Regulation of arachidonic acid release and cytosolic phospholipase A(2) activation

The 85-kDa cytosolic PLA(2) (cPLA(2)) mediates agonist-induced arachidonic acid release in many cell models, including mouse peritoneal macrophages. c PLA(2) is regulated by an increase in intracellular calcium, which binds to an amino-terminal C2 domain and induces its translocation to the nuclear e nvelope and endoplasmic reticulum, Phosphorylation of cPLA(2) on S505 by mi togen-activated protein kinases (MAPK) also contributes to activation, In m acrophages, zymosan induces a transient increase in intracellular calcium a nd activation of MAPK, which together fully activate cPLA(2) and synergisti cally promote arachidonic acid release, There are alternative pathways for regulating cPLA(2) in macrophages because PMA and okadaic acid induce arach idonic acid release without increasing calcium. The baculovirus expression system is a useful model to study cPLA(2) activation. Sf9 cells expressing cPLA(2) release arachidonic acid to either A23187 or okadaic acid. cPLA(2) is phosphorylated on multiple sites in Sf9 cells, and phosphorylation of S7 27 is preferentially induced by okadaic acid. However, the phosphorylation sites are non-essential and only S505 phosphorylation partially contributes to cPLA(2) activation in this model. Although okadaic acid does not increa se intracellular calcium in Sf9 cells, calcium binding by the C2 domain is necessary for arachidonic acid release. A23187 and okadaic acid activate cP LA(2) by different mechanisms, yet both induce translocation to the nuclear envelope in Sf9 cells. The results demonstrate that alternative regulatory pathways can lead to cPLA(2) activation and arachidonic acid release.