Insulin Release in RINm5F Cells and Glyceraldehyde Activation of Protein Kinase C

In the insulin-secreting, glucose-insensitive islet cell subclone RINm5F, the distribution of protein kinase C (PKC) activity in the cytosol and membrane fractions was determined, and the activation of the enzyme, as reflected in its translocation to the membrane fraction, was characterized in conjunction with insulin release. DL-Glyceraldehyde (15 mM) evoked a rapid redistribution of PKC from the cytosol to the membrane fraction; insulin release increased concomitantly. When monitored over 5 min with 15 mM glyceraldehyde, membrane stabilization of PKC reached a maximum at 30 s and decreased thereafter; insulin release occurred at a high rate for the first 15 s and diminished thereafter. With 2–20 mM glyceraldehyde, a dose-dependent increase in membrane stabilization of PKC occurred and was accompanied by a matching increase in insulin release. Exogenous 1,2-dioctanoyl-sn-glycerol (100 μM) induced a rapid membrane stabilization of PKC and concomitant stimulation of insulin release. Glucose (15 mM) failed to evoke any redistribution of PKC or release of insulin. Depletion of total PKC activity by 95% induced by 18-h incubations with 2 μM 12-O-tetradecanoylphorbol-13-acetate resulted in a 67–91% reduction in glyceraldehyde-induced insulin release. We conclude that in the RINm5F islet β-cell subclone 7) the rapid activation of PKC, which occurs in response to the administration of glyceraldehyde, a nutrient secretagogue, plays an amplifying role in the initiation of stimulated insulin release; and 2) the failure of the activation of PKC may be responsible for the insensitivity to glucose.