Smooth muscle cell arachidonic acid release, migration, and proliferation are markedly attenuated in mice null for calcium-independent phospholipase A₂β

Pharmacologic evidence suggests that the lipid products generated by one or more calcium-independent phospholipases A₂ (iPLA₂s) participate in the regulation of vascular tone through smooth muscle cell (SMC) Ca²⁺ signaling and the release of arachidonic acid. However, the recent identification of new members of the iPLA₂ family, each inhibitable by (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2- one, has rendered definitive identification of the specific enzyme(s) mediating these processes difficult. Accordingly, we used iPLA₂β ^-/- mice to demonstrate that iPLA₂β is responsible for the majority of thapsigargin and ionophore (A23187)-induced arachidonic acid release from SMCs. Both thapsigargin and A23187 stimulated robust [ ³H]arachidonate (AA) release from wild-type aortic SMCs that was dramatically attenuated in iPLA₂β^-/- mice (>80% reduction at 5 min; p < 0.01). Moreover, iPLA₂β^-/- mice displayed defects in SMC Ca²⁺ homeostasis and decreased SMC migration and proliferation in a model of vascular injury. Ca²⁺-store depletion resulted in the rapid entry of external Ca²⁺ into wild-type aortic SMCs that was significantly slower in iPLA₂β- null cells (p < 0.01). Furthermore, SMCs from iPLA₂β-null mesenteric arterial explants demonstrated decreased proliferation and migration. The defects in migration and proliferation in iPLA₂β-null SMCs were restored by 2 μM AA. Remarkably, the cyclooxygenase-2-specific inhibitor, NS-398, prevented AA-induced rescue of SMCmigration and proliferation in iPLA₂β^-/- mice. Moreover, PGE₂ alone rescued proliferation and migration in iPLA₂β^-/- mice. We conclude that iPLA₂β is an important mediator of AA release and prostaglandin E₂ production in SMCs, modulating vascular tone, cellular signaling, proliferation, and migration.