Journal of Biological Chemistry
Volume 277, Issue 13, 29 March 2002, Pages 10876-10882
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MECHANISMS OF SIGNAL TRANSDUCTION
Dexras1/AGS-1 Inhibits Signal Transduction from the Gi-coupled Formyl Peptide Receptor to Erk-1/2 MAP Kinases*

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Dexras1 is a novel GTP-binding protein (G protein) that was recently discovered on the basis of rapid mRNA up-regulation by glucocorticoids in murine AtT-20 corticotroph cells and in several primary tissues. The human homologue of Dexras1, termed activator of G protein signaling-1 (AGS-1), has been reported to stimulate signaling by Gi heterotrimeric G proteins independently of receptor activation. The effects of Dexras1/AGS-1 on receptor-initiated signaling by Gi have not been examined. Here we report that Dexras1 inhibits ligand-dependent signaling by the Gi-coupled N-formyl peptide receptor (FPR). Dexras1 and FPR were transiently co-expressed in both COS-7 and HEK-293 cells. Activation of FPR by ligand (N-formyl-methionine-leucine-phenylalanine (f-MLF)) caused phosphorylation of endogenous Erk-1/2 that was reduced by co-expression of Dexras1. Direct effects of Dexras1 on the activity of co-expressed, epitope-tagged Erk-2 (hemagglutinin (HA)-Erk-2) were measured by immune complex in vitro kinase assay. Expression of Dexras1 alone resulted in a 1.9- to 4.9-fold increase in HA-Erk-2 activity; expression of the unliganded FPR alone resulted in a 6.2- to 8.1-fold increase in HA-Erk-2 activity. Stimulation of FPR by f-MLF produced a further 8- to 10-fold increase in HA-Erk-2 activity over the basal (non-ligand-stimulated) state, and this ligand-dependent activity was attenuated at the time points of maximal activity by co-expression of Dexras1 (reduced 31 ± 6.8% in COS-7 at 10 min and 86 ± 9.2% in HEK-293 at 5 min, p < 0.01 for each). Expression of Dexras1 did not influence protein expression of FPR or Erk, suggesting that the inhibitory effects of Dexras1 reflect a functional alteration in the signaling cascade from FPR to Erk. Expression of Dexras1 had no effect on expression of Giα species, but significantly impaired pertussis toxin-catalyzed ADP-ribosylation of membrane-associated Giα. Expression of Dexras1 also significantly decreased in vitro binding of GTPγS in f-MLF-stimulated membranes of cells co-transfected with FPR. These data suggest that Dexras1 inhibits signal transduction from FPR to Erk-1/2 through an effect that is very proximal to receptor-Gi coupling. While Dexras1 weakly activates Erk in the resting state, more potent effects are evident in the modulation of ligand-stimulated receptor signal transduction, where Dexras1 functions as an inhibitor rather than activator of the Erk mitogen-activated protein kinase signaling cascade.

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Published, JBC Papers in Press, December 21, 2001, DOI 10.1074/jbc.M110397200

*

This work was generously supported by the Veterans Affairs Medical Merit Review (to R. I. D. and T. E. G.) and by National Institutes of Health Grant A143932 (to E. R. P.). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.