Glucose-responsitivity and expression of an ATP-stimulatable,Ca2+-independent phospholipase A2 enzyme in clonal insulinoma cell lines

We have previously reported that pancreatic islet β-cells and clonal HIT insulinoma cells express an ATP-stimulatable Ca²⁺-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 βTC6-f7 and early passage (EP)-βTC6 cells. In contrast, RIN-m5f, βTC3, and late passage (LP)-βTC6 cells exhibited little secretagogue-induced secretion. A haloenollactone suicide substrate (HELSS) which inhibits ASCI-PLA₂ activity ablated secretagogue-induced insulin secretion from βTC6-f7 and EP-βTC6 cells. All insulinoma cell lines studied expressed both cytosolic and membrane-associated Ca²⁺-independent PLA₂ activities which were inhibited by HELSS. The cytosolic enzymatic activity in the glucose-responsive βTC6-f7 and EP-βTC6 cells was activated by ATP and protected against thermal denaturation by ATP, but this was not the case in the glucose-unresponsive RIN-m5f, βTC3, or LP-βTC6 cells. Comparison of the distribution of Ca²⁺-independent PLA₂ activity revealed that membrane-associated activity was higher than cytosolic activity in βTC6-f7 and EP-βTC6 cells but not in RIN-m5f, βTC3, or LP-βTC6 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, βTC3, or LP-βTC6 cells. HIT insulinoma cells were also found to undergo a decline in both glucose-responsitivity and membrane-associated Ca²⁺-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.