We have previously reported that pancreatic islet beta-cells and clonal HIT
insulinoma cells express an
ATP-stimulatable Ca(2+)-independent
phospholipase A2 (ASCI-PLA2)
enzyme and that activation of this
enzyme appears to participate in
glucose-stimulated insulin secretion. To further examine this hypothesis,
glucose-responsitivity and expression of ASCI-PLA2 activity in various
insulinoma cell lines were examined.
Secretagogue-stimulated insulin secretion was observed with beta TC6-f7 and early passage (EP)-beta TC6 cells. In contrast, RIN-m5f, beta TC3, and late passage (LP)-beta TC6 cells exhibited little
secretagogue-induced secretion. A haloenollactone suicide substrate (
HELSS) which inhibits ASCI-PLA2 activity ablated
secretagogue-induced insulin secretion from beta TC6-f7 and EP-beta TC6 cells. All
insulinoma cell lines studied expressed both cytosolic and membrane-associated Ca(2+)-independent PLA2 activities which were inhibited by
HELSS. The cytosolic enzymatic activity in the
glucose-responsive beta TC6-f7 and EP-beta TC6 cells was activated by
ATP and protected against thermal denaturation by
ATP, but this was not the case in the
glucose-unresponsive RIN-m5f, beta TC3, or LP-beta TC6 cells. Comparison of the distribution of Ca(2+)-independent PLA2 activity revealed that membrane-associated activity was higher than cytosolic activity in beta TC6-f7 and EP-beta TC6 cells but not in RIN-m5f, beta TC3, or LP-beta TC6 cells. Insensitivity of cytosolic activity to
ATP may prevent association of the PLA2 activity with membrane substrates and contribute to attenuated
glucose-responsitivity in the RIN-m5f, beta TC3, or LP-beta TC6 cells. HIT
insulinoma cells were also found to undergo a decline in both
glucose-responsitivity and membrane-associated Ca(2+)-independent PLA2 activity upon serial passage in culture, and this was associated with a reduction in membrane content of arachidonate-containing
phospholipids. These and previous results suggest that the
ATP-stimulatable PLA2
enzyme may participate in
glucose-induced insulin secretion.