Insulin-secreting pancreatic islet beta-cells express a Group VIA Ca(2+)-independent
phospholipase A(2) (
iPLA(2)beta) that contains a
calmodulin binding site and protein interaction domains. We identified
Ca(2+)/calmodulin-dependent protein kinase IIbeta (CaMKIIbeta) as a potential iPLA(2)beta-interacting
protein by yeast two-hybrid screening of a cDNA library using
iPLA(2)beta
cDNA as bait. Cloning CaMKIIbeta
cDNA from a rat islet library revealed that one dominant CaMKIIbeta
isoform mRNA is expressed by adult islets and is not observed in brain or neonatal islets and that there is high conservation of the
isoform expressed by rat and human beta-cells. Binary two-hybrid assays using
DNA encoding this
isoform as bait and
iPLA(2)beta
DNA as prey confirmed interaction of the
enzymes, as did assays with CaMKIIbeta as prey and
iPLA(2)beta bait. His-tagged CaMKIIbeta immobilized on
metal affinity matrices bound
iPLA(2)beta, and this did not require exogenous
calmodulin and was not prevented by a
calmodulin antagonist or the Ca(2+)
chelator EGTA. Activities of both
enzymes increased upon their association, and
iPLA(2)beta reaction products reduced CaMKIIbeta activity. Both the
iPLA(2)beta inhibitor
bromoenol lactone and the CaMKIIbeta inhibitor
KN93 reduced arachidonate release from INS-1
insulinoma cells, and both inhibit insulin secretion. CaMKIIbeta and
iPLA(2)beta can be coimmunoprecipitated from INS-1 cells, and
forskolin, which amplifies
glucose-induced insulin secretion, increases the abundance of the immunoprecipitatable complex. These findings suggest that
iPLA(2)beta and CaMKIIbeta form a signaling complex in beta-cells, consistent with reports that both
enzymes participate in insulin secretion and that their expression is coinduced upon differentiation of pancreatic progenitor to endocrine progenitor cells.