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Papers In Press, published online ahead of print August 22, 2006
Department of Medicine, Medical University of South Carolina, Charleston, SC 29425
Corresponding Author: luttrell{at}musc.edu
Although several studies have shown that a subset of insulin-like growth factor (IGF) signals require the activation of heterotrimeric G proteins, the molecular mechanisms underlying IGF-stimulated G protein signaling remain poorly understood. Here, we have studied the mechanism by which endogenous IGF receptors activate the ERK1/2 mitogen-activated protein kinase cascade in HEK293 cells. In these cells, treatment with pertussis toxin and expression of a Gaq/11(305-359) peptide that inhibits Gq/11 signaling additively inhibited IGF-stimulated ERK1/2 activation, indicating that the signal was almost completely G protein-dependent. Treatment with IGF-1 or IGF-2 promoted translocation of green fluorescent protein (GFP)-tagged Sphingosine Kinase (SK) 1 from the cytosol to the plasma membrane, increased endogenous SK activity within 30 sec of stimulation, and caused a statistically significant increase in intracellular and extracellular Sphingosine-1-Phosphate (S1P) concentration. Using a GFP-tagged S1P1 receptor as a biological sensor for the generation of physiologically relevant S1P levels, we found that IGF-1 and IGF-2 induced GFP-S1P receptor internalization and that the effect was blocked by pretreatment with the SK inhibitor, dimethylsphingosine. Treating cells with dimethylsphingosine, silencing SK1 expression by RNA interference and blocking endogenous S1P receptors with the competitive antagonist VPC23019 all significantly inhibited IGF-stimulated ERK1/2 activation, suggesting that IGFs elicit G protein-dependent ERK1/2 activation by stimulating SK1-dependent transactivation of S1P receptors. Given the ubiquity of SK and S1P receptor expression, S1P receptor transactivation may represent a general mechanism for G protein-dependent signaling by non-G protein-coupled receptors.
J. Biol. Chem, 10.1074/jbc.M605339200
Submitted on June 5, 2006
Revised on July 24, 2006
Accepted on August 22, 2006
Insulin-like growth factors mediate heterotrimeric G protein-dependent ERK1/2 activation by transactivating sphingosine-1-phosphate receptors
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