Biphasic regulation of extracellular signal-regulated kinases by scalaradial, A secretory phospholipase A2 inhibitor

Abstract

The marine natural product scalaradial (SLD) is a potent inhibitor of secretory phospholipase A2 (sPLA2). Our previous work has demonstrated that SLD inhibits epidermal growth factor receptor-mediated Akt phosphorylation, and this effect is independent of sPLA2. Here we report the role of SLD in extracellular signal-regulated kinase (ERK)1/2 activation. SLD inhibited ERK1/2 phosphorylation within the first 15 min (early inhibition), then stimulated ERK1/2 phosphorylation after 15 min of SLD treatment (late stimulation) in BEL-7402 cells, displaying biphasic regulatory features. Other PLA2 inhibitors such as the cytosolic and Ca2+-independent PLA2 inhibitor methyl arachidonyl fluorophosphonate, and another sPLA2 inhibitor, thioetheramide-phosphatidylcholine, only transiently inhibited ERK1/2 phosphorylation and did not display the stimulatory effect. The early inhibition of ERK1/2 phosphorylation by SLD was reversed by the PLA2 metabolite arachidonic acid, while the late stimulation was abrogated by constitutively active myristolated-Akt. Furthermore, SLD dose- and time-dependently inhibited the phosphorylation of Raf-1 on Ser 259, which is an established event by which Akt inhibits ERK 1/2 activation. Taken together, these data demonstrate a biphasic regulation of ERK1/2 phosphorylation by SLD in a time-dependent manner, i.e., early inhibition and late stimulation. The early inhibition of ERK1/2 phosphorylation is mediated by sPLA2, at least in part, and the late stimulation is effected through SLD inhibition of Akt. These findings provide further insight into the mechanisms underlying the pharmacological effect of SLD.