Protein kinase Cζ is required for oleic acid-induced secretion of glucagon-like peptide-1 by intestinal endocrine L cells

Long-chain, monounsaturated fatty acids (FAs) stimulate secretion of the incretin hormone, glucagon-like peptide-1 (GLP-1) from the intestinal L cell. Because the atypical protein kinase C (PKC), PKC zeta, is involved in FA signaling in many cells, the role of PKC zeta in FA-induced GLP-1 secretion was investigated, using the murine GLUTag L cell line and primary rat intestinal L cells. GLUTag cells expressed mRNA for several PKC isoforms, including PKC zeta, and PKC zeta protein was localized throughout the cytoplasm in GLUTag and primary L cells as well as normal mouse and rat L cells. Treatment with oleic acid (150-1000 mu M) for 2 h increased GLP-1 secretion (P < 0.001), and this was abrogated by the PKC zeta inhibitor ZI (P < 0.05) and PKC zeta small interfering RNA transfection (P < 0.05) but not inhibition of classical/novel PKC isoforms. Although most PKC zeta was localized in the particulate compartment of GLUTag cells, oleate treatment did not alter PKC zeta levels or activity in this cell fraction. GLUTag cells expressed mRNA for the Gq-coupled FA receptor GPR120; however, oleic acid did not induce any changes in Akt, MAPK, or calcium, and pretreatment with LY294002 and PD98059 to inhibit phosphatidylinositol 3-kinase and MAPK, respectively, did not prevent the effects of oleic acid. Finally, GLUTag cells also released GLP-1 in response to arachidonic acid (P < 0.001) but were not affected by other long- chain FAs. These findings demonstrate that PKC zeta is required for oleic acid-induced GLP-1 secretion. This enzyme may therefore serve as a therapeutic target to enhance GLP-1 release in type 2 diabetes.