ANGIOTENSIN-II STIMULATES ERK VIA 2 PATHWAYS IN EPITHELIAL-CELLS - PROTEIN-KINASE-C SUPPRESSES A G-PROTEIN COUPLED RECEPTOR EGF RECEPTOR TRANSACTIVATION PATHWAY

In GN4 rat liver epithelial cells, angiotensin II (Ang II) produces in tracellular calcium and protein kinase C (PKC) signals and stimulates ERK and JNK activity. JNK activation appears to be mediated by a calci um-dependent tyrosine kinase (CADTK). To define the ERK pathway, we es tablished GN4 cells expressing are inhibitory Ras(N17), Induction of R as(N17) blocked EGF-but not Ang II-or phorbol ester (TPA)-dependent ER K activation. In control cells, Ang II and TPA produced minimal increa ses in Ras-GTP level and Raf kinase activity. PKC depletion by chromic TPA exposure abolished TPA-dependemt ERK activation but failed to dim inish the effect of Ang II. In PKC-depleted cells, Ang II increased Ra s-GTP level and activated Raf and ERK in a Ras-dependent manner In PKC depleted cells, Ang II stimulated Shc and Cbl tyrosine phosphorylatio n, suggesting that without PKC, Ang II activates another tyrosine kina se. PKC-depletion did not alter Ang II-dependent tyrosine phosphorylat ion or activity of p125(FAK), CADTK, Fyn or Src, but PKC depletion or incubation with GF109203X resulted in Ang II-dependent EGF receptor ty rosine phosphorylation, In PKC-depleted cells, EGF receptor-specific t yrosine kinase inhibitors blocked Ang II-dependent EGF receptor and Cb l tyrosine phosphorylation, and ERK activation. In summary, Ang II ear n activate ERK via two pathways; the latent EGF receptor, Ras-dependen t pathway is equipotent to the Ras-independent pathway, but is masked by PKC action. The prominence of this G-protein coupled receptor to EG F receptor pathway may vary between cell types depending upon modifier s such as PKC.