Galpha(i2)-mRNA and -protein regulation as a mechanism for heterologous sensitization and desensitization of insulin secretion.
Prolonged exposure of cells to an agonist of a G-protein-coupled receptor usually results in an attenuation of the cellular response. To elucidate the cellular mechanisms of sensitization or desensitization in an insulin secretory cell system (INS-1 cells), we investigated a regulatory link between G-protein alpha(s)- and alpha(i2)-subunits mRNA, their protein levels and insulin secretion as the biological effect using various compounds. Incubation with epinephrine (50 microM) for 8 h decreased alpha(s)- and alpha(i2)-mRNA levels to 58% and 72%, respectively, which is reversed after a longer incubation. From results using isoprenaline and the alpha2-agonist UK 14,304 epinephrine is shown to mediate its actions via alpha2- but not beta-adrenoceptors. The insulin inhibitory neuropeptide galanin (50 nM) caused a decrease of alpha(s)- and alpha(i2)-mRNA levels, whereas insulinotropic compounds (incretin hormones) such as GIP or GLP-1 (both 10 nM) led to an increase of alpha(s)- and alpha(i2)-mRNA levels. By using the Ca2+ channel blocker verapamil (50 microM) alpha(i2)-mRNA changes clearly depend on Ca2+ influx. The effects on alpha(i2)-mRNA were accompanied by a parallel, albeit weaker effect on the protein level (only GIP and UK 14,304 were investigated). The changes in alpha(i2)-mRNA levels by either compound were paralleled by inverse changes in insulin secretion: preincubation with UK 14,304 for 8 h led to an increased insulin secretion when challenged by either GLP-1, GIP or glucose (8.3 mM). This was similar for galanin, another potent inhibitor of insulin release. On the other hand, exposure to the incretins GIP or GLP-1 for 8 h induced a smaller insulin release when challenged afterwards by either UK 14,304, galanin, GIP, GLP-1, or glucose. Thus the influence on insulin secretion of various compounds is reciprocal to the regulation of alpha(i2)-mRNA levels but not alpha(s)-mRNA levels. There is, therefore, evidence from all the manoeuvres used that alpha(i2)-mRNA regulation may play a role in heterologous sensitization and desensitization of insulin secretion.
Kesper
S
S
Department of Pharmacology, Institute of Pharmaceutical Sciences, University of Münster, Germany.
Rucha
J
J
Neye
H
H
Mazenot
C
C
Verspohl
E J
EJ
D000818:Animals; D002460:Cell Line; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000378:metabolism; D005786:Gene Expression Regulation; D020962:Heterotrimeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*
D007328:Insulin; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; C498015:GNAI2 protein, human; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D019205:GTP-Binding Protein alpha Subunits, Gs; D020962:Heterotrimeric GTP-Binding Proteins