Glucogan-like peptide 1-evoked activation mechanisms of protein kinase C through Ca²⁺ signal in insulin secreting cells

Abstract

Insulin secretion from pancreatic β cells is modulated by numerous extracellular signaling molecules. Glucagon-like peptide 1 (GLP-1) among them is a powerful insulinotropic peptide. Upon binding of its receptor, GLP-I increases cyclic adenosine monophosphate (cAMP) levels via a G-protein-coupled activation of adenylate cyclase (AC), leading to activation of protein kinase A (PKA). GLP-1 through cAMP-PKA signaling pathway modulates Ca²⁺ signals. Our recent work has demonstrated that depolarization-evoked Ca²⁺ influx via voltage dependent Ca²⁺ channels in insulin secreting cells (INS-1 cells) induces activation of conventional PKC (cPKC) as well as structurally Ca²⁺-independent novel PKC (nPKC). We investigated whether GLP-1 as a Ca²⁺ modulator can activate PKCα and PKCε as representatives of cPKC and nPKC in INS-1 cells. GLP-1 and 8-Br-cAMP (Br) evoked-Ca²⁺ signal induced GFP-tagged PKCα and PKCε. This result indicated that GLP-1-evoked Ca²⁺ signals can activate PKCs. We verified this by showing in three deferent ways: firstly, GLP-1-induced translocation of GFP-tagged myristoylated alanine-rich C kinase substrate, secondly, GLP-1-induced translocation of GFP-tagged C1 domain of PKCγ as a diacylglycerol (DAG) sensor, thirdly GLP-1-induced translocation of GFP-tagged pleckstrin homology domain as a marker of phospholipase C (PLC) activation. Thus, we concluded from these observations that GLP-1 -evoked Ca²⁺ signal generates DAG through PLC activation thereby activating both PKCα and PKCε. [Jpn J Physiol 54 Suppl:S74 (2004)]