Evidence for the involvement of Gi2 in activation of extracellular signal-regulated kinases in hepatocytes

Targeting the Gi2α expression by a specific ribozyme inhibited the PGF2α -induced ERK1/2 activation in hepatocytes. On the other hand a non-cleaving form of the Gi2α ribozyme did not significantly decrease the ERK1/2 activation. In ribozyme-treated cells the Gi2α protein level was reduced, while the Gqα level was not affected thus confirming the specificity of the ribozyme.The present data suggest an important role of Gi2 in PGF2α -induced ERK1/2 signaling in hepatocytes.The extracellular regulated kinases ERK1 (p44mapk) and ERK2 (p42mapk) are believed to be implicated in regulation of cell growth and differentiation [1,2]. They are activated in response to stimulation both of heptahelical G protein coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). Epidermal growth factor (EGF), hepatocyte growth factor (HGF), PGF2α, norepinephrine, and several other agents activate ERK1/2 in hepatocytes [3,4,5,6]. Furthermore, it was observed in these cells that pretreatment with pertussis toxin (PTX) decreased activation of ERK1/2 in response to various agents acting on RTKs or GPCRs [6,7,8,9]. The data suggest an involvement of Gi protein(s) in the mechanisms of ERK1/2 activation in hepatocytes. However, it is not known which Gi protein(s) that mediate this effect. To approach this issue we have targeted the α subunit of Gi2 by a catalytic RNA (ribozyme) [10,11]. The effect of the ribozyme on PGF2α -induced ERK1/2 activation, which is strongly sensitive to PTX, was subsequently assessed.Pretreatment of hepatocytes with pertussis toxin [12] was reported to decrease ERK1/2 activation by agents acting both on heptahelical G protein coupled receptors as well as receptor tyrosine kinases [6,7,8]. These observations are summarized in Fig. 1. In addition these data show the persistence with time of the marked inhibitory effect of PTX on ERK1/2 activation induced by PGF2α . The EGF- and HGF-induced responses are on the other hand only partially decreased. These findings sug