PI 3-kinase γ and protein kinase C-ζ mediate RAS-independent activation of MAP kinase by a Gi 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-Delta SOS cells). LPA also induced the activation of MAP kinase kinase (MEK), but not that of RAF1, in CHO-Delta SOS 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 gamma isoform of PI3K. Furthermore, LPA induced the activation of the atypical zeta isoform of protein kinase C (PKC-zeta) in CHO-Delta SOS cells in a manner that was sensitive to wortmannin or to the dominant-negative mutant of PI3K gamma, and overexpression of a dominant-negative mutant of PKC-zeta 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 PI3K gamma-dependent activation of atypical PKC-zeta.