Glucocorticoids act within minutes to inhibit recruitment of signalling factors to activated EGF receptors through a receptor-dependent, transcription-independent mechanism.
Recruitment to activated tyrosine kinase growth factor receptors of Grb2 and p21(ras) leads to downstream activation of the kinases Raf, MAPK/Erk kinase (Mek) and, subsequently, extracellular signal-regulated kinase (Erk). Activated Erk phosphorylates specific serine residues within cytosolic phospholipase A(2) (PLA(2)), promoting enzyme translocation to membranes and facilitating liberation of arachidonic acid (AA). In the A549 human adenocarcinoma cell line dexamethasone inhibited epidermal growth factor (EGF)-stimulated cytosolic PLA(2) (cPLA(2)) activation and AA release by blocking the recruitment of Grb2 to the activated EGF receptor (EGF-R) through a glucocorticoid receptor (GR)-dependent (RU486-sensitive), transcription-independent (actinomycin-insensitive), mechanism. The dexamethasone-induced block of Grb2 recruitment was parallelled by changes in phosphorylation status and subcellular localization of lipocortin 1 (LC1) and an increase in the amount of the tyrosine phosphoprotein co-localized with EGF-R. Like dexamethasone, peptides containing E-Q-E-Y-V from the N-terminal domain of LC1 also blocked ligand-induced association of Grb2, p21(ras) and Raf. Our results point to an unsuspected rapid effect of glucocorticoids, mediated by occupation of GR but not by changes in gene transcription, which is brought about by competition between LC1 and Grb2 leading to a failure of recruitment off signalling factors to EGF-R
Croxtall
J D
JD
Department of Biochemical Pharmacology, The William Harvey Research Institute, St. Bartholomew's and the Royal London School of Medicine & Dentistry (Queen Mary and Westfield College), Charterhouse Square, London EC1M 6BQ. j.croxtall@qmw.ac.uk
Choudhury
Q
Q
Flower
R J
RJ
D048868:Adaptor Proteins, Signal Transducing; D017305:Annexin A1 / Q000096:biosynthesis* / Q000378:metabolism; D001667:Binding, Competitive; D003907:Dexamethasone / Q000494:pharmacology*; D004789:Enzyme Activation; D066246:ErbB Receptors / Q000378:metabolism*; D051380:GRB2 Adaptor Protein; D005938:Glucocorticoids / Q000494:pharmacology*; D006801:Humans; D020935:MAP Kinase Signaling System / Q000502:physiology; D010741:Phospholipases A / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D048868:Adaptor Proteins, Signal Transducing; D017305:Annexin A1; D051380:GRB2 Adaptor Protein; C495948:GRB2 protein, human; D005938:Glucocorticoids; D011506:Proteins; D011965:Receptors, Glucocorticoid; D003907:Dexamethasone; D066246:ErbB Receptors; D010741:Phospholipases A