Heterologous desensitization of insulin secretion by GIP (glucose-dependent insulinotropic peptide) in INS-1 cells: the significance of Galphai2 and investigations on the mechanism involved.
Heterologous desensitization is a term that describes the observation that chronic exposure of a cell to an agonist attenuates its response to other agonists. To characterize the cellular mechanisms that might be responsible for heterologous desensitization in an insulin secretory cell system (INS-1), we investigated the link between G-protein alphai2 level and insulin secretion as the biological effect after prolonged incubation with glucose-dependent insulinotropic polypeptide (GIP). Persistent activation (8 h) of the GIP signalling pathway decreased the GLP (glucagon-like peptide)-1 dependent insulin secretion (specific radioimmunoassay) accompanied by an upregulation of G-protein alphai2 protein level to about 126% whereas G-protein alphai3 and alphas protein levels remained unchanged (assessed by Western blots using specific antibodies). This was accompanied by similar changes in Galphai2 mRNA. By using either the CaM kinase II inhibitor KN-62, the calcineurin inhibitor FK 506 or the protein kinase A (PKA) inhibitor Rp-8-Br-cAMPS, the GIP-mediated Galphai2 mRNA increase was fully reversed. Heterologous desensitization of GLP-1-dependent insulin secretion by pretreatment with GIP, however, was not inhibited by calcium/calmodulin-dependent enzymes (using KN-62 and FK 506), but only by suppressing the cAMP/PKA signalling pathway using Rp-8-Br-cAMPS. The outcome is not disturbed by effects initiated by these compounds per se since an 8-h preincubation of cells did not affect glucose-induced insulin secretion. We, therefore, suggest that heterologous desensitization in INS-1 cells may be mediated by Galphai2 changes but depend on the cAMP/PKA signalling pathway probably distant form the Galphai2 protein.
Rucha
A
A
Institute of Pharmaceutical and Medicinal Chemistry, University of Münster, Department of Pharmacology, Germany.
Verspohl
E J
EJ
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine / Q000031:analogs & derivatives* / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D000068438:Brimonidine Tartrate; D045744:Cell Line, Tumor; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000235:genetics* / Q000378:metabolism*; D005749:Gastric Inhibitory Polypeptide / Q000494:pharmacology*; D015870:Gene Expression / Q000187:drug effects; D005934:Glucagon / Q000378:metabolism; D052216:Glucagon-Like Peptide 1; D007166:Immunosuppressive Agents / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics* / Q000378:metabolism*; D011810:Quinoxalines / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology; D016559:Tacrolimus / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology
C105593:8-bromoadenosine-3',5'-cyclic monophosphorothioate; D000316:Adrenergic alpha-Agonists; D004791:Enzyme Inhibitors; D007166:Immunosuppressive Agents; D007328:Insulin; D010446:Peptide Fragments; D011498:Protein Precursors; D011518:Proto-Oncogene Proteins; D011810:Quinoxalines; D012333:RNA, Messenger; D013873:Thionucleotides; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000068438:Brimonidine Tartrate; D005749:Gastric Inhibitory Polypeptide; C063302:KN 62; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D017868:Cyclic AMP-Dependent Protein Kinases; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; C498017:Gnai2 protein, rat; D016559:Tacrolimus