 12-myristate 13 acetate (PMA), an established PKC stimulator, and cholera toxin (CTX), established to activate the Gs protein and presumed to activate PKA, to treat cultured rat pituitary cells followed by LH measurements. Treatment with both drugs severely impaired GnRH-stimulated LH secretion whereas neither drug alone reduced LH secretion. Dibutyryl cAMP did not duplicate the effects of cholera toxin suggesting that the CTX action could not be explained by an increase in cAMP. These results suggest that more than one intracellular signaling pathway requires activation in order to induce desensitization; one pathway involves PKC and the other involves a pathway stimulated by cholera toxin, presumably Gs protein, which does not involve PKA.
Cassina
M P
MP
Department of Physiology and Biophysics, University of Alabama at Birmingham, 35294-0005, USA.
Musgrove
L C
LC
Duck
L W
LW
Sellers
J C
JC
Neill
J D
JD
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D003994:Bucladesine / Q000494:pharmacology; D002478:Cells, Cultured; D002772:Cholera Toxin / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D019204:GTP-Binding Proteins / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007986:Luteinizing Hormone / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D011966:Receptors, LHRH / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D010446:Peptide Fragments; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D003994:Bucladesine; D007986:Luteinizing Hormone; D002772:Cholera Toxin; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate