P. Ambs et al., ROLE OF CYTOSOLIC PHOSPHOLIPASE A(2) IN ARACHIDONIC-ACID RELEASE OF RAT-LIVER MACROPHAGES - REGULATION BY CA2+ AND PHOSPHORYLATION, Biochemical journal, 311, 1995, pp. 189-195
In this study we have verified the existence of a cytosolic phospholip
ase A(2) (cPLA(2)) in rat-liver macrophages. Stimulation of these cell
s with phorbol 12-myristate 13-acetate (PMA), zymosan and lipopolysacc
haride (LPS), but not with the Ca2+-ionophore A23187, leads to phospho
rylation of cPLA(2) and activation of mitogen-activated protein (MAP)
kinase, supporting the hypothesis that MAP kinase is involved in cPLA(
2) phosphorylation. We show furthermore, that the tyrosine kinase inhi
bitor genistein prevents the LPS- but not the PMA- or zymosan-induced
phosphorylation of cPLA(2) and activation of MAP kinase, indicating th
at tyrosine kinases participate in LPS-but not in PMA- and zymosan-ind
uced cPLA(2) phosphorylation and MAP kinase activation. Phosphorylatio
n of cPLA(2) does not strongly correlate with stimulation of the arach
idonic acid (AA) cascade: (1) A23187, a potent stimulator of AA releas
e, fails to induce cPLA(2) phosphorylation; (2) withdrawal of extracel
lular Ca2+, which inhibits PMA-stimulated AA release (Dieter, Schulze-
Specking and Decker (1988) fur. J. Biochem. 177, 61-67), has no effect
on PMA-induced phosphorylation of cPLA(2); (3) LPS induces cPLA(2) ph
osphorylation within minutes, whereas increased AA release upon treatm
ent with LPS is detectable for the first time after 4 h; and (4) genis
tein, which prevents LPS-induced cPLA(2) phosphorylation, does not inh
ibit AA release in response to LPS. From these data we suggest that a
rise in intracellular Ca2+, but not phosphorylation of cPLA(2), is ess
ential for activation of the AA cascade in rat-liver macrophages.