Bradykinin-stimulated arachidonic acid release from MDCK cells is not protein kinase C dependent

Bradykinin (BK)-induced release of arachidonic acid (AA) fromMadin-Darby canine kidney (MDCK) D1 cells was investigated. Phorbol12-myristate 13-acetate (PMA) caused a synergistic increase in BK- andA-23187-induced release of AA but alone had no effect on this release.Inhibition of protein kinase C (PKC) with bisindolmaleimide I (BIS)abolished the synergistic effects of PMA but did not affect AA releasecaused by BK or A-23187 alone. Downregulation of PKC with 100 nM PMAresulted in a reduction of AA release induced by BK or A-23187addition, which corresponded to a decrease in cytoplasmic phospholipaseA₂(cPLA₂) activity as measured incell extracts. Although Western blotting revealed no differences in cPLA₂ expression as a result ofPMA treatment, phosphorylation of the enzyme, as assessed byphosphoserine content, was significantly reduced in PKC-depleted cells.These results imply that, with PKC downregulation, subsequent BKstimulation results in aCa²⁺-dependent translocation of aless phosphorylated, less active form ofcPLA₂. Any stimulation of PKC byBK addition did not appear as a significant event in onset reponsesleading to AA release. On the other hand, inhibition of themitogen-activated protein kinase (MAPK) cascade with the MAPK kinaseinhibitor, PD-98059, significantly decreased BK-induced release of AA,a finding that, with our other results, points to the existence of aPKC-independent route for stimulation of MAPK and the propagation ofonset responses.