Epidermal growth factor and angiotensin II regulation of extracellular signal-regulated protein kinase in rat liver epithelial WB cells

Activation of extracellular signal-regulated protein kinase (ERK) is consid ered essential for mitogenesis. In the present study, rat liver epithelial WB cells were used to investigate the relative roles of Ca2+ protein kinase C (PKC), and protein tyrosine phosphorylation in mitogenesis and activatio n of the ERK pathway stimulated by epidermal growth factor (EGF) and angiot ensin II (Ang II). The sensitivity of the ERK pathway to Ca2+ was studied b y using 1,2-bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) t o chelate intracellular Ca2+ and a low extracellular Ca2+ concentration to prevent Ca2+ influx. Agonist-induced PKC activation was diminished by inhib ition of PKC by GF-109203X (bisindolylmaleimide) or by down-regulation of P KC by long-term treatment of the cells with phorbol myristate acetate (PMA) . Our results show that although activation of PKC was critical for mitogen esis induced by Ang II or EGF, the initial activation of ERK by both agonis ts in these cells was essentially independent of PKC activation and was ins ensitive to Ca2+ mobilization. This is in contrast to the findings in some cell types that exhibit a marked dependency on mobilization of Ca2+ and/or PKC activation. On the other hand, an obligatory tyrosine phosphorylation s tep for activation of ERK was indicated by the use of protein tyrosine kina se inhibitors, which profoundly inhibited the activation of ERK by EGF, Ang II, and PMA. Additional experiments indicated that tyrosine phosphorylatio n by a cytosolic tyrosine kinase may represent a general mechanism for G-pr otein coupled receptor mediated ERK activation.(C) 1999 Elsevier Science In c.