Activation of ERK1/2 by platelet-activating factor receptor is independent of receptor internalisation and G-protein activation

Abstract

Platelet-activating factor (PAF) is a potent pro-inflammatory phospholipid mediator involved in a broad range of physiological and pathophysiological processes. The receptor of PAF (PAFR) is a heptahelical G-protein-coupled receptor. We have shown previously that upon agonist stimulation, PAFR internalised through clathrin-coated vesicles in an arrestin-dependent, but G-protein-coupling-independent manner. In the current report, we demonstrate that PAF stimulates Erk1/2 phosphorylation and: (1) dominant negative mutants of arrestins and dynamin do not influence Erk1/2 activation, (2) hypertonic conditions do not decrease the extent of Erk1/2 phosphorylation, (3) internalisation-deficient and/or G-protein-coupling-deficient mutants of PAFR activate Erk1/2 as efficiently as the wild-type PAFR, and (4) inhibition of epidermal growth factor receptor (EGFR) does not block Erk1/2 activation. Taken together, our results suggest that PAFR-mediated activation of mitogen-activated protein kinases Erk1/2 does not require receptor endocytosis, receptor tyrosine kinase transactivation or G-protein activation. In addition, our studies reveal that PAFR-mediated signals of G-protein activation, receptor internalisation and MAPK activation are differentially regulated by receptor structure and/or conformation.