The tyrosine kinase class of receptors induces mitogen-activated protein kinase (MAPK) activation through the sequential interaction of the signaling proteins Grb2, Sos, Ras, Raf, and MEK. Receptors coupled to heterotrimeric guanine triphosphate-binding protein (G protein) stimulate MAPK through G _{β γ} subunits, but the subsequent intervening molecules are still poorly defined. Overexpression of phosphoinositide 3-kinase γ (PI3Kγ) in COS-7 cells activated MAPK in a G _{β γ} -dependent fashion, and expression of a catalytically inactive mutant of PI3Kγ abolished the stimulation of MAPK by G _{β γ} or in response to stimulation of muscarinic (m2) G protein-coupled receptors. Signaling from PI3Kγ to MAPK appears to require a tyrosine kinase, Shc, Grb2, Sos, Ras, and Raf. These findings indicate that PI3Kγ mediates G _{β γ} -dependent regulation of the MAPK signaling pathway.

The enzymatic activity of mitogen-activated protein kinases (MAP kinases) increases in response to agents acting on a variety of cell surface receptors, including receptors linked to heterotrimeric G proteins of the G_i and G_q family. Recently, it has been shown that stimulation of β-adrenergic receptors, which are typical of those that act through G_s to activate adenylyl cyclases, potently activates MAP kinases in the heart, resulting in the hypertrophy of the cardiac muscle (Lazou, A., Bogoyevitch, M. A., Clerk, A., Fuller, S. J., Marshall, C. J., and Sudgen, P. H.(1994) Circ. Res. 75, 938-941). We have observed that exposure of COS-7 cells to a β-adrenergic agonist, isoproterenol, raises intracellular levels of cAMP and effectively activates protein kinase A (PKA) and an epitope-tagged MAP kinase. However, MAP kinase stimulation by isoproterenol was neither mimicked by expression of an activated mutant of Gα_s, nor by treatment with PKA-stimulating agents. Moreover, pretreatment of COS-7 with a permeable cAMP analog, 8-Br-cAMP, markedly decreased MAP kinase activation by either isoproterenol or epidermal growth factor. Thus, in COS-7 cells cAMP and PKA do not appear to mediate MAP kinase activation by β-adrenergic receptors. Signaling from β-adrenergic receptors to MAP kinase was inhibited by transfection of a chimeric molecule consisting of the CD8 receptor and the carboxyl terminus of the β-adrenergic receptor kinase, which includes the βγ-binding domain. MAP kinase activation by isoproterenol was not affected by depletion of protein kinase C, but it was completely abolished by expression of Ras-inhibiting molecules. We conclude that signaling from β-adrenergic receptors to MAP kinase involves an activating signal mediated by βγ subunits acting on a Ras-dependent pathway and a Gα_s-induced inhibitory signal mediated by cAMP and PKA. The balance between these two opposing mechanisms of regulation would be expected to control the MAP kinase response to β-adrenergic agonists as well as to other biologically active agents known to act on G_s coupled receptors, including a number of hormones, neurotransmitters, and lipid mediators.

Sequestration of c-Fos at the nuclear envelope (NE) through interaction with A-type lamins suppresses AP-1–dependent transcription. We show here that c-Fos accumulation within the extraction-resistant nuclear fraction (ERNF) and its interaction with lamin A are reduced and enhanced by gain-of and loss-of ERK1/2 activity, respectively. Moreover, hindering ERK1/2-dependent phosphorylation of c-Fos attenuates its release from the ERNF induced by serum and promotes its interaction with lamin A. Accordingly, serum stimulation rapidly releases preexisting c-Fos from the NE via ERK1/2-dependent phosphorylation, leading to a fast activation of AP-1 before de novo c-Fos synthesis. Moreover, lamin A–null cells exhibit increased AP-1 activity and reduced levels of c-Fos phosphorylation. We also find that active ERK1/2 interacts with lamin A and colocalizes with c-Fos and A-type lamins at the NE. Thus, NE-bound ERK1/2 functions as a molecular switch for rapid mitogen-dependent AP-1 activation through phosphorylation-induced release of preexisting c-Fos from its inhibitory interaction with lamin A/C.