Activation of Intracellular Kinases in Xenopus Oocytes by p21^ras and Phospholipases: a Comparative Study

Abstract

Signal transduction induced by generations of second messengers from membrane phospholipids is a major regulatory mechanism in the control of cell proliferation. Indeed, oncogenic p21^ras alters the intracellular levels of phospholipid metabolites in both mammalian cells and Xenopus oocytes. However, it is still controversial whether this alteration it is biologically significant. We have analyzed the ras-induced signal transduction pathway in Xenopus oocytes and have correlated its mechanism of activation with that of the three most relevant phospholipases (PLs). After microinjection, ras-p21 induces a rapid PLD activation followed by a late PLA₂ activation. By contrast, phosphatidylcholine-specific PLC was not activated under similar conditions. When each of these PLs was studied for its ability to activate intracellular signalling kinases, all of them were found to activate maturation-promoting factor efficiently. However, only PLD was able to activate MAP kinase and S6 kinase II, a similar pattern to that induced by p21^ras proteins. Thus, the comparison of activated enzymes after microinjection of p21^ras or PLs indicated that only PLD microinjection mimetized p21^ras signalling. Finally, inhibition of the endogenous PLD activity by neomycin substantially reduced the biological activity of p21^ras. All these results suggest that PLD activation may constitute a relevant step in ras-induced germinal vesicle breakdown in Xenopus oocytes.