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Activation of protein kinase C partially alleviates noradrenaline inhibition of insulin secretion.

ABSTRACT: The sympathetic neurotransmitter noradrenaline (NA) fully inhibited both phases of glucose-stimulated insulin secretion from rat islets of Langerhans. The secretory response to the protein kinase C (PKC) activator, 4 beta-phorbol myristate acetate (4 beta PMA), in the absence of exogenous glucose was also abolished by NA. However, at 20 mM glucose 4 beta PMA partially alleviated the inhibitory effect of NA both on insulin release and on cyclic AMP generation. Inhibition of insulin release by NA, albeit much decreased, was still observed in the presence of maximal stimulatory concentrations of both 4 beta PMA and dibutyryl cyclic AMP. The relieving effect of 4 beta PMA on the inhibition of insulin secretion by NA was not overcome by the competitive antagonist of cyclic AMP-dependent protein kinase, Rp-adenosine 3',5'-cyclic phosphorothioate. Down-regulation of islet PKC activity by overnight exposure to 4 beta PMA did not affect the inhibitory capacity of NA. These results suggest that NA inhibits insulin release independently of interaction with PKC, but that activation of this enzyme decreases the inhibitory effect of NA at stimulatory concentrations of glucose. This protective effect of 4 beta PMA could not be attributed to a decrease in NA inhibition of cyclic AMP generation.

SUBMITTER: Persaud SJ

PROVIDER: S-EPMC1132195 | biostudies-other | 1993 Jan

REPOSITORIES: biostudies-other

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Project description:OBJECTIVE:The mechanism by which G-protein-coupled receptor 40 (GPR40) signaling amplifies glucose-stimulated insulin secretion through activation of protein kinase C (PKC) is unknown. We examined whether a GPR40 agonist, GW9508, could stimulate conventional and novel isoforms of PKC at two glucose concentrations (3 mM and 20 mM) in INS-1D cells. METHODS:Using epifluorescence microscopy, we monitored relative changes in the cytosolic fluorescence intensity of Fura2 as a marker of change in intracellular Ca2+ ([Ca2+]i) and relative increases in green fluorescent protein (GFP)-tagged myristoylated alanine-rich C kinase substrate (MARCKS-GFP) as a marker of PKC activation in response to GW9508 at 3 mM and 20 mM glucose. To assess the activation of the two PKC isoforms, relative increases in membrane fluorescence intensity of PKCα-GFP and PKCε-GFP were measured by total internal reflection fluorescence microscopy. Specific inhibitors of each PKC isotype were constructed and synthesized as peptide fusions with the third α-helix of the homeodomain of Antennapedia. RESULTS:At 3 mM glucose, GW9508 induced sustained MARCKS-GFP translocation to the cytosol, irrespective of changes in [Ca2+]i. At 20 mM glucose, GW9508 induced sustained MARCKS-GFP translocation but also transient translocation that followed sharp increases in [Ca2+]i. Although PKCα translocation was rarely observed, PKCε translocation to the plasma membrane was sustained by GW9508 at 3 mM glucose. At 20 mM glucose, GW9508 induced transient translocation of PKCα and sustained translocation as well as transient translocation of PKCε. While the inhibitors (75 μM) of each PKC isotype reduced GW9508-potentiated, glucose-stimulated insulin secretion in INS-1D cells, the PKCε inhibitor had a more potent effect. CONCLUSION:GW9508 activated PKCε but not PKCα at a substimulatory concentration of glucose. Both PKC isotypes were activated at a stimulatory concentration of glucose and contributed to glucose-stimulated insulin secretion in insulin-producing cells.

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Activation of protein kinase C partially alleviates noradrenaline inhibition of insulin secretion.

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