Glucagon-like peptide 1 activates protein kinase C through Ca2+-dependent activation of phospholipase C in insulin-secreting cells.
Although the stimulatory effect of glucagon-like peptide 1 (GLP-1), a cAMP-generating agonist, on Ca(2+) signal and insulin secretion is well established, the underlying mechanisms remain to be fully elucidated. We recently discovered that Ca(2+) influx alone can activate conventional protein kinase C (PKC) as well as novel PKC in insulin-secreting (INS-1) cells. Building on this earlier finding, here we examined whether GLP-1-evoked Ca(2+) signaling can activate PKCalpha and PKCepsilon at a substimulatory concentration of glucose (3 mm) in INS-1 cells. We first showed that GLP-1 translocated endogenous PKCalpha and PKCepsilon from the cytosol to the plasma membrane. Next, we assessed the phosphorylation state of the PKC substrate, myristoylated alanine-rich C kinase substrate (MARCKS), by using MARCKS-GFP. GLP-1 translocated MARCKS-GFP to the cytosol in a Ca(2+)-dependent manner, and the GLP-1-evoked translocation of MARCKS-GFP was blocked by PKC inhibitors, either a broad PKC inhibitor, bisindolylmaleimide I, or a PKCepsilon inhibitor peptide, antennapedia peptide-fused pseudosubstrate PKCepsilon-(149-164) (antp-PKCepsilon) and a conventional PKC inhibitor, Gö-6976. Furthermore, forskolin-induced translocation of MARCKS-GFP was almost completely inhibited by U73122, a putative inhibitor of phospholipase C. These observations were verified in two different ways by demonstrating 1) forskolin-induced translocation of the GFP-tagged C1 domain of PKCgamma and 2) translocation of PKCalpha-DsRed and PKCepsilon-GFP. In addition, PKC inhibitors reduced forskolin-induced insulin secretion in both INS-1 cells and rat islets. Thus, GLP-1 can activate PKCalpha and PKCepsilon, and these GLP-1-activated PKCs may contribute considerably to insulin secretion at a substimulatory concentration of glucose.
Suzuki
Yuko
Y
Department of Physiology, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu 431-3192, Japan.
Zhang
Hui
H
Saito
Naoaki
N
Kojima
Itaru
I
Urano
Tetsumei
T
Mogami
Hideo
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism / Q000502:physiology*; D007328:Insulin / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D011493:Protein Kinase C / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction; D010738:Type C Phospholipases / Q000378:metabolism*
D004791:Enzyme Inhibitors; D007328:Insulin; D047908:Intracellular Signaling Peptides and Proteins; C000619712:Marcks protein, rat; D008565:Membrane Proteins; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D052216:Glucagon-Like Peptide 1; D011493:Protein Kinase C; D010738:Type C Phospholipases; D002118:Calcium