Release of secretory phospholipase A₂ from rat neuronal cells and its possible function in the regulation of catecholamine secretion

Here we show that secretory phospholipase A₂ (sPLA₂) that is immunochemically indistinguishable from type II sPLA₂ is (i) stored in neuroendocrine cells, (ii) released in response to neurotransmitters or depolarization, and (iii) involved in the regulation of catecholamine secretion by these cells. Rat brain synaptic vesicle fractions contained PLA₂ activity, which was neutralized completely by an antibody raised against rat type II sPLA₂. sPLA₂ immunoreactive with anti-(type II sPLA₂) antibody was released from synaptosomes in response to depolarization evoked by a high concentration of potassium in the presence of Ca²⁺. Rat pheochromocytoma PC12 cells, which differentiated into adherent cells similar to sympathetic neurons in response to nerve growth factor, were used for the detailed analysis of the dynamics and function of sPLA₂ in neuronal cells. Antibody against rat type II sPLA₂ precipitated ∼80% of the PLA₂ activity in PC12 cell lysates. Transcript for type II sPLA₂ was detected in PC12 cells by reverse transcriptase-PCR. When neuronally differentiated PC12 cells were stimulated with carbamylcholine or potassium, sPLA₂ was released into the medium and reached a maximal ∼40% release by 15 min. Inhibitors specific to type II sPLA₂ suppressed catecholamine secretion by PC12 cells which had been activated by carbamylcholine. Furthermore, treatment of PC12 cells with exogenous type II sPLA₂ alone elicited catecholamine secretion. These observations indicate that sPLA₂ released from neuronal cells may regulate the degranulation process leading to release of neurotransmitters and are compatible with our earlier finding that this enzyme is involved in the degranulation of rat mast cells.