Activation of p42/p44 mitogen-activated protein kinase by angiotensin II, vasopressin, norepinephrine, and prostaglandin F2α in hepatocytes is sustained, and like the effect of epidermal growth factor, mediated through pertussis toxin-sensitive mechanisms

Abstract

Several agents that act through G-protein-coupled receptors and also stimulate phosphoinositide-specific phospholipase C (PI-PLC), including angiotensin II, vasopressin, norepinephrine, and prostaglandin (PG) F2alpha, activated the ERK1 (p44mapk) and ERK2 (p42mapk) members of the mitogen-activated protein (MAP) kinase family in primary cultures of rat hepatocytes, measured as phosphorylation of myelin basic protein (MBP) by a partially purified enzyme, immunoblotting, and in-gel assays. All these agonists induced a peak activation (two to threefold increase in MBP-phosphorylation) at 3-5 min, followed by a brief decrease, and then a sustained elevation or a second increase of the MAP kinase activity that lasted for several hours. Although all the above agents also stimulated PI-PLC, implicating a Gq-dependent pathway, the elevations of the concentration of inositol (1,4,5)-trisphosphate did not correlate well with the MAP kinase activity. Furthermore, pretreatment of the cells with pertussis toxin markedly reduced the MAP kinase activation by angiotensin II, vasopressin, norepinephrine, or PGF2alpha. In addition, hepatocytes pretreated with pertussis toxin showed a diminished MAP kinase response to epidermal growth factor (EGF). The results indicate that agonists acting via G-protein-coupled receptors have the ability to induce sustained activation of MAP kinase in hepatocytes, and suggest that Gi-dependent mechanisms are required for full activation of the MAP kinase signal transduction pathway by G-protein-coupled receptors as well as the EGF receptor.

β3- and α1-Adrenergic Erk1/2 Activation Is Src- but Not Gi-mediated in Brown Adipocytes

“…An ability of this pathway to activate Erk1/2 has also been observed in other experimental systems, both with ectopically expressed ␣ 1 receptors (19,27) and in cells endogenously expressing these receptors (25,32,54). In the present system, the mediation is probably initially through elevated Ca 2ϩ levels, as demonstrated in other ␣ 1 -induced systems (19,25,26).…”

“…3A). Also ␣ 1 -induced Erk1/2 activation was mediated in a G i -independent manner, confirming the nonessentially of this mediator protein for Erk1/2 activation (although such an ␣ 1 -pathway has been described in primary hepatocytes (32)). An ability of G i to activate Erk1/2 has been observed in several other systems (7-9, 32), although a noninvolvement of G i in the mediation has also been reported in other G-protein-coupled systems (33,34).…”

Conjugated bile acids promote ERK1/2 and AKT activation via a pertussis toxin-sensitive mechanism in murine and human hepatocytes

“…Studies by several laboratories have argued that activation of ERBB1, in addition to exogenous ligand-or ROS-induced activation of the receptor, can be due to the actions of GPCRs, which result in either a paracrine/autocrine form of receptor tyrosine kinase activation or a direct activation of ERBB1 via nonreceptor tyrosine kinases. [15][16][17][18][19][20][21][22][23][24] In our studies, incubation of hepatocytes with either marimastat or GM6001, to inhibit cell surface proteases that release ERBB1 receptor ligands, did not alter conjugated bile acid-induced ERK1/2 and AKT activation (unpublished studies and Zhang et al 30 ), whereas inhibition of Src family tyrosine kinase function blunted activation of ERK1/2 and AKT. These data would suggest a direct intra-membrane interaction between GPCR and ERBB1 activation, and enhanced activity in downstream signaling pathways ( Supplementary Fig.…”

“…[12][13][14] Several studies over the last 10 years have shown that ligand-independent activation of ERBB1 can occur via G-protein-coupled receptor-induced transactivation. [15][16][17][18][19][20] In these studies, activation of membrane-associated non-receptor tyrosine kinases, such as src family members, has been linked to G-protein-coupled receptors (GPCR)-induced ERBB1 tyrosine phosphorylation as well as activation of the receptors by paracrine mechanisms. Because bile acids have been shown to activate some G-protein-coupled receptors in non-hepatocyte cell types, which has been linked to altered proliferation states, some of our prior data showing activation of the ERK1/2 and AKT pathways could have been influenced by bile acid-induced activation of hepatic GPCRs, which in turn promote activation of receptor tyrosine kinases.…”

Single Nucleotide Polymorphisms in G Protein Signaling Pathway Genes in Preeclampsia

“…3,25 Ang II may also exert some of its effects via G i protein signaling. 26 RGS2 acts as a GTPase activating protein, negatively regulating both G q/11 as well as G i/o proteins by enhancing the termination of the activated G protein state. 18,20,27 Additionally, it has been suggested that RGS2 may act as a downstream effector of the NO-cGMP pathway, resulting in vascular relaxation by attenuating vasoconstrictor signaling, whereas suppression of this mechanism may facilitate development of hypertension.…”