PI 3‐kinase γ and protein kinase C‐ζ mediate RAS‐independent activation of MAP kinase by a G i protein‐coupled receptor

Receptors coupled to the inhibitory G protein G i , such as that for lysophosphatidic acid (LPA), have been shown to activate MAP kinase through a RAS‐dependent pathway. However, LPA (but not insulin) has now been shown to activate MAP kinase in a RAS‐independent manner in CHO cells that overexpress a dominant‐negative mutant of the guanine nucleotide exchange protein SOS (CHO‐ΔSOS cells). LPA also induced the activation of MAP kinase kinase (MEK), but not that of RAF1, in CHO‐ΔSOS cells. The RAS‐independent activation of MAP kinase by LPA was blocked by inhibitors of phosphatidylinositol 3‐kinase (PI3K) or by overexpression of a dominant‐negative mutant of the γ isoform of PI3K. Furthermore, LPA induced the activation of the atypical ζ isoform of protein kinase C (PKC‐ζ) in CHO‐ΔSOS cells in a manner that was sensitive to wortmannin or to the dominant‐negative mutant of PI3Kγ, and overexpression of a dominant‐negative mutant of PKC‐ζ inhibited LPA‐induced activation of MAP kinase. These observations indicate that G i protein‐coupled receptors induce activation of MEK and MAP kinase through a RAS‐independent pathway that involves PI3Kγ‐dependent activation of atypical PKC‐ζ.