We previously demonstrated that secretory phospholipase A 2 (sPLA 2 ) and lysophosphatidylcholine (LPC) exhibit neurotrophin-like neuritogenic activity in the rat pheochromocytoma cell line PC12. In this study, we further analyzed the mechanism whereby sPLA 2 displays neurite-inducing activity. Exogenously added mammalian group X sPLA 2 (sPLA 2 -X), but not group IB and IIA sPLA 2 s, induced neuritogenesis, which correlated with the ability of sPLA 2 -X to liberate LPC into the culture media. In accordance, blocking the effect of LPC by supplementation of bovine serum albumin or phospholipase B attenuated neuritogenesis by sPLA 2 or LPC. Overproduction or suppression of G2A, a G-protein-coupled receptor involved in LPC signaling, resulted in the enhancement or reduction of neuritogenesis induced by sPLA 2 treatment. These results indicate that the neuritogenic effect of sPLA 2 is mediated by generation of LPC and subsequent activation of G2A.

Cultured cerebellar granule neurons (CGNs) require membrane depolarization or neurotrophic factors for their survival in vitro and undergo apoptosis when deprived of these survival-promoting stimuli. Here, we show that secretory phospholipases A 2 s (sPLA 2 s) rescue CGNs from apoptosis after potassium deprivation. The neurotrophic effect required the enzymatic activity of sPLA 2 s, since catalytically inactive mutants of sPLA 2 s failed to protect CGNs from apoptosis. Consistently, the ability of sPLA 2 s to protect CGNs from apoptosis correlated with the extent of sPLA 2 -induced arachidonic acid release from live CGNs. The survival-promoting effect of sPLA 2 was inhibited by depletion of extracellular Ca 2+ or by the presence of L-type Ca 2+ channel blocker nicardipine, suggesting that Ca 2+ influx occurs upon sPLA 2 treatment. Among the mammalian sPLA 2 s tested, only group X sPLA 2 , but not group IB nor IIA sPLA 2 s, displayed neurotrophic activity. These results suggest a novel, unexpected neurotrophin-like role of sPLA 2 in the nervous system.

Cultured cerebellar granule neurons (CGNs) undergo apoptosis when deprived of depolarizing stimulation and provide an in vitro model system with which to study the effects of neurotrophic substances. Our previous results showed that secretory phospholipases A(2) (sPLA(2)s) protect CGNs from apoptotic cell death under the nondepolarizing condition. In this study, we further analyzed the mechanism whereby sPLA(2) exhibits this effect. Among the primary metabolites of sPLA(2) tested, lysophosphatidylcholine (LPC), but not other lysophospholipids, remarkably rescued CGNs from apoptosis. In contrast, neither arachidonic nor oleic acids displayed neurotrophic effect. Release of LPC into the culture media occurred in response to sPLA(2) treatment, and degradation or sequestration of LPC attenuated the survival-promoting effects of sPLA(2) and LPC. The neurotrophic effect of LPC required the presence of extracellular Ca(2+) and L-type Ca(2+) channel activity, suggesting that Ca(2+) influx across the plasma membrane is evoked by LPC. sPLA(2)- or LPC-induced promotion of CGN survival was suppressed by inhibitors of protein kinase A and phospholipase C, suggesting that they play a role in mediating survival-promoting signal of sPLA(2). The results presented here demonstrate a novel, unexpected neurotrophin-like effect of LPC in the central nervous system.

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