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and nPKC isoforms
1 Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, Massachusetts 02118; and 2 Immunology Division, Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, Pennsylvania 19104
Culturing clonal
-cells (HIT-T15)
overnight in the presence of phorbol ester [phorbol myristate acetate
(PMA)] enhanced insulin secretion while causing downregulation of some
protein kinase C (PKC) isoforms and most PKC activity. We show here
that this enhanced secretion required the retention of PMA in the cell. Hence, it could not be because of long-lived phosphorylation of cellular substrates by the isoforms that were downregulated, namely PKC-
, -
II, and -
, but could be because of the continued
activation of the two remaining diacylglycerol-sensitive isoforms
and µ. The enhanced secretion did not involve changes in glucose
metabolism, cell membrane potential, or intracellular Ca2+
handling, suggesting a distal effect. PMA washout caused the loss of
the enhanced response, but secretion was then stimulated by acute
readdition of PMA or bombesin. The magnitude of this restimulation
appeared dependent on the mass of PKC-
, which was rapidly
resynthesized during PMA washout. Therefore, stimulation of insulin
secretion by PMA, and presumably by endogenous diacylglycerol, involves
the activation of PKC isoforms
and/or µ, and also PKC-
.
insulin secretion; protein kinase C isoforms; phorbol 12-myristate 13-acetate; bombesin
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